The validity of using an approach-avoidance test to measure the strength of aversion to carbon dioxide in rats

Approach-avoidance tests are often used to estimate the strength of aversion, by making food rewards available during exposure to a stimulus at a known level of food deprivation. It is reasoned that if subjects are unwilling to tolerate the stimulus even when severely food deprived, their aversion to the stimulus must be strong. However, this interpretation assumes that the effects of the competing motivations are additive. The present study tested the validity of this assumption by measuring whether rats’ tolerance of CO2 increased monotonically with food deprivation. In two experiments, 21 male Wistar rats were exposed to a gradually increasing CO2 concentration while eating sweet food items. Rats were tested at seven deprivation levels ranging from 0 to 24 h (Experiment 1), or from 0 to approximately 10 h (Experiment 2). Subjects always left the gas chamber, but the concentration at which they left did not increase monotonically with deprivation level, instead showing a bitonic response (F1,64 = 5.30, P = 0.025 when deprivation was defined in terms of food intake in the previous 24 h; F1,61 = 6.07, P = 0.017 when defined as time since last eating). This result indicates an interaction between hunger and CO2 aversion, contrary to the assumption of additivity. We conclude that the approach-avoidance method cannot be used to ‘titrate’ aversion to CO2 against hunger, perhaps because hunger causes anxiety.