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Nonlinguistic perceptual deficits associated with reading and language disorders
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Nonlinguistic perceptual deficits associated with reading and language disorders Beverly A Wright*, Richard W Bowen† and Steven G Zecker‡ Recent behavioral evidence supports the idea that some individuals with reading and language disorders are impaired in their perception of nonlinguistic auditory and visual information. More sophisticated measurement paradigms and analysis techniques are leading to a clearer understanding of these deficits and to possibilities for their remediation. Addresses *‡ Department of Communication Sciences and Disorders, Audiology and Hearing Sciences Program*, and Program in Learning Disabilities‡, 2299 North Campus Drive, Northwestern University, Evanston, IL 60208, USA *e-mail: [email protected] ‡ e-mail: [email protected] † Department of Psychology and Parmly Hearing Institute, Loyola University Chicago, Chicago, IL 60626, USA; e-mail: [email protected] Current Opinion in Neurobiology 2000, 10:482–486 0959-4388/00/$ — see front matter © 2000 Elsevier Science Ltd. All rights reserved.
Introduction Approximately one person in twelve, who is otherwise unimpaired, has extreme difficulties either reading or understanding spoken language, or both [1]. We review recent behavioral evidence, adding to earlier reports, showing that a subset of this population is also impaired in their perception of nonlinguistic information. These data indicate that some reading and language disorders may be caused, exacerbated or, at a minimum, marked by deficits in nonlinguistic auditory and visual perception. The existence of these general perceptual problems provides evidence against the idea that the impairments in reading and language disorders are restricted to linguistic processing [2,3]. However, it is still possible that in many affected individuals, disorders in general perception combine with disorders in linguistic ability. Our review covers reports published over the past five years — with particular emphasis on those published since 1998 — on behavioral measures of the perception of nonlinguistic auditory and visual information by individuals with developmental reading and language disorders. (For previous related reviews, see [4–8].) We highlight six major developments in this recent literature that we believe will significantly affect future research. These developments concern the perception of sequential sounds, sound-frequency discrimination, detection of target sounds in noise, the visual magnocellular-deficit hypothesis, individual differences, and remediation.
Perception of sequential sounds The first major development is that there has been considerable new support for, and refinement of, the idea that the perception of sequential sounds is impaired in individuals
with a reading or language disorder. Many papers on auditory perception have addressed either the detection or discrimination of sounds presented in rapid succession or the perception of the presentation order of such sounds. Three groups have reported that individuals with reading and language disorders are impaired in their ability to detect and discriminate sequential sounds. First, Wright et al. [9] demonstrated that in individuals with a language disorder, detection of a brief tone presented before a burst of noise was much more impaired than detection of that tone presented after the noise (but see [10•]). Impairments were reduced when the noise did not contain the tone frequency. This study provided the first behavioral evidence that in language disorder the interference between successively presented sounds is temporally asymmetric and frequency specific. Second, Kujala et al. [11] reported that in reading disorder, the detection of altered time intervals between tone bursts was impaired for a four-tone, but not for a two-tone sequence. This result appears to indicate a deficit in the discrimination of the time between pairs of sounds when they are preceded or followed by other sounds. Third, Menell et al. [12] reported that the ability to detect amplitude fluctuations in a wideband noise was uniformly impaired across a range of fluctuation rates in reading disorder, revealing an inefficiency in the processing of such changes. The impaired perception of successive stimuli probably does not result from overall poorer temporal resolution, because individuals with reading disorder have been shown to be normal in their ability to detect a brief silent gap in ongoing noise [13,14]. The recent literature also contains evidence that individuals with reading and language disorders are poor at reporting the order of sequential sounds. Of particular interest is a report by Helenius et al. [15••] that showed that individuals with a reading disorder experienced a segregation of alternating sound frequencies into separate, nonordered frequency streams (as opposed to hearing a connected, ordered pattern) at slower alternation rates than did controls. To the extent that impaired listeners continued to hear both frequencies at the fastest presentation rates, these data appear to indicate that difficulty in sound ordering may not always result simply from an inability to discriminate those sounds, as previously proposed [16]. In addition, several other research groups replicated earlier reports [16,17] showing that individuals with a reading or language disorder have difficulty indicating the order of two sounds presented in rapid succession. Those groups investigated whether and how that ordering difficulty was affected by training [18,19], dichotic presentation [20], disability subtype [21•], heredity [22], and physiology [23]. Thus, this recent literature confirms that individuals with a reading or language disorder have difficulty perceiving
Nonlinguistic perceptual deficits associated with reading and language disorders Wright, Bowen and Zecker
sequential sounds, and that they experience this difficulty in a number of contexts. At the same time, intriguing results from newly applied measurement methods are providing a more refined characterization of this deficit.
