Causal evidence for posterior parietal cortex involvement in visual-to-motor transformations of reach targets

It has been posited that the posterior parietal cortex (PPC) is involved in the visual-to-motor transformation for reach planning. Such a transformation is required because in general the retinal information and the arm motor command do not align, for example in the case of non-zero eye/head orientations. Here, we present behavioral data from a patient with unilateral optic ataxia consecutive to damage to the superior parietal lobule including the intraparietal sulcus in the right hemisphere, who we asked to reach to visual targets under different head roll angles. An accurate visual-to-motor transformation has to integrate head roll to compensate for the rotated retinal location of the target, resulting in a head roll-independent pattern of reach endpoints. If however, head roll is not compensated for, reach endpoints should vary across different head rolls, reflecting a reach plan based on the rotated retinal target location. Remarkably, the patient compensated for head roll when reaching to targets presented within his intact right visual field (VF) (not different from controls) but not for reaches to targets in the contralesional left VF. The amount of compensation was the same irrespective of whether the initial hand position was located in the left or right VF, showing that this transformation concerns only the target location and not the hand-target motor vector. We interpret these findings as causal evidence for the involvement of the PPC in integrating head roll signals in the visual-to-motor transformation of the reach target.