EEG evoked responses to repetitive auditory stimulation in normal and Down's syndrome infants

Distinct patterns of repetition suppression in Fragile X syndrome, down syndrome, tuberous sclerosis complex and mutations in SYNGAP1

Sensory processing is the gateway to information processing and more complex processes such as learning. Alterations in sensory processing is a common phenotype of many genetic syndromes associated with intellectual disability (ID). It is currently unknown whether sensory processing alterations converge or diverge on brain responses between syndromes. Here, we compare for the first time four genetic conditions with ID using the same basic sensory learning paradigm. One hundred and five participants, aged between 3 and 30 years old, composing four clinical ID groups and one control group, were recruited: Fragile X syndrome (FXS; n = 14), tuberous sclerosis complex (TSC; n = 9), Down syndrome (DS; n = 19), SYNGAP1 mutations (n = 8) and Neurotypical controls (NT; n = 55)). All groups included female and male participants. Brain responses were recorded using electroencephalography (EEG) during an audio-visual task that involved three repetitions of the pronunciation of the phoneme /a/. Event Related Potentials (ERP) were used to: 1) compare peak-to-peak amplitudes between groups, 2) evaluate the presence of repetition suppression within each group and 3) compare the relative repetition suppression between groups. Our results revealed larger overall amplitudes in FXS. A repetition suppression (RS) pattern was found in the NT group, FXS and DS, suggesting spared repetition suppression in a multimodal task in these two ID syndromes. Interestingly, FXS presented a stronger RS on one peak-to-peak value in comparison with the NT. The results of our study reveal the distinctiveness of ERP and RS brain responses in ID syndromes. Further studies should be conducted to understand the molecular mechanisms involved in these patterns of responses.

Development of Auditory Evoked Responses in Normally Developing Preschool Children and Children with Autism Spectrum Disorder

The cortical responses to auditory stimuli undergo rapid and dramatic changes during the first 3 years of life in normally developing (ND) children, with decreases in latency and changes in amplitude in the primary peaks. However, most previous studies have focused on children >3 years of age. The analysis of data from the early stages of development is challenging because the temporal pattern of the evoked responses changes with age (e.g., additional peaks emerge with increasing age) and peak latency decreases with age. This study used the topography of the auditory evoked magnetic field (AEF) to identify the auditory components in ND children between 6 and 68 months (n = 48). The latencies of the peaks in the AEF produced by a tone burst (ISI 2 ± 0.2 s) during sleep decreased with age, consistent with previous reports in awake children. The peak latencies of the AEFs in ND children and children with autism spectrum disorder (ASD) were compared. Previous studies indicate that the latencies of the initial components of the auditory evoked potential (AEP) and the AEF are delayed in children with ASD when compared to age-matched ND children >4 years of age. We speculated whether the AEF latencies decrease with age in children diagnosed with ASD as in ND children, but with uniformly longer latencies before the age of about 4 years. Contrary to this hypothesis, the peak latencies did not decrease with age in the ASD group (24-62 months, n = 16) during sleep (unlike in the age-matched controls), although the mean latencies were longer in the ASD group as in previous studies. These results are consistent with previous studies indicating delays in auditory latencies, and they indicate a different maturational pattern in ASD children and ND children. Longitudinal studies are needed to confirm whether the AEF latencies diverge with age, starting at around 3 years, in these 2 groups of children.

Reduced habituation of auditory evoked potentials indicate cortical hyper-excitability in Fragile X Syndrome

Sensory hypersensitivities are common, clinically distressing features of Fragile X Syndrome (FXS). Preclinical evidence suggests this abnormality may result from synaptic hyper-excitability in sensory systems. This model predicts reduced sensory habituation to repeated stimulus presentation. Fourteen adolescents and adults with FXS and 15 age-matched controls participated in a modified auditory gating task using trains of 4 identical tones during dense array electroencephalography (EEG). Event-related potential and single trial time-frequency analyses revealed decreased habituation of the N1 event-related potential response in FXS, and increased gamma power coupled with decreases in gamma phase-locking during the early-stimulus registration period. EEG abnormalities in FXS were associated with parent reports of heightened sensory sensitivities and social communication deficits. Reduced habituation and altered gamma power and phase-locking to auditory cues demonstrated here in FXS patients parallels preclinical findings with Fmr1 KO mice. Thus, the EEG abnormalities seen in FXS patients support the model of neocortical hyper-excitability in FXS, and may provide useful translational biomarkers for evaluating novel treatment strategies targeting its neural substrate.