Sound-frequency discrimination The second major development has been the demonstration in some studies that individuals with a reading or language disorder are impaired in their ability to hear differences in sound frequency. This research represents a new topic of investigation into nonlinguistic perception which has emerged during the review period. The results from such investigations have been mixed. According to some reports, this ability is impaired in reading disorder. Affected individuals were poor at the discrimination of tonal frequencies [13,24,25], the detection of slow (but not fast) rates of frequency modulation [26], the discrimination of frequency-modulation depth [27], and the discrimination of pitches produced by amplitude fluctuations [25]. Furthermore, the ability to detect slow rates of frequency modulation correlated with reading skill in students in a regular-education classroom [28••]. (In a related development, individuals with reading disorder were found to be impaired at tactile discrimination of grating orientation [29•].) In other reports, however, there were only nonsignificant trends for poorer frequency discrimination [30] and frequency-modulation detection [10•,30]. There was also no evidence of difficulty with frequency-transition discrimination [3] or fundamental-frequency discrimination based on temporal cues [10•] in reading and language disorders. Overall, this recent literature raises the possibility that, in addition to affecting the processing of sequential sounds, perceptual problems in reading and language disorders affect the processing of sound frequency as well.
Detection of target sounds in noise The third major development has been that individuals with a reading or language disorder have been shown to be impaired, in some cases, in their ability to hear nonlinguistic target sounds presented within noise. This work represents another new area of investigation initiated during the review period. Individuals with a reading or language disorder were reported to have difficulties detecting brief, but not long, tones within noise using monaural detection cues. Wright et al. [9] observed that, compared to controls, children with a language disorder were impaired in their ability to detect a brief tone in noise, particularly when the noise did not contain the tone frequency. This pattern of results is indicative of poor frequency resolution. In contrast, several groups reported that the detection of a long tone (>84 ms) presented within noise [9,13,30], or temporally centered in a silent period between noises [31], was normal in reading and language disorders. In addition, there have been reports suggesing that individuals with a reading disorder are impaired in their ability to hear long tones within noise using binaural detection cues. Low-frequency tones within noise are easier to
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detect when they are presented in opposite phase to the two ears. The ability to use that interaural time difference was reported to be impaired in reading disorder for a 1 kHz tone [13], but not for a 0.2 kHz tone [30]. Finally, Dougherty et al. [32] reported impaired discrimination in reading disorder of rising from falling pitches created by interaural time differences in narrow spectral regions of a wideband noise. Added to the evidence of impairments in hearing sequential sounds and frequency differences, these new data on impairments in the detection of nonlinguistic target sounds in noise suggest that auditory function may be much more broadly affected in reading and language disorders than was previously suspected.
Visual magnocellular-deficit hypothesis The fourth major development is that there has been new support for, but also new challenges to, the hypothesis that there is a deficit in the visual magnocellular (large-neuron) system in certain forms of reading disorder [4,33,34]. Some researchers have even proposed that there is a deficit in an auditory magnocellular system as well [4,24–27]. Because the visual magnocellular system responds to rapidly changing stimuli, one prediction of the magnocellular-deficit hypothesis is that the perception of visual motion should be impaired in reading disorder. In support of this hypothesis, as predicted, the results of several recent studies revealed poor detection of coherent visual motion [35–38], poor discrimination of the speed of visual motion [39,40], and abnormal perception of illusory visual motion [35,41••] in reading disorder. Similar impairments were also reported in the perception of sound motion [42,43]. The recent literature has also presented three challenges to the magnocellular-deficit hypothesis. The first challenge was to the prediction of the hypothesis that a deficit in the magnocellular system should be associated with reduced sensitivity to low spatial frequencies (coarse images) favored by that system [35,44–46]. In a review of the literature, Skottun [47•] found only inconsistent evidence for this prediction. The second challenge was to a specific version of the magnocellular-deficit hypothesis that proposes that in reading disorder the magnocellular system fails to inhibit a parallel, parvocellular (small-neuron) system during saccades, resulting in a smearing of successive visual images [34]. In contrast to this view, two research groups [41••,48] pointed out that the magnocellular system is actually inhibited by the parvocellular system (see e.g. [49,50]), and that the parvocellular system is itself not affected during a saccade (see e.g. [51]). The third challenge was to the prediction of the magnocellular-deficit hypothesis that motion sensitivity should be impaired in reading disorder. Raymond and Sorenson [52], in a study of motion coherence, reported that the ability to judge the global direction of motion in a brief movie of a random-dot texture was poorer in individuals with a reading disorder than in controls when the motion was presented over multiple frames (as in [35–38]), but not when the motion was presented over only two frames. This is the first evidence that perceptual integration, not motion perception,
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may be impaired in reading disorder. In summary, the recent literature contains new data supporting the visual magnocellular-deficit hypothesis. However, the hypothesis itself is undergoing data-driven refinement.
