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The influence of internal brain state on behavioral performance is well illustrated by the gap-saccade task, in which saccades might be initiated with short latency (express saccade) or with long latency (regular saccade) even though the external visual condition is identical. Accumulated evidence has demonstrated that the internal brain state is different before the initiation of an express saccade than of a regular saccade. However, the reported origin of the fluctuation of internal brain state is disputed among previous studies, e.g., the fixation disengagement theory versus the oculomotor preparation theory. In the present study, we examined these two theories by analyzing the rate and direction of fixational saccades, i.e., small amplitude saccades during fixation period, because they could be modulated by internal brain state. Since fixation disengagement is not spatially tuned, it might affect the rate but not direction of fixational saccade. In contrast, oculomotor preparation can contain the spatial information for upcoming saccade, thus, it might have a distinct effect on fixational saccade direction. We found that the different spatiotemporal characteristics of fixational saccades among tasks with different gap durations reveals different driven force to change the internal brain state. Under short gap duration (100 ms), fixation disengagement plays a primary role in switching internal brain state. Conversely, under medium (200 ms) and long (400 ms) gap durations, oculomotor preparation plays a primary role. These results suggest that both fixation disengagement and oculomotor preparation can change the internal brain state, but their relative contributions are gap-duration dependent.

Brain as a dynamic system responds to stimulations with specific patterns affected by its inherent ongoing dynamics. The patterns are manifested across different levels of organization—from spiking activity of neurons to collective oscillations in local field potential (LFP) and electroencephalogram (EEG). The multilevel and multifaceted response activities show patterns seemingly distinct and non-comparable from each other, but they should be coherently related because they are generated from the same underlying neural dynamic system. A coherent understanding of the interrelationships between different levels/aspects of activity features is important for understanding the complex brain functions. Here, based on analysis of data from human EEG, monkey LFP and neuronal spiking, we demonstrated that the brain response activities from different levels of neural system are highly coherent: the external stimulus simultaneously generated event-related potentials, event-related desynchronization, and variation in neuronal spiking activities that precisely match with each other in the temporal unfolding. Based on a biologically plausible but generic network of conductance-based integrate-and-fire excitatory and inhibitory neurons with dense connections, we showed that the multiple key features can be simultaneously produced at critical dynamical regimes supported by excitation–inhibition (E–I) balance. The elucidation of the inherent coherency of various neural response activities and demonstration of a simple dynamical neural circuit system having the ability to simultaneously produce multiple features suggest the plausibility of understanding high-level brain function and cognition from elementary and generic neuronal dynamics.

Background: Among the elderly, dementia is a common and disabling disorder with primary manifestations of cognitive impairments. Diagnosis and intervention in its early stages is the key to effective treatment. Nowadays, the test of cognitive function relies mainly on neuropsychological tests, such as the Mini-Mental State Examination (MMSE) and Montreal Cognitive Assessment (MoCA). However, they have noticeable shortcomings, e.g., the biases of subjective judgments from physicians and the cost of the labor of these well-trained physicians. Thus, advanced and objective methods are urgently needed to evaluate cognitive functions. Methods: We developed a cognitive assessment system through measuring the saccadic eye movements in three tasks. The cognitive functions were evaluated by both our system and the neuropsychological tests in 310 subjects, and the evaluating results were directly compared. Results: In general, most saccadic parameters correlate well with the MMSE and MoCA scores. Moreover, some subjects with high MMSE and MoCA scores have high error rates in performing these three saccadic tasks due to various errors. The primary error types vary among tasks, indicating that different tasks assess certain specific brain functions preferentially. Thus, to improve the accuracy of evaluation through saccadic tasks, we built a weighted model to combine the saccadic parameters of the three saccadic tasks, and our model showed a good diagnosis performance in detecting patients with cognitive impairment. Conclusion: The comprehensive analysis of saccadic parameters in multiple tasks could be a reliable, objective, and sensitive method to evaluate cognitive function and thus to help diagnose cognitive impairments.

