The exhaustive additivity displayed by nitrous oxide has implications for cognitive-energetical theory

The cognitive-energetical approach, which relies on the discrete stage mode l of additive factors logic, asserts that basal energetical mechanisms such as arousal act via particular information processing stages. The anaesthet ic gas nitrous oxide (N2O) produces additivity at four of the five perceptu al and central stages, but its effect on the remaining stage, feature extra ction, is unknown. We investigated this stage using 12 subjects who perform ed a visual oddball experiment in which two levels of stimulus quality, thr ee levels of breathing mixture (air, 25% and N2O, 35%) and three levels of stimulus probability were combined factorially. Reaction time (RT) and P300 were collected simultaneously. The RT results showed additivity between N2 O, stimulus quality and probability. P300 latency also showed additivity be tween N2O and stimulus quality. Since the discrete stage model cannot easil y account for the exhaustive additivity displayed by N2O on perceptual and central stages, we performed a continuous cascade model simulation to deter mine whether it is better able to account for this phenomenon. We found tha t exhaustive additivity could be reproduced by adding a time delay to the a ctivation rate of the first stage, which we interpreted as evidence that N2 O causes slowing prior to stage processing. To account for these results, w e propose a two-tiered energetical model in which a lower GABAerg ic reticu lar system (influenced by N2O) modulates the activity of upper 'arousal-lik e' multidimensional ascending thalamocortical systems. The applicability of this model to drugs such as the barbiturates, the benzodiazepines and etha nol, as well as the aging process, is discussed. (C) 2000 Elsevier Science B.V. All rights reserved.