Propagating waves of cortical activity have been observed in several species under anaesthesia; however, the extent to which they occur in awake animals was unclear. Here, the authors present a method to detect propagating waves using data from single-trial voltage-sensitive dye imaging in awake monkeys. They show that propagating waves are initiated in both the primary and secondary visual cortices in response to sensory input. Using network modelling, they propose that the horizontal fibre network may be crucially involved in the propagation of these waves.