Slowing due to acute hypoxia originates early in the visual system

Experiments in the visual modality show that acute hypoxia slows the earliest stage of information processing-preprocessing. It is unknown, however, whether a later stage, feature extraction, is also slowed.

To answer this question, an additive factors method (AFM) experiment was conducted which employed seven well trained subjects whose arterial oxyhaemoglobin saturation was controlled at 63% with low oxygen mixtures. The subjects responded to oddball names presented on a computer screen and both reaction time (RT) and the event-related brain potential P300 were measured. The luminance and quality of the names was varied factorially to influence the preprocessing and feature extraction stages, respectively.

RT and P300 latency showed the same pattern of results: stimulus luminance and signal quality were additive, indicating that AFM assumptions were met; hypoxia and stimulus luminance were interactive but hypoxia and signal quality were additive.

In conjunction with other evidence, we interpret these results to indicate that the locus of slowing produced by hypoxia is largely at the preprocessing stage, at least in the visual modality. Slowing at the preprocessing stage can be explained by hypoxia shifting the function relating RT and stimulus luminance to the right by a constant amount.