Encoding of visual targets during 3D reaching movements in human and non-human primates

The aim of my thesis was to investigate how reaching for visual targets placed in 3D space influences the coordinate frames and the kinematics in non-human and human primates. To this end, I conducted three studies. The first study was conducted on non-human primates to find the predominant reference frame of cells in a specific reach related area of the PPC (area PEc) while reaching towards targets placed at different depths and directions; we tested whether PEc reaching cells displayed hand- and/or body-centered coding of reach targets. We found that the majority of PEc neurons encoded targets in a mixed body/hand-centered reference frame. Our findings highlight a role for area PEc as intermediate node between the visually dominated area V6A and the somatosensory dominated area PE. The second study was conducted on healthy human subjects to find the reference frame used while reaching towards targets placed at different depths and directions. Our results revealed reach error patterns based on both eye- and space-centered coordinate systems: in depth more biased towards a space-centered representation and in direction mixed between space- and eye-centered representation. The third study was conducted on a patient with a parietal cortex lesion who showed optic ataxia symptoms. Optic ataxia patients show deficits in visuo-manual guidance especially when reaching to targets located in the periphery of the visual field. By manipulating gaze position and hand position of visual reaching targets, placed at different depth and directions, we investigated how reaching in peripheral and central viewing conditions influenced the trajectories and reach errors of the patient and controls. Our results suggest that the reaching inaccuracies observed, in particular in the configurations where the direction of gaze and reach differed, are due to a disruption of the online correction mechanism and that the PPC is involved in these automatic corrections.