Perceptual Cycles

Brain rhythms not only modulate but also drive perception, resulting in striking illusions of flicker and reverberation. Recently popularized single-trial analyses of electrophysiological signals can be used to probe the causal influence of spontaneous brain states (such as the phase of oscillatory cycles) on perception. Spectral analyses applied to ‘high temporal resolution’ behavioral measurements reveal behavioral, perceptual and attentional oscillations. The partiality to the ‘standard’ oscillatory nomenclature (delta, theta, alpha, beta, gamma) and the community's fixation on specific rhythms (alpha, gamma) are becoming less prominent. As a result, perceptual rhythms are found in a wide range of frequencies (so far, often restricted to below 15 Hz). Alpha (∼10 Hz) and theta (∼7 Hz) are the most commonly reported frequencies for perceptual cycles. Brain function involves oscillations at various frequencies. This could imply that perception and cognition operate periodically, as a succession of cycles mirroring the underlying oscillations. This age-old notion of discrete perception has resurfaced in recent years, fueled by advances in neuroscientific techniques. Contrary to earlier views of discrete perception as a unitary sampling rhythm, contemporary evidence points not to one but several rhythms of perception that may depend on sensory modality, task, stimulus properties, or brain region. In vision, for example, a sensory alpha rhythm (∼10 Hz) may coexist with at least one more rhythm performing attentional sampling at around 7 Hz. How these multiple periodic functions are orchestrated, and how internal sampling rhythms coordinate with overt sampling behavior, remain open questions. Brain function involves oscillations at various frequencies. This could imply that perception and cognition operate periodically, as a succession of cycles mirroring the underlying oscillations. This age-old notion of discrete perception has resurfaced in recent years, fueled by advances in neuroscientific techniques. Contrary to earlier views of discrete perception as a unitary sampling rhythm, contemporary evidence points not to one but several rhythms of perception that may depend on sensory modality, task, stimulus properties, or brain region. In vision, for example, a sensory alpha rhythm (∼10 Hz) may coexist with at least one more rhythm performing attentional sampling at around 7 Hz. How these multiple periodic functions are orchestrated, and how internal sampling rhythms coordinate with overt sampling behavior, remain open questions. retrospective formation of a continuous motion percept, based on discrete shifts of stimulus position. This process, dependent on attention, could contribute to the stability of visual experience despite rhythmic sampling. the periodicity of attention processes turns the classic ‘spotlight’ of attention into a ‘blinking spotlight’ that can rhythmically sample a single location, or rapidly switch between multiple targets. an ‘extreme’ form of rhythmic perception that temporally structures perceptual events into disjoint epochs (or frames or snapshots). Discrete perception can be defined as a rhythmicity of temporal parsing. recording techniques providing a real-time view of large-scale brain activity, and at a sufficient temporal resolution to measure oscillations at 40 Hz or higher. a process by which brain oscillations follow a rhythmic temporal structure present in the sensory environment. Entrainment can be opposed to spontaneous oscillations that reflect the inherent rhythmic organization of perception. the rate of repetition of a brain oscillation, in cycles per second or Hz. the average brain response to a unitary increase of sensory input intensity (e.g., luminance). Oscillatory impulse response functions can produce perceptual reverberation phenomena. the consequence of rhythmic perception is to produce perceptual cycles, in other words oscillatory cycles whose phase directly influences perceptual abilities (also called epochs, frames, or snapshots in the context of discrete perception) instantaneous position of a brain oscillation along its oscillatory cycle, expressed in radians (circular variable). By convention, the oscillatory peak corresponds to zero phase and the trough to a phase of π radians. sampling or processing of sensory inputs that is rhythmically modulated following the phase of one or more brain rhythms (also called cyclic or periodic perception) in the context of discrete perception, a perceptual cycle can be understood as a ‘snapshot’ (or an epoch, or frame), the temporal unit of perceptual experience. All events and attributes within a snapshot are experienced as simultaneous (including temporally defined attributes such as visual motion, flicker, or auditory pitch). erroneous interpretation of the direction of a periodically evolving stimulus by a system performing discrete sampling. Aliasing occurs when the rate of the periodic stimulus exceeds the Nyquist frequency (the theoretical sampling limit) of the system. the determination of temporal relations between events (simultaneity, asynchrony, temporal order). Discrete perception can be defined as a rhythmicity of temporal parsing. a form of stroboscopic perception caused by some recreational and prescription drugs. Moving objects are perceived together with a series of multiple discrete ‘copies’ of the object, trailing in their wake. Trailing could happen either because the drugs drive our normally subliminal brain oscillations above the perceptual threshold, or because they inhibit the neural mechanisms that normally serve to conceal the perceptual cycles.