Motion and vision. III. Stabilized pattern adaptation.

It has been suggested that local variations of retinal sensitivity may be responsible for elevating the threshold in pattern-adaptation experiments of the Blakemore-Campbell type. Subjects are unable to scan high-contrast gratings uniformly enough to eliminate this possibility. To control this effect, we performed grating-adaptation experiments under stabilized-image conditions, while both adapting and test targets were moved at retinal velocities determined by the experimenter. By means of an afterimage technique, we also measured the strength of the retinal sensitivity mask that forms under these conditions. Varying the spatial frequency and velocity of the adapting stimulus, we inferred the spatial and temporal properties of the principal mechanism that contributes to the afterimage. We found that the Blakemore-Campbell effect persists at adapting velocities that are fast enough to rule out local variations of retinal sensitivity. More surprisingly, even the clearly visible afterimages that occur at a retinal velocity of 0.1 deg/s seem to have no effect on pattern adaptation. (Sensitivity masking can raise the adapted threshold, but only at adapting velocities slower than normal eye movements). By manipulating the image velocity, we were able to shift the spatial frequencies of some threshold-elevation curves, but these shifts were not great enough to suggest that velocity tuning plays important role in pattern adaptation.