Brightness illusions evoke pupil constriction and a visual cortex response in rats

Luminance causes the pupillary light response (PLR), a constriction of the pupil. Surprisingly, mere illusions of brightness evoke a pupil constriction in humans. Since brightness illusions depend on scene and context, this suggests that high-level visual processing has a top-down influence onto the brainstem neurons controlling the PLR. The influence of high-level vision on this brainstem-mediated reflex is thought to be restricted to primates that perform foveal fixation. Here, we report that the same brightness illusions that evoke a pupil constriction in humans also do so in rats (N = 14). This response occurred only for stimuli that matched the spectral sensitivity of the rat retina. We recorded 32-channel cortex-wide EEG and found that illusory brightness stimuli also evoked a larger primary visual cortex event-related potential. This neural response preceded pupil constriction by ∼335 msec. Thus, primary visual cortex may contribute to the PLR evoked by passive viewing of brightness illusions. Our results resolve whether this phenomenon is confined to primates by showing that it can also be found in rats