Learning to see 3-D: psychophysics and brain electrical activity.

We investigated human perceptual learning with stereoscopic stimuli presented below threshold. Different visual patterns were shown as dynamic random dot stereograms in a forced-choice design in order to determine the psychophysical thresholds of 16 adults. Brain electrical activity was recorded from 30 electrodes over parieto-occipital areas while stereograms were presented with horizontal disparities below threshold. During the observation of sub-threshold stimuli, we tested repeatedly whether implicit perceptual learning occurred. More than half of the subjects learned to see stereoscopic targets. This was accompanied by topographic changes in the pattern of activation of neural assemblies in the visual cortex where the center of activity shifted towards the right hemisphere. Subjects who did not improve in perception, displayed no such effects.