Trends in Cognitive Sciences
Is perception discrete or continuous?
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.
References (60)
- et al.
Intermittency in reaction time and perception, and evoked response correlates of image quality
Acta Psychol.
(1969) Evidence of internal clocks in the human operator
Acta Psychol.
(1967)- et al.
Iconic persistence and perceptual moment as determinants of temporal integration in vision
Vision Res.
(1978) The relation of decision time to brainwave frequency and to age
Electroencephalogr. Clin. Neurophysiol.
(1964)Time as coding space?
Curr. Opin. Neurobiol.
(1999)- et al.
Oscillatory gamma activity in humans and its role in object representation
Trends Cogn. Sci.
(1999) Theta returns
Curr. Opin. Neurobiol.
(2001)- et al.
Different frequencies for different scales of cortical integration: from local gamma to long-range alpha/theta synchronization
Int. J. Psychophysiol.
(2000) Oscillatory and non-oscillatory synchronizations in the visual cortex and their possible roles in associations of visual features
Prog. Brain Res.
(1994)Temporal mechanisms in perception
Int. Rev. Neurobiol.
(1994)