Desjardins D, Anderson D R
Bascom Palmer Eye Institute, Department of Ophthalmology, University of Miami School of Medicine, Florida 33101.
Invest Ophthalmol Vis Sci. 1988 Jun;29(6):915-21.
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.
酷柏视Dicon AP 2000不是将刺激投射到现有的背景上，而是通过嵌入视野计碗状暗腔内的发光二极管（LED）产生测试刺激。在视野的几乎所有区域，使用Dicon仪器测得的可见度阈值似乎与使用投射式视野计测得的相当，但在视网膜最敏感的区域，我们发现阈值刺激（Is）有时比围绕刺激的“背景”（Ib）更暗，从而产生负的差值阈值（δL = Is - Ib）。换句话说，当暗腔成像在视网膜非常敏感的部分时，视觉机制可能会将强度低于仅淹没周围视网膜的“背景”光（Ib）的刺激（Is）的开启视为刺激。在发生这种情况的视网膜区域，我们发现阈值重新测试的变异性远大于阈值刺激比“背景”更强的位置。当我们通过在LED上方放置一个漫射器来修改仪器，以在测试位置提供与周围环境相当的预先存在的“背景”时，更大的变异性消失了，即使在视网膜最敏感的区域，阈值结果也类似于投射式视野检查的结果。缩短刺激持续时间也降低了变异性。
