Intact reading in patients with profound early visual dysfunction

Abstract

Despite substantial neuroscientific evidence for a region of visual cortex dedicated to the processing of written words, many studies continue to reject explanations of letter-by-letter (LBL) reading in terms of impaired word form representations or parallel letter processing in favour of more general deficits of visual function. In the current paper, we demonstrate that whilst LBL reading is often associated with general visual deficits, these deficits are not necessarily sufficient to cause reading impairment and have led to accounts of LBL reading which are based largely on evidence of association rather than causation. We describe two patients with posterior cortical atrophy (PCA) who exhibit remarkably preserved whole word and letter reading despite profound visual dysfunction. Relative to controls, both patients demonstrated impaired performance on tests of early visual, visuoperceptual and visuospatial processing; visual acuity was the only skill preserved in both individuals. By contrast, both patients were able to read aloud words with perfect to near-perfect accuracy. Reading performance was also rapid with no overall significant difference in response latencies relative to age- and education-matched controls. Furthermore, the patients violated a key prediction of general visual accounts of LBL reading - that pre-lexical impairments should result in prominent word length effects; in the two reported patients, evidence for abnormal word length effects was equivocal or absent, and certainly an order of magnitude different to that reported for LBL readers. We argue that general visual accounts cannot explain the pattern of reading data reported, and attribute the preserved reading performance to preserved direct access to intact word form representations and/or parallel letter processing mechanisms. The current data emphasise the need for much clearer evidence of causality when attempting to draw connections between specific aspects of visual processing and different types of acquired peripheral dyslexia.

Keywords: Alzheimer's disease (AD); Letter-by-letter reading; Posterior cortical atrophy (PCA); Pure alexia; Word form dyslexia.

Fig. 1

Neuroanatomical features in FOL and CLA. Representative brain MRI sections for each patient show the distribution of atrophy in each case. Coronal sections (upper panels in each case) are in the plane of the mid-temporal lobe (mt), temporo-parietal junction (tpj) and posterior parietal lobe (pp), respectively; the left hemisphere is shown on the right for all coronal sections. Sagittal sections (lower panels in each case) are through the left (Lh) and right (Rh) cerebral hemispheres.

Fig. 2

Mean reading latencies for words of different length across all corpora for (A) patient FOL and her matched controls, and (B) patient CLA and her matched controls, with estimated upper and lower control confidence intervals.

Fig. 3

Mean response latencies for target letters under different flanking conditions (letter, shape and number) and spatial conditions (crowded and spaced) for (A) patient FOL and her matched controls, and (B) patient CLA and her matched controls.

Fig. 4

Mean reading latencies for words of different length compared to 5 example LBL readers reported by Mycroft et al. (2009).