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T R Vidyasagar

Publications and source records attributed to T R Vidyasagar.

At least 19 recordsLinked to original sources
PubMed2002

Convergence of parvocellular and magnocellular information channels in the primary visual cortex of the macaque.

We investigated whether responses of single cells in the striate cortex of anaesthetized macaque monkeys exhibit signatures of both parvocellular (P) and magnocellular (M) inputs from the dorsal lateral geniculate nucleus (dLGN). We used a palette of 128 isoluminant hues at four different saturation levels to test responses to chromatic stimuli against a white background. Spectral selectivity with these isoluminant stimuli was taken as an indication of P inputs. The presence of magnocellular inputs to a given cortical cell was deduced from its responses to a battery of tests, including assessment of achromatic contrast sensitivity, relative strengths of chromatic and luminance borders in driving the cell at different velocities and conduction velocity of their retino-geniculo-cortical afferents. At least a quarter of the cells in our cortical sample appear to receive convergent P and M inputs. We cannot however, exclude the possibility that some of these cells could be receiving a convergent input from the third parallel channel from the dLGN, namely the koniocellular (K) rather than the P channel. The neurons with convergent P and M inputs were recorded not only from supragranular and infragranular layers but also from the principal geniculate input recipient layer 4. Thus, our results challenge classical ideas of strict parallelism between different information streams at the level of the primate striate cortex.

Afferent Pathways↗
PubMed2001

Primary visual cortex and memory. Retinal position specificity and lack of size constancy at early stages of learning a visual memory task in the macaque.

Two monkeys were trained in a novel version of a delayed match-to-sample (DMS) task. They were required to fixate on a small spot at the center of the monitor and distinguish whether two gratings presented one after the other with delays up to 1.5 s in a specific visual field location were similar or not. It was found that such learning fails to transfer readily to other retinal locations. In fact, the learning was sensitive even to very small retinal displacements of the visual stimuli. Such acute retinal position specificity implies that at least a component of the learning in this particular memory task occurs at an early visual area such as the striate cortex, which has a fine-grain topographical representation. Furthermore, at early stages of learning the DMS task, when the monkeys had not generalized the learning to stimuli of different sizes, they failed to show size constancy. That is, when the display was placed at a different distance but with the same absolute size, the performance dropped. The performance was almost fully restored when, at the new display location, stimuli were changed to fit the original retinal size. This indicates that a crucial component of the learning does occur at a site even prior to size constancy. These results show that, under certain situations, an early visual area such as the primary visual cortex may be involved even in complex behaviours such as a memory task as more than just a feature-detecting area or a relay station.

Animals↗
PubMed2001

From attentional gating in macaque primary visual cortex to dyslexia in humans.

Selective attention is an important aspect of brain function that we need in coping with the immense and constant barrage of sensory information. One model of attention (Feature Integration Theory) that suggests an early selection of spatial locations of objects via an attentional spotlight would also solve the 'binding problem' (that is how do different attributes of each object get correctly bound together?). Our experiments have demonstrated modulation of specific locations of interest at the level of the primary visual cortex both in visual discrimination and memory tasks, where the actual locations of the targets was also important in being able to perform the task. It is suggested that the feedback mediating the modulation arises from the posterior parietal cortex, which would also be consistent with its known role in attentional control. In primates, the magnocellular (M) and parvocellular (P) pathways are the two major streams of inputs from the retina, carrying distinctly different types of information and they remain fairly segregated in their projections to the primary visual cortex and further into the extra-striate regions. The P inputs go mainly into the ventral (temporal) stream, while the dorsal (parietal) stream is dominated by M inputs. A theory of attentional gating is proposed here where the M dominated dorsal stream gates the P inputs into the ventral stream. This framework is used to provide a neural explanation of the processes involved in reading and in learning to read. This scheme also explains how a magnocellular deficit could cause the common reading impairment, dyslexia.

Animals↗
PubMed2000

Membrane properties and spike generation in rat visual cortical cells during reversible cooling.

