Electrical activity of the human brain can be measured by placing electrodes on a person's scalp and amplifying the electrical signals that penetrate the skull and scalp tissue. The voltage variation that is recorded by the scalp electrodes is called the electroencephalogram (EEG) and the fluctuations that are related to the processing of a specific stimulus event are called event-related brain potentials (ERPs). ERPs typically reflect postsynaptic potentials generated by large groups of neurons that are activated simultaneously by a particular stimulus or event. The small ERP signal can be extracted from the ongoing EEG by averaging the ERP over many trials so that randomly occurring fluctuations in the EEG cancel each other out. The resulting ERP waveform consists of several positive (P) and negative (N) peaks that are related to various aspects of sensory, cognitive, and motor processing.

Researchers have examined the electrophysiological expression of attention in vision and audition by recording ERPs during attention-orienting tasks. In vision, attention-related behavioral effects are accompanied by amplitude enhancements of the early visual P1 and N1 components for attended stimuli compared to unattended stimuli (e.g., Mangun & Hillyard, 1991). In audition, attention-related behavioral effects are accompanied by enhanced negativities beginning around the onset of the auditory N1 (for a review, see Nataanen, 1992).

We are currently studying visual, auditory, and cross-modal attention (and inhibition of return) by recording ERPs during spatial cueing experiments. For example, in one study, we found that visual IOR is associated with a reduction of the P1 amplitude on valid-cue trials (McDonald, Ward & Kiehl, in press).