Background and ObjectivesAmong the elderly, dementia is a common and disabling disorder with primary manifestations of cognitive impairments. Diagnosis and intervention in its early stages is the key to effective treatment. Practically, the test of cognitive function relies mainly on neuropsychological tests, such as the Mini-Mental State Examination (MMSE) and Montreal Cognitive Assessment (MoCA). Although these tests are widely used at the present, there are noticeable shortcomings, e.g., the biases of subjective judgments from physicians and the cost of the labor of these well-trained physicians. Thus, advanced and objective methods are urgently needed to evaluate cognitive functions. Accumulative evidence indicates that the saccades in certain tasks are highly correlated with the performance in some cognitive functions. However, only a few studies directly compared saccades with the performance in neuropsychological tests depicted by their scores. Thus, the reliability of using saccades as a behavioral biomarker to evaluate cognitive functions has rarely been explored. Methods310 subjects performed three sequential designed oculomotor tasks, pro-saccade (PS), anti-saccade (AS) and memory-guided saccade (MGS) and the saccadic parameters including error rate, saccadic reaction time and spatial error are studied.ResultsIn general, most saccadic parameters correlate well with the MMSE and MoCA scores. Moreover, some subjects with high MMSE and MoCA scores have very high error rates in performing these three tasks due to various errors in saccade control. The primary error types vary among tasks, indicating that different tasks assess certain specific brain functions preferentially. Thus, to improve the accuracy of evaluation through saccadic tasks, we built a weighted model to combine the saccadic parameters of the three saccadic tasks. The receiver operating characteristic (ROC) curve analysis shows that the discrimination between cognitive impairment patients and control subjects is better through the output of our model than the MMSE test. ConclusionMeasuring saccades in multiple tasks could be a reliable, objective and sensitive method to evaluate cognitive function and thus to help diagnosing cognitive impairments.

Efficiently mapping sensory stimuli onto motor programs is crucial for rapidly choosing appropriate behavioral responses. While neuronal mechanisms underlying simple, one-to-one sensorimotor mapping have been extensively studied, how the brain achieves complex, many-to-one sensorimotor mapping remains unclear. Here, we recorded single neuron activity from the lateral intraparietal (LIP) cortex of monkeys trained to map multiple spatial positions of visual cue onto two opposite saccades. We found that LIP neurons’ activity was consistent with directly mapping multiple cue positions to the associated saccadic direction (SDir) regardless of whether the visual cue appeared in or outside neurons’ receptive fields. Unlike the explicit encoding of the visual categories, such cue–target mapping (CTM)–related activity covaried with the associated SDirs. Furthermore, the CTM was preferentially mediated by visual neurons identified by memory-guided saccade. These results indicate that LIP plays a crucial role in the early stage of many-to-one sensorimotor transformation.

Summary Saccadic eye movements may not stop steadily but fluctuate briefly, known as saccadic dynamic overshoot (SDO). The reported relationships between SDO and saccadic parameters of main saccade and the effect of aging on SDO are controversial. In addition, it is not clear whether aging-related disease, such as mild cognitive impairment (MCI) and Parkinson disease (PD), causes the specific change of SDO. To address these questions, we analyzed the spatiotemporal features of SDO in young healthy subjects, elderly healthy subjects, and subjects with PD and MCI in three oculomotor tasks. We found two types of SDOs—simple and complex SDO. We confirmed that the frequency and amplitude of SDO were positively correlated with the peak velocity and deceleration of main saccades and increased in elderly subjects; however, they were not significantly different among the three elderly groups. Our results support the previous argument that the oculomotor structure in brainstem and cerebellum directly develop SDO., Graphical abstract, Highlights • We classify two types of saccadic dynamic overshoot (SDO): SDOsimple and SDOcomplex • Saccades with SDO have higher peak velocity and deceleration than saccades without SDO • Elderly subjects show a higher frequency and amplitude of SDO than young subjects • Saccades with SDOcomplex occur more frequently in reflexive than voluntary saccades, Behavioral neuroscience; Cognitive neuroscience

Sensory encoding (how stimuli evoke sensory responses) is known to progress from low- to high-level features. Decoding (how responses lead to perception) is less understood but is often assumed to follow the same hierarchy. Accordingly, orientation decoding must occur in low-level areas such as V1, without cross-fixation interactions. However, a study, Ding, Cueva, Tsodyks, and Qian (2017), provided evidence against the assumption and proposed that visual decoding may often follow a high-to-low-level hierarchy in working memory, where higher-to-lower-level constraints introduce interactions among lower-level features. If two orientations on opposite sides of the fixation are both task relevant and enter working memory, then they should interact with each other. We indeed found the predicted cross-fixation interactions (repulsion and correlation) between orientations. Control experiments and analyses ruled out alternative explanations such as reporting bias and adaptation across trials on the same side of the fixation. Moreover, we explained the data using a retrospective high-to-low-level Bayesian decoding framework.

