Context-specific grasp movement representation in macaque ventral premotor cortex.

Hand grasping requires the transformation of sensory signals to hand movements. Neurons in area F5 (ventral premotor cortex) represent specific grasp movements (e.g., precision grip) as well as object features like orientation, and are involved in movement preparation and execution. Here, we examined how F5 neurons represent context-dependent grasping actions in macaques. We used a delayed grasping task in which animals grasped a handle either with a power or a precision grip depending on context information. Additionally, object orientation was varied to investigate how visual object features are integrated with context information. In 420 neurons from two animals, object orientation and grip type were equally encoded during the instruction epoch (27% and 26% of all cells, respectively). While orientation representation dropped during movement execution, grip type representation increased (20% vs 43%). According to tuning onset and offset, we classified neurons as sensory, sensorimotor, or motor. Grip type tuning was predominantly sensorimotor (28%) or motor (25%), whereas orientation-tuned cells were mainly sensory (11%) or sensorimotor (15%) and often also represented grip type (86%). Conversely, only 44% of grip-type tuned cells were also orientation-tuned. Furthermore, we found marked differences in the incidence of preferred conditions (power vs precision grips and middle vs extreme orientations) and in the anatomical distribution of the various cell classes. These results reveal important differences in how grip type and object orientation is processed in F5 and suggest that anatomically and functionally separable cell classes collaborate to generate hand grasping commands.

Context-specific grasp movement representation in the macaque anterior intraparietal area.

To perform grasping movements, the hand is shaped according to the form of the target object and the intended manipulation, which in turn depends on the context of the action. The anterior intraparietal cortex (AIP) is strongly involved in the sensorimotor transformation of grasping movements, but the extent to which it encodes context-specific information for hand grasping is unclear. To explore this issue, we recorded 571 single-units in AIP of two macaques during a delayed grasping task, in which animals were instructed by an external context cue (LED) to perform power or precision grips on a handle that was presented in various orientations. While 55% of the recorded neurons encoded the object orientation from the cue epoch on, the number of cells encoding the grip type increased from 25% during the cue epoch to 58% during movement execution. Furthermore, a classification of cells according to the time of their tuning onset revealed differences in the function and anatomical location of early- versus late-tuned cells. In a cue separation task, when the object was presented first, neurons representing power or precision grips were activated simultaneously until the actual grip type was instructed. In contrast, when the grasp type instruction was presented before the object, type information was only weakly represented in AIP, but was strongly encoded after the grasp target was revealed. We conclude that AIP encodes context specific hand grasping movements to perceived objects, but in the absence of a grasp target, the encoding of context information is weak.