Brain activity during wakefulness is characterized by fluctuations in neural responses at different time scales. Whether these fluctuations play any role in modulating the coding of sensory information and the accuracy of behavioral responses is poorly understood. By simultaneously recording the responses of multiple neurons I will show that slow changes in local population synchrony in monkey visual cortex impair the coding of sensory information and perceptual performance. These changes also occur in executive areas, such as prefrontal cortex, while monkeys freely explore their environment during foraging. However, while slow fluctuations in population synchrony are detrimental for sensory coding, they play a beneficial role at more rapid time scales. Indeed, by simultaneously recording visual cortical populations in multiple areas (V1 and V4) we recently discovered that the precise temporal coordination between the spikes of three of more neurons carries information about perceptual reports in the absence of firing rate modulation. Altogether, these results demonstrate differential impacts of synchrony in local cortical networks at slow and rapid time scales.