Abstract
The CooperVision Dicon AP 2000 does not project the stimulus onto an existing background, but produces the test stimuli with light emitting diodes (LEDs) recessed in a dark cavity in the perimetric bowl. Nearly everywhere in the visual field, the visibility threshold with the Dicon instrument seems equivalent to that obtained with projection perimeters, but in the most sensitive retinal areas we found the threshold stimulus (Is) to be sometimes dimmer than the "background" (Ib), which surrounds the stimulus, making a negative differential threshold (delta L = Is - Ib). In other words, where a dark cavity is imaged on a very sensitive portion of the retina, the visual mechanism may perceive the switching on of a stimulus of a lower intensity (Is) than the "background" light (Ib) that floods only the surrounding retina. In retinal regions where this happens, we found the variability of retesting of threshold to be much greater than at locations with threshold stimuli that are more intense than the "background". When we modified the instrument by placing a diffuser over the LED to provide a preexistent "background" at the test location equivalent to that of the surround, the greater variability disappeared, and the threshold results resembled those of projection perimetry even at the most sensitive retinal areas. Variability was also reduced with shortened stimulus durations.