Relationship between the event-related brain potential P300 and inert gas narcosis

It is known that inert gas narcosis slows both reaction time (RT) and the event-related brain potential P300 in a correlated and dose-dependent manner. On the assumption that P300 reflects the time to evaluate a stimulus and RT reflects this time plus the time to select and execute a response, these results have been taken to indicate that the locus of the slowing produced by narcosis is early and influences stimulus evaluation processes. If this is the case, RT and P300 should be slowed identically when cognitive workload is manipulated. To test this prediction, subjects breathed 35% nitrous oxide and responded to differences in the intensity of either visual (Exp. 1, n = 8) or auditory (Exp. 2, n = 10) two-choice oddball stimuli with accuracy controlled at a high level. In both experiments narcosis slowed RT and P300 additively by increasing the intercept rather than the slope of the workload function, but these two measures diverged in one important respect; RT was slowed more than P300, as indicated by an RT-P300 difference analysis. It is concluded that, contrary to previous assumptions, narcosis produces two sources of slowing; one stimulus-related and indexed by P300 and the other response-related and indexed by the RT-P300 difference. Possible mechanisms underlying these sources are discussed.