The relation of vernier and depth discriminations to field brightness.

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Abstract
This study measured vernier and real depth thresholds under various brightnesses of the visual field. The test rods were 2 vertical steel rods placed one above the other with a 3 mm. separation between them, at a distance of 4622 mm. from the subjects'' eyes. The bottom rod was adjustable in the frontal plane in the vernier situation; in the median plane in the real depth situation. A constant stimulus method was used with each threshold based on 200 responses. The following 6 field brightnesses were used: 0.04, 0.07, 0.16, 0.90, 4.8, and 192 millilamberts. Comparison of the vernier and real depth thresholds indicates that depth discrimination is more influenced by brightness changes than is vernier discrimination. There is a regular curvilinear change in threshold values with increasing brightnesses in both vernier and real depth discriminations. It is postulated that at the lowest illuminations the essential factor in both vernier and depth discrimination is the retinal resolution of the single eye. At the higher illuminations differential factors may be assumed to be operating in the 2 types of discrimination.

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