The role of high-level visual areas in short- and longer-lasting forms of neural plasticity.

Striate and extrastriate neurons present short-term synaptic depression and facilitation in response to brief stimulations. Recent psychophysical studies have shed light on some possible relationships between these short-term forms of neural plasticity and of psychophysical behavior. It has been shown that a brief adaptation to directional motion biases the perceived direction of a subsequently presented ambiguous test pattern towards the same direction to that of the adaptation (rapid visual motion priming--rVMP), but only after brief (40ms) adaptation-test blank intervals. Although when the adaptation duration is increased, the perceived motion direction of the ambiguous test pattern is biased towards the opposite direction to that of the adaptation pattern (rapid motion aftereffect--rMAE). In the present study we stimulated MT and MST neurons via the presentation of contracting and expanding circular gratings. Our aim was to assess whether rapid effects exist at these higher levels of processing where neurons respond to optic flow, and if such effects are present determine their timescale. Results revealed strong rMAEs and perceptual sensitization (PS), which is a long-lasting facilitation that increases gradually when using intermediate and long adaptation-test blank intervals. We did not observe any effect of rVMP. Our results are considered to reflect the competition between coexistent forms of short- and long-term synaptic depression and facilitation implemented at different visual cortical circuitries.