Developmental and acquired dyslexia: A comparison

Jorm (1979a) has drawn attention to similarities between developmental dyslexia and acquired deep dyslexia, an analogy which has been criticized by A. W. Ellis (1979). A series of three experiments compared the two syndromes, using the techniques applied by Patterson and Marcel (1977) to adult deep dyslexics, to study a group of 15 boys suffering from developmental dyslexia. Patterson and Marcel's patients were able to perform a lexical decision task but showed no evidence of phonemic encoding of nonwords; our dyslexic children performed this task very slowly and with reduced accuracy but showed clear evidence of phonemic coding of the nonword items. Patterson and Marcel observed that their patients could not read out orthographically regular nonwords; our dyslexic children were able to do this task, although more slowly and somewhat less accurately than their chronological age or reading age controls. Finally, Patterson and Marcel observed that highly imageable words were more likely to be read correctly than words of equal frequency but low imageability; we observed a similar effect in both our dyslexic group and in their reading age controls. This implies that the imageability effect may not be peculiar to dyslexics but may be characteristic of normal reading under certain conditions. It is concluded that developmental dyslexics differ from the patients studied by Patterson and Marcel in demonstrating a pattern of reading which, though slow, is qualitatively similar to the reading of normal readers of a younger age. As such, our results do not support Jorm's position.     

Morphological errors in acquired dyslexia: A case of mistaken identity

Morphological errors in oral reading have been used to support theories of lexical access in which root morphemes and affixes are independently represented in the orthographic lexicon. No support for such theories was found in this study, which examined the morphological errors made by two patients with different patterns of acquired dyslexia. Instead, it was shown that the probability of making a morphological error depended upon the absolute levels of imageability and word frequency of the affixed word and the relative levels of its root morpheme. These properties also affected the oral reading of words without affixes. It was concluded that so-called “morphological” errors reflect the properties of the reading system which influence lexical orthographic access for both affixed and unaffixed words and further reflect the functional level of the lexical pathways available to the patient.     

Paying attention to reading errors in acquired dyslexia

The important role of selective attention in normal reading is illustrated by a relatively rare form of acquired dyslexia known as 'attentional dyslexia'. Theories of the functional deficit underlying this disorder differ as to the locus of the attentional dysfunction. A recent study by Mayall and Humphreys has contributed to this debate by studying letter migration errors in an attentional dyslexic.     

An attractor network model of serial recall

We present a neural network model of verbal working memory which attempts to illustrate how a few simple assumptions about neural computation can shed light on cognitive phenomena associated with the serial recall of verbal material. We assume that neural representations are distributed, that neural connectivity is massively recurrent, and that synaptic efficacy is modified based on the correlation between pre- and post-synaptic activity (Hebbian learning). Together these assumptions give rise to emergent computational properties that are relevant to working memory, including short-term maintenance of information, time-based decay, and similarity-based interference. We instantiate these principles in a specific model of serial recall and show how it can both simulate and explain a number of standard cognitive phenomena associated with the task, including the effects of serial position, word length, articulatory suppression (and its interaction with word length), and phonological similarity.     

An attractor network in the hippocampus: theory and neurophysiology

A quantitative computational theory of the operation of the CA3 system as an attractor or autoassociation network is described. Based on the proposal that CA3-CA3 autoassociative networks are important for episodic or event memory in which space is a component (place in rodents and spatial view in primates), it has been shown behaviorally that the CA3 supports spatial rapid one-trial learning and learning of arbitrary associations and pattern completion where space is a component. Consistent with the theory, single neurons in the primate CA3 respond to combinations of spatial view and object, and spatial view and reward. Furthermore, single CA3 neurons reflect the recall of a place from an object in a one-trial object-place event memory task. CA3 neurons also reflect in their firing a memory of spatial view that is retained and updated by idiothetic information to implement path integration when the spatial view is obscured. Based on the computational proposal that the dentate gyrus produces sparse representations by competitive learning and via the mossy fiber pathway forces new representations on the CA3 during learning (encoding), it has been shown behaviorally that the dentate gyrus supports spatial pattern separation during learning, and that the mossy fiber system to CA3 connections are involved in learning but not in recall. The perforant path input to CA3 is quantitatively appropriate to provide the cue for recall in CA3. The concept that the CA1 recodes information from CA3 and sets up associatively learned back-projections to neocortex to allow subsequent retrieval of information to neocortex provides a quantitative account of the large number of hippocampo-neocortical back-projections.     

