“Classifying-together” phenomenon in fish (Xenotoca eiseni)

When animals are previously exposed to two different visual stimuli simultaneously, their learning performance at discriminating those stimuli delays: a phenomenon called “classifying-together” or “Bateson effect”. The current study addressed whether a teleost fish, Xenotoca eiseni, was sensitive to this effect. Three experiments were designed, by handling the visual stimuli (full red disk, amputated red disk, red cross) and the presence of an exposure phase, before performing a discriminative learning task (Exp. 1: full red disk vs. amputated red disk; Exp. 2: full red disk vs. red cross). In the exposure phase, three conditions per pair of training stimuli were arranged: “congruence”, where fish were exposed and trained to choose the same stimulus; “wide-incongruence”, where fish were exposed to one stimulus and trained to choose the other one; “narrow-incongruence”, where fish were exposed to both the stimuli and trained to choose one of them. In the absence of exposure (Exp. 3), the discriminative learning task was carried out to establish a baseline performance as regards both the figural discriminations. Results showed that fish ran into retardation effects at learning when trained to choose a novel stimulus with respect to the one experienced during the exposure-phase (wide-incongruence condition), as well as after being simultaneously exposed to both the stimuli (narrow-incongruence condition). Furthermore, there were no facilitation effects due to the congruence, if compared with the baseline. This study provides the first evidence about the consistency of the classifying-together effect in a fish species, highlighting visual similarities on discrimination processes.