When stereopsis does not improve with increasing contrast1Portions of this work were presented at The European Conference on Visual Perception, Helsinki, 1997.1

It is well known that stereoacuity for conventional (1st-order) stimuli improves with increasing contrast with an approximate slope of -0.5 on log-log axes (Halpern DL, Blake RR. Perception 1988;17:483-495; Legge GE, Gu Y. Vis Res 1989;29:989-1004). In the experiments reported here a variety of stimuli were used (Gabor patches, amplitude modulated stimuli and 1D noise patches) and tasks (stereoacuity and Dmax) to determine if 2nd-order stereopsis shows a similar square root dependence. The results consistently demonstrate that the effect of contrast on stereopsis is quite different for the 2nd-order stimuli. Increases in stimulus contrast have little effect on performance; the resulting slopes are very shallow. The pattern of results is similar when the interocular contrast ratio is varied, demonstrating that 2nd-order processing is more resilient to stimulus differences in the two eyes than 1st-order.