Late, not early mismatch responses to changes in frequency are reduced or deviant in children with dyslexia: an event-related potential study

BACKGROUND

Developmental disorders of oral and written language have been linked to deficits in the processing of auditory information. However, findings have been inconsistent, both for behavioural and electrophysiological measures.

METHODS

In this study, we examined event-related potentials (ERPs) in 20 6- to 14-year-old children with developmental dyslexia and 20 age-matched controls, divided into younger (6-11 years, n = 10) and older (11-14 years, n = 10) age bands. We focused on early (mismatch negativity; MMN) and late (late discriminative negativity; LDN) conventional mismatch responses and associated measures derived from time-frequency analysis (inter-trial coherence and event-related spectral perturbation). Responses were elicited using an auditory oddball task, whereby a stream of 1000-Hz standards was interspersed with rare large (1,200 Hz) and small (1,030 Hz) frequency deviants.

RESULTS

Conventional analyses revealed no significant differences between groups in the size of the MMN to either large or small frequency deviants. However, the younger age band of children with dyslexia showed an enhanced inter-trial coherence in the theta frequency band over the time window corresponding to the MMN to small deviants. By contrast, these same children showed a reduced-amplitude LDN for the small deviants relative to their age-matched controls, whilst the older children with dyslexia showed a shorter and less intense period of event-related desynchronization over this time window.

CONCLUSIONS

Initial detection and discrimination of auditory frequency change appears normal or even enhanced in children with dyslexia. Rather, deficits in late-stage auditory processing appear to be a feature of this population.

Children with specific language impairment are not impaired in the acquisition and retention of Pavlovian delay and trace conditioning of the eyeblink response

Three converging lines of evidence have suggested that cerebellar abnormality is implicated in developmental language and literacy problems. First, some brain imaging studies have linked abnormalities in cerebellar grey matter to dyslexia and specific language impairment (SLI). Second, theoretical accounts of both dyslexia and SLI have postulated impairments of procedural learning and automatisation of skills, functions that are known to be mediated by the cerebellum. Third, motor learning has been shown to be abnormal in some studies of both disorders. We assessed the integrity of face related regions of the cerebellum using Pavlovian eyeblink conditioning in 7-11year-old children with SLI. We found no relationship between oral language skills or literacy skills with either delay or trace conditioning in the children. We conclude that this elementary form of associative learning is intact in children with impaired language or literacy development.

Is auditory discrimination mature by middle childhood? A study using time-frequency analysis of mismatch responses from 7 years to adulthood

Behavioural and electrophysiological studies give differing impressions of when auditory discrimination is mature. Ability to discriminate frequency and speech contrasts reaches adult levels only around 12 years of age, yet an electrophysiological index of auditory discrimination, the mismatch negativity (MMN), is reported to be as large in children as in adults. Auditory ERPs were measured in 30 children (7 to 12 years), 23 teenagers (13 to 16 years) and 32 adults (35 to 56 years) in an oddball paradigm with tone or syllable stimuli. For each stimulus type, a standard stimulus (1000 Hz tone or syllable [ba]) occurred on 70% of trials, and one of two deviants (1030 or 1200 Hz tone, or syllables [da] or [bi]) equiprobably on the remaining trials. For the traditional MMN interval of 100–250 ms post-onset, size of mismatch responses increased with age, whereas the opposite trend was seen for an interval from 300 to 550 ms post-onset, corresponding to the late discriminative negativity (LDN). Time-frequency analysis of single trials revealed that the MMN resulted from phase-synchronization of oscillations in the theta (4–7 Hz) range, with greater synchronization in adults than children. Furthermore, the amount of synchronization was significantly correlated with frequency discrimination threshold. These results show that neurophysiological processes underlying auditory discrimination continue to develop through childhood and adolescence. Previous reports of adult-like MMN amplitudes in children may be artefactual results of using peak measurements when comparing groups that differ in variance.

Delayed retention of new word-forms is better in children than adults regardless of language ability: a factorial two-way study

Background

Nonword repetition, the ability to retain and repeat unfamiliar sequences of phonemes is usually impaired in children with specific language impairment (SLI), but it is unclear whether this explains slow language learning. Traditional nonword repetition tests involve a single presentation of nonwords for immediate repetition. Here we considered whether rate of learning of novel phonological sequences was impaired when the same items were presented repeatedly.

