FACILITATION OF LUMINANCE GRATING DETECTION BY INDUCED GRATINGS

Grating induction causes a homogeneous test field surrounded by sinewa ve gratings to possess an induced counterphase grating [McCourt M. E. (1982). Vision Research, 22, 119]. There is currently no consensus abo ut the stage of visual processing at which illusory phenomena such as simultaneous brightness contrast are signaled, We measured the masking efficacy of induced gratings by measuring contrast detection threshol ds for targets (sinewave luminance gratings) added in phase to both re al and induced gratings which were matched in apparent contrast. At sp atial frequencies below c. 0.5 c/deg, target detection and discriminat ion were comparably facilitated by both real and induced low-contrast pedestals (0.5-2%). At higher spatial frequencies (above 1.0 c/deg) fa cilitation continued to be observed for targets added in-phase to real grating pedestals, but occurred only for targets added out-of-phase w ith induced pedestal gratings. Higher inducing frequencies by themselv es were not responsible for the observed phase shift of facilitation, however, since both real and induced pedestals produced similar target contrast discrimination functions when inducing frequency was varied by manipulating viewing distance (which holds the ratio of inducing gr ating period and test field height constant), The results imply the ex istence of at least two types of lateral interactive processes: one pr oducing in-phase facilitation, and a second producing out-of-phase fac ilitation, The relative contribution of each process depends upon the ratio of inducing grating period and test field height. Copyright (C) 1996 Elsevier Science Ltd.