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Abstract

The function of brain oscillations is seen in the context of various sensory, behavioural or cognitive states, and for the visual system a number of hypotheses have been proposed based
on synchronized activity of different populations of neurons . A prerequisite for the type of
synchronisation suggested is that visual stimuli are capable of affecting (resetting) the phase of
brain oscillations.
A change in the phase of brain waves assumingly also occurs in the “Berger effect” : If observers
open their eyes, the amplitude of EEG oscillations in the alpha band (8 13 Hz) decreases
or disappears completely. One interpretation is that due to visual stimulation oscillations in different
neurons or neuronal populations are desynchronised.
For a functional interpretation of brain oscillations it therefore seems crucial to nd out
whether or not the phase of these brain oscillations can be affected by visual stimuli. To answer
this question one has to examine whether brain waves are generated by linear or nonlinear
mechanisms. If they are due to linear band pass ltering or linear oscillators, no phase resetting
is possible, as the superposition law holds: the response to a stimulus is just superimposed on
the ongoing oscillation, no phase resetting is possible. In contrast, in nonlinear oscillators (as
described e.g. by the van der Pol equation), phases can be reset e.g. by an impulse.
We analysed the question of linearity of alpha waves by investigating whether or not the superposition
law holds: Light ashes were presented randomly, as usual, or at particular phases
of the alpha waves. The result: the evoked potential to a ash, given at a particular phase,
basically is the superposition of the alpha wave and the evoked potential to ashes presented
randomly (which is without alpha contribution). This holds only, however, if one assumes
that within 200 to 300 ms after a light ash the amplitude of alpha waves decreases in a ash
intensity dependent degree.
Conclusion: the phase of alpha, perhaps also of gamma waves cannot be reset by visual
stimuli. This questions existing theories about the function of these waves. The “Berger effect”
is not due to event related de- synchronisation. The amplitude of alpha waves after a ash are
reduced in the same way as that of evoked potentials, due to the loss of sensitivity by light
adaptation.