The neurons that let us see what isn’t there

Illusions may be entertaining, but they also provide critical insights into how we perceive the world around us. Hyeyoung Shin, an assistant professor of neuroscience at Seoul National University, leads a groundbreaking study published in Nature Neuroscience, which identifies a unique set of neurons in the visual cortex, referred to as IC-encoders. This research highlights their essential function in the perception of visual illusions. The collaboration among researchers from the University of California, Berkeley, the Allen Institute in Seattle, and Seoul National University has revealed how our brains perceive edges that do not actually exist. A well-known example is the Kanizsa triangle, where three 'Pac-Man' shapes create the illusion of a bright white triangle. However, when obscured, it becomes clear that there is no real boundary, only a uniform background. For over two decades, neurophysiological studies involving primates and advanced imaging techniques in humans and mice have indicated that neurons in the primary visual cortex (V1) and other higher visual areas respond to both genuine and illusory contours. This suggests that when edges are absent from visual stimuli, the brain compensates by 'filling in' these gaps. This phenomenon poses a challenge to the traditional 'bottom-up' perspective of perception, which views the visual system merely as a passive recorder of retinal input. Instead, early stages of visual processing involve a dynamic interplay of local and distant neural signals, alongside the application of prior knowledge—like statistical patterns in the visual environment—to inform our perceptions. Essentially, the brain makes educated 'assumptions' about what we observe based on previous experiences. The latest findings pinpoint the specific neurons responsible for encoding these critical inferences, achieved through a combination of advanced brain imaging techniques and a novel causal testing method that involved direct stimulation of the neurons to assess their responses.