SPONTANEOUS EEG THETA-ACTIVITY CONTROLS FRONTAL VISUAL-EVOKED POTENTIAL AMPLITUDES

Frontal visual evoked potentials (VEPs) were studied in order to exten d the application of a recently introduced algorithm for selective ave raging of evoked potentials. This algorithm is based on the inverse re lationship between amplitudes of alpha or theta components of the spon taneous EEG activity and evoked potential (EP) amplitudes. Stimuli wer e only applied if the root mean square (RMS) value of the ongoing EEG at the lead F-4 was below an individual threshold level ('selective st imulation'). For this comparison, the EEG was filtered in one of the f requency ranges 'alpha', 'theta' and 'alpha and theta', respectively. 'Alpha' and 'alpha-and-theta-dependent' selective stimulation conditio ns resulted in significant amplitude increases (P < 0.05) at the input reference channel F-4 and partly at ipsihemispherical temporal and pa rietal leads and at Cz. The largest increase of 35% at F-4 (P < 0.01) was Obtained with visual stimulation during low prestimulus theta acti vity. We conclude that spontaneous theta activity of the frontal corte x may be a factor influencing the amplitudes of frontal VEPs. A sophis ticated analysis of frontal EPs, mainly in the framework of cognitive studies, should consider the theta activity prior to stimulation. (C) 1998 Elsevier Science Ireland Ltd.