The scalp topography of potentials associated with missing visual or auditory stimuli

Averaged potentials time-locked to regularly presented visual and auditory stimuli and to the occasional random deletion of a stimulus were recorded from a scalp electrode array overlying the left hemicranium. The major components of visual and auditory evoked potentials and of the potentials associated with missing stimuli (MSP) were measured and their amplitude distributions depicted in the form of isopotential maps. The N1 components of the VEP and AEP had distributions compatible with sources in and near the respective primary cortical projection areas. The P2 components were more widely distributed and could be attributed in part to generators within modality specific association areas. The MSP comprised two main components, an initial negativity (NMSP) and a later positive wave (PMSP). The NMSP distributions were different in the visual and auditory modalities, and were similar to the respective EP topographies. The NMSP appeared to reflect a more powerful contribution of association areas than did the evoked responses. The PMSP topography was modality unspecific with distributions which were maximal over the parietal region. The possible functional significance of the NMSP and PMSP was considered in the light of their timing and topography.