Anne B. Sereno

Shape selectivity in primate lateral intraparietal cortex

The extrastriate visual cortex can be divided into functionally distinct temporal and parietal regions, which have been implicated in feature-related (‘what’) and spatial (‘where’) vision, respectively. Neuropsychological studies of patients with damage to either the temporal or the parietal regions provide support for this functional distinction. Given the prevailing modular theoretical framework and the fact that prefrontal cortex receives inputs from both temporal and parietal streams,, recent studies have focused on the role of prefrontal cortex in understanding where and how information about object identity is integrated with (or remains segregated from) information about object location. Here we show that many neurons in primate posterior parietal cortex (the ‘where’ pathway) show sensory shape selectivities to simple, two-dimensional geometric shapes while the animal performs a simple fixation task. In a delayed match-to-sample paradigm, many neuronal units also show significant differences in delay-period activity, and these differences depend on the shape of the sample. These results indicate that units in posterior parietal cortex contribute to attending to and remembering shape features in a way that is independent of eye movements, reaching, or object manipulation. These units show shape selectivity equivalent to any shown in the ventral pathway.

Antisaccades and smooth pursuit eye movements in schizophrenia.

Saccadic and smooth pursuit eye movements were recorded in schizophrenic patients, nonschizophrenic psychiatric patients, and normal controls. Both schizophrenic subjects and psychiatric controls demonstrated greater increases in error rates and greater delays in generating antisaccades than did normal controls. Schizophrenic patients with impaired smooth pursuit tracking showed greater increases in error rates in the antisaccade task than did schizophrenic patients with normal pursuit. Among psychiatric controls, increased errors on the antisaccade task were unrelated to pursuit performance. The small size of this group, however, reduces the power to detect a relation between smooth pursuit tracking and performance on the antisaccade task. Although most patients were receiving one or more medications, some of which can affect eye movements, medication state in this study did not account for differences we report in dependent variables.

Control of voluntary and reflexive saccades in Parkinson’s disease

Abstract Eight patients with idiopathic Parkinson’s disease (PD) were compared with a group of age-matched controls on both reflexive saccade and antisaccade tasks. While reflexive, visually guided saccades led to equivalent performance in both groups, PD patients were slower, made more errors, and showed reduced gain on antisaccades (AS). This is consistent with previous results showing that PD patients have no difficulty with reflexive saccades but show deficiencies in a number of voluntary saccade paradigms. Moreover, visual information in the form of landmarks improves AS performance more for PD patients than controls, a finding analogous to results seen with other motor acts such as target-directed pointing. Results are discussed in terms of a two-process model of attention and eye movements.

Inhibition of return in manual and saccadic response systems

When nonpredictive exogenous visual cues are used to reflexively orient covert visual spatial attention, the initial early facilitation for detecting stimuli at cued versus uncued spatial locations develops into inhibition by 300 msec following the cue, a pattern referred to asinhibition of return (IOR). Experiments were carried out comparing the magnitude and time course for development of IOR effects when manual versus saccadic responses were required. The results showed that both manual and saccadic responses result in equivalent amounts of facilitation following initial exposure to a spatial cue. However, IOR developed more quickly for saccadic responses, such that, at certain cue-target SO As, saccadic responses to targets were inhibited, whereas manual responses were still facilitated. The findings are interpreted in terms of a premotor theory of visual attention.

Levodopa slows prosaccades and improves antisaccades: an eye movement study in Parkinson’s disease

Background: The integrity of frontal systems responsible for voluntary control and their interaction with subcortical regions involved in reflexive responses were studied in patients with Parkinson’s disease (PD). Previous studies have shown that patients with PD have impaired executive function, including deficits in attention, motor planning and decision making. Methods: Executive function was measured through eye movements: reflexive (stimulus driven) prosaccades and voluntary (internally guided) antisaccades. Patients with advanced idiopathic PD, off and on their optimal levodopa therapy, were tested on a prosaccade and an antisaccade task and compared with matched controls. Results: Levodopa significantly increased response time for reflexive prosaccades and reduced error rate for voluntary antisaccades. Conclusions: Consistent with our proposed model, patients with PD in the medicated state are better able to plan and execute voluntary eye movements. These findings suggest levodopa improves function of the voluntary frontostriatal system, which is deficient in PD.

