The primary goal of the project is to study the effect of Ketamine on cortical neurophysiological function in Treatment Resistant Depression(TRD) patients. There are three key preclinical findings regarding Ketamine antidepressant effects that motivate the current study: a) low dose Ketamine causes early increase in glutamate neurotransmission; b) Ketamine initiates synaptic plasticity; c) ketamine infusion leads to rapid improvement in depression symptoms; d) specific expression and/or methylation markers measured in blood cells isolated treated ex vivo with ketamine will significantly correlate with the clinical response to treatment in patients with treatment-resistant depression; The proposal essentially employs robust and non-invasive neurophysiological techniques, Auditory Steady State Response(ASSR)-gamma oscillatory response and Transcranial Magnetic Stimulation(TMS) cortical excitability to investigate the above findings in patients with treatment resistant depression.