Pure alexia and prosopagnosia traditionally have been seen as prime examples of dissociated, category-specific agnosias affecting reading and face recognition, respectively. More recent accounts have moved towards domain-independent explanations that postulate potential cross-links between different types of visual agnosia. According to one proposal, abnormal crowding, i.e. the impairment of recognition when features of adjacent objects are positioned too closely to each other, might provide a unified account for the perceptual deficits experienced by an agnosic patient. An alternative approach is based on the notion of complementary visual subsystems favouring the processing of abstract categories and specific exemplars, respectively. To test predictions of these two approaches with regard to pure alexia and prosopagnosia, we present previously unpublished data on digit recognition and visual crowding from two in the neuropsychological literature extensively studied patients, KD and MT (e.g., Campbell et al., 1986; Landis and Regard, 1988; Rentschler et al., 1994). Patient MT, diagnosed with pure alexia, showed pronounced abnormal foveal crowding, whereas KD, diagnosed with prosopagnosia, did not. These results form a distinct double dissociation with the performance of the two patients in other perceptual classification tasks involving Gabor micropatterns and textures, as well as Glass patterns, which revealed a significantly greater impairment in KD relative to MT. Based on an analysis of the specific task demands we argue that prosopagnosia and pure alexia may involve complementary deficits in instantiation and abstraction, respectively, during perceptual classification, beyond any category specificity. Such an explanation appears in line with previous distinctions between a predominantly left-hemispheric, abstract-category and a predominantly right-hemispheric, specific-exemplar subsystem underlying object recognition.

传统上，单纯的无氧和围绝症被视为分别影响阅读和面部识别的分离，特定类别的失智的主要例子。最近的研究已经转向独立于域的解释，这些解释假定了不同类型的视觉失认之间潜在的交叉联系。根据一项建议，异常拥挤，即当相邻对象的特征彼此过于靠近时的识别能力受损，可能为不可知病患者经历的感知缺陷提供统一的解释。另一种方法是基于互补视觉子系统的概念，该子系统分别支持抽象类别和特定示例的处理。为了测试关于这两种方法在纯净性贫血和围绝经症方面的预测，我们在神经心理学文献中广泛研究了KD和MT的患者中，从两个尚未发表的数字识别和视觉拥挤方面提供了尚未发表的数据（例如，Campbell等，1986； Landis和Regard，1988； Rentschler等，1994）。被诊断患有单纯性无氧血症的MT患者表现出明显的中央凹拥挤，而被诊断为患有绝经的KD则没有。这些结果与两名患者在其他涉及Gabor微图案和纹理以及玻璃图案的感知分类任务中的表现形成了明显的双重分离，这表明相对于MT，KD的损害更大。根据对特定任务需求的分析，我们认为，更年期和单纯的无氧血症可能分别在实例化和抽象上涉及互补缺陷，在感知分类中，没有任何类别的特殊性。这样的解释与先前的区分对象识别的主要是左半球，抽象类别和主要是右半球，特定示例子系统之间的区别是一致的。