Direction of Rotation and Decay of the Spiral Aftereffect 1

Abstract

Previous findings have suggested that there are a number of differences between the expansion spiral aftereffect and the contraction spiral aftereffect. A neurophysiological hypothesis has been proposed to account for these differences. A more simple explanation in terms of quality of fixation was investigated in the present study. In order to test the fixation hypothesis, use was made of Spigel's observation that an interval of darkness following exposure to a rotating spiral was in some way associated with a delay of the decay of the aftereffect. It was predicted on the basis of the previous findings that (a) an interval of darkness introduced following rotation of the spiral would result in a significant delay of the decay of the aftereffect. On the basis of the fixation hypothesis it was predicted (b) that there would be no significant difference between the expansion aftereffect and the contraction aftereffect in the delay of the decay of the aftereffect produced by post-rotation darkness. Prediction (a) was confirmed but not Prediction (b), casting doubt on the fixation hypothesis.