The development of visual- and auditory processing in Rett syndrome: An ERP study

Brain & Development 28 (26) 487 494 www.elsevier.com/locate/braindev The development of visual- and auditory processing in Rett syndrome: An ERP study Johannes E.A. Stauder a, *, Eric E.J. Smeets b,c,d, Saskia G.M. van Mil a, Leopold G.M. Curfs a,b,c a Department of Psychology, Section Neurocognition, Maastricht University, Maastricht, The Netherlands b Department of Clinical Genetics, Academic Hospital Maastricht, Maastricht, The Netherlands c Research Institute Growth and Development (GROW), Maastricht University, Maastricht, The Netherlands d Center of Human Genetics, University Hospital Gasthuisberg, Leuven, Belgium Received 26 October 24; received in revised form 23 January 26; accepted 4 February 26 Abstract Rett syndrome is a neurodevelopmental disorder that occurs almost exclusively in females. It is characterized by a progressive loss of intellectual functioning and motor skills, and the development of stereotypic hand movements, that occur after a period of normal development. Event-related potentials were recorded to a passive auditory- and visual oddball task in 17 females with Rett syndrome aged between 2 and 6 years, and age-matched controls. Overall the participants with Rett syndrome had longer ERP latencies and smaller ERP amplitudes than the Control group suggesting slowed information processing and reduced brain activation. The Rett groups also failed to show typical developmental changes in event-related brain activity and revealed a marked decline in ERP task modulation with increasing age. Ó 26 Elsevier B.V. All rights reserved. Keywords: Rett syndrome; Passive oddball task; Visual and auditory processing; ERP; Life span development 1. Introduction * Corresponding author. Tel.: +31 43 3881933; fax: +31 43 388412. E-mail address: h.stauder@psychology.unimaas.nl (J.E.A. Stauder). Rett syndrome is characterized by regression of language and psychomotor development at a certain age period, autistic behavior, dementia, seizures, and the loss of purposeful use of the hands [1 3]. Features like breathing dysfunction, electroencephalographic abnormalities, spasticity, peripheral vasomotor disturbance, scoliosis, and growth retardation are common in Rett syndrome, although not all persons with Rett share these features to the same extent [4]. The syndrome occurs almost exclusively in females and has a prevalence of 1 in 1 1, live born girls []. With the present clinical knowledge and molecular diagnosis the incidence is probably much higher. Rett syndrome is usually sporadic and only 1% of the reported cases are familial. In most cases, Rett syndrome is caused by mutations in MECP2, the gene encodes the X-linked methyl-cpg-binding protein 2 (MeCP2) located at Xq28 [6,7]. 1.1. Event-related brain activity In recording electrical brain activity during task performance it is possible to extract event-related potentials (ERPs) that are manifestations of brain activities that occur in preparation for, or in response to, discrete events that can be internal or external to the subject [8]. An ERP consists of amplitude changes over time characterized by a characteristic polarity (positive P 387-764/$ - see front matter Ó 26 Elsevier B.V. All rights reserved. doi:1.116/j.braindev.26.2.11

488 J.E.A. Stauder et al. / Brain & Development 28 (26) 487 494 or negative N peaks), latency, and scalp distribution. The timing of a cognitive process modulated by a certain task can be inferred from the latency of the corresponding peak, and the amplitude of the peak refers to the degree of activation or strength of the process [9]. An early ERP component like the P1 reflects visual encoding and decreases in amplitude with age during childhood [1]. The later occuring Novelty P3 is elicited by unexpected novel stimuli and is thought to reflect involuntary switching of attention to deviant events [11]. In children the Novelty ERP is characterized by a negative Nc wave and a positive Pc wave. The transition from the childhood Nc/Pc complex to the adult Novelty P3 is not complete until after the mid-teens [12,13]. To date only two reports addressed information processing in Rett syndrome using ERPs. In using auditoryand visual evoked potential paradigms Bader et al. [14] reported P3 