Many experiments have successfully demonstrated that prosthetic devices for restoring lost body functions can in principle be controlled by brain signals. However, stable long-term application of these devices, required for paralyzed patients, may suffer substantially from on-going signal changes for example adapting neural activities or movements of the electrodes recording brain activity. These changes currently require tedious re-learning procedures which are conducted and supervised under laboratory conditions, hampering the everyday use of such devices. As an efficient alternative to current methods we here propose an on-line adaptation scheme that exploits a hypothetical secondary signal source from brain regions reflecting the user's affective evaluation of the current neuro- prosthetic's performance. For demonstrating the feasibility of our idea, we simulate a typical prosthetic setup controlling a virtual robotic arm. Hereby we use the additional, hypothetical evaluation signal to adapt the decoding of the intended arm movement which is subjected to large non-stationarities. Even with weak signals and high noise levels typically encountered in recording brain activities, our simulations show that prosthetic devices can be adapted successfully during everyday usage, requiring no special training procedures. Furthermore, the adaptation is shown to be stable against large changes in neural encoding and/or in the recording itself.