The Boring wife/mother-in-law (F), the Necker Cube (G) and Rubin's Maltese Cross (H) are ambiguous figures that result in different interpretations. Herman's grid (E) generates illusory gray points at each intersection of the white lines. In the Poggendorff illusion (D), the context disrupts the impression of continuity. In the Ebbinghaus (A), Ponzo (B) and Müller-Lyer (C) illusions, same-sized patterns are misevaluated because of the context. Progressively, VIs assumed a place of increasing importance in the literature as a fertile, practical and valid way to explore the underlying mechanisms of perception in normal or pathological conditions. These visual effects were named visual illusions (VIs) or optical illusions. Similarly, Necker and, later, Boring and Rubin described ambiguous figures that could lead to different, mutually exclusive interpretations (Necker, 1832 Boring, 1930 Rubin, 1958) (See Figure Figure1). Physiologists, such as Poggendorff, Herman, Müller-Lyer, Ponzo and Ebbinghaus, noticed that our appreciation of contrast, size or continuity can be distorted by contextual information (Zölner, 1860 Hermann, 1870 Müller-Lyer, 1889 Ponzo, 1910). Historically, the first scientific descriptions of misleading visual effects date back to the late 19th to early 20th century. Illusionists also frequently resort to human perceptual properties in their magic tricks (Martinez-Conde and Macknik, 2007) to the extent that the study of these tricks has become the object of an original sub-field in cognitive sciences called “neuro-magic.” A first example is when artists use artifices to mislead perception and induce, for example, a paradoxical “realistic” feeling of depth or movement. Visual arts provide eloquent illustrations of common mistakes made in vision. However, recent findings encourage moving beyond this presupposed primacy of vision over other senses, notably, when looking at the details of visual misperceptions.
Vision has long been considered one of the main routes humans use to understand the world (Glezer, 1995). Implications for psychopathology (in terms of positive symptoms, subjective experience or behavior disruptions) are critically discussed.
Notably, it offers a holistic and convincing explanation for the perceptual changes observed in schizophrenia that might be ideally tested using illusory paradigms, as well as potential paths to explore neural mechanisms. The Bayesian formalism considers perception as the optimal combination between sensory evidence and prior knowledge, thereby highlighting the interweaving of perceptions and beliefs. We propose an integration of these findings into a common hierarchical Bayesian inference framework.
The current paper aimed to review how illusions have been used to explore and reveal the core features of visual perception in schizophrenia from a psychophysical, neurophysiological and functional point of view. Although still debated, the hypothesis of a modified, and typically diminished, susceptibility to illusions in schizophrenia patients is supported by a growing number of studies. This makes illusion a valuable tool with which to explore normal perception and its impairments. Illusion, namely a mismatch between the objective and perceived properties of an object present in the environment, is a common feature of visual perception, both in normal and pathological conditions.
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