Individual differences The fifth major development has been that the analysis of individual differences in perceptual performance has provided insights into the multidimensional nature of reading and language disorders. Within the review period, several research groups have scrutinized the large variability in performance seen on perceptual tasks. The results of this work have provided four basic insights into the nature of reading and language disorders. First, deficits in auditory [21•,53] and visual [35,41••,44] perception may be present only in particular subtypes of disability. Second, individual differences in performance on one or more particular auditory [13,15••,19,27], visual [36,54], or both auditory and visual [26,28••] tasks may correlate with, and help account for, individual differences in reading and language skills. Third, there may be both shared and unique contributions to reading disorder from impairments in the auditory and visual modalities [24,26,28••]. Fourth, although there appears to be a genetic component to some abilities impaired in reading disorder, the ability to order nonlinguistic sounds may not be heritable [22]. Thus, analyses of individual differences in performance on nonlinguistic perceptual tasks are leading to a better understanding of the multidimensional character of reading and language disorders.
Remediation Finally, in the sixth major development, two research groups have reported that treatments designed to overcome nonlinguistic perceptual impairments also have assisted in the remediation of reading and language disorders. First, Merzenich, Tallal, and colleagues [18,19,55] tested a treatment designed to improve the ability of children with language disorder to hear brief sounds presented in rapid succession — a skill necessary for speech perception. They trained affected children adaptively on sound-segmentation tasks using computer-presented exercises with both nonlinguistic and linguistic stimuli, and on more general language exercises presented with modified speech. In contrast, control children played video games rather than the adaptive training game, and received equivalent general language training presented with unmodified speech. Trained children improved their performance on tests of sound segmentation, speech perception and language comprehension more than controls. Furthermore, the observed improvements in nonlinguistic segmentation ability and language processing were significantly correlated. The authors conclude that the observed remediation indicates that perceptual learning in affected individuals is essentially normal, and that language disorder may itself result from learning a defective representation of the auditory environment. Second, Stein et al. [56•] tested a treatment designed to steady the vision of a subset of children with reading disorder who have poor binocular control. They provided these
children with monocularly occluded glasses. Children who received occluded glasses achieved binocular control more quickly than did control children who received unoccluded glasses. Furthermore, children who received the occluded glasses and achieved binocular control increased their reading age over the period of use at twice the rate of controls. The authors suggest that the acquisition of binocular stability and the associated improvement in reading skills indicate that monocular occlusion may be a useful treatment for children with reading disorder who have unstable binocular control. More extensive testing of both of these treatments is necessary. Nevertheless, their apparent success provides some evidence for a direct link between perceptual deficits and reading and language performance.
Conclusions The recent developments in the study of nonlinguistic perceptual deficits in reading and language disorders reviewed here show four clear, positive trends. First, there is an expanding investigation of auditory function in these disorders. The literature contains not only new studies of the processing of sequential sounds, but also novel investigations of frequency discrimination and the detection of target sounds in noise. Second, there is a continuing effort to refine two long standing and influential hypotheses that hold that perceptual deficits arise from difficulties in the processing of sequential sounds, or from a deficiency in the visual magnocellular system. Third, there is a growing interest in individual differences that is leading to new insights into the multidimensional nature of reading and language disorders and is helping to resolve conflicting results. Fourth, there is a promising movement toward applying information about perceptual deficits in reading and language disorders to methods of remediation. These trends set the stage for future progress. They indicate that nonlinguistic perceptual testing in reading and language disorders will become increasingly important. Additional normative data should be collected, both to clarify the pervasiveness of impairments in nonlinguistic perception, and to assist in diagnosis. Furthermore, multiple aspects of auditory and visual perception, as well as linguistic performance, should be tested in the same individuals. This approach would both increase our understanding of the multidimensional nature of these perceptual deficits and help to reconcile independent hypotheses about these deficits that have appeared in the auditory and visual literatures. Future research on reading and language disorders should be aimed at identifying and characterizing all nonlinguistic perceptual deficits, and determining the impact of those deficits on reading and language processing.