Primates frequently make saccades direct fovea on interesting objects to receive acute visual information. However, saccade displaces the images on retina and disrupts the visual constancy. One possible mechanism to retain visual constancy is by integrating the presaccadic and postsaccadic visual information right at the time of saccade, which makes the timing of saccade crucial. So far, the saccadic timing signals have been found only in the subcortical regions, for example, the cerebellum and superior colliculus, but not in the neocortex. Here we report 2 types of saccadic timing signals in macaque lateral intraparietal area (LIP). First, many presaccadic response neurons started to decline activity either right around the start (saccade-on-decay) or the end (saccade-off-decay) of saccades. Notably, the time difference between saccade-off-decay and saccade-on-decay was highly correlated with the mean duration of saccades but not with the individual ones, and both saccade-off-decay and saccade-on-decay were better aligned with saccade end than saccade start-reflecting prediction. Second, the peak activity plateau of a group of postsaccadic response neurons was highly correlated with the actual duration of saccade-reflecting reality. While the predicted timing signals might facilitate the integration of visual information across saccades in LIP, the actual duration signals might calibrate the prediction errors.

Summary Humans and monkeys have access to an accurate representation of visual space despite a constantly moving eye. One mechanism by which the brain accomplishes this is by remapping visual receptive fields around the time of a saccade. In this process a neuron can be excited by a probe stimulus in the current receptive field, and also simultaneously by a probe stimulus in the location that will be brought into the neuron's receptive field by the saccade (the future receptive field), even before saccade begins. Here we show that perisaccadic neuronal excitability is not limited to the current and future receptive fields but encompasses the entire region of visual space across which the current receptive field will be swept by the saccade. A computational model shows that this receptive field expansion is consistent with the propagation of a wave of activity across the cerebral cortex as saccade planning and remapping proceed.

Objectives. We aimed to investigate the prevalence of restless leg syndrome (RLS) and exploring the contributing factors that affect the development of RLS in Parkinson’s disease (PD) patients. Methods. A cross-sectional study was conducted consisting of 178 consecutive PD patients from our hospital between October 2015 and August 2016. We divided the participants into two groups, which were PD with RLS and PD with non-RLS. Then, we recorded their demographics and clinical data to draw a comparison between PD with RLS and PD with non-RLS. Results. 23 (12.92%) were diagnosed with RLS among all the enrolled PD patients. Unified Parkinson’s Disease Rating Scale III (UPDRS III) and Hamilton Depression Scale (HAMD) scores, probable rapid eye movement sleep behavior disorder (PRBD), and daily levodopa equivalent dose (LED) in the PD with the RLS group were significantly different from those in the PD with the non-RLS group. Daily LED and the scores of UPDRS III and HAMD in PD patients with RLS were all higher than those in PD patients with non-RLS. PRBD, daily LED, and HAMD scores were significantly independent factors contributing to the development of RLS (OR=4.678, 95% CI 1.372~15.944, P=0.014; OR=1.003, 95% CI 1.001~1.005, P=0.019; OR=1.094, 95% CI 1.002~1.193, P=0.045). The severity of RLS was positively correlated with the duration of PD and daily LED (r=0.438, P=0.036; r=0.637, P=0.001). Conclusion. PRBD existence, daily LED, and HAMD scores are independent factors for developing RLS in PD patients. PRBD existence is firstly proposed as an independent factor in developing RLS among PD patients. RLS severity in PD patients are positively associated with the duration of PD and daily LED.

