Journal Section: Behavior/systems/cognitive 1 2 Title: Suppression of Cortical Neural Variability Is Stimulus-and State-dependent 3 4 Abbreviated Title: Suppression of Variability Is Stimulus-and State-dependent 5 6

38 39 Internally-generated, spontaneous activity is ubiquitous in the cortex, yet it does 40 not appear to have a significant negative impact on sensory processing. Various 41 studies have found that stimulus onset reduces the variability of cortical 42 responses, but the characteristics of this suppression remained unexplored. By 43 recording multiunit activity from awake and anesthetized rats, we investigated if 44 and how this noise suppression depends on properties of the stimulus and on 45 the state of the cortex. In agreement with theoretical predictions, we found that 46 the degree of noise suppression in awake rats has a non-monotonic dependence 47 on the temporal frequency of a flickering visual stimulus, with an optimal 48 frequency for noise suppression around 2Hz. This effect cannot be explained by 49 features of the power spectrum of the spontaneous neural activity. The non50 monotonic frequency dependence of the suppression of variability gradually 51 disappears under increasing levels of anesthesia, and shifts to a monotonic 52 pattern of increasing suppression with decreasing frequency. Signal-to-noise 53 ratios show a similar, although inverted, dependence on cortical state and 54 frequency. These results suggest the existence of an active noise suppression 55 mechanism in the awake cortical system that is tuned to support signal 56 propagation and coding. 57