The effects of memory scanning on the late Nd and P300: an interference study

Singhal and Fowler (2004) showed that the late negative difference (Nd) waveform elicited during dichotic listening was attenuated by concurrent visual short term memory (STM) scanning, but not long term memory (LTM) scanning. P300 was reduced by both tasks. The present study compared the effects of STM load on the late Nd and P300 by combining dichotic listening and visual memory scanning with varying set sizes. The results showed that the late Nd was sensitive to the introduction of the scanning task, but not to an increase in load. Furthermore, both the auditory and visual P300s were reduced when a second task was introduced, but only the visual P300 decreased as a function of memory-set size. These data suggest that (a) the auditory late Nd reflects working memory, but not memory scanning, (b) late Nd and P300 reflect parallel but distinct working memory processes, and (c) stimulus modality is an important determinant of P300 amplitude.