Phillip M. Gilley

Deprivation-induced cortical reorganization in children with cochlear implants

A basic finding in developmental neurophysiology is that some areas of the cortex cortical areas will reorganize following a period of stimulus deprivation. In this review, we discuss mainly electroencephalography (EEG) studies of normal and deprivation-induced abnormal development of the central auditory pathways in children and in animal models.We describe age cut-off for sensitive periods for central auditory development in congenitally deaf children who are fitted with a cochlear implant. We speculate on mechanisms of decoupling and reorganization which may underlie the end of the sensitive period. Finally, we describe new magentoencephalography (MEG) evidence of somatosensory cross-modal plasticity following long-term auditory deprivation.

Developmental changes in refractoriness of the cortical auditory evoked potential

OBJECTIVE This study examined morphological changes in the cortical auditory evoked potential (CAEP) waveform as a function of varying stimulation rate. Stimuli were presented in a paradigm which indirectly assesses the refractory properties of the underlying neuronal generators. METHODS CAEPs were recorded in 50 normal-hearing children (3-12 years) and 10 young adults (24-26 years). A speech sound was presented in a stimulus train with sequentially decreasing inter-stimulus intervals (ISIs) of 2000, 1000, 560, and 360ms. Latencies and amplitudes of the P1, N1, and P2 components at the Cz electrode were examined as a function of stimulus rate and age. RESULTS Results revealed significant changes in the CAEP as a function of age and stimulation rate. At younger ages the N1-P2 component was elicited only at the slowest stimulation rates, and was more clearly apparent at successively faster stimulation rates as age increased. CONCLUSIONS We have described a stimulus paradigm that allows examination of the development of refractoriness by highlighting the interaction between age and rate on CAEP morphology. SIGNIFICANCE Complex maturational patterns of CAEP components are best understood when the effects of both age and stimulus rate on the CAEP waveform are considered.

Cortical reorganization in children with cochlear implants

Congenital deafness leads to atypical organization of the auditory nervous system. However, the extent to which auditory pathways reorganize during deafness is not well understood. We recorded cortical auditory evoked potentials in normal hearing children and in congenitally deaf children fitted with cochlear implants. High-density EEG and source modeling revealed principal activity from auditory cortex in normal hearing and early implanted children. However, children implanted after a critical period of seven years revealed activity from parietotemporal cortex in response to auditory stimulation, demonstrating reorganized cortical pathways. Reorganization of central auditory pathways is limited by the age at which implantation occurs, and may help explain the benefits and limitations of implantation in congenitally deaf children.

Minimization of cochlear implant stimulus artifact in cortical auditory evoked potentials

OBJECTIVE To compare two methods of minimizing cochlear implant artifact in cortical auditory evoked potential (CAEP) recordings. METHODS Two experiments were conducted. In the first, we assessed the use of independent component analysis (ICA) as a pre-processing filter. In the second, we explored the use of an optimized differential reference (ODR) for minimizing artifacts. RESULTS Both ICA and the ODR can minimize the artifact and allow measurement of CAEP responses. CONCLUSIONS When using a large number of recording electrodes ICA can be used to minimize the implant artifact. When using a single electrode montage an optimized differential reference is adequate to minimize the artifact. SIGNIFICANCE The use of an optimized differential reference could allow cortical evoked potentials to be used in routine clinical assessment of auditory pathway development in children and adults fit with cochlear implants.

The influence of a sensitive period for auditory-visual integration in children with cochlear implants

PURPOSE Children who experience long periods of auditory deprivation are susceptible to large-scale reorganization of auditory cortical areas responsible for the perception of speech and language. One consequence of this reorganization is that integration of combined auditory and visual information may be altered after hearing is restored with a cochlear implant. Our goal was to investigate the effects of reorganization in a task that examines performance during multisensory integration. METHODS Reaction times to the detection of basic auditory (A), visual (V), and combined auditory-visual (AV) stimuli were examined in a group of normally hearing children, and in two groups of cochlear implanted children: (1) early implanted children in whom cortical auditory evoked potentials (CAEPs) fell within normal developmental limits, and (2) late implanted children in whom CAEPs were outside of normal developmental limits. Millers test of the race model inequality was performed for each group in order to examine the effects of auditory deprivation on multisensory integration abilities after implantation. RESULTS Results revealed a significant violation of the race model inequality in the normally hearing and early implanted children, but not in the group of late implanted children. CONCLUSION These results suggest that coactivation to multi-modal sensory input cannot explain the decreased reaction times to multi-modal input in late implanted children. These results are discussed in regards to current models for coactivation to redundant sensory information.

Simultaneous versus sequential bilateral implantation in young children: Effects on central auditory system development and plasticity

The aim of this study was to determine whether children who received early, simultaneous, bilateral cochlear implants showed more rapid development of the central auditory pathways compared to children who received early, sequential, bilateral implants. In 20 children, over the first 15 months following bilateral implantation, we assessed longitudinal changes in the morphology and latency of the P1 cortical response, which is generated within the auditory cortex. Our results showed that by 3.5 months post-implantation, mean P1 latencies for both groups of children were within normal limits. Overall, the developmental trajectory of the P1 response did not differ significantly for the two groups over the 15-month period. Our results suggest that bilateral implantation, whether sequential or simultaneous, occurring within a sensitive period of 3.5 years, takes advantage of the high degree of plasticity in the developing central auditory nervous system.

