Distributed and Dynamic Neural Encoding of Multiple Motion Directions of Transparently Moving Stimuli in Cortical Area MT

Abstract

Segmenting visual scenes into distinct objects and surfaces is a fundamental visual function. To better understand the underlying neural mechanism, we investigated how neurons in the middle temporal cortex (MT) of macaque monkeys represent overlapping random-dot stimuli moving transparently in slightly different directions. It has been shown that the neuronal response elicited by two stimuli approximately follows the average of the responses elicited by the constituent stimulus components presented alone. In this scheme of response pooling, the ability to segment two simultaneously presented motion directions is limited by the width of the tuning curve to motion in a single direction. We found that, although the population-averaged neuronal tuning showed response averaging, subgroups of neurons showed distinct patterns of response tuning and were capable of representing component directions that were separated by a small angle-less than the tuning width to unidirectional stimuli. One group of neurons preferentially represented the component direction at a specific side of the bidirectional stimuli, weighting one stimulus component more strongly than the other. Another group of neurons pooled the component responses nonlinearly and showed two separate peaks in their tuning curves even when the average of the component responses was unimodal. We also show for the first time that the direction tuning of MT neurons evolved from initially representing the vector-averaged direction of slightly different stimuli to gradually representing the component directions. Our results reveal important neural processes underlying image segmentation and suggest that information about slightly different stimulus components is computed dynamically and distributed across neurons.

Spatial arrangement drastically changes the neural representation of multiple visual stimuli that compete in more than one feature domain

“…Procedures for surgical preparation and electrophysiological recordings were routine and similar to those described previously (Xiao et al, 2015). During sterile surgery with the animal under isoflurane anesthesia, a head post and recording cylinder were implanted to allow recording from neurons in cortical area MT.…”

“…We previously found that the tuning curves of some MT neurons in response to overlapping bi-directional stimuli can show a directional "side-bias" toward one of the two direction components (Xiao and Huang, 2015). A subgroup of neurons prefers the stimulus component at the clockwise side of two motion directions, whereas another group of neurons prefers the component direction at the counter-clockwise side.…”

“…Work in our laboratory has shown that the direction tuning curves of MT neurons in response to overlapping random-dot stimuli moving transparently in two different directions can also be described as a weighted sum of the responses elicited by the individual stimulus components (Xiao et al, 2014;Xiao and Huang, 2015). When two stimulus components have different signal strengths in one feature domain, defined either by motion coherence or luminance contrast, MT neurons pool the stimulus component that has a stronger signal strength with a greater weight (Xiao et al, 2014).…”

Wireless Optogenetic Nanonetworks for Brain Stimulation: Device Model and Charging Protocols

“…(b) The raster plot simulation generated based on the experiment. As shown in the raster plot, both directions affect the neuron spike frequency response [29]. (c) and (d) presents the simulation results from the number of misfiring before and after the frequency transition for both the Predictive Sliding Detection Window and the Charge and Fire protocols.…”

Information-theoretic interpretation of tuning curves for multiple motion directions

“…The basic experimental fact relevant to our modeling project involves two sets of overlapping random dot stimuli moving in two separate directions [2]. Primate observers are able to segregate the two direction components and perceive motion transparency.…”