The Use of Acoustically Evoked Potentials for the Study of
Hearing in Fishes
Hong Young Yan
Marine Research Station, Institute of Zoology, Academia Sinica, Jiashi, I-Lan County
The scientific study on fish hearing dates back to the turn of the 20th century.
Over the years several methods have been developed to investigate fish hearing.
Behavioral methods usually involve training fish by using electric shock or food
rewards to respond upon hearing a sound. Tavolga and Wodingsky (1963) develop an
instrumental avoidance conditioning method in which fish is trained and learn to cross
a barrier in the tank upon hearing a sound to avoid electric shock. In the classical
conditioning method, fish respond with ventilatory suppression (Fay, 1969). A third
behavioral method is operant conditioning which involves positive reinforcement of
training fish to peck paddles in response to sound (Yan and Popper 1991). The
electrophysiological methods have less limitation associated with training subjects.
Measurement of microphonics from auditory organs while presenting acoustic stimuli
to the test subjects is widely used to measure auditory sensitivity of fishes (Sand and
Enger 1973). The microphonics recording allow faster data gathering than behavioral
methods albeit with some constraints: 1) preparation is complex and invasive surgery
is required; 2) the placement of electrodes is restricted to specific endoragns and thus
responses recorded do not necessarily represent the whole auditory pathways. Another
electrophysiological recording method is the auditory brainstem response (ABR)
which is a non-invasive far-field recording of synchronous neural activity in the
eighth nerve and brainstem auditory nuclei elicited by acoustic stimuli (Jewett 1970).
The purpose of my presentation is to summarize the fish ABR recording technique
developed in my laboratory (Kenyon et al. 1998) and to highlight the results obtained
from various experiments (Yan 1998, Yan et al. 2000; Yan and Curtsinger 2000)
carried out by this protocol. New insights gained from the studies are shared to point
to some questions on fish hearing for future study.
Materials and Methods
The layout of the setup for fish ABR system is illustrated in a block diagram in
Figure 1 (also see Kenyon et al. 1998). The system is built with components available
in the market (with various electronic modules manufactured by Tucker Davis
Technologies, Gainsville, Florida) and only required very little effort in programming.
To test the hypothesis that a pulsating gas bubble inside a fish body can assist in
hearing, three anabantoids: blue gourami (Trichogaster trichopterus), kissing gourami
(Helostoma temminckii), dwarf gourami (Colisa lalia), goldfish (Carassius auratus),
oyster toadfish (Opsanus tau) and a mormyrid weakly electric fish (Brienomyrus
brachyistius) were used as test subjects. The audiograms were measured before
(baseline) and after gas removal either from suprabranchial chamber (in gourami),
gasbladder (goldfish, blue gourami, oyster toadfish) or otic gasbladder (mormyrid
weakly electric fish). The X-ray radiographs were taken to verify complete removal of
PC1 Electrode (Ref.)
P15 Preamp. Fish
MA1 Sump pump
MS1 Figure 1. Block diagram of the auditory brainstem response recording system.
Components inside the soundproof chambers are enclosed by bold lines. Signal
conditioning modules are enclosed by dashed lines.
In comparing with either microphonics or single unit recording methods, the
fish ABR protocol is easy to set up. This non-invasive technique allows repeated
recordings from the same subjects subject to various experimental treatments. An
additional advantage of fish ABR method is that on average an audiogram can be
obtained within 2-3 hours (Kenyon et al. 1998).
The removal of gas from the suprabranchial chambers of gouramis resulted in
significant elevation of hearing thresholds (Yan 1998). The largest elevation of
threshold all occurred at the best hearing frequencies of three gouramis species. The
removal of gas from the gasbladder of blue gourami, however, did not lead to any
change of hearing threshold (Yan et al. 2000). The removal of gas from the gasbladder
of goldfish showed significant change of thresholds and the replenishment of gas
inside the gasbladder 7 days after gas removal restored baseline hearing sensitivity
(Yan et al. 2000). The removal of gas from the heart-shaped gasbladder of oyster
toadfish did not alter overall audiogram (Yan et al. 2000). The removal of gas from
both sides of otic gasbladder in mormyrid resulted in significant elevation of hearing
thresholds (Yan and Curtsinger 2000). However, removal from only one of the two
otic gasbladders did not result in any significant change of thresholds (Yan and
The results of gas removal experiments support the hypothesis that fish
hearing is enhanced by the direct coupling (e.g., otic gasbladder in mormyrids,
suprabranchial chamber in gouramis) or mechanical coupling (e.g., goldfish) of gas
holding structure to the inner ears. However, removal of gas did not result in
reduction of hearing frequency bandwidth in gouramis, goldfish, and mormyrid. This
new finding pointing to the possible existence of different frequency coding system in
hearing specialist species than generalist species even without the aid of gas-holding
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