Abnormalities in central auditory maturation in children with language-based learning problems

OBJECTIVE To examine maturation of the central auditory pathways in children with language-based learning problems (LP). METHODS Cortical auditory evoked potentials (CAEPs) recorded from 26 children with LP were compared to CAEPs recorded from 38 typical children. CAEP responses were recorded in response to a speech sound, /uh/, which was presented in a stimulus train with decreasing inter-stimulus intervals (ISIs) of 2000, 1000, 560, and 360 ms. RESULTS We identified three atypical morphological categories of CAEP responses in the LP group. Category 1 responses revealed delayed P1 latencies and absent N1/P2 components. Category 2 responses revealed typical P1 responses, but delayed N1 and P2 responses. Category 3 responses revealed generally low-amplitude CAEP responses. A fourth sub-group of LP children had normal CAEP responses. CONCLUSIONS Overall, the majority of children with LP had abnormal CAEP responses. These children fell into distinct categories based on the abnormalities in maturational patterns of their CAEP responses. SIGNIFICANCE We describe a rate sensitive stimulation paradigm which may be used to identify and categorize LP children who exhibit abnormal patterns of central auditory maturation.

A PDA-based Research Platform for Cochlear Implants

Currently researchers interested in developing new signal processing algorithms for commercially available cochlear implants must rely on coding these algorithms in low-level assembly language. We propose a personal digital assistant (PDA) based research platform for developing and testing in real-time new signal processing strategies for cochlear implants. Software development can be done either in C or in LabVIEW. The C implementation can be further optimized using Intels primitive routines. In this paper, we report on the real-time implementation of a 16-channel noise-band vocoder algorithm, which is a similar algorithm used in commercially available implant processors. We further report on EEG recordings on the PDA acquired through a compact-flash data acquisition card.

Oscillatory decoupling differentiates auditory encoding deficits in children with listening problems

OBJECTIVE We sought to examine whether oscillatory EEG responses to a speech stimulus in both quiet and noise were different in children with listening problems than in children with normal hearing. METHODS We employed a high-resolution spectral-temporal analysis of the cortical auditory evoked potential in response to a 150 ms speech sound /da/ in quiet and 3 dB SNR in 21 typically developing children (mean age=10.7 years, standard deviation=1.7) and 44 children with reported listening problems (LP) with absence of hearing loss (mean age=10.3 years, standard deviation=1.6). Children with LP were assessed for auditory processing disorder (APD) by which 24 children had APD, and 20 children did not. Peak latencies, magnitudes, and frequencies were compared between these groups. RESULTS Children with LP had frequency shifts in the theta, and alpha bands (p<0.05), and children with LP+APD had additional frequency (p<0.01) and latency shifts (p<0.05) in the upper beta and in the lower gamma bands. CONCLUSIONS These results provide evidence for differences in higher level modulatory processing in children with LP, and that APD is driven by differences in early auditory encoding. SIGNIFICANCE These findings may better guide future research toward improving the differential diagnosis and treatment of listening problems in this population of children.

Analysing speech problems in a longitudinal case study of logopenic variant PPA

Background: Primary progressive aphasia (PPA) is a neurodegenerative form of dementia in which gradually worsening language impairments are the prominent feature in the initial stages. PPA is commonly differentiated into three variants: nonfluent agrammatic (PPA-NVF), semantic (PPA-SV), and logopenic (PPA-LV). Aims: This article provides a longitudinal description of changes in picture description produced by a woman with PPA-LV, introduces a reliable new measure that captures those changes, and relates the measured changes to raters’ perceptions of changes in discourse quality. Method & Procedures: Seven oral descriptions of the Boston Diagnostic Aphasia Examination (BDAE) Cookie Theft picture were digitally recorded over the course of 27 months and later transcribed. Transcriptions were analysed using a new adaptation of the Linguistic Communication Measure (LCM) and the Linguistic Communication Measure-Revised Cantonese (LCM-RC) designed to be sensitive to the features of PPA-LV. We have named this third form the LCM, the Linguistic Communication Measure–-Speech Sounds (LCM-SS). Audio recordings of the seven picture descriptions plus three produced by typical speakers of similar age were rated for goodness by 15 raters. Outcomes & Results: Goodness ratings of the participants’ speech samples decreased steadily over the 27 months. Although our previous measures of discourse quality (LCM, LCMC-RC) appeared to work well for capturing many of the speakers with vascular aphasia, they failed to capture the nature of this participant’s decline: Her lexical access slowed over time, but did not become more error-prone, and morphosyntactic components did not worsen, with errors remaining low to almost absent. However, speech sound errors and repetitions increased steadily over the 27 months. The new measure, LCM-SS, succeeded in capturing this pattern of decline: Several of the LCM-SS measures were highly correlated to ratings of goodness, and two of the LCM-SS indices (sound errors and grammatical errors) accounted for 98% of the variance in the goodness ratings. Conclusions: Over the course of 27 months, the most significant change in this participant’s Cookie Theft descriptions was the steady increase in sound errors, in the context of decreased efficiency in lexical retrieval and relatively stable grammatical form. This pattern was also highly related to listeners’ perceptions of the quality of discourse. Neither of the previous versions of the LCM captured this debilitating increase in sound errors, but adding the index of sound errors to those previous versions resulted in an analysis method that was sensitive to the linguistic features exhibited by this participant with PPA-LV.