Biomedical subjects

J W Bisley

Publications and source records attributed to J W Bisley.

6 recordsLinked to original sources

PubMed2001

Microstimulation of cortical area MT affects performance on a visual working memory task.

We applied electrical stimulation to physiologically identified sites in macaque middle temporal area (MT) to examine its role in short-term storage of recently encoded information about stimulus motion. We used a behavioral task in which monkeys compared the directions of two moving random-dot stimuli, sample and test, separated by a 1.5-s delay. Four sample directions were used for each site, and the animals had to indicate whether the direction of motion in the sample was the same as or different to the direction of motion in the test. We found that the effect of stimulation of the same directional column in MT depended on the behavioral state of the animal. Although stimulation had strong effects when applied during the encoding and the storage components of the task, these effects were not equivalent. Stimulation applied during the presentation of the sample produced signals interpreted by the monkeys as directional motion. However, the same stimulation introduced during the period of storage no longer produced signals interpreted as unambiguous directional information. We conclude that the directional information used by the monkeys in the working memory task is likely to be provided by neurons in MT, and the use of this information appears to be dependent on the portion of the task during which stimulation was delivered. Finally, the disruptive effects of stimulation during the delay suggest that MT neurons not only participate in the encoding of visual motion information but also in its storage by either maintaining an active connection with the circuitry involved in storage or being an integral component of that circuitry.

Animals↗

PubMed2001

Motion information is spatially localized in a visual working-memory task.

We asked if the information about stimulus motion used in a visual working-memory task is localized in space. Monkeys compared the directions of two moving random-dot stimuli, sample and test, separated by a temporal delay and reported whether the stimuli moved in the same or in different directions. By presenting the two comparison stimuli in separate locations in the visual field, we determined whether information about stimulus direction was spatially localized during the storage and retrieval/comparison components of the task. Two psychophysical measures of direction discrimination provided nearly identical estimates of the critical spatial separation between sample and test stimuli that lead to a loss in threshold. Direction range thresholds measured with dot stimuli consisting of a range of local directional vectors were affected by spatial separation when a random-motion mask was introduced during the delay into the location of the upcoming test. The selective masking at the test location suggests that the information about the remembered direction was localized and available at that location. Direction difference thresholds, measured with coherently moving random dots, were also affected by separation between the two comparison stimuli. The separation at which performance was affected in both tasks increased with retinal eccentricity in parallel with the increase in receptive-field size in neurons in cortical area MT. The loss with transfer of visual information between different spatial locations suggests a contribution of cortical areas with localized receptive fields to the performance of the memory task. The similarity in the spatial scale of the storage mechanism derived psychophysically and the receptive field size of neurons in area MT suggest that MT neurons are central to this task.

Animals↗

PubMed2000

The multiple roles of visual cortical areas MT/MST in remembering the direction of visual motion.

Although the role of cortical areas MT and MST (MT/MST) in the processing of directional motion information is well established, little is known about the way these areas contribute to the execution of complex behavioral tasks requiring the use of such information. We tested monkeys with unilateral lesions of these areas on a visual working memory task in which motion signals not only had to be encoded, but also stored for brief periods of time and then retrieved. The monkeys compared the directions of motion of two random-dot stimuli, sample and test, separated by a temporal delay. By increasing the temporal delay and spatially separating the two stimuli, placing one in the affected visual field and the other in the intact visual field, we were able to assess the contribution of MT/MST to specific components of the task: encoding (sample), retention (delay) and encoding/retrieval/comparison (test). We found that the effects of MT/MST lesions on specific components depended upon the demands of the task and the nature of the visual motion stimuli. Whenever stimuli consisted of random dots moving in a broad range of directions, MT/MST lesions appeared to affect encoding. Furthermore, when the lesions affected encoding of the sample, retention of the direction of stimulus motion was also affected. However, when the stimulus was coherent and the emphasis of the task was on the comparison of small direction differences, the absence of MT/MST had major impact on the retrieval/comparison component of the task and not on encoding or storage.

Animals↗

PubMed2000

Slowly adapting type I afferents from the sides and end of the finger respond to stimuli on the center of the fingerpad.

