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. 2011 May 31:2:118.
doi: 10.3389/fpsyg.2011.00118. eCollection 2011.

Alpha oscillations and early stages of visual encoding

Wolfgang Klimesch  1 , Robert Fellinger, Roman Freunberger
Affiliations

Alpha oscillations and early stages of visual encoding

Wolfgang Klimesch et al. Front Psychol. .

Abstract

For a long time alpha oscillations have been functionally linked to the processing of visual information. Here we propose an new theory about the functional meaning of alpha. The central idea is that synchronized alpha reflects a basic processing mode that controls access to information stored in a complex long-term memory system, which we term knowledge system in order to emphasize that it comprises not only declarative memories but any kind of knowledge comprising also procedural information. Based on this theoretical background, we assume that during early stages of perception, alpha "directs the flow of information" to those neural structures which represent information that is relevant for encoding. The physiological function of alpha is interpreted in terms of inhibition. We assume that alpha enables access to stored information by inhibiting task-irrelevant neuronal structures and by timing cortical activity in task relevant neuronal structures. We discuss a variety findings showing that evoked alpha and phase locking reflect successful encoding of global stimulus features in an early post-stimulus interval of about 0-150 ms.

Keywords: alpha; knowledge system; memory; oscillations; perception.

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Figures

Figure 1
Figure 1
The event-related alpha power response is task, not stimulus dependent. Alpha power changes were determined relative to a pre-stimulus reference interval and were calculated as percentage or z-values. An increase in power is termed event-related synchronization (ERS), a decrease event-related desynchronization (ERD). All of the results shown here are for the upper alpha band (10–12 Hz) and from posterior recording sites [with the exception of the data shown (D) which were recorded from temporal sites]. (A) In a word recognition task, a brief and transient increase in power can be observed in response to the onset of a correctly recognized word. Then, a large ERD develops, reaching a maximum at around 600 ms post-stimulus. Data are replotted from Klimesch et al. (2000). (B) In a visual oddball task a significant ERD can be observed already around 500 ms pre-stimulus. Data are replotted from Klimesch et al. (1998a). (C) An immediate onset of ERD can be observed in a task, in which light flashes were presented when alpha power exceeded a predetermined (individually determined) threshold. Data are replotted from Woertz et al. (2004). (D) In a memory scanning task, ERS can be observed during the presentation of the memory set (i.e., during encoding) but ERD during the processing of the probe item (i.e., during retrieval). Data are replotted from Klimesch et al. (1999).
Figure 2
Figure 2
Findings from an object recognition task show that the onset of ERD is not related to the onset of stimulation. Data are from Freunberger et al. (2008b). (A) Example of stimuli (upper panel). (B) A large ERD (indicated by an arrow) develops during the time window, in which an object is recognized. The upper panel shows time–frequency power plots for object- and control-items. The lower panel shows absolute power values for the lower and upper alpha band (Reprinted with permission).
Figure 3
Figure 3
Instantaneous phase alignment (IPA) as analyzed by Gruber et al. (2005) for the data of a memory retrieval task. (A) Example of the ERP with the P1-component. (B) The frequency characteristic of the ERP is reflected by a measure termed evoked power. It represents the filtered ERP. (C) Significant phase locking can be observed in a broad frequency range. (D) Significant IPA can be observed particularly for the time window of the P1 and N1. The white line represents the ERP. For the P1, phase alignment is around the positive peak, for the N1 it is most pronounced for the positive going slope beyond the negative peak. Note that the IPA is significant only for the broad alpha frequency range. (E) Time–frequency representation of absolute phase. Note the vertical red and blue bands (around 100–200 ms post-stimulus) representing the positive and negative peak of aligned frequencies. These bands correspond to the positive and negative peaks of the ERP, i.e., to the P1 and N1 respectively (Reprinted with permission).
Figure 4
Figure 4
P1 latency differences can be described in terms of a traveling alpha wave. Data are from a stroop task analyzed in Klimesch et al. (2007a). (A) A systematic and consistent travel speed was observed only for the extended alpha band and during the time window of the P1–N1 complex at around 0–200 ms post-stimulus. (B) The direction of the traveling wave can be determined by visual inspection of ERP's (Reprinted with permission).
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