A shared neural substrate for action verbs and observed actions in human posterior 3 parietal cortex. Abstract 1High-level sensory and motor cortical areas are activated when processing the meaning of language, but it 2 is unknown whether, and how, words share a neural substrate with corresponding sensorimotor 3 representations. We recorded from single neurons in human posterior parietal cortex (PPC) while 4 participants viewed action verbs and corresponding action videos from multiple views. We find that PPC 5 neurons exhibit a common neural substrate for action verbs and observed actions. Further, videos were 6 encoded with mixtures of invariant and idiosyncratic responses across views. Action verbs elicited 7 selective responses from a fraction of these invariant and idiosyncratic neurons, without preference, thus 8 associating with a statistical sampling of the diverse sensory representations related to the corresponding 9 action concept. Controls indicated the results are not the product of visual imagery nor arbitrary learned 10 associations. Our results suggest that language may activate the consolidated visual experience of the 11 reader. 12

Cognition relies on transforming sensory inputs into a more generalizable understanding. Mirror neurons are proposed to underlie this process, yet they fail to explain many key features of human thinking and learning. Here we hypothesize that mirror-like responses are one limited view into a more general framework by which internal models of the world are built and used. We recorded populations of single neurons in the human posterior parietal cortex as a participant felt or observed diverse tactile stimuli. We found that mirror-like responses were fragile and embedded within a richer population response that encoded generalizable and compositional features of the stimuli. We speculate that populations of neurons support versatile understanding, not through mirroring, but instead by encoding representational building blocks of cognition.

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In the human posterior parietal cortex (PPC), single units encode high-dimensional information with partially mixed representations that enable small populations of neurons to encode many variables relevant to movement planning, execution, cognition, and perception. Here we test whether a PPC neuronal population previously demonstrated to encode visual and motor information is similarly selective in the somatosensory domain. We recorded from 1423 neurons within the PPC of a human clinical trial participant during objective touch presentation and during tactile imagery. Neurons encoded experienced touch with bilateral receptive fields, organized by body part, and covered all tested regions. Tactile imagery evoked body part specific responses that shared a neural substrate with experienced touch. Our results are the first neuron level evidence of touch encoding in human PPC and its cognitive engagement during tactile imagery which may reflect semantic processing, sensory anticipation, and imagined touch.