A hierarchical neuronal network for planning behavior:
"Planning a goal-directed sequence of behavior is a higher function of the human brain that relies on the integrity of prefrontal cortical areas. In the Tower of London test, a puzzle in which beads sliding on pegs must be moved to match a designated goal configuration, patients with lesioned prefrontal cortex show deficits in planning a goal-directed sequence of moves. We propose a neuronal network model of sequence planning that passes this test and, when lesioned, fails in a way that mimics prefrontal patients’ behavior. Our model comprises a descending planning system with hierarchically organized plan, operation, and gesture levels, and an ascending evaluative system that analyzes the problem and computes internal reward signals that index the correct/erroneous status of the plan. Multiple parallel pathways connecting the evaluative and planning systems amend the plan and adapt it to the current problem. The model illustrates how specialized hierarchically organized neuronal assemblies may collectively emulate central executive or supervisory functions of the human brain."
The Tower of London test (1) consists of moving three colored beads, mounted on vertical rods of unequal length, from an initial position to a prespecified goal (Fig. 1). To solve this task, various levels of motor programming are needed (Fig. 2). At the first and lowest level, here called the “gesture” level, sensory–motor coordination is needed to point to the location of the beads. At a second level, called the “operation” level, a sequence of two elementary gestures (an operation) must be programmed to move a bead from its initial to its final destination. The operation level suffices to solve the simplest Tower of London problems, where each bead can be brought directly to its desired final destination. More difficult problems, however, call for the planning of nondirect, provisory moves and their evaluation by trial and error. These problems pose specific difficulty to human patients with anterior lesions: they experience little or no difficulty executing individual moves but have trouble organizing them into a goal-directed sequence (1–3). We suggest that a third level of programming, the “plan” level, is needed to solve such problems. At this level, sequences of operations (plans) must be selected, executed, evaluated, and accepted or withdrawn depending on their ability to bring the problem to a solution. Our model implements these functions.
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