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This study investigates the cortical correlates of motor response control and monitoring, using the Theory of Event Coding (TEC) as a framework to investigate signals related to low‐level sensory processing of motor reafference and high‐level response monitoring, including verification of response outcomes with the internal model. We used a visuomotor paradigm with two targets at different distances from the participant. For the recorded movement‐related cortical potentials (MRCPs), we analyzed their different components and assessed the movement phases during which they are active. Residual iteration decomposition (RIDE) and multivariate pattern analysis (MVPA) were used for this analysis. Using RIDE, we separated MRCPs into signals related to different parallel processes of visuomotor transformation: stimulus processing (S‐cluster), motor response preparation and execution (R‐cluster), and intermediate processes (C‐cluster). We revealed sequential activation in the R‐cluster, with execution‐related negative components and positive contralateral peaks reflecting reafference processing. We also identified the motor post‐imperative negative variation within the R‐cluster, highlighting the response outcome evaluation process included in the action file. Our findings extend the understanding of C‐cluster signals, typically associated with stimulus–response mapping, by demonstrating C‐activation from the preparatory stages through to response termination, highlighting its participation in action monitoring. In addition, we highlighted the ability of MVPA to identify movement‐related attribute encoding: where statistical analysis showed independence of stimulus processing activity from movement distance, MVPA revealed distance‐related differences in the S‐cluster within a time window aligned with the lateralized readiness potential (LRP). This highlights the importance of integrating RIDE and MVPA to uncover the intricate neural dynamics of motor control, sensory integration, and response monitoring. [ABSTRACT FROM AUTHOR]

Various functional modulations of the stretch reflex help to stabilize actions, but the computational mechanism behind its context-dependent tuning remains unclear. While many studies have demonstrated that motor contexts associated with the task goal cause functional modulation of the stretch reflex of upper limbs, it is not well understood how visual contexts independent of the task requirements affect the stretch reflex. To explore this issue, we conducted two experiments testing 20 healthy human participants (age range 20-45, average 31.3 ± 9.0), in which visual contexts were manipulated in a visually guided reaching task. During wrist flexion movements toward a visual target, a mechanical load was applied to the wrist joint to evoke stretch reflex of wrist flexor muscle (flexor carpi radialis). The first experiment (n = 10) examined the effect of altering the visuomotor transformation on the stretch reflex that was evaluated with surface electromyogram. We found that the amplitude of the stretch reflex decreased (p = 0.024) when a rotational transformation of 90_ was introduced between the hand movement and the visual cursor, whereas the amplitude did not significantly change (p = 0.26) when the rotational transformation was accompanied by a head rotation so that the configuration of visual feedback was maintained in visual coordinates. The results suggest that the stretch reflex was regulated depending on whether the visuomotor mapping had already been acquired or not. In the second experiment (n = 10), we examined how uncertainty in the visual target or hand cursor affects the stretch reflex by removing these visual stimuli. We found that the reflex amplitude was reduced by the disappearance of the hand cursor (p = 0.039), but was not affected by removal of the visual target (p = 0.27), suggesting that the visual state of the body and target contribute differently to the reflex tuning. These findings support the idea that visual updating of the body state is crucial for regulation of quick motor control driven by proprioceptive signals. [ABSTRACT FROM AUTHOR]

Fundamental to human movement is the ability to interact with objects in our environment. How one reaches an object depends on the object's shape and intended interaction afforded by the object, e.g., grasp and transport. Extensive research has revealed that the motor intention of reach-to-grasp can be decoded from cortical activities using EEG signals. The goal of the present study is to determine the extent to which information encoded in the EEG signals is shared between two limbs to enable cross-hand decoding. We performed an experiment in which human subjects (n = 10) were tasked to interact with a novel object with multiple affordances using either right or left hands. The object had two vertical handles attached to a horizontal base. A visual cue instructs what action (lift or touch) and whether the left or right handle should be used for each trial. EEG was recorded and processed from bilateral frontal-central-parietal regions (30 channels). We trained LDA classifiers using data from trials performed by one limb and tested the classification accuracy using data from trials performed by the contralateral limb. We found that the type of hand-object interaction can be decoded with approximately 59 and 69% peak accuracy in the planning and execution stages, respectively. Interestingly, the decoding accuracy of the reaching directions was dependent on how EEG channels in the testing dataset were spatially mirrored, and whether directions were labeled in the extrinsic (object-centered) or intrinsic (body-centered) coordinates. [ABSTRACT FROM AUTHOR]

Fundamental to human movement is the ability to interact with objects in our environment. How one reaches an object depends on the object’s shape and intended interaction afforded by the object, e.g., grasp and transport. Extensive research has revealed that the motor intention of reach-to-grasp can be decoded from cortical activities using EEG signals. The goal of the present study is to determine the extent to which information encoded in the EEG signals is shared between two limbs to enable cross-hand decoding. We performed an experiment in which human subjects (n = 10) were tasked to interact with a novel object with multiple affordances using either right or left hands. The object had two vertical handles attached to a horizontal base. A visual cue instructs what action (lift or touch) and whether the left or right handle should be used for each trial. EEG was recorded and processed from bilateral frontal-central-parietal regions (30 channels). We trained LDA classifiers using data from trials performed by one limb and tested the classification accuracy using data from trials performed by the contralateral limb. We found that the type of hand-object interaction can be decoded with approximately 59 and 69% peak accuracy in the planning and execution stages, respectively. Interestingly, the decoding accuracy of the reaching directions was dependent on how EEG channels in the testing dataset were spatially mirrored, and whether directions were labeled in the extrinsic (object-centered) or intrinsic (body-centered) coordinates.

Currently, P300-BCIs are mostly used for spelling tasks, where the number of commands is equal to the number of stimuli that evoke event-related potentials (ERPs). Increasing this number slows down the BCI operation because each stimulus has to be presented several times for better classification. Furthermore, P300 spellers typically do not utilize potentially useful imagery-based approaches, such as the motor imagery successfully practiced in motor rehabilitation. Here, we tested a P300-BCI with a motor-imagery component. In this BCI, the number of commands was increased by adding mental strategies instead of increasing the number of targets. Our BCI had only two stimuli and four commands. The subjects either counted target appearances mentally or imagined hand movements toward the targets. In this design, the motor-imagery paradigm enacted a visuomotor transformation known to engage cortical and subcortical networks participating in motor control. The operation of these networks suffers in neurological conditions such as stroke, so we view this BCI as a potential tool for the rehabilitation of patients. As an initial step toward the development of this clinical method, sixteen healthy participants were tested. Consistent with our expectation that mental strategies would result in distinct EEG activities, ERPs were different depending on whether subjects counted stimuli or imagined movements. These differences were especially clear in the late ERP components localized in the frontal and centro-parietal regions. We conclude that (1) the P300 paradigm is suitable for enacting visuomotor transformations and (2) P300-based BCIs with multiple mental strategies could be used in applications where the number of possible outputs needs to be increased while keeping the number of targets constant. As such, our approach adds to both the development of versatile BCIs and clinical approaches to rehabilitation. [ABSTRACT FROM AUTHOR]

