Dementia Research Centre, Department of Neurodegeneration, Institute of Neurology, University College London, London, UK.
Cogn Neuropsychol. 2009 Jul;26(5):471-98. doi: 10.1080/02643290903465819.
Visual crowding is a form of masking in which target identification is hindered by excessive feature integration from other stimuli in the vicinity. It has previously been suggested that excessive visual crowding constitutes one specific form of early-visual-processing deficit, which may be observed in individuals with posterior cortical atrophy (PCA). This study investigated whether excessive visual crowding plays a significant role in the acquired dyslexia of two PCA patients, whose reading was characterized by visual paralexias. The patients were administered a series of letter, flanked letter, and word recognition tasks, and the effects of letter spacing and letter confusability upon response accuracy and latency were measured. In both patients, the results showed (a) evidence of excessive visual crowding, (b) a significant interaction between letter spacing and confusability on flanked letter identification tasks, and (c) effects of letter confusability affecting flanked but not unflanked letter identification. However, only mild improvements in reading accuracy were achieved in the experimental manipulations of interletter spacing within words because these manipulations had a dual effect: Increasing spacing improved individual letter identification but damaged whole-word form and/or parallel letter processing. We consider the implications of these results for the characterization of dyslexia in PCA, the design of reading rehabilitation strategies, and the relationship between visual crowding and letter confusability. In particular, we argue that the reading deficits observed in our patients cannot be accounted for solely in terms of a very low signal-to-noise ratio for letter identification, and that an additional crowding deficit is implicated in which excessive integration of fundamental letter features leads to the formation of incorrect letter percepts.
视觉拥挤是一种掩蔽形式，其中目标识别会受到附近其他刺激的过多特征整合的阻碍。之前有人提出，过度的视觉拥挤构成了早期视觉处理缺陷的一种特定形式，这种缺陷可能在皮质后萎缩（PCA）患者中观察到。本研究调查了过度的视觉拥挤是否在两位 PCA 患者的获得性阅读障碍中起重要作用，这两位患者的阅读表现为视觉混淆。患者接受了一系列字母、侧翼字母和单词识别任务，测量了字母间距和字母混淆度对反应准确性和潜伏期的影响。在两位患者中，结果均显示：（a）存在过度的视觉拥挤证据；（b）在侧翼字母识别任务中，字母间距和混淆度之间存在显著的交互作用；（c）字母混淆度影响侧翼但不影响非侧翼字母识别。然而，在单词内字母间距的实验操作中，阅读准确性仅得到了轻微的提高，因为这些操作具有双重作用：增加间距可以提高单个字母的识别，但会破坏整个单词的形状和/或平行字母处理。我们考虑了这些结果对 PCA 中阅读障碍的特征描述、阅读康复策略的设计以及视觉拥挤和字母混淆度之间关系的影响。特别是，我们认为我们患者观察到的阅读障碍不能仅仅用字母识别的信噪比非常低来解释，还存在额外的拥挤缺陷，其中过多的基本字母特征的整合导致形成不正确的字母感知。
