Is perception discrete or continuous?

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Abstract

How does conscious perception evolve following stimulus presentation? The idea that perception relies on discrete processing epochs has been often considered, but never widely accepted. The alternative, a continuous translation of the external world into explicit perception, although more intuitive and subjectively appealing, cannot satisfactorily account for a large body of psychophysical data. Cortical and thalamocortical oscillations in different frequency bands could provide a neuronal basis for such discrete processes, but are rarely analyzed in this context. This article reconciles the unduly abandoned topic of discrete perception with current views and advances in neuroscience.

Section snippets

Psychophysical evidence for discrete processes in visual perception

The concept of ‘discrete perception’ should not be confounded with that of an integration period (Box 1): whereas the latter only determines the temporal resolution of perception, the former is intrinsically linked to the process of perception itself. Discrete perception implies that two distinct events will be judged as simultaneous or sequential depending not only on the time interval between them, but also on their temporal relationship to some intrinsic discrete neuronal process.

Frequency and phase dependence of behavioral variables

From the early days of perception research, these postulated discrete psychological phenomena have been linked to brain oscillations, in particular the alpha rhythm 18, 19, in the 8–12 Hz frequency range. Studies relating behavioral periodicities to the EEG alpha rhythm have been more or less successful [19], depending on the particular aspect of the EEG under consideration. There is, in general, a consistent relationship between the frequency of the alpha rhythm and that of the behavioral

Multiplexing representations

Based partly on the usual observation of cortical oscillations in the hippocampus, and partly on the set size effects on RT (a constant increase of ∼25 ms per ‘to-be-remembered’ item) obtained during short-term memory tasks [36], Lisman and Idiart [37] proposed that cortical oscillations at two distinct frequencies could be used to simultaneously maintain several items in short-term memory (Fig. 3). The slow waves (4–12 Hz) would define the span of memory, on which the fast waves (30–60 Hz)

Conclusion

The backbone and originality of our argument rests on the idea that the temporal evolution of perception can provide a constrained framework to investigate the computations leading to awareness. If perception is discrete, and this remains to be clearly demonstrated, then one or more intrinsic neuronal mechanisms must reflect this organization, that is, must be global (i.e. able to link distinct areas or brain regions; this, however, has been disputed, e.g. [27]), quasi-periodic (i.e. possess at

Acknowledgements

The authors would like to thank D. Eagleman for critical comments on the manuscript. The authors' research is supported by the CNRS, the NSF, the NIMH, the NIH and the Keck Foundation.

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