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Can Visual Stimuli Affect the Phase of Brain Oscillations?


Kirschfeld,  K
Former Department Comparative Neurobiology, Max Planck Institute for Biological Cybernetics, Max Planck Society;

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Kirschfeld, K. (2004). Can Visual Stimuli Affect the Phase of Brain Oscillations?. Poster presented at 7th Tübingen Perception Conference (TWK 2004), Tübingen, Germany.

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.