The Ebbinghaus illusion magnitude was shown to negatively correlate with V1 cortex size (de Haas, Kanai, Jalkanen, & Rees,
2012; Schwarzkopf & Rees,
2013; Schwarzkopf, Song, & Rees,
2011). However, whether this conclusion extends to other visual illusions may be questioned given the large individual differences we found. Our experiments tested a battery of illusions rather than investigating the mechanisms of one illusion as is common practice (but see Coren et al.,
1976; Thurstone,
1944). The factor structure is not only sparse for illusions but also for vision in general. For example, a distinct factor structure was found for contrast: sensitivities correlated between 0.2 and 0.4 c/°, between 0.4 and 1.2 c/° and between 1.2 to 3 c/°, but correlations were weak between these different ranges (Peterzell,
2016; Peterzell, Schefrin, Tregear, & Werner,
2000). Similarly, Emery and colleagues (
2017a,
2017b) observed several small factors underlying individual differences in hue scaling. In addition, Bosten and Mollon (
2010) found “no noteworthy general trait of susceptibility” (p. 1663) for contrast perception. When comparing different spatial tasks, such as bisection discrimination and Vernier offset discrimination, only low correlations were found (Cappe et al.,
2014). Likewise, there was little evidence for a common factor for oculomotor tasks (Bargary et al.,
2017) and for binocular rivalry and other bistable paradigms (Brascamp et al.,
2018; Cao et al.,
2018; Wexler,
2005). Chamberlain, Van der Hallen, Huygelier, Van de Cruys, and Wagemans (
2017) also showed poor evidence for a common factor for local and global visual processing. In addition, the effects of priors in perceptual tasks seem not to follow a single mechanism (Tulver, Aru, Rutiku, & Bachmann,
2019). Hence, these studies found very specific factors, similar to the very specific factors we found for visual illusions, and are rather arguing against a general factor for vision as proposed previously (e.g., Halpern, Andrews, & Purves,
1999). It appears that even studies that had a narrowly defined hypothesis by including several tasks that tap into a specific functional ability or theoretical construct of perception have often not succeeded in finding evidence to support the existence of a stable factor in perception (Tulver,
2019).