Day Mhairi, Seidel Dirk, Gray Lyle S, Strang Niall C
Department of Vision Sciences, Glasgow Caledonian University, Cowcaddens Road, Glasgow, UK.
Vision Res. 2009 Jan;49(2):211-8. doi: 10.1016/j.visres.2008.10.010. Epub 2008 Nov 22.
The magnitude of accommodation microfluctuations increases in emmetropic subjects viewing low luminance targets or viewing a target through small artificial pupils. Larger microfluctuations reported in myopia may result from an abnormally large depth of focus (DoF). The effect of modulating the size of the DoF has not been investigated in myopic subjects and may help to explain the cause of the increased DoF. Accommodation microfluctuations were recorded under two experimental conditions. Firstly, 12 emmetropes (EMMs), and 24 myopes (MYOs) viewed a Maltese Cross target with luminance levels of 0.002, 0.2, 6 and 600cd/m(2) and in darkness, and second, 14 EMMs and 16 MYOs viewed a Maltese Cross target through pupil diameters of 0.5, 1, 2, 3, 4 and 5mm presented in Maxwellian view. The magnitude of the accommodation microfluctuations increased significantly with a target luminance of 0.002cd/m(2) (p<.03) and pinhole diameters of <2mm (p<.05). For all other luminance levels and pupil diameters the magnitude was constant. For both conditions, MYOs had significantly larger microfluctuations than EMMs (p<.01). Considerable inter-subject variability was observed in the degree to which the magnitude of the microfluctuations increased, for both the 0.002cd/m(2) luminance and 0.5mm pupils, however, this was not correlated with refractive error. The increase in the magnitude of the microfluctuations while viewing a low luminance target (0.002cd/m(2)) may be due to a shallower contrast gradient in the cortical image, with a consequent increase in DoF. The microfluctuations also increase when viewing through small pupils (<2mm), which increases the DoF without altering the contrast gradient. The larger microfluctuations found in the MYOs consolidates the theory that MYOs have a larger DoF than EMMs and therefore have a higher threshold for retinal image blur.
正视眼受试者在观察低亮度目标或通过小的人工瞳孔观察目标时，调节微波动的幅度会增加。近视患者中报告的较大微波动可能源于异常大的焦深（DoF）。调节焦深大小的影响在近视患者中尚未得到研究，这可能有助于解释焦深增加的原因。在两种实验条件下记录了调节微波动。首先，12名正视眼者（EMMs）和24名近视者（MYOs）在亮度水平分别为0.002、0.2、6和600cd/m²以及黑暗环境下观察一个马耳他十字目标；其次，14名EMMs和16名MYOs通过麦克斯韦观察法中呈现的直径为0.5、1、2、3、4和5mm的瞳孔观察一个马耳他十字目标。当目标亮度为0.002cd/m²（p<0.03）和针孔直径小于2mm（p<0.05）时，调节微波动的幅度显著增加。对于所有其他亮度水平和瞳孔直径，幅度保持恒定。在这两种情况下，MYOs的微波动都明显大于EMMs（p<0.01）。对于0.002cd/m²的亮度和0.5mm的瞳孔，观察到微波动幅度增加的程度在受试者之间存在相当大的差异，然而，这与屈光不正无关。观察低亮度目标（0.002cd/m²）时微波动幅度的增加可能是由于皮质图像中的对比度梯度变浅，从而导致焦深增加。通过小瞳孔（<2mm）观察时微波动也会增加，这在不改变对比度梯度的情况下增加了焦深。在MYOs中发现的较大微波动巩固了这样一种理论，即MYOs的焦深比EMMs大，因此视网膜图像模糊的阈值更高。
