SPEECH INDUCED CHANGES IN CORTICOSPINAL EXCITABILITY

The aim of the experiments was to investigate the effects of speech on the excitability of corticospinal pathways to human hand muscles. Sin gle transcranial magnetic stimuli were given randomly over the hand ar ea of either the left or right motor cortex of 10 right-handed and 3 l eft-handed normal volunteers. Electromyographic responses were recorde d in the relaxed first dorsal interosseous muscle while the subjects ( a) read aloud a piece of text, (b) read silently, (c) spoke spontaneou sly, or (d) made sounds without speaking. The only consistent effect a cross subjects occurred during task a, which significantly increased t he size of responses evoked in the dominant hand of all subjects, but had either no effect (8 subjects) or a smaller effect in the nondomima nt hand. Tasks b and d had no reliable effect, whereas task c tended t o increase response size in bo th hands. Control measurements suggest that the effects in task a were caused by changes in cortical rather t han spinal excitability. This is the first demonstration of lateralize d speech effects on the excitability of cortical arm areas. The result s provide a useful adjunct to other tests of cerebral dominance, using only single- rather than repetitive-pulse cortical stimulation.