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Contribution of conditioned opioid analgesia to the shock-induced associative US-preexposure deficit

The modulatory effect of conditioned opioid analgesia on learning in the US-preexposure paradigm was examined in three experiments using water-deprived rats. In Experiment 1, it was found that tailflick latencies increased immediately after the rats were exposed to a context in which footshock had previously been administered. Prolonged nonreinforced exposure to the context attenuated this analgesia. Experiment 2 tested the possibility that the effectiveness of CS-US pairings in an excitatory context might be reduced by a conditioned analgesic response that lessens the perceived intensity of the US. Administration of the opiate antagonist naloxone prior to CS-US pairings in the excitatory context reduced the US-preexposure deficit—that is, the retarded response to the CS—but did not eliminate it, suggesting that part of the observed deficit resulted from conditioned activation of the endogenous opioid system. In Experiment 3, it was found that exposure to the excitatory context immediately prior to a CS-US pairing in an associatively neutral context resulted in a conditioned response deficit, indicating that the analgesia elicited by the excitatory context was sufficient to reduce US effectiveness. In combination with other recent reports, these results suggest that the associative deficit resulting from preexposure to a shock US may, in certain instances, represent the sum of several different associative processes.     

Auditory delayed discriminations by the dolphin: Nonequivalence with delayed-matching performance

Working memory in a bottlenosed dolphin was tested in both indirect and direct auditory delayed-discrimination tasks in which a correct spatial response was conditional upon the nature of a preceding sound. In the indirect task, either one of two possible sounds was briefly presented. After a prescribed delay, the dolphin was cued to go either to a left-hand or right-hand paddle pair. Responses to the outer paddle of a pair were rewarded following sound A, and responses to the inner paddle of a pair were rewarded after sound B. In the direct delayed-discrimination task, only one paddle pair was used in each session. In both tasks, the delay interval between the discriminative sound stimulus and the opportunity for a spatial response was progressively increased over sessions until the animal failed to meet a specified performance criterion or self-terminated a session. Delay limits of about 30 and 60 sec were obtained in the indirect and direct tasks, respectively. The increase in delay limit in the latter task was attributable to the use of an overt mediational response during the longer delays. In both cases, however, the obtained delay limits fell considerably short of the 2- to 3-min limits obtained in auditory delayed-matching studies using the same test sounds and the same subject. The task differences indicate that working memory functions cannot depend upon memory of the predelay stimulus alone, but must be determined in part by additional processes.     

Delayed matching of visual materials by a bottlenosed dolphin aided by auditory symbols

A bottlenosed dolphin (Tursiops truncatus) with good underwater and aerial visual acuity was tested in visual matching-to-sample (MTS) paradigms. Attempts to train visual identity MTS directly, using two simple two-dimensional patterns as sample stimuli and as alternatives (comparison stimuli), met with little success, in keeping with previously observed difficulties of this auditory-specialized species for learning complex tasks utilizing simple visual materials. Pairing of each visual sample with a unique sound, to produce a compound auditory-visual sample, while retaining the two visual alternatives, resulted in the dolphin’s learning both auditory-visual symbolic matching and visual-visual identity matching. At 0-sec delay, performance with the auditory element of the sample alone was equivalent (76%) to performance with the visual element alone; performance with the compound was distinctly better (95%–98% correct). Testing with longer delays using the visual element alone resulted in successful matching through to a maximum delay of 34 sec. These results provided the first demonstration of delayed MTS in a dolphin using visual materials, and complemented other data showing the ready capability of this species for delayedauditory MTS. It appeared that the dolphin solved the visual MTS task by forming auditory codes to represent the visual materials, and that these auditory codes were eventually replaced with purely visual codes.