Acknowledgements This work was supported by Grant R29 DC02997 from the National Institutes of Health: National Institute On Deafness And Other Communication Disorders awarded to BA Wright.
Nonlinguistic perceptual deficits associated with reading and language disorders Wright, Bowen and Zecker
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20. Harel S, Nachson I: Dichotic listening to temporal tonal stimuli by good and poor readers. Percept Mot Skills 1997, 84:467-473. 21. Heath SM, Hogben JH, Clark CD: Auditory temporal processing in • disabled readers with and without oral language delay. J Child Psychol Psychiatry 1999, 40:637-647. Compared to controls, children with both reading and language disorders, but not those with only reading disorder, are impaired in their ability to perceive the presentation order of brief sounds. These data provide evidence for disability subtypes that may ultimately help to clarify why different investigations of perceptual ability in reading and language disorders sometimes yield conflicting results. 22. Bishop D, Bishop S, Bright P, James C, Delaney T, Tallal P: Different origin of auditory and phonological processing problems in children with language impairment: evidence from a twin study. J Speech Lang Hear Res 1999, 42:155-168. 23. Nagarajan S, Mahncke H, Salz T, Tallal P, Roberts T, Merzenich M: Cortical auditory signal processing in poor readers. Proc Natl Acad Sci USA 1999, 96:6483-6488. 24. Baldeweg T, Richardson A, Watkins S, Foale C, Gruzelier J: Impaired auditory frequency discrimination in dyslexia detected with mismatch evoked potentials. Ann Neurol 1999, 45:495-503. 25. Hari R, Saaskilahti A, Helenius P, Uutela K: Non-impaired auditory phase locking in dyslexic adults. Neuroreport 1999, 10:2347-2348. 26. Witton C, Talcott J, Hansen P, Richardson A, Griffiths T, Rees A, Stein J, Green G: Sensitivity to dynamic auditory and visual stimuli predicts nonword reading ability in both dyslexic and normal readers. Curr Biol 1998, 8:791-797. 27.
Stein J, McAnally K: Auditory temporal processing in developmental dyslexics. Irish J Psychol 1995, 16:220-228.
28. Talcott J, Witton C, McLean M, Hansen P, Rees A, Green G, Stein J: •• Dynamic sensory sensitivity and children’s word decoding skills. Proc Natl Acad Sci USA 2000, 97:2952-2957. Unselected 10-year old children (n=32) without a reading disorder were tested on a battery of perceptual and psychometric tests and tests of component literacy skills. After controlling for intelligence and reading ability, auditory frequency-modulation sensitivity explained independent variance in phonological skill, while visual sensitivity to coherent motion explained independent variance in orthographic skill. These results suggest that auditory and visual function may play separate roles in determining reading ability. 29. Grant A, Zangaladze A, Thiagarajah C, Sathian K: Tactile perception • in developmental dyslexia: a psychophysical study using gratings. Neuropsychologia 1999, 37:1201-1211. Adults with a reading disorder and controls were compared on their ability to discriminate the orientation of passively applied tactile gratings with equally spaced ridges and grooves of various widths. Observers with a reading disorder required larger scale gratings to achieve criterion performance than did controls. This report extends the range of nonlinguistic perceptual deficits to include tactile discrimination of grating orientation. 30. Hill N, Bailey P, Griffiths Y, Snowling M: Frequency acuity and binaural masking release in dyslexic listeners. J Acoust Soc Am 1999, 106:53-58. 31. Helzer J, Champlin C, Gillam R: Auditory temporal resolution in specifically language-impaired and age-matched children. Percept Mot Skills 1996, 83:1171-1181. 32. Dougherty R, Cynader M, Bjornson B, Edgell D, Giaschi D: Dichotic pitch: a new stimulus distinguishes normal and dyslexic auditory function. Neuroreport 1998, 9:3001-3005. 33. Lovegrove W, Bowling A, Badcock D, Blackwood M: Specific reading disability: differences in contrast sensitivity as a function of spatial frequency. Science 1980, 210:439-440. 34. Breitmeyer B: Sustained (P) and transient (M) channels in vision: a review and implications for reading. In Visual Processes in Reading and Reading Disabilities. Edited by Willows D, Kruk R, Corcos E. Hillsdale, NJ: Erlbaum; 1993:95-110. 35. Slaghius W, Ryan J: Spatio-temporal contrast sensitivity, coherent motion, and visible persistence in developmental dyslexia. Vision Res 1999, 39:651-668. 36. Cornelissen P, Hansen P, Hutton J, Evangelinou V, Stein J: Magnocellular visual function and children’s single word reading. Vision Res 1998, 38:471-482. 37.
