Preorthographical constraints on visual word recognition: Evidence from a case study of developmental surface dyslexia

We investigated the visual word recognition ability of M.T., a young boy with surface dyslexia, by means of a paradigm that measures performance as a function of the eye fixation position within the word, known as the “viewing-position effect” paradigm. In well-achieving readers, the viewing-position effect is mainly determined by factors affecting letter visibility and by lexical constraints on word recognition. We further quantified M.T.'s sensory limitations on letter visibility by computing visual-span profiles—that is, the number of letters recognizable at a glance. Finally, in an ideal-observer's perspective, M.T.'s performance was compared with a parameter-free model combining M.T.'s letter visibility data with a simple lexical matching rule. The results showed that M.T. did not use the whole visual information available on letter identities to recognize words and that preorthographical factors constrained his word recognition performance. The results can be best accounted for by a reduction of the number of letters processed in parallel.     

"Fragment errors" in deep dysgraphia: further support for a lexical hypothesis

In addition to various lexical errors, the writing of patients with deep dysgraphia may include a large number of segmental spelling errors, which increase towards the end of the word. Frequently, these errors involve deletion of two or more letters resulting in so-called "fragment errors". Different positions have been brought forward regarding their origin, including rapid decay of activation in the graphemic buffer and an impairment of more central (i.e., lexical or semantic) processing. We present data from a patient (M.D.) with deep dysgraphia who showed an increase of segmental spelling errors towards the end of the word. Several tasks were carried out to explore M.D.'s underlying functional impairment. Errors affected word-final positions in tasks like backward spelling and fragment completion. In a delayed copying task, length of the delay had no influence. In addition, when asked to recall three serially presented letters, a task which had not been carried out before, M.D. exhibited a preference for the first and the third letter and poor performance for the second letter. M.D.'s performance on these tasks contradicts the rapid decay account and instead supports a lexical-semantic account of segmental errors in deep dysgraphia. In addition, the results fit well with an implemented computational model of deep dysgraphia and segmental spelling errors.     

PERCEPTUA.ND LEXICA.ACTORS IN A CAS.F LETTER-BY-LETTER READING

We report the case of a letter-by-letter reader (MJ) who showed normal processing of single letters and who could normally access the orthographic input lexicon when presented with letter names for aural recognition, or when allow ed enough tim e to process a visually presented letter string. However, MJ showed severe difficulties in simultaneously processing multiple letters and other simple visual stimuli. Furthermore, she does not have normal access to lexical orthographic representations and their meanings when stimuli are presented for too brief a time to allow for serial processing of the letter string. We found no evidence of (partial) lexical or semantic access without corresponding recognition of the letters in a word: No signs of implicit reading were observed w hen the input stimuli were controlled for the relevant visual features; “implicit reading” was only obtained under conditions that allowed sophisticated guessing. This pattern of results is interpreted as indicating that LBL reading (at least in MJ) results from damage to prelexical processing mechanisms. In MJ's case, the deficit reflects the degraded transfer of information from a normal visual processing system in the right hemisphere to a normal language processing system in the left hemisphere.     

Lexical recovery from extinction: Interactions between visual form and stored knowledge modulate visual selection

The effects of lexical knowledge on extinction were examined in a patient with bilateral parietal lesions and left extinction under double simultaneous stimulation: GK. GK was bilaterally presented with two letters that could form either a word or a nonword. In Experiments 1-3, the task was to identify each letter. GK showed better identification of left-side letters in words than in nonwords, whilst the identification of left-side letters in nonwords was worse than that of single letters presented in the same spatial positions (i.e., there was a word superiority effect under conditions in which extinction occurred). This lexical effect on completely correct responses tended to be larger for words with lower-case letters (Experiments 2 and 3) than for words with upper-case letters (Experiment 1). Different results arose when detection was measured. When letters could group by proximity and common contrast polarity, no word superiority effect was apparent. However, a word superiority effect re-emerged when low-level grouping was reduced by using letters with opposite contrast polarity (one white and one black on a grey background). The results are discussed in terms of the impact of different factors on selection in detection and identification tasks, and in terms of the modulatory roles of familiar form and stored knowledge on visual selection.     

