A `passive' event-related potential?

Event potentials (ERPs) were recorded in response to visual patterns in two conditions: (1) where no instructions were given to subjects; and (2) where subjects pressed a button whenever a rare stimulus was detected. P3 amplitudes for the No Instructions condition were smaller than for the Active condition, but the latency of the P3s did not differ across conditions. In both conditions, the P3 was either absent or significantly smaller for frequent stimuli than for rare stimuli. It appears a `passive' visual P3 is measurable, and it occurs at the same latency as a P3 recorded in association with an active response. Thus, it may be possible to record P3s in response to visual stimulation in subjects who cannot follow instructions to respond to rare events.