Sensory-spatial transformations in the left posterior parietal cortex may contribute to reach timing

The posterior parietal cortex (PPC) contains viewer-centered spatial maps important for reaching movements. It is known that spatial reaching deficits emerge when this region is damaged, yet less is known about temporal deficits that may also emerge because of a failure in sensory-spatial transformations. This work introduces a new geometric measure to quantify multimodal sensory transformation and integration deficits affecting the tempo of reaching trajectories that are induced by injury to the left PPC. Erratic rates of positional change involving faulty maps from rotational angular displacements to translational linear displacements contributed to temporal abnormalities in the reach. Such disruptions were quantified with a time-invariant geometric measure. This measure, paired with an experimental paradigm that manipulated the source of visual guidance for reaches, was used to compare the performance of normal controls to those from a patient (T.R.) who had a lesion in his left-PPC. For controls, the source of visual guidance significantly scaled the tempo of target-directed reaches but did not change the geometric measure. This was not the case in patient T.R., who altered this measure. With continuous, extrapersonal visual feedback of the target, however, these abnormalities improved. Vision of the target rather than vision of his moving hand also improved his arm-joint rotations for posture control. These results show that the left PPC is critically important for visuo-motor transformations that specifically rely on extrapersonal cues to align rotational-arm and linear-hand displacements and to continuously integrate their rates of change. The intactness of this system contributes to the fluidity of the reach's tempo.