Introduction: Interacting with the environment requires the planning and execution of reach‐to‐target movements along given reach trajectory paths. Human neural mechanisms for the motor planning of linear, or point‐to‐point, reaching movements are relatively well studied. However, the corresponding representations for curved and more complex reaching movements require further investigation. Additionally, the visual and proprioceptive feedback of hand positioning can be spatially and sequentially coupled in alignment (e.g., directly reaching for an object), termed coupled visuomotor feedback, or spatially decoupled (e.g., dragging the computer mouse forward to move the cursor upward), termed decoupled visuomotor feedback. During reach planning, visuomotor processing routes may differ across feedback types. Methods: We investigated the involvement of the frontoparietal regions, including the superior parietal lobule (SPL), dorsal premotor cortex (PMd), and dorsolateral prefrontal cortex (dlPFC), in curved reach planning under different feedback conditions. Participants engaged in two delayed‐response reaching tasks with identical starting and target position sets but different reach trajectory paths (linear or curved) under two feedback conditions (coupled or decoupled). Neural responses in frontoparietal regions were analyzed using a combination of functional near‐infrared spectroscopy and electroencephalography. Results: The results revealed that, regarding the cue period, curved reach planning had a higher hemodynamic response in the left SPL and bilateral PMd and a smaller high‐beta power in the left parietal regions than linear reach planning. Regarding the delay period, higher hemodynamic responses during curved reach planning were observed in the right dlPFC for decoupled feedback than those for coupled feedback. Conclusion: These findings suggest the crucial involvement of both SPL and PMd activities in trajectory‐path processing for curved reach planning. Moreover, the dlPFC may be especially involved in the planning of curved reaching movements under decoupled feedback conditions. Thus, this study provides insight into the neural mechanisms underlying reaching function via different feedback conditions. [ABSTRACT FROM AUTHOR]

When required, humans can generate very short latency reaches toward visual targets, such as catching a falling cellphone. During such rapid reaches, express arm responses are the first wave of upper limb muscle recruitment, occurring -80-100 ms after target appearance. There is accumulating evidence that express arm responses arise from signaling along the tecto-reticulo-spinal tract, but the involvement of the reticulospinal tract has not been well studied. Since the reticulospinal tract projects bilaterally, we studied whether express arm responses would be generated bilaterally. Human participants (n = 14; 7 females) performed visually guided reaches in a modified emerging target paradigm where either arm could intercept the target. We recorded electromyographic activity bilaterally from the pectoralis major muscle. Our analysis focused on target locations where participants reached with the right arm on some trials, and the left arm on others. In support of the involvement of the reticulospinal tract, express arm responses persisted bilaterally regardless of which arm reached to the target. The latency and magnitude of the express arm response did not depend on whether the arm was chosen to reach or not. However, on the reaching arm, the magnitude of the express arm response was correlated to the level of anticipatory activity. The bilateral generation of express arm responses supports the involvement of the reticulospinal tract. We surmise that the correlation between anticipatory activity and the magnitude of express arm responses on the reaching arm arises from convergence of cortically derived signals with a parallel subcortical pathway mediating the express arm response. NEW & NOTEWORTHY Express arm responses have been proposed to arise from the tecto-reticulo-spinal tract originating within the superior colliculus, but the involvement of the reticulospinal tract has not been well studied. Here, we show these responses appear bilaterally in a task where either arm can reach to a newly appearing stimulus. Our results suggest that the most rapid visuomotor transformations for reaching are performed by a subcortical pathway. [ABSTRACT FROM AUTHOR]

Visually guided behavior relies on the integration of sensory input and information held in working memory (WM). Yet it remains unclear how this is accomplished at the level of neural circuits. We studied the direct visual cortical inputs to neurons within a visuomotor area of prefrontal cortex in behaving monkeys. We show that the efficacy of visual input to prefrontal cortex is gated by information held in WM. Surprisingly, visual input to prefrontal neurons was found to target those with both visual and motor properties, rather than preferentially targeting other visual neurons. Furthermore, activity evoked from visual cortex was larger in magnitude, more synchronous, and more rapid, when monkeys remembered locations that matched the location of visual input. These results indicate that WM directly influences the circuitry that transforms visual input into visually guided behavior.

Goal‐directed movements involve a series of neural computations that compare the sensory representations of goal location and effector position, and transform these into motor commands. Neurons in posterior parietal cortex (PPC) control several effectors (e.g., eye, hand, foot) and encode goal location in a variety of spatial coordinate systems, including those anchored to gaze direction, and to the positions of the head, shoulder, or hand. However, there is little evidence on whether reference frames depend also on the effector and/or type of motor response. We addressed this issue in macaque PPC area V6A, where previous reports using a fixate‐to‐reach in depth task, from different starting arm positions, indicated that most units use mixed body/hand‐centered coordinates. Here, we applied singular value decomposition and gradient analyses to characterize the reference frames in V6A while the animals, instead of arm reaching, performed a nonspatial motor response (hand lift). We found that most neurons used mixed body/hand coordinates, instead of "pure" body‐, or hand‐centered coordinates. During the task progress the effect of hand position on activity became stronger compared to target location. Activity consistent with body‐centered coding was present only in a subset of neurons active early in the task. Applying the same analyses to a population of V6A neurons recorded during the fixate‐to‐reach task yielded similar results. These findings suggest that V6A neurons use consistent reference frames between spatial and nonspatial motor responses, a functional property that may allow the integration of spatial awareness and movement control. [ABSTRACT FROM AUTHOR]

Prior expectations of movement instructions can promote preliminary action planning and influence choices. We investigated how action priors affect action-goal encoding in premotor and parietal cortices and if they bias subsequent free choice. Monkeys planned reaches according to visual cues that indicated relative probabilities of two possible goals. On instructed trials, the reach goal was determined by a secondary cue respecting these probabilities. On rarely interspersed free-choice trials without instruction, both goals offered equal reward. Action priors induced graded free-choice biases and graded frontoparietal motor-goal activity, complementarily in two subclasses of neurons. Down-regulating neurons co-encoded both possible goals and decreased opposite-to-preferred responses with decreasing prior, possibly supporting a process of choice by elimination. Up-regulating neurons showed increased preferred-direction responses with increasing prior, likely supporting a process of computing net likelihood. Action-selection signals emerged earliest in down-regulating neurons of premotor cortex, arguing for an initiation of selection in the frontal lobe.

