Integration of size and depth information of an approaching object in collision-sensitive neurons of the bullfrog (Rana catesbeiana)

Our behavioral experiment suggested that frogs process not only retinal size but also depth information of an approaching object. Here, physiological experiments using single unit recordings were performed to test this hypothesis. Looming stimulation was presented with a computer display placed either 20 cm or 40 cm from the animal. Based on the correlation between time-to-collision and visual parameter l / v (l: object half size, v: approaching velocity), an angular threshold size of the response of a collision-sensitive neuron was calculated. The results revealed two types of collision-sensitive neurons in the frog optic tectum. One showed similar angular threshold size in both cases. The other encoded a specific retinal threshold size only when the visual stimulus was presented at a particular distance from the animal.