Visual Cross-Modal Re-Organization in Children with Cochlear Implants
Background: Visual cross-modal re-organization is a neurophysiological phenomenon in which the intact sensory modality of vision will recruit from the deprived sensory modality of audition (as in deafness) to compensate for communicative performance. Visual cross-modal reorganization has been shown to take place in deaf adults and animals, and is related to deficits in auditory performance in cochlear implanted adults due to occupation of auditory resources by visual processes. Though this cortical re-organization has been studied in adults, little is known if it occurs in children with cochlear implants, and if it does take place, whether it may act as another source of variability in speech perception and language outcomes for this population.
Purpose: Two studies were developed to investigate visual cross-modal re-organization in children with cochlear implants. The first study examined visual development in normal hearing children in response to continuous morphing of a radially modulated visual grating. The second study consisted of presenting the same complex visual stimuli to cochlear implanted children and comparing visual responses with the normal hearing children. Next, visual responses of the cochlear implanted children were correlated with speech perception in background noise to examine a possible functional relationship.
Methods: In the first study, visual evoked potentials (VEPs) were recorded in response to continuous morphing of a radially modulated visual grating using high-density EEG in normal hearing children grouped as 5-7 year olds, 8-10 year olds, and 11-15 year olds. VEP amplitude and latency were compared statistically across groups to illustrate developmental changes across age groups. Source localization analyses were calculated using sLORETA, and current density reconstructions (CDRs) were created to view visual cortical generators in each age group. In the second study, the same complex visual stimuli were presented to cochlear implanted children aged 5-15 years. VEPs were recorded via high-density EEG and amplitude and latency statistically compared to the VEP responses of the normal hearing children. CDR imagery was created to view the visual cortical generators in the cochlear implanted children and compared to the responses in the normal hearing children in order to view activation of non-visual areas, indicative of visual cross-modal re-organization. Next, speech perception in background noise was measured. The scores of the cochlear implanted children were then correlated with the visual response as represented by the VEP N1 component to investigate a possible relationship between auditory function and visual cross-modal re-organization.
Results: In Study 1, no significant age-related differences were observed across the age groups in response to complex visual stimuli. Instead, two morphological patterns were found in each age group. Source localization showed no major differences in activated visual regions across patterns or age. In Study 2, cochlear implanted children showed larger VEP amplitudes in comparison to normal hearing children, as well as activation of the right temporal cortex, including auditory regions. In addition, there was determined to be a significantly negative correlation between speech perception and background noise and visual cross-modal re-organization in the children with cochlear implants.
Conclusions: Children with cochlear implants presented evidence of visual cross-modal reorganization as compared to normal hearing children and measured by both VEP responses and cortical source localization analysis. Furthermore, re-organization was significantly and negatively correlated with speech perception in background noise, demonstrating that visual cross-modal reorganization should be considered as another source of variability in speech perception performance in cochlear implanted children.