activity in nine girls with Rett suggesting remaining perceptual and discrimination properties. McCulloch et al. [1] investigated 11 girls and young women (age range 24 years) with Rett syndrome of which three subjects participated in a face discrimination paradigm, and four subjects in a face recognition paradigm. Unlike the study by Bader et al. [14] they were not able to identify any ERP components in their Rett participants. Other developmental disorders have been studied more extensively with ERPs. Ikeda et al. [16] examined a group of mentally retarded adults and controls with an auditory oddball task. They found a dysfunction of automatic auditory processing in mentally retarded persons but the Novelty P3 was similar in both groups. Courchesne found that the Nc response is entirely absent or strikingly aberrant in infantile autism [13]. Karrer et al. [17] studied 6-month-old infants with Down syndrome during a visual recognition memory task. They found that infants with Down syndrome had larger Nc amplitudes as compared to infants without Down syndrome [18]. In adults with Prader Willi syndrome (PWS) and normal controls ERPs were recorded to a visual- and auditory oddball task [19]. The PWS group revealed an abnormal deflation of the P3 component to the visual, and even to a larger extent to the auditory oddball task. The present ERP study presents a life span approach to study the development of auditory- and visual processing using passive oddball paradigms in 2- to 6-year-old females with Rett syndrome and agematched controls. 2. Materials and methods 2.1. Participants The females with Rett syndrome (n = 17) were aged between 2 and 6 years with a mean age of 17. years. The female Control group (n = 18) was aged between 4 and 6 years with a mean age of 1.7 years. The persons with Rett syndrome were tested at the respective clinics and the controls were tested at home or at the University of Maastricht. The participants with Rett were selected by a clinician (E.J. Smeets, second author) for excluding seizures, and severe irregular breathing that could interfere with the EEG recordings. During presentation of the passive oddball tasks one of the experimenters continuously observed the participants for assuring attention to the screen (visual task). In case the participants moved away from the screen, made sudden movements, or closed the eyes, the recording was paused. Because there was too much noise in the EEG signal after recording, one girl with Rett syndrome was excluded from the analyses for both the auditory- and visual task, one adult with Rett for the auditory task and another adult with Rett for the visual task. In order to assess developmental ERP changes the Rett and Control group were split up in three age groups of similar size and age range that are presented in Table 1 and depicted in Figs. 1 and 2. 2.2. Experimental tasks During EEG recording a visual- and an auditory oddball task was presented. Stimulus presentation and behavioral data acquisition were automatically controlled by Experimental Run Time System (ERTS) software. The visual stimuli were presented on a 1 in. CRT monitor and the participants sat cm from the monitor. The auditory stimuli were presented via headphones. In both passive oddball tasks there was no instruction and for the visual task the experimenter checked constantly whether the participant focused at the center of the screen. Both tasks were preceded by a test session of 1 items in order to get familiarized to the stimulus presentation. The visual oddball stimuli consisted of a green circle on a black background or a red square on a black background. There were also novel stimuli that consisted of unique and random configurations of small colored blocks on a white background. The visual stimuli were presented in two sessions during 1 ms with an inter-stimulus interval of 1 ms. Each session contained a randomized sequence of 8 frequent items (green circle), 2 rare items (red square), and 1 novels. The auditory oddball task presented two pure sinus tones with a clearly distinguishable pitch of, respectively, 44 and 88 Hz (7 db, SPL). The tones were presented in two sessions of each eighty 44 Hz tones and twenty 88 Hz tones. The tones were randomly presented during 3 ms with an inter-stimulus interval of 11 ms.