Cornelissen P, Hansen P: Motion detection, letter position encoding, and single word reading. Ann Dyslexia 1998, 48:155-188.
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majority of studies on spatial contrast sensitivity in reading disorder show either no difference between affected individuals and controls or show deficits in affected individuals at high, but not low, spatial frequencies. 48. Skottun B, Parke L: The possible relationship between visual deficits and dyslexia: examination of a critical assumption. J Learning Disabil 1999, 32:2-5. 49. Burr D, Holt J, Johnstone J, Ross J: Selective depression of motion sensitivity during saccades. J Physiol 1982, 333:1-15.
41. Cestnick L, Coltheart M: The relationship between language •• processing and visual-processing deficits in developmental dyslexia. Cognition 1999, 71:231-255. In children with a reading disability, impaired apparent motion perception was correlated with non-word reading (phonological processing) but not with exception-word reading (lexical processing). The authors offer two explanations: impaired left-to-right processing of letters in non-words, or a coincidental neuro-developmental deficit in magnocellular layers of the physically adjacent lateral geniculate nucleus (motion impairment) and medial geniculate nucleus (phonological impairment) of the thalamus.
51. Burr D, Morrone M, Ross J: Selective suppression of the magnocellular visual pathway during saccadic eye movements. Nature 1994, 371:511-513.
42. Visto J, Cranford J, Scudder R: Dynamic temporal processing of nonspeech acoustic information by children with specific language impairment. J Speech Hear Res 1996, 39:510-517.
53. Rosen S: Language disorders: a problem with auditory processing? Curr Biol 1999, 9:R698-R700.
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54. Eden G, Stein J, Wood H, Wood F: Temporal and spatial processing in reading disabled and normal children. Cortex 1995, 31:451-468.
44. Borsting E, Ridder W, Dudeck K, Kelly C, Matsui L, Motoyama J: The presence of a magnocellular defect depends on the type of dyslexia. Vision Res 1996, 36:1047-1053.
55. Tallal P, Merzenich M, Miller S, Jenkins W: Language learning impairments: integrating basic science, technology, and remediation. Exp Brain Res 1998, 123:210-219.
45. Gross-Glenn K, Skottun BC, Glenn W, Kushch A, Lingua R, Dunbar M, Jallad B, Lubs HA, Levin B, Rabin M et al.: Contrast sensitivity in dyslexia. Vis Neurosci 1995, 12:153-163.
56. Stein J, Richardson A, Fowler M: Monocular occlusion can improve • binocular control and reading in dyslexics. Brain 2000, 123:164-170. A large group (n=143) of children having both a reading disorder and unstable binocular control were randomly assigned to wear spectacles with or without the left lens occluded. Compared to children with the unoccluded spectacles, children with the occluded spectacles were more likely to gain binocular control and to improve reading skills after 3 months. After 9 months, children who gained and maintained stable fixation through occlusion had nearly double the rate of reading improvement of those whose vision was unoccluded or those who lost stable fixation despite using occluded spectacles. The authors conclude that monocular occlusion is a treatment that rapidly improves binocular control and leads to faster reading improvement.
46. Cornelissen P, Richardson A, Mason A, Fowler S, Stein J: Contrast sensitivity and coherent motion detection measured at photopic luminance levels in dyslexics and controls. Vision Res 1995, 35:1483-1494. 47. Skottun B: The magnocellular deficit theory of dyslexia: the • evidence from contrast sensitivity. Vision Res 2000, 40:111-127. The magnocellular-deficit hypothesis predicts that, in reading disorder, sensitivity to visual spatial contrast should be impaired at low, but not at high, spatial frequencies. Contrary to this prediction, this review reports that the
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Nonlinguistic perceptual deficits associated with reading and language disorders Beverly A Wright*, Richard W Bowen† and Steven G Zecker‡ Recent behavioral evidence supports the idea that some individuals with reading and language disorders are impaired in their perception of nonlinguistic auditory and visual information. More sophisticated measurement paradigms and analysis techniques are leading to a clearer understanding of these deficits and to possibilities for their remediation. Addresses *‡ Department of Communication Sciences and Disorders, Audiology and Hearing Sciences Program*, and Program in Learning Disabilities‡, 2299 North Campus Drive, Northwestern University, Evanston, IL 60208, USA *e-mail: [email protected] ‡ e-mail: [email protected] † Department of Psychology and Parmly Hearing Institute, Loyola University Chicago, Chicago, IL 60626, USA; e-mail: [email protected] Current Opinion in Neurobiology 2000, 10:482–486 0959-4388/00/$ — see front matter © 2000 Elsevier Science Ltd. All rights reserved.
