When background noise moves to center stage
In order to understand natural speech it is essential to be able to detect modulations in amplitude and pitch that characterize spoken language and to detect the brief pauses that structure its temporal flow. The team of researchers led by Grothe studied the response characteristics of nerve cells in the inferior colliculus (one of the areas in the brain involved in processing auditory information) of gerbils. They found that these cells loose their ability to respond selectively to temporally variable signal sequences as animals age. As a consequence of this reduction in temporal selectivity, the “receptive fields” of the neurons were less heterogeneous. A nerve cell’s receptive field is defined by the physical parameters of the signals to which it is sensitive. Different cells have different receptive fields, and this heterogeneity is presumably the basis for the processing and interpretation of sensory inputs.
The more heterogeneous the receptive fields of different nerve cells, the larger the information content of the signal that is transmitted to later processing stations in the brain. “Consequently, the results of our study imply that nerve cells of older animals provide less information about natural stimuli than those of younger animals,” explains Grothe. “Such a loss in coding capacity could significantly impair signal processing in complex acoustic situations.” As a result, older people placed in a noisy acoustic environment, such as a crowded restaurant, may perceive only a confusing hubbub of sounds because they fail to separate individual voices from background noise.
The researchers speculate that such alterations in central processing of acoustic information are due to a disturbance in the relative concentrations of so-called neurotransmitters in the aging brain. Neurotransmitters are biochemical compounds that are secreted by activated nerve cells and stimulate or inhibit the firing of neighboring nerve cells. If the balance between inhibitory and excitatory transmitters is perturbed, signal transmission may be distorted. If this hypothesis is valid, conventional approaches to the treatment of age-related hearing loss are in need of revision. “If the alterations in central processing that we observe are indeed due to changes in the equilibrium between neurotransmitters, then drugs that correct the balance could supplement and perhaps improve current therapies,” says Grothe. “One could also combine such a pharmacological approach with the use of hearing aids, and thus provide more effective treatments for hearing loss.” (göd/PH)
Impaired Auditory Temporal Selectivity in the Inferior Colliculus of Aged Mongolian Gerbils.
L. Khouri, N.A. Lesica, B. Grothe
Journal of Neuroscience, 31(27):9958-9970; 6 July 2011