Jude Mitchell

The visual pathways that guide actions do not necessarily mediate conscious perception. Patients with primary visual cortex (V1) damage lose conscious perception but often retain unconscious abilities (e.g. blindsight). Here, we asked if saccade accuracy and post-saccadic following responses (PFRs) that automatically track target motion upon saccade landing are retained when conscious perception is lost. We contrasted these behaviors in the blind and intact fields of 11 chronic V1-stroke patients, and in 8 visually intact controls. Saccade accuracy was relatively normal in all cases. Stroke patients also had normal PFR in their intact fields, but no PFR in their blind fields. Thus, V1 damage did not spare the unconscious visual processing necessary for automatic, post-saccadic smooth eye movements. Importantly, visual training that recovered motion perception in the blind field did not restore the PFR, suggesting a clear dissociation between pathways mediating perceptual restoration...

The visual pathways that guide actions do not necessarily mediate conscious perception. Patients with primary visual cortex (V1) damage lose conscious perception but often retain unconscious abilities (e.g. blindsight). Here, we asked if saccade accuracy and post-saccadic following responses (PFRs) that automatically track target motion upon saccade landing are retained when conscious perception is lost. We contrasted these behaviors in the blind and intact fields of 11 chronic V1-stroke patients, and in 8 visually intact controls. Saccade accuracy was relatively normal in all cases. Stroke patients also had normal PFR in their intact fields, but no PFR in their blind fields. Thus, V1 damage did not spare the unconscious visual processing necessary for automatic, post-saccadic smooth eye movements. Importantly, visual training that recovered motion perception in the blind field did not restore the PFR, suggesting a clear dissociation between pathways mediating perceptual restoration...

The visual pathways that guide actions do not necessarily mediate conscious perception. Patients with primary visual cortex (V1) damage lose conscious perception but often retain unconscious abilities (e.g. blindsight). Here, we asked if saccade accuracy and post-saccadic following responses (PFRs) that automatically track target motion upon saccade landing are retained when conscious perception is lost. We contrasted these behaviors in the blind and intact fields of 11 chronic V1-stroke patients, and in 8 visually intact controls. Saccade accuracy was relatively normal in all cases. Stroke patients also had normal PFR in their intact fields, but no PFR in their blind fields. Thus, V1 damage did not spare the unconscious visual processing necessary for automatic, post-saccadic smooth eye movements. Importantly, visual training that recovered motion perception in the blind field did not restore the PFR, suggesting a clear dissociation between pathways mediating perceptual restoration...

This paper describes a neural network model that directs saccades back to targets after they disappear and other saccades intervene. This is a simple example of knowing where something is after it is no longer visible and the observer has moved. These tasks require ...

Some solutions are outlined to the problem of using readily available types of germanium power transistor at ambient temperatures as high as 85°C, such as might be found in aircraft. Germanium transistors can be used at ambients within 5°C of their maximum storage temperatures if the power dissipated in the transistor is kept low and there is provision to handle

Conversational turn-taking is an integral part of language development, as it reflects a confluence of social factors that mitigate communication. Humans coordinate the timing of speech based on the behaviour of another speaker, a behaviour that is learned during infancy. While adults in several primate species engage in vocal turn-taking, the degree to which similar learning processes underlie its development in these non-human species or are unique to language is not clear. We recorded the natural vocal interactions of common marmosets (Callithrix jacchus) occurring with both their sibling twins and parents over the first year of life and observed at least two parallels with language development. First, marmoset turn-taking is a learned vocal behaviour. Second, marmoset parents potentially played a direct role in guiding the development of turn-taking by providing feedback to their offspring when errors occurred during vocal interactions similarly to what has been observed in huma...

Smooth pursuit eye movements stabilize slow-moving objects on the retina by matching eye velocity with target velocity. Two critical components are required to generate smooth pursuit: first, because it is a voluntary eye movement, the subject must select a target to pursue in order to engage the tracking system; and second, generating smooth pursuit requires a moving stimulus. We examined whether this behavior also exists in the common marmoset, a New World primate that is increasingly attracting attention as a genetic model for mental disease and systems neuroscience. We measured smooth pursuit in two marmosets, previously trained to perform fixation tasks, using the standard Rashbass step-ramp pursuit paradigm. We first measured the aspects of visual motion that drive pursuit eye movements. Smooth eye movements were in the same direction as target motion indicating that pursuit was driven by target movement rather than by displacement. Both the open-loop acceleration and closed-loop eye velocity exhibited a linear relationship with target velocity for slow-moving targets, but this relationship declined for higher speeds. We next examined whether marmoset pursuit eye movements depend on an active engagement of the pursuit system by measuring smooth eye movements evoked by small perturbations of motion from fixation or during pursuit. Pursuit eye movements were much larger during pursuit than from fixation, indicating that pursuit is actively gated. Several practical advantages of the marmoset brain, including the accessibility of areas MT and FEF at the cortical surface, merit its utilization for studying pursuit movements.

The common marmoset (Callithrix jacchus) has been valuable as a primate model in biomedical research. Interest in this species has grown recently, in part due to the successful demonstration of transgenic marmosets. Here we examine the prospects of the marmoset model for visual neuroscience research, adopting a comparative framework to place the marmoset within a broader evolutionary context. The marmoset's small brain bears most of the organizational features of other primates, and its smooth surface offers practical advantages over the macaque for areal mapping, laminar electrode penetration, and two-photon and optical imaging. Behaviorally, marmosets are more limited at performing regimented psychophysical tasks, but do readily accept the head restraint that is necessary for accurate eye tracking and neurophysiology, and can perform simple discriminations. Their natural gaze behavior closely resembles that of other primates, with a tendency to focus on objects of social inter...

