The tilt illusion: Phenomenology and functional implications

Abstract

The perceived orientation of a line or grating is affected by the orientation structure of the surrounding image: the tilt illusion. Here, I offer a selective review of the literature on the tilt illusion, focusing on functional aspects. The review explores the merits of mechanistic accounts of the tilt illusion based upon sensory gain control in which neuronal responses are normalized by the pooled activity of other units. The role of inhibition between orientation-selective neurons is discussed, and it is argued that their associated disinhibition must also be taken into account in order to model the full angular dependence of the tilt illusion on surround orientation. Parallels are drawn with adaptation as modulation by the temporal rather than spatial context within which an image fragment is processed. The chromatic selectivity of the tilt illusion and the extent of its dependence on the visibility of the surround are used to infer characteristics of the neuronal normalization pools and the loci in the cortical processing hierarchy at which gain control operates. Finally, recent evidence is discussed as to the possible clinical relevance of the tilt illusion as a biomarker for schizophrenia.

Reference Repulsion Is Not a Perceptual Illusion

“…Although we did find a genuine perceptual bias for stimuli oriented further away from the discrimination boundary, this effect had a very different tuning profile than the reference repulsion bias in reproduction responses, and did not necessitate an explicit discrimination judgment of the biased stimulus against the boundary (Experiment 3). In terms of orientation tuning, the perceptual bias rather resembled a tilt illusion (Clifford, 2014;Gibson, 1937;Schwartz, Hsu & Dayan, 2007;Wenderoth & Johnstone, 1987) or tilt-aftereffect (Gibson & Radner, 1937;Webster, 2015). Consequently, our experiments provide evidence against the perceptual nature of reference repulsion, indicating that making fine discrimination judgments does not alter the appearance of visual stimuli, but instead leads to a post-perceptual working memory or decision bias.…”

Interaction of contexts in context-dependent orientation estimation

“…Next, to address the possibility that the effect is mediated by an interaction between near and far surrounds (e.g., lateral inhibition, Clifford, 2014;O'Toole & Wenderoth, 1977), we used a near 525 surround tilted at ±45°, with the far surround oriented horizontally or vertically as before. In this experiment, the orientation difference between the near and the far surround is always the same (45° for both vertical and horizontal far surrounds), equating the interaction between the near and the far surrounds.…”

“…(We quantitatively verified this claim by finding the orientations with the most Fourier energy in the "contrast" of the stimuli, where "contrast" was calculated as the squared difference between the stimuli image and the background value.) Importantly, the sign of TI depends on the orientation difference between the surround and the target: at ~5 to ~65° 585 orientation differences, the shift in the perceived orientation is in the opposite direction relative to the surround orientation ("repulsive effect"), whereas orientation differences of ~80° lead to a shift of the perceived orientation in the direction of the surround ("attractive effect", also known as the indirect tilt illusion) (Clifford, 2014). Therefore, under the "A+V" condition, the first-order orientation content (e.g., repulsive from -45°) leads to the same sign of TI as the second-order orientation (repulsive from -22.5°, 590 or attractive toward +67.5°).…”

A decision-time account of individual variability in context-dependent orientation estimation

“…When the context surrounds a target, this phenomenon is referred to as the tilt illusion (TI, Fig. 1A, Clifford, 2014;Gibson, 1937), and when the context precedes a target in time, it is referred to as the tilt aftereffect 35 (TAE, Fig. 1B, Gibson & Radner, 1937;Webster, 2015).…”