On the use of metacontrast to assess magnocellular function in dyslexic readers.

It has been proposed that dyslexia is the result of a deficit in the magnocellular system. Reduced metacontrast masking in dyslexic readers has been taken as support for this view. In metacontrast, a masking stimulus reduces the visibility of a spatially adjacent target stimulus when the target stimulus precedes the masking stimulus by about 30-100 msec. Recent evidence indicates that the latency difference between the magnocellular and parvocellular subcortical pathways is at most 20 msec and may be as small as only 5 msec, or even less. This makes it difficult to attribute the latency in metacontrast to the latency differences between the magnocellular and parvocellular systems. It is therefore problematic to attribute reduced metacontrast masking to a deficit in the magnocellular system.     

On the use of the Ternus test to assess magnocellular function

Ternus stimuli give rise to two mutually exclusive visual experiences: with long interstimulus intervals (ISIs) the elements in the stimulus are perceived as moving together as a group ('group movement'), while at shorter ISIs only a single element appears to be moving ('element movement' or 'end-to-end movement'). It has been hypothesized that group and element movements, respectively, reflect magnocellular and parvocellular activity. On this basis, Ternus tests have been used to assess magnocellular function in dyslexic individuals. This use of Ternus stimuli is examined in the present report. On the basis of amplitude spectra of the stimuli and of a review of previous studies it is concluded that to use Ternus tests to assess magnocellular function is problematic.     

On using Vernier acuity to assess magnocellular sensitivity

A recent study [Keri, S., & Benedek, G. (2009). Visual pathway deficit in female fragile × premutation carriers: A potential endophenotype. Brain and Cognition, 69, 291–295] has found Vernier acuity deficiencies together with contrast sensitivity defects consistent with a magnocellular deficit in female fragile × premutation carriers. This may appear to support the notion that Vernier acuity may serve as a test of magnocellular sensitivity. However, Vernier acuity deficiencies have been reported in other conditions (e.g., schizophrenia, amblyopia and cortical visual impairment) where there is little evidence for magnocellular deficits. The observation that Vernier acuity deficiencies can occur without magnocellular deficits indicates that Vernier acuity is not a reliable test of magnocellular sensitivity.     

On the use of discrimination to assess memory

A number of studies have made use of sensory discrimination to assess memory. These experiments have typically employed a temporal two-alternative forced choice paradigm in which the interstimulus interval (ISI) is varied. Many of these tests have shown that ISI has little or no effect on discrimination performance. This has been taken as evidence for perfect or very good memory. An alternative interpretation would be that discrimination tests lack sufficient sensitivity to reveal memory effects. In the present study, computer simulations based on signal detection theory were used to estimate the relative effects of memory noise and direct noise (i.e., noise inherent in the discrimination task) on discrimination performance. These simulations indicate that the amount of memory noise has to be quite large to cause a marked elevation in discrimination thresholds and that sensory discrimination is a relatively insensitive test of memory.     

Auditory word identification in dyslexic and normally achieving readers

The integrity of phonological representation/processing in dyslexic children was explored with a gating task in which children listened to successively longer segments (gates) of a word. At each gate, the task was to decide what the entire word was. Responses were scored for overall accuracy as well as the children's sensitivity to coarticulation from the final consonant. As a group, dyslexic children were less able than normally achieving readers to detect coarticulation present in the vowel portion of the word, particularly on the most difficult items, namely those ending in a nasal sound. Hierarchical regression and path analyses indicated that phonological awareness mediated the relation of gating and general language ability to word and pseudoword reading ability.     

The development of grapheme-phoneme correspondence in normal and dyslexic readers

The present experiment was concerned with the development of grapheme-phoneme conversion ability in normal and reading-age matched dyslexic readers. The use of grapheme-phoneme correspondences was observed in a recognition memory task for pronounceable nonwords. The nonwords were presented in either the visual or auditory modality and had to be recognized immediately from the converse modality, thus necessitating decoding of stimuli across modalities. The use of grapheme-phoneme correspondences increased with reading age in the normal readers but not in the dyslexics. It was postulated that dyslexics have a specific difficulty in grapheme-phoneme conversion. For them an increase in reading age is attributable mainly to an increase in size of sight vocabulary.     

Processing syntactic functions of words in normal and dyslexic readers

Readers with dyslexia were compared with normal readers for the ability to identify the grammatical function of words in the course of sentence processing by means of electrophysiological measures along with behavior measures. Participants were 18 dyslexic and 18 normally reading, native Hebrew-speaking male university students, aged 18 to 27 years. Obtained results confirmed the hypothesis (Leikin & Breznitz, 1999) that Hebrew readers used lexical-morphological properties of words to identify their grammatical roles. However, the morphologically based strategy acquired full expression only in the presence of the verb, which in the Hebrew sentence fulfills the central role. Seemingly, readers use a few different procedures to identify the grammatical roles of words. Selection of a particular strategy seems to be influenced by diverse factors, including syntactic and lexical-morphological characteristics of the stimuli and level of reading skills. In addition, the results showed significant differences in sentence processing between normal and dyslexic readers, as reflected by ERP measures. These results are in line with the hypothesis, suggesting the existence of a syntactic processing weakness in readers with dyslexia.     

