Event-related potential measures of the inhibition of information processing: I. Selective attention in the waking state
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This article reviews the effects of selective attention on event-related potentials (ERPs). Attention has little, if any, effect on short-latency exogenous ERPs. The longer-latency ERPs can be markedly affected by manipulation of the subject's level of attention. For example, a late positive wave, P300, appears to occur only if subjects actively detect an infrequently occurring target stimulus. However, a number of other late positive waves may also occur independently of the direction of attention, particularly if elicited by highly biologically or psychologically relevant stimuli. Attention may also interact with an earlier, apparently exogenous, negative waveform, N1. This could be due to the overlapping and summating effect of an attentional-related waveform, the processing negativity. The presentation of a physically deviant stimulus occurring among a train of homogeneous standard stimuli will elicit another negative wave, the mismatch negativity (MMN). The MMN has traditionally been thought to occur independently of attention. More recent studies have, however, shown that attention can modulate the MMN. This may, however, be explained by the summating effects of other overlapping components. Interpreting the scalp-recorded ERP can therefore require judicious care. Design of experiments must take into account the fact that the magnitude of attentional effects will depend on a number of different influences, some of which are very subtle and complex. A problem with any study in the waking and alert human is that the subject may not be able to completely ignore stimuli, in spite of instructions to do so. For this reason, the study of unconscious states, such as sleep, may prove to be especially fruitful in understanding the effects of attention in the waking state.
