A number of experiments were conducted to investigate the interaction
of the ON and OFF pathways in the processing of global-motion signals.
The stimulus employed was a variant of that used by Newsome and Pare
[(1988) Journal of Neuroscience, 8, 2201-2211] in which a small subset
of dots move in a common (global-motion) direction in a field of rand
omly moving dots. The threshold measure was the number of dots require
d to move in the global-motion direction for that direction to be dete
cted, We found that: (1) the extraction of a global-motion signal carr
ied by light dots (luminance above the background) was impaired by the
addition of dark dots (luminance below the background) which did not
carry the signal (noise dots); (2) sub-threshold summation occurs for
global-motion signals carried by light and dark dots; and (3) a signal
dot which changed luminance polarity (went from light to dark) did no
t result in a motion signal-either in the global-motion direction or i
n the opposite direction (reverse apparent motion). From these finding
s we conclude that the inputs to the motion sensitive cells have match
ed spatial opponency (the ON and OFF pathways remain separate at this
level) but that they then combine to form a single pathway prior to th
e extraction of the global-motion signal. These findings are contrary
to those predicted by models which advocate squaring or full-wave rect
ification prior to global motion processing.