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Abstract

Purpose:
We developed a model according to which alpha waves are generated by a feedback loop. The gain in the loop is so high that it acts as a band–pass filter with maximal frequency in the range 8 – 12 Hz (alpha band). According to this model light adaptation should lower the gain in the loop, with the consequence that evoked potentials and alpha–wave amplitudes decrease. This has been verified experimentally (K. Kirschfeld, Program No. 985.18. 2004 Abstract Viewer Washington, DC: Society for Neuroscience). In contrast to light adaptation, attention is expected to increase the gain in the same loop, together with the amplitude of alpha waves. The purpose of this study is to test this prediction.
Methods:
Observers were asked to press one of two buttons depending upon whether a repetitively presented white square has been presented above or below a fixation point. Alpha waves were recorded at the occiput (electrode position Oz).
Results:
Alpha wave amplitudes are high in the time window in which a stimulus is to be expected, and low outside this time window. The amplitude is the higher, the higher the probability of the target’s occurrence. The change between low and high alpha amplitudes can be rather fast, less than a second in each direction.
Conclusions:
The effect is so robust and strong that it is likely that the mechanism which generates alpha waves is functionally relevant to the adjustment of sensitivity (gain) for neural processing, in which process light adaptation and attention have opposite effects.