Two experiments examined the positive extradimensional shift effect of overtraining in concurrent mixed discriminations. In Experiment 1 rats were trained on two concurrent simultaneous discriminations and then either reversed on both tasks (Group W), or reversed on one task while a second novel discrimination was introduced (Group eMS). After initial training, two novel discrimination tasks were presented to a third group of rats (Group EDS). Overtraining facilitated extradimensional shift in Group eMS, whereas it neither facilitated nor retarded shift learning in Group EDS. In Experiment 2 using two concurrent go/no-go discriminations with the same design as in Experiment 1, overtraining facilitated extradimensional shift in Group eMS, whereas it delayed the corresponding shift in Group EDS. These findings make it clear that an important factor that determines the animals performance is not only stimulus properties but also the nature of the response required.The old dispute about whether or not overtraining facilitates reversal and non reversal (Le., extradimensional) shifts in a single discrimination task has generally been resolved by conceding that it facilitates reversal shift, whereas it delays or does not facilitate at least non reversal (Le., extradimensional) shift with a few exceptions, in which overtraining facilitates extradimensional shift (Hall, 1974;Mandler, 1968;Mandler & Hooper, 1967;Waller 1970Waller , 1971). However, recently some studies, using a . whole-partial reversal procedure, have reported that overtraining facilitated reversal shift (Le., a positive overtraining reversal effect: positive ORE) under one training condition, say, of a whole reversal condition, in which both discrimination tasks were reversed, whereas it retarded reversal shift (Le., a negative ORE) under the other training condition, say, of a partial reversal condition, in which only one of them was reversed, in concurrent discriminations (Nakagawa, 1978(Nakagawa, , 1986 Correspondence concerning this article should be sent to Esho Nakagawa,

The present two experiments conducted a limited parametric study of the overtraining variable using a whole-partial reversal procedure in rats. Rats received two concurrent two-choice discriminations (Experiment 1) , two concurrent go-no go successive discriminations (Experiment 2), and then received reversal training under a given condition of a whole reversal or a partial reversal either immediately after criterion training , following 2, 10, or 20 days of overtraining in Experiments 1 and 2. In Experiment 1, after 20 days of overtraining, a whole reversal , in which both discrimination tasks were reversed , produced more rapid reversal than a partial condition , in which one of the two discriminations was reversed whereas the other was maintained as in original learning. Conversely, after 2 days of overtraining rats in the partial condition reversed faster than did those in the whole condition. Experiment 2 essentially replicated results of Experiment 1. Experiments 1 and 2 show that a stimulus function transfer and stimulus substitutability, respectively, are most clearly evident after 20 days of overtraining. These results indicate that additional prereversal training (i.e., overtraining) is an obvious operational precondition for stimulus classes formation in simple concurrent discriminations in rats.There are two separate definitions of a stimulus class: The first one pertains to functional equivalence, and the second pertains to the control of a specific response by one member of the stimulus class. Goldiamond (1962) argues that both are necessary for a set of stimuli to be considered a stimulus class. This argument is a very important and even fundamental issue in behavior analysis, and one that has received far too little experimental attention.Nakagawa (1986, 1992), using a whole-partial reversal procedure which compared rats' performance on whole reversal sessions (both stimulus pairs reversed, from A+C-, 8+0-to A-C+ , 8-0+ for example) with that on partial reversal sessions (only one pair reversed, from A-C+, 8-0+ to A+C-, 8-0+ for example) , reported that rats could form stimulus classes during overtraining in concurrent discriminations. In Nakagawa's experiments , rats were trained to criterion or overt rained on two Requests for reprints should be sent to Esho Nakagawa,

