Mouse primary visual cortex neurons respond to the illusory “darker than black” in neon color spreading

Illusions are a powerful tool for studying the single neuron correlates of perception. Here, we introduce the neon color spreading (NCS) illusion in mice and report the neuronal correlates of illusory brightness, which has heretofore only been studied using human fMRI. We designed a novel NCS paradigm to evoke the percept of an illusory drifting grating and analyzed the activity of 520 single units in the mouse primary visual cortex (V1). A substantial proportion of V1 single units (60.5%) responded to illusory gratings with direction tuning matched to their preferred direction, which was determined using physically presented luminance-defined gratings (LDG). Moreover, by presenting LDG gratings with a 180° phase shift relative to NCs gratings, we show that spatial phase tuning shifted 180° for most single units. This finding conclusively demonstrates that V1 single units respond to illusory brightness. Using this novel mouse paradigm, we show that responses to illusory gratings have a lower magnitude and are delayed relative to physical gratings. We determined where V1 single units fell in the V1 cellular hierarchy (based on their susceptibility to surround suppression, their putative classification as interneuron or pyramidal neuron, and designation as a simple or complex cell) and found that higher-level V1 single units are more responsive to NCS stimuli. These findings resolve the debate of whether V1 is involved in illusory brightness processing and reveal a V1 hierarchical organization in which higher-level neurons are pivotal to the processing of illusory qualities, such as brightness