Implicit recognition in pure alexia: The Saffran effect—a tale of two systems or two procedures?

Can the right hemisphere read? A behavioral and disconnectome study on implicit reading in a patient with pure alexia

Patients with pure alexia have major difficulties in reading aloud. However, they often perform above chance level in reading tasks that do not require overt articulation of the target word - like lexical decision or semantic judgment - a phenomenon usually known as "implicit reading." There is no agreement in the literature on whether implicit reading should be attributed to relative sparing of some left hemisphere (LH) reading centers or rather to signs of compensatory endeavors by the right hemisphere (RH). We report the case of an 81-year-old patient (AA) with pure alexia due to a lesion involving the left occipital lobe and the temporal infero-mesial areas, as well as the posterior callosal pathways. Although AA's reading was severely impaired and proceeded letter by letter, she showed an above-chance-level performance for frequent concrete words in a tachistoscopic lexical decision task. A structural disconnectome analysis revealed that AA's lesion not only affected the left occipital cortex and the splenium: it also disconnected white-matter tracts meant to connect the visual word-form system to decision-related frontal areas within the LH. We suggest that the RH, rather than the LH, may be responsible for patient AA's implicit reading.

Fast word reading in pure alexia: "fast, yet serial"

Pure alexia is a severe impairment of word reading in which individuals process letters serially with a pronounced length effect. Yet, there is considerable variation in the performance of alexic readers with generally very slow, but also occasionally fast responses, an observation addressed rarely in previous reports. It has been suggested that "fast" responses in pure alexia reflect residual parallel letter processing or that they may even be subserved by an independent reading system. Four experiments assessed fast and slow reading in a participant (DN) with pure alexia. Two behavioral experiments investigated frequency, neighborhood, and length effects in forced fast reading. Two further experiments measured eye movements when DN was forced to read quickly, or could respond faster because words were easier to process. Taken together, there was little support for the proposal that "qualitatively different" mechanisms or reading strategies underlie both types of responses in DN. Instead, fast responses are argued to be generated by the same serial-reading strategy.

Recovery in a letter-by-letter reader: more efficiency at the expense of normal reading strategy

Although changes in reading performance of recovering letter-by-letter readers have been described in some detail, no prior research has provided an in-depth analysis of the underlying adaptive word processing strategies. Our work examined the reading performance of a letter-by-letter reader, FH, over a period of 15 months, using eye movement methodology to delineate the recovery process at two different time points (T1, T2). A central question is whether recovery is characterized either by moving back towards normal word processing or by refinement and possibly automatization of an existing pathological strategy that was developed in response to the impairment. More specifically, we hypothesized that letter-by-letter reading may be executed with at least four different strategies and our work sought to distinguish between these alternatives. During recovery significant improvements in reading performance were achieved. A shift of fixation positions from the far left to the extreme right of target words was combined with many small and very few longer regressive saccades. Apparently, 'letter-by-letter reading' took the form of local clustering, most likely corresponding to the formation of sublexical units of analysis. This pattern was more pronounced at T2, suggesting that improvements in reading efficiency may come at the expense of making it harder to eventually return to normal reading.

Pure alexia and covert reading: Evidence from Stroop tasks

Patients with pure alexia (also referred to as letter-by-letter readers) show a marked word-length effect when naming visually presented words, evidenced by a monotonic increase in response time (or decrease in accuracy) as a function of the number of letters in the string. Interestingly, despite the difficulty in overtly reporting the identity of some words, many patients exhibit fast and above-chance access to lexical and/or semantic information for the same words. To explore the extent of this covert reading, we examined the degree of interference afforded by the inconsistent (word identity and colour label do not match) versus neutral condition in a Stroop task in a pure alexic patient, EL. EL shows evidence of covert reading on a semantic categorisation task and a lexical decision task. She also demonstrates covert reading by exhibiting Stroop interference of the same magnitude as a matched control subject, when naming the colour of the ink in which a word is printed. When the word shares some but not all letters with the colour name (BLOW instead of BLUE), neither subject shows interference. In contrast with the control subject, EL does not show Stroop interference when various orthographic changes (degraded visual input, cursive font) or phonological or semantic changes are made to the word. These findings indicate that although some implicit processing of words may be possible, this processing is rather rudimentary. Not surprising, this implicit activation may be insufficient to support overt word identification. We explain these findings in the context of a single, integrated account of pure alexia.

