Evaluation of exemplar-based generalization and the abstraction of categorical information.

This article reformulates and reanalyzes a problem originally put forth by Homa, Sterling, and Trepel (1981). The question is whether a pure, exemplar-based abstraction process is an adequate model of category learning or whether it is necessary to postulate an additional prototype-abstraction process. Based on quantitative discrepancies from a pure, exemplar-based model, Homa et al. argued that it was necessary to recognize the operation of a prototype-abstraction process in order to fully explain their results. However, Homa et al. never actually fit the exemplar plus prototype model to the data to determine if indeed the additional prototype process could explain the deviations from the pure exemplar model. The present article compared the pure exemplar model with a mixed (exemplar plus prototype) model and did not find consistent evidence requiring the postulation of an additional prototype-abstraction process. These results point out the difficulty of distinguishing alternative classification models and underscore the need for careful analytic work in this area.

Tonic immobility: effects of dopamine receptor blockade and stimulation.

Blockade of dopaminergic receptors by haloperidol enhanced the duration of tonic immobility in chickens. Apomorphine, a dopamine receptor agonist, produced short durations. Apomorphine also produced an increase in stabilimeter activity. These data suggest dopaminergic involvement in tonic immobility, and support a competing response interpretation of the apomorphine effect.