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. 2022 Apr 27;289(1973):20212599. doi: 10.1098/rspb.2021.2599

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© 2022 The Authors.

Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited.

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Figure 2. — The cumulative probability apes select either familiar or unfamiliar faces during each experimental trial, on average. Here, ‘time since trial began’ represents the time taken for the apes to touch the black dot on the screen after it appeared, replacing one of the two photographs. (a) In experiment 1, the apes were significantly faster to touch the dot when it replaced an unfamiliar face showing a neutral expression as compared with when it replaced a familiar face also showing a neutral expression. (b) In experiment 2, the apes did not show a significant difference in response latency when the dot replaced an unfamiliar face showing a surprised expression as compared with when it replaced a familiar face also showing a surprised expression. (c) In experiment 3, the apes were significantly faster to touch the dot when it replaced an unfamiliar face wearing a surgical face mask as compared with when it replaced a familiar face also wearing a surgical face mask. Thick lines represent the mean response latency among apes, while the shaded areas show 95% confidence intervals.