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Are Words with Effector Specific Motor Related Meaning Represented Somatotopically on the Motor Cortex?

Traditionally, language was proposed to be mediated by various left hemisphere perisylvian structures and the associated role of the motor cortices was limited to tasks such as articulation. Recent theoretical models have proposed that effector specific words with motor related meaning are represented somatotopically on the primary motor (Brodmann’s Area 4) and premotor (Brodmann’s Area 6) cortices. For example, it has been reported that when verbs associated with the hand (e.g., pick) are processed, the primary and premotor areas involved with moving the hand are engaged. However, fundamental methodological problems exist within the reported research. This thesis aimed to address and correct the inconsistencies and methodological limitations within the existing literature to provide more conclusive evidence regarding the involvement of the primary and premotor cortices in processing verbs with motor related meaning. This thesis also aimed to investigate whether the names of effectors (nouns) also involve processing by the motor cortices, either generally or somatotopically. Three behavioural dual task experiments and one fMRI experiment were conducted. Results indicated no evidence of somatotopically organised overlapping activation in the primary or premotor cortex between the various semantic categories of words and related effector movements. However, in the fMRI experiment, motor related verbs in general yielded significant overlapping activity between reading all effector related verbs and moving all effectors in the pre-supplementary motor area of the premotor cortex. These findings indicate that an embodied language involving somatotopic representations of effector specific verbs on the primary or premotor cortex is unlikely to be the case. Rather there appears to be a more general representation of effector related verbs in a more cognitive than motor area of the premotor cortex. The findings of this thesis are consistent a wealth of evidence supporting the motor cortices being generally associated with motor related language and with the idea that semantic representations are distributed throughout the brain according to the embodied cognitive framework, rather than being localised to amodal regions that process all words.