Our brain makes saccadic eye movements up to 3–5 times per second, which disrupt visual images on the retina, yet we perceive the world to be stable. It has been suggested that this perceptual stability is achieved via peri-saccadic remapping of neuronal receptive fields (RFs), that is, before a saccade, neurons start to respond to visual stimuli at their future RFs to be brought up by the impending eye movement. The recent experimental study further unveiled that the remapping of neuronal RF in Lateral Intraparietal Area (LIP) is an elongation process along the saccadic trajectory. Despite that peri-saccadic remapping is known to be closely associated to visual stability, a deep understanding of how it works remains largely unknown. Here, to address this question, we formulate visual stability as an optimization problem, and explore, from a computational point of view, how the response property of a LIP neuron should vary during a saccade, in order to predict the locations of visual images disrupted by the saccade. Through building a biologically plausible network and training it for the corresponding prediction task, we find that the optimal recurrent connections between LIP neurons are asymmetric, biased to the opposite direction of the saccade, such that when combined with the corollary discharge signal responsible for the saccade, a visual stimulus triggers a cortical wave in LIP propagating in the opposite direction of the saccade, manifesting the elongated remapping of neuronal RF. Our model reproduces the experimental data on peri-saccadic remapping, and demonstrates that peri-saccadic remapping accounts for visual stability.

The site of overt attention (fixation point) defines a spatial reference center that affects visuomotor response as indicated by the stimulus-response-compatibility (SRC) effect: When subjects press, e.g., a left key to report stimuli, their reaction time is shorter when stimuli appear to the left than to the right of the fixation. Covert attention to a peripheral site appears to define a similar reference center but previous studies did not control for confounding spatiotemporal factors or investigate the relationship between overt- and covert-attention-defined centers. Using an eye tracker to monitor fixation, we found an SRC effect relative to the site of covert attention induced by a flashed cue dot, and a concurrent reduction, but not elimination, of the overt-attention SRC effect. The two SRC effects jointly determined the overall motor reaction time. Since trials with different cue locations were randomly interleaved, the integration of the two reference centers must be updated online. When the cue was invalid and diminished covert attention, the covert-attention SRC effect disappeared and the overt-attention SRC effect retained full strength, excluding non-attention-based interpretations. We conclude that both covert- and overt-attention sites define visual reference centers that simultaneously contribute to motor response.

Two spatial reference systems, i.e., the observer-centered (egocentric) and object-centered (allocentric) references, are most commonly used to locate the position of the external objects in space. Although we sense the world as a unified entity, visual processing is asymmetric between upper and lower visual fields (VFs). For example, the goal-directed reaching responses are more efficient in the lower VF. Such asymmetry suggests that the visual space might be composed of different realms regarding perception and action. Since the peripersonal realm includes the space that one can reach, mostly in the lower VF, it is highly likely that the peripersonal realm might mainly be represented in the egocentric reference for visuomotor operation. In contrast, the extrapersonal realm takes place away from the observer and is mostly observed in the upper VF, which is presumably represented in the allocentric reference for orientation in topographically defined space. This theory, however, has not been thoroughly tested experimentally. In the present study, we assessed the contributions of the egocentric and allocentric reference systems on visual discrimination in the upper and lower VFs through measuring the manual reaction times (RTs) of human subjects. We found that: (a) the influence of a target's egocentric location on visual discrimination was stronger in the lower VF; and (b) the influence of a target's allocentric location on visual discrimination was stronger in the upper VF. These results support the hypothesis that the upper and lower VFs are primarily represented in the allocentric and egocentric references, respectively.

Neuronal responses to prolonged stimulation attenuate over time. Here, we ask a fundamental question: is adaptation a simple process for the neural system during which sustained input is ignored, or is it actually part of a strategy for the neural system to adjust its encoding properties dynamically? After simultaneously recording the activities of a group of bullfrog's retinal ganglion cells (dimming detectors) in response to sustained dimming stimulation, we applied a combination of information analysis approaches to explore the time-dependent nature of information encoding during the adaptation. We found that at the early stage of the adaptation, the stimulus information was mainly encoded in firing rates, whereas at the late stage of the adaptation, it was more encoded in neural correlations. Such a transition in encoding properties is not a simple consequence of the attenuation of neuronal firing rates, but rather involves an active change in the neural correlation strengths, suggesting that it is a strategy adopted by the neural system for functional purposes. Our results reveal that in encoding a prolonged stimulation, the neural system may utilize concerted, but less active, firings of neurons to encode information.