Methodology/Principal Findings

Three complex nonwords were each presented for repetition five times in two sessions (A and B) separated by one hour. We studied both adults and children from (i) families with a child with SLI and (ii) families whose children did not have SLI. This gave a 2×2 design with familial SLI as one factor, and age (up to or above 18 years) as the other. Overall, participants from families with SLI were poorer at nonword repetition than their peers from typical-language families, and there was a trend for children with SLI to show less within-session learning than typically developing children. However, between-session retention, measured as the difference between the last trial from session 1 and the first trial of session 2, showed a significant age effect, η² = .139, p = .004, regardless of family SLI status. Adult participants showed a decrease in score from the last trial of session A to the first trial of session B, whereas children maintained their level of performance, regardless of whether or not they had SLI.

Conclusions/Significance

Poor nonword repetition in SLI appears to reflect inadequate encoding of phonological information, rather than problems retaining encoded information. Furthermore, the nonword learning task is consistent with the notion of a sensitive period in language learning: Children show better retention over a delay for new phonological sequences than adults, regardless of overall level of language ability.

Auditory deficit as a consequence rather than endophenotype of specific language impairment: electrophysiological evidence

Background

Are developmental language disorders caused by poor auditory discrimination? This is a popular theory, but behavioural evidence has been inconclusive. Here we studied children with specific language impairment, measuring the brain’s electrophysiological response to sounds in a passive paradigm. We focused on the T-complex, an event-related peak that has different origins and developmental course from the well-known vertex response.

Methods

We analysed auditory event-related potentials to tones and syllables from 16 children and 16 adolescents with specific language impairment who were compared with 32 typically-developing controls, matched for gender, IQ and age.

Results

We replicated prior findings of significant reduction in Ta amplitude for both children and adolescents with specific language impairment, which was particularly marked for syllables. The topography of the T-complex to syllables indicated a less focal response in those with language impairments. To distinguish causal models, we considered correlations between size of the Ta response and measures of language and literacy in parents as well as children. The best-fitting model was one in which auditory deficit was a consequence rather than a cause of difficulties in phonological processing.

Conclusions

The T-complex to syllables has abnormal size and topography in children with specific language impairment, but this is more likely to be a consequence rather than a cause of difficulties in phonological processing.

Co-localisation of abnormal brain structure and function in specific language impairment

We assessed the relationship between brain structure and function in 10 individuals with specific language impairment (SLI), compared to six unaffected siblings, and 16 unrelated control participants with typical language. Voxel-based morphometry indicated that grey matter in the SLI group, relative to controls, was increased in the left inferior frontal cortex and decreased in the right caudate nucleus and superior temporal cortex bilaterally. The unaffected siblings also showed reduced grey matter in the caudate nucleus relative to controls. In an auditory covert naming task, the SLI group showed reduced activation in the left inferior frontal cortex, right putamen, and in the superior temporal cortex bilaterally. Despite spatially coincident structural and functional abnormalities in frontal and temporal areas, the relationships between structure and function in these regions were different. These findings suggest multiple structural and functional abnormalities in SLI that are differently associated with receptive and expressive language processing.

Measurement of mismatch negativity in individuals: a study using single-trial analysis

Mismatch negativity (MMN) is measured by subtracting the averaged response to a set of standard stimuli from the averaged response to rarer deviant stimuli, and taking the amplitude of this difference wave in a given time window. This method is problematic when used to evaluate individuals, because there is no estimate of variance. We describe a new approach, in which independent components with high trial-by-trial variance are first removed. Next, each deviant response has the preceding standard response subtracted, giving a set of single trial difference waves. We illustrate this approach in analysis of MMN to brief tones in 17 adults. The best criterion for MMN combined t-test with an index of inter-trial coherence, giving significant MMN in 14 (82%) of individuals. Single-trial methods can indicate which people show MMN. However, in some clinically normal individuals there was no MMN, despite good behavioral discrimination of stimuli.

Mismatch response to polysyllabic nonwords: a neurophysiological signature of language learning capacity

Background

The ability to repeat polysyllabic nonwords such as “blonterstaping” has frequently been shown to correlate with language learning ability but it is not clear why such a correlation should exist. Three alternative explanations have been offered, stated in terms of differences in: (a) perceptual ability; (b) efficiency of phonological loop functioning; (c) pre-existing vocabulary knowledge and/or articulatory skills. In the present study, we used event-related potentials to assess the contributions from these three factors to explaining individual variation in nonword repetition ability.

