Cortex special issue: Language and the motor system

To propose a relationship between language and the motor system is certainly not a novel idea. Theories about the gestural origin of language can be traced back to the writings of Vico and Condillac in the eighteenth century (Gentilucci and Corballis, 2006), and the philosopher Ludwig Wittgenstein famously emphasized that language is “woven” into action (Wittgenstein, 1953). The interest in language-action relationships has been increasing over the recent years in light of a number of important and exciting new discoveries in the field of neuroscience and cognitive psychology. In particular, the observation of mirror neurons in the monkey brain (for a recent review see Rizzolatti and Sinigaglia, 2010) and of brain activity in humans demonstrating the same functional integration of perceptual and motor mechanisms (review in Molenberghs et al., 2012), has been instrumental in opening a newperspective on the brain basis of language (Pulvermuller and Fadiga, 2010). Whereas the dominant view in classical aphasiology had been that superior temporal cortex (“Wernicke’s area”) provides the unique engine for speech perception and comprehension (Benson, 1979), investigations with functional neuroimaging in normal subjects have shown that even during themost automatic speech perception processes inferior fronto-central areas are being sparked (Zatorre et al., 1992). These observations and complementary ones about temporal activation during language production (Wise et al., 1991) have suggested the possibility that understanding of speech and language may critically involve motor processes. Much of the discussion highlighting action-perception integration in language processing has focused on the role of Broca’s area, considered to play a crucial role in the hierarchical organization of both language and action (Greenfield, 1991). According to most researchers, Broca’s area can be considered the human homolog of area F5 in the monkey, where mirror neurons were first observed (Petrides et al., 2005). Mirror neurons activate during specific actions and equally when the individuals sees or hears somebody else perform the same type of action. Because of their sensorimotor function, mirror neurons must be part of circuits that link perceptual mechanisms to motor neurons (Garagnani et al., 2007). Multiple streams of research in the neuroscience of language converge on an important role of similar action-perception circuits, and particularly of premotor regions, in human language function. For example, phonological features of speech sounds are reflected inmotor cortex activation so that the action system likely plays a double role, both in programming articulations and in contributing to the analysis of speech sounds (Pulvermuller et al., 2006). “Semantic somatotopy” has been demonstrated in the processing of action-related language, that is, the meaning of action-related words and sentences becomes manifest in the “lighting up” of specific motor areas e the hand area for a word like “pick”, but the leg area for “kick” (Hauk et al., 2004; Tettamanti et al., 2005). At the same time, behavioral experiments performed in cognitive science demonstrate that compatibility between overt action and action language leads to better understanding, whereas a divergence between the two leads to slowing or errors (Glenberg and Kaschak, 2002). Taken together, these new findings from a range of disciplines show that the motor system reflects both phonological and semantic features of language processing. The crucial question whether motor systems of the brain are also critical and necessary for language processing has been addressed by work in neuropsychology and behavioral neurology. In a range of brain diseases affecting fronto-central cortex and/or functionally closely related areas (stroke affecting frontal regions, motor neuron disease, progressive supranuclear palsy, frontal-variant of fronto-temporal dementia, Parkinson’s