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The present experiment examined the influence of overtraining on the nonshift and reversal shift of a matching-tosample (MTS) discrimination. Rats were trained to criterion or were overtrained on the MTS discrimination. After completing Phase 1 training , they received extinction to criterion , and then were given either a nonshift (Group Nonshift) problem , in which the rule was not changed from Phase 1 but novel stimuli were used, or a reversal shift (Group Reversal) problem , in which the rule was changed from Phase 1 but the stimuli were not changed . Overtraining facil itated Phase 3 shift learning of both Group Nonshift and Group Reversal. There were no significant differences in the rate of learning in Phase 3 shift between Groups Nonshift and Reversal after criterion training and overtraining. These findings suggest that a common response to a configuration of stimuli with the same response assignment formed in Phase 1 remain effective intact after extinction.There are many studies on stimulus class formation in pigeons and rats. They make it clear that both species treat stimuli as being equivalent if they signal the same outcome in either concurrent discriminations (Delius, Ameling, Lea, & Staddon , 1995; Dube, Callahan , & Mcllvane, 1993;Nakagawa, 1978Nakagawa, , 1986Nakagawa, , 1992aNakagawa, , 1998Nakagawa, , 1999aNakagawa, , 1999bNakagawa, , 1999cNakagawa, , 1999dNakagawa, , 2000b a) or matching-(or nonmatching)-to-sample discriminations (Aggleton , 1985;Mumby, Pine, & Wood, 1990;Nakagawa, 1992bNakagawa, , 1993aNakagawa, , 1993bNakagawa, , 1999bNakagawa, , 2000aNakagawa, , 2000c Nakagawa, b, 2001Rothblat & Hayes, 1987; Urcuioli, 1977; Urcuioli & Nevin , 1975;Zentall & Hogan, 1974, 1976 Zentall , Sherburne, Steirn, Randall , Roper, & Urcuioli , 1992; Zentall , Steirn, Sherburne, & Urcuioli , 1991). Nakagawa (200 1 c) has examined effects of overtraining on nonshift and shift (i.e., reversal shift) learning in a matching-(or nonmatching)-tosample discrimination. Nakagawa (2001 c) has reported that overtraining facil itated both nonshift and reversal shift (i.e., shift-1) learnings, and that Requests for reprints should be sent to E. Nakagawa,

Two experiments examined whether or not rats formed stimulus classes on a basis of same response (i.e., shared common response) during overtraining. In Experiment 1, rats were trained on two discriminations in a straight runway, then trained on conditional successive discriminations in a Y maze. Group C, in which rats were required to choose the same goal box when the original positive stimulus or the negative one was presented on the entrance of each goal box, learned shift problems faster than Group IC, in which rats were required to choose the right goal box when one of positive stimuli was presented and to choose the left goal box when the other was presented as well as negative stimuli, after overtraining, but not after criterion training . In Experiment 2, rats were trained on two discriminations in the Y maze, and then they were trained on either whole reversal (Group W) in which two tasks were reversed or partial reversal (Group P) in which one of two tasks was reversed in a straight runway. Group W learned their reversal faster than Group P after overtraining , but not after criterion training. These results indicate that the same response is a factor affecting stimulus classes formation in rats.There are two main views on the formation mechanism of stimulus classes: The one is categorization processes based on simple similarity between stimuli (Bhatt & Wasserman, 1989;Fersen & Lea, 1990; Vaughan & Herrstein, 1987), the other is stimulus associations on the basis of reinforcement concordance (Deli us, Ameling, Lea, & Staddon, 1995;Edwards, Jagielo, Zentall, & Hogan, 1982;Nakagawa, 1986Nakagawa, , 1992Nakagawa, , 1998 Urcuioli, Zentall, Jackson, Smith, & Steirn, 1989, 1998. However, it is important to note that such stimulus associations mechanism as postulated in Nakagawa (1986, 1992, 1998) and Delius et al. (1995) is not necessarily in conflict with categorization processes based on simple similarity between stimuli. Conversely, it seems likely that in nature both principles may often act in consonance. A specific question, however, remains. What precise conditions favor the formation of stimulusRequests for reprints should be sent to E. Nakagawa,