J.E.A. Stauder et al. / Brain & Development 28 (26) 487 494 489 Table 1 The number of participants (N), mean age in years, and range in years of the three age groups (I, II, and III) for the participants with Rett syndrome and normal Controls Rett syndrome Control Group I Group II Group III Group I Group II Group III N 4 6 4 7 7 Mean age 4.4 8.7 31.1 4.4 9. 28.8 Age range 2.1.7 6.1 11. 1. 6.1 4..3 6.1 11.9 1.1.6 Auditory task Control Rett Age group I 1 1 - - -1-1 N2 Age group II 1 1 - - -1-1 Age group III 1 P3 1 - - FREQUENT RARE -1-1 Fig. 1. ERPs to the auditory task at electrode location Cz. The horizontal axis indicates the time after stimulus onset in milliseconds (ms) and the vertical axis the ERP amplitude in microvolts (lv). The left column depicts the Control group and the right column the Rett group. Age group I is depicted at the top row, Age group II in the middle row, and Age group III at the bottom row. The solid line represents the frequent stimulus and the dotted line the rare stimulus. The N2 and P2 components that are included in the analyses are indicated with an arrow. 2.3. Procedure At arrival of the participant the procedure was explained while the electro-cap and EOG electrodes were installed. During task performance two experimenters stayed in the same room as the participant. One observed the participant and the other monitored the EEG signal on the laptop computer. The visual task

49 J.E.A. Stauder et al. / Brain & Development 28 (26) 487 494 Visual task Control Rett Age group I 2 1 - P1 2 1 - -1-1 -2-2 Age group II 2 1 Nc 2 1 -. - -1-1 -2-2 Age group III 2 1 Novelty P3 2 1 - - -1 NOVEL FREQUENT RARE -2-1 -2 Fig. 2. ERPs to the visual task at electrode location Cz. The horizontal axis indicates the time after stimulus onset in milliseconds (ms) and the vertical axis the ERP amplitude in microvolts (lv). The left column depicts the Control group and the right column the Rett group. Age group I is depicted at the top row, Age group II in the middle row, and Age group III at the bottom row. The solid line represents the novel stimulus, the dashed line the frequent stimulus and the dotted line the rare stimulus. The P1, Nc, and Novelty P3 components that are included in the analyses are indicated with an arrow. was always presented first because focusing on the screen implies more effort than passive listening to tone sequences. Between sessions there were short breaks and the entire recording took about one and a half hour. 2.4. EEG recordings The neuroscan NuAmps portable DC amplifiers recorded the EEG and Electro Oculogram (EOG) signals. The electro-cap (Electro-cap inc., USA) comprised 31 tin electrodes at the standard locations: Fz, Cz, Pz, Oz, F7, F8, F3, F4, C3, C4, T3, T4, T, T6, P3, P4, O1, O2, Fp1, Fp2, FT7, FT8, FC3, FC4, TP7, TP8, CP3, CP4, FCz, and CPz. The ground was located halfway between Fz FCz, and the reference electrode was placed at the left mastoid. For the vertical EOG, there were electrodes above and beneath the left eye, and for the horizontal EOG the electrodes were placed at the outer canthi of the eyes. Electrode impedance was kept below kx, and below 8 kx for the EOG electrodes. The impedance was measured with a portable impedance meter (F-EZM4A, Grass Inc. USA). Prior to EEG recording impedance was also checked with the impedance meter of the NuAmps amplifiers. The

