ATTENTIONAL INFLUENCE ON THE P50 COMPONENT OF THE AUDITORY EVENT-RELATED BRAIN POTENTIAL

To determine if attentional factors influence the suppression of the auditory P50 in a conditioning-testing paradigm, known as the 'sensory gating' effect, we tested 10 healthy young adults in four experimental conditions. The first condition was the traditional passive conditioning-testing paradigm in which a pair of identical auditory clicks is administered at an interstimulus interval fixed at 500 ms. The effect of interest is a reduction of P50 amplitude in response to the second stimulus. In the next condition, the second stimulus could be one of two possible frequencies and subjects were required to count one and to ignore the other. The third and fourth experimental conditions involved a motor response. In the third condition, subjects were required to make a unimanual button press to the occurrence of the second stimulus. In the fourth condition, subjects were required to discriminate among two possible second stimuli, and make a unimanual button press to the occurrence of the designated stimulus. Subjects also completed four matched blocks of single stimulus (i.e., unpaired) presentations to provide a baseline for assessing the effect of the warning stimulus on the evoked response. We found, in agreement with previous results, that passive exposure to the paired stimuli produced a suppression of P50 amplitude to the second stimulus. However, we also found that suppression of P50 amplitude was not evident when subjects selectively counted the designated stimuli, and was reduced in magnitude when a simple motor response was required and when a selective motor response was based on stimulus discrimination. In addition, we observed that the amplitude of the P50 was larger with unpaired single stimuli than it was either with the first or second stimulus of a pair, regardless of processing demands. Furthermore, variations in processing demands did not affect P50 amplitude when a single stimulus was presented. This pattern of results suggests that the 'sensory gating' effect is not a simple 'hard-wired' inhibitory phenomenon. Rather, it may be one manifestation of an attention regulation process that is activated by a warning stimulus and has either inhibitory or excitatory effects on neural transmission, determined by variations in processing demands. Presentation of a warning stimulus may have an additional, unselective suppressing effect, operating independently of this attention regulating process.