Visual processing in pure alexia: A case study

Abstract

Whether pure alexia is a selective disorder that affects reading only, or if it reflects a more general visual disturbance, is highly debated. We have investigated the selectivity of visual deficits in a pure alexic patient (NN) using a combination of psychophysical measures, mathematical modelling and more standard experimental paradigms. NN's naming and categorization of line drawings were normal with regards to both errors and reaction times (RTs). Psychophysical experiments revealed that NN's recognition of single letters at fixation was clearly impaired, and recognition of single digits was also affected. His visual apprehension span was markedly reduced for letters and digits. His reduced visual processing capacity was also evident when reporting letters from words. In an object decision task with fragmented pictures, NN's performance was abnormal. Thus, even in a pure alexic patient with intact recognition of line drawings, we find evidence of a general visual deficit not selective to letters or words. This finding is important because it raises the possibility that other pure alexics might have similar non-selective impairments when tested thoroughly. We argue that the general visual deficit in NN can be accounted for in terms of inefficient build-up of sensory representations, and that this low level deficit can explain the pattern of spared and impaired abilities in this patient.

Introduction

Pure alexia is an acquired disorder of reading characterised by slow and effortful reading of words and text. Patients with pure alexia usually show a linear relationship between the number of letters in a word and the time taken to read it, an effect known as the word length effect (WLE). Other language functions – including writing – are unaffected. Pure alexia can be distinguished from global alexia, where patients are completely unable to identify even single letters (Binder and Mohr, 1992, Leff et al., 2001). Also, although the terms pure alexia and letter-by-letter (LBL) reading are often used interchangeably, they may refer to different entities. Pure alexia or alexia without agraphia refers to an acquired disorder of reading that leaves writing and other language functions intact. WLEs or LBL reading may be observed in patients suffering from other disorders (e.g., Price and Humphreys, 1992 for a discussion, see also Cumming et al., 2006). We use the term pure alexia to refer to a reading disorder in the absence of aphasia and agraphia, and this paper is concerned with theories of pure alexia, and not (necessarily) LBL reading.

Theories of pure alexia are usually divided into (i) alphabetical accounts, attributing the reading deficit to damage in a specialized system for word or letter processing, or the disconnection of this system from visual input (e.g., Cohen et al., 2004), and (ii) visual accounts, suggesting that a deficit that affects visual processing in general is at the core of the disorder (e.g., Behrmann et al., 1998a). As noted by Cumming et al. (2006; p. 1132) regarding these competing accounts, “the jury is still out because as yet relatively few studies of pure alexia have included adequate assessment of non-reading visual tasks”.

A classical view of pure alexia within cognitive neuropsychology is that it results from damage to a word form system, that “parses (multiply and in parallel) letter strings into ordered familiar units and characterizes these units visually” (Warrington and Shallice, 1980, p. 109). A more recent version of this hypothesis proposes that pure alexia arises after damage to an area in the left fusiform gyrus, often referred to as the visual word form area (VWFA), which is thought to be responsible for extracting abstract letter identities (Cohen et al., 2004). Most of the evidence for the existence of the VWFA comes from functional imaging studies of normal subjects, but so far there is little consensus regarding the existence of such an area, or which cognitive operations it may perform (e.g., Price and Devlin, 2003, Price and Devlin, 2004, Cohen and Dehaene, 2004, Starrfelt and Gerlach, 2007). There are some patient studies specifically addressing the selectivity of deficits after focal lesions in this particular brain region (Cohen et al., 2003, Hillis et al., 2005, Gaillard et al., 2006), but their results are inconsistent. The anatomical side of this question is beyond the scope of this paper, as our patient's lesion extends beyond the putative VWFA, but the cognitive issue of the selectivity of pure alexia is addressed by comparing a pure alexic patient's performance with letters, words, and digits, as well as objects.

Another main hypothesis suggests that pure alexia is the result of a deficit in processing many visual items in parallel (simultanagnosia) (Kinsbourne and Warrington, 1962, Farah, 1990). Indeed, pure alexia may even be referred to as ventral simultanagnosia (Duncan et al., 2003, Farah, 2004). Duncan et al. (2003) addressed the simultanagnosia hypothesis of pure alexia rather directly by using psychophysical measures and mathematical modelling – methods also employed in the present study. They found that their pure alexic patient did not have a severe problem with perception of multiple items as such, but showed decreased speed of processing even for single stimuli. Thus, a primary deficit in simultaneous perception did not seem to accurately describe the patient's deficit. However, Duncan et al. (2003) only used letters as stimuli in their experiments. Given that their patient was alexic the results leave open the question of whether the reported pattern of deficits characterizes the patient's visual perception in general. We try to overcome this limitation by measuring our patient's processing speed and visual span of apprehension with two kinds of stimuli – letters and digits. Although letters and digits are similar symbols, number reading can be selectively spared in alexia with agraphia (Anderson et al., 1990, Starrfelt, 2007). Also, it is sometimes assumed that number reading can be spared in pure alexia, as reading of multidigit numbers in free vision has been reported to be preserved in some of these patients (Warrington and Shallice, 1980, Leff et al., 2001). Thus, as a first sensitive test of the selectivity if NN's deficits, we compare his performance with letters and digits in displays of single and multiple stimuli. Following a suggestion by Duncan et al. (2003), we also aim to test how the processing of letters within words may be affected by capacity limitations.

The third main account of pure alexia suggests that a general visual deficit is at the core of this disorder. Behrmann et al. (1998a) have shown that pure alexic patients' object recognition abilities may depend on visual complexity, and that pure alexic patients show perceptual difficulties “under impoverished perceptual conditions where there is less support from organisational cues” (Sekuler and Behrmann, 1996, p. 968). They suggest that reading is one such impoverished condition, and therefore seems disproportionately affected. We address this question in the present study, by investigating NN's picture recognition abilities both under normal and “impoverished” perceptual conditions.