Crewther David P, Kiely Patricia M, Crewther Sheila G
Brain Sciences Institute, Swinburne University of Technology, Melbourne, Australia.
Clin Exp Optom. 2006 Nov;89(6):368-73. doi: 10.1111/j.1444-0938.2006.00070.x.
In this study, monocular thresholds and binocular summation for abrupt onset/offset versus gradually revealed phantom letter E (illusory contours) stimuli are compared to determine the suitability of these stimuli for assessment of the integrity of two of the major retinal streams: the magnocellular and the parvocellular pathways. Such tests are important in progressive retinal disease where disease severity may differ between the classes of retinal ganglion cells and between the two eyes. Abrupt onset phantom contours have long been considered to activate the magnocellular visual pathway and we propose that gradually revealed high contrast ramped onset/offset stimuli are more likely to promote the more sustained processing of the parvocellular stream.
Contrast discrimination thresholds for monocular and binocular viewing were compared in a counter-balanced order in 70 young normal subjects, using tests of contrast threshold for a flicker-defined letter E produced by alternation of light and dark dots. Three onset/offset conditions were used - abrupt onset that was maintained for 34 milliseconds (four frames of 8.5 milliseconds) then discontinued, ramped onset over 34 milliseconds (four frames) with offset over 34 milliseconds and ramped onset over 85 milliseconds (10 frames) with offset over 85 milliseconds.
Contrast thresholds for identification of the orientation of the E, when presented with four frames ramped onset and offset when compared to the four frames abrupt onset/offset were three times higher, irrespective of monocular or binocular viewing conditions. Threshold contrasts were seven times higher when the 10 frames ramped onset/offset stimuli were compared to abrupt four frames onset/offset. Binocular contrast thresholds were reduced by approximately 40 per cent compared to monocular thresholds for all conditions. The binocular increase in contrast sensitivity is approximately equal for abrupt transiently presented stimuli and for gradually presented more sustained stimuli.
The results indicate that the same mechanisms of monocular processing and binocular summation are used for identification of a flickering contrast-defined phantom contour under presentation conditions, which are characteristic of the temporal and contrast preferences of the primate magnocellular and parvocellular visual pathways. This suggests that the phantom contour E test may be useful for clinical differentiation of the integrity of the M and P retinal ganglion-derived visual pathways, regardless of whether it is applied monocularly or binocularly.
在本研究中，对突然出现/消失与逐渐显示的幻视字母E（虚幻轮廓）刺激的单眼阈值和双眼总和进行比较，以确定这些刺激对于评估两条主要视网膜通路（大细胞通路和小细胞通路）完整性的适用性。此类测试在进行性视网膜疾病中很重要，因为视网膜疾病的严重程度在不同类别的视网膜神经节细胞之间以及双眼之间可能有所不同。长期以来，突然出现的幻视轮廓被认为会激活大细胞视觉通路，我们提出逐渐显示的高对比度渐变出现/消失刺激更有可能促进小细胞通路更持续的处理。
在70名年轻正常受试者中，采用由明暗点交替产生的闪烁定义字母E的对比度阈值测试，以平衡的顺序比较单眼和双眼观察的对比度辨别阈值。使用了三种出现/消失条件——突然出现并持续34毫秒（8.5毫秒的四帧）然后停止，在34毫秒（四帧）内渐变出现并在34毫秒内消失，以及在85毫秒（10帧）内渐变出现并在85毫秒内消失。
与四帧突然出现/消失相比，当以四帧渐变出现和消失呈现时，识别E方向的对比度阈值高出三倍，无论单眼或双眼观察条件如何。与突然的四帧出现/消失刺激相比，当10帧渐变出现/消失刺激时，阈值对比度高出七倍。在所有条件下，双眼对比度阈值比单眼阈值降低了约40%。对于突然短暂呈现的刺激和逐渐呈现的更持续的刺激，双眼对比度敏感性的增加大致相等。
结果表明，在呈现条件下，相同的单眼处理和双眼总和机制用于识别闪烁对比度定义的幻视轮廓，这些条件是灵长类大细胞和小细胞视觉通路的时间和对比度偏好的特征。这表明幻视轮廓E测试可能有助于临床区分M和P视网膜神经节衍生视觉通路的完整性，无论它是单眼还是双眼应用。
