How the motor system handles verbs and nouns
The embodiment approach to language is relatively recent and contrasts with a more classical view of language processing, where words are viewed as amodal. Embodiment theory claims that language understanding involves the activation of the same neural substrates, sensory and motor, activated when one experiences the action or the object to which a word refers. As for language referring to action, i.e. action verbs or sentences, a variety of behavioral and neurophysiological approaches have shown evidence for activation of the motor system during processing of action-related language material. For example there are behavioral studies showing slower reaction times when the effector used to respond is also involved in actual execution of the action expressed by the presented linguistic material. Other behavioural studies have demonstrated that the execution of a motor response is facilitated during the comprehension of sentences that describe actions taking place in the same direction as the motor response Together these findings were interpreted as resulting from an interaction between activation of the motor system for language understanding and activation for response.
Further evidence comes from neurophysiological and brain imaging techniques, EEG and fMRI, which showed that presentation of verbs associated with different effectors, results in somatotopic activation of motor areas and common activation in Broca’s area for action-related sentences and their action counterparts. There is also evidence from TMS work showing a modulation in the motor evoked potential for a muscle of a given effector, associated with the presented verb.
The mechanism through which words referring to actions, i.e. verbs, could elicit the motor representations for action itself can be explained in terms of the mirror neuron mechanism.
More recent evidence show that a similar modulation of the motor system also occurs during understanding of concrete nouns, as it is for concrete verbs. In a recent behavioral study nouns expressing objects related to hand, foot or abstract were used as stimuli, using a go-nogo paradigm with an early and delayed go-signal delivery. The results showed that participants at the early go-signal, gave slower right-hand responses for hand-related nouns compared to foot-related nouns. The opposite pattern was found for the left hand. These findings demonstrate an early lateralized modulation of the motor system during noun processing, most likely crucial for noun comprehension. More recently a Transcranial Magnetic Stimulation (TMS) study was carried out to compare modulation of the motor system when subjects read nouns referring to objects which are Artificial or Natural and which are Graspable or Ungraspable. TMS was applied to the primary motor cortex representation of the first dorsal interosseous (FDI) muscle of the right hand at 150ms after noun presentation. Analyses of Motor Evoked Potentials (MEPs) revealed that across the duration of the task, nouns referring to graspable artifacts (tools) were associated with significantly greater MEP areas. Analyses of the initial presentation of items revealed a main effect of graspability. Again the findings are in line with an embodied view of nouns, with MEP measures modulated according to whether nouns referred to natural objects or artifacts (tools), confirming tools as a special class of items in motor terms. Additionally these data support a difference for nouns expressing graspable versus non graspable objects, an effect which for natural objects is restricted to initial presentation of items. It may be hypothesized that as verbs recruit mirror neurons, nouns recruit canonical neurons, a set of neurons which, besides motor properties similar to those of mirror neurons, are sensitive to the presentation of objects.