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

Striate and extrastriate neurons present short-term synaptic depression and facilitation in response to brief stimulations. Recent studies have shed light on some possible relationships between these short-term forms of neural plasticity and patterns 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 adaptation-test blank intervals. Increasing the adaptation duration, the perceived motion direction of the ambiguous test pattern is biased towards the opposite direction [rapid motion aftereffect (rMAE)]. In our study we used contracting and expanding circular gratings to selectively elicit activity at the level in which neurons respond to optic flow components (ie, MT and MST neurons). We attempted to assess whether the rapid effects exist and, if so, their timescale. Results revealed strong rMAEs, a long lasting facilitation that arises gradually with intermediate and long blank intervals (perceptual sensitization), but we did not observe rVMP. We interpreted results on the basis of competition between coexistent forms of short- and long-term synaptic depression and facilitation implemented at different visual cortical circuitries.