Evidence that the slowing caused by acute hypoxia is modality dependent

BACKGROUND: AFM (Additive Factors Method) experiments conducted with visual stimuli suggest that the slowing produced by acute hypoxia is located at the earliest preprocessing stage of information processing and that later stages are unaffected (the bottleneck hypothesis). METHODS: To determine the contribution of degraded visual functioning to slowing, we bypassed this modality and measured reaction time in an AFM paradigm to auditory (Experiment 1) and kinesthetic (Experiment 2) stimuli. In both experiments hypoxia was induced with low oxygen mixtures and arterial blood oxygen saturation (SaO2) was controlled at 65%. Task difficulty was manipulated in Experiment 1 with tones that differed in intensity and in Experiment 2 with lifted cylinders that differed in weight. RESULTS: The results for Experiment 1 showed an interaction between task difficulty and hypoxia, indicating slowing of the preprocessing stage. Slowing was not found in Experiment 2. The absence of slowing in Experiment 2 is surprising and indicates that slowing may be confined to vision and audition and may not involve later, more central, stages. We discuss the need to measure cerebral oxygenation in order to understand the sharp differences between the the bottleneck hypothesis, developed by controlling SaO2, and the more traditional behavioral model which postulates multiple cognitive deficits.