The question of whether a binary mixture of amino acids is detected by fish as a unique odor or whether the qualities of the individual components are retained within the mixture was investigated in channel (Ictalurus punctatus) and brown bullhead (Ameiurus nebulosus) catfish, species that are highly similar in their olfactory receptor and behavioral responses to amino acid odorants. Catfish respond with greater appetitive food-searching (swimming) behavior to amino-acid-conditioned olfactory stimuli than to non-conditioned amino acids. In the present study, appetitive food-searching behavior was measured by counting the number of turns of the fish greater than 90 degrees within 90 s of stimulus onset and, in some tests, by video tracking. The two methods yielded highly correlated results. Channel catfish conditioned to a binary mixture composed of equimolar amino acids responded with searching behavior to the amino acid that produced the larger-amplitude electro-olfactogram (EOG) response as they did to the conditioned stimulus. In further studies, bullhead catfish were conditioned either to a binary mixture or to a single amino acid and tested to determine whether a binary mixture was detected as the component evoking the larger EOG response. In all initial tests (trials 1–3), the more stimulatory component of a binary mixture was not discriminated from the binary mixture; however, the less stimulatory component and all other amino acids tested were discriminated from the mixture. By increasing the concentration of the originally less potent component in a binary mixture, making it the more stimulatory compound, it was now detected as not significantly different from the binary mixture; however, the original more potent component (i.e. now the less potent stimulus) was detected as significantly different from the mixture. However, with 5–10 additional discrimination training trials, the less stimulatory component in a binary mixture influenced the perception of the binary mixture because the binary mixture was no longer detected only as its more stimulatory component. The data suggest that a two-step learning process occurs within the olfactory bulb and possibly higher-order telencephalic nuclei.

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