Long lasting aftereffect of a single prism adaptation: Directionally biased shift in proprioception and late onset shift of internal egocentric reference frame.

We aimed to dissociate components in prism adaptation and its aftereffect by using prism adaptation training in healthy humans. Arm proprioceptive aftereffects are usually measured by indicating the subjective straight ahead direction with eyes closed (S). This measure however could be affected by other components besides proprioception, such as an efferent motor component and internal egocentric reference frame. Here we report a very long lasting proprioceptive shift, detected by two measuring methods, that is a component of the adaptation aftereffects to left wedge prism glasses. In order to minimize possible active motor components, arm passive proprioceptive midsagittal judgment was measured (P). The subject's arm was passively brought from the right or left lateral position, and stopped by subjects' verbal order. The results from these different measurements of midsagittal judgment were compared for 7 days after prism adaptation. Surprisingly, we found two distinctly separate aftereffects of proprioceptive shift depending on the directions of the passive arm movement. The shift of the midsagittal plane appeared only when tested from the left (Pl). This indicates that our strong prism adaptation procedure affected proprioception in a directionally biased way and not a spatially ubiquitous way. Further, the early aftereffect seen in active straight ahead pointing (S) was mostly similar to this biased shift in proprioception (Pl). However the long lasting aftereffect in straight ahead pointing was independently maintained up to day 7, when the passive proprioception had returned to pretest level. These results indicate that active straight ahead pointing (S) involves other components in addition to the passively measurable proprioceptive component. We suggest a late onset shift in the internal egocentric reference frame is involved in S. Possible neural mechanisms for these phenomena are discussed.