Analog VLSI-Based Modeling of the Primate Oculomotor System

One way to understand a neurobiological system is by building a simulacrum that replicates its behavior in real time using similar constraints. Analog very large-scale integrated (VLSI) electronic circuit technology provides such an enabling technology. We here describe a neuromorphic system that is part of a long-term effort to understand the primate oculomotor system. It requires both fast sensory processing and fast motor control to interact with the world. A one-dimensional hardware model of the primate eye has been built that simulates the physical dynamics of the biological system. It is driven by two different analog VLSI chips, one mimicking cortical visual processing for target selection and tracking and another modeling brain stem circuits that drive the eye muscles. Our oculomotor plant demonstrates both smooth pursuit movements, driven by a retinal velocity error signal, and saccadic eye movements, controlled by retinal position error, and can reproduce several behavioral, stimulation, lesion, and adaptation experiments performed on primates

A non-spatial bias favouring fixated stimuli revealed in patients with spatial neglect

The cardinal feature of spatial neglect is severely impaired exploration of the contralesional space, a failure resulting in unawareness of many contralesional stimuli. This deficit is exacerbated by a reflexive attentional bias toward ipsilesional items. Here we show that, in addition to these spatially lateralized failures, neglect patients also exhibit a severe bias favouring stimuli presented at fixation. We tested neglect patients and matched healthy and right-hemisphere damaged patients without neglect in a task requiring saccade execution to targets in the left or right hemifield. Targets were presented alone or simultaneously with a distracter that appeared in the same hemifield, in the opposite hemifield, or at fixation. We found two fundamental biases in saccade initiation of neglect patients: irrelevant distracters presented in the preserved hemifield tended to capture gaze reflexively, resulting in a large number of saccades erroneously directed toward the distracter. Additionally, distracters presented at fixation severely disrupted saccade initiation irrespective of saccade direction, leading to disproportionately increased latencies of left and right saccades. This latency increase was specific to oculomotor responses of neglect patients and was not observed when a manual response was required. These results show that, in addition to their failure to inhibit reflexive glances toward ipsilesional items neglect patients exhibit a strong oculomotor bias favouring fixated stimuli. We conclude that impaired initiation of saccades in any direction contributes to the deficits of spatial exploration that characterize spatial neglec

Microsaccade-induced prolongation of saccade latencies depends on microsaccade amplitude

Fixations consist of small movements including microsaccades, i.e., rapid flicks in eye position that replace the retinal image by up to 1 degree of visual angle. Recently, we showed in a delayed-saccade task (1) that the rate of microsaccades decreased in the course of saccade preparation and (2) that microsaccades occurring around the time of a go signal were associated with prolonged saccade latencies (Rolfs et al., 2006). A re-analysis of the same data set revealed a strong dependence of these findings on microsaccade amplitude. First, microsaccade amplitude dropped to a minimum just before the generation of a saccade. Second, the delay of response saccades was a function of microsaccade amplitude: Microsaccades with larger amplitudes were followed by longer response latencies. These finding were predicted by a recently proposed model that attributes microsaccade generation to fixation-related activity in a saccadic motor map that is in competition with the generation of large saccades (Rolfs et al., 2008). We propose, therefore, that microsaccade statistics provide a behavioral correlate of fixation-related activity in the oculomotor system

Effects of aging on regular and express latencies of vergence

Vergence eye movements are frequent in every day life and important for depth perception. Yet, studies of vergence in elderly are rare. We examined convergence and divergence between targets placed along median line at 20, 40 or 150cm. Thirteen elderly (70±11years) and ten young (25±3years) adults participated in the study. The gap paradigm (i.e., the fixed stimulus is extinguished prior to target onset) and the overlap paradigm (the fixed stimulus remains illuminated after target onset) were used to elicit reflexive or voluntary eye movements. Latency of convergence and divergence increased with age in both gap and overlap conditions. Both young subjects and elderly showed shorter latency of vergence under the gap condition than under the overlap condition. In the overlap condition, for elderly only, convergence resulted in longer latencies than divergence. In the gap condition express convergence was rare, while express latencies (80–120ms) were frequent for divergence starting from a near fixation point (at 20cm). The rates of express divergence were similar for young (23%) and elderly (21%). These results were in line with prior studies of saccades and indicated different aging effects on regular and express latencies

Frontal eye field, where art thou? Anatomy, function, and non-invasive manipulation of frontal regions involved in eye movements and associated cognitive operations

