Four experiments investigated the content of the memory used by rats in mediating retention intervals interpolated during performance in a 12-arm radial maze. The delay occurred following either the 2nd, 4th, 6th, 8th, or 10th choice. A 15-min delay had the greatest disruptive effect when interpolated in the middle of the choice sequence and less of an effect when it occurred either earlier or later. This pattern of results was obtained when either a free- or forced-choice procedure was used prior to the delay and regardless of whether postdelay testing consisted of completion of the maze or two-alternative forced-choice tests. Assuming that the disruptive effect of a delay is a function of memory load, this implies that the rats used information about previously visited arms (retrospective memory) following an earlier interpolated delay but information about anticipated choices (prospective memory) following a delay interpolated late in the choice sequence. There appeared to be a recency effect only in the early and middle delay conditions. This provides converging evidence for the dual-code hypothesis. No evidence for prospective memory was obtained following a 60-min delay.
We thank George W. Barlow, Stephen E. Glickman, and Sara Shettleworth for thoughtful critical comments on this article. This article is based, in part, on a thesis submitted to the University of California, Berkeley in partial fulfillment of the requirements for the doctoral degree in psychology for Cynthia M. Langley.
Two pigeons matched to sample in a three-key operant conditioning chamber. In Experiment I, two different kinds of samples were presented on the center key. Element samples were members of one of two sample sets -colors (a red or blue disk) or lines (a vertical or horizontal orientation of a set of white lines). These samples were followed by their respective sample sets on the side keys as comparison stimuli. Compound samples consisted of a set of lines superimposed on a colored disk. Following these samples, either sample set could appear as comparison stimuli. Matching to compound samples was less accurate than matching to element samples. One interpretation is that sharing of attention among elements of a compound sample weakened stimulus control by each element. A different interpretation is that an element sample controlled matching better because it was physically identical to a comparison stimulus whereas a compound sample was not. Experiments II-IV evaluated this "generalization decrement" alternative by testing element-vs. compound sample control with both element and compound comparison stimuli. Irrelevant elements were added to form compound comparison stimuli, some of which were physically identical to a preceding compound sample, but never identical to an element sample. In all experiments, the addition of irrelevant elements of comparison stimuli reduced sample control. However, the generalization decrement hypothesis failed to predict how differences in performance maintained by element and compound samples were affected by different tests of sample control. Matching accuracy appeared to be independently determined by the number of elements in a sample and whether irrelevant elements were present during tests of sample control.
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