Effects of middle ear pressure on frequency representation in the central auditory system

Influence of transient pressure changes on speech intelligibility: Implications for next-generation train travel

High-speed trains are operated in increasingly complex railway networks and continual improvement of driver assistance systems is necessary to maintain safety. Speech offers the opportunity to provide information to the driver without disrupting visual attention. However, it is not known whether the transient pressure changes inside trains passing through tunnels interfere with speech intelligibility. Our primary goal was to test whether the most severe pressure variations occurring in high-speed trains (25 hPa in 2 s) affect speech intelligibility in individuals with normal hearing ability and secondly whether a potential effect would depend on the direction of the pressure change. A cross-over design was used to compare speech intelligibility, measured with the monosyllable word test by Wallenberg and Kollmeier, in steady ambient pressure versus subsequent to pressure events, both realised in a pressure chamber. Since data for a power calculation did not exist, we conducted a pilot study with 20 participants to estimate variance of intra-individual differences. The upper 80% confidence limit guided sample size of the main campaign, which was performed with 72 participants to identify a 10% difference while limiting alpha (5%) and beta error (10%). On average, a participant understood 0.7 fewer words following a pressure change event compared to listening in steady ambient pressure. However, this intra-individual differences varied strongly between participants, standard deviation (SD) ± 4.5 words, resulting in a negligible effect size of 0.1 and the Wilcoxon signed rank test (Z = -1.26; p = 0.21) did not distinguish it from chance. When comparing decreasing and increasing pressure events an average of 0.2 fewer words were understood (± 3.9 SD). The most severe pressure changes expected to occur in high-speed trains passing through tunnels do not interfere with speech intelligibility and are in itself not a risk factor for loss of verbal information transmission.

The effect of nitrous oxide on hearing

There is a belief that the administration of nitrous oxide (N2O) increases hearing acuity. This increase can be interpreted as hearing low intensity sounds more loudly. This study examined auditory threshold levels, acoustic impedance, acoustic perception, memory, and discrimination to determine if hearing was altered by end-tidal 10% or 20% nitrous oxide. Subjects also qualitatively interpreted three intensity levels using a subjective intensity test. Observations were made in the breathing mediums of room air in the control stages (Stages 1 and 4), compared to N2O and supplemental oxygen (Stages 2 and 3), in 16 human subjects in a quiet room. Breathing end-tidal 10% and 20% N2O significantly increased middle ear pressure, and produced a significant effect on the subjective interpretation of a sound's intensity. There was no significant effect on auditory threshold at a range of frequencies. It appears that 10% or 20% N2O inhalation does not lead to the commonly held view of increased hearing acuity as measured in terms of auditory threshold. Rather, at these levels of N2O inhalation, subjects experience a state similar to a pre-sleep stage, whereby the hearing diminishes but remains active for loud intensity sounds.