We studied the effects of reversible cooling between 35 and 7 C on membrane properties and spike generation of cells in slices of rat visual cortex. Cooling led to a depolarization of the neurones and an increase of the input resistance, thus bringing the cells closer to spiking threshold. Excitability, measured with intracellular current steps, increased with cooling. Synaptic stimuli were most efficient in producing spikes at room temperature, but strong stimulation could evoke spikes even below 10 C. Spike width and total area increased with cooling, and spike amplitude was maximal between 12 and 20 C. Repetitive firing was enhanced in some cells by cooling to 20-25 C, but was always suppressed at lower temperatures. With cooling, passive potassium conductance decreased and the voltage-gated potassium current had a higher activation threshold and lower amplitude. At the same time, neither passive sodium conductance nor the activation threshold of voltage-dependent sodium channels changed. Therefore changing the temperature modifies the ratio between potassium and sodium conductances, and thus alters basic membrane properties. Data from two cells recorded in slices of cat visual cortex suggest a similar temperature dependence of the membrane properties of neocortical neurones to that described above in the rat. These results provide a framework for comparison of the data recorded at different temperatures, but also show the limitations of extending the conclusions drawn from in vitro data obtained at room temperature to physiological temperatures. Further, when cooling is used as an inactivation tool in vivo, it should be taken into account that the mechanism of inactivation is a depolarization block. Only a region cooled below 10 C is reliably silenced, but it is always surrounded by a domain of hyperexcitable cells.

Action Potentials↗
PubMed2000

Synaptic transmission in the neocortex during reversible cooling.

We studied the effects of reversible cooling on synaptic transmission in slices of rat visual cortex. Cooling had marked monotonic effects on the temporal properties of synaptic transmission. It increased the latency of excitatory postsynaptic potentials and prolonged their time-course. Effects were non-monotonic on other properties, such as amplitude of excitatory postsynaptic potentials and generation of spikes. The amplitude of excitatory postsynaptic potentials increased, decreased, or remain unchanged while cooling down to about 20 degrees C, but thereafter it declined gradually in all cells studied. The effect of moderate cooling on spike generation was increased excitability, most probably due to the ease with which a depolarized membrane potential could be brought to spike threshold by a sufficiently strong excitatory postsynaptic potential. Stimuli that were subthreshold above 30 degrees C could readily generate spikes at room temperature. Only at well below 10 degrees C could action potentials be completely suppressed. Paired-pulse facilitation was less at lower temperatures, indicating that synaptic dynamics are different at room temperature as compared with physiological temperatures. These results have important implications for extrapolating in vitro data obtained at room temperatures to higher temperatures. The data also emphasize that inactivation by cooling might be a useful tool for studying interactions between brain regions, but the data recorded within the cooled area do not allow reliable conclusions to be drawn about neural operations at normal temperatures.

Action Potentials↗
PubMed1999

Impaired visual search in dyslexia relates to the role of the magnocellular pathway in attention.

We tested the hypothesis that in a cluttered visual scene, the magnocellular (M) pathway is crucial for focusing attention serially on the objects in the field. Since developmental dyslexia is commonly associated with an M pathway deficit, we compared reading impaired children and age-matched normal readers in a search task that required the detection of a target defined by the conjunction of two features, namely form and colour, that are processed by the parvocellular dominated ventral neocortical stream. The dyslexic group's performance was significantly poorer than the controls when there were a large number of distractor items. The scheme of selective attention proposed from these results provides a neural mechanism that underlies reading and explains the pathophysiology of dyslexia.

Analysis of Variance↗
PubMed1999

A neuronal model of attentional spotlight: parietal guiding the temporal.

