Covert word reading induces a late response in the hand motor system of the language dominant hemisphere

Recent work has demonstrated that overt reading influences the excitability of the language-dominant hand motor cortex. However, this effect was related to speech output, whereas results on silent reading have been inconsistent, and have not allowed for systematic investigation of the different stages of word recognition. To investigate a possible modulation of the cortical excitability mediating hand movements through different stages of covert reading, motor evoked potentials (MEP) from hand muscles in right-handed subjects were recorded. We showed a significant increase of the excitability of the hand motor cortex of the dominant hemisphere during late stages of covert word reading, whereas processing of abstract shapes had no effect and covert articulation induced a decrease in hand motor cortex excitability. There was no significant change of MEP amplitudes during earlier stages of covert reading in the dominant hemisphere or in the non-dominant hemisphere. Our results demonstrate a functional connection between cortical networks mediating linguistic processing and hand movements without concurrent activation of the motor cortex through overt articulation at late stages of word reading, which have been shown to involve converging activation of classic left frontal language regions. We speculate that the effect reported here is related to a cortical network mediating gestures which are a part of verbal communication. This supports recent theories on language evolution which postulate that language emerged through manual gestures.