induced moving retinal image is ignored. In addition, the contrast between stimuli and background can significantly affect the type of behavior. In response to a wormlike stripe, common toads orient and snap towards the edge leading in the direction of motion, given that the stripe is black and the background white. If the stimulus/background contrast is reversed, the toad prefers the trailing edge of the white stripe and often snaps behind it. Obviously, "off"-effects (rapid change in luminance from bright to dark) by the moving contrast borders play a guiding role. Generally, white square objects moving against a black background are more attractive as prey than black objects on white. However, this tendency is plastic and reverses seasonally, where black objects against a white background are much more effective at eliciting prey-catching behavior in the fall and winter (Ewert 1980). Feature detectors and the visual system To understand the neural mechanisms underlying the toad's behavioral responses, Ewert performed a series of recording and stimulation experiments. First and foremost, the results allowed him to understand the way the visual system is constructed and connected to the central nervous system. Secondly, he discovered areas of the brain that were responsible for differential analysis of stimuli. First, the retina is connected to the optic tectum by at least three types of ganglion cells, each with an excitatory receptive field and a surrounding inhibitory receptive field, but they differ in the diameter of their central excitatory receptive fields. Diameters in Class II (R2) ganglion cells are approximately four degrees visual angle. Those in Class III (R3) cells are about eight degrees and Class IV (R4) ganglion cells range from twelve to fifteen degrees. As stimuli move across the toad's visual field, information is sent to the optic tectum in the toad's midbrain. The optic tectum exists as an ordered localization system, in the form of a topographical map. Each point on the map corresponds to a particular region of the toad's retina and thus its entire visual field. Likewise, when a spot on the tectum was electrically stimulated, the toad would turn toward a corresponding part of its visual field, providing further evidence of the direct spatial connections. Among Ewert's many experimental goals was the identification of feature detectors, neurons that respond selectively to specific features of a sensory stimulus. Results showed that there were no "worm-detectors" or "enemy-detectors" at the level of the