New study finds evidence for memory replay phenomenon in macaque monkeys

Human’s memory system works like a recorder: one can compress information and fast-forward to any time point within memory space. This capacity has long been thought to be uniquely human. A new study led by Sze Chai Kwok, an Associate Professor at East China Normal University and a member of the NYU-ECNU Institute of Brain and Cognitive Science at NYU Shanghai reported, for the first time, that this capacity is also conserved in the monkeys. This study has been published in the journal eLife.

In the study, researchers trained six monkeys to perform a temporal-order judgement task where the monkeys were asked in a trial to choose a frame among two alternatives that was presented earlier in a video they had watched. Results suggested that macaque monkeys might adopt a time-compressed, replay-like pattern to search within the representation of continuous information. Comparing the reaction time and the location of chosen frame within the video in different trials, researchers found that monkeys were faster in identifying frames that were located earlier in the video, suggesting the process was conducted in a forward direction. Moreover, the time required to respond was about 10 times shorter than the duration of the videos themselves, implying they were able to compress information during the memory recall. These aspects are highly similar to humans.

Based on these initial results, researchers were further interested to know if monkeys’ mental time travel capacity is as sophisticated and flexible as humans. To study this question, they used representational similarity analysis and compared monkeys’ data with those from humans. They found that unlike humans, monkeys’ compression of replay is not sophisticated enough to allow them to skip over irrelevant information by compressing the encoded video. In fact, monkeys had to replay the whole video clip in a frame-by-frame manner in mind to locate the two probe frames and recall their temporal order.

“This study reveals several human-like elements of similarity but also profound differences between humans and non-human primates during memory recall of episodic details. The results would help link extant research on replay in the rodents with those in the primate species,” said Kwok.

One of the co-authors of the study, Yong-di Zhou is also a member of the NYU-ECNU Institute of Brain and Cognitive Science at NYU Shanghai.

Journal Reference:

Zuo, Sǂ., Wang, Lǂ., Shin, J. H., Cai, Y., Zhang, B., Lee, S. W., Appiah, K., Zhou, Y.,& Kwok, S. C. (2020). Behavioral evidence for memory replay of video episodes in the macaque. eLife9, e54519. (ǂ Joint first)