Extended High Frequency Hearing Loss, Not Cochlear Synaptopathy, Predicts Speech Recognition in a Population Cohort

Age-related hearing loss is the leading sensory deficit in older adults, yet audiometric thresholds at conventional frequencies often poorly predict speech understanding. Two competing hypotheses have emerged: extended high-frequency (eHF) hearing loss beyond 8 kHz may unmask variance in speech performance, while hidden hearing loss from cochlear synaptopathy---detectable via auditory brainstem response (ABR) wave I amplitude reduction---may degrade temporal coding independent of audiometry. Here, in 526 ears from 263 tinnitus-free adults in the Swedish Tinnitus Outreach Project (STOP) cohort, we show that eHF pure-tone average (10-16 kHz) is the single most age-sensitive auditory measure, explaining 64% of age-related variance (R{superscript 2} = 0.64) compared to only 16% for conventional audiometry (R{superscript 2} = 0.16). Moreover, eHF thresholds robustly predict both word and phoneme recognition in speech-weighted noise (+4 dB SNR), explaining 34-36% of speech variance (R{superscript 2} = 0.34-0.36)--substantially exceeding conventional pure-tone average (22-25%) and all ABR features (5-13%). In contrast, ABR Wave I amplitude--the putative marker of cochlear synaptopathy--contributes no additional explanatory power even in high-reliability recordings (ICC = 0.96). These findings challenge the translational relevance of cochlear synaptopathy to age-related speech deficits and suggest conduction delays, not synaptic loss, as the peripheral neural mechanism underlying speech comprehension decline in aging.