Internal gain modulations, but not changes in stimulus contrast, preserve the neural code

Neurons in primary visual cortex (V1) are strongly modulated both by stimulus contrast and by fluctuations of internal inputs. An important question is whether population codes are preserved under these conditions. Changes in stimulus contrast are thought to leave population codes invariant, whereas the effect of internal gain modulations remains unknown. To address these questions we studied how the direction-of-motion of oriented gratings is encoded in layer 2/3 of mouse V1. Surprisingly, we found that, because contrast gain responses across cells are heterogeneous, a change in contrast alters the information distribution profile across cells leading to the failure of contrast invariance. Remarkably, internal input fluctuations that cause commensurate firing rate modulations at the single-cell level, do respect population code invariance. These observations have important implications for visual information encoding, and argue that the brain strives to maintain the stability of the neural code in the face of fluctuating internal inputs.