From word superiority to word inferiority: Visual processing of letters and words in pure alexia

Visual processing and naming of individual letters and short words were investigated in four patients with pure alexia. To test processing at different levels, the same stimuli were studied across a naming task and a visual perception task. The normal word superiority effect was eliminated in both tasks for all patients, and this pattern was more pronounced in the more severely affected patients. The relationship between performance with single letters and words was, however, not straightforward: One patient performed within the normal range on the letter perception task, while being severely impaired in letter naming and word processing, and performance with letters and words was dissociated in all four patients, with word reading being more severely impaired than letter recognition. This suggests that the word reading deficit in pure alexia may not be reduced to an impairment in single letter perception.     

Developmental disorders of verbal short-term memory and their relation to sentence comprehension: A reply to Howard and Butterworth

The effects of lexical knowledge on extinction were examined in a patient with bilateral parietal lesions and left extinction under double simultaneous stimulation: GK. GK was bilaterally presented with two letters that could form either a word or a nonword. In Experiments 1–3, the task was to identify each letter. GK showed better identification of left-side letters in words than in nonwords, whilst the identification of left-side letters in nonwords was worse than that of single letters presented in the same spatial positions (i.e., there was a word superiority effect under conditions in which extinction occurred). This lexical effect on completely correct responses tended to be larger for words with lower-case letters (Experiments 2 and 3) than for words with upper-case letters (Experiment 1). Different results arose when detection was measured. When letters could group by proximity and common contrast polarity, no word superiority effect was apparent. However, a word superiority effect re-emerged when low-level grouping was reduced by using letters with opposite contrast polarity (one white and one black on a grey background). The results are discussed in terms of the impact of different factors on selection in detection and identification tasks, and in terms of the modulatory roles of familiar form and stored knowledge on visual selection.     

The word-length effect in reading: A review

The finding that visual processing of a word correlates with the number of its letters has an extensive history. In healthy subjects, a variety of methods, including perceptual thresholds, naming and lexical decision times, and ocular motor parameters, show modest effects that interact with high-order effects like frequency. Whether this indicates serial processing of letters under some conditions or indexes low-level visual factors related to word length is unclear. Word-length effects are larger in pure alexia, where they probably reflect a serial letter-by-letter strategy, due to failure of lexical whole-word processing and variable dysfunction in letter encoding. In pure alexia, the word-length effect is systematically related to mean naming latency, with the word-length effect becoming proportionally greater as naming latency becomes more delayed in severe cases. Other conditions may also generate enhanced word-length effects. This occurs in right hemianopia: Computer simulations suggest a criterion of 160?ms/letter to distinguish hemianopic dyslexia from pure alexia. Normal reading development is accompanied by a decrease in word-length effects, whereas persistently elevated word-length effects are characteristic of developmental dyslexia. Little is known about word-length effects in other reading disorders. We conclude that the word-length effect captures the efficiency of the perceptual reading process in development, normal reading, and a number of reading disorders, even if its mechanistic implications are not always clear.     

ERP correlates of transposed-letter priming effects: the role of vowels versus consonants

One key issue for any computational model of visual-word recognition is the choice of an input coding scheme for assigning letter position. Recent research has shown that pseudowords created by transposing two letters are very effective at activating the lexical representation of their base words (e.g., relovution activates REVOLUTION ). We report a masked priming lexical decision experiment in which the pseudoword primes were created by transposing/replacing two consonants or two vowels while event-related potentials were recorded. The results showed a modulation of the amplitude at an early window (150–250 ms) and at the N400 component for vowels but not for consonant transpositions. In addition, the peak latencies were faster for transposed than replaced consonants. These results suggest that consonants and vowels play a different role during the process of visual word recognition. We examine the implications for the choice of an input coding scheme in models of visual-word recognition.     

