Pure Alexic Patients Research Articles

Visual complexity in letter-by-letter reading: Pure alexia is not pure

Standard accounts of pure alexia have favoured the view that this acquired disorder of reading arises from damage to a left posterior occipital cortex mechanism dedicated to the processing of alphanumeric symbols. We challenge these accounts in two experiments and demonstrate that patients with this reading deficit are also impaired at object identification. In the first experiment, we show that a single subject, EL, who shows all the hallmark features of pure alexia, is impaired at picture identification across a large set of stimuli. As the visual complexity of pictures increases, so ELs reaction time to identify the stimuli increases disproportionately relative to the control subjects. In the second experiment, we confirm these findings with a larger group of five pure alexic patients using a selected subset of high- and low-visual complexity pictures. These findings suggest that the deficit giving rise to pure alexia is not restricted to orthographic symbols per se but, rather, is a consequence of damage to a more general-purpose visual processing mechanism.

PURE ALEXIA AND THE VIEWING POSITION EFFECT IN PRINTED WORDS

In the present article, we investigated the reading ability of CP, a pure alexic patient, using an experimental paradigm that is known to elicit the viewing position effect in norm al readers. The viewing position effect consists of a systematic variation of word recognition performance as a function of fixation location w ithin a word: Word recognition is best when the eyes fixate slightly left from the word centre and decreases when the eyes deviate from this optimal viewing position. A mathematical model (Nazir, O'Regan, & Jacobs, 1991), which provides a good description and quantification of the prototypical shape of the viewing position effect, served to interpret CP's reading performance. The results show ed that, like normal readers, CP was able to process all letters of a w ord in one fixation. However, in contrast to normal readers, reading performance was optimal when CP w as fixating the right half of the word. This somewhat abnormal pattern of performance was due to (1) poor perceptual processing in the right visual field, and (2) poor processing of letters situated towards the end of the word, independent of visual field presentation. A similar pattern of perform ance w as obtained with normal readers under experimental conditions in which lexical know ledge was of restricted use. We suggest that CP's reading impairment stems from a dysfunction in the coupling between incoming visual information and stored lexical information. This dysfunction is thought to uncover a prelexical level of word processing, where letter information is weighted differently as a function of letter position in a word-centred space.

Chinese Pure alexia

Two Chinese pure alexic patients are reported. C.Y.T. had a circumscribed lesion in the left occipital lobe, the same location as the classic cases of pure alexia in other languages. Y.Y.X. was diagnosed with Balint's syndrome characterized by bilateral damage in the parietal and occipital lobes. The Chinese script is logographic and non-alphabetic. Nevertheless, the neuroanatomy of early processing in reading Chinese appears to be the same for readers of alphabetical and non-alphabetical scripts. Both patients displayed a ‘radical-by-radical’ reading strategy which is analogous (and possibly functionally equivalent) to ‘letter-by-letter’ reading in alphabetic patients. There was an association between constructional apraxia and a deficit in processing the visual form of Chinese characters for Y.Y.X. However, C.Y.T. had intact visuospatial function and displayed no impairment to the processing of character form. The implications of the data from pure alexic patients to our understanding of oral reading in Chinese are discussed.

Perceptual Cues in Pure Alexia

This study provides evidence that pure alexia, or letter-by-letter reading, may be attributed to a general perceptual de® cit that extends beyond an orthographic disorder. The perceptual problem may be unmasked when appropriate perceptual cues are not available to aid in the derivation of an integrated structural description. Four pure alexic patients and eight nonbrain-damaged controls participated in this study. In the ® rst two experiments, subjects’ reading abilities were assessed on a naming latency and a lexical decision task. Experiment 3 replicated Farah and Wallace’s (1991) results that the pure alexia de® cit was not speci® c to orthography. Experiments 4 and 5 further explored the nature of the perceptual disorder using nonorthographic stimuli. In Experiment 4, patient performance on a target detection task was unaŒ ected by the number of parts comprising the object but was impaired when the perceptual cue of good continuation was absent. Patient performance also declined when the perceptual cue of symmetry was not available to aid in the integration of occluded object parts in Experiment 5. Overall, the results imply that pure alexia is most likely to arise from a more general, nonorthographic de® cit, and that the nature of the disorder is revealed when the perceptual context lacks strong perceptual cues.

A connectionist model of alexia: Covert recognition and case mixing effects

A connectionist model was developed to simulate the production of phonological, semantic and orthographic lexical representations from an orthographic input code. When lesioned, the network displayed many of the characteristics of the neurological disorder, pure alexia. These included: greater accuracy in the recognition of letters compared to words, and (in some cases) spared semantic categorization and lexical decision ability relative to naming. Performance was superior for high relative to low frequency words and (for some lesions) for ‘high imageability’ words with more semantic referents. Similar to alexic patients, errors in the model tended to be visually rather than semantically related to target words and disrupted input had a greater effect on naming than on lexical decision performance. There were also tendencies apparent in the model which have not so far been reported for alexic patients but are found in other neurological patients, including category specificity effects and superior performance on superordinate relative to subordinate semantic categorization.Noise was added to the input units of the unlesioned and lesioned model in an attempt to simulate case mixing effects on both normal and alexic reading. The unlesioned model demonstrated similar effects to normal readers with mixed case stimuli (Besner & McCann, 1987; Mayall & Humphreys, 1996). Further, effects on the lesioned model were comparable to those found with alexic patients (Bub & Arguin, 1995; Mayall & Humphreys, submitted).The similarity of the performance characteristics of the model to those of pure alexic patients suggests that the architectures of the modelled and the human reading system encompass common properties, and that covert recognition in pure alexia can be attributed to the architecture of the word processing system.

