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The effects of temporal and event uncertainty in determining the waveforms of the auditory event related potential (ERP).

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's 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.