86
D.R. Ketten
sidered a significant evolutionary development that accommodates a longer
basilar membrane in a confined space. The longer membrane allows more
frequencies to be encoded, and consequently, mammals generally have
expanded upper frequency ranges and better high-frequency sensitivity
compared to other vertebrates.
Basilar membrane dimensions are thought to be an important component of the resonance characteristics of the cochlea (von Bekesy 1960). In
mammalian cochleae, thickness and width vary inversely from base to apex.
The construction of the basilar membrane mechanically tunes the ear to a
specific set of frequencies. The highest frequency each animal hears is
encoded at the base of the cochlear spiral, where the membrane is narrow,
thick, and relatively stiff. Moving towards the apex of the spiral, as the membrane becomes broader and more compliant, progressively lower frequencies are encoded (Table 2.1; Figs. 2.6,2.7,2.8). Interspecific differences in
hearing ranges are dictated largely by differences in stiffness and mass that
are the result of differences in basilar membrane thickness and width along
the cochlear spiral. For an animal to "hear" a sound, its basilar membrane
must have a point along the membrane that resonates at the sound's constituent frequencies. Therefore, mammalian basilar membranes are essentially banks of tonotopically arranged resonators, arrayed high to low
from base to apex, rather like a guitar with densely packed strings covering multiple octaves.
For any input signal within the hearing range of the animal, the entire
basilar membrane will respond to some degree. At anyone moment, each
region of the membrane will have a different amount of deflection and a
different phase related to the input signal. Over time, changes in amplitude
and phase at each point give the impression of a traveling response wave
along the cochlea, but because membrane segments with resonance characteristics closest to frequencies in the signal have greater displacements
FIGURE 2.8. Basilar membrane ratios. Average thickness/width basilar membrane
ratios are plotted as a percentage of cochlear length for five land mammals and five
cetaceans. High ratios reflect a thicker, stiffer membrane capable of responding to
ultrasonic frequencies. Differences in the basal basilar membrane ratios among the
echolocators are consitent with the peak frequency differnces among species.
Plateaus followed by steep declines in the porpoise and bat curves reflect foveal
regions. Basal ratios in the low-frequency cochleae are similar to the mid-cochlear
ratios of higher frequency animals. The fin whale has a basal ratio similar to two
other mysticetes but a steeper slope and a significantly lower apical ratio. Species
included in the plot are: harbour porpoise, Phocoena phocoena; bottlenose dolphin,
Tursiops truncatus; Northern right whale, Eubalaena glacialis; humpback whale,
Megaptera novaeangliae; fin whale, Balaenoptera physalus; horseshoe bat, Rhinolophus ferrumequinum; human, Homo sapiens; kangaroo rat, Dipodomys merriami;
mole rat, Spalax ehrenbergi; cat, Felis domesticus.
D.R. Ketten
sidered a significant evolutionary development that accommodates a longer
basilar membrane in a confined space. The longer membrane allows more
frequencies to be encoded, and consequently, mammals generally have
expanded upper frequency ranges and better high-frequency sensitivity
compared to other vertebrates.
Basilar membrane dimensions are thought to be an important component of the resonance characteristics of the cochlea (von Bekesy 1960). In
mammalian cochleae, thickness and width vary inversely from base to apex.
The construction of the basilar membrane mechanically tunes the ear to a
specific set of frequencies. The highest frequency each animal hears is
encoded at the base of the cochlear spiral, where the membrane is narrow,
thick, and relatively stiff. Moving towards the apex of the spiral, as the membrane becomes broader and more compliant, progressively lower frequencies are encoded (Table 2.1; Figs. 2.6,2.7,2.8). Interspecific differences in
hearing ranges are dictated largely by differences in stiffness and mass that
are the result of differences in basilar membrane thickness and width along
the cochlear spiral. For an animal to "hear" a sound, its basilar membrane
must have a point along the membrane that resonates at the sound's constituent frequencies. Therefore, mammalian basilar membranes are essentially banks of tonotopically arranged resonators, arrayed high to low
from base to apex, rather like a guitar with densely packed strings covering multiple octaves.
For any input signal within the hearing range of the animal, the entire
basilar membrane will respond to some degree. At anyone moment, each
region of the membrane will have a different amount of deflection and a
different phase related to the input signal. Over time, changes in amplitude
and phase at each point give the impression of a traveling response wave
along the cochlea, but because membrane segments with resonance characteristics closest to frequencies in the signal have greater displacements
FIGURE 2.8. Basilar membrane ratios. Average thickness/width basilar membrane
ratios are plotted as a percentage of cochlear length for five land mammals and five
cetaceans. High ratios reflect a thicker, stiffer membrane capable of responding to
ultrasonic frequencies. Differences in the basal basilar membrane ratios among the
echolocators are consitent with the peak frequency differnces among species.
Plateaus followed by steep declines in the porpoise and bat curves reflect foveal
regions. Basal ratios in the low-frequency cochleae are similar to the mid-cochlear
ratios of higher frequency animals. The fin whale has a basal ratio similar to two
other mysticetes but a steeper slope and a significantly lower apical ratio. Species
included in the plot are: harbour porpoise, Phocoena phocoena; bottlenose dolphin,
Tursiops truncatus; Northern right whale, Eubalaena glacialis; humpback whale,
Megaptera novaeangliae; fin whale, Balaenoptera physalus; horseshoe bat, Rhinolophus ferrumequinum; human, Homo sapiens; kangaroo rat, Dipodomys merriami;
mole rat, Spalax ehrenbergi; cat, Felis domesticus.