Is the size aftereffect direction selective?

We investigated whether the size aftereffect (apparent spatial-frequency shift after adaptation to slightly different frequencies) is direction selective; i.e., whether it is stronger for test stimuli moving in the adapting direction than the opposite direction. We used drifting sinusoidal gratings of various spatiotemporal frequencies for both adaptation and test stimuli, and the perceived test frequency was estimated by means of a matching technique with a staircase method. For the purpose of comparison, the post-adaptation threshold elevation was measured in addition to the size aftereffect. The results revealed that the direction of stimuli had no influence on the magnitude of the size aftereffect for a wide range of spatiotemporal frequencies, whereas the post-adaptation threshold elevation showed clear direction selectivity. Although there was a significant direction selectivity for the size aftereffect at low spatial and high temporal frequencies, the selectivity was much weaker than that seen in the threshold elevation data. These findings are discussed in relation to the validity of a unified account of selective adaptation at and above threshold contrast and the notion of the separate processing of pattern and motion information.