SPATIAL SCALE INTERACTIONS IN STEREO SENSITIVITY AND THE NEURAL REPRESENTATION OF BINOCULAR DISPARITY

How are binocular disparities encoded and represented in the human vis ual system? An 'encoding cube' diagram is introduced to visualise diff erences between competing models. To distinguish the models experiment ally, the depth-increment-detection function (discriminating disparity d from d +/- Delta d) was measured as a function of standing disparit y (d) with spatially filtered random-dot stereograms of different cent re spatial frequencies. Stereothresholds degraded more quickly as stan ding disparity was increased with stimuli defined by high rather than low centre spatial frequency. This is consistent with a close correlat ion between the spatial scale of detection mechanisms and the disparit ies they process. It is shown that a simple model, where discriminatio n is limited by the noisy ratio of outputs of three disparity-selectiv e mechanisms at each spatial scale, can account for the data. It is no t necessary to invoke a population code for disparity to model the dep th-increment-detection function. This type of encoding scheme implies insensitivity to large interocular phase differences. Might the system have developed a strategy to disambiguate or shift the matches made a t fine scales with those made at the coarse scales at large standing d isparities? In agreement with Rohaly and Wilson, no evidence was found that this is so. Such a scheme would predict that stereothresholds de termined with targets composed of compounds of high and low frequency should be superior to those of either component alone. Although a smal l stereoacuity benefit was found at small disparities, the more striki ng result was that stereothresholds for compound-frequency targets wer e actually degraded at large standing disparities. The results argue a gainst neural shifting of the matching range of fine scales by coarse- scale matches posited by certain stereo models.