Direct evidence for the economy of action: glucose and the perception of geographical slant.

When locomoting in a physically challenging environment, the body draws upon available energy reserves to accommodate increased metabolic demand. Ingested glucose supplements the body's energy resources, whereas non-caloric sweetener does not. Two experiments demonstrate that participants who had consumed a glucose-containing drink perceived the slant of a hill to be less steep than did participants who had consumed a drink containing non-caloric sweetener. The glucose manipulation influenced participants' explicit awareness of hill slant but, as predicted, it did not affect a visually guided action of orienting a tilting palmboard to be parallel to the hill. Measured individual differences in factors related to bioenergetic state, such as fatigue, sleep quality, fitness, mood, and stress, also affected perception: lower energetic states were associated with steeper perceptions of hill slant. This research shows that the perception of the spatial layout of the environment is influenced by the energetic resources available for locomotion within it. Our findings are consistent with the view that spatial perceptions are influenced by bioenergetic factors.