Thesis defense Wenjun Zhang (Donders series 430)

Recent years, mice have become an important animal model for studies on visual processing. Relatively new technologies, like optogenetics or two-photon imaging, which can stimulate neurons and measure neural activity are often combined with behavioral and/or perceptual tests in mouse models. This thesis focuses on the investigation of visual reflexes of mice in a virtual environment. As free movement and more natural conditions are increasingly becoming a standard method for studying behavior of animals, we developed a method to measure visual function in a virtual reality set-up. Moving random dot patterns were projected with a large field of view, the mice were put on a Styrofoam ball and their head was fixed to the setup. We found that mice responded to moving random dot patterns by running in the same motion direction. The gain of this response depended on speed, luminance contrasts, and dot size. When we covered one eye and stimulating with either leftward or rightward motion, we found that the mice showed a bias by responding only to one of the motion directions of the stimulus during the early phase. After 700 milliseconds the bias disappears, and the mice react to both directions. These phenomena might be explained by different contributions of subcortical and cortical motion processing pathways. So, we explored the underlying mechanisms with binocular and monocular conditions by using a transparent motion stimulus in order to investigate whether mice integrate the two components or choose one of the two motion directions. Our results show that mice respond to the two different motion directions in binocular and monocular directions. Examination of the response to transparent stimuli revealed first an integration phase, where mice ran straight. This was followed by a winner-take-all phase, where mice ran into one of two possible motion directions. Finally, we also investigated the reflex of mice to a reverse-phi motion stimulus. For phi motion patterns, different directions caused the mice to compensate with body movements reflexively. When we show reverse-phi motion, mice respond by moving opposite to the displacement direction of the dots. The results show that mice perceive reverse-phi motion similar to humans. The ultimate objective of this thesis is to provide a better understanding of the visual processing mechanisms of mice. Our results will lead to more basic knowledge and may be used in translational studies for finding the neurobiological basis and new treatment of diseases.