Parvalbumin-expressing interneurons improve sensory discrimination by shaping noise geometry in primate V1

The ability to discriminate sensory stimuli is fundamental to sensory-guided behavior. Sensory discrimination by neural populations is constrained by the magnitude of trial-to-trial spiking variability, or noise, and its geometric alignment with the stimulus encoding directions of the population response. Computational models predict that cortical inhibition shapes the noise geometry, but causal evidence from primate cortex is lacking. We used optogenetic stimulation of parvalbumin-expressing inhibitory interneurons (PV-cells) combined with high-density extracellular recordings in primary visual cortex of awake marmoset monkeys to test this prediction. Stimulating PV-cells improved the discriminability of V1 population responses, without expanding the stimulus-related signal magnitude. Instead, PV-cell activation compressed shared trial-to-trial variability and rotated it away from the stimulus-coding direction. Our results establish a causal role for inhibition in shaping the geometry of neural population responses to improve sensory discrimination in the primate visual cortex.