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The cortico- basal- ganglia circuit plays a critical role in decision making on the basis of probabilistic information. Computational models have suggested how this circuit could compute the probabilities of actions being appropriate according to Bayes' theorem. These models predict that the subthalamic nucleus (STN) provides feedback that normalizes the neural representation of probabilities, such that if the probability of one action increases, the probabilities of all other available actions decrease. Here we report the results of an experiment testing a prediction of this theory that disrupting information processing in the STN with deep brain stimulation should abolish the normalization of the neural representation of probabilities. In our experiment, we asked patients with Parkinson's disease to saccade to a target that could appear in one of two locations, and the probability of the target appearing in each location was periodically changed. When the stimulator was switched off, the target probability affected the reaction times (RT) of patients in a similar way to healthy participants. Specifically, the RTs were shorter for more probable targets and, importantly, they were longer for the unlikely targets. When the stimulator was switched on, the patients were still faster for more probable targets, but critically they did not increase RTs as the target was becoming less likely. This pattern of results is consistent with the prediction of the model that the patients on DBS no longer normalized their neural representation of prior probabilities. We discuss alternative explanations for the data in the context of other published results. [ABSTRACT FROM AUTHOR]

The promise of new treatments for Huntington's disease (HD) has intensified interest in markers of preclinical onset and progression. Recent research has shown that elementary saccadic tasks may exhibit subtle preclinical abnormalities. Other studies have shown cognitive dysfunction to be a major component of early-HD phenotype. It was hypothesized that the synergistic combination of an oculomotor response with a cognitively demanding paradigm might therefore offer a functional marker that could be superior both to simpler saccadic paradigms and to conventional cognitive tests. The performance of 12 early-symptomatic HD patients and 17 presymptomatic gene carriers on three saccadic paradigms were studied: prosaccades, color-cued centrally triggered saccades and a second-order conditional rule task on which the direction of the saccade was cued by the combination of central and peripheral cues. Eye movements were recorded using a highly portable, patient-friendly infrared recording device. The analysis of the latency distributions showed that the second-order conditional rule task could discriminate between clinical groups, had a predictive potential within the presymptomatic group and appeared to be more sensitive than easier tasks. The simplicity of portable saccadometry, combined with the sensitivity of a complex oculomotor task, may therefore offer a promising functional marker in HD. [ABSTRACT FROM AUTHOR]

The article offers step-by-step instructions for examining eye movements.

It remains an important question whether neural function is mediated entirely by its tailored circuitry. A persistent debate in retinal colour vision is whether the centre and the surround of a ganglion cell receptive field receive dominant inputs either from L or M cones in an antagonistic manner (the selective wiring model) or mixed inputs (the mixed wiring model). Despite many anatomical, physiological and psychophysical experiments, a decisive conclusion has not been reached. An in-depth examination of what the pure mixed wiring mechanisms predicts is therefore important. These two models make different predictions both for the fovea and for the peripheral retina. Recently, a dynamic cellular model of the primate fovea was developed [ Momiji et al. (2006 ) Vis. Res., 46, 365–381]. Unlike earlier models, it explicitly incorporates spatial non-uniformities, such as the random arrangement of L and M cones. Here, a related model is developed for the peripheral retina by incorporating anatomically reasonable degrees of convergence between cones, bipolar cells and ganglion cells. These two models, in which selective wiring mechanisms are absent, are applied to describe both foveal and peripheral colour vision. In numerical simulations, peripheral ganglion cells are less colour sensitive than foveal counterparts, but none-the-less display comparative sensitivities. Furthermore, peripheral colour sensitivity increases with temporal frequency, relative to foveal sensitivity. These results are congruent with recent physiological experiments. [ABSTRACT FROM AUTHOR]

Right-hemisphere patients with left neglect often demonstrate abnormal visual search, re-examining stimuli to the right while ignoring those to the left. But re-fixations alone do not reveal if patients misjudge whether they have searched a location before. Here, we not only tracked the eye movements of 16 neglect patients during search, but also asked them to click a response button only when they judged they were fixating a target for the very first time. ''Re-clicking'' on previously found targets would indicate that patients erroneously respond to these as new discoveries. Lesions were mapped with high-resolution MRI. Neglect patients with damage involving the right intraparietal sulcus or right inferior frontal lobe ''re-clicked'' on previously found targets on the right at a pathological rate, whereas those with medial occipito-temporal lesions did not. For the intraparietal sulcus patients, the probability of erroneous re-clicks on an old target increased with time since first discovering it; whereas for frontal patients it was independent of search time, suggesting different underlying mechanisms in these two types of patient. Re-click deficits correlated with degree of leftward neglect, mainly due to both being severe in intraparietal cases. These results demonstrate that misjudging previously searched locations for new ones can contribute to pathological search in neglect, with potentially different mechanisms being involved in intraparietal versus inferior frontal patients. When combined with a spatial bias to the right, such deficits might explain why many neglect patients often re-examine rightward locations, at the expense of items to their left. [ABSTRACT FROM AUTHOR]

