Asymmetric suppression of components in binary aldehyde mixtures : Behavioral studies in the laboratory rat

The aim of the present study was to assess component interaction in the perception of the 2 aldehydes butanal and heptanal when presented in binary mixtures to rats. A further aim was to develop a behavioral paradigm for testing suppression of components in mixtures using rodent subjects. Thirsty rats were initially trained to discriminate between the 2 aldehydes butanal and heptanal in an olfactometer using a go/no-go discrimination task. This involved rats learning to place their noses in a sniff port where odors were presented and to lick a tube for water reward when one of the aldehydes was presented (S+) while withholding licking at the tube to the other, unrewarded, aldehyde (S). A mixture condition was then introduced into the task, whereby a proportion of trials involved presentation of a combination of the 2 aldehydes as an additional unrewarded condition. Rats readily learned to withhold licking on trials when the mixture was presented. The concentration of the nonrewarded (S) aldehyde in the mixture was then systematically decreased, whereas the concentration of the S+ component was held constant. This eventually caused the S+ component in the mixture to suppress detection of the S, as shown by an increasing number of lick responses (false alarms) on trials when the mixture was presented. These suppressing effects occurred well above the detection threshold for the S aldehyde presented alone. Results showed asymmetric suppression in the mixture condition such that butanal suppressed detection of heptanal at much lower concentrations than vice versa. A second experiment showed that when both butanal and heptanal were present in a binary mixture at the same concentration (10 6 volume %), then rats responded to the mixture as if only butanal was present. These findings are discussed in terms of butanal having higher mobility and being able to compete more effectively than heptanal for occupation of shared receptor sites.