Lennox-Gastaut syndrome with late-onset and prominent reflex seizures in trisomy 21 patients

PURPOSE

Lennox-Gastaut syndrome (LGS) is a severe epileptic condition characterized by multiple seizure types including tonic seizures, slow spike-and-wave discharges on electroencephalography (EEG), and cognitive impairment. LGS can occur in apparently healthy subjects or in patients with preexisting brain damage. The onset peaks between 3 and 5 years of age and the prognosis is usually poor. Herein we report 13 subjects with trisomy 21 who developed LGS.

METHODS

We retrospectively reviewed the clinical and EEG data of consecutive patients with LGS and trisomy 21 referred to five epilepsy centers over the last 30 years.

RESULTS

Data for 13 patients (8 male, 5 female) were collected. The mean age at onset was 9.1 years (range 5-16). The mean age at last follow-up was 23.5 years (range 11-43 years). Seizure onset was after age 8 years in eight (62%) patients and between age 5 and 8 in the other five. In none of the cases did a West syndrome precede the onset of LGS. Nine of 13 patients (69%) had unambiguous reflex seizures, mostly precipitated by sudden unexpected sensory stimulations, usually preceding or accompanying the onset of a full-blown LGS picture. Interictal and ictal EEG findings were typical for LGS. All patients were drug-resistant.

DISCUSSION

Patients with trisomy 21 may present a peculiar LGS, characterized by late onset and high occurrence of reflex seizures. Mechanisms underlying this particular presentation of LGS may include dendritic rarefaction and decreased interneurons, as well as functional abnormalities leading to overall decreased brain inhibition in these patients.

Multimodal inhibition of return effects in adults with and without Down syndrome

Data from a previous study (Welsh & Elliott, 2001) has been reanalyzed to explore inhibition of return (IOR) effects in adults with and without Down syndrome (DS). Participants were required to react and move with either the left or right hand as quickly as possible to 1 of 2 target locations based on either a visual or a verbal cue. Although persons with DS demonstrated a different pattern of information processing capabilities, they demonstrated the same magnitude of IOR across all conditions of presentation as their peers without DS. This pattern of results provides further support for the multimodal and response-based nature of IOR. Moreover, the results indicate that the inhibitory processes that underlie IOR in the average population seem to be functional in persons with DS.

Pain expression and stimulus localisation in individuals with Down's syndrome

BACKGROUND

Individuals with Down's syndrome do not always exhibit signs of distress in reaction to noxious stimuli comparable with the general population. This pilot study was designed to measure the ability of individuals with Down's syndrome to detect and express sensation in comparison with healthy volunteers.

METHODS

In the first test, the latency of pain detection to self-administered cold stimuli on the wrist and on the temple was measured. The second test was designed to assess ability to localise cold stimuli on sites on the hand, on the face, and in the mouth.

FINDINGS

75 control individuals and 26 individuals with Down's syndrome were tested. Individuals with Down's syndrome had significantly longer median latencies than controls: Down's syndrome median (quartiles) 28.7 s (1st 18.0, 3rd 47.6); controls 20.6 s (1st 12.4, 3rd 31.0); p=0.0005. In addition, more individuals with Down's syndrome had difficulties in localising the cold stimulus. The differences in distribution for precise localisation were significant for the hand (Down's syndrome [p<0.0005] 54%; control 99%), the mouth (31%; 84%), and the face (54%; 97%).

INTERPRETATION

Individuals with Down's syndrome are not insensitive to pain. However, they do express pain or discomfort more slowly and less precisely than the general population. This implies that medical teams managing these patients should use pain-control procedures, even in the absence of obvious pain manifestations.

Event-related brain potentials during an extended visual recognition memory task depict delayed development of cerebral inhibitory processes among 6-month-old infants with Down syndrome

Development of cerebral inhibitory processes among individuals with Down syndrome (DS) may be delayed at an early age. In support of this hypothesis, sensory-evoked potentials (EPs) and event-related brain potentials (ERPs) have previously delineated altered habituation to stimuli among infants with DS. The purpose of the current study was to provide extended experience with visual stimuli among 6-month-old infants with and without DS (nDS) to determine if altered ERP and behavioral response decrements would be evident even after repeated presentations of stimuli. An 80/20% oddball paradigm was employed. Infants with DS and nDS were matched according to age and gender. Infants with DS demonstrated significantly larger Nc areas, Nc peak amplitudes, Nc2 areas and, inversely, significantly smaller peak Pb amplitudes when compared to infants nDS. Contrasts of the two study groups were most robust within ERP measures from frontal (Fz) and parietal (Pz) recording sites. Infants with DS also demonstrated a significantly slower decrement of most ERP components with repetitive stimulus experience. Most noteworthy was the observation of little or no decrement of ERP components at Fz among infants with DS. Both infants with DS and nDS demonstrated significantly larger Nc peak amplitudes, Nc areas, Nc2 areas, Pb peak amplitudes and NSW areas to rare stimuli. While significant probability and experiential trends were observed in visual fixation measures across both study groups, there were no significant differences of visual attention between infants with DS or nDS. These data demonstrate the value of ERPs within the study of atypical cognitive development during infancy and support the concept of altered inhibitory processes in the brain of infants with DS.

