Primate Striate and Prestriate Cortical Neurons During Discrimination:
II. Separable Temporal Codes for Color and Pattern.
McClurkin, John W., Jennifer A. Zarbock, Lance M. Optican.
Laboratory of Sensorimotor Research, National Eye Institute.
APStracts 2:0273N, 1995.
SUMMARY AND CONCLUSIONS
1. In the previous paper, we reported our analysis of the responses of neurons
in cortical areas V1, V2, and V4 to a set of stimuli that consisted of all 36
combinations of six colors and six patterns. Neurons in all three cortical
areas simultaneously encoded information about both the color and pattern of
the stimulus in the number and temporal distribution of spikes in their
responses. To account for this ability, we propose that a neuron's response
consists of separable temporal codes representing the color and pattern of the
stimulus that are multiplexed together. 2. We used non-linear regression to
fit the model parameters to the data. We used the responses to 30 of the 36
stimuli as a training set to estimate the parameters of the model, and the
responses to the remaining six stimuli as a test set. After training, the
model fitted the responses to stimuli in the training sets very well, and
predicted the responses to stimuli in the test sets. Thus, neuronal responses
to colored patterns contain separate temporal codes representing color and
pattern. 3. After establishing the model parameters, we obtained the waveforms
that represented each neuron's temporal codes for the six colors and six
patterns of our stimulus set. We then proceeded with a series of analyses to
determine whether these waveforms were viable candidates for neuronal codes.
Cluster analysis revealed that there were only a few different classes of
waveforms representing each color and pattern, and there were many neurons in
each class. Further, neurons which used similar waveforms to represent one
color or pattern also tended to use similar waveforms to represent other
colors or patterns. The waveforms representing five of the six colors and
three of the six patterns were similar in the two monkeys used in this study.
4. We compared the shapes of the code waveforms across cortical areas, and
found no differences among areas in the shapes of the waveforms representing
four of the six colors. In contrast, we found that there were differences
among areas in the shapes of the waveforms representing all six patterns.
These results suggest that messages about color are encoded at an early level
and are then propagated upward, but that messages about pattern are altered in
each successive cortical area. 5. Our results offer a neurophysiological
explanation for the psychophysical evidence that color and form are processed
by different channels. We propose that the psychophysical channels for color
and pattern arise from the separability of the temporal codes for color and
pattern in the responses of single neurons. This hypothesis implies that
psychophysical channels correspond to classes of temporal codes rather than to
classes of neurons.
Received 3 November 1994; accepted in final form 23 August 1995.
APS Manuscript Number J697-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 23 September 1995.