The psychological organization of "uncertainty" responses and "middle" responses: a dissociation in capuchin monkeys (Cebus apella)

Abstract

Some studies of nonhuman animals' metacognitive capacity encourage competing low-level, behavioral descriptions of trial-decline responses by animals in uncertainty-monitoring tasks. To evaluate the force of these behavioral descriptions, the authors presented 6 capuchin monkeys (Cebus apella) with 2 density discrimination tasks between sparse and dense stimuli. In one task, difficult trials with stimuli near the middle of the density continuum could be declined through an "uncertainty" response. In the other task, making a "middle" response to the same stimuli was rewarded. In Experiment 1, capuchins essentially did not use the uncertainty response, but they did use the middle response. In Experiment 2, the authors replicated this result with 5 of 6 monkeys while equating the overall pace and reinforcement structure of the 2 tasks, although 1 monkey also showed appropriate use of the uncertainty response. These results challenge a purely associative interpretation of some uncertainty-monitoring performances by monkeys while sharpening the theoretical question concerning the nature of the psychological signal that occasions uncertainty responses.

Figure 1

A trial from the psychophysical discrimination task. The pixel box at the top center of the screen was colored yellow and was the stimulus to be classified sparse (“S”) or dense (“D”). The third response (“?”) cleared the screen and moved to the next trial—this was the uncertainty response. In the alternative version of the task, the third response option was the letter “M”, and it was the correct response for pixel boxes of an intermediate or middle density. This was the middle response. The cursor is the small circle in the bottom center—it was colored red.

Figure 2

Mean percentage of sparse, dense, and uncertainty or middle responses in the Uncertainty (A) and Middle (B) task of Experiment 1.

Figure 3

Performance in Experiment 1 separated by task order completion. The top row shows performance for monkeys who completed the SUD task (A) followed by the SMD task (B). The bottom row shows performance for monkeys who completed the SMD task (C) followed by the SUD task (D).

Figure 4

Approximate rewards per minute that could be earned by different decisional strategies under the reinforcement and timeout contingencies used in Experiments 1 and 2. The simulations that produced these estimates are described in the text.

Figure 5

Mean percentage of sparse, dense, and uncertainty or middle responses in the two cycles of the Uncertainty (A,C) or Middle (B,D) tasks of Experiment 2. The top and bottom, graphs, respectively, come from the first and second Uncertainty-Middle cycle.

Figure 6

Mean percentage of sparse, dense, and uncertainty responses for Logan (A) compared to the five other capuchins (B) for the second Uncertainty-Middle cycle of Experiment 2.