Primary motor cortex excitability is modulated by tactile adaptation in primary somatosensory cortex

We used transcranial magnetic stimulation (TMS) to study sensorimotor integration in humans in the context of a tactile adaptation paradigm. When two identical successive stimuli activate the same neuronal population a decrease in the overall neural response is observed. This type of neurophysiological response is known as an adaptation effect. Thus, when two tactile events are repeated on exactly the same skin region those neurons that have a strictly somatotopic response should show less activity. Moreover, previous reports suggest that the neurophysiological response of the primary motor cortex (M1) can be inhibited when a single tactile stimulus is presented 20 - 100 ms before a TMS pulse. This inhibition is measured as a decrease in the amplitude of the motor evoked potentials (MEPs) recorded from a given muscle. This phenomenon has been named short-latency afferent inhibition (SAI). Given this, we reasoned that repeating tactile events on the same finger (e.g. the index) should produce more adaptation in the primary somatosensory cortex (S1) than presenting stimuli to the middle and then index finger, which should in turn result in weaker inhibition of M1. Here, we tested whether SAI is sensitive to tactile adaptation effects and if so, whether these effects vary as a function of the stimulated finger pairs. We presented two consecutive electrocutaneous stimuli on homologous (i.e., left index finger stimulated twice) and non-homologous fingers (i.e., left middle and left index fingers) with fixed inter-stimulus intervals. Our results show a smaller reduction in the amplitude of MEPs recorded from the first dorsal interrosseus of the hand that received the tactile stimuli when stimuli were delivered to the same finger than to two different fingers. We interpret the presence of less SAI during same-finger than different-finger tactile stimulation as evidence that the excitability of the primary motor cortex can be differentially modulated by tactile adaptation effects that occur primarily within S1.