Changes in Bone-Conduction Thresholds Produced by Masking in the Non-Test Ear

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Abstract
Two experiments were conducted to determine the effects of central masking on both air and bone-conduction thresholds. In the first investigation, 10 normal listeners were tested by air conduction and by bone conduction via the mastoid process and the frontal bone as various levels of narrow-band noise were presented to the contralateral ear. Generally, as the noise level increased, there was a small but gradual increase in the threshold on the test ear. However, the threshold shift for frontal bone measurements was always greater than comparable air-conduction or bone-conduction thresholds from the mastoid process. The additional shift in threshold for frontal bone measurements may have been the result of changing from binaural stimulation in quiet to monaural stimulation during the masking conditions. This proposition was investigated in Experiment II. The test ears of 6 subjects were occluded with plugs which induced an appreciable increase in bone-conduction sensitivity. Thus, the frontal bone threshold as well as the mastoid threshold was monaural, even in the quiet condition. If the above proposition were correct, it was predicted that identical shifts in threshold due to central masking would be found for frontal bone measurements as for mastoid measurements. The threshold shifts for air conduction and bone conduction via mastoid or frontal bone vibrator placements were found to be similar.

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