Introduction Approximately one person in twelve, who is otherwise unimpaired, has extreme difficulties either reading or understanding spoken language, or both [1]. We review recent behavioral evidence, adding to earlier reports, showing that a subset of this population is also impaired in their perception of nonlinguistic information. These data indicate that some reading and language disorders may be caused, exacerbated or, at a minimum, marked by deficits in nonlinguistic auditory and visual perception. The existence of these general perceptual problems provides evidence against the idea that the impairments in reading and language disorders are restricted to linguistic processing [2,3]. However, it is still possible that in many affected individuals, disorders in general perception combine with disorders in linguistic ability. Our review covers reports published over the past five years — with particular emphasis on those published since 1998 — on behavioral measures of the perception of nonlinguistic auditory and visual information by individuals with developmental reading and language disorders. (For previous related reviews, see [4–8].) We highlight six major developments in this recent literature that we believe will significantly affect future research. These developments concern the perception of sequential sounds, sound-frequency discrimination, detection of target sounds in noise, the visual magnocellular-deficit hypothesis, individual differences, and remediation.
Perception of sequential sounds The first major development is that there has been considerable new support for, and refinement of, the idea that the perception of sequential sounds is impaired in individuals
with a reading or language disorder. Many papers on auditory perception have addressed either the detection or discrimination of sounds presented in rapid succession or the perception of the presentation order of such sounds. Three groups have reported that individuals with reading and language disorders are impaired in their ability to detect and discriminate sequential sounds. First, Wright et al. [9] demonstrated that in individuals with a language disorder, detection of a brief tone presented before a burst of noise was much more impaired than detection of that tone presented after the noise (but see [10•]). Impairments were reduced when the noise did not contain the tone frequency. This study provided the first behavioral evidence that in language disorder the interference between successively presented sounds is temporally asymmetric and frequency specific. Second, Kujala et al. [11] reported that in reading disorder, the detection of altered time intervals between tone bursts was impaired for a four-tone, but not for a two-tone sequence. This result appears to indicate a deficit in the discrimination of the time between pairs of sounds when they are preceded or followed by other sounds. Third, Menell et al. [12] reported that the ability to detect amplitude fluctuations in a wideband noise was uniformly impaired across a range of fluctuation rates in reading disorder, revealing an inefficiency in the processing of such changes. The impaired perception of successive stimuli probably does not result from overall poorer temporal resolution, because individuals with reading disorder have been shown to be normal in their ability to detect a brief silent gap in ongoing noise [13,14]. The recent literature also contains evidence that individuals with reading and language disorders are poor at reporting the order of sequential sounds. Of particular interest is a report by Helenius et al. [15••] that showed that individuals with a reading disorder experienced a segregation of alternating sound frequencies into separate, nonordered frequency streams (as opposed to hearing a connected, ordered pattern) at slower alternation rates than did controls. To the extent that impaired listeners continued to hear both frequencies at the fastest presentation rates, these data appear to indicate that difficulty in sound ordering may not always result simply from an inability to discriminate those sounds, as previously proposed [16]. In addition, several other research groups replicated earlier reports [16,17] showing that individuals with a reading or language disorder have difficulty indicating the order of two sounds presented in rapid succession. Those groups investigated whether and how that ordering difficulty was affected by training [18,19], dichotic presentation [20], disability subtype [21•], heredity [22], and physiology [23]. Thus, this recent literature confirms that individuals with a reading or language disorder have difficulty perceiving
Nonlinguistic perceptual deficits associated with reading and language disorders Wright, Bowen and Zecker
sequential sounds, and that they experience this difficulty in a number of contexts. At the same time, intriguing results from newly applied measurement methods are providing a more refined characterization of this deficit.
