WE investigated human perceptual learning with stereoscopic stimuli present
ed below threshold. Different visual patterns were shown as dynamic random
dot stereograms in a forced-choice design in order to determine the psychop
hysical thresholds of 16 adults. Brain electrical activity was recorded fro
m 30 electrodes over parieto-occipital areas while stereograms were present
ed with horizontal disparities below threshold. During the observation of s
ub-threshold stimuli, we tested repeatedly whether implicit perceptual lear
ning occurred. More than half of the subjects learned to see stereoscopic t
argets. This was accompanied by topographic changes in the pattern of activ
ation of neural assemblies in the visual cortex where the center of activit
y shifted towards the right hemisphere. Subjects who did not improve in per
ception, displayed no such effects. (C) 1999 Lippincott Williams & Wilkins.