Modulation of responses elicited by moving bars within the
classical receptive fields (CRF) of cat area 17 neurons were
studied as a function of the direction and velocity of drifting
gratings in the surrounds. Several different types of modulation
were observed; collectively, the responses of most cells, both
simple and complex, were strongly modulated by surround motion.
None of these cells appear to signal relative velocity between
the CRF and its surround. The gain and spatiotemporal structure
of the CRF mechanism were estimated using contrast-response
functions and reverse correlation with spatiotemporal ternary
white noise, respectively. These measurements were made in the
presence of surround gratings shown to significantly modify
responses elicited from the CRF. In all cases, the gain of the
CRF mechanism was driven up or down relative to controls but
the receptive-field structure did not change in any way. We
conclude that neurons in cat area 17 act like scalable filters,
meaning that their gains can be adjusted by stimuli in the
surrounds without altering the properties of the CRF. This was
verified by showing that velocity tuning curves were also
unmodified by stimuli in the surround that did change the gain.
Based in part on these data, we discuss the notion that primary
visual cortex makes use of a double-opponent mechanism for the
representation of local discontinuities in motion and orientation.