An add-on device for use in combination with a stereotactic apparatus is herein disclosed, the device including a stereotactic adapter, an optical support comprising lenses and a display support adapted to accommodate a virtual reality display. The stereotactic adapter can adapt to different commercial models of stereotactic systems, and is particularly intended for implementing a method for conducting cognitive and/or behavioral tests in subjects during neurosurgery, wherein a virtual reality stimulus is provided to said subject during awake phases of neurosurgery.