J.E.A. Stauder et al. / Brain & Development 28 (26) 487 494 491 Table 2 The time windows (in milliseconds, ms) used for determining the peak measures of the ERP components (P1, N2, NC/Novelty P3, and P3) and age groups (I, II, and III) of the participants with Rett and controls Visual task Auditory task Peak P1 Peak Nc/ Novelty P3 Area Peak N2 Peak P3 Area Rett Group I 2 ms positive 4 9 ms positive 4 9 ms 4 7 ms negative X 4 9 ms Group II 1 2 ms positive 4 8 ms positive 4 9 ms 4 7 ms negative X 4 9 ms Group III 1 ms positive 7 ms positive 4 9 ms 3 ms negative 4 6 ms positive 3 6 ms Control Group I 3 ms positive 2 6 ms negative 2 6 ms 3 67 ms negative X 38 67 ms Group II 3 ms positive 2 6 ms negative 2 6 ms 3 67 ms negative X 38 67 ms Group III 2 ms positive 2 6 ms positive 2 6 ms 3 ms negative 4 6 ms positive 3 6 ms continuous EEG acquisition was controlled by Neuroscan 4.2 software with a band-pass filter of. 3 Hz. Presentation of the stimuli was controlled by the Experimental Run Time System (ERTS, Version 3.18, BeriSoft Cooperation, Germany). 2.. EEG analysis EOG artifacts were removed from the EEG by means of linear regression (Neuroscan 4.2). Next the EEG data were epoched and baseline corrected using a 1 ms prestimulus and 1 ms post-stimulus period. EEG epochs containing data of which the absolute voltage in any channel exceeded ±3 lv for the Rett patients and ±2 lv for the controls were omitted from the ERP averages for each stimulus condition. ERP peaks were scored within a given time window resulting in an (peak-) amplitude and (peak-) latency after stimulus onset. The respective windows for each ERP component are presented in Table 2. 2.6. Statistical analyses To reduce the number of analyses only the midline electrodes (Fz, FCz, Cz, CPz, Pz, and Oz) were included. These EEG channels also revealed the most apparent task modulations effects. A regression analysis across all age groups was performed resulting in a Pearson s Product Moment Correlation Coefficient for each electrode and task condition. All latency and amplitude data were included, except for the Nc component, that was only present in the two youngest groups, and for the Novelty P3 and auditory P3 that were only present in the adult group. The regression analyses were performed using Statistical Package for Social Sciences (SPSS, version 11) with two-tailed significance levels at p 6.. 3. Results Fig. 1 depicts the ERPs to the auditory tasks of the frequent (solid line) and rare (dotted line) stimulus condition for the three age groups (I, II, and III). The N2 and P3 ERP components that are included in the analyses are indicated by arrows. The Control groups showed typical developmental changes for the N2 with decreasing amplitudes and shorter peak latencies with increasing age. The P3 component is only visible in the adult Control group. In the Rett group the changes with development were less obvious and the morphology of the ERP waveforms in the Rett groups differed considerably from the Control groups for all age groups. Persons with Rett syndrome also revealed reduced ERP differences between task conditions (dotted vs. solid lines). For the visual task (Fig. 2) there were also marked differences between the Rett- and the Control group, especially for the ERP components P1, Nc, and Novelty P3 (arrows). The P1 clearly changed with age and was only present in the Control groups I, II, and III. The Nc to novel stimulation is typically seen in children (group I + II) and changed to the Novelty P3 (III) in adults. The youngest children with Rett showed a strongly reduced Nc component for the novel task condition, but the older Rett groups (II and III) failed to show any typical activation to the novel stimulation. Thus both the auditory and visual modality revealed obvious deviant brain activation in the participants with Rett. The regression analyses demonstrated significant developmental changes for most ERP measures in the Control group, but only for auditory N2 latency in the group with Rett syndrome. 3.1. Auditory N2-latency N2 latency decreased with age for the Control group and the Rett group. The Control group showed significant effects at the entire midline for both the rare and the frequent auditory conditions. The Rett group showed significant effects at Fz, FCz, Cz, and Pz for the rare condition and on FCz, Cz, CPz, Pz, and Oz for the frequent condition (also see Table 3).