Auditory evoked potentials in Down's syndrome

Short-, middle- and long-latency auditory evoked potentials (SAEPs, MAEPs and LAEPs) were examined in 12 subjects with Down's syndrome and in 12 age-matched normal subjects. In comparison with the normal subjects, Down subjects showed shorter latencies for SAEP peaks II, III, IV and V (and correspondingly shorter interpeak intervals I-II and I-III) so long as stimulus intensity was at least 45 dB SL. The MAEP peak Na had a longer latency in Down subjects than in normal subjects, but not the Pa latency. In passive oddball experiments for LAEPs, the latencies of all components from N1 to P3 were progressively longer in Down subjects, and the N2-P3 amplitude increased slightly between the first and fourth blocks of stimuli (whereas in the normal subjects it decreased). These alterations in auditory evoked potentials, which may correlate with cerebral alterations in organization and responsiveness responsible for deficient information processing, may constitute an electrophysiological pattern that is characteristic of Down's syndrome.

Age, sex and mental retardation related changes of brainstem auditory evoked potentials in Down's syndrome

Brainstem auditory evoked potentials (BAEPs) were recorded in 51 Down's syndrome (DS) subjects and compared with those of 38 normal controls; the correlations between the BAEP measures and age, sex, and degree of mental retardation were then evaluated. The DS patients showed a significant reduction in wave V latency and amplitude and in I-III, III-V, and I-V interpeak intervals. An age-related shortening of the I-V interpeak interval found in DS patients was interpreted as being a result of changes in central inhibitory/excitatory mechanisms. In both groups, female subjects presented an I-V interval shorter than that of males but this difference was greater in the DS subjects than in the normal population. The DS patients with severe mental retardation showed significantly longer I-V interpeak intervals than those with moderate retardation; this could be due to the presence of additional central nervous system abnormalities.

Reflex seizures are frequent in patients with Down syndrome and epilepsy

In a retrospective study of 30 Down syndrome (DS) patients with epilepsy, we found 6 cases (20%) with reflex seizures. One patient had benign myoclonic epilepsy of infancy with clinical photosensitivity. The other 5 cases had all startle-induced epileptic seizures and a form of symptomatic epilepsy. Three patients had a Lenox-Gastaut syndrome, one had generalized symptomatic epilepsy, and one had partial symptomatic epilepsy (PSE). Reflex epilepsy was also used as a classification category in the PSE case, as most or all seizures were stimulus-related in this patient. Seizures precipitated by stimuli were stereotyped in 4 patients, but 2 patients responded to stimuli with different types of seizures. The actual occurrence of reflex seizures in DS patients with epilepsy is probably underestimated. These cases seem to confirm previous reports showing deficiencies in cortical inhibition in the brain of DS patients.

Short-latency somatosensory evoked potentials following median nerve stimulation in Down's syndrome

Short-latency somatosensory evoked potentials (SEPs) following median nerve stimulation were recorded in 42 patients with Down's syndrome and in 42 age- and sex-matched normal subjects. There were no significant differences between the 2 groups in the absolute peak latencies of N9, N11 and N13 components. However, interpeak latencies, N9-N11, N11-N13 and N9-N13, were prolonged significantly in Down's syndrome. These findings suggest impaired impulse conduction in the proximal part of the brachial plexus, posterior roots and/or posterior column-medial lemniscal pathway. Interpeak latency N13-N20, representing conduction time from cervical cord to sensory cortex, was not significantly different between the 2 groups. Cortical potentials N20 and P25 in the parietal area and P20 and N25 in the frontal area were of significantly larger amplitude in Down's syndrome. P25 had double peaks in 16 of 42 normal subjects, but these were not apparent in any of the patients.

Down syndrome--a disruption of homeostasis

A major question in human genetics concerns the relationship between the extra chromosome material in the Down syndrome (DS) and its effects. It is suggested here that a generalized disruption of evolved genetic balance in cells of affected individuals leads to decreased developmental and physiological buffering against genetic and environmental forces. Examples of consequences in DS of this model of disruption of homeostasis are presented: i) increased variance for metric traits, ii) amplified instability of developmental pathways, iii) reduced precision of physiological homeostatic controls, and iv) generalized increased morbidity. Evolution has selected for interacting systems. When this evolved balance is disrupted, as in autosomal aneuploidy, the organism is generally disrupted. The model emphasizes the role of environment in producing much of the DS phenotype. Traits less buffered than others in the general population are the ones most disturbed in DS and account for much of the DS phenotype.

