ReviewEEG alpha oscillations: The inhibition–timing hypothesis
Section snippets
Introduction: basic principles underlying the inhibition–timing hypothesis
Osocillations reflect rhythmic changes in the (relative) level of depolarization in the (dendritic and somatic) membrane potentials of masses of neurons. Consequently, they reflect phases of low versus high excitability. The basic principle (shown in Fig. 1) can be illustrated by considering the phase of oscillatory activity together with the level of excitation in excitatory neurons. For simplicity, we assume that oscillatory activity is induced by inhibitory cells and reflects rhythmic
Alpha desynchronization (ERD) as a functional correlate of brain activation
The well-known alpha response – power suppression during eyes opening (described since the early days of EEG research, cf. Berger, 1929) – suggests that light stimulation (bottom-up processing) is responsible for the decrease of the large amplitudes which can be observed particularly at posterior recording sites during closed eyes. Most interestingly, this interpretation is questioned by the simple fact that alpha suppression can also be observed solely in response to eyes opening (a top-down
The timing aspect
As discussed in Section 2, an increase in rhythmic activity results in two different effects, in a general decrease in firing rate and an increase in rhythmic discharges (cf. Figs. 1A with B). We assume that the first effect reflects inhibition as discussed in Section 3. The second effect, however, underlies the timing of neuronal activity and will be discussed in this section.
The crucial aspect – as outlined in Fig. 1 – is that an increase in inhibition (driving an oscillation) is accompanied
The physiological basis of alpha oscillations
Since several decades, it was suggested that the thalamus plays a key role in the generation of cortical oscillations and sleep spindles in particular (Andersen and Andersson, 1968). Thus, it was tempting to assume that alpha, as the dominant oscillation in the human scalp EEG, also is generated by thalamic nuclei. This view has led to the idea that the cortex might be passively driven by a ‘thalamic pacemaker’ (cf. Basar et al., 1997, for a review). The seminal work by Lopes da Silva et al.,
Elaboration of the alpha inhibition–timing (AIT) hypothesis and critical questions
We have focused on two basic aspects, one referring to the state, the other to the timing of information processing. With respect to the first aspect, experimental evidence shows consistently that ERD reaches a maximum during a time window in which (conscious) task-related processes take place, whereas ERS can be observed when certain aspects of task performance are under top-down control. Although both cases reflect states of information processing, their functional difference can be best
Conclusions
The general line of our argumentation is that alpha – like other oscillations – is an active phenomenon but reflects – in contrast to other oscillations, a certain type of top-down process. The active role of alpha is seen in a mechanism that also may underlie the functional role of other oscillations: Synchronization in the alpha frequency range helps neurons in distributed networks to effectively activate common target cells. We think that this timing mechanism plays an important role in the
Acknowledgment
This research was supported by the Austrian Science Fund (FWF), P-16849-B022.
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