492 J.E.A. Stauder et al. / Brain & Development 28 (26) 487 494 Table 3 Correlations of the auditory N2-latency with age for the Control and Rett group at the midline electrodes Fz, FCz, Cz, CPz, Pz and Oz Fz FCz Cz CPz Pz Oz corr. p-value corr. p-value corr. p-value corr. p-value corr. p-value corr. p-value Control n =18 Rare.714.1 *.692.1 *.639.4 *.633. *.83.11 *.34.22 * Frequent.674.2 *.664.3 *.63.3 *.6.3 *.66.3 *.668.2 * Rett n =1 Rare.683. *.69.4 *.49.34 *.41.92.7.31 *.369.176 Frequent.8.3.21.46 *.618.14 *.6.28 *.638.1 *.8.24 * Significant correlations (p <.) are indicated by an asterisks (*). Table 4 Correlations of the auditory N2-amplitude with age for the Control and Rett group at the midline electrodes Fz, FCz, Cz, CPz, Pz and Oz Fz FCz Cz CPz Pz Oz corr. p-value corr. p-value corr. p-value corr. p-value corr. p-value corr. p-value Control n =18 Rare.49.37 *.49.39 *.18.28 *.37.21 *.26.2 *.32.23 * Frequent.6.3 *.6.3 *.613.7 *.79.12 *.3.17 *.11.3 * Rett n =1 Rare.33.23.297.282.2.463.422.117.147.6.66.81 Frequent.416.123.372.173.293.29.427.112.327.234.232.46 Significant correlations (p <.) are indicated by an asterisks (*). 3.2. Auditory N2-amplitude The Control group showed a decrease in amplitude with age. This means that N2 amplitude became less negative with increasing age. This trend was significant for the rare and the frequent condition at the entire midline. Although the ERPs in Fig. 1 might suggest a similar trend in the Rett groups, their correlations were much smaller and failed to reach significance (also see Table 4). 3.3. Auditory P3-amplitude Only the oldest Control group (III) revealed a clear P3 component. For the adult Control group (n =7) there was no age trend for the rare condition. However, for the frequent condition the amplitude increased with growing age. Significant correlations with age were found at Fz (r =.77), FCz (r =.81), Cz (r =.81), CPz (r =.83), and Pz (r =.84). For the Rett group (n = 6) no significant correlations with age were found. 3.4. Visual P1-latency For the Control group P1 latency increased with age and the rare stimulus showed a significant positive correlation between age and latency at Oz (r =.61, p <.1). No such trend was found for the Rett group. 3.. Visual P1-amplitude P1 amplitude got smaller with age for the Control group. There was a significant negative correlation between age and amplitude for the novel condition at Oz (r =.6, p <.). No developmental trend was found for the Rett group. 3.6. Visual Nc-amplitude The Control group (n = 11) showed a decrease in amplitude with age, which means that the amplitude became less negative with increasing age. This correlation was significant at Cz (r =.76), CPz (r =.76), and Pz (r =.7). Again there were no significant correlations for the Rett group (n = 9). 4. Discussion The present study investigated developmental changes in the auditory N2 and P3 components and the visual P1, Nc, and novelty P3 components in 17 females with Rett syndrome between 2 and 6 years of age, and 18 age-matched controls with normal development. Overall the Control group showed shorter ERP latencies and larger ERP amplitudes as compared to the Rett group, suggesting that the persons with Rett needed more time to process information and had reduced and/or less synchronized processing in response to

J.E.A. Stauder et al. / Brain & Development 28 (26) 487 494 493 visual and auditory stimulation. The regression analyses with age revealed significant and typical developmental changes for the ERP measures in the Control group. The Rett group only showed a significant developmental change for N2 latency. For the visual P1 component there was an increase in ERP latency and a decrease in ERP amplitude with age for the Control group that was comparable to findings reported by Batty and Taylor [1]. The Rett group did not reveal such trends. This suggests that for Rett females early visual processing or encoding does not become easier with age and that the process of visual encoding is less efficient the Rett group. The auditory N2 is thought to reflect the detection of some type of mismatch between stimulus features or between the stimulus and some previously formed template and is sensitive to stimulus probability and discrimination difficulty [8]. Batty and Taylor [1] found that N2 latencies and amplitudes decreased during childhood and reached adult levels at the age of 9 years. In the present study N2 latencies decrease with age for both the Control and the Rett group. This was the only significant change with age for the Rett group and