Do animals form task-specific representations, or do those representations take a general form that can be applied to qualitatively different tasks? Rhesus monkeys (Macaca mulatta) learned the ordering of stimulus lists using two different serial tasks, in order to test whether prior experience in each task could be transfered to the other, enhancing performance. The simultaneous chaining paradigm delivered rewards only after subjects responded in the correct order to all stimuli displayed on a touch sensitive video monitor. The transitive inference paradigm presented pairs of items and delivered rewards when subjects selected the item with the lower ordinal rank. After learning a list in one paradigm, subjects’ knowledge of that list was tested using the other paradigm. Performance was enhanced from the very start of transfer training. Transitive inference performance was characterized by ‘symbolic distance effects,’ whereby the ordinal distance between stimuli in the implied list ordering was strongly predictive of the probability of a correct response. The patterns of error displayed by subjects in both tasks were best explained by a spatially coded representation of list items, regardless of which task was used to learn the list. Our analysis permits properties of this representation to be investigated without the confound of verbal reasoning.
In the current study, we examined the planning abilities of rhesus monkeys (Macaca mulatta) by training them on a five-item list composed of coloured photographs and then testing them on switch and mask trials. In contrast to previous studies where monkeys made responses using a joystick, in the current study, monkeys made responses directly to a touch screen. On switch trials, after a response to the first list item, the on-screen positions of two list items were exchanged. Performance on trials in which the second and third list items were exchanged was poorer compared to normal (non-switch) trials for all subjects. When the third and fourth items were exchanged, however, only one subject continued to show performance deficits. On mask trials, following a response to the first item, the remaining items were covered by opaque white squares. When two items were masked, all four subjects responded to each masked item at a level significantly above chance. When three items were masked, however, only one subjected was able to respond to all three masked items at a level significantly above chance. The results of the present study indicate that three of our four monkeys planned one response ahead while a single monkey planned two responses ahead. The significance of these findings is discussed in relation to previous studies on planning in chimpanzees and monkeys.
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