On the apparent collapse of stereopsis in random-dot-stereograms at isoluminance

We have investigated the apparent collapse of stereopsis obtained with random-dot-stereograms at isoluminance. Contrast thresholds for both depth and form discrimination of targets in random-dot- and figural stereograms were measured at a number of disparitics, using both isoluminant and isochromatic stimuli. All contrast thresholds for stereoscopic tasks were normalised to contrast thresholds for detecting the appropriate stimulus. We found that at isoluminance contrast thresholds for depth judgements were not higher for random-dot compared to figural stereograms, even when normalised to the same thresholds obtained with isochromatic stimuli. On the other hand contrast thresholds for three-dimensional form judgements were much higher than those for depth judgements in isoluminant, compared to isochromatic random-dot-stereograms. This specific impairment of stereoscopic form (as opposed to depth) processing at isoluminance was confirmed in a further experiment in which subjects were required to judge the presence and orientation of depth corrugations in a disparity-modulated random-dot-stereogram.