QUANTITATIVE ELECTROPHYSIOLOGICAL STUDIES OF MENTAL TASKS

A number of pilot studies were performed to evaluate the utility in hu man studies of the so-called 'tracer strategy' previously used extensi vely in numerous experiments by the senior author of this paper. This strategy utilized presentation of visual or auditory information inten sity modulated at some specific repetition rate or 'tracer frequency'. Representation, storage and retrieval by the brain of information thu s presented is identified by the appearance of 'labelled responses' de fined as power in the EEG spectrum at the tracer frequency or averaged ERP waveshapes extracted from brain electrical activity by triggering on the onset of the modulation cycle. This method has been applied to scalp recordings obtained during performance of (1) audio-visual cont inuous pursuit tasks in which the target and pursuer were labelled at different frequencies and (2) delayed match from sample tasks in which sets of letters, numbers or faces modulated at a specific frequency h ad to be retrieved from working memory. Results showed statistically s ignificant appearance of labelled responses in different scalp regions , depending upon the nature of the cognitive task.