Two experiments indicated that pigeons commonly code samples associated with same comparison in many-to-one matching. Experiment 1 showed that retention was similar for pigeons matching four sample stimuli (two hues and two lines) to a different pair of comparisons (hues or lines). Accuracy was slightly higher with hue than with line samples, but this did not interact with delay. Alternative samples from each dimension and associatively different samples from the different dimensions both produced intertrial interference. In Experiment 2, pigeons learned new comparison associations to two samples from a prior many-to-one task. Later, their ability to match the remaining samples to the new comparisons was tested. Positive transfer occurred when samples previously paired with the same comparison replaced one another. Negative transfer occurred when opposing samples were interchanged. Apparently, the associatively related samples in many-to-one matching evoke similar representations.

Pigeons were presented with an analog of a radial-arm-maze task involving 5 response keys. In Experiment 1, pigeons were exposed to either a linear (L) or a two-dimensional matrix (M) array of the 5 keys, and the keys were either each of a distinctive hue (H) or were all white (W). Acquisition was facilitated both by M and by H. In Experiment 2, increasing the number of pecks that constituted a choice from 5 to 20 for half of the LW birds had little effect on performance. In Experiment 3, a delay was interpolated after a number of choices that varied across trials. All groups except the LW 5-peck group showed little disruption in performance with increasing delay (up to a maximum of I hr). Inverted-U-shaped point-of-delay interpolation functions were found for each group. The data suggest that pigeons retrospectively coded choices already made when the delay occurred early in the trial and prospectively coded choices yet to be made when the delay occurred late in the trial. Experiment 1We developed a five-choice version of the Wilkie and Kennedy (1985) task to explore pigeons' spatial coding ability. 358This document is copyrighted by the American Psychological Association or one of its allied publishers.This article is intended solely for the personal use of the individual user and is not to be disseminated broadly.

Coding strategies developed in the acquisition of delayed conditional discriminations can be assessed by independently manipulating sample and comparison memory load. Two stimulus dimensions that can affect memory load were examined: Number of stimuli in the sample and comparison sets (two vs. four) was manipulated between groups in a 2 x2 design, and discriminability of sample and comparison stimuli (hues vs. lines) was manipulated between counterbalancing subgroups and within subjects. The results indicated large effects of sample discriminability but not of comparison discriminability, evidence for retrospective coding. There was also a significant effect of number of stimuli in the comparison set (although only with hard-to-discriminate samples) but not of number of stimuli in the sample set, evidence for prospective coding. These findings suggest evidence for retrospective coding with easy-to-discriminate samples, independently of number of stimuli in the comparison set, and evidence for prospective coding with hard-todiscriminate samples. Research in animal memory has recently focused on the identification of the events that are represented in memory during the retention interval. For example, after having learned the contingencies between the samples and comparisons in delayed matching-to-sample (DMTS), a subject can perform correctly on the retention test by remembering the sample it saw at the beginning of the trial or, alternatively, which comparison stimulus to respond to after the retention interval. In other words, either retrospective coding of the samples or prospective coding of the comparisons can mediate delayed matching performances (Honig & Thompson, 1982; Wasserman, 1986). A considerable body of literature developed over the last few years appears to have established retrospective and prospective coding as distinct, separable memory processes (e.g.

Hippocampal processing is often crucial for normal spatial learning and memory in both birds and mammals, suggesting a general similarity in avian and mammalian hippocampal function. However, few studies using birds have examined the effect of hippocampal lesions on spatial tasks analogous to those typically used with mammals. Therefore, we examined how hippocampal lesions would affect the performance of pigeons in a dry version of the water maze. Experiment 1 showed that hippocampal-lesioned birds were impaired in acquiring the location of hidden food in the maze. Experiment 2 showed that hippocampal-lesioned birds were not impaired when a single cue indicated the location of hidden food. These results support the notion that avian and mammalian hippocampal functions are quite similar, in terms of the tasks for which their processing is crucial and the tasks for which it is not.

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