Three types of looming-selective neurons have been found in the nucleus rotundus of pigeons, each computing a different optical variable related to image expansion of objects approaching on a direct collision course with the bird. None of these neurons respond to simulated approach toward stationary objects. A detailed analysis of these neurons' firing pattern to approaching objects of different sizes and velocities shows that one group of neurons signals relative rate of expansion tau (tau), a second group signals absolute rate of expansion rho (rho), and a third group signals yet another optical variable eta (eta). The rho parameter is required for the computation of both tau and eta, whose respective ecological functions probably provide precise 'time-to-collision' information and 'early warning' detection for large approaching objects.