Preserved adjustment but impaired awareness in a sensory-motor conflict following prefrontal lesions

Control of action occurs at different stages of the executive process, in p articular at those of sensory-motor integration and conscious monitoring. T he aim of this study was to determine the implication of the prefrontal cor tex in the control of action. For that purpose, we compared the performance of 15 patients with frontal lobe lesions and 15 matched controls on an exp erimental paradigm generating a conflict between the action planned and the sensory-motor feedback. Subjects had to trace a sagittal line with a stylu s on a graphic tablet. The hand was hidden by a mirror on which the traced line, processed by a computer, was projected. Without informing the subject s, the line traced was modified by introducing a bias to the right, which i ncreased progressively from 2 to 42. To succeed the task, subjects had to m odify their motor program and deviate their hand in the opposite direction. The sensory-motor adjustment to the bias was evaluated by the surface betw een the line traced and the ideal line to compensate for the deviation. The awareness of the conflict was measured by the angle of the bias at which s ubjects expressed the feeling that the line they traced was not the same as the line they saw. The deviation was similarly compensated for by patients and controls until 24 degrees. Then 14 controls but only 3 patients were a ware of a conflict. After that, the variability of performance increased si gnificantly for the unaware patients. These results suggest that the prefro ntal cortex is required at the level of conscious monitoring of actions, bu t nor at the level of sensory-motor integration.