Down's syndrome, aging, and Alzheimer's disease: a clinical review

This review has been directed towards those aspects of DS which bear upon pathological aging. Clinical dementia in DS has heretofore been studied largely by retrospective methods with variable findings. A prospective study utilizing techniques designed to measure cognitive performance in a poorly verbal, retarded population is badly needed. There is definitive evidence for Alzheimer's disease changes in the brains of DS patients with some suggestion of altered topography compared to the general population. Immunological studies have established a T-cell deficiency in DS that may be linked to precocious aging of thymic-dependent processes. Both antiviral and nonantiviral effects of interferon are accentuated in cell culture systems utilizing DS tissue, presumably as a consequence of the localization of the interferon gene(s) on chromosome 21. Multiple endocrine studies confirm the high frequency of autoimmune disease, an abnormality that may be related to the problems of immune surveillance in DS. Precocious aging has been noted in regards to measures of skin elasticity, fenestration of cardiac valves, and premature cataracts. The 21st chromosome has been implicated in the elevated activity of superoxide dismutase, a finding of significance in regard to potential intracellular damage from increased levels of peroxide. Several studies have suggested a compensatory increase in glutathione peroxidase.

Life-span alterations in visually evoked potentials and inhibitory function

Visually evoked potentials (VEPs) elicited by patterned and unpatterned flashes were recorded from 211 healthy males aged 4-90 years. A measure of similarity between the two kinds of VEPs was obtained by correlating the digital values comprising the two waveforms. Across the life-span, correlations followed a U-shaped curve; patterned and unpatterned flash VEPs were most alike for the youngest and oldest subjects. This age effect, localized to scalp areas overlying visual cortex, is compatible with a concept of reduced inhibitory functioning within the visual systems of the young and the old. At central scalp, patterned and unpatterned flash VEP waveforms were more effectively differentiated by the right hemisphere. This observation agrees that the right hemisphere specializes in analyses of spatial material.

The evolution of visual and auditory evoked potentials in infants with perinatal disorder

The evolutionary changes of evoked potentials (EPs) were studied from the neonatal period up to 1 year of age in 41 infants with various perinatal disorders. Abnormal EPs in the first week of life recovered quickly. In infants with normal outcome, abnormal EPs became normal within a month. In infants with cerebral palsy (CP) or mental retardation (MR), EPs recovered within 2-3 months of age. Infants with more severe neurological damage showed abnormal EPs even beyond 6 months of age. Abnormal EPs beyond 2 weeks of age indicated poor prognosis. As for the wave form of EPs, absent responses or abnormal wave form reflected more severe brain dysfunction. AEPs tended to show more profound abnormalities than VEP. However, some infants with absent AEP in the first week of life had a favorable prognosis. AEPs seemed to be more easily affected by brain dysfunction.

Auditory evoked potential development in early childhood: a longitudinal study

Serial recordings of auditory evoked potentials (AEPs) to clicks were obtained using a vertex-mastoid derivation from 16 normal children during sleep over an age span from near birth to age 3. The AEP components studied were: N0 (38 +/- 10 msec), P1 (79 +/- 24 msec), N1 (109 +/- 39 msec), P2 (186 +/- 35 msec), N2 (409 +/- 97 msec), P3A (554 +/- 116 msec), P3B (757 +/- 121 msec) and P3 (728 +/- 128 msec). Amplitudes and latencies of the components were calculated and regressions of the measures on age were computed for the group as a whole, for each subject and for subsets of the data based on sleep stage, sex, order of stimulus presentation and a rearing/race factor. For the group as a whole the latencies of P1, P2, P3, and P3B decreased with age. The amplitudes of P1N1 and the N2P3 waves increased with age. Most change occurred during the first year of life. In general, the changes with age were also found to hold across all of the factors examined, although individuals varied widely in the degree to which they conformed to the trends found for the data as a whole. The amount contributed by each of the factors mentioned above to the total variance was estimated. The proportions varied for different EP components but, in general, age, sleep state, and subject factors other than rearing/race and sex accounted for most variance. One half to 5/6 of the unexplained variance in AEP latencies and amplitudes (i.e., that not due to age, sleep state, etc.) occurred across rather than within subjects. For both the group as a whole and for individual children, P2 and N2 latencies were found to exhibit the greatest stability across time. The results of the longitudinal study reported here were in good agreement with those of a previous study from this laboratory which utilized a cross-sectional design.