Sound-frequency discrimination The second major development has been the demonstration in some studies that individuals with a reading or language disorder are impaired in their ability to hear differences in sound frequency. This research represents a new topic of investigation into nonlinguistic perception which has emerged during the review period. The results from such investigations have been mixed. According to some reports, this ability is impaired in reading disorder. Affected individuals were poor at the discrimination of tonal frequencies [13,24,25], the detection of slow (but not fast) rates of frequency modulation [26], the discrimination of frequency-modulation depth [27], and the discrimination of pitches produced by amplitude fluctuations [25]. Furthermore, the ability to detect slow rates of frequency modulation correlated with reading skill in students in a regular-education classroom [28••]. (In a related development, individuals with reading disorder were found to be impaired at tactile discrimination of grating orientation [29•].) In other reports, however, there were only nonsignificant trends for poorer frequency discrimination [30] and frequency-modulation detection [10•,30]. There was also no evidence of difficulty with frequency-transition discrimination [3] or fundamental-frequency discrimination based on temporal cues [10•] in reading and language disorders. Overall, this recent literature raises the possibility that, in addition to affecting the processing of sequential sounds, perceptual problems in reading and language disorders affect the processing of sound frequency as well.
Detection of target sounds in noise The third major development has been that individuals with a reading or language disorder have been shown to be impaired, in some cases, in their ability to hear nonlinguistic target sounds presented within noise. This work represents another new area of investigation initiated during the review period. Individuals with a reading or language disorder were reported to have difficulties detecting brief, but not long, tones within noise using monaural detection cues. Wright et al. [9] observed that, compared to controls, children with a language disorder were impaired in their ability to detect a brief tone in noise, particularly when the noise did not contain the tone frequency. This pattern of results is indicative of poor frequency resolution. In contrast, several groups reported that the detection of a long tone (>84 ms) presented within noise [9,13,30], or temporally centered in a silent period between noises [31], was normal in reading and language disorders. In addition, there have been reports suggesing that individuals with a reading disorder are impaired in their ability to hear long tones within noise using binaural detection cues. Low-frequency tones within noise are easier to
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detect when they are presented in opposite phase to the two ears. The ability to use that interaural time difference was reported to be impaired in reading disorder for a 1 kHz tone [13], but not for a 0.2 kHz tone [30]. Finally, Dougherty et al. [32] reported impaired discrimination in reading disorder of rising from falling pitches created by interaural time differences in narrow spectral regions of a wideband noise. Added to the evidence of impairments in hearing sequential sounds and frequency differences, these new data on impairments in the detection of nonlinguistic target sounds in noise suggest that auditory function may be much more broadly affected in reading and language disorders than was previously suspected.
Visual magnocellular-deficit hypothesis The fourth major development is that there has been new support for, but also new challenges to, the hypothesis that there is a deficit in the visual magnocellular (large-neuron) system in certain forms of reading disorder [4,33,34]. Some researchers have even proposed that there is a deficit in an auditory magnocellular system as well [4,24–27]. Because the visual magnocellular system responds to rapidly changing stimuli, one prediction of the magnocellular-deficit hypothesis is that the perception of visual motion should be impaired in reading disorder. In support of this hypothesis, as predicted, the results of several recent studies revealed poor detection of coherent visual motion [35–38], poor discrimination of the speed of visual motion [39,40], and abnormal perception of illusory visual motion [35,41••] in reading disorder. Similar impairments were also reported in the perception of sound motion [42,43]. The recent literature has also presented three challenges to the magnocellular-deficit hypothesis. The first challenge was to the prediction of the hypothesis that a deficit in the magnocellular system should be associated with reduced sensitivity to low spatial frequencies (coarse images) favored by that system [35,44–46]. In a review of the literature, Skottun [47•] found only inconsistent evidence for this prediction. The second challenge was to a specific version of the magnocellular-deficit hypothesis that proposes that in reading disorder the magnocellular system fails to inhibit a parallel, parvocellular (small-neuron) system during saccades, resulting in a smearing of successive visual images [34]. In contrast to this view, two research groups [41••,48] pointed out that the magnocellular system is actually inhibited by the parvocellular system (see e.g. [49,50]), and that the parvocellular system is itself not affected during a saccade (see e.g. [51]). The third challenge was to the prediction of the magnocellular-deficit hypothesis that motion sensitivity should be impaired in reading disorder. Raymond and Sorenson [52], in a study of motion coherence, reported that the ability to judge the global direction of motion in a brief movie of a random-dot texture was poorer in individuals with a reading disorder than in controls when the motion was presented over multiple frames (as in [35–38]), but not when the motion was presented over only two frames. This is the first evidence that perceptual integration, not motion perception,
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may be impaired in reading disorder. In summary, the recent literature contains new data supporting the visual magnocellular-deficit hypothesis. However, the hypothesis itself is undergoing data-driven refinement.
