SIMILAR MECHANISMS UNDERLIE SIMULTANEOUS BRIGHTNESS CONTRAST AND GRATING INDUCTION

The experiments explore whether the mechanism(s) underlying grating in duction (GI) can also account for simultaneous brightness contrast (SB C). At each of three test field heights (1, 3 and 6 deg), point-by-poi nt brightness matches were obtained from two subjects for test field w idths of 32 deg (GI condition), 14, 12, 8, 6, 3 and 1 deg, The point-b y-point brightness matches were quantitatively compared, using GI cond ition matches as a standard, to assess systematic alterations in the s tructure and average magnitude of brightness and darkness induction wi thin the test fields as a function of changing test field height and w idth. In the wider test fields induction structure was present and was generally well-accounted for by the GI condition sinewave predictions . As test field width decreased the sinewave amplitude of the induced structure in the test field decreased (i.e., flattened), and eventuall y became negative (i.e., showed a reverse cusping) at the narrower tes t field widths, As expected, both subjects showed a decrease in overal l levels of brightness and darkness induction with increasing test fie ld height. For any particular test field height, however, relative bri ghtness increased with decreasing test field width, This brightness in crease began at larger test field widths as test field height increase d, The results are parsimoniously accounted for by the output of a wei ghted, octave-interval array of seven difference-of-gaussian filters. This array of filters differs from those previously employed to model various aspects of spatial vision in that it includes filters tuned to much lower spatial frequencies. The two-dimensional output of this sa me array of filters also accounts for the GI demonstrations of Zaidi [ (1989) Vision Research, 29, 691-697], Shapley and Reid's [(1985) Proce edings of the National Academy of Sciences USA, 82, 5983-5986] contras t and assimilation demonstration, and the induced spots seen at the st reet intersections of the Hermann Grid. The physiological plausibility of the filter array explanation of brightness induction is discussed, along with a consideration of its relationship to other models of bri ghtness perception. (C) 1997 Elsevier Science Ltd.