The breakdown of parallel letter processing in letter-by-letter dyslexia

Two critical issues were examined regarding letter-by-letter (LBL) dyslexia: (1) What is the nature of the functional impairment responsible for the incapacity of LBL patients to overtly recognise words on the sole basis of parallel letter processing? (2) What is the purpose of sequential letter processing? Four experiments focusing on these issues were conducted in LH, an LBL dyslexic. Expt 1 showed facilitatory effects of increased phonographic neighbourhood size, lexical frequency, and imageability on the word naming performance of LH. These high-order effects reflect a modulation of parallel letter processing in LH and demonstrate that he is able to rapidly access phonological, lexical, and semantic knowledge during reading. Congruently, Expt 2 demonstrated that all three high-order effects are eliminated when words are presented one letter at a time, from left to right. Expt 3 showed that these high-level effects are also abolished if target words are made of letters that are highly confusable (i.e., visually similar) to other letters of the alphabet. These observations suggest that LBL dyslexia may rest on an impairment at the letter encoding level that causes an excessive level of background noise in the activation of higher-order representations (i.e., letter combinations) when letters are processed in parallel. An additional experiment (Expt 4) shows that the letter confusability effect is also eliminated when words are presented one letter at a time, from left to right. This latter finding suggests that compensatory sequential processing invoked by LBL dyslexics serves to avoid the confusion between visually similar letters, which is present with parallel letter processing, and to amplify the signal-to-noise ratio required to achieve overt word identification.     

The breakdown of parallel letter processing in letter-by-letter dyslexia

Two critical issues were examined regarding letter-by-letter (LBL) dyslexia: (1) What is the nature of the functional impairment responsible for the incapacity of LBL patients to overtly recognise words on the sole basis of parallel letter processing? (2) What is the purpose of sequential letter processing? Four experiments focusing on these issues were conducted in LH, an LBL dyslexic. Expt 1 showed facilitatory effects of increased phonographic neighbourhood size, lexical frequency, and imageability on the word naming performance of LH. These high-order effects reflect a modulation of parallel letter processing in LH and demonstrate that he is able to rapidly access phonological, lexical, and semantic knowledge during reading. Congruently, Expt 2 demonstrated that all three high-order effects are eliminated when words are presented one letter at a time, from left to right. Expt 3 showed that these high-level effects are also abolished if target words are made of letters that are highly confusable (i.e., visually similar) to other letters of the alphabet. These observations suggest that LBL dyslexia may rest on an impairment at the letter encoding level that causes an excessive level of background noise in the activation of higher-order representations (i.e., letter combinations) when letters are processed in parallel. An additional experiment (Expt 4) shows that the letter confusability effect is also eliminated when words are presented one letter at a time, from left to right. This latter finding suggests that compensatory sequential processing invoked by LBL dyslexics serves to avoid the confusion between visually similar letters, which is present with parallel letter processing, and to amplify the signal-to-noise ratio required to achieve overt word identification.     

“Fragment errors” in deep dysgraphia: Further support for a lexical hypothesis

In addition to various lexical errors, the writing of patients with deep dysgraphia may include a large number of segmental spelling errors, which increase towards the end of the word. Frequently, these errors involve deletion of two or more letters resulting in so-called “fragment errors”. Different positions have been brought forward regarding their origin, including rapid decay of activation in the graphemic buffer and an impairment of more central (i.e., lexical or semantic) processing. We present data from a patient (M.D.) with deep dysgraphia who showed an increase of segmental spelling errors towards the end of the word. Several tasks were carried out to explore M.D.'s underlying functional impairment. Errors affected word-final positions in tasks like backward spelling and fragment completion. In a delayed copying task, length of the delay had no influence. In addition, when asked to recall three serially presented letters, a task which had not been carried out before, M.D. exhibited a preference for the first and the third letter and poor performance for the second letter. M.D.'s performance on these tasks contradicts the rapid decay account and instead supports a lexical-semantic account of segmental errors in deep dysgraphia. In addition, the results fit well with an implemented computational model of deep dysgraphia and segmental spelling errors.     

