Abstract
For decades, researchers have debated whether mental representations are symbolic or grounded in sensory inputs and motor programs. Certainly, aspects of mental representations are grounded. However, does the brain also contain abstract concept representations that mediate between perception and action in a flexible manner not tied to the details of sensory inputs and motor programs? Such conceptual pointers would be useful when concepts remain constant despite changes in appearance and associated actions. We evaluated whether human participants acquire such representations using fMRI. Participants completed a probabilistic concept learning task in which sensory, motor, and category variables were not perfectly coupled or entirely independent, making it possible to observe evidence for abstract representations or purely grounded representations. To assess how the learned concept structure is represented in the brain, we examined brain regions implicated in flexible cognition (e.g., pFC and parietal cortex) that are most likely to encode an abstract representation removed from sensory-motor details. We also examined sensory-motor regions that might encode grounded sensory-motor-based representations tuned for categorization. Using a cognitive model to estimate participants' category rule and multivariate pattern analysis of fMRI data, we found the left pFC and human middle temporal visual area (MT)/V5 coded for category in the absence of information coding for stimulus or response. Because category was based on the stimulus, finding an abstract representation of category was not inevitable. Our results suggest that certain brain areas support categorization behavior by constructing concept representations in a format akin to a symbol that differs from stimulus-motor codes.
Highlights
Concepts organize our experiences into representations that can be applied across domains to support higher-order cognition
How does the brain organize sensory input into an appropriate representation for categorization? Are concepts a combination of sensory signals and motor plans, or does the brain construct a separate concept representation, abstracted away from sensory–motor codes? Despite much research on how people organize sensory information into a format suited for categorization (e.g., Love, Medin, & Gureckis, 2004; Kruschke, 1992; Nosofsky, 1986) and its neural basis (e.g., Zeithamova et al, 2019; Bowman & Zeithamova, 2018; Mack, Love, & Preston, 2016; Folstein, Palmeri, & Gauthier, 2013; Mack, Preston, & Love, 2013; Davis, Love, & Preston, 2012a, 2012b; Cromer, Roy, & Miller, 2010; Seger & Miller, 2010; Freedman & Assad, 2006; Sigala & Logothetis, 2002), few have explicitly examined whether category representations exist independently of sensory–motor information (Figure 1A)
To assess how the learned concept structure is represented in the brain, we focused on brain regions implicated in flexible cognition, including pFC and parietal cortex, which are strong candidates for representing the abstract concept structure without being tied to sensory– motor variables, and sensory–motor regions that are involved in stimulus processing and may encode grounded representations such as category-modulated stimulus representations as the basis of concept knowledge
Summary
Concepts organize our experiences into representations that can be applied across domains to support higher-order cognition. How does the brain organize sensory input into an appropriate representation for categorization? Are concepts a combination of sensory signals and motor plans, or does the brain construct a separate concept representation, abstracted away from sensory–motor codes? Certain aspects of concepts are more abstracted from first-hand experience and act more like symbols or pointers, which can support flexible cognition. The same object can appear entirely different in some contexts, such as a camouflaging stick insect appearing as a leaf or when a caterpillar changes into a butterfly In such cases where sensory information is unreliable or exhibits changes, an amodal symbol working as an abstract pointer may aid reasoning and understanding. Sensory–motor variables and categories are often correlated in the real world, and the brain may never need to represent “category” in a way that can be disentangled from perception and action
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