The role of the right posterior parietal cortex in prism adaptation and its aftereffects

Adaptation to optical prisms (Prismatic Adaptation, PA) displacing the visual scene laterally, on one side of visual space, is both a procedure for investigating visuo-motor plasticity and a powerful tool for the rehabilitation of Unilateral Spatial Neglect (USN). Two processes are involved in PA: i) recalibration (the reduction of the error of manual pointings toward the direction of the prism-induced displacement of the visual scene); ii) the successive realignment after prisms’ removal, indexed by the Aftereffects (AEs, in egocentric straight-ahead pointing tasks, the deviation in a direction opposite to the visual displacement previously induced by prisms). This study investigated the role of the posterior parietal cortex (PPC) of the right hemisphere in PA and AEs, by means of low frequency repetitive Transcranial Magnetic Stimulation (rTMS). Proprioceptive and Visuo-proprioceptive egocentric straight-ahead pointing tasks were used to assess the presence and magnitude of AEs. The primary right visual cortex (V1) was also stimulated, to assess the selectivity of the PPC effects on the two processes of PA (recalibration and realignment) in comparison with a cortical region involved in visual processing. Results showed a slower adaptation to prisms when rTMS was delivered before PA, regardless of target site (right PPC or V1). AEs were reduced only by PPC rTMS applied before or after PA, as compared to a sham stimulation. These findings suggest a functional and neural dissociation between realignment and recalibration. Indeed, PA interference was induced by rTMS to both the PPC and V1, indicating that recalibration is supported by a parieto-occipital network. Conversely, AEs were disrupted only by rTMS delivered to the PPC, thus unveiling a relevant role of this region in the development and maintenance of the realignment.