A rose is a rose or a nose: A deficit in initial letter identification

Abstract

We describe the letter and word identification performance of a patient with a posterior left-hemisphere lesion, a right homonymous hemianopia, and unimpaired performance on tests for visuo-spatial neglect. His frequent errors in single word reading were almost all of the variety suggested by the title, i.e. rose → “nose”. The likelihood of his correctly identifying a word was significantly affected by lexical constraints on the initial letter, with words like rose (_ose can be many different words) yielding lower performance than words with a unique first letter (_oap can only be soap). The patient also made errors in identifying single isolated letters, and was particularly likely to misidentify the initial character in random strings of letters. By striking contrast, he identified letters in positions 2-N of words or strings with good accuracy. A variety of reading tasks and stimulus types were employed in an attempt to characterise and understand this pattern of performance.     

Sequential and parallel letter processing in letter-by-letter dyslexia

Four experiments are reported that focus on the issue of sequential vs. parallel letter processing in letter-by-letter (LBL) dyslexia; these were conducted on patient IH. Expt. 1 showed a large linear reduction of word naming times with an increase in the number of orthographic neighbours of the target (i.e., words of the same length differing by just one letter; N size). Given the large negative linear correlation existing between word length and N size, this result raises the possibility that the large word length effect diagnostic of LBL dyslexia may be, in fact, an artefact of uncontrolled N size. Expt. 2 falsified this possibility by showing that the word length effect is unaffected by whether N size is controlled for or not. This result also suggested that the facilitatory effect of increased N size in LBL dyslexia is based on the parallel processing of the constituent letters of the target. Further supporting a contribution of parallel letter processing to overt word recognition performance in the disorder, Expt. 3 showed significant but independent effects of word length and letter confusability (i.e., similarity of the constituent letters of the target word with other letters of the alphabet). The letter confusability effect therefore appears to rest on the parallel analysis of the letters in the target word. Finally, Expt. 4 showed that the facilitatory effect of N size is prevented with high letter-confusability targets. These observations suggest that LBL dyslexia rests on an impairment of letter encoding that results in an excessive level of background noise in the activation of lexical-orthographic representations when letters are processed in parallel. This prevents overt identification of the target and forces sequential letter processing in order to achieve this goal.     

Sequential and parallel letter processing in letter-by-letter dyslexia

Four experiments are reported that focus on the issue of sequential vs. parallel letter processing in letter-by-letter (LBL) dyslexia; these were conducted on patient IH. Expt. 1 showed a large linear reduction of word naming times with an increase in the number of orthographic neighbours of the target (i.e., words of the same length differing by just one letter; N size). Given the large negative linear correlation existing between word length and N size, this result raises the possibility that the large word length effect diagnostic of LBL dyslexia may be, in fact, an artefact of uncontrolled N size. Expt. 2 falsified this possibility by showing that the word length effect is unaffected by whether N size is controlled for or not. This result also suggested that the facilitatory effect of increased N size in LBL dyslexia is based on the parallel processing of the constituent letters of the target. Further supporting a contribution of parallel letter processing to overt word recognition performance in the disorder, Expt. 3 showed significant but independent effects of word length and letter confusability (i.e., similarity of the constituent letters of the target word with other letters of the alphabet). The letter confusability effect therefore appears to rest on the parallel analysis of the letters in the target word. Finally, Expt. 4 showed that the facilitatory effect of N size is prevented with high letter-confusability targets. These observations suggest that LBL dyslexia rests on an impairment of letter encoding that results in an excessive level of background noise in the activation of lexical-orthographic representations when letters are processed in parallel. This prevents overt identification of the target and forces sequential letter processing in order to achieve this goal.     

