Serotonergic modulation of local network processing in V1 mirrors previously reported signatures of local network modulation by spatial attention.

Sensory processing is influenced by neuromodulators such as serotonin, thought to relay behavioral state. Recent work has shown that the modulatory effect of serotonin itself differs with the animal's behavioral state. In primates, including humans (Beliveau et al., 2017), the serotonin system is anatomically important in the primary visual cortex (V1) (Jacob and Nienborg, 2018). We previously reported that in awake fixating macaques, serotonin reduces the spiking activity by decreasing response gain in V1 (Seillier et al., 2017). But the effect of serotonin on the local network is unknown. Here, we simultaneously recorded single-unit activity and local field potentials (LFP) while iontophoretically applying serotonin in V1 of alert monkeys fixating on a video screen for juice rewards. The reduction in spiking response we observed previously is the opposite of the known increase of spiking activity with of spatial attention. Conversely, in the local network (LFP) the application of serotonin resulted in changes mirroring the local network effects of previous reports in macaques directing spatial attention to the receptive field (Chalk et al., 2010). It reduced the LFP power and the spike-field coherence, and the LFP became less predictive of spiking activity, consistent with reduced functional connectivity. We speculate that together, these effects may reflect the sensory side of a serotonergic contribution to quiet vigilance: the lower gain reduces the salience of stimuli to suppress an orienting reflex to novel stimuli while at the network level, visual processing is in a state comparable to that of spatial attention. This article is protected by copyright. All rights reserved.