Visual Illusions and Psychopathology

Neurocognitive deficits have been widely investigated using visual illusions as illusory paradigms demonstrate different contextual influences on visual perception (Chen et al., 2015; Corbett & Enns, 2006; Roberts et al., 2005). Visual illusions are thus valuable tools for revealing core features of pathological conditions.

Schizophrenia

Atypical integration processes in schizophrenia have been examined using visual illusions (Clifford, 2014; Palmer et al., 2018; Thakkar et al., 2020). Indeed, several studies have demonstrated a diminished susceptibility to visual illusions amongst patients with schizophrenia relative to healthy controls (King et al., 2017; Notredame et al., 2014). People with schizotypal personality traits perform significantly better at making accurate judgements of contrasts under contextual suppression (Dakin et al., 2005), are less prone to the perception of illusory motion (Crawford et al., 2010), and are less susceptible to visual size illusions (Uhlhaas et al., 2004).

While individuals along the schizophreniform spectrum display apparent resistance to perceptual distortions, they concurrently experience a wide range of cognitive deficits (e.g., working memory, object naming, concentration, Liddle, 1987). This supports the idea that schizophrenia might be better characterized by a subset of specific processing abnormalities, rather than by a generalized neurocognitive impairment (Dakin et al., 2005; Tibber et al., 2013). Specific perceptual anomalies in schizophrenia have also been highlighted using different classes of visual illusions that tap on distinct neurocognitive processes. For example, while patients with schizophrenia show a heightened resistance to contrast illusions, they are indistinguishable from healthy controls in judging brightness illusions (Tibber et al., 2013), arguing against a broad deficit in low-level perceptual integration since not all illusions are affected. In fact, other studies have emphasized the role of high-level perceptual deficits in schizophrenia (e.g., problems in contextual processing that manifest as greater resistance to the Ebbinghaus illusion, Massaro & Anderson, 1971; Uhlhaas et al., 2006).

However, the lack of consistency in the tasks’ methodologies has posed a significant challenge to advancing our theoretical understanding of the role of visual perception and reality construction in the psychopathology of schizophrenia (King et al., 2017). Specifically, the finding of an increased perceptual accuracy towards high-level illusions has failed to replicate in several other studies (e.g., Parnas et al., 2001; Spencer & Ghorashi, 2014; Yang et al., 2013), and other kinds of illusions (e.g., the Poggendorff illusion) have simply not been sufficiently tested (Kantrowitz et al., 2009).

Autistic Spectrum Disorder

Individuals with autistic spectrum disorder (ASD) comprise another clinical group that demonstrates a similar immunity to perceptual biases, supporting the existence of difficulties in global processing and, conversely, an enhanced preference for idiosyncratic and detailed information (Happe, 1996). Hence, individuals with ASD appear as protected as the clinical schizophrenia population against the contextual influences of illusions in biasing perception, allowing them to perceive elements accurately in a local fashion (Gori et al., 2016; referred to as a ‘weak central coherence,’ Mitchell et al., 2010; Walter et al., 2009). Some work has also been successful in delineating the underlying cognitive mechanisms employed by different illusions, revealing that autistic traits in a typical population were related to greater resistance to the Müller-Lyer illusion, but not the Ebbinghaus or Ponzo illusions (Chouinard et al., 2013).

One possible explanation for this dissociation relates to the extent of global processing engaged by the illusions, with the Müller-Lyer illusion (a within-object illusion where contextual elements and the target stimulus are physically joined) requiring more cognitive resources for the local binding of features than the Ebbinghaus and Ponzo illusions (a between-object illusion where contextual elements and the target stimulus are physically separate, Ben-Shalom & Ganel, 2012).

Regardless, findings of illusion resistance amongst ASD face similar low replicability rates as with the literature on schizophrenia, even when the same illusion tasks are used (Hoy et al., 2004; Ropar & Mitchell, 1999). These mixed findings are attributed not only to the heterogenous nature of ASD as a clinical population, but also to the large variability in experimental instructions (e.g., asking whether lines: “looked the same length”; vs. “were the same length,” see Happe and Frith (2006)) and the subsequent understanding of the task requirements (Chouinard et al., 2013).

References

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