Air-puff-induced facilitation of motor cortical excitability studied in patients with discrete brain lesions

Air-puff stimulation applied to a fingertip is known to exert a location-sp ecific facilitatory effect on the size of the motor evoked potentials elici ted in hand muscles by transcranial magnetic stimulation, In order to clari fy its nature and the pathway responsible for its generation, we studied 27 patients with discrete lesions in the brain (16, 9 and 2 patients with les ions in the cerebral cortex, thalamus and brainstem, respectively), Facilit ation was absent in patients with lesions affecting the primary sensorimoto r area, whereas it was preserved in patients with cortical lesions that spa red this area. Facilitation was abolished with thalamic lesions that totall y destroyed the nucleus ventralis posterolateralis (VPL), but was preserved with lesions that at least partly spared it. Lesions of the spinothalamic tract did not impair facilitation, The size of the N20-P25 component of the somatosensory evoked potential showed a mild correlation with the amount o f facilitation, The facilitation is mainly mediated by sensory inputs that ascend the dorsal column and reach the cortex through VPL, These are fed in to the primary motor area via the primary sensory area, especially its ante rior portion, corresponding to Brodmann areas 3 and 1 (possibly also area 2 ), without involving other cortical regions. The spinothalamic tract and di rect thalamic inputs into the motor cortex do not contribute much to this e ffect, Some patients could generate voluntary movements despite the absence of the facilitatory effect. The present method will enable us to investiga te in humans the function of one of the somatotopically organized sensory f eedback input pathways into the motor cortex, and will be useful in monitor ing ongoing finger movements during object manipulation.