Methodology/Principal Findings

59 adults who were subdivided according to whether they were good or poor nonword-repeaters participated. Electrophysiologically measured mismatch responses were recorded to changes in consonants as participants passively listened to a repeating four syllable CV-string. The consonant change could occur in one of four positions along the CV-string and we predicted that: (a) if nonword repetition depended purely on auditory discrimination ability, then reduced mismatch responses to all four consonant changes would be observed in the poor nonword-repeaters, (b) if it depended on encoding or decay of information in a capacity-limited phonological store, then a position specific decrease in mismatch response would be observed, (c) if neither cognitive capacity was involved, then the two groups of participants would provide equivalent mismatch responses. Consistent with our second hypothesis, a position specific difference located on the third syllable was observed in the late discriminative negativity (LDN) window (230–630 ms post-syllable onset).

Conclusions/Significance

Our data thus confirm that people who are poorer at nonword repetition are less efficient in early processing of polysyllabic speech materials, but this impairment is not attributable to deficits in low level auditory discrimination. We conclude by discussing the significance of the observed relationship between LDN amplitude and nonword repetition ability and describe how this relatively little understood ERP component provides a biological window onto processes required for successful language learning.

Duration of auditory sensory memory in parents of children with SLI: a mismatch negativity study

In a previous behavioral study, we showed that parents of children with SLI had a subclinical deficit in phonological short-term memory. Here, we tested the hypothesis that they also have a deficit in nonverbal auditory sensory memory. We measured auditory sensory memory using a paradigm involving an electrophysiological component called the mismatch negativity (MMN). The MMN is a measure of the brain's ability to detect a difference between a frequent standard stimulus (1000 Hz tone) and a rare deviant one (1200 Hz tone). Memory effects were assessed by varying the inter-stimulus interval (ISI) between the standard and deviant. We predicted that parents of children with SLI would have a smaller MMN than parents of typically developing children at a long ISI (3000 ms), but not at a short one (800 ms). This was broadly confirmed. However, individual differences in MMN amplitude did not correlate with measures of phonological short-term memory. Attenuation of MMN amplitude at the longer ISI thus did not provide unambiguous support for the hypothesis of a reduced auditory sensory memory in parents of affected children. We conclude by reviewing possible explanations for the observed group effects.

Cortical evoked potentials due to motion contrast in the blind hemifield

Subcortical visual inputs to motion-selective cortex in primates survive after damage to the primary visual cortex (area 17/V1). Activation of human motion cortex was examined using scalp electrodes in a V1-damaged hemianope. Blind field motion-onset visual evoked potentials (VEPs) shared many of the characteristics associated with sighted vision but were smaller in amplitude and had longer latencies. The representative negative wave (C(II) peak) showed typical dependency on stimulus contrast, its peak latency increased and amplitude decreased as contrast decreased, reflecting the difficulty with which directional information could be detected. VEPs were present at contrasts below 25% when blind field motion was imperceptible even though direction guessing was paradoxically accurate. Subcortical inputs to motion cortex contribute to visual experience but not to conscious perception.

Reversible inactivations of the cerebellum with muscimol prevent the acquisition and extinction of conditioned nictitating membrane responses in the rabbit

Lesions of the cerebellum severely impair the classically conditioned nictitating membrane response (NMR) in rabbits. Thus, the cerebellum is essential for the production of conditioned responses (CRs), either because it is actively involved in NMR conditioning or because damage to it causes motor or other general deficits. To distinguish between these alternatives, the cerebellum may be inactivated during training. Inactivation of the cerebellum during acquisition training might result in the absence of CRs on initial trials of subsequent training without the neuronal blockade. The blockade may have prevented learning but it may have produced other deficits that require time or further training to overcome. This problem can be addressed by inactivating the cerebellum during extinction training. If inactivation during extinction training results in the immediate production of CRs when training is resumed without the blockade, then it may be concluded that extinction learning was prevented by the blockade-the presence of CRs argues against any deficits not associated with learning. We used muscimol to inactivate the cerebellum and test its involvement in acquisition and extinction of NMR conditioning in the same subjects. We injected muscimol close to the interpositus nucleus of the cerebellum 1 h before each of four daily training sessions of delay conditioning. Almost no CRs were produced in these training sessions-there was little or no acquisition of NMR conditioning during cerebellar inactivation. The subjects were then trained for four daily sessions without injections of muscimol. There were no CRs on initial trials of the first session of retraining, but all subjects produced CRs by the end of this session. The subjects then received four daily sessions of extinction training with muscimol inactivation of the nuclei-no CRs were produced. Extinction training then continued for four daily sessions without muscimol inactivation. On the first of these sessions, all subjects immediately produced high levels of CRs. These responses then extinguished within and between sessions with characteristic beginning-of-session spontaneous recovery. There was little or no extinction of NMR conditioning during cerebellar inactivation. After inactivation, the muscimol- inactivated subjects went on to acquire and extinguish NM responses at rates similar to those of appropriate controls. We conclude that cerebellar circuitry is essential for, and actively engaged in, both acquisition and extinction of this simple form of motor learning.

