In a dimmed room, directly under the cone of light from a lamp, there is a comfortable chair which invites the visitor to take a rest. Once he takes a seat, he finds himself facing a mirror which seems at first glance to be in no way unusual. On several tripods positioned around this chair, there are diverse measuring instruments, which are operated by a feedback controller. In the background, behind the installation, there are further technical apparatuses that the visitor only notices due to the display panels with their multicoloured lights and constantly changing data diagrams. "Face the interface - interface the face" sums up the basic principle of the “Psychophysical Mirror" by Horst Prehn, and programmatically unites a distinctly artistic approach and medical- scientific studies. As the most vital of the human extremities, the head concentrates within it not only all synchronisation and control systems, but beyond this also provides man's most versatile and interesting communication surface: the face. Every emotion, every mood and every intention can be read in it as if in an open book. In front of the mirror, face to face with himself, the participant becomes the principal witness of a mysterious transformation of his own mirror image. This occurs by wiring the person to the computer, i.e. breath frequency, the surface moisture of the skin, brain activity and the heartbeat of the test person are all registered by different sensors. The results are analysed qualitatively and quantitatively and transferred into a large number of parametric impulses. At the same time, a video camera records the actual physiognomy of the participant as he is looking into the mirror. The video image serves as a point of departure and is altered by modulations of the computer software. With the aid of a special reproduction technique, the optoelectronic mirror image is superimposed on the real mirror image, resulting in a closed control circuit. The viewer experiences his reflection in the mirror in a view that has, so to speak, been fed back. When he finally frees himself from the fascination of the immediate experience, he is reminded involuntarily of that "mirror, mirror on the wall,...", and his mind is filled with innumerable mystical and mythical associations. Semi-transparent mirror, multisensory psychophysical interface, video system, computer