Rats exhibit asymmetrical retention functions for hedonic and nonhedonic samples in many-to-one symbolic delayed matching to sample.

Rats were initially trained in a symbolic delayed matching-to-sample task either to discriminate hedonic samples that consisted of food or no food or to discriminate tone samples that differed in frequency and location. The retention functions for both the hedonic and tone samples were asymmetric, with forgetting of the food sample or the high-frequency tone occurring more rapidly than forgetting of the no-food sample or the low-frequency tone. Next, many-to-one (MTO) training was given in which tone samples were added for the rats initially trained with hedonic samples, and hedonic samples were added for the rats initially trained with tone samples.

Rats exhibit asymmetrical retention functions for hedonic samples.

Rats were trained in a symbolic delayed matching-to-sample task to discriminate hedonic sample stimuli that consisted of food or no food. Retention functions decreased more rapidly on trials initiated by a food sample than on trials initiated by a no-food sample when retention intervals were manipulated within session. The asymmetrical functions could not be explained in terms of mediation of choice responding by magazine head-entry behavior during the retention interval.

Individual differences in the perception of similarity and difference.

Thematically related concepts like coffee and milk are judged to be more similar than thematically unrelated concepts like coffee and lemonade. We investigated whether thematic relations exert a small effect that occurs consistently across participants (i.e.

Metamorphosis: essence, appearance, and behavior in the categorization of natural kinds.

The transformation paradigm (Rips, 1989) was used to contrast causal homeostasis and strict essentialist beliefs about biological kinds. Participants read scenarios describing animals that changed their appearance and behavior through either accidental mutation or developmental maturation and then rated the animals on the basis of similarity, typicality, and category membership both before and after the change. Experiment 1 in the present study replicated the dissociation of typicality and categorization reported by Rips (1989) but also revealed systematic individual differences in categorization.