Procedures for generating arbitrary matching-to-sample performances may generate only conditional discriminations. Rational grounds for this distinction are proposed, based on the properties that any equivalence relation must possess. Empirical tests are described for determining whether subjects trained on conditional discriminations are also engaged in true matching to sample. A series of studies then leads to the conclusion that proof of true matching to sample by monkeys, pigeons, or baboons is yet to be provided. Whether the absence of such proof reflects experiential factors or species-defined limitations is not presently clear.Key words: conditional discrimination, matching to sample, stimulus equivalence, stimulus classes, stimulus control, key press, monkeys, baboons, children A commonly used experimental arrangement presents a subject with two discriminative stimuli simultaneously, perhaps a vertical and a horizontal line, while a third stimulus, perhaps a green or red hue, determines the positive or negative function of each line. Proce-

A detailed analysis is presented of the ways in which control by the negative stimulus in two-comparison conditional discriminations may be expected to affect the outcome of tests for the properties of equivalence relations. Control by the negative stimulus should produce the following results: (a) no observable effect on symmetry tests; (b) reflexivity test results should look like "oddity" rather than "identity"; and (c) transitivity tests that involve an odd number of nodes should yield results that are 100% opposite to tests that involve an even number of nodes. The analysis also considers the effects of variation in the type of comparison-stimulus control between and within baseline conditional discriminations. Methods are suggested for experimentally regulating the type of control, and for verifying the predictions that the analysis generates. If suggested experiments continue to support the analysis, investigators who use two-comparison conditional discriminations to study equivalence relations will either have to control explicitly whether the positive or the negative comparison governs their subjects' choices, or they will have to abandon two comparisons and use three or more comparisons instead.

With a customary arrangement of three horizontally aligned stimulus/response keys, two rhesus monkeys learned conditional hue and line discriminations--an "identity-matching" procedure. First, sample stimuli were presented on the center key, and comparison stimuli were presented on the two side keys. Next, the sample was allowed to appear on any one of the three keys, with the comparisons on the remaining two. The change from fixed to variable sample and comparison locations caused the horizontal and vertical lines to lose control over the animals' responses; the conditional hue discrimination remained intact. Accurate description of controlling stimuli in a matching-to-sample procedure may therefore require that their spatial location be specified.

A discriminated Sidman avoidance procedure used by Forgione (1970) was replicated using a head-poke response instead of a leverpress as the avoidance operant. The resultant data were described in terms of the five dependent measures reported by Forgione. Head-poke avoidance was found to be more efficient than its leverpress counterpart and compared very favorably with the lever-disabling (or shock-timer-on) procedure used by Forgione to break up inefficient leverpress behavior patterns.The purpose of the present study was to explore the feasibility of using a head-poke response (Bolles & Seelbach, 1964) in investigations of discriminated free operant (Sidman) avoidance with rats. In this study, a head poke was defined as the entry of a rat's head into a small hole far enough to disrupt a photocell beam. Like the leverpress, the head poke is objectively measured and is capable of being emitted over a long period of time. But more important for present purposes, the topography of the head poke (in contrast to the topography of the leverpress) would seem to make it ideally suited for free operant avoidance.Leverpress avoidance appears to involve a set of response topographies and operant contingencies that interact to retard the development of efficient avoidance behavior (Forgione, 1970). Specifically, rats tend to hover in the vicinity of the lever or to lean on it throughout the response-shock interval (R-S interval). When the R-B interval terminates and shock occurs, the animal often hits the lever and thus recycles the R-S interval. If the R-S interval exceeds the shock-shock interval (S-S interval), this behavior is reinforced by reduction in shock frequency. The behavior is inefficient, however, to the extent that it comes under the control of shocks terminating the R-S interval. Presumably, more efficient behavior could be shaped by reinforcing only those responses emitted in the absence of shock. In support of this notion, Forgione (1970) found that disabling