221
5 Future Directions
1. Do the sensory roles of the lagena and the saccule differ substantially for otophysines (fishes with otophysic connections that enhance detection of the pressure
component of underwater sound) versus non-otophysines (like the oyster toadfish) that lack otophysic connections? This seems likely, but requires carefully
conducted comparative studies. Fay (1984) provided the first data on the directional responses from the utricle, saccule, and lagena in the goldfish, using the
same frequency stimulus for all three. Those data are a clear indication that all
three endorgans can have overlapping frequency responses, and that each could
contribute to directional sound analyses. However, there are distinct differences
in the size and shape of the saccule and lagena in different fish species. The overall areas of the sensory epithelia for the lagena and the saccule in goldfish are
nearly equivalent and the endorgans lie directly adjacent to each other (Platt
1977; Edds-Walton and Popper 2000). In most teleosts investigated thus far, the
lagena is the smallest of the three otolithic endorgans and is located caudal to the
saccule. In toadfish, the lagenar nerve joins VIII with the afferents from the posterior canal crista. Does the relative size and/or location of the lagena reliably
reflect its role in audition versus gravistatic/postural functions?
2. What are the roles of the nuclei in the secondary octaval populations? Although
they are clearly involved in the ascending auditory circuit, and maybe other sensory systems as well (McCormick 2011), the location and small size of these
nuclei (though the individual cells can be large, particularly in the dorsal division) provide a huge technical challenge for electrophysiology. Answering this
particular research question may best be approached by the use of a slice preparation centered at the entrance of VIII.
3. Can otophysine fishes determine the direction of a sound source, despite
enhanced “unidirectional” input from the gas bladder? Zeddies et al. (2012) have
shown that local particle motion is the key parameter used by the midshipman
fish (a non-otophysine) to localize a sound source (conspecific vocalization from
a speaker). A similar study (with a carefully quantified sound field) is needed,
ideally using a vocal otophysine fish, but also using goldfish or carp. There are
anecdotal stories of trained goldfishes or carp coming to a feeding site when a
“dinner bell” attracts them. In that context, there are multiple cues for the fish,
including visual observations of the feeder (human or mechanical) and chemical
cues in the water. Will the goldfish localize a sound source without other cues to
attract them in a natural setting?
4. Does input from the gas bladder facilitate sound source localization in fishes that
lack a mechanical connection between the gas bladder and the ear? Coffin et al.
(2014) have provided evidence that midshipman fish use the indirect input from
the gas bladder to compute the direction of a sound source. In addition, a recent
model of hearing in toadfish (Rozin et al. 2013) that incorporates the proximity
of the gas bladder and the morphology of the otic capsule (Edds-Walton et al.
2015) provides mathematical evidence that indirect input from the gas bladder
What the Toadfish Ear Tells the Toadfish Brain About Sound
5 Future Directions
1. Do the sensory roles of the lagena and the saccule differ substantially for otophysines (fishes with otophysic connections that enhance detection of the pressure
component of underwater sound) versus non-otophysines (like the oyster toadfish) that lack otophysic connections? This seems likely, but requires carefully
conducted comparative studies. Fay (1984) provided the first data on the directional responses from the utricle, saccule, and lagena in the goldfish, using the
same frequency stimulus for all three. Those data are a clear indication that all
three endorgans can have overlapping frequency responses, and that each could
contribute to directional sound analyses. However, there are distinct differences
in the size and shape of the saccule and lagena in different fish species. The overall areas of the sensory epithelia for the lagena and the saccule in goldfish are
nearly equivalent and the endorgans lie directly adjacent to each other (Platt
1977; Edds-Walton and Popper 2000). In most teleosts investigated thus far, the
lagena is the smallest of the three otolithic endorgans and is located caudal to the
saccule. In toadfish, the lagenar nerve joins VIII with the afferents from the posterior canal crista. Does the relative size and/or location of the lagena reliably
reflect its role in audition versus gravistatic/postural functions?
2. What are the roles of the nuclei in the secondary octaval populations? Although
they are clearly involved in the ascending auditory circuit, and maybe other sensory systems as well (McCormick 2011), the location and small size of these
nuclei (though the individual cells can be large, particularly in the dorsal division) provide a huge technical challenge for electrophysiology. Answering this
particular research question may best be approached by the use of a slice preparation centered at the entrance of VIII.
3. Can otophysine fishes determine the direction of a sound source, despite
enhanced “unidirectional” input from the gas bladder? Zeddies et al. (2012) have
shown that local particle motion is the key parameter used by the midshipman
fish (a non-otophysine) to localize a sound source (conspecific vocalization from
a speaker). A similar study (with a carefully quantified sound field) is needed,
ideally using a vocal otophysine fish, but also using goldfish or carp. There are
anecdotal stories of trained goldfishes or carp coming to a feeding site when a
“dinner bell” attracts them. In that context, there are multiple cues for the fish,
including visual observations of the feeder (human or mechanical) and chemical
cues in the water. Will the goldfish localize a sound source without other cues to
attract them in a natural setting?
4. Does input from the gas bladder facilitate sound source localization in fishes that
lack a mechanical connection between the gas bladder and the ear? Coffin et al.
(2014) have provided evidence that midshipman fish use the indirect input from
the gas bladder to compute the direction of a sound source. In addition, a recent
model of hearing in toadfish (Rozin et al. 2013) that incorporates the proximity
of the gas bladder and the morphology of the otic capsule (Edds-Walton et al.
2015) provides mathematical evidence that indirect input from the gas bladder
What the Toadfish Ear Tells the Toadfish Brain About Sound