The Use of Acoustically Evoked Potentials for the Study of Hearing

							 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
                                     262 Taiwan
                          Email: hyyan@gate.sinica.edu.tw


Introduction


       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

gas.
                                                                        Speaker             Reservoir

                 power amp.


                 oscilloscope                 Microphone


                 TG6

                 PC1                                                              Electrode (Ref.)
                                                            Electrode (Rec.)
                                         P15 Preamp.

  Computer       AD1

                                         P15 Preamp.                       Fish
                 DA1
                                                                        Hydrophone
                 PA4


                 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.


Results



       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

Curtsinger 2000).



Discussion



          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

device.



References



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Jewett, D.L. (1970) Volume conducted potentials in response to auditory stimuli as

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Kenyon, T.N, Ladich, F. & Yan, H.Y. (1998) A comparative study of hearing ability in

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Sand, O. & Enger, P.S. (1973) Evidence for an auditory function of the swimbladder

in

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thresholds

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