Biomedical subjects

Peter H Schiller

Publications and source records attributed to Peter H Schiller.

16 recordsLinked to original sources

PubMed2005

Mapping cortical activity elicited with electrical microstimulation using FMRI in the macaque.

Over the last two centuries, electrical microstimulation has been used to demonstrate causal links between neural activity and specific behaviors and cognitive functions. However, to establish these links it is imperative to characterize the cortical activity patterns that are elicited by stimulation locally around the electrode and in other functionally connected areas. We have developed a technique to record brain activity using the blood oxygen level dependent (BOLD) signal while applying electrical microstimulation to the primate brain. We find that the spread of activity around the electrode tip in macaque area V1 was larger than expected from calculations based on passive spread of current and therefore may reflect functional spread by way of horizontal connections. Consistent with this functional transynaptic spread we also obtained activation in expected projection sites in extrastriate visual areas, demonstrating the utility of our technique in uncovering in vivo functional connectivity maps.

Action Potentials↗

PubMed2005

What is the coordinate frame utilized for the generation of express saccades in monkeys?

The latencies of saccades to suddenly appearing eccentric targets can have a bimodal distribution, with an early, express peak, and a late, regular peak (Fischer and Boch 1983, Brain Res 260: 21-26). Express saccades usually are a product of learning. The purpose of this study was to determine whether this learning is specific to the relative position of the target in space, the orbital position of the eye, or the vector of the saccade to be produced. Further, it was asked whether and how the frequency with which express saccades are generated is influenced by the immediately preceding saccadic vector and the familiarity of the targets. To this end, rhesus monkeys were trained to make saccadic eye movements to single targets and to two sequential targets that appeared at various positions relative to the head, relative to the initial fixation spot and relative to each other. The results show that the frequency with which express saccades are generated is determined by the saccadic vector that has to be generated and not by the relative position of a target in space, the orbital position of the eye, the immediately preceding saccadic vector, or the familiarity of the targets.

Animals↗

PubMed2005

Neural mechanisms underlying target selection with saccadic eye movements.

In exploring the visual scene we make about three saccadic eye movements per second. During each fixation, in addition to analyzing the object at which we are looking, a decision has to be made as to where to look next. Although we perform this task with the greatest of ease, the computations to perform the task are complex and involve numerous brain structures. We have applied several investigative tools that include single-cell recordings, microstimulation, pharmacological manipulations and lesions to learn more about the neural control of visually guided eye saccadic movements. Electrical stimulation of the superior colliculus (SC), areas V1 and V2, the lateral intraparietal sulcus (LIP), the frontal eye fields (FEF) and the medial eye fields (MEF) produces saccadic eye movements at low current levels. After ablation of the SC, electrical microstimulation of V1, V2, and LIP no longer elicits saccadic eye movements whereas stimulation of the FEF and MEF continues to be effective. Ablation of the SC but not of the FEF eliminates short-latency saccadic eye movements to visual targets called "express saccades," whereas lesions of the FEF selectively interfere with target selection. Bilateral removal of both the SC and the FEF causes major, long lasting deficits: all visually elicited saccadic eye movements are eliminated. In intact monkeys, subthreshold electrical microstimulation of the FEF and MEF as well as the lower layers of V1 and V2 and of some subregions of LIP greatly facilitates the choice of targets presented in the receptive fields of the stimulated neurons. By contrast, stimulation of the upper layers of V1 and V2 and other sub-regions of LIP produces a dramatic interference in target selection. Examination of the role of inhibitory circuits in eye-movement generation reveals that local infusion of muscimol, a GABA (gamma-aminobutyric acid) agonist, or bicuculline, a GABA antagonist, interferes with target selection in V1. On the other hand, infusion of bicuculline into the FEF produces facilitation in target choice and irrepressible saccades. It appears therefore that inhibitory circuits play a central role in visual analysis in V1 and in the generation of saccadic eye movements in the FEF. It is proposed that two major streams can be discerned in visually guided eye-movement control, the posterior from occipital and parietal cortex that reaches the brainstem via the SC and the anterior from the FEF and MEF that has direct access to the brainstem oculomotor centers.

Animals↗

PubMed2005

The Hermann grid illusion revisited.

