Cortical neurons show irregular spiking activity during rest and sensory stimulation. This could mean that the cortex is in an asynchronous cortical state in which the membrane potential (Vm) is just below the spike threshold and irregular spiking results from converging uncorrelated inputs, or that irregular spikes are due to infrequent correlated inputs that induce large Vm fluctuations. The authors assessed these possibilities using intracellular recordings in the visual cortex of monkeys performing a visual fixation task. The recordings showed that during fixation (in the absence of visual stimulation), the cortex is in a synchronous state and that it shifts to an asynchronous, high-conductance state during visual stimulation.