As animals forage for food and water or evade predators, they must rapidly decide what visual features in the environment deserve attention. In vertebrates, this visuomotor computation is implemented within the neural circuits of the optic tectum (superior colliculus in mammals). However, the mechanisms by which tectum decides whether to approach or evade remain unclear, and also which neural mechanisms underlie this behavioral choice. To address this problem, we used an eye-brain-spinal cord preparation to evaluate how the lamprey responds to visual inputs with distinct stimulus-dependent motor patterns. Using ventral root activity as a behavioral readout, we classified 2 main types of fictive motor responses: (i) a unilateral burst response corresponding to orientation of the head toward slowly expanding or moving stimuli, particularly within the anterior visual field, and (ii) a unilateral or bilateral burst response triggering fictive avoidance in response to rapidly expanding looming stimuli or moving bars. A selective pharmacological blockade revealed that the brainstem-projecting neurons in the deep layer of the tectum in interaction with local inhibitory interneurons are responsible for selecting between these 2 visually triggered motor actions conveyed through downstream reticulospinal circuits. We suggest that these visual decision-making circuits had evolved in the common ancestor of vertebrates and have been conserved throughout vertebrate phylogeny. [ABSTRACT FROM AUTHOR]

The present study tested whether remapping of visuomotor correspondence alters automatic motor responses induced by visual stimuli. We hypothesized that the congruency effect, an automatic modulation of motor responses based on stimulus-response congruency, changes in accordance with a new visuomotor correspondence acquired through an adaptation task. To induce visuomotor adaptation, participants performed a tracking task with 30° or 150° rotation of the visual feedback. The congruency effect was evaluated multiple times by a visual response task where participants moved their finger left or right. We predicted that the congruency effect, as a measure of automatic responses, would be almost reversed after adaptation to the 150° rotation, because a visual stimulus spatially opposite to the participant's own action would become a "congruent" stimulus in a 150°-rotated environment but not in a 30°-rotation environment. The results show that visuomotor adaptation to the 150° rotation did modulate the congruency effect in accordance with the acquired visuomotor correspondence, but did not completely reverse the effect. When the effect was assessed after the manipulation, which was assumed to switch an internal model back to its normal state, there was no change in automatic motor responses. Furthermore, we found that after effects developed as the training proceeded but decreased over time. These findings suggest that the visuomotor system subserving automatic modulation in motor responses is based on the currently active internal model and, therefore, highly adaptive. In addition, the mechanism underlying after effects in a visuomotor task is discussed in terms of a switching function of internal models. [ABSTRACT FROM AUTHOR]

We previously examined adaptive changes of eye-hand coordination during learning of a visuomotor rotation. Gazes during reaching movements were initially directed to a feedback cursor in early practice, but were gradually shifted toward the target with more practice, indicating an emerging gaze anchoring behavior. This adaptive pattern reflected a functional change of gaze control from exploring the cursor-hand relation to guiding the hand to the task goal. The present study further examined the effects of hemispace and joint coordination associated with target directions on this behavior. Young adults performed center-out reaching movements to four targets with their right hand on a horizontal digitizer, while looking at a rotated visual feedback cursor on a computer monitor. To examine the effect of hemispace related to visual stimuli, two out of the four targets were located in the ipsilateral workspace relative to the hand used, the other two in the contralateral workspace. To examine the effect of hemispace related to manual actions, two among the four targets were related to reaches made in the ipsilateral workspace, the other two to reaches made in the contralateral workspace. Furthermore, to examine the effect of the complexity of joint coordination, two among the four targets were reaches involving a direct path from the start to the target involving elbow movements (simple), whereas the other two targets were reaches involving both shoulder and elbow movements (complex). The results showed that the gaze anchoring behavior gradually emerged during practice for reaches made in all target directions. The speed of this change was affected mainly by the hemispace related to manual actions, whereas the other two effects were minimal. The gaze anchoring occurred faster for the ipsilateral reaches than for the contralateral reaches; gazes prior to the gaze anchoring were also directed less at the cursor vicinity but more at the mid-area between the starting point and the target. These results suggest that ipsilateral reaches result in a better predictability of the cursor-hand relation under the visuomotor rotation, thereby prompting an earlier functional change of gaze control through practice from a reactive to a predictive control. [ABSTRACT FROM AUTHOR]

Neurons in the posterior parietal cortex of macaques show spatial tuning during several phases of an instructed delay reaching task, but their reference frames have been studied mostly during fixed periods without addressing how they evolve across task phases. In parietal area V6A, we reported recently that during the late delay and hand movement periods, most neurons represent target location either in body-centered frame of reference, or in mixed body/hand-centered coordinates, with no evidence of hand-centered representations. Here, we characterized the spatial representations of V6A neurons in earlier task epochs, i.e., immediately after target fixation and in the subsequent main part of the delay and examined whether the reference frames of individual neurons are stable across the task. We report no evidence of hand-centered coding also in the earlier phases of the task. Shortly, after target fixation and throughout the main part of the delay period, V6A neurons used either body-centered or mixed body/hand-centered reference frames. Most of the cells showed consistent reference frames across epochs. Interestingly, a population trend of shifting from mixed body/hand-centered frames to 'pure' body-centered coordinates was found as the task progressed. These findings suggest that, similar to other parietal areas, in V6A, the reference frames show a limited degree of temporal evolution. The stronger presence of mixed coding at the early task stages could reflect the early involvement of V6A in eye-hand coordination, whereas the increase in spatiotopic representations towards movement execution could be related to its role in online movement control. [ABSTRACT FROM AUTHOR]

We previously reported that visuomotor activity in the superior colliculus (SC) - a key midbrain structure for the generation of rapid eye movements - preferentially encodes target position relative to the eye (Te) during low-latency head-unrestrained gaze shifts (DeSouza et al., 2011). Here, we trained two monkeys to perform head-unrestrained gaze shifts after a variable post-stimulus delay (400-700 ms), to test whether temporally separated SC visual and motor responses show different spatial codes. Target positions, final gaze positions and various frames of reference (eye, head, and space) were dissociated through natural (untrained) trial-to-trial variations in behaviour. 3D eye and head orientations were recorded, and 2D response field data were fitted against multiple models by use of a statistical method reported previously (Keith et al., 2009). Of 60 neurons, 17 showed a visual response, 12 showed a motor response, and 31 showed both visual and motor responses. The combined visual response field population (n = 48) showed a significant preference for Te, which was also preferred in each visual subpopulation. In contrast, the motor response field population (n = 43) showed a preference for final (relative to initial) gaze position models, and the Te model was statistically eliminated in the motor-only population. There was also a significant shift of coding from the visual to motor response within visuomotor neurons. These data confirm that SC response fields are gaze-centred, and show a target-to-gaze transformation between visual and motor responses. Thus, visuomotor transformations can occur between, and even within, neurons within a single frame of reference and brain structure. [ABSTRACT FROM AUTHOR]

When we learn a novel task, the motor system needs to acquire both feedforward and feedback control. Currently, little is known about how the learning of these two mechanisms relate to each other. In the present study, we tested whether feedforward and feedback control need to be learned separately, or whether they are learned as common mechanism when a new control policy is acquired. Participants were trained to reach to two lateral and one central target in an environment with mirror (left-right)-reversed visual feedback. One group was allowed to make online movement corrections, whereas the other group only received visual information after the end of the movement. Learning of feedforward control was assessed by measuring the accuracy of the initial movement direction to lateral targets. Feedback control was measured in the responses to sudden visual perturbations of the cursor when reaching to the central target. Although feedforward control improved in both groups, it was significantly better when online corrections were not allowed. In contrast, feedback control only adaptively changed in participants who received online feedback and remained unchanged in the group without online corrections. Our findings suggest that when a new control policy is acquired, feedforward and feedback control are learned separately, and that there may be a trade-off in learning between feedback and feedforward controllers. [ABSTRACT FROM AUTHOR]

