Evolution of crossmodal reorganization of the voice area in cochlear‐implanted deaf patients

Abstract

Psychophysical and neuroimaging studies in both animal and human subjects have clearly demonstrated that cortical plasticity following sensory deprivation leads to a brain functional reorganization that favors the spared modalities. In postlingually deaf patients, the use of a cochlear implant (CI) allows a recovery of the auditory function, which will probably counteract the cortical crossmodal reorganization induced by hearing loss. To study the dynamics of such reversed crossmodal plasticity, we designed a longitudinal neuroimaging study involving the follow-up of 10 postlingually deaf adult CI users engaged in a visual speechreading task. While speechreading activates Broca's area in normally hearing subjects (NHS), the activity level elicited in this region in CI patients is abnormally low and increases progressively with post-implantation time. Furthermore, speechreading in CI patients induces abnormal crossmodal activations in right anterior regions of the superior temporal cortex normally devoted to processing human voice stimuli (temporal voice-sensitive areas-TVA). These abnormal activity levels diminish with post-implantation time and tend towards the levels observed in NHS. First, our study revealed that the neuroplasticity after cochlear implantation involves not only auditory but also visual and audiovisual speech processing networks. Second, our results suggest that during deafness, the functional links between cortical regions specialized in face and voice processing are reallocated to support speech-related visual processing through cross-modal reorganization. Such reorganization allows a more efficient audiovisual integration of speech after cochlear implantation. These compensatory sensory strategies are later completed by the progressive restoration of the visuo-audio-motor speech processing loop, including Broca's area.

Adaptive benefit of cross-modal plasticity following cochlear implantation in deaf adults

“…A significant reduction in cross-modal activation to visual speech has previously been documented from approximately 1 wk to 8 mo post-CI within anterior portions of the right superior temporal sulcus (9). Thus, we next examined changes in the amplitude of cross-modal activation to visual speech within the left and the right STC separately.…”

“…Data pertaining to changes over time in activation of auditory brain regions by visual speech are not available from existing studies for both CI users and NH control subjects (9). We therefore asked whether the observed change over time in right STC activation to visual speech differed between CI users and NH controls.…”

“…Existing evidence from a PET study in adult CI users showed that greater activation of auditory brain regions during lip-reading predicted poorer speech understanding abilities with a CI (8) and that this activity reduced from an earlier to a later stage of CI rehabilitation (9). Subsequently, it has been assumed that activation of auditory cortex by visual language can limit its capacity for auditory processing (3) and that a reduction in cross-modal activation of auditory cortex to visual speech after implantation may be crucial for successful hearing restoration (9).…”

“…Subsequently, it has been assumed that activation of auditory cortex by visual language can limit its capacity for auditory processing (3) and that a reduction in cross-modal activation of auditory cortex to visual speech after implantation may be crucial for successful hearing restoration (9). Such assumptions have led to clinical recommendations for deaf individuals undergoing cochlear implantation to avoid the use of visual language to maintain the ability of auditory brain regions to process auditory speech and thereby optimize CI success.…”

Physiology, Psychoacoustics and Cognition in Normal and Impaired Hearing

“…Secondly, the contribution of the visual cortex to speech recognition increased over time after implantation (Giraud et al 2001b), suggesting that the CI users were actively using enhanced audio-visual integration to facilitate their learning of the novel speech sounds received through the CI. In contrast, Rouger et al (2012) suggested a negative impact of cross modal plastic-ity: they found that the right temporal voice area (TVA) was abnormally activated in CI users by a visual speech-reading task and that this activity declined over time after implantation while the activity in Broca's area (normally activated by speech reading) increased over time after implantation. Coez et al (2008) also used PET to study activation by voice stimuli of the TVA in CI users with poor and good speech understanding.…”