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Our visual system to seamlessly informs us about and guide us through the outside world so that we typically hardly notice its actions. However, our brain has limited processing capacity, and must filter visual input to extract the more biologically meaningful features from the totality of the visual scene. Optical illusions, in which a perception about an external scene does not match the physical reality, emerge from this filtering process. Illusions thus reveal a dissociation between the physical world and our perception of it, allowing a glimpse into the workings of the mind.

The Ebbinghaus illusion is a widely studied optical effect in which the perceived size of a circle is affected by circles of a different size surrounding it. In this illustration, most people perceive the orange circle in the right hand group as larger than the orange circle on the left. This perception is variable among individuals, with the strength of the Ebbinghaus illusion reflecting both developmental and environmental influences. The effect is absent in young subjects and in some individuals with autism, and it also varies in strength among subjects from different cultures. It can also be abolished entirely if the central circle is an object of known scale, such as a coin.

A report from last month’s Nature Neuroscience (Schwarzkopf et al.) reported an intriguing correlation between the strength of the Ebbinghaus illusion and individual functional variation in the brain. Visual information enters the body through the retina, where it is partially processed and relayed toward the primary visual cortex (V1) at the rear of the brain. The surface area devoted to V1 is known to vary by up to threefold in the general population.

Schwarzkopf et al hypothesized that the individual variations in V1 area might contribute to variability in optical perception. To test this idea, they measured the strength of the Ebbinghaus illusion in large numbers of volunteers and then performed functional magnetic resonance imaging on each to measure the area of V1. They found a strong negative correlation between V1 area and the extent to which volunteers perceived the size illusion – the smaller the V1 area, the more pronounced the visual illusion. They found a similarly strong negative correlation between V1 area and the perceptual strength of a second illusion — even though there was no inter-individual correlation in the strengths of the two illusions. There was weak or no correlation between illusion perceptual strength and the surface area of other cortical regions.

Thus the strength of the Ebbinghaus illusion correlates with a specific neuronal property, the functionally defined primary visual cortex, suggesting that variability in V1 could affect conscious experience of the physical world. Since the strength of the Ebbinghaus illusion varies with age and cultural background, this observation further suggests that usage can modify perception or even aggregate neuronal behavior at a very basic level. In terms of the anatomical basis for the correlation observed, the authors speculate that a larger V1 might allow finer-grained perception, reducing crosstalk effects such as those which create this illusion.

Illustration obtained from Wikimedia Commons, as uploaded by user Fibonacci

Schwarzkopf DS, Song C, Rees G. The surface area of human V1 predicts the subjective experience of object size. Nat Neurosci 14:28-30.