Further Observations on Contralateral Remote Masking and Related Phenomena

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Abstract
Previous knowledge is reviewed relevant to contralateral remote masking (CRM)—the elevation in threshold of a low-frequency sinusoid the presence of a high-frequency band of noise in the opposite ear—and series of additional observations is presented. CRM is nearly as great (1) in ears with paralyzed middle-ear muscles as in normal ears, (2) for bone-conducted as for air-conducted test tones, or (3) when a 50-msec tone pulse occurs simultaneously with the onset of the masking pulse as when it is presented half a second later. Furthermore, (4) the gradual decrease of CRM with time, in the presence of a sustained masker, is not affected by abrupt changes in frequency or level of the masker, and (5) only a low negative correlation between CRM and auditory fatigue exists. These facts all indicate that the middle-ear muscles play only a minor role in CRM. The course of adaptation of CRM is shown to parallel the course of development of perstimulatory fatigue. It appears, therefore, that CRM represents primarily central masking arising at one or more centers receiving afferent innervation from both right and left ears, and that the change in time of CRM can be ascribed to adaptation processes either in the noise channel or, via the efferent system, in the contralateral channel. Implications of this formulation in regard to auditory fatigue from diotic and dichotic exposure to noise is discussed.

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