The Hermann grid illusion consists of smudges perceived at the intersections of a white grid presented on a black background. In 1960 the effect was first explained by a theory advanced by Baumgartner suggesting the illusory effect is due to differences in the discharge characteristics of retinal ganglion cells when their receptive fields fall along the intersections versus when they fall along non-intersecting regions of the grid. Since then, others have claimed that this theory might not be adequate, suggesting that a model based on cortical mechanisms is necessary [Lingelbach et al, 1985 Perception 14(1) A7; Spillmann, 1994 Perception 23 691 708; Geier et al, 2004 Perception 33 Supplement, 53; Westheimer, 2004 Vision Research 44 2457 2465]. We present in this paper the following evidence to show that the retinal ganglion cell theory is untenable: (i) varying the makeup of the grid in a manner that does not materially affect the putative differential responses of the ganglion cells can reduce or eliminate the illusory effect; (ii) varying the grid such as to affect the putative differential responses of the ganglion cells does not eliminate the illusory effect; and (iii) the actual spatial layout of the retinal ganglion cell receptive fields is other than that assumed by the theory. To account for the Hermann grid illusion we propose an alternative theory according to which the illusory effect is brought about by the manner in which S1 type simple cells (as defined by Schiller et al, 1976 Journal of Neurophysiology 39 1320-1333) in primary visual cortex respond to the grid. This theory adequately handles many of the facts delineated in this paper.

Contrast Sensitivity↗

PubMed2005

Delaying visually guided saccades by microstimulation of macaque V1: spatial properties of delay fields.

Electrical microstimulation of macaque primary visual cortex (area V1) is known to delay the execution of saccadic eye movements made to a punctate visual target placed into the receptive field of the stimulated neurons. We examined the spatial extent of this delay effect, which we call a delay field, by placing a 0.2 degrees visual target at various locations relative to the receptive field of the stimulated neurons and by stimulating different sites within the operculum of V1. A 100-ms train of stimulation consisting of current pulses at or less than 100 microA was delivered immediately before monkeys generated a saccadic eye movement to the visual target. The region of tissue activated was within 0.5 mm from the electrode tip. The depth of stimulation for a given site ranged from 0.9 to 2.0 mm below the cortical surface. The location of the receptive fields of the stimulated neurons ranged from 1.8 to 4.4 degrees of eccentricity from the center of gaze. Within this range, the size of the delay field increased from 0.1 to 0.55 degrees of visual angle. The shape of the field was roughly circular. The size of the delay field increased as the stimulation site was located further from the foveal representation of V1. These results are consistent with the finding that phosphenes evoked by electrical stimulation of human V1 are circular and increase in size as the stimulating electrode is placed more distant from the foveal representation of V1.

Animals↗

PubMed2004

How do target predictability and precueing affect the production of express saccades in monkeys?

The extent to which target predictability and precueing affect express saccade generation was determined in Rhesus monkeys. Target predictability, as manipulated by the probability with which targets appeared at various locations, had a strong influence on express saccade generation. Pre-cueing the location of the appearance of an impending single target with an identical stimulus was effective in increasing express saccade generation when there was a gap of 50-150 ms between fixation spot termination and target onset. However, precueing was not effective when the gap time was set to 0 ms in the single target task, when several simultaneous targets appeared requiring a visual discrimination to be made using an oddity task, or when the precue was not identical to the target. These findings indicate that express saccades are facilitated by a restricted set of conditions that increase the predictability of target location and identity.

Animals↗

PubMed2004

Microstimulation of V1 delays the execution of visually guided saccades.

