WITHIN-SUBJECTS EXPERIMENTS ON BLOCKING AND FACILITATION IN HONEYBEES (Apis mellifera)

The parsimonious assumption of early learning theorists (Hull, 1929; Spence, 1936) that the components of a compound stimulus gain and lose associative strength independently with reinforcement and nonreinforcement has generally been regarded as untenable in the light of experiments on “blocking” in rats and a variety of other animals, but those experiments have been clouded from the outset by a variety of control problems. In the prototypical case (Kamin, 1969), a blocking group has reinforced trials with stimulus A (A+ trials) that are followed by reinforced trials with a compound of stimuli A and B (AB+ trials), while a control group has reinforced trials with the compound alone. In a subsequent test, the blocking group responds less to B than does the control group, although the contiguity of B with reinforcement has been the same for both. In his work on “cue validity,” Wagner (1969) soon found that subsequent responding to B is less also when the A+ training trials do not precede the AB+ trials but are interspersed among them, an effect that has been described as “concurrent” blocking to distinguish it from the prototypical “forward” blocking. Kamin’s control procedure – the elimination of A+ trials – leaves us uncertain as to whether it is the conditioning of A that is responsible for the difference, or only the additional experience with reinforcement apart from A, or the additional experience with A apart from reinforcement. Wagner (1969) substituted unsignalled reinforcements for the A+ trials, which equated experience with reinforcement, but not with A. In later experiments, A was backwardly paired or explicitly unpaired with reinforcement, thus equating experience both with A and with reinforcement, or reinforced trials with a different stimulus (C+ trials) were substituted for the A+ trials; in their work with honeybees, Smith and Cobey (1994) notably used all three of these control procedures. Although none of the control procedures, either alone or in combination, can provide unequivocal evidence of blocking, their various shortcomings have commonly been lost sight of because the same outcome -- less responding by the blocking group than by the control group in the test with B -- has usually (but not always) been reported for all of them. The traditional explanation of blocking is that the conditioning of B on AB+ trials is impaired by the conditioning of A, with which B must compete for associative strength (Rescorla & Wagner, 1972) or for the attention assumed to support the acquisition of associative strength (Sutherland & Mackintosh, 1971). A more recent suggestion (Miller & Matzel, 1988) is that the competition is not for associative strength (the old independence assumption is here resurrected) but for control of performance. The pairing of A and B is held to establish a so-called “comparator” relation between them which determines that responding to each alone will be an increasing function of its own associative strength relative to that of the other. A limitation of all conventional between-groups designs, as Rescorla (1981) noted to little effect some decades ago, is that the two treatments compared may differentially affect responding to B quite apart from experience with it. As the most obvious example, the enhancement of contextual conditioning by unsignalled reinforcement in the training of a control group like Wagner’s might be expected to produce greater responding in the test with B even where the associative strength of B is the same in both groups. Just such an experiment, and several others that had given the appearance of blocking in honeybees (Blaser, Couvillon, & Bitterman, 2004), was recently repeated (Blaser, Couvillon, & Bitterman, 2006) with only one difference in procedure: B was absent in training and encountered for the first time in the test. In every case, even so, there was less responding to B by the group whose treatment was otherwise the same as that of the blocking group than by the group whose treatment was otherwise the same as that of the control group, an effect we have termed “pseudoblocking.” These results demonstrate that different treatments of non-target stimuli may in fact influence responding to B in the test quite apart from experience with B in the training. They suggest that a demonstration of blocking in any conventional between-groups experiment can carry little conviction unless accompanied by null results for parallel B-absent groups. Fortunately, this difficulty can be avoided by comparing the treatments within subjects, a much more efficient strategy as well, since a single group takes the place of four. In a within-subjects experiment with pigeons by Rescorla (1981), differentially reinforced trials with stimuli A and C (A+ and C- trials) were followed by consistently reinforced trials with compounds of A and B (AB+ trials) and of C and D (CD+ trials); in a subsequent test, the animals responded less to B than to D. This design confounds the possibility of a facilitating effect of the C- training on responding to D (examined early by Rescorla, 1971) with the possibility of a blocking effect of the A+ training on responding to B, both of which are anticipated by current theories of blocking. Confounded designs have since been used also in conventional between-groups experiments to study the tenability of the independence assumption for honeybees (Couvillon, Arakaki, & Bitterman, 1997; Couvillon, Campos, Bass, & Bitterman, 2001). In within-subjects experiments, if not in between-groups experiments, blocking and facilitation effects can readily be separated, the C- trials eliminated to look for blocking alone and the A+ trials eliminated to look for facilitation alone. What may be the first unambiguous evidence of blocking in any animal has, in fact, recently been provided by pure within-subjects blocking experiments (A+ trials, followed by AB+ and CD+ trials, and then by a test with B and D) on conditioned fear in rats (McNally & Cole, 2006; Cole & McNally, 2007). Reported here are some within-subjects experiments with free-flying honeybees that were designed to look for blocking and facilitation unquestionably dependent on experience with the putative blockers and facilitators. For honeybees, it should be noted, even conventional between-groups experiments have as yet produced no indisputable indications of blocking (Gerber & Ulrich, 1999; Guerrieri, Lachnit, Gerber, & Giurfa, 2005). The concurrent procedure, expected by contemporary theories to produce both blocking and facilitation, was used in these initial within-subjects experiments because of its operational convenience. The strategy for each set of stimuli – odors and colors -- was to begin with a confounded design and then, if a confounded effect appeared, to analyze it in pure blocking and facilitation experiments.