Neural representations of concrete concepts enable identification of individuals during naturalistic story listening

Different people listening to the same story may converge upon a largely shared interpretation while still developing idiosyncratic experiences atop that shared foundation. What semantic properties support this individualized experience of natural language? Here, we investigate how the "concreteness" of word meanings -- i.e., the extent to which a concept is derived from sensory experience -- relates to variability in the neural representations of language. Leveraging a large dataset of participants who each listened to four auditory stories while undergoing functional MRI, we demonstrate that an individuals neural representations of concrete concepts are reliable across stories and unique to the individual. In contrast, we find that neural representations of abstract concepts are variable both within individuals and across the population. Using natural language processing tools, we show that concrete words exhibit similar neural signatures despite spanning larger distances within a high-dimensional semantic space, which potentially reflects an underlying signature of sensory experience -- namely, imageability -- shared by concrete words but absent from abstract words. Our findings situate the concrete-abstract semantic axis as a core dimension that supports reliable yet individualized representations of natural language.