DISPARITY-SENSITIVE CELLS IN THE OWL HAVE A CHARACTERISTIC DISPARITY

WE experience the visual world as being three-dimensional. A major sou rce of depth information derives from the slightly different views of each eye, leading to small variations in the retinal images ('disparit ies'). Neurons sensitive to visual disparities are thought to form the neural basis of stereo vision1-10. Barn owls2,3 as well as several ma mmalian species1,4-10 have neurons that are sensitive to visual dispar ities. But how visual disparities are represented in the brain has bee n a matter of discussion ever since the first disparity-sensitive neur ons were found some 25 years ago. Here we adopt a new approach to this problem and study the neural computation of visual disparities with a paradigm borrowed from auditory research. The measurement of interaur al time difference (ITD) has many similarities with the measurement of visual disparity on the formal, algorithmic level. We speculate that the similarities might extend to the level of neural computation. The neural representation of ITD is well understood11-18, and we have stud ied the representation of disparities with visual stimuli analogous to those successfully used in acoustic experiments. For example, ITD is converted in the brain to a pathlength on an axon that, owing to the f inite conduction velocity in neurons, exactly matches the external ITD . This pathlength is called 'characteristic delay,12. Our results sugg est that there is an analogue of the characteristic delay in stereo vi sion which we propose to call 'characteristic disparity'.