Brief communication Test stimulus characteristics determine the perceived speed of the dynamic motion aftereVect

Abstract Using a speed-matching task, we measured the speed tuning of the dynamic motion aftereVect (MAE). The results of our Wrst experi-ment, in which we co-varied dot speed in the adaptation and test stimuli, revealed a speed tuning function. We sought to tease apart whatcontribution, if any, the test stimulus makes towards the observed speed tuning. This was examined by independently manipulating dotspeed in the adaptation and test stimuli, and measuring the eVect this had on the perceived speed of the dynamic MAE. The resultsrevealed that the speed tuning of the dynamic MAE is determined, not by the speed of the adaptation stimulus, but by the local motioncharacteristics of the dynamic test stimulus. The role of the test stimulus in determining the perceived speed of the dynamic MAE wasconWrmed by showing that, if one uses a test stimulus containing two sources of local speed information, observers report seeing a trans-parent MAE; this is despite the fact that adaptation is induced using a single-speed stimulus. Thus while the adaptation stimul us necessar-ily determines perceived direction of the dynamic MAE, its perceived speed is determined by the test stimulus. This dissociation of speedand direction supports the notion that the processing of these two visual attributes may be partially independent.© 2006 Elsevier Ltd. All rights reserved.

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