A dual-route perspective on eye movements of dyslexic readers

This study assessed eye movement abnormalities of adolescent dyslexic readers and interpreted the findings by linking the dual-route model of single word reading with the E-Z Reader model of eye movement control during silent sentence reading. A dysfunction of the lexical route was assumed to account for a reduced number of words which received only a single fixation or which were skipped and for the increased number of words with multiple fixations and a marked effect of word length on gaze duration. This pattern was interpreted as a frequent failure of orthographic whole-word recognition (based on orthographic lexicon entries) and on reliance on serial sublexical processing instead. Inefficiency of the lexical route was inferred from prolonged gaze durations for singly fixated words. These findings were related to the E-Z Reader model of eye movement control. Slow activation of word phonology accounted for the low skipping rate of dyslexic readers. Frequent reliance on sublexical decoding was inferred from a tendency to fixate word beginnings and from short forward saccades. Overall, the linkage of the dual-route model of single word reading and a model of eye movement control led to a useful framework for understanding eye movement abnormalities of dyslexic readers.     

Letter identification and lateral masking in dyslexic and average readers

Lateral masking and letter identification in dyslexic and average readers were investigated with a methodology that corrected for some of the weaknesses of the Geiger and Lettvin studies (1986, 1987). Target letters were presented alone or embedded within three-letter arrays at retinal locations from 0 degrees to 15 degrees to the right or left of a fixation point. A foveal letter, which was the same as or different from a target letter, appeared as a distractor, and comparisons were made between scaled and unscaled stimuli. Dyslexic readers were found to be better than average readers at detecting scaled letters embedded in an array in some of the peripheral locations tested. Unlike the results of Geiger and Lettvin, however, this finding of better letter detection in the periphery by dyslexics was limited to selected conditions of the study. Reading groups were also found to differ in responding to the foveal distractor in the letter task. Detection of letters by average readers was affected by the type of distractor, but this variable was not found to influence the dyslexic readers. These findings suggest some differences between dyslexic and average readers in attention to stimuli presented at multiple locations in the visual field.     

Syntactic processing of Hebrew sentences in normal and dyslexic readers: electrophysiological evidence

The authors examined differences in brain activity as measured by amplitudes and latencies of event-related potential (ERP) components in Hebrew-speaking adult dyslexic and normal readers. The participants were measured while processing words' syntactic functions during reading of sentences with subject-verb-object syntactic order. The results suggested that among dyslexic and normal readers, N100 and P300 ERP components were sensitive to certain constituents of syntactic analysis for target words in accordance with their grammatical roles. The findings further demonstrated significant differences in ERP measures between dyslexic and normal readers. Compared with normal readers, dyslexic readers exhibited consistently higher amplitudes and longer latencies in both ERP components for the subject of the sentence. Significant, though less consistent, ERP variations were observed for other sentence elements. In addition, dyslexic readers differed from normal readers in their processing strategies. For normal readers, the verb-oriented, morphologically based strategy was found to be the most efficient for sentence processing in Hebrew, whereas the dyslexic readers demonstrated a more primitive mode of identification of words' grammatical roles, namely, the word-order strategy. The results support the hypothesis that there is a syntactic processing "weakness" in dyslexics.     

Reduced orthographic learning in dyslexic adult readers: evidence from patterns of letter search

Visual letter search performance was investigated in a group of dyslexic adult readers using a task that required detection of a cued letter target embedded within a random five-letter string. Compared to a group of skilled readers, dyslexic readers were significantly slower at correctly identifying targets located in the first and second string position, illustrating significantly reduced leftward facilitation than is typically observed. Furthermore, compared to skilled readers, dyslexic readers showed reduced sensitivity to positional letter frequency. They failed to exhibit significantly faster response times to correctly detect target letters appearing in the most, compared to least, frequent letter position within five-letter words, and response times correlated with positional letter frequency only for the initial, and not the final, letter position. These results are compatible with the SERIOL (sequential encoding regulated by inputs to oscillations within letter units) model of orthographic processing proposed by Whitney and Cornelissen (2005). Furthermore, they suggest that dyslexic readers are less efficient than skilled readers at learning to extract statistical regularities from orthographic input.     

Reduced orthographic learning in dyslexic adult readers: Evidence from patterns of letter search

Visual letter search performance was investigated in a group of dyslexic adult readers using a task that required detection of a cued letter target embedded within a random five-letter string. Compared to a group of skilled readers, dyslexic readers were significantly slower at correctly identifying targets located in the first and second string position, illustrating significantly reduced leftward facilitation than is typically observed. Furthermore, compared to skilled readers, dyslexic readers showed reduced sensitivity to positional letter frequency. They failed to exhibit significantly faster response times to correctly detect target letters appearing in the most, compared to least, frequent letter position within five-letter words, and response times correlated with positional letter frequency only for the initial, and not the final, letter position. These results are compatible with the SERIOL (sequential encoding regulated by inputs to oscillations within letter units) model of orthographic processing proposed by Whitney and Cornelissen (2005). Furthermore, they suggest that dyslexic readers are less efficient than skilled readers at learning to extract statistical regularities from orthographic input.     

