Figure 1. Face discrimination task and modulation by age.

a, procedure. In each trial, a face picture of an individual (cue) was centrally presented on the display for 750 ms, followed by an inter trial interval (ITI) of 500 ms and a presentation of two horizontally aligned face pictures of the same individual (match), but not the identical picture as the cue, and a different individual (distractor). Chimpanzees indicated their choice by touching either the match or the distractor stimulus. The correct answer (match) depicts a face picture of the same individual as shown in the cue stimulus. b, Proportion of correct responses. Performance scores (correct trials / number of trials) were average according to age group (YC, OC) and the stimulus class (chimpanzee (C, blue boxes), human (H, light red boxes)). Boxplots describe the samples of individual runs. Dots and text labels mark the mean performance of the participants.