11/13/2006

A Candidate Neural Mechanism for Cross-Frequency Phase Coupling

In a recent article from the Journal of Neuroscience, authors Palva, Palva & Kaila present compelling evidence for the idea that neural oscillations in various frequency bands are temporally multiplexed.

The authors begin with a brief synthesis of the previous literature on synchronized oscillations, suggesting that beta and gamma waves seem to be related to active maintenance of information, while theta and alpha waves are involved in top-down modulation of information held online. Cross-frequency phase coupling among these different oscillations is thought by some to result in working memory.

Importantly, Palva et al distinguish between two types of cross-frequency phase coupling: n:m phase synchrony, which "indicates amplitude-independent phase locking of n cycles of one oscillation to m cycles of another oscillation," and "nested oscillations, which reflect the locking of the amplitude fluctuations of faster oscillations to the phase of a slower oscillation."

The authors performed MEG imaging on 17 subjects while they performed three tasks: the first two involves "active rest," in which subjects were told to actively clear their "visual and auditory fields" respectively, and the third task involved iterative mental arithmetic of two or three numbers. The MEG data was analyzed with Morlet wavelets, with the time-domain spread of wavelet transforms further reduced by a "finite-impulse response" filtering technique. Frequencies were said to show "phase coupling" if their peak frequency differed by a constant amount and if the phases of those oscillations were not randomly distributed. The authors also analyzed amplitude relationships between coupled frequencies.

The results from the active rest conditions showed that n:m phase synchrony existed between all frequency bands, either at ratios of 1:2 (between alpha and beta) or 1:3 (primarily between gamma and alpha). The locations of these oscillations differed as well: alpha-beta phase coupling occurred widely throughout cortex, whereas gamma-alpha phase coupling occurred primarily over occipitoparietal and somatomotor regions.

Results from the mental arithmetic task showed increased gamma-beta, beta-alpha, and gamma-alpha phase coupling relative to rest. Furthermore, the use of 3 digits instead of 2 in the mental arithmetic task was associated with enhanced gamma-alpha phase coupling. The amplitudes of theta band oscillations were significantly increased in the 3 digit condition, compared to the 2 digit condition, but only over prefrontal regions; throughout the rest of cortex, theta oscillations were actually suppressed.

The authors conclude with speculation on how these phase-coupled oscillations may arise from neural circuits (probably the question several of you are asking yourselves). They suggest that one type of Layer V pyramidal neuron may be particularly suited to gamma/alpha phase coupling, because it has dendrites that span all cortical layers. Excitatory input to the proximal and basal dendrites comes primarily from thalamic nuclei and from regions "lower" in the cognitive hierarchy, whereas the excitatory input to distal apical dendrites comes primarily from regions higher in the cognitive hierarchy. Burst firing of these cells is evoked only when input from both the distal and proximal dendrites arrive within 5ms of one another.

Much of this excitatory input converges on gamma-band rhythms, but the mechanisms underlying action potentials in distal dendrites (involving calcium) are rather slow and cannot fire much faster than 10 HZ (alpha). Thus, Palva et al. suggest these neurons may act as "phase-couplers" which could be important for attention and binding. Consistent with this speculation is the fact that subcortical projects from these L5 cells include the pulvinar and superior colliculus, areas thought to be important for early attentional processes.

The authors also mention fast rhythmic bursting cells, which show bistable spiking patterns: they can fire single spikes, or change into a burst firing pattern of very fast (300 Hz - aka "high gamma") action potentials clustering at gamma rhythms. Palva et al. note that these cells are well represented in thalamocortical loops.

Related Posts:
The Argument for Multiplexed Synchrony
High Gamma Modulation in Cortex

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