Phonetic categorization relies on motor simulation, but combinatorial phonological computations are abstract

To identify a spoken word (e.g., dog), people must categorize the speech steam onto distinct units (e.g., contrast dog/fog,) and extract their combinatorial structure (e.g., distinguish dog/god). However, the mechanisms that support these two core functions are not fully understood. Here, we explore this question using transcranial magnetic stimulation (TMS). We show that speech categorization engages the motor system, as stimulating the lip motor area has opposite effects on labial (ba/pa)- and coronal (da/ta) sounds. In contrast, the combinatorial computation of syllable structure engages Broca’s area, as its stimulation disrupts sensitivity to syllable structure (compared to motor stimulation). We conclude that the two ingredients of language—categorization and combination—are distinct functions in human brains.