Schematic examples of the stimulus display.

<p>(A) oddity discrimination for Experiment 1 (preliminary training and transfer tests) and Experiments 2–5, and (B) visual search task for Experiment 1.</p

Mean accuracy in transfer tests using novel front-view and three-quarter-view faces in Experiment 3A.

<p>Examples of the stimuli are shown below the graph. Error bars indicate the standard errors of means across test stimuli. **<i>p</i><0.01. The chimpanzee showed significant transfer of eye-gaze discrimination from front-view to three-quarter-view faces, but the accuracy was significantly lower than that for the new front-view faces.</p

Experimental apparatuses.

<p>The eye tracker on Pan’s head (A) and the eye (left top) and scene-camera image (B). Cross mark indicates point-of-regard (POR). Also, see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0059785#pone.0059785.s002" target="_blank">Video S2</a>.</p

Mean accuracy for each condition in Experiment 3C.

<p>Examples of the stimuli are shown below the graph. The target stimuli contained eyes with the iris located at the center. White bar indicates the results for baseline trials, and black bars show those for test trials. Error bars indicate the standard errors of means across test stimuli. *<i>p</i><0.05. The chimpanzee significantly chose the face with iris at the center of eye region more than the other types of faces irrespective of “eye contact”.</p

Mean accuracy in each of the baseline and test conditions in Experiment 2.

<p>Examples of the stimuli are shown below the graph. Pos: positive polarity, Neg: negative polarity. Broken lines: 5% significance levels of binomial tests for each test condition. Chloe exhibited the so-called “bloodshot illusion.” Furthermore, her behavior was controlled by the contrast polarity.</p

Experimental procedures.

<p>(A) Schematic of the experimental setting. (B) Time flow of an experimental session.</p

Actions observed during the experiments.

<p>Actions observed during the experiments.</p

Mean response times in correct trials for each condition as a function of the number of stimulus items in the visual-search testing in Experiment 1.

<p>Error bars indicate standard errors of means across sessions. *<i>p</i><0.05; **<i>p</i><0.01. Chloe showed faster response times when the direct-gaze face than when the averted-gaze face was the target irrespective of the facial configurations.</p

Average of (A) fixation duration (ms) and (B) saccade length (degree) for each scene area during the greeting and task phases (first 50 s).

<p>Average of (A) fixation duration (ms) and (B) saccade length (degree) for each scene area during the greeting and task phases (first 50 s).</p

Fixation duration (ms) and saccade length (degree) during each experimental phase.

<p>Fixation duration (ms) and saccade length (degree) during each experimental phase.</p