Macaque monkeys learn by observation in the ghost display condition in the object-in-place task with differential reward to the observer

Abstract

Observational learning has been investigated in monkeys mainly using conspecifics or humans as models to observe. Some studies attempted to clarify the social agent's role and to test whether non-human primates could learn from observation of a non-social agent, usually mentioned as a 'ghost display' condition, but they reported conflicting results. To address this question, we trained three rhesus monkeys in an object-in-place task consisting of the presentation of five subsequent problems composed of two objects, one rewarded and one unrewarded, for six times, or runs. Three types of learning conditions were tested. In the individual learning condition, the monkeys performed the first run, learned from it and improved their performance in the following runs. In the social and non-social learning conditions, they observed respectively a human model and a computer performing the first run and learned by the observation of their successes or errors. In all three conditions, the monkeys themselves received the reward after correct choices only. One-trial learning occurred in all three conditions. The monkeys performed over chance in the second run in all conditions, providing evidence of non-social observational learning with differential reward in macaque monkeys using a "ghost display" condition in a cognitive task.

Figure 1

Task. (A) Examples of stimuli displayed as objects in a problem and stimuli displayed as feedback around the chosen object. (B) Example of the temporal sequence of a trial. The periods of the trial were identical in the three different conditions MA, HI, and CI. In the CI condition, the central target was red instead of white only in the first run. (C) Example of temporal sequence of the six runs. Five problems composed the first run and were subsequently represented in the same order for six times. Six runs composed one complete session.

Figure 2

Learning curves for the three monkeys. The curves show the mean percentage of correct choices for the six runs in the 60 sessions for the three different conditions. Vertical bars represent the separation between runs always performed by the monkeys (on the right) and first runs performed by different agents depending on the experimental condition (on the left). Dashed lines represent chance level (50%). As expected, the first run performances do not differ from chance in any case. The mean percentage of correct choices in the second run is above chance in all conditions and for all three monkeys, showing that learning occurred after a single trial. Error bars represent standard error means (SEM).

Figure 3

Performance in the second run after correct responses and errors made during the first run. The scatterplot shows the percentage of correct responses in the second run for the three monkeys in the three conditions, MA, HI, and CI. Trials were divided into two groups based on the performance in the first run, correct response or error. AC: after correct; AE: after error. Stars indicate significant differences (*p < 0.05 two-sample test for equality of proportions with continuity correction; **p < 0.01, two-sample test for equality of proportions with continuity correction).