Paired-coil Measures

Abstract

Abstract Effective connectivity between two areas of the human brain can be studied by testing the effects of conditioning stimulation at one site on the effect of test stimulation at another site by using two coils (dual coil, or paired-coil transcranial magnetic stimulation (TMS)). TMS over the bilateral primary motor cortices (M1) induces interhemispheric inhibition (IHI) and weak interhemispheric facilitation (IHF), largely mediated by the corpus callosum. IHI consists of short-interval (~ 10 ms) IHI (SIHI) and long-interval (~ 50 ms) IHI (LIHI). Abnormalities of IHI have been identified in a variety of brain disorders. The cerebello-dentato-thalamo-motor cortical connection is studied with a conditioning stimulus over one cerebellar hemisphere and a test stimulus over the contralateral M1 at interstimulus intervals of 5–8 ms (cerebellar brain inhibition (CBI)). CBI is caused by inhibitory Purkinje cell activation by the cerebellar stimulus, which inhibits the dentato-thalamo-M1 facilitatory projection. Abnormally reduced CBI is a hallmark of disorders of the cerebello-dentato-thalamo-motor cortical connection. Sensory inputs from peripheral nerve stimulation also affect M1 excitability in the form of short-latency afferent inhibition (SAI, interstimulus interval ~ 20 ms) and long-latency afferent inhibition (LAI, interstimulus interval ~ 200 ms). SAI is a marker of central cholinergic function. SAI is reduced in disorders with central cholinergic dysfunction, such as Alzheimer’s disease.