Neurobiology. Look but don't touch, or vice versa.
Explore the source record for details and available documents.
Animals↗
Biomedical subjects
M Shadlen
Publications and source records attributed to M Shadlen.
Dark-reared cats: unresponsive cells become visually responsive with microiontophoresis of an excitatory amino acid.
The visual system of kittens reared in total darkness is grossly abnormal. Although estimates vary, substantial proportions of cells in the visual cortex of these animals are unresponsive to visual stimulation. Additional cells are weakly responsive or erratic. We have considered the possibility that these neurons receive subthreshold input which might be evident if an excitatory neurochemical agent is applied during extracellular recording with a microelectrode. To test this notion, we have recorded from cells in the striate cortex of dark-reared kittens during microiontophoretic application of an excitatory amino acid, DL-homocysteate (DLH). Using this technique, we find that virtually all cells in the visual cortex of dark-reared kittens are responsive to visual stimulation. Prior to application of DLH, 27% of the cells were unresponsive to visual stimuli. Following iontophoresis of DLH, half of these cells responded with excitatory discharge to visual stimuli and the other half exhibited an inhibitory response in that the elevated maintained activity was suppressed during presentation of a visual stimulus. Additional cells from these animals, which were initially visually responsive, were also studied. For some of these units, responses were weak prior to administration of DLH and we were able to obtain a more clear estimate of selectivity for stimulus orientation during microiontophoresis of the drug. In these cases, and for the few cells which were initially responsive and orientation selective, we observed no major differences in selectivity before and after DLH application.
Animals↗
Mechanisms of human motion perception revealed by a new cyclopean illusion.
A new cyclopean illusion of motion may bear on neural mechanisms of direction selectivity. Stationary flickering patterns were presented to each eye, and the resulting fused pattern was perceived to be moving. To determine direction of motion, the visual system seems to integrate image components differing by 90 degrees in spatial and temporal phase. On the other hand, image speed seems to be derived from displacement of features over time. A model of neural direction selectivity is discussed in light of these results.
Fourier Analysis↗
A comparison of inhibition in orientation and spatial frequency selectivity of cat visual cortex.
Neurones in the visual cortex are highly selective for orientation and spatial frequency of visual stimuli. There is strong neurophysiological evidence that orientation selectivity is enhanced by inhibitory interconnections between columns in the cortex which have different orientation sensitivities, an idea which is supported by experiments using neuropharmacological manipulation or complex visual stimuli. It has also been proposed that selectivity for spatial frequency is mediated in part by a similar mechanism to that for orientation, although evidence for this is based on special use of visual stimuli, which hampers interpretation of the findings. We have therefore examined selectivity for both orientation and spatial frequency using a technique which allows direct inferences about inhibitory processes. Our method uses microiontophoresis of an excitatory amino acid to elevate maintained discharge of single neurones in the visual cortex. We then present visual stimuli both within and outside the range of orientations and spatial frequencies which cause a cell to respond with increased discharge. Our results show that orientations presented on either side of the responsive range usually produce clear suppression of maintained discharge. In marked contrast, spatial frequencies shown to either side of the responsive range have little or no effect on maintained activity. We conclude that there is an intracortical organization of inhibitory connections between cells tuned to different orientations but not different spatial frequencies.
Animals↗
Parallel processing of motion and colour information.
When the two eyes are confronted with sufficiently different versions of the visual environment, one or the other eye dominates perception in alternation. A similar situation may be created in the laboratory by presenting images to the left and right eyes which differ in orientation or colour. Although perception is dominated by one eye during rivalry, there are a number of instances in which visual processes nevertheless continue to integrate information from the suppressed eye. For example the interocular transfer of the motion after-effect is undiminished when induced during binocular rivalry. Thus motion information processing may occur in parallel with the rivalry process. Here we describe a novel example in which the visual system simultaneously exhibits binocular rivalry and vision that integrates signals from both eyes. This apparent contradiction is resolved by postulating parallel visual processes devoted to the analyses of colour and motion information. Counterphased gratings are viewed dichoptically such that for one eye the grating is composed of alternating yellow and black stripes (luminance) while for the other it is composed of alternating red and green stripes (chrominance). When the gratings are fused, a moving grating is perceived. A consistent direction of motion can only be achieved if left and right monocular signals are integrated by the nervous system. Yet the apparent colour of the binocular percept alternates between red-green and yellow-black. These observations demonstrate the segregation of processing by the early motion system from that affording the perception of colour. Although, in this stimulus, colour information in itself can play no part in the cyclopean perception of motion direction, colour is carried along perceptually (filled in) by the moving pattern which is integrated from both eyes.
Color Perception↗