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With backgrounds in both medical (M.D.) and psychological (Ph.D.) fields, I am interested in both normal function and pathology of the brain. My previous research involved the electrophysiology of emotion (pain) in humans and of cognitive behaviors (spatial learning, schedule task, and conditioning) in rodents. Particularly, my studies in recent years have focused on the role of the medbrain dopaminergic neurons in associative learning. I have recently found that rat dopamine neurons not only code the value of acquisition with excitatory response but also code extinction with an inhibitory response during classical conditioning. This finding confirmed, for first time at the cellular level, Pavlov's theory that extinction is a new inhibitory learning rather than unlearning. Based on this finding, we developed a temporal difference (TD) learning model that explains mechanisms of new learning, unlearning, and forgetting and provides a unified theory of extinction (J. Neurosci., 25:6235-42, 2005 and 28:9619-31, 2008). I further investigated the mechanisms and neuronal network of the dopamine system underlying associative learning (e.g. J. Neurosci., 25:4725-32, 2005). At Janelia in my work with the Josh Dudman lab, we are developing the techniques of single-cell recording in conscious mice and combining genetic model and microinjection techniques to fully investigate the neuronal network of the dopamine system for controlling associative learning.