In matching behavior, the subject is first presented with a standard stimulus, and then required to select the corresponding stimulus from a set (usually two) of comparison stimuli. There has been increasing contemporary interest in this situation (Blough, 1957(Blough, , 1959Ferster, 1960), perhaps because it shares a number of common dimensions with several other important research areas. If the number of standard stimuli is made large, the matching procedure relates closely to the "learning set" problems of Harlow (1959). If time intervals are introduced between the presentation of the standard and comparison stimuli, contact is made with studies of delayed responding. If, after a specified history, the subject is required to match "novel" stimuli, the procedure is relevant to investigations of generalization and concept formation. Finally, if the set of comparison stimuli is suitably chosen, psychophysical functions are generated.Preliminary data are reported here on a procedure which can be easily adapted to the study of all of these problems. METHOD Subjects and AppartusThe subjects were three White Carneaux barren hens. Throughout the experiment, the birds were maintained at 80% of their free-feeding weights. Grit and water were continuously available in the home cages. Maple peas were given in the home cages when feeding outside the experimental situation was required. The reinforcement grain mixture consisted of 50% Kaffir, 40% vetch, and 10% hempseed. Both the living quarters and the experimental room were air-conditioned.Experimental sessions were scheduled daily for each bird, except on those occasions when the animals' body weights differed from the 80% free-feeding value by more than 4 15 grams.The response chamber was in the form of an isoceles right triangle approximately 24 by 24 by 35 inches. The hypotenuse of the triangle was a plate-glass front which permitted observations of the Ss' behavior. The legs of the triangle were two solid walls, painted flat black. One was furnished with an access door; on the other were mounted the reinforcement magazine and three milk-plastic response keys, each I inch in diameter. The entire ceiling of the box was of translucent plastic, which diffused the light from three 25-watt bulbs. Behind each key was a light box with three 7.5-watt Christmas-tree bulbs.

The contingency between conditional and unconditional stimuli in classical conditioning paradigms, and between responses and consequences in instrumental conditioning paradigms, is analyzed. The results are represented in two- and three-dimensional spaces in which points correspond to procedures, or procedures and outcomes. Traditional statistical and psychological measures of association are applied to data in classical conditioning. Root mean square contingency, Ø, is proposed as a measure of contingency characterizing classical conditioning effects at asymptote. In instrumental training procedures, traditional measures of association are inappropriate, since one degree of freedom-response probability-is yielded to the subject. Further analysis of instrumental contingencies yields a surprising result. The well established "Matching Law" in free-operant concurrent schedules subsumes the "Probability Matching" finding of mathematical learning theory, and both are equivalent to zero contingency between responses and consequences.

Pigeons were exposed to three successive matching-to-sample procedures. On a given trial, the sample (red, green or blue light) appeared on a center key; observing responses to this key produced the comparison stimuli on two side keys. Seven different experimental conditions could govern the temporal relations between the sample and comparison stimuli. In the "simultaneous" condition, the center key response was followed immediately by illumination of the side key comparison stimuli, with the center key remaining on. In "zero delay" the center key response simultaneously turned the side keys on and the center key off, while in the "variable delay" conditions, intervals of 1, 2, 4, 10, and 24 sec were interposed between the offset of the sample and the appearance of the comparison stimuli on the side keys. In all conditions, a response to the side key of matching hue produced reinforcement, while a response to the non-matching side key was followed by a blackout. In procedure I all seven experimental conditions were presented in randomly permutated order. After nine sessions of exposure (at 191 trials per session, for a total of 1719 trials) the birds gave no evidence of acquisition in any of the conditions. They were therefore transferred to Procedure II, which required them to match only in the "simultaneous" condition, with both the sample and comparison stimuli present at the same time. With the exception of one bird, all subjects acquired this performance to near 100% levels. Next, in Procedure III, they were once more exposed to presentation of all seven experimental conditions in random order. In contrast to Procedure I, they now acquired the delay performance, and were able to match effectively at delays of about 4 sec.

The matching-to-sample experimental procedure was altered by reinforcing the selection of the non-matching comparison hue rather than the matching stimulus. Six birds were trained with red, green, and blue alternatives and a simultaneous presentation of stimuli in which the sample was present at the same time as the choice stimuli. The acquisition functions began well above the chance level but displayed a very slow improvement thereafter, which was different from that shown under matching conditions. Transfer of the oddity performance was tested by substituting a yellow light whenever a blue stimulus had previously been programmed. The results from the transfer test are considered in terms of both a “coding hypothesis” and the stimulus rules which appear to govern the performance of the “oddity” task.