Biomedical subjects

P H Schiller

Publications and source records attributed to P H Schiller.

At least 19 recordsLinked to original sources

PubMed2004

Phosphene induction and the generation of saccadic eye movements by striate cortex.

The purpose of this review is to critically examine phosphene induction and saccadic eye movement generation by electrical microstimulation of striate cortex (area V1) in humans and monkeys. The following issues are addressed: 1) Properties of electrical stimulation as they pertain to the activation of V1 elements; 2) the induction of phosphenes in sighted and blind human subjects elicited by electrical stimulation using various stimulation parameters and electrode types; 3) the induction of phosphenes with electrical microstimulation of V1 in monkeys; 4) the generation of saccadic eye movements with electrical microstimulation of V1 in monkeys; and 5) the tasks involved for the development of a cortical visual prosthesis for the blind. In this review it is concluded that electrical microstimulation of area V1 in trained monkeys can be used to accelerate the development of an effective prosthetic device for the blind.

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PubMed2001

Look and see: how the brain moves your eyes about.

Two major cortical streams are involved in the generation of visually guided saccadic eye movements: the anterior and the posterior. The anterior stream from the frontal and medial eye fields has direct access to brainstem oculomotor centers. The posterior stream from the occipital cortices reaches brainstem oculomotor centers through the superior colliculus. The parietal cortex interconnects with both streams. Our findings suggest that the posterior stream plays an unique role in the execution of rapid, short-latency eye movements called 'express saccades'. Both the anterior and posterior streams play a role in the selection of targets to which saccades are to be generated, but do so in different ways. Areas V1, V2 and LIP contribute to decisions involved in where to look as well as where not to look. In addition, area LIP is involved in decisions about how long to maintain fixation prior to the execution of a saccade. Area V4 does not appear to be directly involved in eye-movement generation. In the anterior stream, the frontal eye fields, and to a lesser extent the medial eye fields, are involved in the correct execution of saccades subsequent to decisions made about where to look and where not to look.

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PubMed2001

Eye movements modulate visual receptive fields of V4 neurons.

The receptive field, defined as the spatiotemporal selectivity of neurons to sensory stimuli, is central to our understanding of the neuronal mechanisms of perception. However, despite the fact that eye movements are critical during normal vision, the influence of eye movements on the structure of receptive fields has never been characterized. Here, we map the receptive fields of macaque area V4 neurons during saccadic eye movements and find that receptive fields are remarkably dynamic. Specifically, before the initiation of a saccadic eye movement, receptive fields shrink and shift towards the saccade target. These spatiotemporal dynamics may enhance information processing of relevant stimuli during the scanning of a visual scene, thereby assisting the selection of saccade targets and accelerating the analysis of the visual scene during free viewing.

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PubMed2000

The effects of anterior arcuate and dorsomedial frontal cortex lesions on visually guided eye movements in the rhesus monkey: 1. Single and sequential targets.

This study examined the effects of anterior arcuate and dorsomedial frontal cortex lesions on visually guided eye movements in three rhesus monkeys. Lesions of the anterior bank of the arcuate, where the frontal eye fields reside, produced major deficits in the execution of saccadic eye movements to sequentially presented targets that did not recover even after 1 year after the lesions. Ablations of the dorsomedial frontal cortex, wherein the medial eye fields reside, produced much smaller and shorter-duration deficits on this task. Deficits after paired lesions of the anterior arcuate and dorsomedial frontal cortex were of approximately the same magnitude as after single anterior arcuate lesions. Anterior arcuate lesions also increased saccadic reaction times to single visual targets and decreased saccadic velocities that recovered gradually over a period of 2-5 months. Dorsomedial frontal cortex lesions produced only small increases in saccadic latencies that recovered rapidly. None of the lesions produced deficits in executing combined saccadic and pursuit eye movements to moving targets. The results suggest that the anterior arcuate area plays a central role in the execution of sequences of eye movements to successively appearing targets.

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PubMed2000

The effects of anterior arcuate and dorsomedial frontal cortex lesions on visually guided eye movements: 2. Paired and multiple targets.

