Multisensorische Repräsentation von Eigenbewegung im menschlichen Gehirn

While moving through our environment we receive visual, auditory, proprioceptive, vestibular and sometimes tactile information about the position, velocity and acceleration of our body. Only a successful integration of these signals allows for a coherent perception of self-motion. Information from all modalities together provides the most reliable representation. However, previous studies demonstrated that one can use pure visual, vestibular or proprioceptive signals for distance estimation. The aim of my thesis was to analyse the role of auditory signals for self-motion perception and to determine which brain areas process audio-visual self-motion signals. For this purpose I carried out psychophysical tests and recorded brain activities using functional magnetic resonance imaging (fMRI). In my first study I investigated whether auditory self-motion information can be used to estimate and reproduce the distances of forward movements. Participants were presented with a visually simulated forward-motion across a ground plane (passive displacement). The frequency of an associated auditory stimulus was proportional to the simulated speed. Subjects had to reproduce the distance of the displacement with a joystick (active displacement). During the active displacements they received either audio-visual or pure visual or pure auditory motion signals. I found that reproduction was most precise when the participants only heard the tone while it was least precise when they only saw the ground plane. In a subsequent experiment in some trials the relationship between optical velocity and tone frequency was differently scaled during the active displacements, i.e., the tone frequency was either higher or lower than during the passive displacements (catch trials). I found that the re-scaling affected the subjects’ performance:When the frequency was lower subjects used higher speeds resulting in a substantial overshoot of travelled distance, whereas a higher frequency resulted in an undershoot of travelled distance. I conclude that during self motion tone frequency can be used as a velocity cue and helps to estimate and reproduce travel distance. During self-motion an image of the environment is shifted on the retina. This image motion – called optic flow – provides us with information about the direction and velocity of the displacement. It induces reflexive, compensatory eye movements which stabilize part of the image on the retina. In my second study I observed that a simulated forward motion across a ground plane (as used in Study I) induces such reflexive eye movements. They are composed of slow (following) and fast (resetting) phases. I found that subjects controlled the speed of the slow eye movements more precisely when they controlled the driving speed with a joystick. Probably the proprioceptive feedback from the joystick facilitated eye movement control. Moreover, I found that participants also moved their eyes in the direction of the ground plane motion when they did not see the plane but only received auditory velocity cues. In a third study I investigated by means of fMRI which brain regions are involved in the processing of audio-visual self-motion signals. Since only spatially and temporally congruent signals are integrated optimally into a common percept I investigated to what extend the congruency of signals influences brain activity. The visual stimulus consisted of an alternately expanding and contracting cloud of random dots simulating a forward and backward motion. Auditory stimuli consisted of a sinusoidal tone which simulated a forward and backward motion in a congruent bimodal condition. In an incongruent bimodal condition the tone simulated a frontoparallel motion while the visual stimulus simulated a forward and backward motion. The contrast of bimodal versus unimodal stimulation activated amongst others regions around the precentral sulcus, the superior temporal sulcus as well as the intraparietal sulcus. Compared to incongruent stimulation the congruent stimulus activated a part of the precentral sulcus. Taken together, I showed in my thesis that auditory self-motion information plays an important role for the estimation and reproduction of travelled distances. Audiovisual self-motion information is processed in a parieto-frontal brain network. Spatially congruent signals are processed in a brain area which might be an equivalent of the polysensory zone (PZ) in the macaque brain.