One of the main drawbacks of the fixation-based gaze interfaces is that they are unable to distinguish top-down attention (or selection, a gaze with a purpose) from stimulus driven bottom-up attention (or navigation, a stare without any intentions) without time durations or unnatural eye movements. We found that using the bistable image called the Necker's cube as a button user interface (UI) helps to remedy the limitation. When users switch two rivaling percepts of the Necker's cube at will, unique eye movements are triggered and these characteristics can be used to indicate a button press or a selecting action. In this paper, we introduce (1) the cognitive phenomenon called "percept switch" for gaze interaction, and (2) propose "perceptual switch" or the Necker's cube user interface (UI) which uses "percept switch" as the indication of a selection. Our preliminary experiment confirms that perceptual switch can be used to distinguish voluntary gaze selection from random navigation, and discusses that the visual elements of the Necker's cube such as size and biased visual cues could be adjusted for the optimal use of individual users.