Perceptual learning of stereoacuity.

It has often been noted that with repeated exposure to random-dot stereograms the time required to perceived depth decreases. Further, with extensive practice, stereoacuity thresholds have been shown to decrease. For both types of learning some researchers have reported specificity of the improvements to retinal location, and have thus suggested that the learning may be localised at early levels of visual processing, such as in primary visual cortex. However, these studies have not adequately ruled out the possibility that the specificity shown may be due to the operation of selective-spatial-attention mechanisms. In the present study this possibility was examined by training observers to judge the relative depth of a pair of stereograms presented equally often in two spatial locations, but stimuli were only presented with one direction of disparity (ie crossed or uncrossed) in any one location. Results indicated that, as expected, observers' judgements improved with practice. However, this improvement transferred completely to stimuli presented with the other direction of disparity in each location. Thus, it is argued that previous findings of retinal-location-specific improvements in stereoacuity may well be due to selective-spatial-attention mechanisms, rather than to learning localised at an early level of visual processing.