Our behavioral state constantly fluctuates. It does so both on long timescales throughout the sleep-wake cycle and on much faster, seconds-long timescales. These fluctuations profoundly impact how we perceive and act upon the world. Think, for example, of the difference between paying attention to and spacing out during a lecture. I am interested in understanding how brain states are controlled and the mechanisms by which they alter perception and decision-making. My work has focused on the impact of cholinergic projections from the basal forebrain on behavior and cortical activity patterns. Using a combination of behavior, multisite extracellular recordings, anatomical tracing and optogenetics, we have shown that these projections can bidirectionally modulate visual perception and coding in the visual cortex on subsecond timescales, such that increases in cholinergic activity result in better behavioral performance and enhanced sensory coding. They do so by decorrelating cortical activity and increasing the reliability of sensory-evoked activity, improving the signal-to-noise ratio of V1 responses. We have also shown through calcium imaging of genetically identified cell types that the effects of manipulating cholinergic projections from the basal forebrain are compatible with how the activity of their cell bodies is modulated by perceptual decision-making behavior.