Quantitative electrophysiological studies of mental tasks.

A number of pilot studies were performed to evaluate the utility in human studies of the so-called 'tracer strategy' previously used extensively in numerous experiments by the senior author of this paper. This strategy utilized presentation of visual or auditory information intensity modulated at some specific repetition rate or 'tracer frequency'. Representation, storage and retrieval by the brain of information thus presented is identified by the appearance of 'labelled responses' defined 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 continuous 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 had to be retrieved from working memory. Results showed statistically significant appearance of labelled responses in different scalp regions, depending upon the nature of the cognitive task.