Cerebellar cortex and eyeblink conditioning: a reexamination

We examined the effects of cerebellar cortical lesions upon conditioned nictitating membrane responses in rabbits. Using extended postoperative conditioning and unpaired presentations of the conditioned stimuli (CSs), we confirmed that combined lesions of lobules HVI and ansiform lobe abolished conditioned responses (CRs) established to light and white noise CSs. Extended retraining enabled some slight recovery of CR frequencies. Less extensive cortical lesions produced initial abolition of CRs but allowed more complete recoveries. Although CR frequencies and amplitudes were profoundly depressed by cortical lesions, unconditioned response (UR) amplitudes to periorbital electrical stimulation were enhanced. The dissociation of lesion effects upon conditioned and unconditioned responses is consistent with the suggestion that cerebellar cortical mechanisms are important for the learning and execution of eyeblink conditioning.

Visual projections to the pontine nuclei in the rabbit: orthograde and retrograde tracing studies with WGA-HRP

Visual projections to the pontine nuclei in the rabbit were examined by means of both orthograde and retrograde tracing of WGA-HRP. The tecto-pontine projection was examined following microinjections of WGA-HRP in the right superior colliculus. The projection to the pontine nuclei is strictly ipsilateral and terminates at middle and caudal levels of the pons. The projection is absent in rostral pontine nuclei. The strongest projection is to the dorsal border of the dorsolateral pontine nuclei and is the only projection seen when the primary injection site is confined to superficial laminae. When the primary injection site also includes intermediate and deep laminae, patches of labelled terminals are also seen within dorsolateral, lateral, peduncular, paramedian, and ventral pontine nuclei as well as in the contralateral nucleus reticularis tegmenti pontis. The striate corticopontine projection was also examined with orthograde tracing of WGA-HRP. The striate corticopontine projection is ipsilateral. Most labelled terminals were seen in dorsolateral and lateral pontine nuclei throughout the rostral half of pons with some additional terminal labelling in paramedian and peduncular nuclei. Labelled terminals were also seen in ventral pontine nuclei throughout the middle and caudal levels of the pons. In a retrograde tracing study, visual projections to the pontine nuclei were examined following microinjections of WGA-HRP into the pontine nuclei. Labelled cells were seen ipsilaterally in superficial and deep laminae of the superior colliculus and in layer V of striate and surrounding occipital cortex. The pontine nuclei also receive ipsilateral projections from the ventral lateral geniculate, the nucleus of the optic tract, anterior and posterior pretectal nuclei, and the dorsal and medial terminal nuclei of the accessory optic system. These pathways are potential sources of visual input to the cerebellum.

Hippocampal lesions and trace conditioning in the rabbit

Trace conditioning of the nictitating membrane response (NMR) was examined in rabbits with lesions of the dorsal hippocampus and fimbria-fornix. Using a white noise conditional stimulus and an electrical shock unconditional stimulus, the number and amplitude of conditional responses (CRs) was similar in hippocampus-lesioned and control subjects. At some stages of conditioning, the latencies of CRs from hippocampus-lesioned subjects were slightly shorter than those of the controls. We suggest that the hippocampus is not essential for trace conditioning but may exert a modulatory influence on the timing of the CR.

Classical conditioning of the nictitating membrane response of the rabbit. IV. Lesions of the inferior olive

The nictitating membrane response (NMR) of 15 rabbits was conditioned to light and white noise conditional stimuli (CSs) using a periorbital shock unconditional stimulus (US). Unilateral lesions of the inferior olive were then made. Lesions restricted to the medial parts of rostral dorsal accessory olive (DAO) and principal olive (PO) abolished conditioning and prevented subsequent acquisition on either side. Unconditional responses to the US were intact. Lesions in all other parts of the olive did not impair conditioning. The effective lesions were located in that part of the olive which supplies somatosensory information from the face to cerebellar lobule HVI. Lobule HVI is also essential for NMR conditioning. We suggest that this region of the inferior olive is part of a circuit which provides US information to the cerebellar cortex during NMR conditioning.