Visuomotor adaptation involves the learning of a new mapping between a spatial goal and well-learned movements. In order to learn a new visuomotor transformation, visual attention is needed to monitor movements and their visual consequences. Once a transformation is learnt, it can be executed automatically without attentional control. Using steady-state visual evoked potentials (SSVEPs) measured from EEG activity, we examined how visual attention changes during the early phase of visuomotor adaptation. SSVEPs were elicited by a green disc flickering at 15 Hz which was either the movement target or the cursor that participants controlled. Participants performed an adapted continuous visuomotor adaptation task with either 60° or 120° screen cursor rotation, and changes in 15-Hz SSVEP power were examined. Participants' performance improved over time in all conditions, with the rate of learning significantly influenced by the degree of rotation. SSVEPs at 15 Hz showed a significant change over time with adaptation for 60° rotations, but not for 120° rotations, such that SSVEPs elicited by the stimuli were significantly lower for 60° compared with 120° rotation conditions over the last adaptation blocks. This suggests that visual attention to the movement target and feedback reduces over time as performance improves during visuomotor adaptation for easier rotations, but must be maintained throughout the task for more difficult 120° rotations that might require more strategic control. [ABSTRACT FROM AUTHOR]

Smooth pursuit eye movements are driven by retinal motion and enable us to view moving targets with high acuity. Complicating the generation of these movements is the fact that different eye and head rotations can produce different retinal stimuli but giving rise to identical smooth pursuit trajectories. However, because our eyes accurately pursue targets regardless of eye and head orientation (Blohm G, Lefèvre P. J Neurophysiol 104: 2103-2115, 2010), the brain must somehow take these signals into account. To learn about the neural mechanisms potentially underlying this visual-to-motor transformation, we trained a physiologically inspired neural network model to combine twodimensional (2D) retinal motion signals with three-dimensional (3D) eye and head orientation and velocity signals to generate a spatially correct 3D pursuit command. We then simulated conditions of 1) head roll-induced ocular counterroll, 2) oblique gaze-induced retinal rotations, 3) eccentric gazes (invoking the half-angle rule), and 4) optokinetic nystagmus to investigate how units in the intermediate layers of the network accounted for different 3D constraints. Simultaneously, we simulated electrophysiological recordings (visual and motor tunings) and microstimulation experiments to quantify the reference frames of signals at each processing stage. We found a gradual retinal-to-intermediate-to-spatial feedforward transformation through the hidden layers. Our model is the first to describe the general 3D transformation for smooth pursuit mediated by eye- and head-dependent gain modulation. Based on several testable experimental predictions, our model provides a mechanism by which the brain could perform the 3D visuomotor transformation for smooth pursuit. [ABSTRACT FROM AUTHOR]

Eye-hand coordination is crucial for our ability to interact with the world around us. However, much of the visually guided reaches that we perform require a spatial decoupling between gaze direction and hand orientation. These complex decoupled reaching movements are in contrast to more standard eye and hand reaching movements in which the eyes and the hand are coupled. The superior parietal lobule (SPL) receives converging eye and hand signals; however, what is yet to be understood is how the activity within this region is modulated during decoupled eye and hand reaches. To address this, we recorded local field potentials within SPL from two rhesus macaques during coupled vs. decoupled eye and hand movements. Overall we observed a distinct separation in synchrony within the lower 10- to 20-Hz beta range from that in the higher 30- to 40-Hz gamma range. Specifically, within the early planning phase, beta synchrony dominated; however, the onset of this sustained beta oscillation occurred later during eye-hand decoupled vs. coupled reaches. As the task progressed, there was a switch to low-frequency and gamma-dominated responses, specifically for decoupled reaches. More importantly, we observed local field potential activity to be a stronger task (coupled vs. decoupled) and state (planning vs. execution) predictor than that of single units alone. Our results provide further insight into the computations of SPL for visuomotor transformations and highlight the necessity of accounting for the decoupled eye-hand nature of a motor task when interpreting movement control research data. [ABSTRACT FROM AUTHOR]

Previous studies have demonstrated a great uncertainty in evaluating one's own voluntary actions when visual feedback is suspended. We now compare these limitations in younger and older adults during active or passive limb movements. Participants put their dominant hand on a robot arm and performed movements actively or the relaxed limb was moved passively. Either a distorted visual feedback or no visual feedback at all was provided during the movement. Perception of limb movements was attenuated through visual feedback. This effect was more pronounced in older adults. However, no difference between active and passive movements was found. The results provide evidence for the limited awareness of body effects, even in the absence of voluntary actions.

Among vertebrates, birds have a particularly well-developed retinopetal system, i.e., the centrifugal system projecting from the brain to the retina. Primary, secondary and tertiary neurons of the retinopetal system connect serially in a one-to-one-to-one fashion, thereby assembling several thousand retinopetal modules projecting in parallel to the retina. The signal conveyed by the retinopetal system enhances the visual responses of retinal ganglion cells (RGCs), and each retinopetal module may facilitate the RGCs in a topographically restricted area. We investigated how the retinopetal signal contributes to visual search by first training Japanese quail to search and peck a target stimulus presented on one of three touch-sensor monitors around the bird and then examining the effect of retinopetal signal blockage on the task. Our results indicate that the retinopetal signal improves both stimulus detectability and target discriminability. Thus, this study confirms that the avian retinopetal system functions specifically in attentional facilitation of the retina; this new finding suggests that this system benefits early detection and late selection phases of visual search. The fast and precise target detection facilitated by the retinopetal signal may be transformed into quick and correct orienting to the target through visuomotor transformation mechanisms implemented in the optic tectum. [ABSTRACT FROM AUTHOR]

Abstract: Discrimination of proprioceptive and visual spatial information is a prerequisite for the learning of visuo-motor transformations. This study investigated the individual''s capability to discriminate the directions of seen cursor motions and felt hand movements under a visuo-motor rotation paradigm and its age-related variation. Young and older participants performed 3-stroke arm movements on a digitizing tablet without seeing their arm. The visual feedback of the second stroke was rotated randomly by various angles ranging from −30° to 30° and displayed on a monitor. Older adults were poorer in discrimination than young adults. In both age groups, the felt hand direction was shifted toward the seen cursor direction (i.e., visual capture) by approximately 25% to 30% of the rotation of the visual feedback. Older adults also showed an enhanced visual capture. The results suggest that both the increased sensory noise and the increased assimilation of the bimodal information cause the reduction of discrimination capability in older adults. These findings provide underlying reasons for age-related changes in learning a new visuo-motor transformation. [Copyright &y& Elsevier]

Abstract: When reaching for an object in the environment, the brain often has access to multiple independent estimates of that object’s location. For example, if someone places their coffee cup on a table, then later they know where it is because they see it, but also because they remember how their reaching limb was oriented when they placed the cup. Intuitively, one would expect more accurate reaches if either of these estimates were improved (e.g., if a light were turned on so the cup were more visible). It is now well-established that the brain tends to combine two or more estimates about the same stimulus as a maximum-likelihood estimator (MLE), which is the best thing to do when estimates are unbiased. Even in the presence of small biases, relying on the MLE rule is still often better than choosing a single estimate. For this work, we designed a reaching task in which human subjects could integrate proprioceptive and allocentric (landmark-relative) visual information to reach for a remembered target. Even though both of these modalities contain some level of bias, we demonstrate via simulation that our subjects should use an MLE rule in preference to relying on one modality or the other in isolation. Furthermore, we show that when visual information is poor, subjects do, indeed, combine information in this way. However, when we improve the quality of visual information, subjects counter-intuitively switch to a sub-optimal strategy that occasionally includes reliance on a single modality. [Copyright &y& Elsevier]

