Academic journal article Perception and Psychophysics

The Effect of Eccentricity and the Adapting Level on the Café Wall Illusion

Academic journal article Perception and Psychophysics

The Effect of Eccentricity and the Adapting Level on the Café Wall Illusion

Article excerpt

The café wall pattern is composed of rows of alternating light and dark tiles, and alternate rows are shifted by one fourth of a cycle. The rows of tiles are separated by narrow horizontal mortar lines whose luminance is between those of the dark and the light tiles. Although the mortar lines are physically parallel, they are perceived to be tilted, which is known as the café wall illusion. In this study, an energy-based model for encoding orientation is implemented in order to estimate the strength of the café wall illusion, and it is shown that the estimated orientation depends on the spatial frequency to which each orientation-encoding unit is tuned. The estimation of mortar line orientation from an orientation-encoding unit tuned to a lower spatial frequency was greater than that from a unit tuned to a higher spatial frequency. It is assumed that the perceived mortar line orientation is the result of an integration of responses from the orientation-encoding units tuned to various spatial frequencies. This leads to the prediction that under viewing conditions in which responses from orientation-encoding units tuned to a higher spatial frequency are presumably weakened, the strength of the café wall illusion increases. In agreement with this prediction, it is shown that the café wall illusion is stronger when the café wall image is presented at the periphery or is observed under low luminance levels. On the other hand, the weighted averaging of the estimated mortar orientations across spatial frequencies overestimates the perceived orientation of the mortar lines. This suggests that the final percept of the café wall illusion could be determined by some kind of nonlinear interaction, such as an inhibitory interaction, between orientation-encoding units.

In geometrical illusions, the perceived orientation of contours often deviates from what it really is (e.g., Gregory, 1968; Rock, 1986). The café wall illusion is a well-known geometrical illusion. The café wall pattern (Figure IA) is composed of rows of alternating light and dark tiles, and the rows are shifted by one fourth of a cycle. The rows of tiles are separated by narrow horizontal mortar lines whose luminance is between those of the dark and the light tiles. Although the mortar lines are physically parallel, they are perceived to be tilted (Gregory, 1972; Gregory & Heard, 1979). The café wall pattern is a variation of the Munsterberg figure, in which the mortar lines have the same luminance as the dark tiles (Fraser, 1908; Munsterberg, 1897). When the mortar luminance is darker than the dark tiles (Figure 1 B) or lighter than the light tiles, the strength of the illusion is largely reduced (Gregory & Heard, 1979).

A prominent characteristic of the primary visual cortex is that most neurons have selectivity for patterns of a specific orientation and spatial frequency, and the particular optimum orientation and spatial frequency varies from cell to cell (e.g., R. L. De Valois & K. K. De Valois, 1988; Hubel & Wiesel, 1962). There is also psychophysical evidence that there is a specialized mechanism for encoding orientation and spatial frequency (e.g., Blakemore & Campbell, 1969; Campbell & Kulikowski, 1966; R. L. De Valois & K. K. De Valois, 1988; Phillips & Wilson, 1984). Since the café wall illusion is an orientation-specific phenomenon, it seems reasonable to explain it upon the basis of an orientationencoding mechanism in the visual system. In fact, several studies have suggested that the café wall illusion is induced by outputs from a mechanism that selectively responds to specific orientations in the image (Haig, 1989; Lulich & Stevens, 1989; Morgan & Moulden, 1986).

When the café wall figure is convolved with a difference of Gaussians (DoG) operator, a twisted cord (Fraser, 1908) of alternating light and dark diagonal bands appears (Earle & Maskell, 1993; Lulich & Stevens, 1989; Morgan & Moulden, 1986). …

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