In the new issue of Seed, Douglas Hofstadter talks about “strange loops” – his term for patterns of level-crossing feedback inside some medium (such as neurons) – and their role in consciousness. Likewise, Gerald Edelman has talked about how a “reentrant dynamic core” of neural activity could tightly integrate large groups of neurons through positive feedback cycles. Similarly, many view interactions among neural oscillations as a candidate mechanism for the formation of consciousness – such oscillations can perform abstract computations (as in liquid state machines) and can interact with one another through multiplexing mechanisms (in which, for example, a certain rhythm of neural firing might have its periodicity or phase locked to other rhythms). This theory of nested oscillations and cross-frequency phase coupling is controversial yet has substantial supporting evidence, reviewed here.
Many theories of consciousness emphasize the importance of gamma rhythms: they are prominent in human cortex, are attenuated by sleep and anesthesia, and are sometimes argued to integrate information over large cortical distances (as required by the dynamic core theory). In contrast, a new article by Palva & Palva discusses the potential role of alpha oscillations (~8-13 Hz) in consciousness.
The authors review how alpha band oscillations are often thought to reflect “cortical idling” or inhibition, based on the fact that power in these frequencies decreases with eye opening, alertness, and visual stimulation. But more recent evidence shows that alpha band oscillations increase during “internal” tasks such as mental calculation, mental imagery, and during the retention period of working memory tests, suggesting that the idling and inhibition hypotheses are unlikely to be correct – after all, you’re certainly not merely “idling” when performing mental arithmetic!
More recent work provides a detailed view of how alpha-band oscillations may contribute to cognition. Palva & Palva review evidence that large amplitude alpha oscillations may actually perform a different function than lower-amplitude oscillations in the same frequency band. In a visual search task, for example, alpha-band oscillations are dampened in active visual cortex and enhanced in inactive regions of visual cortex, but both small and large alpha band activity prior to the onset of the visual stimulus was correlated with better subsequent performance! One could conclude that these alpha-band oscillations reflect preparation for the task by generally “calming the waters” in cortex (either through alpha phase locking or alpha amplitude suppression) so that activity due to the upcoming visual stimulus can be readily detected
Palva and Palva suggest that the phase dynamics of alpha oscillations are particularly important for consciousness, because of the way alpha may interact with theta (4-7 Hz), beta (14-30 Hz) and gamma (30-50 Hz) oscillations. For example, the authors observed that “ongoing alpha band oscillations … phase lock selectively to weak somatosensory stimuli that become consciously perceived” as do gamma oscillations, whereas beta and theta phase locking does not appear to be selective for consciously-perceived objects.
Palva & Palva previously found cross-frequency phase coupling in every frequency band from delta to gamma, and that this coupling was stronger during mental calculation as well as high working memory load – in particular, the strength of alpha-gamma coupling. The authors argue that a variety of behaviors are synchronized at alpha rhythms, including human serial recognition/categorization speed, the timing of illusory motion reversals in the wagon wheel illusion, discrimination of odors by rats, and phasic muscular activity (including “alpha tremor“).
Finally, the authors consider that alpha band oscillations may be related to “closed” thalamocortical gates, as in some computational models of working memory, but ultimately warn against this hypothesis based on two facts: thalamic burst firing seems to convey information effectively and may be used in signal detection; and a recently discovered form of thalamic burst firing is triggered by glutamatergic activity and seems important for neocortical alpha oscillations. Both of these findings suggest inaccuracies in a one-to-one mapping between alpha power and a closed thalamic gate.