In defence of empirical reductionism – Rejecting the hidden dualism of irreducible brain complexity

Illusions lay bare the startling fact that our perception of the world is an invention of the brain. Scientists therefore use illusions to study sensory processing. Popular science communication often describes illusions as revealing underlying brain mechanisms. However, such explanations are usually simplistic and sometimes completely wrong. Does the Ebbinghaus illusion (Figure 1A) demonstrate “size contrast,” whereby a target disc surrounded by larger/small circles appears smaller/larger than its true size? No – in most situations this illusion is in truth a perceptual underestimation of target size (Figure 1B). The illusion is better explained by a contour-interaction account rather than object size (Todorović and Jovanović, 2018). One problem with simple, seemingly intuitive explanations is that they ignore the brain’s complexity. Perceptual processing involves dozens of densely interconnected brain regions. The same neural process underlying the distance effects in the Ebbinghaus may be involved at multiple stages. Beyond our subjective experience, measuring illusions must also depend on the observer’s thoughts and how they report their percept. Like any behaviour, illusion measurements are the result of the whole system working together, from the eye’s optics over processing in visual cortex to the higher brain areas involved in making decisions and executing a button press (Figure 1C). Any explanation pinpointing a single neural locus creating a specific percept is therefore extremely implausible.