When two differently colored, superimposed patterns of dots rotate in opposite directions, this yields the percept of two super- imposed transparent surfaces. If observers are cued to attend to one set of dots, they are impaired in making judgments about the other set. Since the two sets of dots are overlapping, the cueing effect cannot be explained by spatial attention.

When two differently colored, superimposed patterns of dots rotate in opposite directions, this yields the percept of two superimposed transparent surfaces. If observers are cued to attend to one set of dots, they are impaired in making judgments about the other set. Since the two sets of dots are overlapping, the cueing effect cannot be explained by spatial attention. This has led to the interpretation that the impairment reflects surface-based attentional selection. However, recent single-unit recording studies in monkeys have found that attention can modulate the gain of neurons tuned for features such as color. Thus, rather than reflecting the selection of a surface, the behavioral effects might simply reflect a reduction in the gain of color channels selective for the color of the uncued set of dots (feature-based attention), as if viewing the surfaces through a colored filter. If so, then the impairment should be eliminated when the two surfaces are made the same color. Instea...

Visuomotor processing is selective - only a small subset of stimuli that impinge on the retinae reach perceptual awareness and/or elicit behavioral responses. Both binocular rivalry and attention involve visual selection, but affect perception quite differently. During rivalry, awareness alternates between different stimuli presented to the two eyes. In contrast, attending to one of the two stimuli impairs discrimination of the ignored stimulus, but without causing it to perceptually disappear. We review experiments demonstrating that, despite their phenomenological differences, attention and rivalry depend upon shared competitive selection mechanisms. These experiments, moreover, reveal stimulus selection that is surface-based and requires coordination between the different neuronal populations that respond as a surface changes its attributes (type of motion) over time. This surface-based selection, in turn biases interocular competition, favoring the eye whose image is consistent ...

The common marmoset (Callithrix jacchus), a small-bodied New World primate, offers several advantages to complement vision research in larger primates. Studies in the anesthetized marmoset have detailed the anatomy and physiology of their visual system (Rosa et al., 2009) while studies of auditory and vocal processing have established their utility for awake and behaving neurophysiological investigations (Lu et al., 2001a,b; Eliades and Wang, 2008a,b; Osmanski and Wang, 2011; Remington et al., 2012). However, a critical unknown is whether marmosets can perform visual tasks under head restraint. This has been essential for studies in macaques, enabling both accurate eye tracking and head stabilization for neurophysiology. In one set of experiments we compared the free viewing behavior of head-fixed marmosets to that of macaques, and found that their saccadic behavior is comparable across a number of saccade metrics and that saccades target similar regions of interest including faces....

In natural viewing, a visual stimulus that is the target of attention is generally surrounded by many irrelevant distracters. Stimuli falling in the receptive field surround can influence the neuronal response evoked by a stimulus appearing within the classical receptive field. Such modulation by task-irrelevant distracters may degrade the target-related neuronal signal. We therefore examined whether directing attention to a target stimulus can reduce the influence of task-irrelevant distracters on neuronal response. We find that in area V4 attention to a stimulus within a neuron's receptive field filters out a large fraction of the suppression induced by distracters appearing in the surround. When attention is instead directed to the surround stimulus, suppression is increased, thereby filtering out part of the neuronal response to the irrelevant distracter positioned within the receptive field. These findings demonstrate that attention modulates the neural mechanisms that give...

Abstract Human observers can attentively track 3–5 stimuli as they move along independent random trajectories among distracter stimuli (Pylyshyn and Storm, 1988; Sears and Pylyshyn, 2000). We developed a multi-object tracking task suitable for monkeys, and ...

Abstract Human observers can attentively track multiple stimuli as they move along independent random trajectories among distracter stimuli (Pylyshyn and Storm, 1988; Sears and Pylyshyn, 2000). We developed a multi-object tracking task suitable for monkeys and ...

Abstract Directing attention to one of two stimuli placed within a neuron's classical receptive field (CRF) biases the neuron's response to more closely resemble the response evoked when the attended stimulus appears alone. It is not known, however, how attention ...

J. F. Mitchell, G. R. Stoner, and J. H. Reynolds (2004) observed that exogenously cuing one of two superimposed transparent surfaces resulted in an enhanced perceptual bias for the cued surface during binocular rivalry. We investigated the neural bases of this effect by recording event-related potentials (ERPs). Subjects viewed two superimposed rotating transparent surfaces and compared the directions of two successive translations, either both of the same surface or one of each surface. Following the first translation, which cued attention to the translating surface, two surface images were removed-one from each eye (dichoptic viewing) or both from one eye (monocular viewing). Subjects were impaired at comparing the first and second translations when they occurred on different surfaces, and the impairment was greater during dichoptic viewing (rivalry). The P1 component (110-160 ms) of the ERP elicited by the second translation of the same surface was larger than for the different surface during dichoptic but not monocular viewing. Larger cueing effects were also observed for the subsequent posterior N1 (160-220 ms) and P2 (250-300 ms) components during rivalry than during monocular viewing. These results are in line with a hybrid model of rivalry whereby cuing one surface initiates an earlier interocular selection process when the competing surfaces are presented to separate eyes.

Abstract Correlated firing among sensory neurons limits the accuracy with which sensory information can be decoded from neuronal populations (Zohary et. al., 1994). Attention might improve sensory processing by reducing correlations in the responses evoked by an ...

Abstract Visual neurons integrate information over large regions of space beyond their classical receptive fields. A stimulus presented alone in this surround region does not elicit a response, but can modulate the response evoked by a simultaneously presented center ...

A growing number of neurophysiological studies have found that spike-LFP coherence (SFC) varies with behavioral state. SFC quantifies the extent to which spike activity is phase-locked to the LFP across frequency. Changes in SFC within a particular frequency band ...