Pure Alexia: Attempted Rehabilitation and Its Implications for Interpretation of the Deficit

On the basis of data indicating the failure to encode letters as abstract orthographic identities in a pure alexic patient (D.M.) coupled with hypotheses about the effect of such a failure on word reading, an attempt at changing the nature of letter processing in D.M. was conducted. The training procedures failed to produce any fundamental change in the operations used by D.M. to encode isolated letters or words. However, rapid and massive benefits occurred in the overall speed of reading as a result of the training program. These appear to result from an increased rate of letter identification and the faster integration of individual letters into letter combinations. The observations gathered throughout this rehabilitation attempt provided evidence which constrains the range of possible explanations for the characteristic features of pure alexia. It is proposed that the letter-by-letter reading procedure which is the hallmark of the disorder may follow from an incapacity to directly encode visual letters as abstract orthographic types.

Rationale and efficacy of a tactile—kinaesthetic treatment for alexia

Abstract Patients with pure alexia typically read words by first naming each letter of the word, either aloud or silently. These patients do not show signs of phonological alexia (poor pseudoword and functor reading). Patient T.L., a pure alexic patient with poor letter-naming abilities, had marked difficulty reading functors and pseudowords, but not spelling them or identifying them when they were spelled to him, a pattern which has been called ‘modality-specific phonological alexia’. We used a multiple-baseline design to evaluate a tactile-kinaesthetic treatment programme designed to improve T.L.'s letter-naming, thus enabling him to read in letter-by-letter fashion. Results showed that the treatment was efficacious, with a 51% improvement achieved in trained word reading and a 40% increase in untrained word reading. Furthermore, the part-of-speech effect was no longer significant following treatment, a result that our hypothesis of T.L.'s underlying impairment predicted.

Single-character processing in a case of pure alexia

The processing of single characters in a pure alexic patient was studied in an attempt to identify the impairment responsible for his reading disorder. Observations from Experiments 1 to 4 suggested a deficit of identification of alphanumeric stimuli without any impairment affecting the elaboration of a structural description of visual stimulation. Experiment 5 indicated that the identification disorder results from a defect in the selective processes—activation and/or inhibition— that must come into play for achieving an appropriate match between a structural description of the stimulation and representations of the identities of known stimuli. The possible implications of this deficit in single-character identification for word reading are discussed.

Multiple oral rereading technique in rehabilitation of pure alexia

Abstract Three pure alexic patients were given reading practice with the multiple oral re-reading (MOR) technique (Moyer 1979). All patients read single words relatively fast, but differed from each other in the reading speed of texts. In addition, two of the patients (HT and TT) had no significant memory or visuospatial deficit whereas one patient (PA) exhibited severe memory and visuospatial problems. The results showed in HT and TT a notable increase in reading speed of texts after therapy when compared to baseline measures. In relationship to text reading, single word reading became significantly faster only in patient HT. Patient PA did not benefit from MOR. The difference in the recovery of word reading versus text reading suggests that MOR does not primarily affect the functioning of the damaged word-form system. Instead, the increase in reading speed may be attributed to top-down processes, which facilitate the recognition of words in a sentence frame. Furthermore, it is suggested that this kind o...

Pure Alexia as a Visual Impairment: A Reconsideration

Abstract We contrast two types of explanation for the syndrome of pure alexia: the visual impairment hypothesis, according to which the reading difficulty is just the most obvious manifestation of an impairment in visual perception; and the word form hypothesis, according to which the reading difficulty results from damage to reading-specific mechanisms. We argue that the word form hypothesis cannot account for certain findings in the literature on pure alexia, and that the visual impairment hypothesis is consistent with the available data. In addition, we present two new studies in support of the visual impairment hypothesis. In the first study, a pure alexic patient was found to be impaired on tests that assess the specific type of visual impairment hypothesised to underlie pure alexia. He was not impaired on the less specific tests that had been used previously to argue that perception is intact in pure alexia. In the second study, the additive factors method was used to identify the locus of impairment during reading in the same pure alexic patient. An interaction was found between the visual quality of the words and the length of the words. This implies that the abnormal word length effect in pure alexia (i.e. the letter-by-letter reading strategy) originates in the same processing stage as the visual quality effect, that is, in visual perception. Finally, we attempt to generalise about which kinds of processes make use of dedicated brain hardware and which do not, and suggest that the visual impairment hypothesis of pure alexia is more consistent with this generalisation than is the word form hypothesis.

Language and Reading in the Right Hemisphere: Highways or Byways of the Brain?

The literature pertaining to the representation of language in the right hemisphere of hemispherectomy and callosotomy patients is reviewed to ascertain whether it provides an empirical basis for the assertions that (1) the right hemisphere participates in the recovery of language in aphasia, (2) the right hemisphere mediates the reading errors of deep dyslexic and pure alexic patients, and (3) the right hemisphere plays a fixed role in normal reading processes. At present, there appears to be some support for the first assertion, limited support for the second (if individual variation in representation can be accepted), and disconfirming evidence for the third in the data from these populations.