Individual differences The fifth major development has been that the analysis of individual differences in perceptual performance has provided insights into the multidimensional nature of reading and language disorders. Within the review period, several research groups have scrutinized the large variability in performance seen on perceptual tasks. The results of this work have provided four basic insights into the nature of reading and language disorders. First, deficits in auditory [21•,53] and visual [35,41••,44] perception may be present only in particular subtypes of disability. Second, individual differences in performance on one or more particular auditory [13,15••,19,27], visual [36,54], or both auditory and visual [26,28••] tasks may correlate with, and help account for, individual differences in reading and language skills. Third, there may be both shared and unique contributions to reading disorder from impairments in the auditory and visual modalities [24,26,28••]. Fourth, although there appears to be a genetic component to some abilities impaired in reading disorder, the ability to order nonlinguistic sounds may not be heritable [22]. Thus, analyses of individual differences in performance on nonlinguistic perceptual tasks are leading to a better understanding of the multidimensional character of reading and language disorders.
Remediation Finally, in the sixth major development, two research groups have reported that treatments designed to overcome nonlinguistic perceptual impairments also have assisted in the remediation of reading and language disorders. First, Merzenich, Tallal, and colleagues [18,19,55] tested a treatment designed to improve the ability of children with language disorder to hear brief sounds presented in rapid succession — a skill necessary for speech perception. They trained affected children adaptively on sound-segmentation tasks using computer-presented exercises with both nonlinguistic and linguistic stimuli, and on more general language exercises presented with modified speech. In contrast, control children played video games rather than the adaptive training game, and received equivalent general language training presented with unmodified speech. Trained children improved their performance on tests of sound segmentation, speech perception and language comprehension more than controls. Furthermore, the observed improvements in nonlinguistic segmentation ability and language processing were significantly correlated. The authors conclude that the observed remediation indicates that perceptual learning in affected individuals is essentially normal, and that language disorder may itself result from learning a defective representation of the auditory environment. Second, Stein et al. [56•] tested a treatment designed to steady the vision of a subset of children with reading disorder who have poor binocular control. They provided these
children with monocularly occluded glasses. Children who received occluded glasses achieved binocular control more quickly than did control children who received unoccluded glasses. Furthermore, children who received the occluded glasses and achieved binocular control increased their reading age over the period of use at twice the rate of controls. The authors suggest that the acquisition of binocular stability and the associated improvement in reading skills indicate that monocular occlusion may be a useful treatment for children with reading disorder who have unstable binocular control. More extensive testing of both of these treatments is necessary. Nevertheless, their apparent success provides some evidence for a direct link between perceptual deficits and reading and language performance.
Conclusions The recent developments in the study of nonlinguistic perceptual deficits in reading and language disorders reviewed here show four clear, positive trends. First, there is an expanding investigation of auditory function in these disorders. The literature contains not only new studies of the processing of sequential sounds, but also novel investigations of frequency discrimination and the detection of target sounds in noise. Second, there is a continuing effort to refine two long standing and influential hypotheses that hold that perceptual deficits arise from difficulties in the processing of sequential sounds, or from a deficiency in the visual magnocellular system. Third, there is a growing interest in individual differences that is leading to new insights into the multidimensional nature of reading and language disorders and is helping to resolve conflicting results. Fourth, there is a promising movement toward applying information about perceptual deficits in reading and language disorders to methods of remediation. These trends set the stage for future progress. They indicate that nonlinguistic perceptual testing in reading and language disorders will become increasingly important. Additional normative data should be collected, both to clarify the pervasiveness of impairments in nonlinguistic perception, and to assist in diagnosis. Furthermore, multiple aspects of auditory and visual perception, as well as linguistic performance, should be tested in the same individuals. This approach would both increase our understanding of the multidimensional nature of these perceptual deficits and help to reconcile independent hypotheses about these deficits that have appeared in the auditory and visual literatures. Future research on reading and language disorders should be aimed at identifying and characterizing all nonlinguistic perceptual deficits, and determining the impact of those deficits on reading and language processing.
Acknowledgements This work was supported by Grant R29 DC02997 from the National Institutes of Health: National Institute On Deafness And Other Communication Disorders awarded to BA Wright.
Nonlinguistic perceptual deficits associated with reading and language disorders Wright, Bowen and Zecker
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