Perception and Cognition

Our everyday experience suggests that the world is exactly as we perceive it. However, the information available via our senses is a priori incomplete, noisy and may allow for more than one interpretation (see Figs. 1 and 2). As a consequence, the distinction between reality and illusion is limited by the capacity of our perceptual system and the reliability of our memories. Perception is thus a core area of psychophysical interaction and the mind-matter debate. Perceptual illusions and perceptual anomalies can be experienced as exceptional perceptual experiences and can lead to altered and sometimes unstable states of consciousness. Perceptual anomalies can also be consequences of altered and/or unstable perceptual and consciousness states.


Figure 1. The convex front side (left) and the concave back side (right) of a Charlie Chaplin mask. Even if we know consciously about the concavity of the mask’s backside (right figure), we perceive it as concave. Source: Max Planck Institute for Biological Cybernetics, Tübingen, Germany (see also Hill and Johnston 2007).

We study the processes underlying stable and unstable perception and perceptual anomalies in order to better understand the psychophysical relations. Particularly, we use ambiguous stimuli to induce unstable perceptual states in highly controlled lab environments. Unstable perceptual states serve as model states for unstable mental states that are often observed in people with exceptional experiences or people in altered states of consciousness. With psychophysical methods, eye-movement recordings and EEG, functional MRI and recently also with methods from artificial intelligence, we try to better understand the processes underlying unstable perceptual and mental states.

Figure 2. Adelsons Checker-Shadow Illusion (see also Adelson EG 1993). Left: The square “B” appears much brighter than the square “A”. Right: A yellow mask isolates areas within the squares A and B from their context. It becomes obvious, that both areas are in fact equiluminant. Source: Michael Bach’s website of visual illusions.

In cooperation with the Department of Psychiatry and Psychotherapy at the Medical Center of the University of Freiburg and the Psychiatric Hospital Strasbourg, France we investigate in this context patients with psychiatric disorders, who also show altered perceptual and conscious states, in order to test our models and hypotheses derived thereof. Concurrently, we also try to better understand the focused disorders.


Adelson EH (1993). Perceptual organization and the judgment of brightness. Science (New York, N.Y.),262(5142), 2042-2044 .

Hill H & Johnston A (2007). The hollow-face illusion: Object-specific knowledge, general assumptions or properties of the stimulus? Perception, 36(2), 199-223.