Superior colliculus encodes visual saliency before the primary visual cortex

Theories of visual attention postulate the existence of a saliency map that guides attention/gaze toward the most visually conspicuous stimuli in complex scenes. This study compared saliency coding in the two dominant visual gateways: the primary visual cortex (V1) and the evolutionarily older visual system that exists in the midbrain superior colliculus. Our results show that neurons in the superficial visual layers of the superior colliculus (SCs) encoded saliency earlier and more robustly than V1 neurons. This was surprising, because the dominant input to the SCs arises from V1. This result is in line with models that place a feature processing stage (V1) before the feature-agnostic saliency map in SCs.