Seeing what is coming: building collision-sensitive neurones

The image of a rapidly approaching object has to elicit a quick response. A n animal needs to know that the object is approaching on a collision course and how imminent a collision is. The relevant information can be computed from the way that the image of the object grows on the retina of one eye. F irm data about the types of neurones that react to such looming stimuli and trigger avoidance reactions come from recent studies on the pigeon and the locust. The neurones responsible;le are tightly tuned to detect objects th at are approaching on a direct collision course. In the pigeon these neuron es signal the time remaining before collision whereas in the locust they ha ve a crucial role in the simple strategy this animal uses to detect an obje ct approaching on a collision course.