Structured sequence learning across sensory modalities in humans and nonhuman primates
Introduction
The ability to recognise and learn predictive dependencies between environmental events is critical to an animal's survival and is central to a wide range of behaviours. For example, statistical learning — the development of sensitivity to distributional regularities in an input — appears to be important for processes as diverse as linguistic processing [1], visual scene analysis [2], motor learning [3] and many other behaviours that require the prediction of future events [4]. An early suggestion was therefore that a single cognitive system for extracting statistical regularities might operate over a number of different domains [5]. In humans, however, direct comparisons across sensory modalities, or between different types of stimuli, suggest clear modality-specific and stimulus-specific constraints on how information is processed [6, 7, 8], pointing to differences in the neural systems that underpin these apparently similar behaviours ([9] and see Figure 1).
Statistical learning experiments, including structured sequence processing tasks and artificial grammar learning paradigms, can be used to explore the ability to extract order-based regularities from sequentially presented stimuli [10, 11], (see [12] for a historical review). This approach has demonstrated that statistical learning abilities likely play a role in language acquisition [1, 11] and syntactic processing [13, 14, 15]. Furthermore, comparative experiments have identified similarities in structured sequence learning across a wide range of nonhuman animals, providing insights into the types of sequence processing abilities that may have been evolutionarily conserved and those which may have adapted to support language in humans (for reviews, see [16, 17, 18]). However, while both auditory and visual sequence processing have been studied in nonhuman animals, direct comparisons across modalities are lacking. Such comparisons will be critical in determining how closely the cognitive systems supporting auditory and visual sequence processing in nonhuman primates resemble those present in humans.
Understanding differences both between species and across modalities can provide important insights about potential cognitive specialisations that occurred during more recent human evolution, and their contributions to the emergence of language. For example, while we might observe striking similarities in the responses of humans and monkeys using certain stimuli and particular tasks, it remains possible that very different patterns of learning may be observed across the species using different stimuli in another modality. Such differences would highlight not only those abilities that appear to be evolutionarily conserved in nonhuman primates, but might point to behavioural abilities and the underlying neural substrates which have functionally differentiated in more recent evolution, and their possible role in language. Identifying such potentially human-unique adaptations will be critical in understanding how humans diverged from other primates, and how language might be supported by the human brain [19•].
In this paper, we summarise how sequence learning has been assessed across sensory modalities in humans, consider how data from nonhuman animals might be compared in similar ways, and discuss how similarities and differences, across sensory modalities and species, might inform us about the cognitive and neural systems that support statistical sequence learning.
Section snippets
Constraints on sequence processing in humans
A wide range of studies using different stimuli and tasks have shown that humans can extract statistical regularities from a wide range of sequentially presented auditory or visual stimuli (summarised in Table 1). These tasks vary in complexity, from learning relatively simple predictive relationships between adjacent sequence elements, to more nonadjacent or long-distance dependencies between stimuli, or embedded patterns involving multiple overlapping nonadjacent dependencies (for reviews see
Sequence learning in primates
In humans, sequence learning is observed reliably across a wide range of tasks and sensory modalities, albeit with input-related constraints. It is therefore unsurprising that similar learning is also observed in other species. The study of nonhuman animals, particularly nonhuman primates, has become a valuable way to investigate the evolutionary origins of cognitive and neural systems that might be related to those that support language in humans [31]. Nonhuman primates have been tested with a
Sequence learning in the brain: across modalities and species
Human neuroimaging experiments using sequence learning and artificial grammar paradigms have identified a broad network of regions involved in sequence processing (see Figure 1). Some of these regions are primarily engaged in only the auditory or visual modality, while other areas are involved in sequence processing regardless of stimulus modality. In particular, a number of regions such as the inferior frontal gyrus including the frontal operculum [20] and Broca's territory tend to be engaged
Conclusion
Understanding how the brain supports complex cognitive operations, like those involved in sequence processing, requires rigorous research to differentiate the mechanisms that have been conserved since our last common ancestor with nonhuman primates from those that have diverged. It is initially tempting to assume that similar patterns in behavioural data point to the presence of a single, domain-general cognitive or neurobiological system. However, in humans there is little evidence to support
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
Conflict of interest statement
Nothing declared.
Acknowledgements
Supported by a Medical Research Council (MRC, U.K.) PhD Studentship to AM; Sir Henry Wellcome Postdoctoral Fellowship to BW (WT110198/Z/15/Z). We thank Blair Armstrong for his help producing the human neuroimaging panel in Figure 1.
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Both the authors contributed equally to this work.