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Biomedical subjects

A A Skavenski

Publications and source records attributed to A A Skavenski.

At least 19 recordsLinked to original sources
PubMed2002

Cross-modal plasticity after monocular enucleation of the adult rabbit.

The mature brain undergoes compensatory reorganization of the primary visual cortex (V1) in response to retinal lesions. This study demonstrates that V1 also supports cross-modal reorganization by observing an increase in tactile responses in V1 after monocular enucleation of the adult rabbit. The proportion of tactile-responsive V1 neurons increased from 0% to 31%, in an area of cortex equivalent to 40 degrees of visual space. Retrograde fiber-tracing analysis suggests that intracortical connections from association areas may underlie these novel responses. Cortical plasticity of this kind may be involved in recovery from sensory system damage and could provide an enhanced sense of touch to the blind.

Action Potentials↗
PubMed2000

Concurrent processing of saccades in visual search.

We provide evidence that the saccadic system can simultaneously program two saccades to different goals. We presented subjects with simple visual search displays in which they were required to make a saccade to an odd-colored target embedded in an array of distractors. When there was strong competition between target and distractor stimuli (due to color priming from previous trials), subjects were more likely to make a saccade to a distractor. Such error saccades were often followed, after a very short inter-saccadic interval ( approximately 10-100 ms), by a second saccade to the target. The brevity of these inter-saccadic intervals suggests that the programming of the two saccades (one to a distractor and one to the target) overlapped in time. Using a saccade-contingent change in the search display, we show that new visual information presented during the initial saccade does not change the goal of the second saccade. This supports the idea that, by the end of the first saccade, programming of the second saccade is already well underway. We also elicited two-saccade responses (similar to those seen in search) using a double-step task, with the first saccade directed to the initial target step and the second saccade directed to the second target step. If the two saccades are programmed in parallel and programming of each saccade is triggered by one of the two target steps, the second saccade should occur at a relatively fixed time after the onset of the second target step, regardless of the timing of the initial saccade. This prediction was confirmed, supporting the idea that the two saccades are programmed in parallel. Finally, we observed that the shortest inter-saccadic intervals typically followed hypometric initial saccades, suggesting that the initial saccade may have been interrupted by the impending second saccade. Using predictions from physiological studies of interrupted saccades, we tested this hypothesis and found that the hypometric initial saccades did not appear to be interrupted in mid-flight. We discuss the significance of our findings for models of the saccadic system.

Humans↗
PubMed1997

Extraretinal eye position signals determine perceived target location when they conflict with visual cues.

To examine the role of extraretinal eye position information (EEPI) in visual perception of target location in normal room illumination, subjects participated in experiments in which EEPI was manipulated using the eye press maneuver with either monocular or binocular viewing. The viewing condition and eye press caused EEPI and retinal information about target location to conflict. Pointing responses in eye press trials were all in the direction of EEPI showing that EEPI is the dominant source of information in egocentric visual space perception. In binocular viewing, version and vergence occur in response to the eye press to maintain fusion and EEPI based on these movements also determine perceived location. An unanticipated finding was that the eye press was variable in its effectiveness in rotating the eye, which contributed to large variability in pointing errors and suggested the method would be a poor choice for future work.

Adult↗
PubMed1992

Adaptation of saccades and fixation to bilateral foveal lesions in adult monkey.

Bilateral foveal lesions were made by laser photocoagulation in adult monkeys. One day post-lesion, animals fixated with a new retinal locus inferior to the fovea (in the visual field) that they used permanently. Fixation stability improved modestly over two days. Initially, saccades maladaptively brought the lesioned foveae to visual targets. Over at least several weeks, saccade trajectories gradually changed bringing targets to intact retina, although some animals never totally adapted. The slow time course of saccadic adaptation to foveal loss suggests a mechanism different from that documented in other studies of saccadic adaptation and from that used by the fixation system.

Adaptation, Ocular↗
PubMed1991

Recovery of visual responses in foveal V1 neurons following bilateral foveal lesions in adult monkey.

