Prabhu, G.; (2008) Neural mechanisms of object-oriented action. Doctoral thesis , University of London.

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Abstract

The ventral premotor cortex (area F5, in the macaque monkey) plays an important role in the control of hand shape during object grasp. F5 neurones can encode the selection of different grasps, and are excited by vision of graspable objects. However, it is not clear how F5 influences the motor outputs to the hand and arm that control hand shape. By using linked neurophysiological studies in macaque monkeys and humans, this thesis examined, firstly, how visual information for action is encoded in area F5 and, secondly, the transmission of this information from F5 to the primary motor cortex (Ml). The relationship of F5 activity to the observation and movement stages of a visuomotor grasping task was examined. Single units from Ml and F5 and electromyographic (EMG) activity were recorded from macaque monkeys. Recordings from F5 revealed evidence for object- and grasp-related activity, suggesting that initially neurones are activated by the visual characteristics of the object and later represent the specific hand configuration for grasp of the object. Ml activity was consistent with encoding of grasp, but was uninfluenced by object. Secondly, using the same task, the pathways used to transmit visuomotor information from F5 to Ml were investigated by intracortical myostimulation of these cortical regions. The facilitation and suppression of the Ml test response from F5 conditioning was highly specific, evoked only in particular muscles and certain object-grasp combinations. Furthermore, the timing of the evoked response supports late I-wave pathways mediating F5-M1 interactions in visuomotor grasp. In the final set of experiments a paired-pulse transcranial magnetic stimulation paradigm, previously shown to enhance the late I-wave components of the corticospinal volley, was used to examine object-orientated grasp in healthy human subjects. The results suggest excitability of cortico-cortical inputs to Ml was transient and contributed to action selection only when immediate sensory information specified which action to make. Results from this thesis expand our knowledge of the role of F5 in visuomotor grasp, by showing F5 single unit activity is compatible with encoding the physical properties of an object to be grasped as well as the motor prototype used for grasp. Evidence is also provided for a possible route for transmission of visuomotor information from F5 to Ml. Overall, there was a highly specific task-related pattern of object/grasp-related activity present in F5 and Ml single units and the EMG activity evoked from stimulation of these regions during the preparation and execution stages of visuomotor grasp.

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