In the absence of physical practice, observation and imagery do not result in updating of internal models for aiming.

The presence of after-effects in adaptation tasks implies that an existing internal model has been updated. Previously, we showed that although observers adapted to a visuomotor perturbation, they did not show after-effects. In this experiment, we tested 2 further observer groups and an actor group. Observers were now actively engaged in watching (encouraged through imagery and movement estimation), with one group physically practising for 25% of the trials (mixed). Participants estimated the hand movements that produced various cursor trajectories and/or their own hand movement from a preceding trial. These trials also allowed us to assess the development of explicit knowledge as a function of the three practice conditions. The pure observation group did not show after-effects, whereas the actor and mixed groups did. The pure observation group improved their ability to estimate hand movement of the video model. Although the actor and mixed groups improved in actual reaching accuracy, they did not improve in explicit estimation. The mixed group was more accurate in reaching during adaptation and showed larger after-effects than the actors. We suggest that observation encourages an explicit mode of learning, enabling performance benefits without corresponding changes to an internal model of the mapping between output and sensory input. However, some physical practice interspersed with observation can change the manner with which learning is achieved, encouraging implicit learning and the updating of an existing internal model.