Classical conditioning of the nictitating membrane response of the rabbit. III. Connections of cerebellar lobule HVI

We report the connections of cerebellar cortical lobule HVI in the rabbit. We have studied the anterograde and retrograde transport of wheatgerm-agglutinated horseradish peroxidase (WGA-HRP) following its injection into HVI to reveal efferent and afferent connections. All of the cases showed strong anterograde transport to the anterior interpositus nucleus (AIP) - indicating that this is the major efferent target of HVI. Retrogradely labelled cells were found in the inferior olivary, spinal trigeminal, lateral reticular, inferior vestibular and pontine nuclei. Within the olive, the medial part of the rostral dorsal accessory olive (DAO) and the adjacent medial part of the principal olive (PO) were consistently labelled in all cases. This area is known to receive somatosensory information from the face and neck. There was no projection to the hemispheral part of lobule VI from visual parts of the olive within the dorsal cap and medial parts of the medial accessory olive. Likely sources of visual and auditory information to HVI are the dorsolateral basilar pontine nuclei and nucleus reticularis tegmenti pontis, which were densely labelled in all cases. These anatomical findings are consistent with the suggestion that, during NMR conditioning, information related to the periorbital shock unconditional stimulus (US) may be provided by climbing fibres to HVI and light and white noise conditional stimulus (CS) information may be supplied by pontine mossy fibres.

Classical conditioning of the nictitating membrane response of the rabbit. II. Lesions of the cerebellar cortex

The nictitating membrane response (NMR) of 20 rabbits was conditioned to light and white noise conditional stimuli (CSs) using a periorbital shock unconditional stimulus (US). Unilateral lesions of the cerebellar cortex, sparing the underlying deep nuclei, were then made. Small lesions of cerebellar cortical lobule HVI abolished conditioning on the side of the lesion to both CSs leaving unconditional responses to the US intact. Larger lesions of the posterior lobe which spared HVI did not impair NMR conditioning. We conclude that cerebellar lobule HVI is essential for NMR conditioning in the rabbit. Degeneration following critical lesions of HVI was seen in a restricted region of the inferior olive - the medial part of the dorsal accessory olive and the adjoining medial part of the dorsal leaf of the principal olive. This region of the olive provides somatosensory information from the face to HVI. We suggest that HVI receives information related to the US via climbing fibres from the olive and CS information via mossy fibres from the pontine nuclei. The critical changes underlying NMR conditioning may be the association of these two inputs at the Purkinje cells of cortical lobule HVI.

Classical conditioning of the nictitating membrane response of the rabbit. I. Lesions of the cerebellar nuclei

The classically conditioned nictitating membrane response (NMR) of the rabbit, a simple form of associative motor learning, is crucially dependent upon the cerebellum. Discrete unilateral lesions of the cerebellar nuclei were made in 20 rabbits. Lesions of the anterior interpositus nucleus (IA) abolished NMR conditioning to light and white noise stimuli on the side of the lesion without affecting unconditional responses. Lesions of the posterior interpositus nucleus, fastigial and dentate nuclei were without effect upon NMR conditioning.

Discrete lesions of the cerebellar cortex abolish the classically conditioned nictitating membrane response of the rabbit

The classically conditioned nictitating membrane response (NMR) of the rabbit is useful for analyses of brain structures and pathways involved in mammalian associative learning. Lesions of the entire cerebellum, its nuclei or its efferent pathway abolish conditioned NMR learning and prevent its reacquisition. Until now it has not been clear whether the cerebellar cortex is an essential part of the circuitry for NMR conditioning. Here we report that small lesions of the cerebellar cortex in the hemispheral portion of lobule VI, with sparing of the underlying nuclei, abolish the ipsilateral conditioned NMR and prevent it from being reacquired.

Trace conditioning of the nictitating membrane response in decorticate rabbits

Rabbits with substantial neocortical lesions were given trace conditioning of the nictitating membrane response over a range of interstimulus intervals (ISIs). Unlike hippocampal lesioned animals, which show deficits in such conditioning, the decorticates were not impaired and they gave greater numbers of conditioned responses at longer ISIs than did sham-operated controls. These results suggest that a neocortical-entorhinal input to the hippocampus may not be crucial for trace conditioning.

Retention of conditioned inhibition of the nictitating membrane response in decorticate rabbits

Rabbits were trained on a Pavlovian conditioned inhibition (CI) task using light as the reinforced conditional stimulus (CS) and the same light compounded with a tone as the non-reinforced CS. The conditional response was the nictitating membrane response (NMR). The subjects then received an extensive neocortical lesion or a sham operation. After 9 weeks postoperative recovery, the animals were retrained on the CI task. Sham operated control animals showed immediate and high levels of CI retention but the decorticates showed a profound initial loss, showing that there is a cortical influence in CI. Reacquisition of CI by the decorticates was rapid and showed considerable savings over de novo acquisition. This supports our earlier finding that the neocortex is not essential for the acquisition of CI.