The Journal of Neuroscience, August 1, 1999, 19(15):6571-6587
The Contribution of Color to Motion Processing in Macaque Middle
Temporal Area
Alexander
Thiele1,
Karen R.
Dobkins2, and
Thomas D.
Albright1, 3
1 The Salk Institute for Biological Studies, San Diego,
California 92186, 2 Department of Psychology, University of
California San Diego, La Jolla, California 92093-0109, and
3 Howard Hughes Medical Institute, La Jolla, California
92093-0646
The chromatic properties of an image yield strong cues for object
boundaries and thus hold the potential to facilitate the detection of
object motion. The extent to which cortical motion detectors exploit
chromatic information, however, remains a matter of debate. To address
this further, we quantified the strength of chromatic input to
directionally selective neurons in the middle temporal area (MT) of
macaque cerebral cortex using an equivalent luminance contrast (EqLC)
paradigm. This paradigm, in which two sinusoidal gratings, one
heterochromatic and the other achromatic, are superimposed and moved in
opposite directions, allows the sensitivity of motion detectors to
heterochromatic stimuli to be quantified and expressed relative to the
benchmark of sensitivity for a luminance-defined stimulus.
The results of these experiments demonstrate that the chromatic
contrast in a moving red-green heterochromatic grating strongly influences directional responses in MT when the luminance contrast in
that same grating is relatively low; for such stimuli, EqLC is at least
5%. When luminance contrast is added to the heterochromatic grating,
however, EqLC wanes sharply and becomes negative (
4%) when luminance contrast is sufficiently high (>17-23%). Thus, the
chromatic properties of an object appear to confer little or no benefit
to motion processing by MT neurons when sufficient luminance contrast
concurrently exists. These data support a simple model in which
chromatic motion processing in MT is almost exclusively determined by
magnocellular input. Additionally, a comparison of neuronal and
psychophysical data suggests that MT may not be the sole contributor to
the perceptual experience elicited by motion of heterochromatic
patterns, or that only a subset of MT neurons serve this function.
Key words:
luminance; chrominance; magnocellular; parvocellular; contrast; extrastriate
Copyright © 1999 Society for Neuroscience 0270-6474/99/19156571-17$05.00/0
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