PRE-ATTENTIVE PERCEPTUAL GATING OF THE SPATIAL FREQUENCY DOUBLING ILLUSION

T. Maddess, M. P. Davey and S. Bedford

Purpose: The spatial frequency doubling (FD) illusion occurs for rapidly flickering low spatial frequency sinusoidal gratings. Under these conditions we perceive the gratings as having twice their original spatial frequency. The FD effect is now the basis for new methods of diagnosis of glaucoma ^{1, 2}. When the contrast modulation is the sum of a fast and a slow flicker rate the FD illusion may disappear depending upon the frequency of the slower modulation. We investigated this apparent perceptual gating process using several methods.

Methods: We first examined the stimulus conditions for which the perceptual gating occurs in up to 8 parts of the visual field. We then used a multi-region stimulus to simultaneously record ERGs and VEPs for stimuli presented in the same regions. We examined contrast dependent phase changes in the evoked potentials previously shown³ to be associated with the FD effect.

Results: Visibility of the FD effect could be perceptually gated on or off by selecting a particular slow frequency component, however, we could not observe a change in either the VEP or ERG signals. Instead, the evoked potential data suggested that the FD percept should be present even when it was not seen.

Conclusions: Past work indicates that the contrast dependent changes in evoked potential phase are associated with seeing the illusion and agree with expectations from retinal gain control mechanisms. Thus the results indicate that we cannot observe the perceptual gating mechanism with our evoked potentials. This may indicate that either (unrecorded) higher brain areas manage the gating, or that the gating occurs by changing what is read-out of V1 from a selection of possible valid percepts.

Keywords: Perceptual Gating, Illusion, Pre-attentive, Multiple Frequency

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