Influence of a visual spatial attention task on auditory early and late Nd and P300

Singhal et al. [Psychophysiology 39 (2002) 236] reported that the amplitude of the late negative difference (Nd) and P300 components, generated by a dichotic listening task, were decreased in parallel by a simultaneously performed visual flying task. In contrast, early Nd was unaffected. To investigate the role of covert spatial attention in this pattern of results, we substituted Posner's attention switching paradigm for the flying task. Fourteen subjects performed the dichotic listening and attention tasks alone and in combination. The results showed that only the invalid/valid condition of the Posner task reduced the amplitude of the late Nd, while the early Nd was unaffected. Contrary to the earlier study, P300 dissociated from the late Nd and its amplitude decreased in both the invalid/valid and neutral conditions of the Posner task. We conclude that the late Nd is sensitive to some aspect of attention switching and that the late Nd and P300 share sensitivity to an amodal system to which the early Nd is insensitive. Within this system, the late Nd is differentiated from P300 by its narrow sensitivity to attentional processing whereas P300 is sensitive to a broader range of processing. These similarities and differences are explained in the context of a modified version of Kok's model [Psychophysiology 38 (2001) 557] of the determinants of P300 amplitude.