Comparing Biological Motion Perception in Two Distinct Human Societies

Cross cultural studies have played a pivotal role in elucidating the extent to which behavioral and mental characteristics depend on specific environmental influences. Surprisingly, little field research has been carried out on a fundamentally important perceptual ability, namely the perception of biological motion. In this report, we present details of studies carried out with the help of volunteers from the Mundurucu indigene, a group of people native to Amazonian territories in Brazil. We employed standard biological motion perception tasks inspired by over 30 years of laboratory research, in which observers attempt to decipher the walking direction of point-light (PL) humans and animals. Do our effortless skills at perceiving biological activity from PL animations, as revealed in laboratory settings, generalize to people who have never before seen representational depictions of human and animal activity? The results of our studies provide a clear answer to this important, previously unanswered question. Mundurucu observers readily perceived the coherent, global shape depicted in PL walkers, and experienced the classic inversion effects that are typically found when such stimuli are turned upside down. In addition, their performance was in accord with important recent findings in the literature, in the abundant ease with which they extracted direction information from local motion invariants alone. We conclude that the effortless, veridical perception of PL biological motion is a spontaneous and universal perceptual ability, occurring both inside and outside traditional laboratory environments.

these analyses, some minor but unavoidable data exclusions were made in order to maintain the integrity of our participant groups in terms of: (i) the requirement for observers to have no previous experience with biomotion stimuli, and (ii) the requirement for observers to have been born and raised entirely in one strict geographically-defined location. Specifically, a number of Mundurucu observers completed the walker trials of Experiment 1 having previously taken part in Experiment 2. As these observers were no longer naive to biological motion tasks on completing the Experiment 1 trials, their data for Experiment 1 was not used in the analyses with the other eighteen observers. Data from one American observer was excluded from analysis on similar grounds. While running in Experiment 1, this observer noted having taken part previously in a study that employed biological motion stimuli. Data from two additional American observers was excluded when they revealed to the experimenter that, while being US citizens, they were born outside the country and had moved to the US as children.

Further details about PL stimulus construction
The human PL walker represented the average walker computed from a database consisting of motion-captured data from 50 women and 50 men. Details about the data acquisition and about the algorithm used to average the 100 walkers can be found elsewhere [1]. A set of 11 markers was used, which represented the head, one shoulder, one hip, the two elbows, two wrists, two knees, and two ankles. The second point-light sequence showed a walking pigeon. Data were recorded from a pigeon that was trained to walk back and forth between two feeders. The pigeon was supplied with 11 retroreflective markers. Both humans and pigeons were recorded by means of an optical motion capture system (Vicon, Oxford Metrics); constant translation was subtracted from the data, and a third-order Fourier series was fitted to the data to smooth them and loop them into a repeatable walking cycle. The third sequence showed a walking cat. The data are based on a high-speed (200 fps) video sequence showing a cat walking on a treadmill. Fourteen feature points were manually sampled from single frames. As with the other sequences, data were approximated with a third-order Fourier series to obtain a generic walking cycle.
Gait frequencies were 0.93 Hz for the human, 1.6 Hz for the pigeon, and 1.7 Hz for the cat.
The display size on the screen was 2.3 x 5 degrees of visual angle for the human, 3.1 x 4.4 degrees for the pigeon, and 5.7 x 3.1 degrees for the cat.

Additional task
Prior to completing the walker task in Experiment 2, observers also completed a contrasting global form perception task. This task is a version of the "pathfinder" display devised by Field et al. [2] and used in earlier work by one of us, where a more detailed description can be found [3,4]. Briefly, on each of 100 test trials (two concurrently-run 50 trial staircases) the entire laptop monitor was filled with short lines whose orientations were randomly determined on each trial. The display was also divided into four equal-sized Threshold values were calculated as the mean over the last 8 trials of a staircase procedure.
Detecting this simple static figure in a noisy background is a rather demanding task, and presumably relies upon early visual processing stages where lateral interactions between orientation-selective neurons are thought to support contour integration.
Performance on this task, defined as the amount of angular jitter at which the observer could locate the pseudo-circular figure with approximately 75% accuracy, did not differ between groups (Welch two-sample t-test, t(31.7) = 0.358; p = 0.72; mean jitter threshold in degrees: Mundurucu 22.86, American 22.49).