Under many circumstances, motor memory needs to be retained for a long period of time to enable accurate behavior. Since the first introduction of the saccadic adaptation paradigm in 1960s, saccadic adaptation protocols have been widely used to study the mechanisms of motor learning and motor memory. However, previous studies reported that the effect of saccadic adaptation on the oculomotor system was rather short (minutes to hours) in human and non-human primates. Here we ask whether the fast decay of the effects of saccadic adaptation is due to the influence of environmental context. To test this hypothesis, we asked human subjects to perform a saccadic adaptation task in a very dark environment. Our data showed that saccade gain remained at the post-adaptation level 24-72h after exposure to the saccadic adaptation task without significant recovery, and that the effect of saccadic adaptation on saccade gain could still be found 2 months later, much longer than previously reported. Our results indicate a vital role for environmental context in the retention of saccadic adaptation.

Although the cerebral cortex is thought to be composed of functionally distinct areas, the actual parcellation of area and assignment of function are still highly controversial. An example is the much-studied lateral intraparietal cortex (LIP). Despite the general agreement that LIP plays an important role in visual-oculomotor transformation, it remains unclear whether the area is primary sensory- or motor-related (the attention-intention debate). Although LIP has been considered as a functionally unitary area, its dorsal (LIPd) and ventral (LIPv) parts differ in local morphology and long-distance connectivity. In particular, LIPv has much stronger connections with two oculomotor centers, the frontal eye field and the deep layers of the superior colliculus, than does LIPd. Such anatomical distinctions imply that compared with LIPd, LIPv might be more involved in oculomotor processing. We tested this hypothesis physiologically with a memory saccade task and a gap saccade task. We found that LIP neurons with persistent memory activities in memory saccade are primarily provoked either by visual stimulation (vision-related) or by both visual and saccadic events (vision-saccade-related) in gap saccade. The distribution changes from predominantly vision-related to predominantly vision-saccade-related as the recording depth increases along the dorsal-ventral dimension. Consistently, the simultaneously recorded local field potential also changes from visual evoked to saccade evoked. Finally, local injection of muscimol (GABA agonist) in LIPv, but not in LIPd, dramatically decreases the proportion of express saccades. With these results, we conclude that LIPd and LIPv are more involved in visual and visual-saccadic processing, respectively.

Cortical fast-spiking (FS) neurons generate high-frequency action potentials (APs) without apparent frequency accommodation, thus providing fast and precise inhibition. However, the maximal firing frequency that they can reach, particularly in primate neocortex, remains unclear. Here, by recording in human, monkey, and mouse neocortical slices, we revealed that FS neurons in human association cortices (mostly temporal) could generate APs at a maximal mean frequency (Fmean) of 338 Hz and a maximal instantaneous frequency (Finst) of 453 Hz, and they increase with age. The maximal firing frequency of FS neurons in the association cortices (frontal and temporal) of monkey was even higher (Fmean 450 Hz, Finst 611 Hz), whereas in the association cortex (entorhinal) of mouse it was much lower (Fmean 215 Hz, Finst 342 Hz). Moreover, FS neurons in mouse primary visual cortex (V1) could fire at higher frequencies (Fmean 415 Hz, Finst 582 Hz) than those in association cortex. We further validated our in vitro data by examining spikes of putative FS neurons in behaving monkey and mouse. Together, our results demonstrate that the maximal firing frequency of FS neurons varies between species and cortical areas.

Objects in the visual world can be represented in both egocentric and allocentric coordinates. Previous studies have found that allocentric representation can affect the accuracy of spatial judgment relative to an egocentric frame, but not vice versa. Here we asked whether egocentric representation influenced the processing speed of allocentric perception. We measured the manual reaction time of human subjects in a position discrimination task in which the behavioral response purely relied on the target's allocentric location, independent of its egocentric position. We used two conditions of stimulus location: the compatible condition-allocentric left and egocentric left or allocentric right and egocentric right; the incompatible condition-allocentric left and egocentric right or allocentric right and egocentric left. We found that egocentric representation markedly influenced allocentric perception in three ways. First, in a given egocentric location, allocentric perception was significantly faster in the compatible condition than in the incompatible condition. Second, as the target became more eccentric in the visual field, the speed of allocentric perception gradually slowed down in the incompatible condition but remained unchanged in the compatible condition. Third, egocentric-allocentric incompatibility slowed allocentric perception more in the left egocentric side than the right egocentric side. These results cannot be explained by interhemispheric visuomotor transformation and stimulus-response compatibility theory. Our findings indicate that each hemisphere preferentially processes and integrates the contralateral egocentric and allocentric spatial information, and the right hemisphere receives more ipsilateral egocentric inputs than left hemisphere does.

