Recent psychophysical and neurophysiological studies have suggested th
at, in mammals, there are interactions between the P (colour processin
g) and M (motion processing) visual pathways, which were previously be
lieved to be parallel and separate. In this study the role colour info
rmation plays in the coding of object motion was determined in the tec
tofugal pathway of pigeons. The responses of motion-sensitive neurons
in the tectum to moving stimuli formed by chromatic contrast were reco
rded extracellularly using standard single-unit recording techniques.
A moving coloured object was presented on a uniform (opponent coloured
) background (e.g. blue-on-yellow, red-on-green and black-on-white). T
hrough systematically manipulation of the luminance contrast between o
bject and background, an equiluminant condition was generated. It was
found that, at chromatic equiluminance, the majority of cells maintain
some level of response. The mean magnitude of the response at equilum
inance was about one-third of the response at maximal contrast to the
same chromatic border. These results suggest that tectal units can det
ect motion of a pattern defined by a pure colour contour, although the
strength of output is considerably weaker than that for the movement
of patterns formed by luminance contrast.