The Effects of Temporal and Event Uncertainty in Determining the Waveforms of the Auditory Event Related Potential

In each of two experimental conditions, subjects were presented with a series of tones; one of two tones (1500 Hz or 1000 Hz) was presented on each trial with a probability respectively of .10 or .90. The subjects counted the rare (p=.10) tones; such counted rare tones are normally associated with a large P300 component. The two conditions differed in that in one the tunes were triggered by the subject\u27s button press; in the other the tones were triggered by computer. Schafer and Marcus (1973) reported that all the components of event‐related potentials (ERPs) elicited by self‐triggered tones were substantially smaller than those elicited by machine‐triggered stimuli. Our paradigm allowed a detailed assessment of the effects of self‐stimulation on specific ERP components as well as the interaction of temporal with event uncertainty. Data were analyzed using a Principal Components technique. Both temporal and event uncertainty appeared to augment a negative component of the ERP with approximately 140 msec latency. Such effects, however, were confounded by the presence of slow negative potentials preceding the button press in the self‐stimulation conditions. As expected, the P300 component was largest for the ERPs elicited by the rare tones. Temporal uncertainty diminished the amplitude of P300 at central electrode sites. A large slow wave was present following P300; its anterior‐posterior distribution was altered by mode of stimulus presentation. It appears that temporal and event uncertainty have distinct effects upon the morphology and distribution of KRP components