A possible mechanism for binocular depth judgements in anurans.

Two experiments were performed to analyze how anurans (Bufo marinus) use binocular cues to gauge the distance of their prey. In the first, bilateral lesions of the nucleus isthmi eliminated the major source of input from the ipsilateral eye to the tectum. These lesions did not disrupt the animals' ability to use binocular cues to judge distance, suggesting that frogs and toads may not employ binocular disparity-selective cells to assess prey distance. They may instead use a scheme more overtly akin to triangulation, with each tectum providing an output signal encoding the angular position of the prey with respect to the contralateral eye and with distance extracted from the difference between these tectal outputs. In the second experiment, prisms imposed large (13.5 degrees) vertical disparities between the two eyes' images. The toads continued to use binocular cues. The added vertical disparities, like added horizontal ones, caused toads to undershoot their prey. Thus the binocular system must tolerate such vertical disparities and fail to distinguish them from horizontal ones.