Perceptual Coherence : Hearing and Seeing

To review, neurons sensitive to motion direction are found in the primary
visual area V1 stemming from the magnocellular pathways that are insensitive
to color and stationary contours, but that make strong transient firing responses
to moving contours. The V1 neurons project to the middle temporal
visual area and then onto the medial superior temporal area. For the higher
regions, the general rule is that the size of the receptive field increases and
the selectivity for complex motion patterns increases. The basic conception
is that the direction-sensitive neurons in V1 with small receptive fields extract
the local motion signals autonomously, and those local motion signals
are integrated in the temporal areas to form objects and surfaces.
On this basis, models for motion aftereffects are composed of two parts.
The first is adaptation in the first-stage local motion detectors. Barlow and
Hill (1963) measured the firing rate to rotating random dot patterns in the
rabbit retina. They found that the rate decreased over a period of about 20 s.
After the pattern was removed, the firing rate dropped below its baseline
and gradually recovered over a period of about 30 s.
The second part is competitive comparison between the local motion detectors
in a second stage of analysis. Motion aftereffects occur when the
unadapted detectors override the adapted detectors. Often the comparison
is between opposing motion detectors so that the aftereffect is reversed motion,
but that is not always the case. The motion aftereffects seem due to the
drop in responsiveness of the adapted neurons. There is little change in the
unadapted neurons.
Motion Integration
Perception of Motion 221
Vidnyanszky, Blaser, and Papathomas (2002) summarized interesting research
illustrating how the segmentation of random dot movements into one
surface or two transparent surfaces affects the motion aftereffect. If there
is a pattern of moving dots, the local direction of movements determines
whether we perceive one coherent surface or two transparent surfaces moving
relative to one another (similar to the two possible perceptions resulting
from the movement of two gratings shown in figure 5.6). The motion aftereffects
are particularly interesting in the latter case because there are two
adapting directions at each point in space, and so we might expect that the
motion aftereffect would also be in two directions. But that is not the outcome
due to the integration of motion from all directions.
Consider the prototypical case: coherent motion of all of the dots to
the right, creating the perception of a single surface moving to the right
(figure 5.11A). That motion would adapt and reduce the firing rates of the
rightward-direction neurons. When followed by a static dot pattern, the
normal balance in the firing rates between the leftward and rightward neurons
that yield no motion would have been disrupted. The leftward neurons