Eye Tracking Dysfunction in Schizophrenia: Characterization and Pathophysiology

Eye tracking dysfunction (ETD) is one of the most widely replicated behavioral deficits in schizophrenia and is over-represented in clinically unaffected first-degree relatives of schizophrenia patients. Here, we provide an overview of research relevant to the characterization and pathophysiology of this impairment. Deficits are most robust in the maintenance phase of pursuit, particularly during the tracking of predictable target movement. Impairments are also found in pursuit initiation and correlate with performance on tests of motion processing, implicating early sensory processing of motion signals. Taken together, the evidence suggests that ETD involves higher-order structures, including the frontal eye fields, which adjust the gain of the pursuit response to visual and anticipated target movement, as well as early parts of the pursuit pathway, including motion areas (the middle temporal area and the adjacent medial superior temporal area). Broader application of localizing behavioral paradigms in patient and family studies would be advantageous for refining the eye tracking phenotype for genetic studies.

Nicotine reduces antisaccade errors in task impaired schizophrenic subjects.

Nicotine and/or smoking have been shown to reduce various cognitive deficits associated with schizophrenia. Here, we examine the effects of nicotine gum on repeated performance on a simple eye movement task. Eight schizophrenic subjects and eight controls participated in three days of testing on saccade (S) and antisaccade (AS) tasks. On each testing day, subjects participated in four testing sessions and received both of two nicotine gum treatments (4 and 6 mg) and both of two control conditions (placebo gum and no gum), each followed by a recovery period. Overall, schizophrenics showed significant impairments on the AS task. However, upon individual examination only four schizophrenics showed significant differences in AS errors or reaction times (RTs) when compared to controls. The other four schizophrenic subjects showed control level performance. All schizophrenic subjects showed normal and better than control level performance on the simple S task. Furthermore, no effects of nicotine were seen on the simple S task. There were significant treatment effects on the AS task. Nicotine treatment significantly decreased errors in the task impaired schizophrenic group and this effect was most pronounced at the 6 mg level. No nicotine effects were demonstrated for non-impaired schizophrenic subjects or controls. This study demonstrates a benefit of short exposure to nicotine in cognitively impaired schizophrenic subjects. These results support previous findings of cognitive benefits of nicotine in schizophrenics.

Spatial selective attention in schizophrenic, affective disorder, and normal subjects

Schizophrenic, affective disorder, and normal subjects performed tasks involving exogenous (automatic) and endogenous (voluntary) attention. In the exogenous attention task, schizophrenic subjects demonstrated a greater benefit in response time than did normal subjects. In the endogenous attention task, however, schizophrenic subjects showed a smaller benefit in response time than did normal subjects. These results are consistent with a model of schizophrenia that predicts a deficit in voluntary (endogenous) control, and a disinhibition and therefore enhancement of the automatic (exogenous) processes of spatial selective attention. Affective disorder subjects did not demonstrate a greater benefit in response time than normal subjects in the exogenous attention task, but did show a smaller benefit in response time than normal subjects in the endogenous attention task. The somewhat similar pattern of behavior of schizophrenic and affective disorder subjects suggests that abnormal spatial selective attentional processes may not be specific to schizophrenia.

Shape Selectivity in Primate Frontal Eye Field

Previous neurophysiological studies of the frontal eye field (FEF) in monkeys have focused on its role in saccade target selection and gaze shift control. It has been argued that FEF neurons indicate the locations of behaviorally significant visual stimuli and are not inherently sensitive to specific features of the visual stimuli per se. Here, for the first time, we directly examined single cell responses to simple, two-dimensional shapes and found that shape selectivity exists in a substantial number of FEF cells during a passive fixation task or during the sample, delay (memory), and eye movement periods in a delayed match to sample (DMTS) task. Our data demonstrate that FEF neurons show sensory and mnemonic selectivity for stimulus shape features whether or not they are behaviorally significant for the task at hand. We also investigated the extent and localization of activation in the FEF using a variety of shape stimuli defined by static or dynamic cues employing functional magentic resonance imaging (fMRI) in anesthetized and paralyzed monkeys. Our fMRI results support the electrophysiological findings by showing significant FEF activation for a variety of shape stimuli and cues in the absence of attentional and motor processing. This shape selectivity in FEF is comparable to previous reports in the ventral pathway, inviting a reconsideration of the functional organization of the visual system.

Programming Saccades: The Role of Attention

Many different lines of research have attempted to answer questions about how the brain produces a saccadic eye movement. Often visual attention is an important factor in the findings of eye movement research. For this reason, it should prove useful to review some basic issues in eye movement research and the role attention plays.