Computational approaches to the architecture and operations of the prefrontal cortical circuit for working memory.
1. This article reviews recent progress in the computational studies towards the architecture and operations of the prefrontal cortical circuit, which are keys to understand the mechanisms of working memory processing. 2. The recurrent excitatory connections form closed-loop circuits, which contribute to the sustainment of delay-period activity. These connections subserve the cortical amplification of the activity. 3. Recent experimental studies (Wilson et al. 1994; Rao et al. 1999, 2000) suggested that at least two architectonically distinct types of intracortical inhibition, isodirectional and cross-directional inhibition, play significant roles in the formation of memory fields. 4. Computer simulations of a prefrontal cortical circuit model (Tanaka 1999, 2000a) showed that the isodirectional inhibition in the model regulated the amplitude of memory fields (i.e., the maximum firing rate) while the cross-directional inhibition contributed to the sharpening of the memory fields or the tuning curves. 5. The above characteristics enable the prefrontal cortical circuit to control memory fields, which would be necessary to general working memory processing. It would also be interesting to know whether different subtypes of the interneurons have distinct roles. 6. Another important issue is how neuromodulators contribute to working memory processing. Recent computer simulations by Durstewitz et al. (1999, 2000) showed that stronger dopamine action required stronger intervening input to destroy working memory, suggesting that dopamine contributes to the stabilization of working memory representation. 7. Further elucidation of these issues based on more detailed anatomical data of the cortical circuitry would make the architecture and operations of the prefrontal cortical circuit be more clearly described.