The effects of spatial and cognitive attention on the late component of the negative difference (late Nd) event-related brain potential (ERP)

The roles of the early and late negative difference (Nd) components of the event-related potential in auditory selective attention were investigated by Ramirez, Bomba, Singhal and Fowler (2005) who reported that the amplitude of the late but not the early Nd was decreased by Posner's spatial attention switching task presented in the visual modality. The purpose of the present experiment was to determine whether the late Nd is also sensitive to deeper cognitive attention switching. Twenty-one subjects performed a dichotic listening task, which generated the Nd components, both alone and in combination with either a spatial or a cognitive attention-switching task involving letter matching. The results showed that the late Nd was unaffected by the cognitive task but was reduced in amplitude by the cued condition of the spatial task but not by a neutral control condition. On the other hand, P300 amplitude was reduced by both tasks. In contrast to previous results, early Nd amplitude was somewhat reduced by the spatial task but only at one electrode site. Also in contrast to previous results, the amplitudes of N100 and P200 were not reduced in a correlated manner by the visual tasks. On the basis of this and other evidence we conclude that the late Nd reflects a selective attention control system involving a memory trace containing spatial information, either in the process of being converted to amodal form or in amodal form. This hypothesis is discussed in the context of a model relating the late Nd and P300. Implications of the uncorrelated reduction in N100 and P200 are also discussed. In addition the results of various dual-task studies done by our lab are discussed in light of Naatanen's (1990) theory of auditory processing.