Abstract
We have previously documented two patients (FOL and CLA) with posterior cortical atrophy who achieved accurate and rapid reading despite deficits in ten measures of visual processing, with two notable exceptions: (1) a measure of visual acuity, (2) a measure of visual crowding. Subsequent longitudinal investigation of these patients was carried out, involving annual tests of early visual, visuoperceptual and visuospatial processing and assessment of reading ability. Follow-up assessments identified the evolution of a particular early visual processing deficit, excessive visual crowding; this deficit has been previously implicated in forms of dyslexia. Consistent with the link between crowding and reading dysfunction, follow-up assessments also revealed deterioration in both patients' reading ability. The current findings demonstrate a neurodegenerative approach towards understanding the relationship between visual crowding and the reading system, and suggest possible mechanisms for how excessive crowding may disrupt word recognition.
Highlights
Visual crowding describes the inhibition of the identification of a target stimulus by the presence of flanking stimuli
In healthy individuals, crowding effects tend to be largely restricted to peripheral vision
In individuals with posterior cortical atrophy (PCA), a neurodegenerative condition characterised by progressive visual impairment, prominent effects have been observed in central vision (Crutch and Warrington, 2007, 2009, Yong et al, 2014a)
Summary
Visual crowding describes the inhibition of the identification of a target stimulus by the presence of flanking stimuli. CLA completed a battery of tests including a background neuropsychological assessment of memory, language, spelling and arithmetic and an assessment of early visual, visuoperceptual and visuospatial processing (see Sections 2.2.1 and 3.1: Visual assessment). CLA was due for a follow-up assessment at 31 months after her initial visit but was no longer suitable for testing due to a sharp deterioration in her condition, she did, complete the reading and crowding assessments at 27 months after her initial visit (see Section 2.2: Experimental procedures). CLA performed well on concrete synonym and spelling tasks for visits where background neuropsychological tests were administered (see Table 1). Visual acuity (CORVISTe): snellen [Visual Angle equivalent at 30 cm viewing distance] Figure-ground (VOSPf) Shape discriminationg
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