Simultaneous bilateral cochlear implants: Developmental advances do not yet achieve normal cortical processing

Abstract

BackgroundSimultaneous bilateral cochlear implantation promotes symmetric development of bilateral auditory pathways but binaural hearing remains abnormal. To evaluate whether bilateral cortical processing remains impaired in such children, cortical activity to unilateral and bilateral stimuli was assessed in a unique cohort of 16 children who received bilateral cochlear implants (CIs) simultaneously at 1.97 ± 0.86 years of age and had ~4 years of CI experience, providing the first opportunity to assess electrically driven cortical development in the absence of reorganized asymmetries from sequential implantation.MethodsCortical activity to unilateral and bilateral stimuli was measured using multichannel electro‐encephalography. Cortical processing in children with bilateral CIs was compared with click‐elicited activity in 13 normal hearing children matched for time‐in‐sound. Source activity was localized using the Time Restricted, Artefact and Coherence source Suppression (TRACS) beamformer method.ResultsConsistent with dominant crossed auditory pathways, normal P1 activity (~100 ms) was weaker to ipsilateral stimuli relative to contralateral and bilateral stimuli and both auditory cortices preferentially responded to the contralateral ear. Right hemisphere dominance was evident overall. Children with bilateral CIs maintained the expected right dominance but differences from normal included: (i) minimal changes between ipsilateral, contralateral and bilateral stimuli, (ii) weaker than normal contralateral stimulus preference, (iii) symmetric activity to bilateral stimuli, and (iv) increased occipital lobe recruitment during bilateral relative to unilateral stimulation. Between‐group contrasts demonstrated lower than normal activity in the inferior parieto‐occipital lobe (suggesting deficits in sensory integration) and greater than normal left frontal lobe activity (suggesting increased attention), even during passive listening.ConclusionsTogether, findings suggest that early simultaneous bilateral cochlear implantation promotes normal‐like auditory symmetry but that abnormalities in cortical processing consequent to deafness and/or electrical stimulation through two independent speech processors persist.

Impact of Consistency in Daily Device Use on Speech Perception Abilities in Children with Cochlear Implants: Datalogging Evidence

“…Such asymmetries evident in physiological measurements often parallel asymmetries observed in behavioral tasks, such as speech perception in quiet (e.g., Gordon and Papsin, 2009;Jiwani et al, 2016), spatial unmasking (Chadha et al, 2011), and lateralization to interaural timing differences (Gordon et al, 2014). These abnormal asymmetries can be prevented by providing two CIs early in life, simultaneously or with minimal interimplant delay (Gordon et al, 2013;Easwar et al, 2017); this is the current recommended standard of care in children with bilateral deafness (Ramsden et al, 2012). In support, none of the children with simultaneously received CIs or those with short interimplant delays demonstrated asymmetries as large as those evident in children with long interimplant delays ( Figure 4C and D).…”

“…On the contrary, several studies have shown a lack of difference in outcomes between CIs in children with simultaneously received devices (Gordon and Papsin, 2009;Easwar et al, 2017) and in adults with unilateral implants in the right/left ear (Morris et al, 2007;Budenz et al, 2011 [younger adults group]). While the lack of an observable right ear advantage could be due to smaller sample sizes in previous studies in children (Gordon and Papsin, 2009: n 5 6; Easwar et al, 2017: n 5 16) relative to the present study (n 5 25), this is less likely the case with studies in adults with larger samples (Morris et al, 2007: right CI n 5 63, left CI n 5 38; Budenz et al, 2011 [younger adults group]: right CI n 5 29, left CI n 5 19).…”

“…The residual effects of interimplant delay on the asymmetry between CIs were also assessed by including a control cohort of children who received their CIs simultaneously in the same surgery. This comparison was made because speech perception skills between the two CIs are often reportedly similar in children receiving CIs simultaneously (Gordon and Papsin, 2009;Easwar et al, 2017), paralleling their consistent use of both CIs (Galvin and Hughes, 2012;Easwar et al, 2016).…”

Delayed access to bilateral input alters cortical organization in children with asymmetric hearing

“…The time-restricted, artefact and coherent source suppression (TRACS) linearly constrained minimum variance type beamformer ( Wong and Gordon, 2009 ) was used to estimate source dipole activity underlying latency windows encompassing the first visually identified positive (P1) and negative (N2) peaks of the immature cortical waveform ( Fig. 2 ), as previously described ( Easwar et al, 2017a , Gordon et al, 2013b , Gordon et al, 2010 ). Briefly, application of the time-restricted artefact suppression algorithm maintained evoked responses while suppressing ~ 97% of the CI artefact corresponding to the four largest singular vector values between − 80 to 10 ms ( Wong and Gordon, 2009 ).…”

Cortical hemispheric asymmetries are present at young ages and further develop into adolescence

“…Stimuli were presented at 50 dB above behavioral threshold (dB SL) and the order of click and tone‐burst presentation was counter balanced. As in previous studies (Easwar, Yamazaki, Deighton, Papsin, & Gordon, ), EEG responses from 62 cephalic electrodes referenced to the right earlobe were recorded using NeuroScan v4.5 with a Synamps‐II amplifier during passive listening (Compumedics USA, Inc., Charlotte, NC). Participants watched a silent subtitled movie, read a book, or played games on a tablet with minimal movement.…”