A test of memory for stimulus sequences in great apes

Abstract

Identifying cognitive capacities underlying the human evolutionary transition is challenging, and many hypotheses exist for what makes humans capable of, for example, producing and understanding language, preparing meals, and having culture on a grand scale. Instead of describing processes whereby information is processed, recent studies have suggested that there are key differences between humans and other animals in how information is recognized and remembered. Such constraints may act as a bottleneck for subsequent information processing and behavior, proving important for understanding differences between humans and other animals. We briefly discuss different sequential aspects of cognition and behavior and the importance of distinguishing between simultaneous and sequential input, and conclude that explicit tests on non-human great apes have been lacking. Here, we test the memory for stimulus sequences-hypothesis by carrying out three tests on bonobos and one test on humans. Our results show that bonobos’ general working memory decays rapidly and that they fail to learn the difference between the order of two stimuli even after more than 2,000 trials, corroborating earlier findings in other animals. However, as expected, humans solve the same sequence discrimination almost immediately. The explicit test on whether bonobos represent stimulus sequences as an unstructured collection of memory traces was not informative as no differences were found between responses to the different probe tests. However, overall, this first empirical study of sequence discrimination on non-human great apes supports the idea that non-human animals, including the closest relatives to humans, lack a memory for stimulus sequences. This may be an ability that sets humans apart from other animals and could be one reason behind the origin of human culture.

On some statistical and cerebral aspects of the limits of Working Memory capacity in Anthropoid Primates, with particular reference to Pan and Homo, and their significance for Human Evolution

“…Interpretations of empirical evidence sometimes involve non-parsimonious inferences. Thus, although some controlled experiments have given rise to findings from which claims have been made that chimpanzees are endowed with remarkable WM abilities for storage (Carruthers, 2013;Inoue and Matsuzawa, 2007;Kawai and Matsuzawa, 2000) and updating (Völter et al, 2019), these claims are in direct conflict with other results and analytical implications published by several other researchers (e.g., Lind et al, 2023;Read, 2008;Washburn et al, 2007). In order to make a comprehensive, unbiased assessment of the WM of great apes, we conducted a critical review (see Read et al, 2022) in which we examined not only the appropriateness of methods that have been used to measure WM in nonhuman taxa of the order Primates (LINNAEUS, 1758), with special emphasis on chimpanzees, but we also reviewed the parsimony of the inferences derived from the data; that is, whether claims that were made derived directly from, or are supported sufficiently, by published data.…”

Sequence representation as an early step in the evolution of language

“…Here, we explore the simpler hypotheses that domain-general sequence representation is a first step towards human language and thinking, and that non-human animals lack such sequence representation because under most circumstances it is not beneficial. This hypothesis is grounded in a suggested taxonomic gap between humans and other animals in recognizing and remembering sequential information [ 23 , 29 , 30 ]. Our reason for taking this tentative taxonomic gap as our starting point is recent empirical studies showing that animals may not be able to faithfully represent sequential information [ 23 , 30 ].…”

“…This hypothesis is grounded in a suggested taxonomic gap between humans and other animals in recognizing and remembering sequential information [ 23 , 29 , 30 ]. Our reason for taking this tentative taxonomic gap as our starting point is recent empirical studies showing that animals may not be able to faithfully represent sequential information [ 23 , 30 ]. Below we expand on this point.…”

“…Recent empirical studies suggest that non-human animals do not rely on faithful sequence representation when discriminating between sequences of stimuli but instead rely on memory traces of stimuli, where the intensity of the memory for each stimulus decays over time. A comprehensive meta-study, incorporating over 100 discrimination experiments in mammals and birds [ 23 ] including, for example, rule learning [ 31 , 32 ], artificial grammar [ 33 – 35 ], sequence discrimination [ 36 , 37 ] and birdsong [ 38 , 39 ], shows that the trace memory model can account well for how animals recognize and remember sequences of stimuli, and there are subsequent consistent results from great apes [ 29 , 30 ]. This points to the importance of considering trace memory as an explanation when limited sequence discrimination is observed in similar studies [ 40 – 44 ].…”

“…Even vocal learners with the capacity to imitate sound sequences do not recognize and remember arbitrary sequences of information faithfully. Instead, they seem to rely on approximate sequence representation for arbitrary stimuli [ 23 , 30 ] and specialized memory mechanisms for vocal learning [ 69 , 70 ]. Thus, while there are surface similarities between combinatorial communication in animals and humans, it is not clear that they rely upon similar biological foundations.…”