Transparent Motion Perception as Detection of Unbalanced Motion
Signals II: Physiology
Ning Qian and Richard A. Andersen, J. Neurosci.
1994, 14:7367-7380.
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Abstract
We investigated how the primate visual system solves the difficult
problem of representing multiple motion vectors in the same part of
the visual space -- the problem of motion transparency. In the
previous paper, we reported that displays with locally well-balanced
motion signals in opposite directions are perceptually non-transparent
(i.e., one does not see two coherent moving surfaces) and that
transparent displays always contain locally unbalanced motion signals.
This is exemplified by our paired and unpaired dot patterns. Although
both types of stimuli contain two sets of dots moving in opposite
directions, the former is locally well-balanced and appears like
flicker while the latter gives a perception of two transparent
surfaces. In this paper we report our physiological recordings from
areas V1 and MT of behaving monkeys, comparing single cell responses
to the paired and the unpaired dot patterns. Although a small
proportion of directionally selective V1 cells responded differently
to the two types of patterns, the average V1 responses could not
reliably distinguish between the paired and the unpaired stimuli. A
large fraction of MT cells, on the other hand, responded significantly
better to the unpaired dot patterns than to the paired ones.
Furthermore, the average response of all MT cells to the unpaired dot
patterns was significantly higher than that to the paired dot
patterns.
These results demonstrate a neural correlate of the perceptual
transparency at the level of MT. On the other hand, V1 cells do not
generally discriminate between the transparent and non-transparent
stimuli, indicating that V1 activity is not well correlated with the
perception of motion transparency. Our results are consistent with a
two stage model for motion processing: the first stage measures local
motion and the second stage introduces suppression if different
directions of motion are present at a local region of the visual
field. The first stage is located primarily in V1 and the second
stage primarily in MT. Finally, we found a strong and negative
correlation between the degree of the opponent-direction suppression
of MT cells and their responses to flicker noise stimuli. This result
suggests that one of the fundamental roles of the opponent-direction
suppression in MT is noise reduction.
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