An analysis of human EEG responses in the alpha range of frequencies.

The diffuse thalamocortical system (DTS) can be represented by a simple 'lumped' model if near synchronous activity of thalamocortical 'circuits' is assumed. Such activity results in scalp potential changes that are manifest in the EEG. The model suggested by the anatomy and physiology of the DTS is also suggested by the dynamic character of the EEG and it is testable since it is possible to excite the DTS with a stimulus input and record the corresponding EEG response. Experiments carried out on 48 human subjects reveal that the model can account for most of the within- and across-subject amplitude variation of EEG evoked responses to sinusoidally modulated diffuse light. It can also explain phenomena specifically associated with stimulation of the DTS, namely 'after-discharges,' 'recruiting responses' and EEG 'alpha activity.' Most importantly, the model makes it possible to carry out a precise analysis of EEG responses to stimulation in terms that are neurophysiologically and psychologically meaningful.