Fast cyclic stimulus flashing modulates perception of bi-stable figure

Many experiments have demonstrated that the rhythms in the brain influence an initial information processing. We investigated whether the alternation rate of the perception of a Necker cube depended on the degree of synchronization between two streams of spikes, one stemming from an external flashing image and the other from the action of an internal impulse stream. Knowing how a flickering stimulus with a given frequency and duration affects the alternation rate of bi-stable perception we could estimate properties of the internal signal. As the internal spike frequency is difficult to control, we varied the frequency of the flicker stimulus. Our results show that the duration of the dominant stimulus perception depends on the frequency or duration of the flashing stimuli. The values of the stimuli, at which the changes of the duration of the perceived image was maximal, we have called ‘extremal’. While changing the flash duration, the extremal parameters repeated periodically at 4ms intervals. Increasing the duration of the extremal stimuli by less than 4 ms shortens the duration of the dominant stimulus perception. Hence we may conclude that it is not the stimulus duration but the accurate coincidence (timing) of the moments of switching on of external stimuli to match the internal stimuli which explains our experimental results.