Expectation of an important event affects responses to irrelevant stimuli of different modalities

Periodic alterations of event-related potentials (ERPs) were studied during "oddball" tasks. Sequences of randomly intermixed frequent (non-target) an d rare (target) stimuli were presented. In visual experiments, these were f lashes of light of two different colors. In auditory tests there were two t ones of different frequencies. The instruction was to keep a mental count o f each target stimulus. To study the alterations of the "state of the brain " produced by target detection, responses to non-targets immediately follow ing targets were compared with responses to an eighth subsequent non-target stimulus. To evaluate the effect of such "brain states" on responses to st imuli of a different modality, additional visual stimuli (probes) were deli vered after both auditory and visual "oddball" stimuli. It was found that r esponses to the eighth presentation of non-target stimulus were preceded by significant negative shift of recorded potential. This shift was smaller b efore the responses to non-targets immediately following the presentation o f target stimuli. The difference was significant both in auditory and visua l tests. Responses to "oddball" stimuli were little affected: only the redu ction of P200 peaks in "after target" responses was significant in visual t ests. Responses to probes showed stronger effects: when visual probes follo wed visual "oddball" stimuli, all three components measured (N100, P130 and P200) were shifted positively in responses to eighth presentations of non- targets. When visual probes were presented in auditory tests, only the ampl itude of the N100 component was significantly affected.