Psychophysical Hallucinations of Orientation and Spatial Frequency

Abstract

After inspection of vertical sinusoidal gratings at least three distinct types of subjective or ‘hallucinated’ patterns can be seen on a uniform test field. One type, here called horizontal streaming (H), is already well-known from the work of MacKay. A second type (V) looks like a roughly sinusoidal grating about 15 octaves above the adapting spatial frequency. Under optimal conditions a second vertical component appears at about 2 octaves below the adapting frequency. The third category of aftereffect consists of diagonal lines (D) at two orientations (about ±40° from vertical). The spatial-frequency band at these two orientations appears to be fairly broad, but roughly similar to the adapting frequency. The duration and strength of D increased, while V declined, at higher adapting spatial frequencies. D and V were increasing functions of adapting contrast, while H appeared abruptly only after the highest adapting contrast. H, D, and V are thus all functionally distinct. A schematic model of cortical organization is proposed to account for these phenomena. Pattern channels selective for a given orientation are grouped together with movement channels selective for the orthogonal direction. Antagonism between channels within such ‘modules’ accounts for the streaming effect (H). Inhibition between modules tuned to different orientations and spatial frequencies accounts for the D and V effects: after adaptation of channels in one module, neighbouring module(s) are released from inhibition to produce a spurious response which is seen as a grating-like object in the adapted part of the visual field. During flickering adaptation a ‘hallucinated’ lattice can be seen superimposed on the adapting grating. It apparently consists of Fourier components more remote from the adapting pattern than D and V are. This disinhibitory effect is strong confirmation of the inhibitory model. The regular and highly organized matrix of channels implied by these experiments may constitute a cortical hypercolumn conducting a coarse, piecewise Fourier transformation of the retinal image.