HUMAN VIGILANCE AND AUDITORY EVOKED RESPONSES

1. 1. In seven normal subjects vigilance fluctuations and related changes in the auditory evoked response were quantitatively analysed during rest lasting 10 min (eyes open) or 80 min (eyes closed) in a soundproof room. Vigilance was visually determined from the EEG on the basis of a sensitive classification. Responses evoked by slowly repeated clicks were summated selectively according to the vigilance level in the 2 sec immediately prior to each stimulus. 2. 2. During a session vigilance decreased noticeably in all subjects whether the eyes were kept open or closed. With closed eyes the vigilance decrease was rapid and in most cases reached real sleep. After this initial fall vigilance increased again and remained, with smaller oscillations, at a certain level of the intermediate stages. 3. 3. The click-evoked response was large and stable in the alert state. With decreasing vigilance a progressive amplitude reduction of three prominent response components (N1a, N1b, P2b) was observed. At stage B2 these components had approximately 25% of their original size at stage O. The transition to real sleep was characterized by a marked increase of N2 and a slight growth of P2a. P1 did not change over the whole vigilance range. 4. 4. No progressive changes in latency or shifts in focus were noticed. There was, however, a decrease in latency and a posterior shift in the focus of N2 at the transition to real sleep. 5. 5. The progressive loss in amplitude is interpreted as a decline in the activity of certain brain functions which are essential for the maintenance of an efficient subject-environment relation. © 1969.