Covert attention modulates saccadic performance, e.g., the abrupt onset of a task-irrelevant visual stimulus grabs attention as measured by a decrease in saccadic reaction time (SRT). The attentional advantage bestowed by the task-irrelevant stimulus is short-lived: SRT is actually longer ∼200 ms after the onset of a stimulus than it is when no stimulus appears, known as inhibition of return. The mechanism by which attention modulates saccadic reaction is not well-understood. Here, we propose two possible mechanisms: by selective routing of the visuomotor signal through different pathways (routing hypothesis) or by general modulation of the speed of visuomotor transformation (shifting hypothesis). To test them, we designed a cue gap paradigm in which a 100-ms gap was introduced between the fixation point disappearance and the target appearance to the conventional cued visual reaction time paradigm. The cue manipulated the location of covert attention, and the gap interval resulted in a bimodal distribution of SRT, with an early mode (express saccade) and a late mode (regular saccade). The routing hypothesis predicts changes in the proportion of express saccades vs. regular saccades, whereas the shifting hypothesis predicts a shift of SRT distribution. The addition of the cue had no effect on mean reaction time of express and regular saccades, but it changed the relative proportion of two modes. These results demonstrate that the covert attention modification of the mean SRT is largely attributed to selective routing between visuomotor pathways rather than general modulation of the speed of visuomotor transformation.

The cerebral cortex must have access to an eye position signal, as humans can report passive changes in eye position in total darkness, and visual responses in many cortical areas are modulated by eye position. The source of this signal is unknown. Here we demonstrate a representation of eye position in monkey primary somatosensory cortex, in the representation of the trigeminal nerve, near cells with a tactile representation of the contralateral brow. The neurons have eye position signals that increase monotonically with increasing orbital eccentricity from near the center of gaze, with directionally selectivity tuned in a Gaussian manner. All directions of eye position are represented in a single hemisphere. The signal is proprioceptive, because it can be obliterated by anesthetizing the contralateral orbit. It is not related to foveal or peripheral visual stimulation, and it represents the position of the eye in the head and not the angle of gaze in space.

Self-organized critical states (SOCs) and stochastic oscillations (SOs) are simultaneously observed in neural systems, which appears to be theoretically contradictory since SOCs are characterized by scale-free avalanche sizes but oscillations indicate typical scales. Here, we show that SOs can emerge in SOCs of small size systems due to temporal correlation between large avalanches at the finite-size cutoff, resulting from the accumulation-release process in SOCs. In contrast, the critical branching process without accumulation-release dynamics cannot exhibit oscillations. The reconciliation of SOCs and SOs is demonstrated both in the sandpile model and robustly in biologically plausible neuronal networks. The oscillations can be suppressed if external inputs eliminate the prominent slow accumulation process, providing a potential explanation of the widely studied Berger effect or event-related desynchronization in neural response. The features of neural oscillations and suppression are confirmed during task processing in monkey eye-movement experiments. Our results suggest that finite-size, columnar neural circuits may play an important role in generating neural oscillations around the critical states, potentially enabling functional advantages of both SOCs and oscillations for sensitive response to transient stimuli.

The lateral intraparietal area (LIP) contains neurons that are active during the memory interval of memory saccades. We call these “persistent neurons.” Here we study the activity of the persistent neurons in memory antisaccades, “motor” (the saccade is made toward the response field, although the response field is not stimulated visually) and “visual” (the response field is stimulated visually, but the movement is away from the field). Most persistent neurons are active during parts of the memory intervals of both visual and motor memory-antisaccades. Typically, these parts significantly overlap each other and together span the entire memory interval. The amplitude of the activity changes systematically during the memory intervals of visual and motor memory antisaccades. These changes are reflected in an antisaccade differential activity, which turns first to the visual direction and then crosses over to the motor direction. Some persistent neurons appear to show the paradoxical activity previously characterized in visual neurons; paradoxical activity accelerates the transition of the neuron's activity from visual to motor. These observations suggest that the persistent neurons reflect working memory for the computation of the antisaccade sensorimotor transformation. Ensembles of persistent neurons with different response fields may make up modules of working memory.

