Co-Planar Stereotaxic Atlas of the Human Brain—3-Dimensional Proportional System: An Approach to Cerebral Imaging, J. Talairach, P. Tournoux. Georg Thieme Verlag, New York (1988), 122 pp., 130 figs. DM 268

A multiple case study was conducted in order to assess three leading theories of developmental dyslexia: (i) the phonological theory, (ii) the magnocellular (auditory and visual) theory and (iii) the cerebellar theory. Sixteen dyslexic and 16 control university students were administered a full battery of psychometric, phonological, auditory, visual and cerebellar tests. Individual data reveal that all 16 dyslexics suffer from a phonological deficit, 10 from an auditory deficit, four from a motor deficit and two from a visual magnocellular deficit. Results suggest that a phonological deficit can appear in the absence of any other sensory or motor disorder, and is sufficient to cause a literacy impairment, as demonstrated by five of the dyslexics. Auditory disorders, when present, aggravate the phonological deficit, hence the literacy impairment. However, auditory deficits cannot be characterized simply as rapid auditory processing problems, as would be predicted by the magnocellular theory. Nor are they restricted to speech. Contrary to the cerebellar theory, we find little support for the notion that motor impairments, when found, have a cerebellar origin or reflect an automaticity deficit. Overall, the present data support the phonological theory of dyslexia, while acknowledging the presence of additional sensory and motor disorders in certain individuals.

http://cogprints.org/2350/1/dyslexia02web.pdf

Theories of developmental dyslexia: insights from a multiple case study of dyslexic adults

Digital Coding of Waveforms

The neural correlates of how attended speech is internally represented are described, shedding light on the ‘cocktail party problem’. The 'cocktail-party problem' — the question of what goes on in our brains when we listen selectively for one person's voice while ignoring many others — has puzzled researchers from various disciplines for years. Using electrophysiological recordings from neurosurgery patients listening to two speakers simultaneously, Nima Mesgarani and Edward Chang determine the neural correlates associated with the internal representation of attended speech. They find that the neural responses in the auditory cortex represent the attended voice robustly, almost as if the second voice were not there. With these patterns established, a simple algorithm trained on various speakers predicts which stimulus a subject is attending to, on the basis of the patterns emerging in the secondary auditory cortex. These results suggest that speech representation in the brain reflects not only the acoustic environment, but also the listener's understanding of these signals. As well as shedding light on a long-standing neurobiological problem, this work may give clues as to how automatic speech recognition might be improved to cope with more than one talker. Humans possess a remarkable ability to attend to a single speaker’s voice in a multi-talker background1,2,3. How the auditory system manages to extract intelligible speech under such acoustically complex and adverse listening conditions is not known, and, indeed, it is not clear how attended speech is internally represented4,5. Here, using multi-electrode surface recordings from the cortex of subjects engaged in a listening task with two simultaneous speakers, we demonstrate that population responses in non-primary human auditory cortex encode critical features of attended speech: speech spectrograms reconstructed based on cortical responses to the mixture of speakers reveal the salient spectral and temporal features of the attended speaker, as if subjects were listening to that speaker alone. A simple classifier trained solely on examples of single speakers can decode both attended words and speaker identity. We find that task performance is well predicted by a rapid increase in attention-modulated neural selectivity across both single-electrode and population-level cortical responses. These findings demonstrate that the cortical representation of speech does not merely reflect the external acoustic environment, but instead gives rise to the perceptual aspects relevant for the listener’s intended goal.

/pdf/selective-cortical-representation-of-attended-speaker-in-4khmmzze02.pdf

Selective cortical representation of attended speaker in multi-talker speech perception

