Course control and tracking: orientation through image stabilization.

Course control and tracking are based on visual detection of the position and movement of objects. A disadvantage of biological movement detectors is that they cannot provide a signal proportional to the speed at which the image of an object moves over the retina. Other image parameters, such as brightness, contrast, and texture, strongly affect the magnitude of the detectors' output signals. To function well, the optomotor control circuit must solve these problems. One possible solution, realized in Diptera, is the principle of "gain control by feedback oscillations" described in this chapter. The optomotor system serves for course control by stabilizing the image of the visual panorama on the eye, and for tracking a moving object by stabilizing the object's image on the eye. When an object moves in front of a structured background, it is impossible for the images of both object and background to be stabilized simultaneously. Arthropods and vertebrates usually employ the same strategy to cope with this problem: saccadic tracking. In Diptera, the neural substrate for saccadic tracking is partially understood.