Marc Jeannerod

The Timing of Natural Prehension Movements

Prehension movements were studied by film in 7 adult subjects. Transportation of the hand to the target-object location had features very similar to any aiming arm movement, that is, it involved a fast-velocity initial phase and a low-velocity final phase. The peak velocity of the movement was highly correlated with its amplitude, although total movement duration tended to remain invariant when target distance was changed. The low-velocity phase consistently began after about 75% of movement time had elapsed. This ration was maintained for different movement amplitudes. Formation of the finger grip occurred during hand transportation. Fingers were first stretched and then began to close in anticipation to contact with the object. The onset of the closure phase was highly correlated to the beginning of the low velocity phase of transportation. This pattern for both transportation and finger grip formation was maintained in conditions whether visual feedback from the moving limb was present or not. Implications of these findings for the central programming of multisegmental movements are discussed.

Impairment of grasping movements following a bilateral posterior parietal lesion.

The observation of a patient (A.T.) with a bilateral posterior parietal lesion of vascular origin is reported. A.T. presented a bilateral (more marked on the right) deficit in grasping simple objects (neutral cylindrical dowels) without deficit in reaching toward the location of these objects. The major symptom was an exaggerated anticipatory opening of the fingers with poor correlation with object size, resulting in awkward grasps. It was present both when the hand was visible to the subject and when it was not. This deficit was much less marked if the neutral objects were replaced by usual objects of the same sizes. Finally, in tasks where she had to indicate with her fingers the size of visual objects presented as virtual images through a mirror, or the size of imagined usual objects, A.T. performed normally. These results are discussed within the framework of a dual representation of objects. Only the pragmatic representation for steering object-oriented actions would be impaired in this patient as a result of posterior parietal damage. By contrast the semantic representation for object identification would be intact.

The mechanism of self-recognition in humans

Recognizing oneself as the owner of a body and the agent of actions requires specific mechanisms which have been elucidated only recently. One of these mechanisms is the monitoring of signals arising from bodily movements, i.e. the central signals which contribute to the generation of the movements and the sensory signals which arise from their execution. The congruence between these two sets of signals is a strong index for determining the experiences of ownership and agency, which are the main constituents of the experience of being an independent self. This mechanism, however, does not account from the frequent cases where an intention is generated but the corresponding action is not executed. In this paper, it is postulated that such covert actions are internally simulated by activating specific cortical networks or representations of the intended actions. This process of action simulation is also extended to the observation and the recognition of actions performed or intended by other agents. The problem of disentangling representations that pertain to self-intended actions from those that pertain to actions executed or intended by others, is a critical one for attributing actions to their respective agents. Failure to recognize ones own actions and misattribution of actions may result from pathological conditions which alter the readability of these representations.

Visuomotor channels: Their integration in goal-directed prehension

Abstract The visuomotor transformations for producing a grasping movement imply simultaneous control of different visual mechanisms. The size, orientation and 3D characteristics of the object have to be encoded for the selection of the appropriate opposition space, within which the opposition forces will be applied on the object surface. These mechanisms also have to combine with those of the transport of the hand to the object location. Finally, biomechanical constraints impose categorical visuomotor decisions for positioning the opposition space according to object changes in size, orientation and spatial location. This paper examines possible interactions between the specialized structures for visuomotor transformation and the internal model that adapts prehension to its goals. PsycINFO classification: 2300; 2323; 2380; 2500; 2540; 3297

Prehension Movements: The Visuomotor Channels Hypothesis Revisited

Publisher Summary Due to its high number of degrees of freedom, the primate hand can perform highly complex movements. The chapter analyzes the evidence for the visuomotor channel hypothesis to determine whether or not there are valid arguments against the existence of parallel processing for the action of grasping an object. Testing this model implies, first, describing the two components with the correct parameter. It also implies that the experiments in which the object properties are changed during the movement are sufficiently selective. The double peak pattern observed on the grip curve might be due to the presentation of two different objects. Indeed, when grasping a single moving object, no such a pattern was observed. When a pure change in object size was produced, there was no effect on the kinematics of the transport component, but there was an effect on movement time, due to a disruption of the coordination. These results suggest that the two components of prehension are controlled by distinct pathways and that these pathways are selectively activated when perturbations are applied to either one or the two components.