Preserved visual language identification despite severe alexia

Patients with letter-by-letter alexia may have residual access to lexical or semantic representations of words despite severely impaired overt word recognition (reading). Here, we report a multilingual patient with severe letter-by-letter alexia who rapidly identified the language of written words and sentences in French and English while he had great difficulty in reading them, judging their lexical status or extracting semantic information. Lexical decision was strongly influenced by the orthographic structure of stimuli: whereas he easily determined the lexical status of illegal nonwords (e.g., 'rsdo'), he had random performance with legal pseudowords (e.g., 'binus'). When asked to determine the language of meaningless letter trigrams with high frequency in the English or French orthography (e.g., 'oth' or 'iqu') his performance was significantly above chance. In contrast, similarly to healthy participants his language decision was at chance with low-frequency trigrams. These findings suggest that written language identification relies on sublexical processing of orthographic rules specific to each language.

When does less yield more? The impact of severity upon implicit recognition in pure alexia

Pure alexia (PA) is characterised by strong effects of word length on reading times and is sometimes accompanied by an overt letter-by-letter (LBL) reading strategy. Past studies have reported "implicit recognition" in some individual PA patients. This is a striking finding because such patients are able to perform semantic classification and lexical decision at above chance levels even when the exposure duration is short enough to prevent explicit identification. In an attempt to determine the prevalence of this "implicit recognition" effect, we assessed semantic categorisation and lexical decision performance using limited exposure durations in 10 PA cases. The majority of the patients showed above chance accuracy in semantic categorisation and lexical decision. Performance on the lexical decision test was influenced by frequency and imageability. In addition, we found that the extent to which patients showed evidence of "implicit recognition" in both tasks was inversely related to the severity of their reading disorder. This result is consistent with hypotheses which suggest that this effect does not constitute an implicit form of unique word identification but is a reflection of the degree of partial activation within the word recognition system. These results also go some way towards explaining the individual variation in the presence of this effect observed across previous case-study investigations in the literature.

Do deep dyslexia, dysphasia and dysgraphia share a common phonological impairment?

This study directly compared four patients who, to varying degrees, showed the characteristics of deep dyslexia, dysphasia and/or dysgraphia--i.e., they made semantic errors in oral reading, repetition and/or spelling to dictation. The "primary systems" hypothesis proposes that these different conditions result from severe impairment to a common phonological system, rather than damage to task-specific mechanisms (i.e. grapheme-phoneme conversion). By this view, deep dyslexic/dysphasic patients should show overlapping deficits but previous studies have not directly compared them. All four patients in the current study showed poor phonological production across different tasks, including repetition, reading aloud and spoken picture naming, in line with the primary systems hypothesis. They also showed severe deficits in tasks that required the manipulation of phonology, such as phoneme addition and deletion. Some of the characteristics of the deep syndromes - namely lexicality and imageability effects - were typically observed in all of the tasks, regardless of whether semantic errors occurred or not, suggesting that the patients' phonological deficits impacted on repetition, reading aloud and spelling to dictation in similar ways. Differences between the syndromes were accounted for by variation in other primary systems--particularly auditory processing. Deep dysphasic symptoms occurred when the impact of phonological input on spoken output was disrupted or reduced, either as a result of auditory/phonological impairment, or for patients with good phonological input analysis, when repetition was delayed. 'Deep' disorders of reading aloud, repetition and spelling can therefore be explained in terms of damage to interacting primary systems such as phonology, semantics and vision, with phonology playing a critical role.