Electrical stimulation delivered to V1 concurrently with the presentation of a visual target interferes with both the selection and the detection of targets positioned in the receptive field of the stimulated neurons. In the present study, we examined the temporal course of this effect by delivering electrical stimulation to V1 of rhesus monkeys at various times before the appearance of a visual target. Each trial was initiated by the appearance of a fixation spot that, once acquired, was followed by the presentation of a visual target in the receptive field of the stimulated neurons. A monkey was reward after making a saccadic eye movement to the target. A delay in saccade generation was obtained when stimulation was delivered while an animal maintained fixation on the fixation spot. No delay occurred when the visual target was placed outside the receptive field of the stimulated neurons. The best parameters for inducing the saccadic delay were: (i). anode-first pulses (as opposed to cathode-first pulses) and (ii). train durations greater than 40 ms and frequencies greater than 100 Hz. The lowest current threshold for producing a saccadic delay occurred at 1.5 mm below the top of superficial V1. The chronaxies of the directly stimulated elements mediating the delay ranged from 0.13 to 0.24 ms. These values overlap with those that have been described for phosphene induction in human V1. We discuss how the elements mediating the saccadic delay might interrupt a visual signal as it passes along the geniculostriate pathway.

Animals↗

PubMed2004

Are express saccades generated under natural viewing conditions?

To assess whether express saccades are generated under everyday conditions, we collected eye movement data from Rhesus monkeys engaged in free viewing under a variety of conditions. The durations of the fixation periods that occurred between saccades were calculated. The results show that while short-duration fixations within the range of express saccades occur quite commonly, the overall distribution is unimodal. This is the case even when all the object elements in the visual scene have the same contrast. The findings suggest that while saccades that fall within the express range occur commonly under natural viewing conditions, bimodal distributions of saccadic latencies are obtainable only under laboratory conditions.

Animals↗

PubMed2003

Temporal factors in target selection with saccadic eye movements.

Decision times involved in selecting visual targets with saccadic eye movements in rhesus monkeys were studied for three tasks in which single targets, paired targets with varied asynchronies, and multiple targets requiring a discrimination were presented. Probability of target choice in the paired-target task was strongly influenced by target luminance and size as specified by the temporal offset required to yield equal probability choice. Among the animals tested, reaction times for target selection in the paired-target task took 12-47 ms longer, and in the discrimination task 17-70 ms longer than for generating saccades to single targets, thereby reflecting the decision times involved. The results provide information about the time-course of decisions involved in selecting visual targets with saccadic eye movements.

Animals↗

PubMed2003

Saccadic eye movements evoked by microstimulation of striate cortex.

Experiments were performed to assess the excitability of neural elements activated while inducing saccadic eye movements electrically from different cortical layers of striate cortex (area V1) in rhesus monkeys. Excitability was assessed by measuring current thresholds, saccadic latencies, chronaxies, and the effectiveness of anode-first vs. cathode-first pulses. Minimum current thresholds for the evocation of saccades (i.e. less than 5 microA) were observed when the deepest layers of V1 were stimulated. The shortest saccadic latencies were also observed at these depths. The shortest latency at 10 times the threshold current was 49 ms on average. The chronaxies of the elements mediating saccades were less in deep V1 (i.e. 0.17 ms) than in superficial V1 (i.e. 0.23 ms). Anode-first pulses were more effective at evoking saccades from superficial V1, whereas cathode-first pulses were more effective at evoking saccades from deep V1. These results indicate that the excitability properties of superficial and deep V1 are distinct for the generation of saccades. Moreover, the excitability of elements mediating saccades in V1 of monkeys is comparable to that of elements mediating phosphenes in human V1.

Animals↗

PubMed2003

Cortical inhibitory circuits in eye-movement generation.

The role inhibitory circuits play in target selection with saccadic eye movements was examined in area V1, the frontal eye fields (FEF) and the lateral intraparietal sulcus (LIP) of the Rhesus Macaque monkey by making local infusions of the GABA agonist muscimol and antagonist bicuculline. In V1, both agents greatly interfered with target selection and visual discrimination of stimuli placed into the receptive field of the affected neurons. In the FEF, bicuculline facilitated target selection without affecting visual discrimination and generated many spontaneous saccades. Muscimol in the FEF interfered with saccadic eye-movement generation. In the LIP, bicuculline was ineffective and muscimol had only a small effect. These findings suggest that in the FEF GABAergic inhibitory circuits play a central role in eye-movement generation whereas in V1 these circuits are essential for visual analysis. Inhibitory circuits in the LIP do not appear to play a central role in target selection and in visual discrimination.

Anesthetics, Local↗

PubMed2002

Spatial frequency and orientation tuning dynamics in area V1.

