N1 (or N100) is a negative wave occurring approximately 100 msec after a stimulus. N1 consists of two components that may overlap in time—an earlier automatic component followed by an attention-related component. The automatic component of N1 is the same in both the relevant and irrelevant channels. In normal subjects, N1 has a greater amplitude when elicited by stimuli in a relevant sensory channel compared with the N1 elicited by stimuli in an irrelevant channel. Therefore, to examine the attentional process, the amplitude of the N1 from the relevant channel is subtracted from that of the irrelevant channel, yielding an Nd (negative difference) component. Nd amplitude thus reflects the subject’s allocation of attentional resources to the appropriate channel.
