Visual perception and motor control in birls and humans

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Stereopsis as a cue to the distance of objects is a rare invention among animals as it requires frontally posi- tioned eyes with a relatively large distance between them. A more common way to obtain depth is by means of motion parallax - the displacement of an object's image on the retina as a result of an observer's motion through space. Even though more ubiquitous than stereopsis, motion parallax is less well studied. Stereopsis and motion parallax rely on comparing views taken from different viewpoints. Knowledge about the distance between them is crucial. While interocular distance is a physiological constant, the distance traveled between subsequent measurements for motion parallax - that is, the speed between object and observer - may not be available. Theoretically, not knowing an observer's speed can be overcome by moving with two different speeds. If the difference between them is known, distance can be calculated. My research aims to test whether this hypothesis provides an explanation for head-bobbing behaviour - that is, the peculiar oscillating movement of the head of walking pigeons, and many other birds. Using a complex virtual-reality setup, we can uncouple and then re-define the relation between the movement of the bird and the retinal image motion which this movement induces. In addition to studying visual control of head-movements, we will also investigate the role of two interial sensor systems: the vestibular system in the head, and the lumbosacral vertebral canal system in the lower spine which we both expect to be involved in the control of head-bobbing behaviour. A comparative study into which birds head-bob and which ones do not will reveal evolutionary origin of head-bobbing. This line of research will likely reveal the functional significance of head-bobbing which has puzzled researchers and lay people alike for a long time. Furthermore, we aim to develop the pigeon into a general model for the study of motor control in an animal which is much more accessible to research than non-human primates and that shares with us the fact that it spends a considerable time of its life on two legs.