Review
Perceptual Cycles

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Trends

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.

Section snippets

Is Perception Discrete or Continuous?

Philosophers, psychologists and neuroscientists have long questioned whether the apparently continuous stream of our mental experience could in fact rely on a disjoint series of discrete ‘moments’ of experience [1] (a more detailed history is given in [2]) similar to the disjoint snapshots of a movie or video clip. My colleague Christof Koch and I evaluated this question more than 13 years ago in the same journal, tentatively concluding that, although hard evidence was clearly lacking at the

Rhythmic versus Discrete Perception

At first sight, it might be argued that rhythmic modulations of perception do not truly equate discrete perception (Figure 1A,B). A genuine discretization would imply that sensory and mental events are chunked into distinct epochs, with nothing in-between (Figure 1B); within each epoch, the passage of time is not directly experienced–only across epochs (note: this does not preclude encoding, within each epoch, a static representation of time-dependent sensory attributes such as visual motion or

Nature of Evidence for Perceptual Cycles

Brain rhythms occur at multiple frequencies, and therefore their perceptual consequences–the perceptual cycles–should display similar periodicities. This logic forces us to abandon the chimera of a unitary central sampling rhythm simultaneously affecting all aspects of perceptual experience, as postulated by early proponents of the discrete perception theory (who often placed the critical frequency in the ‘alpha’ range, around 10 Hz) [1]. Multiple perceptual cycles could in fact coexist in

Why Does the World Seem Continuous?

The perceptual cycles are not usually experienced in daily life–the world appears mostly continuous to us, except for the odd reverberation illusion (Figure 2A–C). The mechanisms responsible for this normal sense of continuity are unknown, but, as explained above, are likely to include apparent motion as well as the potential ‘echoing’ of visual information across successive cycles. What happens when these safeguard mechanisms fail? There are very few reported cases of akinetopsia, or patients

Periodicities Across Modalities

Although a majority of rhythmic perception studies have focused on the visual system (as we have, so far in this review), there are many corresponding reports of perceptual cycles in other sensory modalities. In the tactile domain, for example, EEG oscillatory phase at around 10 Hz was found to modulate the detection of threshold-level somatosensory stimuli [83]. MEG oscillatory phase at around 14 Hz also influenced temporal parsing of two successive tactile stimuli [84].

The investigation of

Is Temporal Parsing a Rhythmic Process?

Perceptual cycles were shown to affect nearly all aspects of perception and cognition. Their influence on temporal parsing mechanisms, however, is of special theoretical importance because it more directly supports the notion of discrete as opposed to rhythmic perception (Figure 1). Although this issue remained undecided for many years 3, 7, 124, recent evidence has accumulated for cyclic temporal parsing in vision. Parsing performance was affected by ongoing MEG and EEG phase around the alpha

Concluding Remarks and Future Directions

The inventory of findings presented here suggests that the study of discrete perception has recently switched gears. After conceding that there might not exist a single common sampling rhythm affecting all of our perceptions, but instead many simultaneous rhythms that periodically modulate various perceptual and cognitive functions in distinct modalities at independent rates, one begins to see perceptual rhythms (almost) everywhere (see, however, Box 3). The frequencies of these periodicities

Acknowledgments

The work described was funded by an European Research Council Consolidator Grant 614244 (P-CYCLES). I am indebted to past and present members of my lab for helping me, directly or indirectly, to put together an extensive list of bibliographic references. In addition, I wish to thank Leila Reddy for detailed comments on the manuscript.

Glossary

Apparent motion
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.
Blinking spotlight
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.
Discrete perception
an ‘extreme’ form of

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