Two waves of a long-lasting aftereffect of prism adaptation measured over 7 days.

Prism adaptation is a useful paradigm to study the integration and reorganization of various sensory modalities involved in sensory-motor tasks. By prolonging the prismatic aftereffect and well-timed observation, we aimed to dissociate the components and mechanisms involved in human prism adaptation by their differential decay and development time courses. Here, we show that a single session of prism adaptation training, combining small increments of prism strength below the subjects' awareness threshold, during a pointing task with a free walk session with total prism exposure duration of 75 min, generated a surprisingly long-lasting aftereffect. The aftereffect was measured by the magnitude of the proprioceptive shift (assessed by straight-ahead pointing in the dark) for 7 days. An aftereffect was observed, which lasted for more than 6 days, by a single prism adaptation session. The aftereffect did not decay gradually. Unlike previous descriptions, the aftereffect showed two separate time-courses of decay and increase. After a significant initial decay within 6 h, the aftereffect increased again from 1 day up to 3 days. The novel decay and delayed development profile of this adaptation aftereffect suggests two separate underlying neural mechanisms with different time scales. Our experimental paradigms promise to reveal directly the temporal characteristics of early versus late long-term neural plasticity in complex human adaptive behavior.