Active region is a dynamic process involving the Flexible coordination of different gaze strategies to achieve behavioral goals, although many complex behaviors rely on an ability to efficiently switch between gaze-control strategies, few studies to date have examined mechanisms of task level oculomotor control in detail. Here, we report few experiments in which subjects alternated between conflicting stimulus saccade mappings within a block of trials. The first experiment showed that there is no performance cost associated witch switching between pro and anti saccades. However, follow-up experiment demonstrate that whenever subjects alternate between arbitrary stimulus-saccade mappings, latency costs are apparent on the first trial after a task change. More detailed analysis of switch costs showed that latencies were particularly elevated for saccacdes directed toward the same location that had been the target for a saccade on the preceeding trial. This saccade "inhibition of return" effect was most marked when unexpected error feedbacks cued task switches, suggesting that saccade selection processes arc modulated by reward We conclude that there are two systems for arcade control that differ in their characteristics following a task switch. The "reflexive" control system can be enabled/disabled in advance of saccade execution without incurring any performance cost. Switch costs are only observed when two or more arbitrary stimulus-saccade mappings have to be coordinated by a "symbolic" control system. [ABSTRACT FROM AUTHOR]

Parkinson's disease patients are known to suffer loss of dopaminergic input to the rostral caudate nucleus. Recent functional magnetic resonance imaging (fMRI) studies have implicated this structure in the transformation of spatial information in memory to guide action, suggesting that memory to motor transformations may be selectively impaired in Parkinson's disease. In order to investigate this possibility we tested a group of Parkinson's disease patients (PDs) using a memory-guided pointing task. Of interest was whether patients showed reduced accuracy in the task as a function of memory load. Twelve PD patients and 13 elderly controls were asked to recall single or four step target sequences with 2 time delays (500 and 3,500 ms). In all memory-guided conditions PD patients showed increased variability in memory-guided movement end-points. This effect was not affected by delay, number of items, or the sequence familiarity. The results are consistent with increased variability in memory-motor transformations in early PD, due to dopamine depletion within the rostral caudate nucleus. [ABSTRACT FROM AUTHOR]

We investigated the organisation of working memory processes by examining how a secondary memory task interferes with the accuracy of memory-guided saccades. A target was flashed at a random location, followed by a Kanji character. Subjects had to remember the location of the target and the Kanji character, and then they had to make a saccade towards the remembered target location. A second Kanji character was displayed and the subject had to decide if it was same or different. The performance of seven non-Kanji reader's were compared with six fluent Kanji readers in the task. Memorisation of Kanji characters was found to interfere with the accuracy of memory-guided saccades made by non-Kanji, but not by Kanji readers. These findings directly contradict accounts of working memory function which propose that spatial and visual object memory are functionally discrete. [ABSTRACT FROM AUTHOR]

The latency of 'reflexive' saccades (made in response to peripheral visual stimuli) was compared to that of 'voluntary' saccades performed in anti-saccade and symbolically cued paradigms. Manipulation of visual events at fixation was carefully controlled across all conditions. Reflexive saccade latency was significantly faster than the latency of all forms of voluntary saccades. Importantly, the latency of saccades made after presentation of a symbolic cue at central fixation (voluntary arrow-cue condition) was greater than that made in the anti-saccade paradigm that requires suppression of a reflexive response. It is suggested that the increase in latency of saccades made in the voluntary arrow-cue condition may reflect differences in programming a 'When' trigger signal for saccades made in the absence of a peripheral stimulus. [ABSTRACT FROM AUTHOR]

Five patients with left-sided visual neglect following focal infarction of the right frontal lobe are presented. Lesion location was assessed using computed tomography or magnetic resonance imaging. The common area of lesion overlap was small, being confined to the dorsal aspect of the inferior frontal gyrus (Brodmann's area 44) and the immediate underlying white matter. This cortical region is part of the homologue of Broca's area in the right hemisphere and is considered to be part of human premotor cortex. The association of neglect with injury to this area suggests it may play an important role in directing attention in visual space. [ABSTRACT FROM AUTHOR]

Presents research which showed that patients with right inferior parietal lobe (IPL) lesions have difficulty initiating leftward movements towards visual targets on the left side of space. Hypotheses about the role of the parietal lobe in spatial cognition; Human IPL operating as a sensorimotor interface.

Presents a clinopathological conference of a woman who died after a bone marrow transplant. History of cognitive impairment; Complication of the transplant; Treatment of dizziness with acyclovir.