Running memory span: A comparison of behavioral capacity limits with those of an attractor neural network

We studied a computational model of short term memory capacity that performs a simulated running memory span task using Hebbian learning and rapid decay of connection strengths to keep recent items active for later recall. This model demonstrates recall performance similar to humans performing the same task, with a capacity limit of approximately three items and a prominent recency effect. The model also shows that this capacity depends on decay to release the model from accumulating interference. Model findings are compared with data from two behavioral experiments that used varying task demands to tax memory capacity limits. Following additional theoretical predictions from the computational model, behavioral data support that when task demands require attention to be spread too thin to keep items available for later recall, capacity limits suffer. These findings are important both for understanding the mechanisms underlying short term memory capacity, and also to memory researchers interested in the role of attention in capacity limitations.     

The analysis of morphological errors in a case of acquired dyslexia

An issue that is persistently raised in studies of subjects who produce morphological errors in reading and other tasks is whether these errors are the consequence of a morphological processing deficit, or whether they in any way reflect morphological principles of organization in the lexicon. We discuss the performance of one such subject on a number of tasks and evaluate standard arguments for attributing aspects of his performance to a morphological processing deficit. Although there are several features of his performance that are suggestive in this regard; we argue that, when these issues are addressed in the context of a sufficiently elaborated theory of lexical processing, a morphological processing deficit cannot be demonstrated. We also survey a number of recent reports that purport to provide evidence for a morphological processing deficit and argue that, in most cases, they fail to support such claims for similar reasons. An important moral to be drawn from a critique of these studies is that in order to make valid inferences concerning the role of morphology in organizing the lexicon, we must consider these errors in the context of theories of the lexicon that take seriously the effects of converging lexical factors in processing.     

Letter-by-letter acquired dyslexia is due to the serial encoding of letters

Letter-by-letter acquired dyslexia (pure alexia) is assumed to be related to the serial encoding of letters, but the evidence for this assumption is somewhat indirect. Here, we demonstrate that the deficit is indeed due to serial encoding by comparing the performance of a letter-by-letter dyslexic reader with the performance of normal readers who were forced to read letter by letter; the data patterns are remarkably similar.     

Diagnosis and rehabilitation attempt of a patient with acquired deep dyslexia

This study presents a patient (JH) suffering from acquired deep dyslexia. According to a cognitive model of reading, JH exhibited a significant impairment in both the phonological and lexical routes to reading. A rehabilitation program was proposed. We decided to focus on the lexical route using a priming paradigm. The written words were associated with a picture and the patient was invited to construct a mental association between the written word and the image. During five sessions, JH was trained to read 50 words. A pre-test, a post-test, and an 8-month follow-up session consisted of reading 140 written words including the 50 trained words. At the post-test, 90% of the trained words were accurately read. At the follow-up session, JH's reading performance for the trained words was still highly accurate at 84%. This strong effect is discussed and ways of increasing the patient's abilities to read are proposed.     

Whole-word phonological representations of disyllabic words in the Chinese lexicon: data from acquired dyslexia

This study addresses the issue of the existence of whole-word phonological representations of disyllabic and multisyllabic words in the Chinese mental lexicon. A Cantonese brain-injured dyslexic individual with semantic deficits, YKM, was assessed on his abilities to read aloud and to comprehend disyllabic words containing homographic heterophonous characters, the pronunciation of which can only be disambiguated in word context. Superior performance on reading to comprehension was found. YKM could produce the target phonological forms without understanding the words. The dissociation is taken as evidence for whole-word representations for these words at the phonological level. The claim is consistent with previous account for discrepancy of the frequencies of tonal errors between reading aloud and object naming in Cantonese reported of another case study of similar deficits. Theoretical arguments for whole-word form representations for all multisyllabic Chinese words are also discussed.     

Self-organizing continuous attractor network models of hippocampal spatial view cells

Single neuron recording studies have demonstrated the existence of hippocampal spatial view neurons which encode information about the spatial location at which a primate is looking in the environment. These neurons are able to maintain their firing even in the absence of visual input. The standard neuronal network approach to model networks with memory that represent continuous spaces is that of continuous attractor networks. It has recently been shown how idiothetic (self-motion) inputs could update the activity packet of neuronal firing for a one-dimensional case (head direction cells), and for a two-dimensional case (place cells which represent the place where a rat is located). In this paper, we describe three models of primate hippocampal spatial view cells, which not only maintain their spatial firing in the absence of visual input, but can also be updated in the dark by idiothetic input. The three models presented in this paper represent different ways in which a continuous attractor network could integrate a number of different kinds of velocity signal (e.g., head rotation and eye movement) simultaneously. The first two models use velocity information from head angular velocity and from eye velocity cells, and make use of a continuous attractor network to integrate this information. A fundamental feature of the first two models is their use of a 'memory trace' learning rule which incorporates a form of temporal average of recent cell activity. Rules of this type are able to build associations between different patterns of neural activities that tend to occur in temporal proximity, and are incorporated in the model to enable the recent change in the continuous attractor to be associated with the contemporaneous idiothetic input. The third model uses positional information from head direction cells and eye position cells to update the representation of where the agent is looking in the dark. In this case the integration of idiothetic velocity signals is performed in the earlier layer of head direction cells.     

