ScienceWeek

BIOMEDICINE: NOISE AS A RISK FACTOR IN AUDITORY DEVELOPMENT

The following points are made by E.F. Chang and M.M. Merzenich (Science 2003 300:498):

1) Soon after the onset of hearing in the rat (postnatal day 12, or P12), a large auditory cortical area dominated by broadly tuned, high-frequency–selective neurons can be defined in the temporal cortex. Through a subsequent 2- to 3-week critical period, the infant rat's auditory cortex undergoes extensive refinement to acquire an adult-like organization.

2) Adult rats exhibit a compact, tonotopically ordered "primary auditory cortex" (A1) that represents the full spectrum of acoustic inputs with sound frequency–selective neural responses. A1 organization is easily distorted within this early postnatal period by exposure to specific acoustic inputs, indicating that the normal development of the auditory cortex is substantially influenced (and potentially strongly biased) by the structure of environmental acoustic inputs in early life. In the human infant, the emergent selective representation of the phonemic structure of the infant's native language is a probable manifestation of this powerful, sound-exposure–based critical-period plasticity.

3) The authors investigated how cortical development is affected by degraded signal-to-noise conditions, specifically conditions that could simulate natural environments that apply to human infant hearing and that could simulate the many possible inherited deficits that contribute to poor signal-to-noise conditions in central auditory processes. Previous attempts at reversibly depriving animals of natural acoustic inputs have been largely unsuccessful. A simple alternative strategy used in the present experiments was the rearing of infant rats in continuous noise applied to effectively mask normal environmental sound inputs.

4) In summary: The mammalian auditory cortex normally undergoes rapid and progressive functional maturation. The authors demonstrate that rearing infant rat pups in continuous, moderate-level noise delayed the emergence of adult-like topographic representational order and the refinement of response selectivity in the primary auditory cortex (A1) long beyond normal developmental benchmarks. When those noise-reared adult rats were subsequently exposed to a pulsed pure-tone stimulus, A1 rapidly reorganized, demonstrating that exposure-driven plasticity characteristic of the critical period was still ongoing. The authors suggest these results demonstrate that A1 organization is shaped by a young animal's exposure to salient, structured acoustic inputs -- and implicate noise as a risk factor for abnormal child development.

Science http://www.sciencemag.org

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PLASTICITY OF AUDITORY CORTEX IN CONGENITALLY DEAF ANIMALS

In general, the brains of animals and humans are organized in layers, the most recently evolved structures added on top of the older structures. The cerebral cortex is the most exterior part of the human brain, and also the most recently evolved region. The cortex is a thin surface layering of nerve cells, the region only several millimeters thick but covering all of the brain surface. This is the part of the central nervous system most intimately involved with the so-called "higher faculties", although the cortex operates in concert with other parts of the brain. The structure is primitive in lower mammals, and is found progressively more pronounced and with greater surface area in primates and man.

In addition to involvement with higher faculties, the cerebral cortex also contains several primary receiving areas for various sensory modalities, these regions of the cortex essentially acting as topographic maps of sensory input, maps that apparently organize input data into patterns meaningful for connected analytical regions. Two outstanding features of the cerebral cortex are its capacity for self-organization ("self-wiring") and its plasticity (in general, the ability of specific loci to alter function in response to previous experience).

In all sensory modalities, self-organization and plasticity depend on external stimuli. During development of the nervous system in a single individual, critical periods apparently exist during which an external influence is required to trigger the subsequent steps of central development. The criticality of these periods is made clear by the demonstrated arrested development that results from sensory deprivation during the critical periods. This aspect of neurological development is of particular importance in congenitally deaf patients, whose deafness can now in certain cases be treated by implants in the auditory sensory organ (cochlear implants).

When adults who are congenitally or prelingually deaf receive cochlear implants, the results are disappointing: these patients never gain language competence and often request that the implants be removed. In contrast, early cochlear implantation in congenitally or prelingually deafened children can lead to nearly perfect acoustic communication and language competence. What is not known, however, is the neurological basis underlying this phenomenon. Since certain types of experiments in humans are obviously not feasible, much research in the area has focused on animal models.

In congenitally deaf cats, the central auditory system is deprived of acoustic input because of degeneration of the auditory sensory organ cell system (the organ of Corti) before the onset of hearing. But auditory neurons that propagate activity to the brain (primary auditory afferents) survive under these conditions and can be stimulated electrically in appropriate experiments.

The following points are made by R. Klinke et al (Science 1999 285:1729):

1) The authors report that by means of an intracochlear implant and an accompanying sound processor, congenitally deaf kittens were exposed to sounds and conditioned to respond to tones. After months of exposure to meaningful stimuli, the cortical activity in chronically implanted cats (i.e., cats with chronically implanted recording electrodes) produced electric field potentials of higher amplitudes and expanded in area, developed long latency responses indicative of intracortical information processing, and showed more evidence of neuron connection efficacy than was observed in naive and unstimulated deaf cats.

2) This activity established by auditory experience in congenitally deaf animals resembles activity found in hearing animals. The authors suggest their results indicate that although without afferent input the auditory cortex remains rudimentary, this deficiency can be overcome by reafferentation (i.e, rewiring of input) to the deprived auditory channel by substitution of the missing cochlear activity. After implantation, a continuous input of relevant acoustic stimuli mimicking normal conditions results in animals displaying exploratory behavior, and animals that are attentive and motivated, factors known to strengthen cortical plasticity. The authors suggest a similar recruitment (i.e., organized activation) of the auditory cortex is likely to be the basis of demonstrated hearing acquisition in prelingually deaf human infants after early cochlear implantation.

Science http://www.sciencemag.org

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