suggests that the detection of a mismatch did get faster with increasing age for children with Rett, similar to the children in the Control group. For the Control group N2 amplitude becomes less negative with increasing age. Such an amplitude trend with age was not found in the Rett group, suggesting that in Rett syndrome the detection of a mismatch between stimuli does not become more efficient with age. The visual Nc component is seen as a sign of enhanced auditory and visual selective attention to surprising, interesting, or important stimuli [13] and the novelty P3 is thought to reflect the involuntary switching of attention to deviant events, or distraction from the primary task [11]. Children do not show a positive novelty P3 wave when confronted with novel stimuli, but instead show negative Nc wave occurring at about 4 ms after stimulus onset. The amplitude of the Nc wave is largest in young children and sharply decreases with age to finally emerge as an adult novelty P3. Courchesne [12] speculated that this agerelated morphological change in ERP waveform implies that children and adults process novel information differently. In accordance with Courchesne s developmental study [12] did the present Control group show a typical decrease in N2 amplitude with age. Children with Rett show a positive component that looks more like a delayed adult Novelty P3, instead of the typical Nc in the Control group. This is in contrast with research on infants with Down syndrome where Karrer et al. [17] reported larger Nc peak amplitudes as compared to normal infants. Children with autism, on the other hand, do show an absent or strikingly aberrant Nc response [13] that is similar to the findings for Rett children in the present study. Finally the adults with Rett do not reveal any novelty P3-like activity to novel stimuli (see Fig. 2), instead they show a very early positive peak before 1 ms that is unlike any known ERP modulation to novel stimulation. These markedly aberrant ERPs to novel stimulation in the Rett group points at a disturbed processing of novel and unexpected stimulation that might constitute a key abnormality in deviant information processing in Rett syndrome. The auditory P3 could only be identified in the oldest Control group. Nevertheless, there was a significant correlation for P3 amplitude with age where the amplitude of the frequent stimulus increases with age in adulthood. This suggests that when people get older it becomes harder to process auditory stimuli and therefore need more activation. The Rett group did not show such trend because of the absence of any identifiable P3-like activity. This difficulty of detecting any major ERP components in Rett patients was also reported by McCulloch et al. [1]. The present study confirms this difficulty for the adults with Rett, but children with Rett clearly do reveal some ERP components (see Figs. 1 and 2). The failure to identify any ERP components by the McCulloch et al. [1] study was thus probably due to the age of their participants and, more important, because of a higher task complexity in their visual face discrimination/recognition paradigm. In using a simple oddball paradigm like in the present study some ERP activity can be identified in adults with Rett, especially in the auditory oddball task. In the visual task only the ERP to the novel condition shows a clear positive peak around 1 ms in the adult Rett group, albeit completely dissimilar from the adult controls. Taken together there are major differences in ERP found between the Rett groups and the controls at any age level. In general, the Rett groups show slower, reduced, and more irregular information processing in both the visual- and auditory modality as compared to the Control group. The finding that the ERP traces the Rett group get more irregular and show less differentiation between task conditions with increasing age suggests that there may be no leveling or improvement in information processing with increasing age. It should be noted that the exclusion of persons with Rett with seizures and severe irregular breathing, in order to obtain good EEG recordings, might obscure the generalization of the present results to overall Rett population. The small number of participants in each age group did not allow for more profound statistical analyses of the data and therefore only descriptive statistics were used. Nevertheless, the present study points at remarkable particularities of auditory and visual information processing in the development of Rett syndrome that may guide future research.