Auditory processing in developmental dyslexia: An exploratory study of an auditory and visual matching training program with Swedish children with developmental dyslexia

This study examined whether training using a nonverbal auditory-visual matching task had a remedial effect on reading skills in developmental dyslexia. The pretest/post-test design was used with Swedish children ( N = 41), between the ages of 7 and 12. Training comprised twice-weekly sessions of 15 minutes, over eight weeks. There was an improvement in auditory-visual matching during the training period. There were also improvements in some reading test scores, especially in reading nonsense words and in reading speed. These improvements in tasks which are thought to rely on phonological processing suggest that such reading difficulties in dyslexia may stem in part from more basic perceptual difficulties, including those required to manage the visual and auditory components of the decoding task. The utility of the concept of auditory structuring is discussed in relation to auditory and phonological processing skills when a child learns to read.     

A case study of an English-Japanese bilingual with monolingual dyslexia

We report the case of AS, a 16 year-old English/Japanese bilingual boy, whose reading/writing difficulties are confined to English only. AS was born in Japan to a highly literate Australian father and English mother, and goes to a Japanese selective senior high school in Japan. His spoken language at home is English. AS's reading in logographic Japanese Kanji and syllabic Kana is equivalent to that of Japanese undergraduates or graduates. In contrast, his performance in various reading and writing tests in English as well as tasks involving phonological processing was very poor, even when compared to his Japanese contemporaries. Yet he has no problem with letter names or letter sounds, and his phoneme categorisation is well within the normal range of English native speakers. In order to account for our data that show a clear dissociation between AS's ability to read English and Japanese, we put forward the ‘hypothesis of granularity and transparency'. It is postulated that any language where orthography-to-phonology mapping is transparent, or even opaque, or any language whose orthographic unit representing sound is coarse (i.e. at a whole character or word level) should not produce a high incidence of developmental phonological dyslexia.     

Spelling dyslexia: A form of cross-cuing

A patient with spelling dyslexia maintained the ability to write, spell, and pronounce spelled nonsense words and words that require knowledge of orthographic rules of language. He was unable to perform grapheme-phoneme conversions except after naming the letter involved. We propose that he uses a letter-naming strategy to circumvent the disconnection of visual areas from the area of visual word images and that his letter-naming strategy represents a compensatory cross-cuing strategy.     

Lesioning a connectionist model of visual search: selective effects on distractor grouping.

A connectionist model of visual search is presented, in which search is determined by patterns of grouping between distractors and between the target and the distractors. Grouping is based on conjunctive relations between simple form elements (corners, line-end terminators), and uses principles of similarity and spatial proximity. In its normal, 'unlesioned' state, the model simulates the search data generated by human subjects when they search for simple form conjunctions amongst either homogeneous or heterogeneous distractors (Humphreys & Muller, in press). In this paper, the performance of the model is examined after the model is subject to various types of 'lesion'. 'Lesioning' is produced either by increasing the internal noise on the activation functions governing the interactions between processing units, or by eliminating processing units from different loci in the model. Increasing the internal noise within the model, or lesioning units within 'high-level' processing stages, can generate the selective effects on search found in a detailed single case study of an agnosic patient (Humphreys, Riddoch, Quinlan, Price & Donnelly, this volume): Namely, there is selective disruption of search for a form conjunction amongst homogeneous distractors relative to search for the same target amongst heterogeneous distractors. This selective effect can be attributed to the 'lesion' disrupting grouping between distractors, which normally facilitates search with homogeneous distractors. The simulations demonstrate that (1) search processes can be disrupted by adding internal noise to search functions, (2) that there can be selective effects on grouping processes, and (3) that these selective effects can be associated with different types of lesion to different stages in the model. The implications of the simulations for understanding visual processing impairments in agnosia are discussed.     

Rapid naming deficits in dyslexia: a stumbling block for the perceptual anchor theory of dyslexia

According to a recent theory of dyslexia, the perceptual anchor theory, children with dyslexia show deficits in classic auditory and phonological tasks not because they have auditory or phonological impairments but because they are unable to form a ‘perceptual anchor’ in tasks that rely on a small set of repeated stimuli. The theory makes the strong prediction that rapid naming deficits should only be present in small sets of repeated items, not in large sets of unrepeated items. The present research tested this prediction by comparing rapid naming performance of a small set of repeated items with that of a large set of unrepeated items. The results were unequivocal. Deficits were found both for small and large sets of objects and numbers. The deficit was actually bigger for large sets than for small sets, which is the opposite of the prediction made by the anchor theory. In conclusion, the perceptual anchor theory does not provide a satisfactory account of some of the major hallmark effects of developmental dyslexia.