The question of how our brains and those of other animals code sensory information is of fundamental importance to neuroscience research. Visual illusions o¡er valuable insight into the mechanisms of perceptual coding. One such illusion, the tilt after-e¡ect (TAE), has been studied extensively since the 1930s, yet a full explanation of the e¡ect has remained elusive. Here, we put forward an explanation of the TAE in terms of a functional role for adaptation in the visual cortex. The proposed model accounts not only for the phenomenology of the TAE, but also for spatial interactions in perceived tilt and the e¡ects of adaptation on the perception of direction of motion and colour. We discuss the implications of the model for understanding the e¡ects of adaptation and surround stimulation on the response properties of cortical neurons.

It has long been accepted that amongst patterns which are bilaterally symmetrical, those which have their axis of symmetry vertical are more saliently symmetrical than patterns whose axis of symmetry is at some other orientation. The evidence regarding the relative salience of other orientations of axis of symmetry is somewhat more equivocal. In experiment 1, subjects were required to discriminate between symmetric or random-dot patterns when the axis of symmetry was at one of eighteen different orientations, spaced 10 degrees apart, both clockwise and counterclockwise of vertical to horizontal. The data indicated that vertical was most salient, then horizontal but that, unlike in the classical oblique effect for contrast sensitivity, performance for precisely diagonal axes was better than that for surrounding axis orientations. Additional data (from experiments 2 and 3) also showed that the salience of vertical and horizontal axes of symmetry can be manipulated extensively by varying the range of stimuli presented, presumably by manipulating the scanning or attentional strategy adopted by the observer. Many previous studies of symmetry perception may have confounded hard-wired salience for vertical symmetry with scanning or attentional strategies.

We investigated the effect of adaptation on orientation discrimination using two experienced observers, then replicated the main effects using a total of 50 naïve subjects. Orientation discrimination around vertical improved after adaptation to either horizontal or vertical gratings, but was impaired by adaptation at 7.5 or 15 degrees from vertical. Improvement was greatest when adapter and test were orthogonal. We show that the results can be understood in terms of a functional model of adaptation in cortical vision.

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