Spatial frequency (SF) and orientation tuning are intrinsic properties of neurons in primary visual cortex (area V1). To investigate the neural mechanisms mediating selectivity in the awake animal, we measured the temporal dynamics of SF and orientation tuning. We adapted a high-speed reverse-correlation method previously used to characterize orientation tuning dynamics in anesthetized animals to estimate efficiently the complete spatiotemporal receptive fields in area V1 of behaving macaques. We found that SF and orientation tuning are largely separable over time in single neurons. However, spatiotemporal receptive fields also contain a small nonseparable component that reflects a significant difference in response latency for low and high SF stimuli. The observed relationship between stimulus SF and latency represents a dynamic shift in SF tuning, and suggests that single V1 neurons might receive convergent input from the magno- and parvocellular processing streams. Although previous studies with anesthetized animals suggested that orientation tuning could change dramatically over time, we find no substantial evidence of dynamic changes in orientation tuning.

Animals↗

PubMed2002

Behavioral assessment of motion parallax and stereopsis as depth cues in rhesus monkeys.

Although human psychophysical results show that motion parallax and stereopsis are both effective depth cues, it is not clear whether the same is true for non-human primates. As an initial step, we assessed the extent to which rhesus monkeys are capable of processing depth information based solely on motion parallax as compared with stereopsis. We constructed a unique display that enabled us to provide depth cues by either stereopsis or motion parallax or both. Our results show that monkeys can process depth information conveyed both by motion parallax and stereopsis. As in humans, motion parallax was somewhat less effective for depth discrimination than was stereopsis. These findings prepare efforts for assessing how motion parallax and stereopsis are co-processed in the visual system.

Animals↗

PubMed2002

Differential effects of laminar stimulation of V1 cortex on target selection by macaque monkeys.

We explored the effects of microstimulation on target selection by delivering stimulation at different depths within V1 (striate cortex) of the rhesus monkey (Macaca mulatta). Stimulation evoked saccadic eye movements that terminated in the receptive-field location of the activated neurons. The current thresholds for saccade evocation were highest (> or = 30 micro A) in the superficial layers and lowest (< or = 10 micro A) in the deep layers. To study target selection, one visual target was presented in the receptive-field location of the stimulated neurons and a second visual target was presented outside this location. Microstimulation delivered in concert with the appearance of the two targets decreased the probability that a monkey would select the target placed in the receptive-field location when the upper layers of V1 were stimulated, and it increased this probability when the lower layers were stimulated. We suggest that microstimulation of the upper layers of V1 disrupts visual signals from retina en route to higher cortical areas, whereas microstimulation of the lower layers activates V1 efferents that innervate the oculomotor system.

Animals↗

PubMedDate not supplied

Neural responses to relative speed in the primary visual cortex of rhesus monkey.

Relative motion information, especially relative speed between different input patterns, is required for solving many complex tasks of the visual system, such as depth perception by motion parallax and motion-induced figure/ground segmentation. However, little is known about the neural substrate for processing relative speed information. To explore the neural mechanisms for relative speed, we recorded single-unit responses to relative motion in the primary visual cortex (area VI) of rhesus monkeys while presenting sets of random-dot arrays moving at different speeds. We found that most VI neurons were sensitive to the existence of a discontinuity in speed, that is, they showed higher responses when relative motion was presented compared to homogenous field motion. Seventy percent of the neurons in our sample responded predominantly to relative rather than to absolute speed. Relative speed tuning curves were similar at different center-surround velocity combinations. These relative motion-sensitive neurons in macaque area VI probably contribute to figure/ground segmentation and motion discontinuity detection.

Action Potentials↗

PubMedDate not supplied

An examination of the variables that affect express saccade generation.

The frequency with which express saccades are generated under a variety of conditions in rhesus monkeys was examined. Increasing the gap time between fixation spot termination and target onset increased express saccade frequency but was progressively less effective in doing so as the number of target positions in the sample was increased. Express saccades were rarely produced when two targets were presented simultaneously and the choice of either of which was rewarded; a temporal asynchrony of only 17 ms between the targets reinstated express saccade generation. Express saccades continued to be generated when the vergence or pursuit systems was coactivated with the saccadic system.

Animals↗