Research on bimanual coordination has shown that the efficiency of programming an action is determined by the way the action is cognitively represented. In tool use, actions can be represented with respect to the spatial goal of the action (e.g., the nail that is to be hit by a hammer) or with respect to the tool and its transformation (i.e., the function that maps external target locations onto corresponding bodily movements). We investigated whether the way of cuing bimanual actions with tools affects their cognitive representation and the efficiency with which they are programmed. In one group of participants, tool transformations were specified by symbolic cues, whereas the targets were indicated by direct spatial cues. In another group of participants, symbolic cues specified the targets of the tool-use actions, whereas tool transformations were indicated by direct spatial cues. In a third group, both targets and tool transformations were cued directly by spatial cues. It was hypothesized that different cognitive representations would result in more or less efficient programming of the action. Results indicated longer reaction times and a higher error rate in the group with symbolic cuing of the targets as compared to the group with symbolic cuing of the transformations. The latter did not differ much from the direct cuing group. These results suggest that it is more efficient to represent bimanual tool-use actions in terms of the tool transformations involved than in terms of the targets at which they are directed. [ABSTRACT FROM AUTHOR]

We review a new computational model developed to understand how evidence about stimulus salience in visual search is translated into a saccade command. The model uses the activity of visually responsive neurons in the frontal eye field as evidence for stimulus salience that is accumulated in a network of stochastic accumulators to produce accurate and timely saccades. We discovered that only when the input to the accumulation process was gated could the model account for the variability in search performance and predict the dynamics of movement neuron discharge rates. This union of cognitive modeling and neurophysiology indicates how the visual-motor transformation can occur, and provides a concrete mapping between neuron function and specific cognitive processes. [ABSTRACT FROM AUTHOR]

Abstract: In the visuomotor mental rotation (VMR) task, participants point to a location that deviates from a visual target by a predetermined angle. A seminal investigation of the VMR task reported a linear increase in reaction time (RT) as a function of increasing angle, for 5°, 10°, 15°, 35°, 70°, 105°, and 140° (). This finding led to the development of the mental rotation model (MRM) and the assertion that response preparation is mediated via the imagined rotation of a movement vector. To determine if the MRM can be extrapolated to perceptually familiar angles (e.g., 90° and 180°) within a range of equally spaced angles, we evaluated two independent sets of angles: 5°, 10°, 15°, 35°, 70°, 105°, and 140° (experiment one) and 30°, 60°, 90°, 120°, 150°, 180°, and 210° (experiment two). Consistent with the MRM, experiment one revealed a linear increase in RT as a function of increasing angle; however, a non-linear relation was revealed for experiment two. RTs were fastest for 180°, followed by 30°, 90°, 60°, 150°, 210°, and 120°. Such results demonstrate that response preparation was not uniquely mediated via a mental rotation process. Instead, the present work provides evidence of a temporally demanding and cognitively mediated response substitution process, wherein the computational demands of response preparation are determined by the complexity of the sensorimotor transformations mediating the response. [ABSTRACT FROM AUTHOR]

This study investigates coordinative constraints when participants execute discrete bimanual tool use actions. Participants moved two levers to targets that were either presented near the proximal parts of the levers or near the distal tips of the levers. In the first case, the tool transformation (i.e. the relationship between hand movement direction and target direction) was compatible, whereas in the second case, it was incompatible. We hypothesized that an egocentric constraint (i.e. a preference for moving the hands and tools in a mirror-symmetrical fashion) would be dominant when targets are presented near the proximal parts of the levers because in this situation, movements can be coded in terms of body-related coordinates. Furthermore, an allocentric constraint (i.e. a preference to move the hands in the same (parallel) direction in extrinsic space) was expected to be dominant when one of the targets or both are presented near the distal parts of the levers because in this condition, movements have to be coded in an external reference frame. The results show that when both targets are presented near the proximal parts of the levers, participants are faster and produce less errors with mirror-symmetrical when compared to parallel movements. Furthermore, the RT mirror-symmetry advantage is eliminated, when both targets are presented near the distal parts of the levers, and it is reversed, when the target for one lever is presented near its distal part and the target for the other lever is presented near its proximal part. These results show that the dominance of egocentric and allocentric coordinative constraints in bimanual tool use depends on whether movements are coded in terms of body-related coordinates or in an external reference frame. [ABSTRACT FROM AUTHOR]

Visual information can specify spatial layout with respect to the observer (egocentric) or with respect to an external frame of reference (allocentric). People can use both of these types of visual spatial information to guide their hands. The question arises if movements based on egocentric and movements based on allocentric visual information comprise 2 independent visuomotor systems. In the experiments reported here, we used visuomotor adaptation to address this question. In an adaptation phase, subjects received distorted-visual feedback about their hand movements (17° rotation and 110% amplitude stretch). In a testing phase (no-visual feedback), we measured how behavior changes in response to the distorted-visual feedback. During adaptation and testing, we used 2 tasks that required processing of either egocentric or allocentric visual information. The results show that behavioral changes are significantly larger when the same task is used during testing and adaptation, compared to when the task is switched. The findings suggest that the human brain employs 2 partially independent visuomotor systems that rely on different types of visual spatial information. [ABSTRACT FROM AUTHOR]

The neural mechanisms underlying visuomotor transformation are examined based on deficits in tracing and copying, as well as functional neuroimaging studies. The developmental process of copying and tracing, as well as lesion studies with adults showing disability in drawing, are reviewed, then two experiments are introduced. In Experiment 1, a behavioral analysis of copying and tracing by individuals with Williams Syndrome (WS) was presented. In Experiment 2, the brain activity involved in copying and tracing was measured in normal adults using functional magnetic resonance imaging. Based on these findings, we propose a model of visuomotor transformation to explain the neural basis of tracing and copying, as well as to provide a possible neural mechanism underlying the copying deficits and closing-in phenomenon observed in WS. [ABSTRACT FROM AUTHOR]

To investigate the effects of interference tasks in the early stages of motor learning, we used a computerized motor skill analyzer (CMSA) for the lower limb that evaluates the accuracy and speed of movements in tracking tasks. Forty-five healthy subjects were randomly divided into three groups: a control group without an interference task, a mirror image interference group, and a point symmetry interference group. The practice and measurement of motor-learning trials were carried out as follows: (1) 10 practice trials followed by 10 measurement trials in the morning of the first day. (2) 10 measurement trials 6–7 awake hours after the first measurement. (3) 10 measurement trials on the morning of day 2. In the learning interference groups, 10 trials of the mirror image or point symmetry task were performed immediately after the first measurement. Although motor skills of the control group improved, those of the learning interference groups did not. This suggests that a second task performed immediately after the first task interferes with the consolidation of motor skill learning demanding complex visuomotor transformation. [ABSTRACT FROM AUTHOR]

