It is widely believed that humans are better than other mammals at
discriminating different frequency sounds. However, mechanisms that could
account for such differences are unknown. Recently the TM has been shown to
support longitudinally propagating waves, and these waves have been implicated
in tuning properties in a mutant mouse model of hearing. In this study we
investigate the possibility that sharper tuning in humans could result from
differences in human TM waves relative to those of other mammals.
TM wave properties were measured in samples taken from human cadavers,
guinea pigs, and mice. Human TM wave measurements were performed 48 hours
post-mortem. To determine what effect this amount of time could have on TM
properties, we aged mouse cochleae and compared the resulting properties to
those from freshly dissected samples. We found that TM fixed charge density
decreased when samples were aged. We studied the effect of charge on TM waves
using KCl to modulate charge shielding and found that this had little to no
effect on wave properties. We also found that TM wave properties in aged samples
were not significantly different from those in fresh preparations.
These experiments led us to conclude that charge does not play an important
role in TM wave properties. Although surprising, this result is consistent with
the fact that TM waves involve shearing displacements, as opposed to bulk
compression, of the TM.
We performed measurements of TM wave decay constants and wave speeds in
humans, guinea pigs and mice and found that in all three species, wave
properties were similar to those previously seen in mice. Namely, apical TM
wave speeds (1 - 3 m/s) were significantly smaller than basal wave speeds (3 -
10 m/s), and basal TM decay constants (0.1 - 0.15 mm) were smaller than apical
decay constants (150 - 250 μm). We also saw that wave speeds increased
with frequency and decay constants decreased with frequency. Overall, TM wave
properties were not significantly different in humans, guinea pigs and mice.
If we use each animal's cochlear map to convert distance measurements into
frequency measurements, a different picture emerges. By scaling TM wave
properties by the cochlear map, human TM wave decay constants (0.03 - 0.05
octaves) are significantly smaller than those in mice (0.1 - 0.2 octaves) and
guinea pigs (0.05 - 0.09). We conclude that this smaller spread of excitation
in TM waves contributes to the sharper frequency selectivity in humans.
Although radial cross-sections of the organs of Corti are similar in these
three species, there are significant differences in their longitudinal
properties that can account for hearing differences. Our findings show
that the wave properties of the TM in combination with the cochlear map can be
used to predict frequency selectivity in the cochlea, suggesting that the
remarkable sharpness of tuning in humans can be explained by the presence of
waves coupled with the cochlear map. Our results demonstrate the importance of
measuring spread of excitation on a physiologically relevant scale.
Thesis Supervisor: Dennis Freeman
Committee: Martha Gray, John Guinan, Christopher Shera