Premotor cortex encoding of dynamic hand force and motor output observation underlying hand-object interaction

We studied encoding of hand force and its relations with observation-related activity in macaques trained in an isometric hand-force application and recalibration task, that required to move a visual cursor on a screen toward eight peripheral targets by exerting a force on an isometric joystick, in absence or presence of an opposing force field. Monkeys also observed the result of their action in play-back, as motion of a visual cursor on the screen. This approach combined in a single experiment isometric action performance and force adjustment with observation of its consequences in the external world, also allowing to determine whether PMd neuronal populations reflected an inverse model that specified the force necessary to move in different directions a visual object, or a forward computation encoding its desired trajectory in visual space. We found that a population of PMd cells encoded the direction of dynamic force and its recalibration when the force condition changed but did not retain memory of such change, probably reflecting an adaptation rather than a learning process. Cells with observation-related activity also modulated by change in hand force were not modulated when the force conditions changed, suggesting that their activity reflected the motion of the visual cursor on the screen, therefore the consequences of force application in the visual space. These results also allow a direct comparison of the relative contribute of different populations of PMd cells with that of cells with similar activity profile in the encoding of hand force and its consequences in the parieto-frontal system.