The physical basis of alpha waves in the electroencephalogram and the origin of the ?Berger effect?

Abstract

Synchronised activity, differing in phase in different populations of neurons, plays an important role in existing theories on the function of brain oscillations (e.g., temporal correlation hypothesis). A prerequisite for this synchronisation is that stimuli are capable of affecting (resetting) the phase of brain oscillations. Such a change in the phase of brain waves is also assumed to underlie the "Berger effect": when observers open their eyes, the amplitude of EEG oscillations in the alpha band (8-13 Hz) decreases significantly. This finding is usually thought to involve a desynchronisation of activity in different neurons. For functional interpretations of brain oscillations in the visual system, it therefore seems to be crucial to find out whether or not the phase of brain oscillations can be affected by visual stimuli. To answer this question, we investigated whether alpha waves are generated by a linear or a nonlinear mechanism. If the mechanism is linear - in contrast to nonlinear ones - phases cannot be reset by a stimulus. It is shown that alpha-wave activity in the EEG comprises both linear and nonlinear components. The generation of alpha waves basically is a linear process and flash-evoked potentials are superimposed on ongoing alpha waves without resetting their phase. One nonlinear component is due to light adaptation, which contributes to the Berger effect. The results call into question theories about brain-wave function based on temporal correlation or event-related desynchronisation.

Comments for Current Interpretation EEG Alpha Activity: A Review and Analysis

“…These different interrelations between amount of alpha amplitude suppression (desynchronization) and other EEG indices in low and high frequency ranges is consistent with the idea that alpha desynchronization serves an integrative role through a corticocortical "gating" [17,33]. This means that the neurophysiological mechanisms by which activation is generated in LAF and HAF subjects are different what was theoretically supported in previous investigations [17,58].…”

“…So it's possible to define 3 main groups of the alpha activity indices: 1) the frequency of the dominant posterior EEG rhythm [11,12], which is assessed as individual alpha peak frequency in an eyes-closed condition (IAPF) [13][14][15]; 2) the amount of alpha amplitude suppression in response to eyes-open [12,16,17] which defines the activation magnitude, and the width of the frequency range in which amplitude suppression occurs, or alpha band width [7,18]; 3) the "autorhythmicity" (or bursting/spindle forming process), which is result of phase resetting process [19][20][21] (Figure 1). through childhood till pubertal age [22,23] and decreases after 40 years of age [24].…”

Interpreting EEG alpha activity

“…Such findings suggest that despite the fact that alpha EEG power and coherence are often used to assess functional connectivity in the human cortex, moderate to large EEG coherence can also arise simply as a function of the volume conduction of current through the tissue of the head (Srinivasan et al, 2007) or by increasing brain temperature (Kiyatkin, 2005). Thus, the age related decline seen in alpha amplitude across all areas of scalp (Chiang et al, 2011;Sebastián et al, 2011;Yordanova & Kolev, 1997) and the phenomenon of so called 'low amplitude alpha rhythm' could simply be associated with reduced conductivity as a function of increasing age (Wendel et al, 2010) or with genetically determined low volume tissue conductivity.…”

“…Exploring EEG alpha oscillations has generated considerable interest with regard to their role in cognitive (Klimesch et al, 1993(Klimesch et al, , 1996 Babenko et al, 2003). The fact that a variety of EEG rhythmical components are described by the same dominant frequency as the alpha rhythm, with distinct frequency and topographical boundaries, adds to the confusion.…”

“…The fact that a variety of EEG rhythmical components are described by the same dominant frequency as the alpha rhythm, with distinct frequency and topographical boundaries, adds to the confusion. As such, speaking of alpha wave activity often implies some change in amplitude across a standard frequency range (e.g., ), invariably without reference to the oscillatory feature referred to as the 'Berger effect' (Kirschfeld, 2005) which in turn has led to inconsistencies regarding the psychophysiological role of alpha activity (Nunez et al, 2001) and this has led to divergent interpretations of the role of alpha activity (see e.g., Cooper LaVaque (1999) has suggested that it is easier to understand the role of alpha wave activity when viewed from an historical perspective. Hence, in an attempt to understand these issues and set them into context we begin with a brief historical reflection on the nature of alpha wave activity.…”