Perceptual cycles

Abstract

Various pieces of experimental evidence using both psychophysical and physiological (EEG) measurements have lead us (and others) to conclude that at least certain aspects of visual perception and attention are intrinsically rhythmic. For example, in a variety of perceptual and attentional tasks, the trial-by-trial outcome was found to depend on the precise phase of pre-stimulus EEG oscillations in specific frequency bands (between 7 and 15Hz). This suggests that there are “good” and “bad” phases for perception and attention; in other words, perception and attention proceed as a succession of cycles. These cycles are normally invisible, but in specific situations they can be directly experienced as an illusory flicker superimposed on the static scene. The brain oscillations that drive these perceptual cycles are not strictly spontaneous, but can also be modulated by visual stimulation. Therefore, by manipulating the structure of the stimulation sequence (e.g. white noise), it is possible to control the instantaneous phase of the relevant perceptual rhythm, and thereby ensure that a given target will be perceived (if presented at the proper phase) or will go unnoticed (at the opposite phase). Better, by taking into account individual differences in oscillatory responses, we can even tailor specific stimulus sequences with an embedded target that can only be perceived by one observer, but not another –a form of “neuro-encryption”.