Interstimulus interval and the selective-attention effect on auditory ERPs: "N1 enhancement" versus processing negativity.

The attention effect on the auditory event-related potential (ERP) in dichotic conditions was studied as a function of the interstimulus interval (ISI). Subjects attended to stimuli delivered to a designated ear and responded to infrequent pitch deviants in this input. The mean ISI was either 80, 160, 480, or 800 ms. Negative difference waves (Nds) were computed by subtracting ERPs to unattended standards from ERPs to the same stimuli when attended. The exogenous N1, as estimated from unattended standard ERPs, was larger contralaterally to the stimulus and inverted in polarity at mastoids. With decreasing ISIs, N1 diminished in amplitude much faster than did the Nd. In addition, N1 latency remained stable, whereas Nd peaked markedly earlier with shorter ISIs, almost perfectly coinciding with the exogenous N1. However, this temporal coincidence found in grand averages proved to be illusory in single subjects. The early Nd showed no contralateral asymmetry at its peak, but asymmetry was apparent during the ascending slope. These lateral asymmetries resembled those of the exogenous N1 but occurred later. The early Nd peak was, at least mainly, caused by an endogenous attention effect, the processing negativity (PN), even with very short ISIs, but an effect on the exogenous N1 could not be excluded.