Event-related Lateralizations of EEG activity during Pointing

Reaching to a visual target requires the solution of the following spatial correspondence problem: The position of the target and the position of the limb are coded in different coordinate frames. The location of the target is initially coded in retinal coordinates, whereas, if vision of the hand is not available, its position is derived from proprioceptive signals. Finally, the hand has to be guided in muscle-centered coordinates. This discordance may be solved by a transformation from retinal to body-centered coordinates in order to accurately direct the hand to the target in extrapersonal space. This thesis investigates the visual to motor transformation process in pointing, using event-related EEG recordings. The EEG provides a real-time measure of brain activity. Thus, by means of EEG recordings, the visual to motor transformation for the reach can be assessed at different stages within the process from target onset to the execution of movement. When we interact with our everyday environment, the transformation from eye to body-centred coordinates is accomplished easy, presumable on the basis of learned visuomotor relations. However, the difficulty in computing accurate visually guided reaches depends on the familiarity of the spatial codes. In many cases, the objects that we want to reach are dynamic. They either move or appear suddenly. This is the case, for example, when we want to hit or catch a ball in a game. This is more efficient if we can predict where the ball will appear in our peripersonal space. In this case, the spatial parameters of the reach can be predefined. On the other hand, if the familiar relation between the position of an object in visual space and its body-centered coordinates is disturbed, performing the reach requires additional visuomotor computation. This can be the case if we wear a new pair of glasses that distort the visual world or in experimental conditions when participants look through prisms that dissociate visual from felt information. The experiments of this thesis aimed at revealing the stages within the visuomotor transformation process that are modified by changes in the diffculty of generating the spatial codes for reaching and by the rearrangement of visuomotor relations. It is shown that the visuomotor transformation process, starting from the representation of the target position in visual space to the execution of the reach, is reflected in lateralized components of the EEG. These contra-ipsilateral hemispheric asymmetries seem to depend on the difficulty of generating the spatial codes for reaching. This is demonstrated by the modification in the ERLs when the pointing direction is predictable, as well as by their susceptibility to disruptions of the familiar coordinate transformation induced by a reversal of the visual field that dissociates visual from felt information. The ERLs also reflect the increased familiarity of the visuomotor codes under reversed vision when pointing repeatedly toward the same target position. Furthermore, ERLs reveal that the reversal of vision seems to change the interhemispheric distribution of activity of neurons with combined visual and motor properties that code the representation of the arms. These ERLs seem to reflect the properties of the networked frontal and parietal areas that are involved in the coding of reaching movements. In this work, the properties of the frontoparietal network for reaching that are relevant for the questions underlying the experiments of this thesis are discussed. Furthermore, the principle of EEG recordings and the computation of the event-related lateralizations are illustrated