The thalamocortical circuit has long been considered important in control of consciousness. Series of works utilizing mice with genetic modifications of the alpha1G T-type Ca2+ channel have helped to settle the long held issue on the critical role, in vivo, of T-type channels in control of neuronal firing mode and consciousness. These have been shown by studies on absence epilepsy, sleep, and alcohol-induced loss of consciousness. As we explore further into the structure and function of the thalamocortical circuit, especially in relation to other brain regions, we begin to find that the thalamus has critical roles in other fundamental brain functions, i.e. in cognition and emotion control. Thus, we showed that the thalamus is deeply involved in control of empathy fear by the anterior cingulate cortex and in extinction of fear memory by the amygdala and prefrontal cortex. In the latter case, we began our studies with a mutant mouse for the phospholipase Cbeta4 gene, a signaling molecule downstream to mGluR1. Using gene manipulation, pharmacology, lesion, single-unit recording via tetrodes, and intracranial electrical stimulations, we found that the mediodorsal thalamus plays a critical switch function in extinction of fear memory through two different firing patterns, burst vs tonic. The demonstration of the opposite functions, suppression vs enhancement, of the two firing modes on extinction of fear memory clearly shows the distinctive physiological roles of the two firing modes of the thalamus in control of cognition, in addition to their roles in control of consciousness. Current studies on this issue will be discussed.