This study examined the effects of anterior arcuate and dorsomedial frontal cortex lesions on the execution of saccadic eye movements made to paired and multiple targets in rhesus monkeys. Identical paired targets were presented with various temporal asynchronies to determine the temporal offset required to yield equal probability choices to either target. In the intact animal equal probability choices were typically obtained when the targets appeared simultaneously. After unilateral anterior arcuate lesions a major shift arose in the temporal offset required to obtain equal probability choices for paired targets that necessitated presenting the target in the hemifield contralateral to the lesion more than 100 ms prior to the target in the ipsilateral hemifield. This deficit was still pronounced 1 year after the lesion. Dorsomedial frontal cortex lesions produced much smaller but significant shifts in target selection that recovered more rapidly. Paired lesions produced deficits similar to those observed with anterior arcuate lesions alone. Major deficits were also observed on a multiple target temporal discrimination task after anterior arcuate but not after dorsomedial frontal cortex lesions. These results suggest that the frontal eye fields that reside in anterior bank of the arcuate sulcus play an important role in temporal processing and in target selection. Dorsomedial frontal cortex, that contains the medial eye fields, plays a much less important role in the execution of these tasks.

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PubMed2000

Eye fields in the frontal lobes of primates.

Two eye fields have been identified in the frontal lobes of primates: one is situated dorsomedially within the frontal cortex and will be referred to as the eye field within the dorsomedial frontal cortex (DMFC); the other resides dorsolaterally within the frontal cortex and is commonly referred to as the frontal eye field (FEF). This review documents the similarities and differences between these eye fields. Although the DMFC and FEF are both active during the execution of saccadic and smooth pursuit eye movements, the FEF is more dedicated to these functions. Lesions of DMFC minimally affect the production of most types of saccadic eye movements and have no effect on the execution of smooth pursuit eye movements. In contrast, lesions of the FEF produce deficits in generating saccades to briefly presented targets, in the production of saccades to two or more sequentially presented targets, in the selection of simultaneously presented targets, and in the execution of smooth pursuit eye movements. For the most part, these deficits are prevalent in both monkeys and humans. Single-unit recording experiments have shown that the DMFC contains neurons that mediate both limb and eye movements, whereas the FEF seems to be involved in the execution of eye movements only. Imaging experiments conducted on humans have corroborated these findings. A feature that distinguishes the DMFC from the FEF is that the DMFC contains a somatotopic map with eyes represented rostrally and hindlimbs represented caudally; the FEF has no such topography. Furthermore, experiments have revealed that the DMFC tends to contain a craniotopic (i.e., head-centered) code for the execution of saccadic eye movements, whereas the FEF contains a retinotopic (i.e., eye-centered) code for the elicitation of saccades. Imaging and unit recording data suggest that the DMFC is more involved in the learning of new tasks than is the FEF. Also with continued training on behavioural tasks the responsivity of the DMFC tends to drop. Accordingly, the DMFC is more involved in learning operations whereas the FEF is more specialized for the execution of saccadic and smooth pursuit eye movements.

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PubMed1999

Express averaging saccades in monkeys.

When monkeys are presented simultaneously with multiple stimuli, they can make one of two types of response. Either they make averaging saccades, that land at intermediate locations between the targets, or target-directed saccades, that land close to one of the targets. The two types of saccades occur at different latencies and are thought to reflect different processes; fast reflexive averaging and slower target selection. We investigated the latency of averaging saccades in five monkeys, with particular emphasis on 'express' latency saccades, which are thought to be inhibited by target selection. Express averaging saccades were made prolifically by the two monkeys that made both express and regular latency saccades, but only when no specific instruction was given regarding the saccade target. When these monkeys had to choose one of the targets, on the basis of its color, they still made averaging saccades. However, the endpoints formed two distributions close to the targets as opposed to one single distribution centered between the targets, as was the case when targets were identical; also, express saccades were almost entirely absent. We conclude that express averaging saccades are a form of spatial and temporal optimization of gaze shifting.

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PubMed1999

Behavioural conditions affecting saccadic eye movements elicited electrically from the frontal lobes of primates.

We assessed the effects of varying the time at which electrical stimulation was delivered to the dorsomedial frontal cortex (DMFC) and the frontal eye fields (FEF) relative to the onset of a visual target. Monkeys were required to fixate the visual target to obtain a drop of apple juice as reward. We found that the probability of eliciting saccades increased with increases in the delay of electrical stimulation relative to target onset. Also, the current threshold to evoke saccades decreased as electrical stimulation was delivered later following target onset. There were major differences in the magnitude of this effect with stimulation of the DMFC versus the FEF. The current threshold to evoke saccades from the DMFC was 16 times greater when electrical stimulation was delivered 200 ms after target onset as compared to when it was delayed 200 ms after target offset. In contrast, the current threshold to evoke saccades from the FEFs was only three times greater when stimulation was delivered under similar conditions. These results suggest that the FEF are more closely connected with the saccade generator for the execution of saccadic eye movements than is the DMFC, even though both regions have direct projections to brainstem oculomotor centres.