Grapheme-to-lexeme feedback in the spelling system: Evidence from a dysgraphic patient

This article presents an argument for grapheme-to-lexeme feedback in the cognitive spelling system, based on the impaired spelling performance of dysgraphic patient CM. The argument relates two features of CM's spelling. First, letters from prior spelling responses intrude into subsequent responses at rates far greater than expected by chance. This letter persistence effect arises at a level of abstract grapheme representations, and apparently results from abnormal persistence of activation. Second, CM makes many formal lexical errors (e.g., carpet → compute). Analyses revealed that a large proportion of these errors are "true" lexical errors originating in lexical selection, rather than "chance" lexical errors that happen by chance to take the form of words. Additional analyses demonstrated that CM's true lexical errors exhibit the letter persistence effect. We argue that this finding can be understood only within a functional architecture in which activation from the grapheme level feeds back to the lexeme level, thereby influencing lexical selection.     

A peripheral reading deficit under conditions of diffuse visual attention

We report a single case study of a patient, FL, who has a peripheral dyslexia with symptoms resembling attentional dyslexia. FL demonstrated impaired identification of letters within strings, and better identification of words than their constituent letters. We found that FL was impaired at both letter counting and same-case letter matching with letter strings, and his matching and naming performance were strongly affected by letter spacing. The effects of these visual variables on performance suggest an early locus to FL's deficit. We propose that letter identification was disrupted by abnormal lateral masking and poor location coding within words. These peripheral processing deficits were reduced when the task required focused attention on the central letter location. Nevertheless, even with impaired letter coding, word representations could be accessed to some degree, via supra-letter units. We discuss the implications of the data for understanding normal reading.     

A peripheral reading deficit under conditions of diffuse visual attention

We report a single case study of a patient, FL, who has a peripheral dyslexia with symptoms resembling attentional dyslexia. FL demonstrated impaired identification of letters within strings, and better identification of words than their constituent letters. We found that FL was impaired at both letter counting and same-case letter matching with letter strings, and his matching and naming performance were strongly affected by letter spacing. The effects of these visual variables on performance suggest an early locus to FL's deficit. We propose that letter identification was disrupted by abnormal lateral masking and poor location coding within words. These peripheral processing deficits were reduced when the task required focused attention on the central letter location. Nevertheless, even with impaired letter coding, word representations could be accessed to some degree, via supra-letter units. We discuss the implications of the data for understanding normal reading.     

Neglect Dyslexia With a Stimulus-Centred Deficit and Without Visuospatial Neglect

This paper reports a single case of ipsilesional left neglect dyslexia and interprets it according to the three-level model of visual word recognition proposed by Caramazza and Hillis (1990). The three levels reflect a progression from the physical stimulus to an abstract representation of a word. RR was not impaired at the first, retinocentric, level, which represents the individual features of letters within a word according to the location of the word in the visual field: She made the same number of errors to words presented in her left visual field as in her right visual field. A deficit at this level should also mean the patient neglects all stimuli. This did not occur with RR: She did not neglect when naming the items in rows of objects and rows of geometric symbols. In addition, although she displayed significant neglect dyslexia when making visual matching judgements on pairs of words and nonwords, she did not do so to pairs of nonsense letter shapes, shapes which display the same level of visual complexity as letters in words. RR was not impaired at the third, graphemic, level, which represents the ordinal positions of letters within a word: She continued to neglect the leftmost (spatial) letter of words presented in mirror-reversed orientation and she did not neglect in oral spelling. By elimination, these results suggest RR's deficit affects a spatial reference frame where the representational space is bounded by the stimulus: A stimulus-centred level of representation. We define five characteristics of a stimulus-centred deficit, as manifest in RR. First, it is not the case that neglect dyslexia occurs because the remaining letters in a string attract or capture attention away from the leftmost letter(s). Second, the deficit is continuous across the letter string. Third, perceptually significant features, such as spaces, define potential words. Fourth, the whole, rather than part, of a letter is neglected. Fifth, category information is preserved. It is concluded that the Caramazza-Hillis model accounts well for RR's data, although we conclude that neglect dyslexia can be present when a more general visuospatial neglect is absent.