Functional Aspects of Border-Signalling Mechanisms

Abstract

Multiple border-signalling mechanisms have been proposed in some models of object segregation. We investigated whether distinct functions of different border mechanisms could be isolated in human psychophysical tasks. The display was a low-spatial-frequency (1.5 cycles deg⁻¹) sinusoidal grating. Two types of centre-surround stimuli were produced by (i) increasing or decreasing the mean luminance of a central patch of the grating to create a signed luminance border (LB), or (ii) by shifting the grating within the central patch 180° to create a sign-reversing border (PSB) without a change in mean luminance. Grating contrast was kept constant at 20%. Observers performed a spatial 2AFC task in each condition. The PSB stimulus served as the comparison stimulus. Test stimuli were either LB or PSB+LB. The mean luminance of the central test patches varied over trials to create border contrasts of −15% to 15% Michelson contrast. In separate sessions, subjects made four types of comparisons: which of the two central areas appeared (1) to differ more in depth from its surround; (2) to have greater modulation contrast of the sinusoid; (3) lighter/darker, (4) to have higher border contrast. Results depended strongly upon test border type and suggested that the mechanism responding to each has a distinct and different perceptual function, and the two may interact. Our results support the hypothesis that surface lightness is calculated by a sign-preserving mechanism, whereas a phase-insensitive mechanism is involved in percepts relating to segregation in depth.