TMS is a safe, inexpensive and noninvasive means to focally stimulate the human brain. Presently, TMS is in extensive use as a means to measure regional brain excitability, which is dependent on local synaptic strength. TMS can be used to temporarily alter synaptic strength as well as to acutely measure levels of cortical excitability and short and long interval inhibition. Since altered synaptic plasticity and an imbalanced inhibitory:excitatory ratio are cited as fundamental abnormalities in ASD, we hypothesize that both severity of ASD-related learning deficits and their improvement after therapy will correlate with TMS measures of synaptic plasticity and E:I ratio. We propose to embed TMS measures of synaptic plasticity and E:I ratio in a 'Proof of Principal' trial of R-baclofen and to examine: