Visual stimulations can activate cortical regions normally devoted to auditory processing in deaf individuals (Prog Brain Res 2011;191:211), and it is generally believed that this cross-modal plasticity leads directly to behavioral advantages and improved communication strategies. Recent advances, however, in brain reorganization and multisensory processing have raised questions about the benefits of such compensatory mechanisms. An extensive reorganization might represent a change that may in some cases hinder rehabilitation for those seeking to restore their hearing perception.
The visual takeover of the cortices typically associated with audition represents a cerebral ability to adapt to change, but it raises an important question regarding the recovery of hearing functions. The question of auditory function following deafness bears special importance for audiologists, given that profound deafness is sometimes reversed with a cochlear implant (CI).
One may wonder how these two modalities interact during tasks that require multisensory processing, such as speech perception, if visual input is redirected to auditory cortical areas. Most speech understanding occurs in a multisensory environment in which visual and auditory cues are present. Given the attack of the auditory cortex by visual information following deafness, it can be hypothesized that visual information may interfere with auditory treatment. This interference could lead to poor speech recognition in some CI patients, namely those who have undergone more adaptive plasticity during the period of auditory deprivation (i.e., before implantation).
An apparent scientific consensus surrounds the ease of CI patients to access visual and auditory information when both cues are related. Some have argued that CI patients might be better at integrating similar auditory and visual information than those with normal hearing. (Proc Natl Acad Sci U S A 2007;104:7295.) These neuroscientists have urged audiologists to use visually oriented therapeutic strategies for rehabilitating CI users. While it may be tempting to embrace an all-encompassing, generic philosophy, other aspects of multisensory integration should be considered first.
Congruent multisensory cues account for only a fraction of the information occurring in normal interactions with our surroundings. The ability to fuse congruent multisensory information (i.e., when auditory and visual cues are related) is as important as the ability to segregate incongruent cues (i.e., when auditory and visual cues are unrelated or contradictory). Evidence regarding incongruent multisensory cues is a bit more ambiguous than with congruent cues. Some data suggest that while visually oriented rehabilitation strategies may be well suited for some CI patients, they might be detrimental for others in certain perceptual conditions.
We investigated the ability of CI users to fuse and segregate conflicting auditory and visual information to shed light on incongruent cues. (Neuropsychologia 2009;49:17; PLoS One 2012;7:e33113.) Our most recent results confirm that normal-like audiovisual fusion and segregation are found in highly skilled CI patients, supporting the idea they are able to make good use of visual and auditory modalities during speech perception. We also found, however, that CI patients who have undergone more adaptive plasticity before implantation rely so heavily on visual cues that they are unable to segregate incongruent auditory and visual information adequately. These data suggest that while visual signals can facilitate speech perception in some multisensory conditions, they might also hinder speech discrimination performance for some CI patients when audiovisual input needs to be separated.
The major conclusion from these findings is the substantiation that not all CI patients can be lumped into a single rehabilitation category. Several factors should be considered when investigating multisensory interaction in CI users. Performance has been shown to depend on duration of deafness, the communication strategy used before implantation, age at onset of deafness, and the extent of cross-modal plasticity prior to cochlear implantation. (Prog Brain Res 2011;191:211.) All these factors interact and influence auditory perceptions following implantation. This suggests that multisensory integration in CI patients varies greatly in auditory perception as a result of these variables. All these important features must be taken into account when considering therapeutic approaches.
Which Strategy Works Best?
Audiologists often ask if they should encourage use of visual cues in their rehabilitation program. We believe that these unilateral statements should be approached with caution, even though some might claim that visually centered programs are beneficial for all. We strongly believe that optimum rehabilitation programs should be individualized to achieve the best level of speech comprehension. A speech-perception training program, for example, might be adapted according to the nature and extent of brain reorganization following hearing loss. An individual for whom a substantial amount of visual information is processed at the level of the auditory cortices might benefit from training that differs from someone whose auditory cortices are uninvolved in processing visual information.
The finest tool for determining audiological intervention is still the openness, expertise, judgment, and care from the audiologist in charge. Future investigations on cross-modal plasticity and multisensory interaction hopefully will provide new and innovative insight into the best rehabilitation approach on the sensory and general developmental levels for different CI users. These specific rehabilitation techniques would be especially beneficial for CI users for whom auditory signals provide limited information in speech comprehension. Given the unique and complex nature of hearing loss and regardless of the outcome of these studies, it is most likely that a generic approach will never be valid for everyone, especially in a case with varying amounts of perceptual situations.
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