A crucial insight into handwriting dynamics is embodied in the idea that stable, robust handwriting movements correspond to attractors of an oscillatory dynamical system. We present a phase dynamic model of visuomotor performance involved in copying simple oriented lines. Our studies on human performance in copying oriented lines revealed a systematic error pattern in orientation of drawn lines, i.e., lines at certain orientation are drawn more accurately than at other values. Furthermore, human subjects exhibit “flips” in direction at certain characteristic orientations. It is argued that this flipping behavior has its roots in the fact that copying process is inherently ambiguous—a line of given orientation may be drawn in two different (mutually opposite) directions producing the same end result. The systematic error patterns seen in human copying performance is probably a result of the attempt of our visuomotor system to cope with this ambiguity and still be able to produce accurate copying movements. The proposed nonlinear phase-dynamic model explains the experimentally observed copying error pattern and also the flipping behavior with remarkable accuracy. [ABSTRACT FROM AUTHOR]

In natural environments, the orienting of attention to an object of interest occurs jointly with selecting it as a potential target for action. This coupling of perceptual selection and motor planning has led to ‘the premotor theory of attention’, which argues that attention and intention share the same neural mechanism. Here we used fMRI to test this hypothesis by examining neural activity in the temporal parietal junction (TPJ) and intraparietal sulcus (IPS) of the posterior parietal cortex (PPC) while subjects searched for a target among distractors with the presence of color singletons. Task-relevant salience of stimuli and the variability of response time were used to characterize the behavior of attention and intention components of the visuomotor transformation performed in these two cortical regions, respectively. We found that TPJ responses were significantly higher when the color singletons were distractors (vs. targets), suggesting that the TPJ was involved in attentional control of interference from task-irrelevant but salient stimuli. In contrast, signals in the IPS were closely related to the variability of response time, with a larger BOLD response associated with longer RTs, suggesting that the IPS plays a pivotal role in intention by translating encoded information into action through evidence accumulation. Our data help to specify the functional division of labor between the IPS and TPJ and to further dissociate process components in visual search. [ABSTRACT FROM AUTHOR]

Abstract: Visually guided reaching entails multiple coordinate frame transformations between retina-centered target location and body-centered limb location. Reaching errors in optic ataxia (OA) may be caused by disruptions to these transformations. Consistent with this proposal, previous studies report that reaching errors in OA depend primarily on the location of a target relative to the patient’s gaze regardless of its location relative to the head or body midline. We attempted to replicate this finding by testing KE, a patient with OA following bilateral parietal and left premotor lesions (as well as significant non-specific white matter disease) on a reaching task that varied the orientation of his head and torso while holding the gaze-relative position of the target constant (always foveated). In contrast to previous reports, we observed that rotating the head or body away from the midline led to decreased reaching accuracy. Further analyses showed that multiple visuomotor transformation steps might have been disrupted in KE. These results suggest that gaze-relative target position is not the sole determinant of reaching errors in all OA patients. [Copyright &y& Elsevier]

Both theoretical and empirical studies suggest that numerical processing is intimately linked to representations of goal-directed hand actions. Further evidence for this possibility is provided here by the results of two experiments, both of which revealed a powerful influence of numerical magnitude on the selection of hand grasping movements. Human participants performed either power or precision grip responses based on the semantic properties (e.g., parity) of visual Arabic numerals, in Experiment 1, or depending on their surface characteristics (e.g., colour), in Experiment 2. In both the experiments, it was found that small numerical values facilitated precision grip (commonly used to grasp small objects), while large numerical value potentiated power grip (commonly used to grasp large objects). These findings reveal that perceiving numbers can automatically prime grasping gestures, in a similar manner to viewing physical objects. This result is coherent with the view that processing of symbolic (numerical) and physical quantitative information converges onto a shared magnitude system representing the coordinates of action. [ABSTRACT FROM AUTHOR]

This study examined the ability of neurologically healthy individuals and individuals with Alzheimer’s disease (AD) to successfully complete procedures involving short-term spatial visuomotor memory tasks, and tasks involving increasingly complex visuomotor transformations. Participants made sliding finger movements over a clear touch-sensitive screen on two separate spatial planes (vertical and horizontal), to visually constant and remembered target positions. Significant main effects were observed between participant groups on reaction time and movement time measures. As well, significant changes in reaction time and movement time were observed within the patient group over the different of any experimental procedures. In addition, as task increased in complexity significant increases in errors were observed in the AD group. Overall, the results reveal that AD patients show substantial declines in their ability to process and integrate visual information to produce motor responses. Therefore, we believe that this psychophysical research provides further evidence that AD, even early stages of AD, can affect anatomical regions supporting vision for action. Copyright © 2007 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

Abstract: Visuomotor transformation processes are essential when accurate reaching movements towards a visual target have to be performed. In contrast, those transformations are not needed for similar, but non-visually guided, arm movements. According to previous studies, these transformations are carried out by neuronal populations located in the parietal and frontal cortical areas (the so-called “dorsal visual stream”). However, it is still debated whether these processes are mediated by the sequential and/or parallel activation of the frontoparietal areas. To investigate this issue, we designed a task where the same visual cue could represent either the target of a reaching/pointing movement or the go-signal for a similar but non-targeting arm movement. By subtracting the event-related potentials (ERPs) recorded from healthy subjects performing the two conditions, we identified the brain processes underlying the visuomotor transformations needed for accurate reaching/pointing movements. We then localized the generators by means of cortical current density (CCD) reconstruction and studied their dynamics from visual cue presentation to movement onset. The results showed simultaneous activation of the parietal and frontal areas from 140 to 260 ms. The results are interpreted as neural correlates of two critical phases of visuomotor integration, namely target selection and movement selection. Our findings suggest that the visuomotor transformation processes required for correct reaching/pointing movements do not rely on a purely sequential activation of the frontoparietal areas, but mainly on a parallel information processing system, where feedback circuits play an important role before movement onset. [Copyright &y& Elsevier]

When we react to the outer world, perceived sensory information is frequently memorized over a temporal interval then transformed into a motor command based on a behavioural rule. In this type of memory-based sensorimotor transformation, working memory is considered to play an important role. It has been suggested that the lateral prefrontal cortex is involved in the process of the working memory. However, the neuronal mechanism for guiding a motor command from the working memory has not been established. To examine how visuospatial working memory is linked with a forthcoming saccade direction, we used an antisaccade paradigm for monkeys in which a behavioural rule was presented in the middle of a delay period. In this task, the subjects were required to maintain cue location and to select a response based on a behavioural rule. We found that a subset of mnemonic neurons in the lateral prefrontal cortex changed their representation from cue to saccade direction. Furthermore, the discriminability for saccade direction of these neurons tended to appear soon after the behavioural rule presentation, indicating their significant dependency on the behavioural rule. These results suggest that a subset of mnemonic neurons in the lateral prefrontal cortex change their activity depending on a behavioural rule to guide a prospective motor command. [ABSTRACT FROM AUTHOR]

Information about the world is often encoded in the brain as topographic maps. These internal representations are not always static but can have a dynamic nature, allowing for constant adjustments that depend on factors like experience or injury. Recently, it has been shown that areas involved in visuomotor transformations also show topographical organization. These findings suggest that it could be possible to observe plastic modifications in specific parts of the representation in response to a local perturbation that affects only a part of the space that is represented. Here, we tested this hypothesis using an adaptation paradigm with hemiprisms. Our results suggest that, initially, the system tries to modify the visuomotor transformation in the whole spatial representation; however, if feedback is available from both hemifields, the system can perform specific regional topographical realignments. The results also suggest that access to the rearranged visuomotor transformation is independent of eye position, in contrast with previous studies that found a kind of conditional learning. Also, whereas prism adaptation experiments using ballistic movements do not show intermanual transfer of learning, the topographical modification found here is available to both hands. These results provide strong evidence for rapid topographical plasticity that can modify space transformations between two different modalities. [ABSTRACT FROM AUTHOR]

