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Stimulus recognition and its relationship to the cerebral event-related potential

From the Department of Neurology, University of California, San Francisco.

Address correspondence and reprint requests to Dr. Douglas S. Goodin, Department of Neurology, Rm. M-794, University of California, San Francisco, 505 Parnassus Ave., San Francisco, CA 94143-0114; e-mail: dsg{at}itsa.ucsf.edu

Background: The relationship between the event-related cerebral potential (ERP) and perceptual awareness by the subject is poorly understood. The authors manipulated the process of stimulus recognition to examine the effect of changes in a subject’s awareness on different components of the ERP and on behavioral (motor) responses.

Methods: Ten subjects listened to a series of 400 tone pips, one of which (frequent tone) was slightly louder than the other (rare tone; 14% of trials). Stimuli were presented in blocks of 10 tones and, following each block, subjects were asked how many rare tones they had detected and if they were aware of any errors that they had made during that block. Cerebral and motor responses were recorded by surface electrodes.

Results: When subjects were aware of having responded erroneously to either the rare or frequent tone, their response times were shorter than for correct responses to that tone. When subjects were unaware of such errors, however, the response times were delayed compared to correct responses and to error responses of which the subject was aware. When the subject made an error by responding to the rare tone as if it were frequent and was aware of the error, the N2 and P3 latencies were delayed. By contrast, when subjects were unaware of such errors, no N2 or P3 components were present. Similar findings were noted when subjects responded to a frequent tone as if it were rare.

Conclusions: These results suggest that the N2 and P3 components of the ERP reflect the awareness of the subject that an unexpected event has occurred, regardless of whether it is an unexpected stimulus or an unexpectedly erroneous response to that stimulus. The recording of ERP may provide a means of measuring perceptual awareness in other contexts.

Received June 11, 2003. Accepted in final form July 29, 2003.