While spatial maps obtained with optical imaging have been shown to correspond to results from single unit recording, the quantitative relationship between the magnitudes of electrophysiological and optical responses is open to question. Single neurons in cat A18 typically show direction-selective contrast response functions which are sigmoidal or even non-monotonic (saturating, or declining, at higher contrasts) - (Ledgeway & Baker, SFN Abst 164.3, 2001). We have used intrinsic signal optical imaging (720 nm, responses averaged over ca 8.5 sec stimulus durations) from cat A18 to measure responses to drifting sinewave gratings at contrasts ranging from 10-80 %. We calculated average difference images for pairs of oppositely moving sinewave gratings at each contrast level tested, and employed a novel strategy to find the optimized smoothing filter for noise reduction. The contrast response functions of magnitudes of direction-selective optical responses were monotonic and convex, quite unlike the saturating...