A system-level brain model of spatial working memory

A system-level model of spatial working memory is described, using the author’s computer science and logical modeling approach. The mental image which is remembered is located in a lateral parietal area, and is part of a larger distributed representation including object identities and object appearances. The system is driven by plans which are stored in a separate module corresponding to ventral prefrontal and then copied to a planning module corresponding to dorsal prefrontal, where they are executed and sequenced. The system is integrated by an event memory module corresponding to the hippocampus, which gathers event information from most of the modules and constructs a representation of the current mental event and the current mental episode. This information is used by the mental imagery modules to construct a unified representation of the current scene. Mental images are maintained by explicit messages from a currently executed plan. Upon a cue to use a saved mental image for motor output or eye movement, a message from the plan causes the image to be temporarily reinstated allowing it to be used for motor control. The system generates predicted MRI image sequences. We developed an energy estimation function based on the different kinds of information processing being carried out and we graphed energy usage in the main modules over time during a spatial working memory experiment. We also made a study of lesioning the model to produce spatial working memory deficits. We first describe the main logical dependencies among the different brain modules giving a systematic way of finding all the vulnerabilities of the system and how lesions in different modules can interfere with the action of the complete system. The interference in most mechanisms is often catastrophic, preventing any spatial working memory behavior at all. We then describe a more gradual approach in which the goal attenuates, causing the plan to fall below a threshold. This approach allowed us to match the experimental results of Conklin et al. in which schizophrenic and schizoptypal subjects could carry out a spatial working memory task with a 0.5 second delay but not when there was a 7 second delay.