Temporal beats in the human visual system.

The human visual system is generally assumed to comprise multiple independent channels tuned to limited ranges of spatial and temporal frequency. However, it is known that spatial beats effectively mask a sinusoidal grating whose spatial frequency coincides with the beat frequency of the mask, even though no Fourier energy exists in the mask at that frequency. We investigated whether analogous masking effects occur in the temporal domain and whether adaptation to either spatial or temporal beats results in threshold elevation. Our results show that temporal beat patterns do not mask a grating temporally modulated at the beat frequency. Adaptation to neither spatial nor temporal beats results in measurable threshold elevation at the beat frequency. Since a luminance non-linearity would lead to temporal as well as spatial distortion products, we conclude that sensitivity to spatial beats does not rely on the presence of distortion products but instead reflects subsequent processing that occurs in the spatial domain only. The most likely site of such processing is cortical.