A comparison of word recognition processes in dyslexic and normal readers at two reading-age levels

This study addressed the question of whether dyslexic children use qualitatively different word identification processes as compared to normal readers at the same stage of reading acquisition. Fifty-two dyslexic children and reading-age matched normal readers were required to pronounce words and pseudowords designed to tap several word recognition and decoding processes. Performance profiles were compared for the two reading groups at two reading ages. Although an invariant acquisition sequence was observed across reading groups, differences in level of performance between dyslexics and reading-age controls varied as a function of reading age. The performance of the more advanced dyslexics was virtually indistinguishable from normal readers on all measures. In contrast, the younger reading age dyslexics differed from normal readers on several measures of spelling-sound correspondences. However, no reading group differences were observed on measures of word recognition. The results indicated that dyslexics and normal readers at the same reading age use essentially the same processes to recognize words, but may differ in knowledge of correspondence rules.     

On the relation between metacontrast masking and suppression of visible persistence

Does the introduction of additional contours in a display sequence (an operation known to reduce the strength of suppression in metacontrast) also reduce suppression of visible persistence? In three experiments, duration of visible persistence was estimated by a method in which successful performance depends on the temporal integration of a pattern whose elements are displayed in two successive frames. In this procedure, the arrival of the trailing frame is known to exert a suppressive influence on the visible persistence of the leading frame. Embedding the elements of the leading frame within additional contours (a line grid) reduced the degree of suppression exerted by the trailing frame. This did not occur when the grid was part of the trailing display. We conclude that suppression of visible persistence and metacontrast masking belong to the same class of events.     

Quality of phonological representations, verbal learning, and phoneme awareness in dyslexic and normal readers

This study of dyslexia was concerned with the quality of phonological representations of lexical items. It extended the studies of verbal learning in dyslexia from learning new vocabulary items (pseudo-names) to the learning of more well-specified variants of known words. The participants were 19 dyslexic adolescents in grades 4 to 6 and 19 younger normal readers in grade 2 matched on single word decoding. The dyslexics were significantly outperformed by the reading-age controls in non-word reading and in phoneme awareness. The dyslexics also took longer time to learn to associate a set of pseudo-names with pictures of persons although the dyslexics learned to associate familiar names with pictures as quickly as the controls did. The acquisition of new phonological representations of words was studied in an imitation task with maximally distinct pronunciations of long, familiar words. The dyslexics gained less than the controls in this task. They also gained less on one measure taken from a phoneme substitution task with the same words as in the distinctness task. The results are interpreted in the light of the hypothesis that poorly specified phonological representations may be an underlying problem in dyslexia.     

Letter identification declines with increasing retinal eccentricity at the same rate for normal and dyslexic readers

It has recently been claimed (Geiger & Lettvin, 1987; Perry, Dember, Warm, & Sacks, 1989) that the acuity/eccentricity function is flatter in dyslexics than in normal subjects, with dyslexics showing better performance in the periphery and worse performance at fixation. In these studies, all target letters were presented to the right of fixation, a procedural flaw inviting subjects to optimize performance by directing attention and/or gaze to the right of the designated fixation point. It is suggested that dyslexic and normal readers may differ in the degree to which they might adopt the optimal strategy in this situation. To overcome this problem, target letters were briefly presented at 16 randomly intermixed locations derived from the orthogonal combination of four eccentricities and four directions from fixation (above, below, right, left). The accuracy of letter identification declined with increasing eccentricity at the same rate for good and poor adult readers and dyslexic teenagers. This finding provides no support for the view that the acuity/eccentricity function might vary with and possibly cause differences in reading level.     

Short-term memory function in young readers

Twelve good readers and 12 poor readers, 10-y4-olds, were given memory span tests, and memory scanning tests, in both auditory and visual modalities. Their concept of a letter pattern was also tested. The major finding was that short-term memory (STM) function deteriorated over time in the poor reading group. When modality was switched the good readers showed a release from proactive inhibition (PI); the poor readers did not. Among good readers, memory scanning in the auditory modality occurred at about the same speed as memory scanning in the visual modality; among the poor readers, auditory speed gradually lagged relative to visual rates. Poor readers were more likely (than good readers) to lack the concept of a letter pattern. Stepwise regressions showed that different patterns of variables were assocated with different types of reading errors. Implicatios for model construction were discussed.     

Decrease in metacontrast masking following adaptation to flicker

Selective adaptation was used to explore the characterisitcs of a metacontrast masking stimulus which contribute to its effectiveness in masking the test stimulus. Subjects adapted for 10 s to a configuration like the masking stimulus that was either continuously on or flickering. Following this they viewed a metacontrast presentation and estimated the brightness of the test stimulus. Prior adaptation to a continuously present stimulus did not appreciably affect metacontrast masking; however, masking was greatly reduced following adaptation to flickering stimuli. These results are consistent with recent models of metacontrast masking based on transient and sustained visual channels.