494 J.E.A. Stauder et al. / Brain & Development 28 (26) 487 494 Acknowledgments We thank the girls, adults, and their parents for their cooperation in this study. References [1] Hagberg B, Aicardi J, Dias K, Ramos O. A progressive syndrome of autism, dementia, ataxia and loss of purposeful hand use in girls: Rett s syndrome: report of 3 cases. Ann Neurol 1983;14:471 9. [2] Hagberg B. Clinical manifestations and stages of Rett syndrome. Ment Retard Dev Disabil Res Rev 22;8:61. [3] Rett A. Uber ein cerebral-atrophisches Syndrom bei Hyperammonämie. Wien Med Wochenschr 1966;116:723 6. [4] Ellaway C, Christodoulou I. Rett syndrome: clinical characteristics and recent genetic advances. Disabil Rehabil 21;23:98 16. [] Kerr AM, Engerstrom IW. The clinical background to the Rett disorder. In: Kerr AM, Engerstrom IW, editors. Rett disorder and the developing brain. Oxford: Oxford University Press; 21. p. 1 26. [6] Schwartzman JS, Bernardino A, Nishimura A, Gomes RR, Zatz M. Rett syndrome in a boy with a 47, XXY karyotype confirmed by a rare mutation in the MECP2 gene. Neuropediatrics 21;32:162 4. [7] Yntema HG, Oudakker AR, Kleefstra T, Hamel BC, van Bokhoven H, Chelly J, et al. In-frame deletion in MECP2 causes mild non-specific mental retardation. Am J Med Genet 22;17:81 3. [8] Fabiani M, Gratton G, Coles MGH. Event-related potentials: methods, theory, and applications. In: Cacioppo JT, Tassinary LG, Berntson GG, editors. Handbook of psychophysiology. Cambridge: University Press; 2. p. 3 84. [9] Kutas M, Dale A. Electrical and magnetic readings of mental functions. In: Rugg MD, editor. Cognitive neuroscience. Sussex: Psychology Press; 1997. p. 197 237. [1] Batty M, Taylor MJ. Visual categorization during childhood: an ERP study. Psychophysiology 22;39:482 9. [11] Goldstein A, Spencer KM, Donchin E. The influence of stimulus deviance and novelty on the P3 and Novelty P3. Psychophysiology 22;39:781 9. [12] Courchesne E. Neurophysiological correlates of cognitive development: changes in long-latency event-related potentials from childhood to adulthood. Electroencephalogr Clin Neurophysiol 1978;4:468 82. [13] Courchesne E. Chronology of postnatal human brain development: event-related potential, positron emission tomography, myelinogenesis, and synaptogenesis studies. In: Rohrbaugh JW, Parasraman R, Johnson Jr R, editors. Event-related brain potentials: basic issues and applications. New York: Oxford University Press; 199. p. 21 41. [14] Bader GG, Witt-Engerström I, Hagberg B. Neurophysiological findings in Rett syndrome, II: visual and auditory brainstem, middle and late evoked potentials. Brain Dev 1989;11:11 4. [1] McCulloch DL, Henderson RM, Saunders KJ, Walley RM. Vision in Rett syndrome: studies using evoked potentials and event-related potentials. In: Kerr A, Witt-Engerström I, editors. Rett disorder and the developing brain. Oxford: Oxford University Press; 21. p. 343 8. [16] Ikeda K, Okuzumi H, Hayashi A, Hashimoto S, Kanno A. Automatic auditory processing and event-related brain potentials in persons with mental retardation. Percept Mot Skills 2;91:114. [17] Karrer JH, Karrer R, Bloom D, Chaney L, Davis R. Eventrelated brain potentials during an extended visual recognition memory task depict delayed development of cerebral inhibitory processes among 6-months-old infants with Down syndrome. Int J Psychophysiol 1998;29:167 2. [18] Kaneko WM, Ehlers CL, Philips EL, Riley EP. Auditory eventrelated potentials in fetal alcohol syndrome and Down s syndrome children. Alcohol Clin Exp Res 1996;2:3 42. [19] Stauder JE, Brinkman MJ, Curfs LM. Multi-modal P3 deflation of event-related brain activity in Prader Willi syndrome. Neurosci Lett 22;327:99 12.