Thomas H. Simpson

Forward masking properties of neurons in the dorsal cochlear nucleus: Possible role in the process of echo suppression

The majority of single unit studies in the auditory system have been carried out using stimuli whose temporal and spectral contexts are held constant. Relatively little attention has been given to the influence of context on unit response properties. Indeed, auditory nerve fiber responses are known to be context-dependent due to the property of forward masking, a phenomenon by which the response to one sound results in a reduction in the response to a subsequent sound. Forward masking might be expected to be even more influential at central levels of the auditory pathway where the responses are reshaped by additional synaptic interactions. The purpose of the present study was to characterize the forward masking properties of neurons in the dorsal cochlear nucleus (DCN). A tool was developed for measuring the response to a probe tone as a function of delay following a previous tone-burst. The frequency of the probe was held constant at the units characteristic frequency while the frequency of the leading tone (masker) was varied. These measures provided a description of neural masking effects in different temporal and spectral contexts. The data yielded two patterns of suppression. In the first pattern (Type A), the suppression of the probe response became evident immediately following offset of the masker; the suppression bandwidth showed a gradual narrowing as the delay between masker and probe was increased. In the second class (Type B), the suppression of the probe response did not become evident until well after offset of the masker; this pattern appeared more circumscribed in that the suppression bandwidth gradually increased as a function of delay up to a maximum then decreased with further increases in delay. The results imply that mechanisms intrinsic to the DCN contribute to further modification and reshaping of the spectral and temporal context of masking effects beyond those seen in the auditory nerve. It is hypothesized that such properties may be specialized for suppressing the response to echoes thus facilitating communication and localization of sound in enclosed spaces.

Factors affecting laterality of standard threshold shift in occupational hearing conservation programs

Abstract Although noiseinduced hearing loss is generally characterized as bilaterally symmetrical, 80% of audiometric shifts meeting Occupational Safety and Health Administration (OSHA] criteria were unilateral in a large industrial sample. The primary factor related to unilateral OSHA shifts was asymmetric baseline hearing level. Better ears at baseline audiogram were more likely to demonstrate OSHA shifts than poorer ears. When hearing levels were symmetric at baseline, left ears were more likely to demonstrate OSHA shifts. Although OSHA shifts were primarily unilateral, cumulative effects of noise exposure were judged to be bilaterally symmetrical due to larger hearing shifts occurring in better ears of subjects with preexisting hearing asymmetries. Recovery from temporary threshold shift in second ears tested was judged not to be a significant determinant for high prevalence of unilateral shifts. Many factors likely interact to confound interpretive value of OSHA shift laterality in occupationally noise-exposed populations. This holds important implications for hearing conservation program professionals considering hearing shift laterality as a factor in shift etiology.