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PubMed1998

Visual representations during saccadic eye movements.

In normal vision, shifts of attention are usually followed by saccadic eye movements. Neurons in extrastriate area V4 are modulated by focal attention when eye movements are withheld, but they also respond in advance of visually guided saccadic eye movements. We have examined the visual selectivity of saccade-related responses of area V4 neurons in monkeys making delayed eye movements to receptive field stimuli of varying orientation. This task did not require the monkey to attend to orientation per se but merely to foveate the receptive field stimulus. We present evidence that the presaccadic enhancement exhibited by V4 neurons, quite separate from the response at stimulus onset, is a resurgent visual representation that seems as selective as the response is when the stimulus first appears. The presaccadic enhancement appears to provide a strengthening of a decaying featural representation immediately before an eye movement is directed to visual targets. We suggest that this reactivation provides a mechanism by which a clear perception of the saccade goal can be maintained during the execution of the saccade, perhaps for the purpose of establishing continuity across eye movements.

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PubMed1998

Saccades induced electrically from the dorsomedial frontal cortex: evidence for a head-centered representation.

The amplitude and direction of saccadic eye movements evoked electrically from the dorsomedial frontal cortex (DMFC) of monkeys vary with starting eye position. This observation has been used to argue that the DMFC codes saccadic eye movements in head-centered coordinates. Whether the amplitude and direction of the evoked saccades are also affected by changes in head position has never been demonstrated. Such a result would argue against a head-centered representation, and instead would suggest a representation anchored to another body part. Tests were conducted on rhesus monkeys to determine whether changing the position of the head with respect to the trunk or changing the position of the head with respect to the gravitational axis alters saccadic parameters. The amplitude and direction of saccadic eye movements remained invariant to such manipulations. These findings confirm the claim that the DMFC encodes saccadic eye movements in head-centered coordinates.

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PubMed1998

The effects of frontal eye field and dorsomedial frontal cortex lesions on visually guided eye movements.

In the frontal lobe of primates, two areas play a role in visually guided eye movements: the frontal eye fields (FEF) and the medial eye fields (MEF) in dorsomedial frontal cortex. Previously, FEF lesions have revealed only mild deficits in saccadic eye movements that recovered rapidly. Deficits in eye movements after MEF ablation have not been shown. We report the effects of ablating these areas singly or in combination, using tests in which animals were trained to make saccadic eye movements to paired or multiple targets presented at various temporal asynchronies. FEF lesions produced large and long-lasting deficits on both tasks. Sequences of eye movements made to successively presented targets were also impaired. Much smaller deficits were observed after MEF lesions. Our findings indicate a major, long-lasting loss in temporal ordering and processing speed for visually guided saccadic eye movement generation after FEF lesions and a significant but smaller and shorter-lasting loss after MEF lesions.

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PubMed1996

Contextual modulation in primary visual cortex.

We studied extra-receptive field contextual modulation in area V1 of awake, behaving macaque monkeys. Contextual modulation was studied using texture displays in which texture covering the receptive field (RF) was the same in all trials, but the perceptual context of this texture could vary depending on the configuration of extra-RF texture elements. We found robust contextual modulation when disparity, color, luminance, and orientation cues variously defined a textured figure centered on the RF of V1 neurons. We found contextual modulation to have a spatial extent of approximately 8 to 10 degrees diameter parafoveally. Contextual modulation correlated with perceptual experience of both binocularly rivalrous texture displays and of displays with a simple example of surface occlusion. We found contextual modulation in V1 to have a characteristic latency of 80-100 msec after stimulus onset, potentially allowing feedback from extrastriate areas to underlie to this effect.

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PubMed1996

On the specificity of neurons and visual areas.

The dominant view during the past 40 years has been that the visual system analyzes the visual scene by breaking it down into basic attributes such as color, form, motion, depth and texture. Individual dedicated neurons and specific visual areas were believed to be devoted to the analysis of each of these attributes. Current research has challenged these views by emphasizing that neurons, especially in the cortex, have multifunctional properties and therefore serve as general-purpose analyzers rather than feature detectors. Consequently, it appears that most extrastriate visual areas, rather than each being devoted to the analysis of a specific basic visual attribute, perform several different tasks and thereby engage in more advanced and complex analyses than had been realized.

