Neural and Cortical Plasticity Reorganization

Animal research in the area of cortical plasticity and reorganization as regards the auditory system is intriguing and bears directly on the last controversy considered in this chapter. The phenomenon of failure to develop perceptual ability, due to early sensory deprivation, has been well recognized for many years, beginning with studies of kittens that were kept in darkness, and including newborn mice that were denied auditory input.19 The outcomes in those experiments showed lack of development of visual cortex and visual perceptual abilities in the first case and noticeable changes in structure of various nuclei in the auditory pathways of the brainstem and midbrain in the latter instance. More recently, reports on declines in speech perception abilities in the unaided ear of adults who are long-term monaural hearing aid users bring this question to the forefront again. Finally, research on apparent cortical reorganization in newborn cats whose auditory cortex is examined several months after they received lesions in the high-frequency region of their cochleas offers some interesting evidence. Observations in these studies have included the fact that the cortical area that commonly codes a range of frequencies was reorganized to be responsive only to the low-frequency information it was receiving. That is, typical high-frequency coding regions were eroded or recruited to code only low-frequency information. This reorganization was noted after only a few months of deprivation. The question of whether this cortex could be awakened to participate in more typical coding patterns has not been answered, of course. Nonetheless, the question of whether peripheral implants can be processed by auditory cortex that has never been stimulated, as in children with congenital deafness, remains. Data concerning adults who begin to wear a hearing aid on a long unaided ear suggest some answers, as many seem to recover speech perception abilities after a period of accommodation, but these are persons who lose hearing as adults and thus have well-developed auditory and language abilities (and brain organization) to assist with accommodation. Positron-emission tomography (PET) scan studies in cochlear implant users20 and other research of central auditory effects with electrical stimulation21 seem to offer the possibility that peripheral stimulation does have the effect of (re)activating central auditory pathways.

Although the early cortical plasticity research could be taken to argue for implantation during the first year of life, it seems to explain why some congenitally deaf children who are implanted early do not make progress. If the auditory cortex and complex brain underpinnings for spoken language cannot be awakened to sensory stimulation in this way, the value of an implant would be expected to be restricted.22

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