Effects of ethanol on postural adjustments in humans

Surface EMGs and a platform capable of horizontal, vertical, and rotational displacements were used to examine the effects of ethanol on human balance control on three levels within the nervous system's postural control hierarchy: the automatic long-latency muscle responses of proprioceptive origin (100 ms latency), the more centrally originating visual and vestibular responses of longer latencies (150 ms or greater), and the higher adaptive levels of command, responsible for attenuating or facilitating responses under changing task conditions. Two types of long-latency response which were previously considered to involve similar neuronal circuitry, the anteroposterior sway response and the lateral sway response, were affected to significantly different extents by ethanol intoxication. The significantly greater increase in latency of the lateral sway response suggests that its supporting pathway is more complex than that mediating the anteroposterior sway response. Though intoxicating amounts of ethanol significantly prolonged the latency and reduced the amplitude of long-latency muscle responses, sway was not increased sufficiently to produce a fall under normal conditions. However, under conditions where proprioceptive and visual cues were absent, five of the six subjects were destabilized sufficiently to fall on their first trial. Underlying this decreased stability were significantly delayed vestibular compensatory responses. However, with repeated trials subjects were able to adapt normally to altered conditions, suggesting that the higher adaptive levels of command were not significantly affected by ethanol intoxication.