Abstract
Spatial hearing is crucial in occupational tasks, yet until 2019, US Army fitness-for-duty requirements only considered hearing thresholds in the better ear. The regulation change introduced the Military Operational Hearing Test (MOHT) for those with worse-ear thresholds exceeding 40 dB HL at 0.5 or 1 kHz, or 60 dB HL at 2 kHz. This led to the development of the Spatial Digit Test (SDT) to assess binaural function in individuals with poor hearing in one ear but sufficiently good hearing in the other ear to pass a speech-in-noise test. The SDT involves digit pairs presented with Interaural Time Delays (ITDs) of +/−800 μs. Two studies were conducted: a validation study with over 200 individuals, and a verification study with more than 130 undergoing MOHT exams. Results show that individuals with normal auditory thresholds perform very well on the SDT, with about 95% correctly identifying at least 8 digits and 75% correctly identifying all 10. In contrast, only about 50% of individuals with monaural losses who took the SDT as part of the MOHT identified 8 or more digits. Results of the SDT were correlated both with subjective hearing complaints and with the Binaural Masking Level Difference. In conclusion, the SDT proves valuable for identifying individuals struggling using ITD cues to segregate and localize simultaneously presented speech signals.
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