We recorded the activity of neurons in the lateral intraparietal area of two monkeys while they performed two similar visual search tasks, one difficult, one easy. Each task began with a period of fixation followed by an array consisting of a single capital T and a number of lowercase t's. The monkey had to find the capital T and report its orientation, upright or inverted, with a hand movement. In the easy task the monkey could explore the array with saccades. In the difficult task the monkey had to continue fixating and find the capital T in the visual periphery. The baseline activity measured during the fixation period, at a time in which the monkey could not know if the impending task would be difficult or easy or where the target would appear, predicted the monkey's probability of success or failure on the task. The baseline activity correlated inversely with the monkey's recent history of success and directly with the intensity of the response to the search array on the current trial. The baseline activity was unrelated to the monkey's spatial locus of attention as determined by the location of the cue in a cued visual reaction time task. We suggest that rather than merely reflecting the noise in the system, the baseline signal reflects the cortical manifestation of modulatory state, motivational, or arousal pathways, which determine the efficiency of cortical sensorimotor processing and the quality of the monkey's performance.

The influence of cognitive context on orienting behaviour can be explored using the mixed memory-prosaccade, memory-antisaccade task. A symbolic cue, such as the colour of a visual stimulus, instructs the subject to make a brief, rapid eye movement (a saccade) either towards the stimulus (prosaccade) or in the opposite direction (antisaccade). Thus, the appropriate sensorimotor transformation must be switched on to execute the instructed task. Despite advances in our understanding of the neuronal processing of antisaccades, it remains unclear how the brain selects and computes the sensorimotor transformation leading to an antisaccade. Here we show that area LIP of the posterior parietal cortex is involved in these processes. LIP's population activity turns from the visual direction to the motor direction during memory-antisaccade trials. About one-third of the visual neurons in LIP produce a brisk, transient discharge in certain memory-antisaccade trials. We call this discharge 'paradoxical' because its timing is visual-like but its direction is motor. The paradoxical discharge shows, first, that switching occurs already at the level of visual cells, as previously proposed by Schlag-Rey and colleagues; and second, that this switching is accomplished very rapidly, within 50 ms from the arrival of the visual signals in LIP.

We studied the effects of small lesions of the oculomotor vermis of the cerebellar cortex on the ability of monkeys to execute and adapt saccadic eye movements. For saccades in one horizontal direction, the lesions led to an initial gross hypometria and a permanent abolition of the capacity for rapid adaptation. Mean saccade amplitude recovered from the initial hypometria, although variability remained high. A series of hundreds of repetitive saccades in the same direction resulted in gradual decrement of amplitude. Saccades in other directions were less strongly affected by the lesions. We suggest the following. (1) The cerebellar cortex is constantly recalibrating the saccadic system, thus compensating for rapid biomechanical changes such as might be caused by muscle fatigue. (2) A mechanism capable of slow recovery from dysmetria is revealed despite the permanent absence of rapid adaptation.

Saccadic eye movements are central to primate behavior and serve to move the eyes to visual objects of interest. Express saccades, unlike regular saccades, occur with very short reaction times, a behavior necessary for speeded reactions in goal-directed behavior. Previous studies have shown that introduction of a blank interval (gap) between the fixation point offset and the saccadic target onset leads to an increase in the number of express saccades and that the superior colliculus plays a crucial role in the generation of express saccades. A longstanding hypothesis asserted that express saccades are mediated largely by a subcortical circuit, circumventing extrastriate visual cortex. An alternative “posterior pathway” hypothesis proposed the involvement of posterior parietal cortex. In the present study, using a gap saccade task, we investigated the role of nonhuman primate's lateral intraparietal cortex (LIP) in generation of express saccades. We show that roughly half of recorded LIP neurons were modulated during the gap interval. Moreover, a group of neurons with persistent activity in a memory-guided saccade task enhanced their activity during express saccades relative to that during regular saccades. After reducing the target's certainty by increasing the potential target locations, neuronal activity remained in the similar level during express saccades but markedly reduced during regular saccades that correlated with the increase of saccadic reaction time in the regular saccade. Our results suggest that area LIP is directly involved in generating saccades in express mode.

