We use theoretical and modeling approaches to study how the brain works. We are especially interested in understanding the neural circuit basis of higher cognitive functions, with a focus on the prefrontal cortex. Our working hypothesis i s that reverberatory dynamics in strongly recurrent (`attractor') networks give rise to `cognitive-type' neural processes, such as persistent activity and competitive ramping activity, that instantiate working memory and decision making functions. Moreove r, such a system endowed with reward-dependent learning may underlie adaptive and complex choice behavior. Other topics that have been covered by our research include the diversity of inhibitory cells in the cortex, synchronous oscillations, timing. Curre ntly, in collaboration with experimentalists, we are pursuing biophysically realistic large-scale circuit models of spiking neurons, to elucidate general principles and cellular mechanisms of key cognitive processes such as working memory, decision makin g, selective attention, executive control, as well as their impairments associated with mental disorders.