Biomedical subjects

T R Vidyasagar

Publications and source records attributed to T R Vidyasagar.

At least 19 recordsLinked to original sources

PubMed1992

Cytoarchitecture and visual field representation in area 17 of the tammar wallaby (Macropus eugenii).

Tritiated proline was injected into one eye in the tammar wallaby and transported label was studied in the cortex after transneuronal passage through the lateral geniculate nucleus. The autoradiographic label and cytoarchitecture were used to anatomically demarcate the borders of area 17. Electrophysiological recordings from single units were done to obtain a retinotopic map of area 17. Single units in area 17 were found to have orientation sensitivity comparable to those seen in placental mammals such as cat and monkey. They could also be classified as simple, complex, and hypercomplex cells. Changes in the cortical areal magnification factor with eccentricity were found to match the drop off in retinal ganglion cell density only along the vertical meridian representation. Along the horizontal meridian, the cortical magnification falls off significantly with eccentricity, whereas the ganglion cell density shows only a mild reduction. Thus central vision, especially the binocular segment, is heavily represented at the cost of the periphery.

Animals↗

PubMed1992

Subcortical mechanisms in orientation sensitivity of cat visual cortical cells.

The orientation biases seen in the responses of neurones of the dorsal lateral geniculate nucleus (dLGN) can be reduced by the local application of the GABA antagonist, bicuculline methiodide. This fact was exploited to investigate whether these biases are important for cortical orientation selectivity by measuring the orientation sensitivity of cortical cells before and during iontophoretic administration of bicuculline in the topographically corresponding region of the dLGN. This procedure led to a significant reduction in the orientation sensitivity of the cortical cell. The results suggest that subcortical orientation biases are at least partly responsible for the orientation sensitivity seen at the level of the striate cortex.

Animals↗

PubMed1992

Postsynaptic potentials in cat visual cortex: dependence on polarization.

During the investigation of visually evoked postsynaptic potentials (PSPs) of visual cortical neurons, we recorded cell activity under different levels of membrane potential. In some cases, however, dependence of these PSPs on the level of membrane polarization appears to be inconsistent with the conventional scheme. One disagreement was the reduction, instead of an increase, of excitatory potentials during hyperpolarization of the cell. The other point was that depolarization of the cell often leads to increase of the amplitude of both excitatory and inhibitory postsynaptic potentials. This inconsistency may suggest the involvement of voltage-dependent ion channels in generating PSPs to visual stimuli. A possible way of separating the excitatory and inhibitory components of the response by polarization of the cell in spite of the presence of voltage-dependent channels and possible implications of this mechanism in the visual cortex are discussed.

Animals↗

PubMed1991

Unit activity in the hippocampus and the parahippocampal temporobasal association cortex related to memory and complex behaviour in the awake monkey.

Monkeys (Macaca fascicularis) were trained on a delayed match-to-sample (DMS) task using delays of upto 20 s. Unit activity was recorded from the hippocampus and the temporo-basal association cortex in the lateral parahippocampal region (partly corresponding to TF and TH) during the DMS task, as well as during a visual object discrimination task and some behavioural situations involving the experimenter. Units were encountered that gave visual responses which were sometimes context-dependent. Changes in discharge rate during the delay period of the DMS task were very rare and when present, very weak. On the other hand, many neurones, including some of those which were unresponsive during the DMS task fired vigorously (or were inhibited) during situations which involved attention, expectation or food consumption. For example, the neurones' firing rate was altered when the cage door was opened or closed, the experimenter entered or left the room or showed the monkey a piece of food before giving it to him. A variety of such responses in complex behavioural situations were seen, sometimes even in neurones which did not respond in the DMS task. Activity changes in neurons of the temporo-basal cortex thus appear to be related to the internal state associated with a stimulus and even some of the responses obtained in the DMS task can be interpreted as being related to changes in the behavioural state rather than to the mnemonic elements of the task.

Action Potentials↗

PubMed1991

Whole cell recording and conductance measurements in cat visual cortex in-vivo.

Long and stable recordings of post-synaptic, action and membrane potentials from visual cortical neurons in-vivo, are possible with the patch-clamp technique. These are comparable to the whole-cell configuration, but with an incomplete seal. EPSPs and IPSPs of normal time course and up to several mV can be recorded. DC potentials ranged from - 30 to - 60 mV and input resistances from 50 to 150 M omega. Injected currents have the same effect as if applied intracellularly. Membrane conductance after electrical stimulation of the lateral geniculate nucleus is increased during the first 20 ms, but decreases from 60 to about 130 ms, during return of the membrane potential to its resting level. The recording method is compared to other intracellular recording techniques in-vivo and in-vitro.

Animals↗

PubMed1990

Pattern adaptation in cat visual cortex is a co-operative phenomenon.

