Chubb Illusion

The Chubb illusion is a visual perception effect in which the apparent contrast of an object varies substantially, for most viewers, depending on the contrast of the field surrounding it, even when the object itself never actually changes. In its classic demonstration, a patch of low-contrast texture that is embedded within a uniform, low-contrast background appears to have noticeably higher contrast than an identical low-contrast patch that has instead been placed within a high-contrast texture field, even though the central patch is exactly the same in both cases. Charles Chubb and his colleagues first documented the effect in 1989, through experiments in which subjects rated how sharp or strong the contrast of the central patch appeared to be across different combinations of foreground and background. The researchers Dale Purves and Beau Lotto proposed an empirical explanation for the effect in 2001, based on the idea of imperfect transmittance, the everyday experience of viewing objects through an imperfect medium such as fog or haze; on this account, the brain interprets the visual scene as though it were being viewed through a semi-transparent medium that is itself altering the apparent contrast of what lies behind it, rather than attributing the effect purely to low-level neural mechanisms. Researchers have used the Chubb illusion and related effects to study how the human visual system processes contrast and context, including research finding that people with schizophrenia show reduced sensitivity to this kind of contextual visual effect.