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. 2017 Feb;40(2):72–82. doi: 10.1016/j.tins.2016.11.004

Figure 1.

Figure 1

Taxonomy of Sequencing Complexity and Primate Abilities. Relationships between events in a sequence can vary in complexity on multiple dimensions (Box 1, main text). (A) One dimension of complexity (Y-axis) that is relevant for language begins with the capacity to evaluate adjacent relationships (or dependencies) between items in a sequence. This requires an incoming element to be at least temporarily held in memory, compared and associated with the next element in the sequence. (B) Another level of complexity, which human infants learn during the first year of life [6], is processing nonadjacent relationships. In this case, a relationship must be learned between two elements separated in time. For example, nonhuman primates have been shown to learn that the first element in a three element-long sequence predicts the final element, while ignoring an intervening element that is uninformative about the nonadjacent relationship between the first and third element [6]. Moreover, squirrel monkeys were able to learn that the pitch of the first tone in a sequence predicted the final tone, separated by a variable number of tones of a different pitch [28]. This requires holding a stimulus in working memory and comparing it to another element over one or more intervening elements so that the nonadjacent dependency can be established, assessed, or monitored. (C) Hierarchical relationships ​, prominent in language, reflect even higher levels of complexity, such as one phrase (e.g., ‘AB’) being nested inside of another ‘AB’ phrase, requiring multiple ‘A-B’ associations to be simultaneously held in memory. Relative to this taxonomy of sequencing complexity, some currently known sequence processing capabilities of nonhuman primates are illustrated (see the section ‘Sequence Learning: A Candidate Language Precursor’ in the main text). Question marks denote experiments in which species showed no evidence of sensitivity to certain types of sequencing violations (see [3] for details), indicating uncertainty as to whether these species are able to process sequences at the given level of complexity (see Outstanding Questions, main text).