Velocity sensitivity and directional selectivity of frog retinal ganglion cells depend on chromaticity of moving stimuli.

Action potentials of single afferent optic-nerve fibers were recorded in the superficial layers of the optic tectum of frogs. Horizontally moving chromatic stimuli were applied. A large range of stimulus velocities and 2 or 3 different wavelengths (450, 500 or 580 nm) of the moving monochromatic light spots were applied. The velocity functions of class 3 neurons varied only slightly with different chromatic stimuli. About half of the neurons of our sample exhibited directional selectivity to one or two of the wavelengths investigated. In some of these neurons the directional selectivity was found over the entire velocity range studied (0.046-18.4 degrees X s-1), while in others it was also dependent upon the angular velocity of the moving chromatic spot. Thus, a new principle of chromatic-signal processing exists in frogs which has so far not been described in other animals: an interrelation between directional selectivity, chromatic composition of the stimulus and angular velocity. We concluded from these findings that the analytic properties of tectal cells, with respect to their possible function in pattern recognition, might receive insufficient description when the stimuli are restricted to the achromatic grey scale. On the other hand, we would like to stress that the peculiar properties of the retinal color channels in frogs, directional selectivity and different time constants of the recovery functions, contribute to the processing of black/white stimuli according to their shape, size and velocity, since the response to the leading edge and the trailing edge traversing the receptive field depends on these factors.