Abstract
The perceived slant (or inclination) of a slanted stereoscopic surface is affected by a surrounding stereo-slanted surface. Previous studies have shown that the effect is generally contrasting, that is the perceived slant of the test stereo-surface is shifted away from that of the surrounding stereo-surface. However previous studies have not established whether a surrounding stereo surface affects test surfaces slanted in the opposite slant direction, and have not measured the mutual contrasting effect between test and surround in order to measure their perceived angular difference. Using an adjustable matching slanted surface in a two-interval-forced-choice procedure, observers measured the perceived slant of both a central test as well as its surrounding surface for a range of combinations of test and surround slants. For each combination of test and surround slant two measures were calculated: (1) the perceived slant difference between the test and surround when the two were presented in isolation and (2) the perceived slant difference between the test and surround when the two were presented in combination, i.e. when affecting each other. The difference between these two measures is termed here the mutual contrasting effect, or MCE. MCEs were plotted as a function of surround slant; as the test-surround slant difference increased from zero, there was a sharp rise in MCEs followed by various degrees of decline at larger test-surround slant differences. Importantly, MCEs were consistently observed with opposite signs of test and surround slant. Our findings suggest that positive and negative stereoscopic slants are encoded by a single bipolar mechanism, one subject to mutual inhibitory interactions between neighboring stereo-slanted surfaces that rapidly rise and gradually decline with the angular difference between test and surround.