Abstract: Regional distributions of ligand binding sites of 12 different neurotransmitter receptors (glutamatergic: AMPA, kainate, NMDA; GABAergic: GABAA, GABAB; cholinergic: muscarinic M2, nicotinic; adrenergic: α1, α2; serotonergic: 5-HT1A, 5-HT2; dopaminergic: D1) were studied in human postmortem brains by means of quantitative receptor autoradiography. Binding site densities were measured in the superior parietal lobule (SPL) (areas 5L, 5M, 5Ci, and different locations within Brodmann''s area (BA) 7), somatosensory (BA 2), and visual cortical areas (BA 17, and different locations within BAs 18 and 19). Similarities of receptor distribution between cortical areas were analyzed by cluster analysis, uni- and multivariate statistics of mean receptor densities (averaged over all cortical layers), and profiles representing the laminar distribution patterns of receptors. A considerable heterogeneity of regional receptor densities and laminar patterns between the sites was found in the SPL and the visual cortex. The most prominent regional differences were found for M2 receptors. In the SPL, rostrocaudally oriented changes of receptor densities were more pronounced than those in mediolateral direction. The receptor distribution in the rostral SPL was more similar to that of the somatosensory cortex, whereas caudal SPL resembled the receptor patterns of the dorsolateral extrastriate visual areas. These results suggest a segregation of the different SPL areas based on receptor distribution features typical for somatosensory or visual areas, which fits to the dual functional role of this cortical region, i.e., the involvement of the human SPL in visuomotor and somatosensory motor transformations. [Copyright &y& Elsevier]

Using transcranial magnetic stimulation, we explored the properties of premotor mirror neurons during the passive observation of a reaching–grasping movement in human subjects. Two different experiments were run using video-clips as visual stimuli. Video-clips showed a normally performed (control stimulus) or an anomalous reaching–grasping movement executed by delaying the time of the appearance of the maximal finger aperture (experiment 1), or substituting it with an unpredictable closure (experiment 2). Motor evoked potentials were recorded at different time-points during the observation of the video-clips. Profiles of cortical excitability were drawn and compared with the kinematic profiles of the corresponding movement. Passive observation of the natural movement evoked a profile of cortical excitability that is in concordance with the timing of the kinematic profile of the shown finger movements. Observation of the uncommon movements did not exert any modulation (experiment 1) or evoked an activity that matched, at the beginning, the modulation obtained with observation of the natural movement (experiment 2). Results show that the resonant motor plan is loaded as whole at the beginning of observation and once started tends to proceed to its completion regardless of changes to the visual cues. The results exclude thepossibility of a temporal fragmentation of the resonant plan, because activation of different populations of mirror neurons for each phase of the ongoing action. They further support the notion of the role of the mirror system as neural substrate for the observing–execution matching system and extend the current knowledge regarding mechanisms that trigger the internal representation of an action. [ABSTRACT FROM AUTHOR]

Very little is known about the role of the cingulate motor area (CMA) in visually guided reaching compared to other cortical motor areas. To investigate the hierarchical role of the caudal CMA (CMAc) during reaching we recorded the activity of neurons in CMAc in comparison to the supplementary motor area proper (SMA) while a monkey performed an instructed delay task that required it to position a cursor over visual targets on a computer screen using two-dimensional (2D) joystick movements. The direction of the monkey’s arm movement was dissociated from the direction of the visual target by periodically reversing the relationship between the direction of movement of the joystick and that of the cursor. Neurons that responded maximally with a particular limb movement direction regardless of target location were classified as limb-dependent, whereas neurons that responded maximally to a particular target direction regardless of the direction of limb movement were classified as target-dependent. Neurons whose activity was directional in one of the two visuomotor mapping conditions and non-directional or inactive in the other were categorized as context-dependent. Limb-dependent activity was observed more frequently than target-dependent activity in both CMAc and SMA proper during both the delay period (preparatory activity; CMAc, 17%; SMA, 31%) and during movement execution (CMAc, 49%, SMA, 48%). A modest percentage of neurons with preparatory activity were target-dependent in both CMAc (11%) and SMA proper (8%) and a similar percentage of neurons in both areas demonstrated target-dependent, movement activity (CMAc, 8%; SMA, 10%). The surprising finding was that a very large percentage of neurons in both areas displayed context-dependent activity either during the preparatory (CMAc, 72%; SMA, 61%) or movement (CMAc, 43%, SMA 42%) epochs of the task. These results show that neural activity in both CMAc and SMA can directly represent movement direction in either limb-centered or target-centered coordinates. The presence of target-dependent activity in CMAc, as well as SMA, suggests that both are involved in the transformation of visual target information into appropriate motor commands. Target-dependent activity has been found in the putamen, SMA, CMAc, dorsal and ventral premotor cortex, as well as primary motor cortex. This indicates that the visuomotor transformations required for visually guided reaching are carried out by a distributed network of interconnected motor areas. The large proportion of neurons with context-dependent activity suggests, however, that while both CMAc and SMA may play a role in the visuomotor transformation of target information into movement parameters, their activity is not solely coding parameters of movement, since their involvement in this process is highly condition-dependent. [ABSTRACT FROM AUTHOR]

The medial parieto-occipital cortex is a central node in the dorsomedial visual stream. Recent physiological studies in the macaque monkey have demonstrated that the medial parieto-occipital cortex contains two areas, the visual area V6 and the visuomotor area V6A. Area V6 is a retinotopically organized visual area that receives form and motion information directly from V1 and is heavily connected with the other areas of the dorsal visual stream, including V6A. Area V6A is a bimodal visual/somatosensory area that elaborates visual information such as form, motion and space suitable for the control of both reaching and grasping movements. Somatosensory and skeletomotor activities in V6A affect the upper limbs and involve both the transport phase of reaching and grasping movements. Finally, V6A is strongly and reciprocally connected with the dorsal premotor cortex controlling arm movements. The picture emerging from these data is that the medial parieto-occipital cortex is well equipped to control both proximal and distal movements in the online visuomotor guidance of prehension. In agreement with this view, selective V6A lesions in monkey produce misreaching and misgrasping with the arm contralateral to the lesion in visually guided movements. These deficits are similar to those observed in optic ataxia patients and suggest that human and monkey superior parietal lobules are homologous structures, and that optic ataxia syndrome is the result of the lesion of a 'human' area V6A. [ABSTRACT FROM AUTHOR]

