Biology subjects

Shapley, R. M.

Publications and source records attributed to Shapley, R. M..

3 recordsLinked to original sources

bioRxiv2019

Distinct spatiotemporal mechanisms underlie extra-classical receptive field modulation in macaque V1 microcircuits

Complex scene perception depends upon the interaction between signals from the classical receptive field (CRF) and the extra-classical receptive field (eCRF) in primary visual cortex (V1) neurons. While much is known about V1 eCRF properties, it remains unknown how the underlying mechanisms map onto the cortical microcircuit. We probed the spatio-temporal dynamics of eCRF modulation using a reverse correlation paradigm, and found three principal eCRF mechanisms: tuned-facilitation, untuned-suppression, and tuned-suppression. Each mechanism had a distinct timing and spatial profile. Laminar analysis showed that the timing, orientation-tuning, and strength of eCRF mechanisms had distinct signatures within magnocellular and parvocellular processing streams in the V1 microcircuit. The existence of multiple eCRF mechanisms provides new insights into how V1 responds to spatial context. Modeling revealed that the differences in timing and scale of these mechanisms predicted distinct patterns of net modulation, reconciling many previous disparate physiological and psychophysical findings.

neuroscience↗

bioRxiv2019

Functional clusters of neurons in layer 6 of macaque V1

AbstracLayer 6 appears to perform a very important role in the function of macaque primary visual cortex, V1, but not enough is understood about the functional characteristics of neurons in the layer 6 population. It is unclear to what extent the population is homogeneous with respect to their visual properties or if one can identify distinct sub-populations. Here we performed a cluster analysis based on measurements of the responses of single neurons in layer 6 of primary visual cortex to achromatic grating stimuli that varied in orientation, direction of motion, spatial and temporal frequency, and contrast. The visual stimuli were presented in a stimulus window that was also varied in size. Using the responses to parametric variation in these stimulus variables we extracted a number of tuning response measures and used them in the cluster analysis. Six main clusters emerged along with some smaller clusters. Additionally we asked whether parameter distributions from each of the clusters were statistically different. There were clear separations of parameters between some of the clusters, particularly for f1/f0 ratio, direction selectivity, and temporal frequency bandwidth but other dimensions also showed differences between clusters. Our data suggest that in layer 6, across the spatial extent of a single cortical hypercolumn, there are multiple parallel circuits that provide information about different aspects of the visual stimulus.

neuroscience↗

bioRxiv2018

Human Sound Localization Depends on Sound Intensity: Implications for Sensory Coding

A fundamental question of human perception is how we perceive target locations in space. Through our eyes and skin, the activation patterns of sensory organs provide rich spatial cues. However, for other sensory dimensions, including sound localization and visual depth perception, spatial locations must be computed by the brain. For instance, interaural time differences (ITDs) of the sounds reaching the ears allow listeners to localize sound in the horizontal plane. Our experiments tested two prevalent theories on how ITDs affect human sound localization: 1) the labelled-line model, encoding space through tuned representations of spatial location; versus 2) the hemispheric-difference model, representing space through spike-rate distances relative to a perceptual anchor. Unlike the labelled-line model, the hemispheric-difference model predicts that with decreasing intensity, sound localization should collapse toward midline reference, and this is what we observed behaviorally. These findings cast doubt on models of human sound localization that rely on a spatially tuned map. Moreover, analogous experimental results in vision indicate that perceived depth depends upon the contrast of the target. Based on our findings, we propose that the brain uses a canonical computation of location across sensory modalities: perceived location is encoded through population spike rate relative to baseline.

neuroscience↗