The planning, control and execution of eye movements in 3D space relies on a distributed system of cortical and subcortical brain regions. Within this network, the Eye Fields have been described in animals as cortical regions in which electrical stimulation is able to trigger eye movements and influence their latency or accuracy. This review focuses on the Frontal Eye Field (FEF) a “hub” region located in Humans in the vicinity of the pre-central sulcus and the dorsal-most portion of the superior frontal sulcus. The straightforward localization of the FEF through electrical stimulation in animals is difficult to translate to the healthy human brain, particularly with non-invasive neuroimaging techniques. Hence, in the first part of this review, we describe attempts made to characterize the anatomical localization of this area in the human brain. The outcome of functional Magnetic Resonance Imaging (fMRI), Magneto-encephalography (MEG) and particularly, non-invasive mapping methods such a Transcranial Magnetic Stimulation (TMS) are described and the variability of FEF localization across individuals and mapping techniques are discussed. In the second part of this review, we will address the role of the FEF. We explore its involvement both in the physiology of fixation, saccade, pursuit, and vergence movements and in associated cognitive processes such as attentional orienting, visual awareness and perceptual modulation. Finally in the third part, we review recent evidence suggesting the high level of malleability and plasticity of these regions and associated networks to non-invasive stimulation. The exploratory, diagnostic, and therapeutic interest of such interventions for the modulation and improvement of perception in 3D space are discussed

Inhibition of saccadic eye movements to locations in spatial working memory

Inhibition of return (IOR) refers to a bias against overt and covert attentional orienting toward previously attended locations. According to the reorienting hypothesis, IOR is generated when attention is withdrawn from the attended location and is prevented from "returning" to it. The present study investigated whether maintenance of attention at the cued location could affect the inhibition of oculomotor orienting to it. To preclude dis-engagement of attention, we asked participants to maintain the cued location in working memory. Maintenance of visuospatial information in memory has been shown to be accomplished through a sustained shift of spatial attention to a memorized location. Our results show that oculomotor IOR occurs at a particular location even when that location is kept in working memory (Experiment 1). Furthermore, we demonstrate that the mere act of maintenance of a location in working memory produces oculomotor inhibition similar to IOR (Experiments 2 and 3). We conclude that the oculomotor system is used for coding and maintaining locations in spatial working memory. In addition, we demonstrate that endogenous attention associated with maintenance of a location in working memory can be dissociated from the attention needed for execution of a saccadic eye movement. © 2009 The Psychonomic Society, Inc

New light on neurocognitive processes linked to autism and attention deficit and hyperactivity disorder in childhood : studies of eye movements in twins

Visual attention and oculomotor response inhibition have been associated with Autism Spectrum Disorder (ASD) and Attention Deficit and Hyperactivity Disorder (ADHD) respectively. The aim of this thesis was to increase our knowledge about these cognitive functions relevant to ASD and ADHD in early infancy and childhood using eye tracking and twin modelling. Study 1 assessed the relative contribution of genetic and environmental influences to attentional networks and visual disengagement (using the gap overlap task) in a sample of twins from the general population, aged 9-14 years. It also assessed whether visual disengagement was associated with autistic traits. Gaze shift latencies across conditions were driven by shared genetic factors. Additionally, there were unique genetic influences to gaze shift latencies in the gap condition. In line with previous work, autistic traits were found to be heritable. There was no association between visual disengagement and autistic traits. Study 2 investigated the relative contribution of genetic and environmental factors to oculomotor response inhibition (using the antisaccade task) and the degree to which oculomotor response inhibition was associated with ADHD traits in the same twin sample. Oculomotor response inhibition in the form of premature anticipatory eye movements was heritable and associated to parent rated inattentive traits. This association was partially due to shared genetic factors. Study 3 investigated how visual disengagement relates to other cognitive developmental processes and behaviors, socioeconomic status and biological sex in early infancy. Gaze shift latencies in the overlap, baseline and gap conditions, of the Gap Overlap task, differed as a function of socioeconomic status and sex. No other associations between visual attention and developmental measures were observed. Thus, in summary, while these findings do not support neither a phenotypic nor a genetic link between visual disengagement and ASD, they support such association between oculomotor response inhibition and inattention (a core component of ADHD). Finally, these findings highlight the influence of sociodemographic factors on individual differences in visual attention in early infancy, thus underscoring the importance of understanding all sources of variation in attentional functions in childhood