Optic ataxia and visual agnosia have been proposed to constitute a double dissociation which provides the main argument for the assimilation of the anatomical distinction between a dorsal and a ventral visual stream to the functional distinction between perception and action. In the present review, we argue that insufficient evidence has been collected to argue for this double dissociation. Several criteria are reviewed: (1) exploration of the visuomotor behavior in central versus peripheral vision has not been matched for the two types of patients; (2) the temporal constraints of visual processes that are impaired in the two neurological conditions appear to play a crucial role in the apparent dissociation; (3) the necessary reductionism of experimental conditions used to study action has led to an overconsideration of optic ataxia as a global deficit for action. Altogether optic ataxia appears to result from a specific impairment of immediate visuomotor control rather than of visually guided action as a whole. These results are discussed in the light of recent research on optic ataxia and on motor control, and directions for future research are proposed. [ABSTRACT FROM AUTHOR]

Two experiments investigated the effects of providing nonveridical knowledge of the results (KR) in a visuomanual task in which participants pointed to briefly (200 ms) presented targets without seeing their hand. By showing after each trial the movement endpoint displaced radially with respect to its true position, we were able to alter progressively the gain of the visuomanual loop. In experiment 1, the KR was provided only for transversal movements and for one target distance, but the effect generalized to all directions and all distances. Moreover, it also generalized to the other hand that had never been biased. In experiment 2, nonveridical KR was supplied for movements along the two major diagonals which require sharply different muscle synergies. The transfer to other directions and to the other hand was equally substantial. It is argued that the results support the vector coding hypothesis, which holds that the input to the motor execution stage is supplied by specifying independently the amplitude and the direction of the vector from the initial to the final position in an extrinsic frame of reference. We also discuss the possible brain structures involved in the biasing action of the KR. [ABSTRACT FROM AUTHOR]

We studied the oscillatory activity of the scalp-recorded EEG in healthy humans performing a task that required a particular eye-movement response choice according to the shape of a visual target. We observed a significant stimulus-aligned activity at the 40 Hz frequency band 100 ms after the appearance of the target only when that target was the end point for the subsequent eye movement (pro-saccade). This activity was most prominent over the central-parietal area of the right hemisphere. When the target indicated a movement to the opposite direction (anti-saccade) or indicated that no movement was required (no-move), this 40 Hz activity was nearly absent. This difference in activity between the pro-saccade and the other two tasks was evident in the single subject ERPs for four of the six subjects studied. In contrast, the movement-aligned 40 Hz activity for the pro-saccade and anti-saccade was almost identical. We speculate that this early stimulus-aligned 40 Hz activity might reflect a fast transformation of a visual stimulus to a motor response (eye movement) that can be performed for the pro-saccade task where stimulus-response compatibility is strong compared to the anti-saccade and no-move tasks. The movement-aligned 40 Hz activity might be related to the motor response preparation per se. We conclude that this task specific transient oscillatory activity could be used as a probe in the study of the temporal dynamics of visuomotor transformations. [ABSTRACT FROM AUTHOR]

Healthy humans performed arm movements in a horizontal plane, from an initial position toward remembered targets, while the movement and the targets were projected on a vertical computer monitor. We analyzed the mean error of movement endpoints and we observed two distinct systematic error patterns. The first pattern resulted in the clustering of movement endpoints toward the diagonals of the four quadrants of an imaginary circular area encompassing all target locations (oblique effect). The second pattern resulted in a tendency of movement endpoints to be closer to the body or equivalently lower than the actual target positions on the computer monitor (y-effect). Both these patterns of systematic error increased in magnitude when a time delay was imposed between target presentation and initiation of movement. In addition, the presence of a stable visual cue in the vicinity of some targets imposed a novel pattern of systematic errors, including minimal errors near the cue and a tendency for other movement endpoints within the cue quadrant to err away from the cue location. A pattern of systematic errors similar to the oblique effect has already been reported in the literature and is attributed to the subject's conceptual categorization of space. Given the properties of the errors in the present work, we discuss the possibility that such conceptual effects could be reflected in a broad variety of visuomotor tasks. Our results also provide insight into the problem of reference frames used in the execution of these aiming movements. Thus, the oblique effect could reflect a hand-centered reference frame while the y-effect could reflect a body or eye-centered reference frame. The presence of the stable visual cue may impose an additional cue-centered (allocentric) reference frame. [ABSTRACT FROM AUTHOR]

Five normal subjects were tested in a simulated grasping task. A cylindrical container filled with water was placed on the center of a horizontal monitor screen. Subjects used a precision grip formed by the thumb and index finger of their right hand. After a preliminary run during which the container was present, it was replaced by an image of the upper surface of the cylinder appearing on the horizontal computer screen on which the real cylinder was placed during the preliminary run. In each trial the image was marked with two contact points which defined an opposition axis in various orientations with respect to the frontal plane. The subjects' task consisted, once shown a stimulus, of judging as quickly as possible whether the previously experienced action of grasping the container full of water and pouring the water out would be easy, difficult or impossible with the fingers placed according to the opposition axis indicated on the circle. Response times were found to be longer for the grasps judged to be more difficult due to the orientation and position of the opposition axis. In a control experiment, three subjects actually performed the grasps with different orientations and positions of the opposition axis. The effects of these parameters on response time followed the same trends as during simulated movements. This result shows that simulated hand movements take into account the same biomechanical limitations as actually performed movements. [ABSTRACT FROM AUTHOR]

Forty-seven normal subjects performed two-dimensional arm movements on a digitizer board using a mouse device. The movements were projected on a computer monitor. Subjects were instructed to move the mouse using the whole arm from a center position to a peripheral target so that the projected movement would pass over the target without stopping on the target. A large number of targets (360) were used to cover the entire directional continuum. The direction of the arm movement was the parameter of interest, which was measured at an initial position, at one third of the distance towards the target, and at the vicinity of the target. Four conditions of delay between target presentation and movement execution were used (0, 2, 4, 6 s). A systematic directional error was observed at the initial portion of the trajectory. This error resulted from a clustering of movement directions on an axis that was perpendicular to the axis of the resting forearm before movement onset. This pattern of errors can be explained by the initial inertial anisotropy of the arm. As the trajectory evolved, a different directional error emerged, resulting from a clustering of movement directions in two orthogonal axes. This pattern of directional error increased in amplitude as the delay increased, in contrast to the error at the initial portion of the trajectory which remained invariant with increasing delay. Finally, the information transmitted by the movement direction was shown to increase with the evolution of the trajectory. The increase in delay resulted in a decrease in directional-information transmission. It is proposed that the directional bias towards the end of the movement trajectory might reflect the action of "movement primitives", that is patterns of muscle activation resulting from spinal interneuronal activation. It is further proposed that the directional bias observed at the vicinity of the target might reflect a loss of cortical directional information with increasing delay between target presentation and movement onset. [ABSTRACT FROM AUTHOR]

A series of step-tracking experiments was conducted before, during, and after a 3-week space mission to assess the effects of prolonged microgravity on a non-postural motor-control task. In- and post-flight accuracy was affected only marginally. However, kinematic analyses revealed a considerable change in the underlying movement dynamics: too-small force and, thus, too-low velocity in the first part of the movements was mainly compensated by lengthening the deceleration phase of the primary movement, so that accuracy was regained at its end. The observed in-flight decrements in peak velocity and peak acceleration point to an underestimation of mass, in agreement with the re-interpretation hypothesis of Bock et. al. Post-flight no reversals of the in-flight changes (negative aftereffects) were found. Instead, there was a general slowing down, which could be due to post-flight physical exhaustion. [ABSTRACT FROM AUTHOR]