Responses to Criticisms of our Paper

Responses of the Ear to Low Frequency Sounds, Infrasound and Wind Turbines

December 15, 2010

A scientific paper, even when peer-reviewed, represents the authors opinions at the time it was written. On this page we will take comments about our paper and discuss them. If we change our opinion on any subject, we will let you know.

#1 The Outer hair cells are not connected to the brain

Dr Robert McCunney, Testimony to the State of Vermont Pubic Service Board, Nov 22, 2010

they have simply introduced concepts about responses of the outer hair cells of the inner ear (which do not send signals to the brain) to exposure to infrasound

and

Moreover, the outer hair cells are not connected to the brain.


These statements by Dr McCunney are absolutely false. There is a huge literature showing that the outer hair cells of animals and humans are connected to the cochlear nucleus of the brain by Type II afferent fibers (see any standard text on auditory physiology, e.g. Physiology of the Ear, Jahn & Santos Sacchi, 2001). Type II afferents from outer hair cells represent about 5% of the fibers in the auditory nerve. The point of Dr McCunney's argument is to say that although the outer hair cells are stimulated by infrasound at levels lower than those you can hear, that stimulation of these cells doesn't really affect you, because they are not connected to the brain. Well, this argument is scientifically dead-on-arrival. Yes, the OHC are connected to the brain, so yes, if the OHC are stimulated by infrasound then indeed the infrasound can jolly well affect you.


#2 Results from the human and the guinea pig are not comparable because their helicotrema works differently


Dr Robert McCunney, Testimony to the State of Vermont Pubic Service Board, Nov 22, 2010

These laboratory animals [guinea pigs], however, have a strikingly different anatomy of the inner ear in comparison to humans, and, as a result, the corresponding implications of these animal studies to humans are dubious.

and

Moreover, in all mammals, one of the limits of low-frequency hearing is the helicotrema (the gap in the basilar membrane that connects the scala tympani and scala vestibuli). The helicotrema acts as a high-pass filter; the larger the helicotrema, the greater low-frequency sound is shunted away from hair cells. The guinea pig has a very small helicotrema (only 7% of the area of the human helicotrema) and therefore unusually good low-frequency hearing.


The argument here is whether measurements showing that the ears of guinea pigs are sensitive to infrasounds has any relevance to the human situation. The implication of this argument is that, with respect to low frequency sensitivity, guinea pigs are “different” and their ears work differently from humans. The argument is complex but most of the data suggest (as I'll show below) that the helicotrema of the guinea pig functions in almost the same manner to that of humans. And even if it didn't it is not a major factor as attenuation by the helicotrema equally affects the sensitivity of both the inner hair cells (which mediate hearing) AND the outer hair cells (which are sensitive to infrasound). So in humans, whatever infrasound level stimulates the inner hair cells (and can therefore be heard), we know that the outer hair cells are sensitive to 30 – 40 dB lower levels, because outer hair cells respond to displacement, while inner hair cells respond to velocity (see our paper for detailed explanation). There is no evidence that human hair cells respond differently to guinea pig hair cells in this respect. Anatomically, the inner hair cell stereocilia of both humans and guinea pigs are not directly coupled to the tectorial membrane but instead are within the fluid of the subtectorial space, while the longest stereocilia of the outer hair cells are directly attached to the tectorial membrane. So the difference between inner and outer hair cells responses is totally independent of what the helicotrema is doing. Even though this helicotrema argument is therefore really irrelevant, we will deal with it anyway.


The idea that the helicotrema had different properties in different species was proposed in a number of papers by Dallos (summarized in his 1973 book, The Auditory Periphery). They measured cochlear microphonics from different animals in response to low stimulus frequencies (20 Hz – 1 kHz) and found that in cats and chinchillas the microphonic decreased at about 12 dB/octave while in guinea pigs it decreased at 6 dB/octave. Based on the observation that the helicotrema was anatomically smaller in the guinea pig it was suggested that the 6 dB/octave difference was due to the guinea pig helicotrema not providing the 6 dB/octave attenuation below 100 Hz that it does for other species (such as humans and cats). However, later experimental studies by Franke and Dancer (1982) and our study (Salt et al, 2009) showed that when the helicotrema was experimentally plugged in guinea pigs (by silicone paste or healon gel respectively in the two studies) the cochlear responses below 100 Hz were increased by about 6 dB/octave (with a 90 degree phase shift) showing that the guinea pig helicotrema had virtually the same properties, when measured experimentally, as believed to exist in the human. It is therefore assumed that the original cochlear microphonic differences reported by Dallos were explained by some other mechanism, such as cochlear input impedance as suggested by Franke & Dancer. So the measured helicotrema characteristics in guinea pigs are similar to those of the human. To ignore these findings is to believe that “speculation” is more reliable than “measured data”.


In our paper, we point out that guinea pig hearing to low frequencies is about 10 dB LESS sensitive than that of humans at the lowest frequency tested, which is 50 Hz. Using low frequency masking of acoustic emissions, Marquardt et al. (2007) show a generally similar decline in measured masking sensitivity in guinea pigs and humans, suggesting that any difference in the low frequency cutoff for the two species is quite small. Thus it cannot be argued that the guinea pig is fundamentally different from the human in this respect.


The conclusion that outer hair cells of the human cochlea are stimulated by infrasound at levels lower than can be heard, and that the responses of these cells are likely transmitted to the brain by “subconscious” pathways (Type II afferent fibers) remains intact. It cannot be concluded that infrasound from wind turbines cannot affect humans living nearby.