The application of TMS in Speech Studies
TMS has a role in developing a more complete understanding of the neurological basis of language. The earliest use of TMS to investigate language by Pascual-Leone et al. (1991) induced speech arrest in pre-surgical epilepsy patients. This early study by Pascual-Leone et al. showed that after stimulation of the left inferior frontal cortex, a reproducible speech arrest was observed in each of the six patients and no speech arrest was seen during any right hemisphere stimulation. Intracarotid amobarbital testing with these same patients revealed left hemisphere language dominance in all six, suggesting that the TMS-induced speech arrest offered a non-invasive alternative for determining language dominance. In later studies it was shown that higher intensities led to stronger speech arrest effects, and surprisingly it was the lower rates of stimulation (4–8Hz) that were more reliable at inducing speech arrest (16–32Hz, Jennum et al. 1994; Michelucci et al. 1994; Pascual-Leone et al. 1991).
Ongoing research
TMS has also been used to measure functional connections between linguistic processes and the motor cortex, by measuring motor excitability during various language tasks and demonstrating a link between action words and motor programs (Pulvermuller et al. 2005, Devlin and Watkins 2007). This suggests a potential evolutionary link between hand gestures and language (Meister et al. 2003); and demonstrates that speech perception potentiates the specific parts of the motor system engaged to produce equivalent movements (Fadiga et al. 2002; Watkins and Paus 2004; Watkins et al. 2003, Devlin and Watkins 2007). Speech production increases motor excitability not only in the face area of the left hemisphere, but also in the hand area (Saarinen et al. 2006).
More recent studies have focused on the topological origin of speech production in the brain, for example, Cappelletti et al., 2008 used rTMS to suppress the excitability of distinct parts of the left prefrontal cortex to assess their role in producing regular and irregular verbs compared to nouns. Response latencies increased for verbs, but were unaffected for nouns, following stimulation to the left anterior midfrontal gyrus. No significant interference specific for verbs resulted after stimulation to two other areas in the left frontal lobe, the posterior midfrontal gyrus and Broca's area. These results therefore reinforce the idea that the left anterior midfrontal cortex is critical for processing verbs.
TMS has been has also been beneficial in evaluating the neural mechanisms of compensation following aphasic brain injury (Devlin and Watkins, 2007).
References
- Pascual-Leone et al., Neurology, 1991
- Jennum et al., Neurology, 1994
- Michelucci et al., Neurology, 1994
- Pulvermuller et al., J Cogn Neurosci, 2005
- Meister et al., Neuropsychologia, 2003
- Fadiga et al., J Cogn Neurosci, 2002 Devlin, J.T. and Watkins, K.E. Stimulating language: insights from TMS. Brain, 130 (3). pp. 610-622. 2007.
- Watkins and Paus, J Cogn Neurosci, 2004
- Watkins et al. 2003
- Saarinen et al. 2006
- Cappelletti et al., J Cogn Neurosci, 2008