The central part of the fingerpad in anesthetized monkeys was stimulated by spheres varying in curvature indented into the skin. Responses were recorded from single slowly adapting type I primary afferent fibers (SAIs) innervating the sides and end of the distal segment of the stimulated finger. Although these afferents had receptive field centers that were remote from the stimulus, their responses were substantial. Increasing the curvature of the stimulus resulted in an increased response for most afferents. In general, responses increased most between stimuli with curvatures of 0 (flat) and 80.6 m(-1), with further increases in curvature having progressively smaller effects on the response. We calculated an index of sensitivity to changes in curvature; this index varied widely among the afferents but for most it was less than the index calculated for afferents innervating the fingerpad in the vicinity of the stimulus. Responses of all the SAIs increased when the contact force of the stimulus increased. An index of sensitivity to changes in contact force varied widely among the afferents but in all cases was greater than the index calculated for SAIs from the fingerpad itself. Neither the curvature sensitivity nor the force sensitivity of an afferent was related in any obvious way to the location of its receptive field center on the digit. There was only a minor correspondence between an afferent's sensitivity to force and its sensitivity to curvature. The large number of afferents innervating the border regions of the digit do respond to stimuli contacting the central fingerpad; they convey some information about the curvature of the stimulus and substantial information about contact force.

Adaptation, Physiological↗

PubMed1997

Encoding of object curvature by tactile afferents from human fingers.

Isolated responses were recorded from fibers in the median nerves of human subjects by using microneurography. Mechanoreceptive afferent fibers with receptive fields on the fingerpads were selected. The fingers were immobilized and spherical stimuli were applied passively to the receptive field with a contact force of 40-, 60-, or 80-g weight. The radii of the spheres were 1.92, 2.94, 5.81, or 12.4 mm or infinity (flat); the corresponding curvatures, given by the reciprocal of the radii, were 694, 340, 172, 80.6, or 0 m-1, respectively. When the spheres were applied to the receptive field center of slowly adapting type I afferents (SAIs), the response increased as the curvature of the sphere increased and also increased as the contact force increased. All SAIs behaved in the same way except for a scaling factor proportional to the sensitivity of the afferent. When a sphere was located at different positions in the receptive field, the shape of the resulting response profile reflected the shape of the sphere; for more curved spheres the profile was higher and narrower (increased peak and decreased width). Slowly adapting type II afferents (SAIIs) showed different response characteristics from the SAIs when spheres were applied to their receptive field centers. As the curvature of the stimulus increased from 80.6 to 172 m-1, the response increased. However, further increases in curvature did not result in further increases in response. An increase in contact force resulted in an increase in the response of SAIIs; this increase was proportionately greater than it was for SAIs. For SAIIs, the shape of the receptive field profile did not change when the curvature of the stimulus changed. For fast-adapting type I afferents (FAIs), the responses were small and did not change systematically with changes in curvature or contact force. Fast-adapting type II afferents (FAIIs) did not respond to our stimuli. Human SAIs, FAIs, and FAIIs behaved like monkey SAIs, FAIs, and FAIIs, respectively. The response of the SAI population contains accurate information about the shape of the sphere and its position of contact on the finger and also indicates contact force. Conversely, whereas SAIIs possess a greater capacity to encode changes in contact force, they provide only coarse information on local shape.

Adolescent↗

PubMed1996

Identification of osmoresponsive neurons in the forebrain of the rat: a Fos study at the ultrastructural level.

The aims of this study are twofold. The first is to describe the ultrastructural morphology of putative osmoreceptors concentrated in the ventral aspect of the lamina terminalis in the rat forebrain. The second is to determine whether or not these neurons lie within an area which lacks a blood-brain barrier, i.e. the organum vasculosum lamina terminalis. The results describe a compact population of neurons in the ventral part of the lamina terminalis which both respond to an osmotic challenge and project directly to the supraoptic nucleus. Injection of horseradish peroxidase into the circulation, as a marker to define areas of the brain without a blood-brain barrier, indicates that these neurons are in the dorsal aspect of the organum vasculosum of the lamina terminalis. An ultrastructural analysis of the neurons in this area, which respond to an osmotic challenge with an elevation of Fos protein, show them to have no specific morphological characteristics which differentiate them from other, non-responsive neurons in the organum vasculosum of the lamina terminalis. However, one possible exception is that osmotically sensitive neurons have a less indented nucleus, suggesting that they are in a more active state than their non-osmotically sensitive neighbours. It is concluded that neurons in this region of the brain are candidate structures for the "receptors" which mediate vasopressin release in response to an osmotic challenge. The response of only a subset of neurons in the organum vasculosum of the lamina terminalis to an osmotic stimulus, despite an apparent morphological homogeneity and the ability of blood borne agents to reach all parts of the structure suggests that osmoresponsiveness is conferred by unique membrane properties or intracellular processing events. The presence of synaptic input to osmoresponsive cells indicates a potential for integration of other inputs at this level.

Animals↗