O'Keefe and Nadel distinguished between two paradigms for navigation, the locale system for map-based navigation, proposed to be in the hippocampus, and the taxon (behavioral orientation) systems for route navigation. We model the taxon system, the map-based system, and their interactions; we argue that the map-based system involves the interaction of hippocampus with other systems.The World Graph (WG) theory expands the idea of a map by developing the hypothesis that cognitive and motivational states interact. We implemented NeWG (Neural WG), a neural network implementation of this theory as well as a modeling environment for the investigation of motivated spatial behavior. NeWG comprises a network of schemas, which incorporate the world graph properties to produce goal-oriented behavior.
We relate taxes to the notion of an affordance. Just as a rat may have basic taxes for approaching food or avoiding a bright light, so does it have a wider repertoire of affordances for possible actions associated with the immediate sensing of its environment. Our model proposes that affordances are extracted by the rat posterior parietal cortex, which guide action selection by the premotor cortex, which is also influenced by hypothalamic drive information.
The determination of movement direction is based on models of frog detour behavior, while the expectations of future reward are implemented using reinforcement learning. In both frog models, prey were represented by excitation coarsely coded across a population, while barriers to movement were encoded by more localized inhibition. The resultant sum of excitation was passed through a winner-take-all circuit to yield the choice of movement direction. In our model, a similar strategy is used in the behavioral orientation system by replacing the direction of the prey (frog) by the direction of the orientation vector (rat). In this way, the specification of the direction of movement is refined by current affordances and motivational information to yield an appropriate course of action.
The model is available at our Brain Models on the Web repository as part of the USC Brain Project (http://www-hbp.usc.edu/).