P51 The influence of hearing impairment on the pupil dilation response to degraded speech
The measurement of pupil size to assess the influence of speech perception conditions on listening effort has been widely applied in the past decade. Several studies directly compared the pupil responses of listeners with hearing impairment with those of normal-hearing listeners. Although the results are not conclusive, the main evidence emerging from these studies is that hearing loss seems to be associated with a less dynamic pupil dilation response across intelligibility conditions. Also, the pre-trial baseline pupil response tends to be the same or slightly smaller than that of age-matched normal-hearing participants. It is currently not clear what the underlying mechanism causing these differences is. In the current study, we compared the baseline pupil size and the pupil dilation response between a group of 17 normal-hearing participants (mean age 46 years, age range 20 – 62 years) and a group of 17 age-matched hearing-impaired participants (mean age 42 years, age-range 20-63 years). The average pure-tone hearing thresholds at 1, 2, and 4 kHz were 6 dB HL for the normal-hearing group, and 50 dB HL for the hearing-impaired group. Participants performed speech perception tests in three degradation conditions: noise-vocoded speech, speech masked with stationary noise, and speech masked with interfering speech. They also listened to speech presented in quiet. Speech intelligibility of the degraded speech conditions was matched using an adaptive procedure targeting 50% sentence intelligibility. The results confirm that hearing impairment is associated with smaller increase in the pupil dilation response in difficult (degraded) conditions compared to easy conditions (speech in quiet). This finding was even observed in the noise-vocoded speech condition in which both the intelligibility level and the degradation level was similar between groups. We will present these findings, including the baseline pupil data. The results will be discussed in the context of the available evidence of the effect of hearing impairment in age-matched groups of participants. We will discuss several factors that may be associated with the mixed results found previously. These include the age-matching of participants, matching of performance level, and the baseline correction applied to the pupil dilation response. We conclude that more research is warranted to assess the effect of hearing impairment in larger, age-matched groups in a range of conditions.