No evidence that monkeys attribute mental states to animated shapes in the Heider-Simmel videos

Abstract

Human Theory of Mind (ToM) is so automatic and pervasive that we spontaneously attribute mental states to animated abstract shapes, as evidenced by the classic Heider-Simmel findings. The extent to which this represents a fundamental characteristic of primate social cognition is debated. Prior research suggests that monkeys spontaneously predict behavior and attribute basic goals to conspecifics, but it remains unclear whether, like humans, they spontaneously ascribe mental states to animated shapes. Here, we address this question by analyzing rhesus monkeys' viewing patterns of the classic Heider-Simmel animations. We hypothesized that if rhesus monkeys also spontaneously attribute mental states to animated shapes, then, like humans, they would have the longest fixation durations for theory of mind animations, medium duration fixation for goal-directed animations, and shortest fixations for animations with random motion. In contrast, if attributing mental states to animations is specific to humans and perhaps other apes, then we predict no differences in looking time across animation categories. Unlike humans, monkeys did not fixate longer on ToM videos. Critically, monkeys' viewing patterns did not correlate with humans' viewing patterns or intentionality ratings from previously published research. The only major difference in viewing patterns between animation categories tracked differences in low-level visual motion. Thus, monkeys do not view the classic Heider-Simmel animations like humans do and we found no evidence that they spontaneously attribute mental states to animated shapes.

Figure 1

Monkeys freely viewed the Heider–Simmel animations. (a) While seated in a primate chair with heads fixed, monkeys viewed 12 animations lasting 18 s each. (b) Abell et al. animations categories: Theory of Mind, Goal Directed, and Random. Each category was composed of four videos. (c) Monkeys viewed the animations in a random order with 30–40 s intertrial intervals.

Figure 2

Monkeys did not show longer fixations for ToM videos. (a) Monkeys’ fixation durations as a function of animation category. Each thick horizontal line is the group mean fixation duration and each symbol represents an individual monkey’s mean fixation duration with ± 95% CI (ms). Abbreviations: ToM, Theory of Mind; GoalDir, Goal Directed; Rand, Random. (b) For comparison, humans’ fixation durations (± 95% CI) and intentionality ratings (± SD) as a function of category. Adapted from Klein et al.. (c) Peak visual motion, left, and standard deviation in visual motion, right, of the animations as a function of category. Visual motion was calculated as in.

Figure 3

Monkeys viewing preferences were idiosyncratic. Monkeys showed viewing preferences for individual videos, not broad categories. Colors represent video categories as in Figs. 1 and 2. See also Supplemental Fig. 1.

Figure 4

Monkeys’ mean fixation duration did not significantly correlate with humans’ fixation durations or intentionality ratings (data from Klein et al.). (a) Monkeys’ mean fixation duration (ms) for each animation as a function of the humans’ mean fixation duration reported in Klein et al.. (b) Monkeys’ mean fixation duration (ms) for each video as a function of humans’ intentionality ratings (0–5; 0 = nondeliberate action and 5 = deliberate action with the goal of affecting the other’s mental state). Each symbol represents data from a single animation. Colors represent animation categories, and match assignments used in Figs. 1, 2 and 3.