Recent studies have reported an attentional feedback that highlights neural responses as early along the visual pathway as the primary visual cortex. Such filtering would help in reducing informational overload and in performing serial visual search by directing attention to individual locations in the visual field. The magnocellular (M) and parvocellular (P) subdivisions are two of the major parallel pathways in primate vision that originate in the retina and carry distinctly different types of information. The M pathway, characterized by its high sensitivity to movement and to low contrast stimuli, forms the predominant visual input into the dorsal, parietal stream in the neocortex. The P inputs, characterized by their colour selectivity and higher spatial resolution, are channeled mainly into the ventral, temporal stream. It is proposed that the attentional spotlight originates in the dorsal stream and helps in serially searching the field for conjunction of the relevant target features in the temporal stream, effectively performing a gating function on all visual inputs. This model predicts that a defect limited to the magnocellular or the dorsal pathway can lead to widespread deficits in cognitive abilities, including those functions that are largely based on parvocellular information. For example, the model provides a neural mechanism linking a peripheral defect in the magnocellular pathway to the reading disabilities in dyslexia. Even though there has been strong evidence for a magnocellular deficit in dyslexia, the paradox has been that the cognitive disability seems to be related to P pathway function. The scheme proposed here shows how M input may be vital for controlling sequential attention during reading.

Animals↗
PubMed1998

Gating of neuronal responses in macaque primary visual cortex by an attentional spotlight.

Neuronal responses were recorded from the striate cortex of monkeys trained to perform visual discrimination at locations in the visual field to which their attention was drawn. A subset of neurons showed vigorous responses to visual stimuli for trials in which the monkey was directing its attention to the respective receptive field location. In trials where attention is directed elsewhere, responses to the same stimuli were significantly reduced. In some cells the early response component was not modulated by attention, but later components were affected by the locus of attention. The results suggest the operation of a feedback in the paradigm that spotlights a topographically restricted area of V1 for further processing at higher levels.

Animals↗
PubMed1996

A linear model fails to predict orientation selectivity of cells in the cat visual cortex.

1. Postsynaptic potentials (PSPs) evoked by visual stimulation in simple cells in the cat visual cortex were recorded using in vivo whole-cell technique. Responses to small spots of light presented at different positions over the receptive field and responses to elongated bars of different orientations centred on the receptive field were recorded. 2. To test whether a linear model can account for orientation selectivity of cortical neurones, responses to elongated bars were compared with responses predicted by a linear model from the receptive field map obtained from flashing spots. 3. The linear model faithfully predicted the preferred orientation, but not the degree of orientation selectivity or the sharpness of orientation tuning. The ratio of optimal to non-optimal responses was always underestimated by the model. 4. Thus non-linear mechanisms, which can include suppression of non-optimal responses and/or amplification of optimal responses, are involved in the generation of orientation selectivity in the primary visual cortex.

Animals↗
PubMed1996

Multiple mechanisms underlying the orientation selectivity of visual cortical neurones.

For over three decades, the mechanism of orientation selectivity of visual cortical neurones has been hotly debated. While intracortical inhibition has been implicated as playing a vital role, it has been difficult to observe it clearly. On the basis of recent findings, we propose a model in which the visual cortex brings together a number of different mechanisms for generating orientation-selective responses. Orientation biases in the thalamo-cortical input fibres provide an initial weak selectivity either directly in the excitatory input or by acting via cortical interneurones. This weak selectivity of postsynaptic potentials is then amplified by voltage-sensitive conductances of the cell membrane and excitatory and inhibitory intracortical circuitry, resulting in the sharp tuning seen in the spike discharges of visual cortical cells.

Models, Biological↗
PubMed1994

Function of GABAA inhibition in specifying spatial frequency and orientation selectivities in cat striate cortex.

Responses of simple and complex cells in cat striate cortex were studied with moving sine-wave gratings before and during application of the GABAA receptor antagonist bicuculline methiodide. Both simple and complex cells exhibited a broadening of their spatial frequency tuning functions under bicuculline. This was especially evident at spatial frequencies lower than the ones the cell was responding to before the drug administration. The effects cannot be explained by response saturation and could be reversed by cessation of the iontophoresis. The results indicate that the band-pass response characteristics of the spatial frequency response functions of striate cells derive largely from intracortical inhibition. The findings have implications also for the orientation selectivity of cortical cells. Since many geniculate cells are tuned for stimulus orientation at higher spatial frequencies, suppression of the low-spatial-frequency component would remove some of the orientation non-specific response in striate cortical cells and contribute to their orientation selectivity.

Animals↗
PubMed1994

Receptive field analysis and orientation selectivity of postsynaptic potentials of simple cells in cat visual cortex.