Low literacy is termed ‘developmental dyslexia’ when reading is significantly behind that expected from the intelligence quotient (IQ) in the presence of other symptoms —incoordination, left ‐right confusions, poor sequencing —that characterize it as a neurological syndrome. 5‐10% of children, particularly boys, are found to be dyslexic. Reading requires the acquisition of good orthographic skills for recognising the visual form of words which allows one to access their meaning directly. It also requires the development of good phonological skills for sounding out unfamiliar words using knowledge of letter sound conversion rules. In the dyslexic brain, temporoparietal language areas on the two sides are symmetrical without the normal left-sided advantage. Also brain ‘warts’ (ectopias) are found, particularly clustered round the left temporoparietal language areas. The visual magnocellular system is responsible for timing visual events when reading. It therefore signals any visual motion that occurs if unintended movements lead to images moving off the fovea (‘retinal slip’). These signals are then used to bring the eyes back on target. Thus, sensitivity to visual motion seems to help determine how well orthographic skill can develop in both good and bad readers. In dyslexics, the development of the visual magnocellular system is impaired: development of the magnocellular layers of the dyslexic lateral geniculate nucleus (LGN) is abnormal; their motion sensitivity is reduced; many dyslexics show unsteady binocular fixation; hence poor visual localization, particularly on the left side (left neglect). Dyslexics’ binocular instability and visual perceptual instability, therefore, can cause the letters they are trying to read to appear to move around and cross over each other. Hence, blanking one eye (monocular occlusion) can improve reading. Thus, good magnocellular function is essential for high motion sensitivity and stable binocular fixation, hence proper development of orthographic skills. Many dyslexics also have auditory:phonological problems. Distinguishing letter sounds

/pdf/the-magnocellular-theory-of-developmental-dyslexia-5d1wvjkewf.pdf

The magnocellular theory of developmental dyslexia.

http://www.utdallas.edu/~otoole/GRAD_CGS/R7_hari.pdf

Impaired processing of rapid stimulus sequences in dyslexia

We searched for a core mechanism underlying the diverse behavioural and sensorimotor deficits in dyslexic subjects. In psychophysical temporal order judgement and line motion illusion tasks, adult dyslexics processed stimuli in the left visual hemifield significantly (~15 ms) more slowly than normal readers, indicating a left-sided `minineglect'. Furthermore, abrupt stimuli captured attention in both visual hemifields less effectively in dyslexics than in normal readers. These abnormalities could reflect right parietal lobe hypofunction, a consequence of a general magnocellular deficit demonstrated previously. Based on these and previous data, we propose a causal chain which could result in several sensory and cognitive deficits observed in dyslexic subjects.

/pdf/left-minineglect-in-dyslexic-adults-2gxhy69fgo.pdf

Left minineglect in dyslexic adults

Sound Categories Are Represented as Distributed Patterns in the Human Auditory Cortex

It is commonly thought that phonological learning is different in young children compared to adults, possibly due to the speech processing system not yet having reached full native-language specialization. However, the neurocognitive mechanisms of phonological learning in children are poorly understood. We employed magnetoencephalography (MEG) to track cortical correlates of incidental learning of meaningless word forms over two days as 6-8-year-olds overtly repeated them. Native (Finnish) pseudowords were compared with words of foreign sound structure (Korean) to investigate whether the cortical learning effects would be more dependent on previous proficiency in the language rather than maturational factors. Half of the items were encountered four times on the first day and once more on the following day. Incidental learning of these recurring word forms manifested as improved repetition accuracy and a correlated reduction of activation in the right superior temporal cortex, similarly for both languages and on both experimental days, and in contrast to a salient left-hemisphere emphasis previously reported in adults. We propose that children, when learning new word forms in either native or foreign language, are not yet constrained by left-hemispheric segmental processing and established sublexical native-language representations. Instead, they may rely more on supra-segmental contours and prosody.

/pdf/children-show-right-lateralized-effects-of-spoken-word-form-ztkyy0b84e.pdf

Children show right-lateralized effects of spoken word-form learning

Auditory cortical processing of speech-like sounds was studied in 9 dyslexic and 11 normal-reading adults. Noise/square-wave sequences, mimicking transitions from a fricative consonant to a vowel, were presented binaurally once every 1.1 sec and the cortical responses were recorded with a whole-scalp neuromagnetometer. The auditory cortices of both hemispheres were less reactive to acoustical changes in dyslexics than in controls, as was evident from the weaker responses to the noise/square-wave transitions. The results demonstrate that dyslexic adults are deficient in processing acoustic changes presented in rapid succession within tens to hundreds of milliseconds. The observed differences could be related to insufficient triggering of automatic auditory attention, resulting, for instance, from a general deficiency of the magnocellular system.

/pdf/auditory-cortical-responses-to-speech-like-stimuli-in-50xnu7gfmw.pdf

Hanna Renvall

Papers

Impaired processing of rapid stimulus sequences in dyslexia

Left minineglect in dyslexic adults

Sound Categories Are Represented as Distributed Patterns in the Human Auditory Cortex

Children show right-lateralized effects of spoken word-form learning

Auditory Cortical Responses to Speech-Like Stimuli in Dyslexic Adults