Sensory thresholds and neurophysiological correlates of human perceptual learning.

The repeated presentation of visual targets yields significant improvements of psychophysical thresholds, and in the present paper corresponding alterations in neurophysiological activity of the human brain are reported. A group of 23 healthy adults was investigated in a psychophysical experiment in which hyperacuity thresholds were determined repeatedly for up to about 25 min with vertically and horizontally oriented targets. Evoked brain activity was recorded during the same experimental session from 30 electrodes over the occipital brain areas. For each subject mean potentials were computed offline for two conditions corresponding to "before learning' (i.e. the first half of the experiment) and "after learning' (i.e. the second half of the experiment), and visual evoked brain activity was compared between these conditions. In the psychophysical experiments vernier thresholds decreased significantly with time, and this improvement in performance could be confirmed in a control session after an interval of 9 weeks. All changes in discrimination performance were specifically bound to stimulus orientation. Learning effects were also observed in electrical brain activity, which displayed significantly smaller component latencies and larger amplitudes as a function of training time. In addition, learning affected the topography of the evoked potential fields suggesting that cortical neurons are activated in a different way before and after perceptual learning.