Academic journal article The Psychological Record

An Investigation of Oddity Concept Learning by Rats

Academic journal article The Psychological Record

An Investigation of Oddity Concept Learning by Rats

Article excerpt

The oddity concept task is representative of what has been defined as a relative class concept as opposed to an absolute class concept (e.g., Thomas, 1980). The defining features of exemplars of absolute class concepts are inherent in each discriminandum (e.g., "tree," "water," and "a person"; Hernnstein, Loveland, & Cable, 1976), but relative properties such as "oddity" are not inherent in the discriminandum that represents a relative class concept. Operationally, this reduces to the need-to-compare (relative) versus no-need-to-compare (absolute) discriminanda in order to affirm whether a discriminandum represents the concept.

The oddity concept appears to have been the most investigated relative class concept using the most different species of animals, including birds (e.g., Lombardi, Fachinelli, & Delius, 1984; Pastore, 1954; Wright & Delius, 1994; Zentall & Hogan, 1974), rodents (e.g., Langworthy & Jennings, 1972; Nakagawa, 1993; Wodinsky & Bitterman, 1953), carnivores (e.g., Strong & Hedges, 1966; Warren, 1960), and primates (e.g., Bernstein, 1961; Levine & Harlow, 1959; Thomas & Frost, 1983). Despite many claims that nonprimate animals have been shown to be able to perform oddity problems successfully on a conceptual basis, it has been suggested that the studies using nonprimate animals likely have all been subject to confounding variables or competing interpretations that render interpretations of successful oddity concept-based performance inconclusive (e.g., Premack, 1978; Steirn & Thomas, 1990; Thomas, 1994, 1996). However, it has not been contended that oddity concept learning is beyond the ability of nonprimate animals, only that the definitive investigation appears to be lacking.

Perhaps the most promising method to investigate oddity concept learning by rats was introduced by Langworthy and Jennings (1972) who described a clever, effective, and inexpensive way to present olfactory discriminanda to rats. It is well known that olfactory discriminanda are inherently more appropriate for rats (e.g., Lu, Slotnick, & Silberberg, 1993; Slotnick & Katz, 1974; Thomas & Noble, 1988), although some of the better known or more recent investigations of rat oddity concept learning have used visual discriminanda (e.g., Wodinsky & Bitterman, 1953; Nakagawa, 1993). Langworthy and Jennings concluded that their rats had shown use of the oddity concept, but they did not provide statistical validation. Thomas and Noble (1988) confirmed that Langworthy and Jenning's findings were statistically significant; however, Thomas and Noble noted that Langworthy and Jennings had only baited the correct food well, the one covered by the odd discriminandum. Baiting only the correct food well left open the possibility that the rats smelled the food and used that as the discriminative cue.

Thomas and Noble (1988) used Langworthy and Jennings' (1972) task but with a considerably modified procedure, intended to be more rigorous, to investigate whether rats could learn and use the oddity concept. Thomas and Noble used 16 odoriferous substances compared to 8 for Langworthy and Jennings; Thomas and Noble administered 300 five-trial problems instead of Langworthy and Jenning's 30 problems each one administered to a criterion (16 of 20 successive trials correct) or a maximum number of trials (100); and Thomas and Noble baited all three food wells. Thomas and Noble's rats showed very good learning set performance (responding correctly, better-than-chance on Trial 2), but they showed no evidence of correct responding on Trial 1. If the animals had acquired the oddity concept, then they should have responded correctly on Trial 1 (see French, 1965).

Because there are good reasons to persist in trying to determine whether rats can use the oddity concept (e.g., having a good rat model for relative class concept learning in psychopharmacological research; see Thomas, 1996), the present investigation was designed to approximate more closely the more extended training on each problem that Langworthy and Jennings had used while also maintaining the rigor of using more odiferous substances and baiting all three food wells on each trial to eliminate the odor of the food as a discriminative cue. …

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