Levi Dennis M, Song Shuang, Pelli Denis G
School of Optometry and Helen Wills Neuroscience Institute, UC Berkeley, Berkeley, CA 94720-2020, USA.
J Vis. 2007 Oct 26;7(2):21.1-17. doi: 10.1167/7.2.21.
We measure acuity, crowding, and reading in amblyopic observers to answer four questions. (1) Is reading with the amblyopic eye impaired because of larger required letter size (i.e., worse acuity) or larger required spacing (i.e., worse crowding)? The size or spacing required to read at top speed is called "critical". For each eye of seven amblyopic observers and the preferred eyes of two normal observers, we measure reading rate as a function of the center-to-center spacing of the letters in central and peripheral vision. From these results, we estimate the critical spacing for reading. We also measured traditional acuity for an isolated letter and the critical spacing for identifying a letter among other letters, which is the classic measure of crowding. For both normals and amblyopes, in both central and peripheral vision, we find that the critical spacing for reading equals the critical spacing for crowding. The identical critical spacings, and very different critical sizes, show that crowding, not acuity, limits reading. (2) Does amblyopia affect peripheral reading? No. We find that amblyopes read normally with their amblyopic eye except that abnormal crowding in the fovea prevents them from reading fine print. (3) Is the normal periphery a good model for the amblyopic fovea? No. Reading centrally, the amblyopic eye has an abnormally large critical spacing but reads all larger spacings at normal rates. This is unlike the normal periphery, in which both critical spacing and maximum reading rate are severely impaired relative to the normal fovea. (4) Can the uncrowded-span theory of reading rate explain amblyopic reading? Yes. The case of amblyopia shows that crowding limits reading solely by determining the uncrowded span: the number of characters that are not crowded. Characters are uncrowded if and only if their spacing is more than critical. The text spacing may be uniform, but the observer's critical spacing increases with distance from fixation, so the uncrowded span extends out to where the spacing is critical. Amblyopes have normal critical spacing in the periphery, so, when the uncrowded span extends into the periphery, it has normal extent, which predicts our finding that reading rate is normal too. This confirms the theory that reading rate is determined by the width of the uncrowded span, independent of the critical spacing within the span. The uncrowded-span model of normal reading fits the amblyopic results well, with a roughly fivefold increase in the critical spacing at fixation. Thus, the entire amblyopic reading deficit is accounted for by crowding.
我们对弱视观察者的视敏度、拥挤现象和阅读能力进行测量，以回答四个问题。（1）弱视眼阅读能力受损是因为所需字母尺寸更大（即视敏度更差）还是所需间距更大（即拥挤现象更严重）？以最快速度阅读所需的尺寸或间距称为“临界值”。对于7名弱视观察者的每只眼睛以及2名正常观察者的优势眼，我们测量阅读速度作为中央和周边视觉中字母中心间距的函数。根据这些结果，我们估计阅读的临界间距。我们还测量了孤立字母的传统视敏度以及在其他字母中识别一个字母的临界间距，这是拥挤现象的经典测量方法。对于正常人和弱视者，在中央和周边视觉中，我们发现阅读的临界间距等于拥挤现象的临界间距。相同的临界间距以及非常不同的临界尺寸表明，限制阅读的是拥挤现象，而非视敏度。（2）弱视会影响周边阅读吗？不会。我们发现，弱视者用弱视眼阅读时正常，只是中央凹处异常的拥挤现象使他们无法阅读小字。（3）正常周边视觉是弱视中央凹的良好模型吗？不是。在中央阅读时，弱视眼的临界间距异常大，但能以正常速度阅读所有更大间距的内容。这与正常周边视觉不同，在正常周边视觉中，相对于正常中央凹，临界间距和最大阅读速度都严重受损。（4）阅读速度的非拥挤跨度理论能解释弱视阅读吗？能。弱视的情况表明，拥挤现象仅通过确定非拥挤跨度来限制阅读：即未被拥挤的字符数量。当且仅当字符间距大于临界值时，字符才不拥挤。文本间距可能是均匀的，但观察者的临界间距会随着与注视点距离的增加而增大，所以非拥挤跨度会延伸到间距为临界值的地方。弱视者周边视觉的临界间距正常，所以当非拥挤跨度延伸到周边视觉时，其范围正常，这预测了我们关于阅读速度也正常的发现。这证实了阅读速度由非拥挤跨度宽度决定的理论，与跨度内的临界间距无关。正常阅读的非拥挤跨度模型很好地拟合了弱视的结果，注视点处的临界间距大约增加了五倍。因此，整个弱视阅读缺陷都可由拥挤现象来解释。
