A selection model for motion transparency in visual cortex
T.J. Sejnowski, M.I. Stewart, S.J. Nowlan
Investigative Ophthalmology and Visual Science Suppl.,
35(4);1828.
Abstract
Purpose: Identify cortical mechanisms for the perception
of motion transparency and motion occlusion using a computational model
based on the response properties of neurons in visual cortex.
Methods: The inputs to the model are 64 frames of moving
stimuli on a 64 x 64 pixel input array with 256 gray levels at each
pixel. Area V1 was modeled by a 49x49 aray of 36 Adelson-Bergen motion
energy units, tuned to 4 directions and 9 combinations of spatial and
temporal frequencies. The model of MT had two parallel pathways, each
receiving inputs from the motion-energy array. The first pathway was
optimized to estimate the local velocity while the second pathway was
optimized to select the most reliable regions to support each global
velocity. The output was 33 global velocity units each receiving inputs
from local velocity units gated multiplicatively by the selection units.
Results: We tested the model on a variety of psychophysical
motion stimuli including plaids, random dots, and barber-pole stimuli, and
compared the response properties of the units in the model to those found
in area MT. Plaid patterns cohered if the spatial requency of the component
gratings were within one octave. When the contrast of the intersection was
varied, the range of contrasts that yielded component motion corresponded
to the range over which humans perceive motion transparency (Stoner,
Ramachandran, and Albright, 1990). The threshold for detecting coherent
motion in correlated random dots was similar to that of primates. Responses
to the barber-pole illusion and rotation elipses were similar to those
reported by humans. Conclusions: We suggest that a
subpopulation of neurons in area MT is primarily concerned with robust
selection rather than representing velocity. In visual scenes with
occluded and transparent objects, the selection units partially segment the
image into disjoint regions sharing a common global velocity. Attention to
a particular velocity in a particular region of the visual field could be
accomplished through bias of the selection units. The selection units had
opponent nonclassical receptive fields, similar to those observed in some
area MT neurons.