How are word meanings represented in the brain?
Focusing on the case of verbs, we have shown that both modality-specific brain areas such as the motor cortex and brain areas outside modality-specific systems, such as the middle temporal gyrus, are activated during understanding of verbs. By focusing on those two nodes of a larger network (i.e. motor cortex and middle temporal cortex), we have substantiated the existence of both abstract and general (in the middle temporal cortex) and specific information (in the motor cortex) about of word meanings.
Using transcranial magnetic stimulation (TMS), we have found that the disruption of middle-temporal activity impairs the ability to understand verbs, while the disruption of motor activity does not. This is in line with our research on brain-damaged patients (stroke patients and patients with amyotrophic lateral sclerosis), which has demonstrated that an individual with damage to the system for action production can still understand the verbs referring to the actions s/he cannot produce.
Further, the disruption of middle-temporal activity suppresses verb-related motor activity, as if in the absence of temporal activation, the motor cortex lacks the critical signal to perform any further computation on verbs.
Those findings showed that the retrieval of general, abstract information in non sensory-motor regions is the necessary step to drive any further computation of word meaning in regions such as the motor cortex, where modality-specific features are represented. The relationship between the middle temporal and the motor cortex could reflect a general principle of functioning that governs the interaction between general, amodal and modality-specific information within the words’ semantic network.