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PubMed1995

Effect of lesions in visual cortical area V4 on the recognition of transformed objects.

The primate visual system has a remarkable capability for recognizing objects irrespective of the multitude of images they form on the retinal surface by virtue of changes in size, perspective, contrast, colour and partial obstruction by other stimuli in the visual scene. There is increasing evidence that this remarkable capacity is brought about by processes that occur earlier in the visual system than had previously been thought. Here I show that after ablation of area V4 in the rhesus monkey, major deficits arise in the recognition of objects that have been transformed in size, in the degree of occlusion, and in the amount of contour information provided. The ability to detect these objects when presented individually was unaffected by these lesions.

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PubMed1994

Stimulation-evoked saccades from the dorsomedial frontal cortex of the rhesus monkey following lesions of the frontal eye fields and superior colliculus.

This study examined whether signals for the generation of eye movements from the dorsomedial frontal cortex (DMFC) reach brainstem oculomotor centers either through the frontal eye fields (FEF) or through the superior colliculi (SC). The DMFC was stimulated when the monkeys studied were intact and after either one FEF or one SC was ablated. Following lesions of either the FEF or SC, the topographic order of the DMFC was largely preserved. After either lesions, stimulation of anterior DMFC sites still evoked saccades that terminated in contralateral space, and stimulation of posterior DMFC sites still evoked saccades that terminated in central space. The probability of evoking saccades decreased and the latency to evoke saccades increased as fixation neared the termination zone (a restricted region within craniotopic space) both before and after either lesion. Ablation of the SC, but not of the FEF, eliminated the saccadic inhibition to visual targets which resulted when the DMFC was stimulated in the intact animal. The findings suggest that additional channels besides those coursing through the FEF and SC are utilized by the DMFC to access the saccade generator in the brainstem.

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PubMed1994

The effects of visual scene composition on the latency of saccadic eye movements of the rhesus monkey.

This study examined how variations in the visual scene affect the generation of bimodal saccadic latency distributions, the first mode of which is called the population of "express saccades". The surface media used to make stimuli visible and the composition of the background were varied to determine the conditions under which express saccades can be generated in rhesus monkeys. The results show that express saccades to singly presented targets can readily be elicited when the stimuli are made visible by virtue of either luminance contrast, color contrast or motion cues. Express saccades are rarely obtained when stimuli are made visible by virtue of only stereoscopic depth or texture cues. Express saccades can, however, be elicited using random-dot stereograms or textures when luminance or chrominance information is added to the target. When single target stimuli are presented simultaneously with a set of non-target stimuli, express saccades are for the most part prevented unless either the non-target stimuli are near threshold or their numerosity is very high, in which case they form a texture-like array. However, when the non-target stimuli are continuously present in the display, express saccades reemerge. These findings suggest that express saccades are not unique to experimental situations in which only a single stimulus appears on an otherwise homogeneous surface; they can readily be generated as long as the target stimulus is made visible by virtue of luminance, chrominance, motion or a combination of more than one surface medium and as long as the target does not appear concurrently with a salient group of other non-target stimuli.

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PubMed1992

The ON and OFF channels of the visual system.

In the vertebrate retina, all photoreceptors hyperpolarize in response to light. In the outer retina, at the bipolar cell level, a dual system is created from the cones forming the ON and OFF channels. In the rod system a similar arrangement is found, but the ON and OFF channels in many species are formed using an amacrine cell network in the inner retina. Physiological experiments in which the ON bipolar cells are pharmacologically blocked reveal that in the primate the two channels remain largely segregated in the geniculostriate system until they reach the cortex, where they converge upon single cells. Behavioral studies show that following ON channel block, notable deficits arise in the detection of light increments but not light decrements. These and related studies suggest that the ON and OFF channels optimize information transfer to the CNS by providing excitatory signals for both increases and decreases in light energy.

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PubMed1991

The role of the primate extrastriate area V4 in vision.

Area V4 is a part of the primate visual cortex. Its role in vision has been extensively debated. Inferences about the functions of this area have now been made by examination of a broad range of visual capacities after ablation of V4 in rhesus monkeys. The results obtained suggest that this area is involved in more complex aspects of visual information processing than had previously been suggested. Monkeys had particularly severe deficits in situations where the task was to select target stimuli that had a lower contrast, smaller size, or slower rate of motion than the array of comparison stimuli from which they had to be discriminated. Extensive training on each specific task resulted in improved performance. However, after V4 ablation, the monkeys could not generalize the specific task to new stimulus configurations and to new spatial locations.

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