Length perception in degeneratio pigmentosa retinae cases and normal subjects with limited visual field

To investigate quantitatively the visual disturbance in cases of degeneratio pigmentosa retinae, the effect of the visual field size on length perception was studied. In the experiment, 3 patients were asked to compare the length of two lines presented on the screen for 5 seconds under monocular and free viewing conditions. The difference threshold value for stimulus length was estimated by the descending method of limit. Three normal subjects participated in this experiment under two conditions; one was the free viewing condition mentioned above and the other was a limited visual field condition. In the latter case, subjects viewed through a pinhole on the Lo-Vac contact lens. With this contact lens, they had almost the same visual field size and visual acuity as the patients. In these cases, subjects could move the eye and head freely. The results suggested that accurate length comparison is possible only when the lines are perceived simultaneously.