The effects of microiontophoretic application of glutamate, GABA and the GABA antagonist, bicuculline methiodide were tested on the degree of adaptation exhibited by striate cortical cells to moving sin wave grating patterns. Application of GABA, which prevents firing of the cell and thereby any fatigue of the cell, did not reduce the degree of adaptation. Administration of either glutamate or GABA, without simultaneous exposure to the adapting high-contrast gratings did not reduce the sensitivity of the cell to subsequent exposure of a low-contrast grating, showing that adaptation is not caused by the excitatory or inhibitory activity of the cell itself. Application of the GABA antagonist, bicuculline did not prevent pattern adaptation, indicating that the lowered sensitivity of the cell is not mediated by a GABAergic inhibition acting on the cell. Thus adaptation of a striate neuron is not due to altered sensitivity of the cell to a constant input but depends upon changes in the input itself. It is most likely that these changes occur in a co-operative cortical network, whose effect on individual cortical cells is mediated by intracortical excitatory connections.

Adaptation, Physiological↗

PubMed1990

Relationship between preferred orientation and ordinal position in neurones of cat striate cortex.

Striate cortical cells were classified according to whether or not their preferred orientation was close to one of the "primary" orientations (horizontal, vertical or radial, i.e. directed to the area centralis) and according to their ordinal position on the afferent pathway from the dorsal lateral geniculate nucleus (dLGN). Among the neurones that could be driven monosynaptically from the dLGN, there was a high representation of those with a preference for the primary orientations. This was particularly evident in the case of C (complex) cells. There was no such preponderance of primary orientations among the polysynaptically activated cells. It is proposed that the asymmetry of distribution seen among the first-order cells reflects the asymmetry seen subcortically in neurones that show orientation biases. It may be that the cortex elaborates a more uniform representation of orientations only at the higher ordinal levels.

Afferent Pathways↗

PubMed1987

A model of striate response properties based on geniculate anisotropies.

The orientation biases seen in the responses of cells in the retina and dLGN are dependent on the spatial frequency of the stimulus, being appreciable only at higher spatial frequencies. An inhibitory mechanism that suppresses the responses to low spatial frequencies would leave a striate cell receiving a biased geniculate input with an orientation sensitivity at the higher spatial frequencies. Such an inhibition could in fact come from one or a small group of LGN cells (through cortical interneurones), since their response extends to spatial frequencies much lower than for cortical cells at the same eccentricity. According to this scheme, a number of other striate response characteristics, e.g., their length and spatial frequency response functions, can also be explained.

Animals↗

PubMed1986

Space and spatial frequency: analysis and representation in the macaque striate cortex.

Simple cells in the macaque striate cortex were tested with bars, edges and gratings. Spatial frequency tuning curves could be predicted from the spatial profiles plotted with bars and edges and the bandwidth could be evaluated more accurately by computing the mean from measured and predicted tuning curves. The results suggest that the mean relative spatial frequency bandwidth (delta f/fo) is nearly constant and of a moderate value. But at each optimal spatial frequency, cells with different bandwidths (about a factor of two) were recorded. The shapes of spatial response profiles resemble the corresponding spatial and spatial frequency characteristics of line and edge detectors evaluated psychophysically. Among the remaining cell types, concentric cells tend to be tuned to lower spatial frequencies and have broader bandwidths, whereas periodic cells prefer higher spatial frequencies and have narrower bandwidths. Thus the mean relative bandwidth tends to decrease significantly with spatial frequency (as required by a system of patch-by-patch Fourier analysis) only when cells with poor orientation selectivity and the non-linear silent periodic cells are included along with the simple cells. Simple cells, on their own, seem to form a quasi-linear contrast processing system which is more biased towards spatial accuracy than spatial frequency selectivity.

Animals↗

PubMed1986

The role of GABAergic inhibition in the response properties of neurones in cat visual area 18.

Bicuculline methiodide was iontophoretically applied to single neurones in cat area 18 to investigate how removal of gamma-aminobutyrate mediated inhibition affects the visual response properties. Moving sinusoidal gratings were used to study spatial and temporal response characteristics. Orientation sensitivity and spatial and temporal frequency tuning curves were determined with and without iontophoretically applied bicuculline. In most neurones, orientation sensitivity and spatial frequency tuning remained largely unaffected, whereas temporal frequency tuning was very much broadened. It is suggested that the dominant excitatory input to area 18 cells is a spatially organized input from area 17 and local inhibition in area 18 sharpens primarily temporal selectivity. An alternative explanation of our results would be that the distribution of synapses mediating temporal tuning in area 18 is fundamentally different from that mediating spatial frequency and orientation tuning, which may be located at sites distant from the cell body and relatively inaccessible to the drug application.

Animals↗

PubMed1985

Relationship between orientation tuning and spatial frequency in neurones of cat area 17.

The orientation bandwidth was measured at different spatial frequencies for simple and complex cells. With increasing spatial frequency, the orientation tuning of simple cells became progressively narrower. This tendency was much less marked in complex cells. The results are interpreted in support of geniculate cells with orthogonal orientation biases providing the excitatory and inhibitory inputs to a simple cell.

Animals↗

PubMed1979