These measurements were begun when the animals were about 7 months old and continued for 1–2 months by use of a slight modification of the jumping stand and the procedure described much earlier (Mitchell et al.,
1979). The main features of the jumping stand are shown in schematic form in
Figure 1. The animal was required to jump from a platform toward the closer of two adjacent stimuli that were visible through a black mask placed on the transparent clear glass (5 mm thick) surface of the jumping stand that was located 67 cm below the jumping platform. The stimuli were two transparent plates upon which black opaque circles of 3 different sizes (5, 14 & 20 mm) were displayed upon a virtual grid of 20 mm dia squares with a distribution density of 30% in a quasi-random fashion (i.e. the circles could not appear on adjacent squares of the grid). The rectangular masks (19 × 14.5 cm) restricted the angular dimensions of the displays to 15.8 × 12.2 deg. Because the arrangement of the patterns on the two stimuli were independently determined, they looked quite different, a feature that reduced the ability of the animals to employ density and size cues for their judgments. Once the discrimination was learned with a large (23 cm) depth difference between the two stimuli, measurements were made of the smallest separation in depth (ΔD) of the stimuli that could be discriminated with the closest stimulus always located 2 cm below the surface of the glass plate onto which the cat jumped. As in the past (Kaye et al.,
1981; Mitchell et al.,
1994) the animal received a minimum of 5 trials at each depth interval (ΔD). If an error was made, the animal was required to make 5 consecutively correct choices or be correct on at least 7 of the maximum of 10 trials provided for each depth interval which were changed in 2 cm steps. The threshold on any day was defined as the smallest depth interval for which the animal achieved this criterion (i.e. at least 70% correct). Typically, animals went from 100% correct performance to chance over only 3 depth intervals. When performance stabilized, measurements were made each day for 10 days of first the binocular depth threshold followed immediately by a monocular threshold with one eye occluded (the occluded eye alternated on successive days) with an opaque hard contact lens. This procedure highlighted any difference between binocular and monocular performance as the two measurements were made daily. For typically reared animals, the monocular thresholds were a factor of 10 times larger than the binocular values, a difference that suggests that monocular depth cues were sparse. The paucity of monocular cues can be seen by the photograph of
Figure 2 which shows the appearance of the stimuli when separated by 1.75 cm, the threshold value for binocular viewing for the best animal of this study at an observation distance of 69 cm.