Department of Ophthalmology, McGill University, Montreal, Quebec, Canada.
Hum Brain Mapp. 2010 Aug;31(8):1233-48. doi: 10.1002/hbm.20931.
Although there is general agreement that the fMRI cortical response is reduced in humans with amblyopia, the deficit is subtle and has little correlation with threshold-based psychophysics. From a purely contrast sensitivity perspective, one would expect fMRI responses to be selectively reduced for stimuli of low contrasts. However, to date, all fMRI stimuli used in studies of amblyopia have been of high contrast. Furthermore, if the deficit is selective for low contrasts, one would expect it to reflect a selective M-cell loss, because M-cells have much higher contrast gain than P-cells and make a larger contribution to the threshold detection of stimuli of low spatial and medium temporal frequencies. To test these two predictions, we compared % BOLD response between the eyes of normals and amblyopes for low- and high-contrast stimuli using a phase-encoded design. The results suggest that the fMRI deficit in amblyopia depends upon stimulus contrast and that it is greater at high contrasts. This is consistent with a selective P-cell contrast deficit at a precortical or early cortical site.
尽管人们普遍认为弱视患者的 fMRI 皮质反应降低，但这种缺陷很细微，与基于阈值的心理物理学相关性很小。从纯粹的对比敏感度的角度来看，人们会期望 fMRI 反应对于低对比度的刺激选择性降低。然而，迄今为止，在弱视研究中使用的所有 fMRI 刺激都是高对比度的。此外，如果缺陷是针对低对比度的，那么人们会期望它反映出选择性的 M 细胞丢失，因为 M 细胞的对比度增益比 P 细胞高得多，并且对低空间和中时间频率刺激的阈值检测有更大的贡献。为了验证这两个预测，我们使用相位编码设计比较了正常人和弱视患者的低对比度和高对比度刺激之间的 %BOLD 反应。结果表明，弱视的 fMRI 缺陷取决于刺激对比度，在高对比度下更大。这与在皮质前或早期皮质部位存在选择性 P 细胞对比缺陷一致。