Top-down and bottom-up effects in pure alexia: evidence from eye movements

The eye movements of a patient with pure alexia, GJ, were recorded as he read sentences in order to explore the roles of top-down and bottom-up information during letter-by-letter reading. Specifically, the effects of word frequency and word predictability were examined. Additional analyses examined the interaction of these effects with the lower level influences of word length and letter confusability. The results indicate that GJ is sensitive to all four of these variables in sentence reading. These findings support an interactive account of reading where letter-by-letter readers use both bottom-up and top-down information to decode words. Due to the disrupted bottom-up processes caused by damage to the Visual Word Form Area or the input connections to it, pure alexic patients rely more heavily on intact top-down information in reading.

Using errorless learning to treat letter-by-letter reading: Contrasting word versus letter-based therapy

Some pure alexic readers have been shown to activate lexical and semantic knowledge under brief presentation conditions. This ability is not seen when letter-by-letter reading accuracy is high or the reading impairment is very severe. It is also unlikely to occur under normal untimed presentation because the pure alexic will make deliberate use of their letter-by-letter strategy. This paper presents data from a moderately severe letter-by-letter reader, FD, who had visual processing problems affecting reading. He also had other mild aphasic characteristics. FD showed implicit reading abilities under brief presentation conditions, being able to make lexical decisions and semantic categorisations well above chance. FD was given two therapy programmes, the first, whole word therapy to exploit this implicit ability and the second to improve letter-by-letter accuracy and speed. FD showed some improvement in reading ability after both therapy programmes, particularly for words of personal interest to him. His letter naming accuracy and reading of visually similar words were the most resistant to change. A striking effect of therapy was the cessation of FD's letter-by-letter reading and the emergence of some of the characteristics of deep dyslexia. Even when therapy concentrated on letter accuracy, FD did not revert back to his original letter-by-letter reading strategy. The results are discussed with reference to the two theories of pure alexia. Some conclusions are drawn about the need for therapists to examine and exploit all residual reading skills when devising therapeutic programmes.

Inducing letter-by-letter dyslexia in normal readers

Letter-by-letter (LBL) dyslexia is an acquired reading disorder characterized by very slow reading and a large linear word length effect. This suggests the use of a sequential LBL strategy, in sharp contrast with the parallel letter processing used by normal subjects. Recently, we have proposed that the reading difficulty of LBL dyslexics is due to a deficit in discriminating visually similar letters based on parallel letter processing [Arguin, M., Fiset, S., & Bub, D. Sequential and parallel letter processing in letter-by-letter dyslexia. Cognitive Neuropsychology, 19, 535-555, 2002]. The visual mechanisms underlying this deficit and the LBL strategy, however, are still unknown. In this article, we propose that LBL dyslexic patients have lost the ability to use, for parallel letter processing, the optimal spatial frequency band for letter and word recognition. We claim that, instead, they rely on lower spatial frequencies for parallel processing, that these lower spatial frequencies produce confusions between visually similar letters, and that the LBL compensatory strategy allows them to extract higher spatial frequencies. The LBL strategy would thus increase the spatial resolution of the visual system, effectively resolving the issue pertaining to between-letter similarity. In Experiments 1 and 2, we succeeded in replicating the main features characterizing LBL dyslexia by having normal individuals read low-contrast, high-pass-filtered words. Experiment 3, conducted in LBL dyslexic L.H., shows that, indeed, the letter confusability effect is based on low spatial frequencies, whereas this effect was not supported by high spatial frequencies.

Patterns of Peripheral Paralexia: Pure Alexia and the Forgotten Visual Dyslexia?

The concept of visual dyslexia put forward by Marshall and Newcombe (1973) is assessed. After a long period of neglect it was resurrected in the late 1990s in a narrow form. In the current paper it is proposed that a wider form of the functional syndrome is useful to include amongst other conditions attentional dyslexia and neglect dyslexia. The variety of sub-forms would correspond to the behavioural effects of the different ways in which the orthographic processing systems can be impaired. What distinguishes the broader form from pure alexia is that the patient lacks the capacity to use a serial letter processing strategy, and so interpretation of visual dyslexia in terms of the impairment to the orthographic processing systems is not contaminated by the use of a compensatory strategy that results in processing operations which are qualitatively very different from the normal and highly opaque. The lack of a serial letter processing strategy makes visual dyslexia a much more transparent functional syndrome.