The role of posterior parietal cortex in spatial representation of time: a TMS study

1. Introduction. The existence of a spatial representation of time, where temporal intervals are represented on a mental temporal line (MTL), oriented in ascending order from left to right, was demonstrated manipulating spatial attention by means of Prismatic Adaptation (PA). In young healthy subjects, prisms adaptation inducing a rightward shift of spatialattention produced an overestimation of time intervals, whereas prisms adaptation inducing a leftward shift of spatialattention produced an underestimation of time intervals [4]. The aimof the present study was to investigate the neural basis mediating the effects of PA on spatial time representation. PosteriorParietalCortex (PPC) is the best candidate to discharge this function. Indeed, neuropsychological and neurophysiological studies designate rightPPC as the site of spacetime interaction [1,3,7]. Concerning the neural bases of PA procedures, left and rightPPC are involved in different phases of PA procedure [2,5, 6]. Here we investigated, by using TMS, the role of the PosteriorParietalCortex (PPC) in spatial representation of time and in cerebral plasticity phenomena mediating prismatic adaptation effects on time processing. To this aim, healthy subjects were submitted to a temporal task before and after PA. TMS was applied on left or right PPC, before or after PA. If the right (left) PPC is the key area leading the effects of PA on time, a reduced effect of PA on time is expected for TMS over right (left) PPC both before and after PA. If the right (left) PPC plays a role in PA procedure and not in mediating PA effects on time, the effect of TMS over right (left) PPC is expected for TMS applied before and not after PA.