Abstract
Aim: To investigate behavioural and developmental factors associated with variation in the normal sensorineural hearing threshold.
Method: The Dunedin Multidisciplinary Health and Development Study, measured audiometric thresholds at ages 5, 7, 9, and 11 for a birth cohort of 1000 people. The binaural 0.5, 1, and 2 kHz average was used. Hearing levels measured under abnormal middle-ear conditions, and those poorer than 15 dBHL, were excluded. We investigated associations between hearing level, speech in noise recognition, neurocognitive ability, linguistic ability, and behavioural problems. To test the hypothesis of a neurodevelopmental mechanism underlying these associations, we investigated the relationship between hearing and growth rates through the lifecourse.
Results: Speech in noise recognition, neurocognitive and language ability were better in those with the very best hearing, and effects were stronger in girls. Behavioural problems were less common in girls with the very best hearing, while no differences were observable in boys. Growth rates during infancy and late adolescence were associated with variation in normal hearing.
Conclusions: 1. People with the very best hearing have better behavioural and language outcomes among those with normal hearing. 2. The associations between hearing level and growth rate are strongest at ages when serum insulin-like growth factor 1 (IGF-1) levels are higher. These two findings provide evidence for the theory that expression of IGF-1 may lead to both improved hearing and development throughout the nervous system, thereby leading to the observed relations between hearing and behaviour.