Synchronous neural oscillations and cognitive processes

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Abstract

The central problem for cognitive neuroscience is to describe how cognitive processes arise from brain processes. This review summarizes the recent evidence that synchronous neural oscillations reveal much about the origin and nature of cognitive processes such as memory, attention and consciousness. Memory processes are most closely related to theta and gamma rhythms, whereas attention seems closely associated with alpha and gamma rhythms. Conscious awareness may arise from synchronous neural oscillations occurring globally throughout the brain rather than from the locally synchronous oscillations that occur when a sensory area encodes a stimulus. These associations between the dynamics of the brain and cognitive processes indicate progress towards a unified theory of brain and cognition.

Section snippets

EEG oscillations and cognitive processes

The EEG varies with activity, both in humans and other animals, and particularly with the sleep-wakefulness cycle. Moreover, spectral power at various frequencies (Box 2) changes with age; alpha power increases as children mature whereas theta and delta power decrease. These changes are linked to the more general increase in cognitive competence with maturation, whereas the reverse changes signal declining mental abilities in old age [11]. Alpha waves have been apparent in EEG recordings ever

Theta in memory encoding

Although theta oscillations are apparent in lower animals such as rats, they are seldom seen directly in EEG recordings from humans and it has been difficult to understand what the classically-observed increases in theta power meant [13]. Recently, however, intracranial EEG (iEEG) recordings made from epileptic patients have revealed strong theta oscillations from many areas of the human brain 14, 15. In these experiments, periods in which theta oscillations were apparent were more frequent

Alpha and gamma and attention

Attention is a dynamic process. The attention ‘spotlight’ typically moves from one location in space to another over a period of less than a second to a few seconds, like the spotlight at a variety show. Very general dynamical models of this process have been proposed [33], and much is known about the attention orienting process on the scale of hundreds of milliseconds [34]. The cyclic organization of infants' attention at around 0.5 Hz to 2 Hz is closely related to how efficiently they process

Gamma and consciousness

Crick and Koch suggested over a decade ago that synchronous neural firing at the gamma frequency might be the neural correlate of visual awareness [53]. Since then several important studies have reported correlations between conscious awareness and synchronous neural activity at various frequencies. In one of these, the EEG was recorded while subjects viewed an ambiguous visual stimulus that could be perceived as either a face or as a meaningless shape [54]. When subjects reported seeing a

Synchronous neural oscillations and cognitive science

The research I have discussed supports the idea that the neural oscillations revealed by the EEG and the MEG are closely related to dynamic processes of cognition. They are consistent with the idea that fundamental cognitive processes arise from the synchronous activity of neurons in the brain. Moreover, specific oscillations can be identified with particular cognitive processes: theta and gamma rhythms with memory encoding and retrieval, alpha and gamma rhythms with attentional suppression and

Acknowledgements

This work was supported by a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada to the author. I thank the anonymous reviewers and the editor for useful critical comments on an earlier version.

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