Cells in the foveal representation of V1 cortex of adult primates became visually responsive after normal sensory input was removed. Immediately after fovea were lesioned bilaterally, a region was found where no cells' activity could be modulated by visual stimulation. Recordings made in that deafferented zone at greater than 2.5 months after lesions revealed that activity of over half of the cells could be modulated by visual stimuli presented to intact peripheral retina. Although response characteristics made cells with recovered driving quite unlike normal cells, the result suggests a level of visual cortical reorganization previously observed only in immature animals.

Animals↗
PubMed1991

Eye movements elicited by electrical stimulation of area PG in the monkey.

1. Eye positions of monkeys were tracked while low-current electrical stimulation was delivered to area PG of the posterior parietal cortex. Stimulation was delivered while monkeys were in darkness, while they were in a dimly illuminated room, or while they actively fixated on small lamps to receive a liquid reward. 2. Resulting eye movements fell into one of three categories, depending roughly on the area stimulated. Stimulation of caudal regions generally resulted in saccades that were of approximately equivalent amplitudes and directions. When more rostral areas were stimulated, saccades were generally produced that directed the eyes toward roughly the same position in the head. Distributed throughout all regions were sites for which elicited saccades did not fall clearly into either of these coordinate bases. Stimulation of lateral areas produced low-velocity eye movements that were directed ipsilaterally from the stimulated hemisphere. 3. Stimulation made while monkeys fixated on target lamps produced saccades with more variability and less amplitude than those produced while monkeys were in darkness. Low-velocity eye movements could only be elicited while monkeys were in darkness. 4. Craniocentric saccades typically brought the eyes to within a 10-20 degrees area, and saccades could not be produced when the initial eye position was near this area. Craniocentric saccades were always greater than 5 degrees in amplitude. 5. It is concluded that area PG is organized into at least two zones that differ in the way by which they code saccades. A caudal region codes saccades in a way similar to that found in the frontal cortex and superior colliculus of primates. A rostral region codes saccades in a craniocentric manner, although it is restricted only to gross redirection of gaze without the accuracy monkeys are capable of using in directing their eyes.

Animals↗
PubMed1985

Accuracy of spatial localizations near the time of saccadic eye movements.

Two-dimensional eye movements were recorded while subjects used a hammer to strike targets that were flashed-on briefly before, during or up to 750 msec after a horizontal saccade. Mean position of hammer blows was 20 min arc (SD = 67 min arc) from the target when the only cue to target location was eye position. Position of responses varied slightly with time of target exposure relative to the saccade. These results show that observers can closely monitor small changes in eye position during and near the time of saccadic eye movements.

Eye Movements↗
PubMed1981

Oculomotor changes in cats reared without experiencing continuous retinal image motion.

Eye movements recorded with the magnetic-field search coil technique from six cats raised in an 8 Hz stroboscopically illuminated environment were found to be normal with the exception of fixation and optokinetic nystagmus. Fixation eye movements were characterized by a 4-5 Hz conjugate pendular nystagmus superimposed on a wandering drift. Optokinetic nystagmus was very asymmetrical on monocular tests. Depriving the oculomotor system of experience differentially effects fixation and smooth pursuit.

Animals↗
PubMed1981

The effect of habituating vestibular and optokinetic nystagmus on each other.

Currently, the vestibulo-ocular and optokinetic reflexes are both thought to require a velocity storage mechanism within their neural pathways. To test whether these storage mechanisms are shared by both reflexes, animals were given programs of stimulation known to change the status of the storage mechanism of one of the type of nystagmus. The other type of nystagmus was examined then to ascertain whether the characteristic of its velocity storage system had remained invariant. Horizontal eye movements of three macaque monkeys were recorded during post-rotatory nystagmus and optokinetic afternystagmus (OKAN) before and after 20 habituating exposures to either vestibular or optokinetic stimulation. Repetitive exposures to vestibular stimulation alone markedly reduced the time constant of post-rotatory nystagmus and this effect was accompanied only occasionally by a reduction in the time constant of OKAN. Repetitive exposure to optokinetic stimulation alone reliably reduced the time constant of OKAN but produced no reliable change in the time constant of post-rotatory nystagmus. These results suggest that the vestibulo-ocular and optokinetic reflexes do not share a single common velocity storage mechanism.

Animals↗