Postsynaptic potentials (PSPs) were recorded from cat striate cortical cells by the whole-cell in vivo recording technique using patch-clamp electrodes. EPSPs and IPSPs evoked by flashing bars on the receptive field at different positions and orientations revealed the spatial structure of the excitatory and inhibitory inputs. The elongation of the excitatory input field (length:width ratio) was found to be minimal (mean ratio of 1.7) and much lower than those reported for spike discharges. Two-dimensional receptive field response profiles of early PSPs were recorded by flashing a small spot of light over a square matrix covering the receptive field. These recordings also showed only mild degrees of elongations of the receptive field. Such elongations could be the result of either an excitatory input from the geniculate that is already biased for orientation or an excitatory convergence from a limited number of LGN fields arranged in a row. In most first-order cells, we found that inhibition was contributing significantly to orientation selectivity. Often prominent IPSPs could be evoked by stimuli of nonoptimum orientations. Presence of inhibition could also be inferred by the way that EPSPs were sharply cut off by inhibition. When the amplitude of an EPSP was measured at different latencies after its onset, the EPSP was found to be very broadly tuned to orientation at the beginning, but showing increasing orientation selectivity with time. It is proposed that this progressive development of orientation selectivity is due to (1) inhibitory inputs arriving after the first wave of excitation, (2) intracortical excitatory inputs from other cells tuned to similar orientations, and (3) voltage-sensitive mechanisms such as NMDA channels.

Animals↗
PubMed1993

Perceptual learning in seeing form from motion.

Some perceptual tasks, such as global stereopsis, have been shown to improve with practice. Paradigms that involve such 'perceptual learning' have been exploited to learn more about the nature and sites of these perceptual tasks in the brain and about plasticity in the adult central nervous system. We found that seeing structure from global motion in some kinematograms composed of tilted line elements required a period of learning. However, such learning was specific neither to the orientation of the line elements nor to the direction of global motion, even though detection of these line elements and the direction of motion was necessary for seeing the global structure in these kinematograms. Our results suggest that the neural site of deriving form from motion is beyond the level of individual motion and pattern detectors. In both its nature and locus along the hierarchy of the visual system, this learning is quite different from other types of perceptual learning reported so far.

Adult↗
PubMed1993

Functional comparison of neuronal properties in the primate posterior hippocampus and parahippocampus (area TF/TH) during different behavioural paradigms involving memory and selective attention.

Monkeys were trained on a delayed match-to-sample (DMS) task. In addition a standardized behavioural trial was performed which involved an experimenter approaching the animal in certain sequence and presenting edible or other objects ('raisin trial'). Neuronal activity of 617 units was recorded in the posterior parahippocampus (PH) and in the posterior hippocampus (H). In many cases, we compared the activity of the same neuron in different tasks. 32.7% of the 455 PH neurons and 28.5% of the 130 H cells responded during the presentation of the visual stimuli in the DMS task. These responses were only mildly influenced by the physical dimensions of the visual stimulus, but often depended on the context in which the stimuli were presented. There was no differential response to the second stimulus that clearly depended on the nature of the first stimulus. 6.2% of the PH units, but none in H, responded in relation to the reward. 4.4% of the PH neurons, but none in H, showed a mild response during the interstimulus interval. 38.1% of 215 PH neurons and 37.8% of 45 H cells responded during one or more phases of the raisin trial. These responses were not related to the physical dimensions of the sensory stimuli. 210 PH and 41 H units were investigated during the DMS task as well as during the raisin trial. 18.1% (PH) and 12.2% (H) of the units responded during the DMS task, but not during the raisin trial; 17.1% (PH) and 36.6% (H) responded vice versa. A response in both trials was found in 17.1% of the PH neurons, but in none of the H cells. There were also other PH unit types showing responses during different aspects of the DMS task and even in other control paradigms, while no such overlap was encountered in H. Our results suggest a function of H and PH in the evaluation of the behavioural significance of sensory information. It may be this aspect which leads to anterograde memory disturbances after lesion of these areas. Since representation of neuronal information was found to be more specific in H, a possible function as an 'evaluation index' is discussed.

Acoustic Stimulation↗
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↗