The middle temporal (MT) area has long been established as a cortical area involved in the encoding of motion information and has been thought to do so in retinotopic coordinates. It was previously shown that memory for motion has a spatial component by demonstrating that subjects do significantly worse on a match-to-sample task when the stimuli to be compared were spatially separated. The distance at which performance deteriorated (the critical spatial separation) increased at increasing eccentricities, suggesting that area MT was involved in the process. In this study, we asked whether optimal performance occurred when the stimuli were in the same retinotopic or spatiotopic coordinates. We found that the performance was best when the stimuli appeared in the same location in space rather than the same retinal location, after an eye movement. We also found that the relationship between retinal eccentricity and the critical spatial separation approximated that of area MT, as found previously. We conclude that area MT plays an important role in the memory for motion process and that this is carried out in spatiotopic coordinates. This conclusion supports the hypothesis that MT processing may have a spatiotopic component.

The sensorimotor processing necessary in complex realistic situations goes beyond straight-forward application of a given sensorimotor transformation. Contextual information may make it necessary to switch to another sensorimotor transformation. We studied the issue of switching by using the mixed memory prosaccade/antisaccade task. Neurons of the lateral intra-parietal area (LIP) might be involved in computing the sensorimotor transformations for both prosaccades and antisaccades. LIP neurons may also be involved in switching to the antisaccade sensorimotor transformation, when an antisaccade is requested. Some neurons in LIP show a paradoxical pattern of activity-motor in space but visual in time. Funahashi, Chafee and Goldman-Rakic reported in 1993 a complimentary pattern of activity in prefrontal cortex-visual in space but motor in time. These odd observations are explained by the hypothesis that (1) the parietal cortex contains a sensorimotor transformation module, and prefrontal cortex a context categorization module, and (2) following target onset, information flows from early visual system to parietal cortex and on to prefrontal cortex; then, a second wave of activation, contingent on a switching signal, arrives back at parietal cortex. The duration of this loop is less than 100 ms. Thus, the paradoxical activities are intermediate representations derived in the cognitive processing involved in switching sensorimotor transformations.

The visual system is tied to the retina. Because the eyes move in the orbit, and the head moves on the body, accurate location of an object in extrapersonal space cannot simply result from a visual signal. Instead, the retinal signal must be combined with an estimate of where the eyes are in the orbit, and where the head is in space, to calculate where that object is relative to the observer. There is abundant evidence for eye position signals in various areas of the visual cortex. However, the source of that eye position signal is unknown. Estimates of eye position can arise from two different sources. One is outflow, an 'efference copy' or 'corollary discharge' which might arise from some eye position signal used to specify eye position for the eye muscles. The second source is inflow, a direct proprioceptive signal from the muscles themselves. Nevertheless, neither a proprioceptive representation of eye position nor corollary discharge of a motor command for eye position has ever been demonstrated unambiguously in the cerebral cortex. We recently discovered the neuronal representation of proprioceptive eye position signal in monkey primary somatosensory cortex.

The sensorimotor processing necessary in complex realistic situations goes beyond straight-forward application of a given sensorimotor transformation. Contextual information may make it necessary to switch to another sensorimotor transformation. We studied the issue of switching by using the mixed memory prosaccade/antisaccade task. Neurons of the lateral intra-parietal area (LIP) might be involved in computing the sensorimotor transformations for both prosaccades and antisaccades. LIP neurons may also be involved in switching to the antisaccade sensorimotor transformation, when an antisaccade is requested. Some neurons in LIP show a paradoxical pattern of activity-motor in space but visual in time. Funahashi, Chafee and Goldman-Rakic reported in 1993 a complimentary pattern of activity in prefrontal cortex-visual in space but motor in time. These odd observations are explained by the hypothesis that (1) the parietal cortex contains a sensorimotor transformation module, and prefrontal cortex a context categorization module, and (2) following target onset, information flows from early visual system to parietal cortex and on to prefrontal cortex; then, a second wave of activation, contingent on a switching signal, arrives back at parietal cortex. The duration of this loop is less than 100 ms. Thus, the paradoxical activities are intermediate representations derived in the cognitive processing involved in switching sensorimotor transformations.