Richard Gregory (1923-2010)

A British psychologist and professor of neuropsychology at the University of Bristol, Richard Gregory adhered to a Helmholtzian belief of perception as an active function that engaged memory as well as internal and cognitive processes. The reception of environmental cues are processed through a hypothetico-deductive process—perception itself was a hypothesis that could be analyzed and explained through visual and cognitive psychology. Gregory applied a scientific method to a micro time scale of observation to impression.

“Following Hermann von Helmholtz, who described visual perceptions as unconscious inferences from sensory data and knowledge derived from the past, perceptions are regarded as similar to predictive hypotheses of science, but are psychologically projected into external space and accepted as our most immediate reality.” (Gregory, 1997).

Much of Gregory’s writing and studies could be seen as in direct response and retaliation to Gibson’s ecological and direct realism theories of perception. Rather, Gregory believed that cues, hints, and environmental evidence inferred prior experiences and preconceptions, which in combination constructed perception. Illusions, then, were “errors of perception” that occur because of the misappropriation of knowledge. “So illusions are important for investigating cognitive processes of vision.” (Gregory, 1997).

Richard Gregory in front of Munsterbert pattern cafe wall
Various ‘border’ (or mortar) luminance contrasts with tiles.

Gregory’s “Café Wall Illusion” illustrates the quantitative way in which he believed perception could be dissected. Based on the visual distortions Munsterbert checkerboard pattern – rectangular checkered tiles shifted at half-cycle from row to row – found on the exterior entrance wall of a local café near his laboratory, this illusion study sought to quantitatively evaluate through variations the visual and psychological mechanisms that caused horizontal parallel lines to appear ‘wedged.’ Gregory concluded that the contrasting tiles as well as its mortar lines created visual registers, or borders that ‘locked’ the luminantly opposite tiles and mortar against each other, creating this effect.

Cafe Wall Illusion experiments conducted by Gregory’s lab with projectors and screens to simulate variations in luminance contrasts between tiles and mortar.

Gregory and his lab were able to develop a set of ‘laws’ surrounding this phenomenon, including such observations as the wedge distortion being the same in direction for all viewers, that the crop of the pattern did not affect distortion, and that the proportions of the rectangle had little to do with the distortion. The factors that did affect the distortion of the café wall tiles were the luminance contrast of the tiles and their difference in luminance to their mortar lines (the farther in luminance, the greater the distortion), the thickness of the mortar lines (the narrower the ‘border-locking’ lines, the greater the distortion), and the angle of viewing the pattern (greater distortion at a skewed or blurred view).

By similar properties of ‘border-locking,’ lines that seemingly spiral in this image are in fact concentric circles.

Through the discovery of these variations, Gregory sought to prove the inextricable link between visual perception and cognitive processes, and believed that the basis for perceptual illusions lie in neural and optical reasons. “We shall attempt a functional explanation – in terms of processes that seem necessary for maintaining registration of borders. It is hoped that underlying physiological mechanisms may soon be identified, explaining how the functions are mediate.” (Gregory, 1979). These illusions could be explained through lags and discrepancies in retinal receptor response, and Gregory’s claims were furthermore supported by “recent finding[s] that visual characteristics such as luminance, colour, and movement are ‘mapped’ in separate cortical regions” (Gregory, 1979).


Gregory, Richard L., Heard, Priscilla. “Border locking and the Café Wall illusion.” Perception, Vol. 8. Brain and Perception Laboratory, University of Bristol. 1979. pp 365–380.

Gregory, Richard L. “Knowledge in perception and illusion.” Phil. Trans. R. Soc. Lond. B. Department of Psychology, University of Bristol. 1997.