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To define the audiogram for a normally hearing common dolphin (Delphinus delphis) using the Auditory Brainstem Response (ABR) technique
This proposal presents a rationale for studying the hearing abilities of the common dolphin (Delphinus delphis) along with a proposed methodology for the generation of a non-invasive ABR audiogram for this species. Introduction The oceans are virtually transparent to sound, and opaque to light and radio waves. At a wavelength of 1 m (1,500 Hz), water is nearly 1,000,000 times more transparent to sound than to radio signals (Pilgrim and Lovell, 2002). This fact underlies the intense interest currently being directed toward the acoustical exploration of the oceans. Naturally produced sounds arise from a number of sources, such as breaking waves, heavy rain, volcanic activity, or from marine animals (bio-acoustic sources). Vocalisations such as whale song, along with the grunts and whistles from sonic fish are especially relevant for communication purposes, and during predator prey interactions (Myrberg, 1981). There are several types of anthropogenic sources used routinely that produce intense levels of noise, from commercial shipping and powered leisure craft, to deliberately produced signals such as the Low Frequency Active Sonar (LFA) used by the military in anti-submarine warfare, or from the airgun arrays used during a seismic survey of the substrate beneath the seafloor by the petroleum industry. These activities can generate noise levels in excess of 253 dB (re 1 uPa at 1 m) (Engås et al., 1996), and are comparable to the noise levels generated by a seafloor volcanic eruption, which can produce a source level of in excess of 255 dB (re 1 uPa) (Northrop, 1974). Recent concerns regarding the impact of anthropogenic sounds on fish and other marine animals has prompted a number of studies into the effects of intense noise exposure on the hearing systems of marine mammals (e.g., Costa et al., 2003; Ketten, 1995; Richardson et al., 1995; Todd et al., 1996; Whitlow et al., 1997). Trauma to the auditory system can result in lesions developing along the VIII nerve pathway, or ruptures in the blood vessels surrounding the inner ear. A number of techniques have been developed to investigate gross physiological damage, though concise evidence of inner ear hair cell loss from odontocetiforms exposed to loud noise has as yet to be presented.
The audiogram In order for an accurate diagnosis of raised hearing thresholds as a result of exposure to intense noise, the audiogram for a normally hearing animal must first be established. Until recently, very little has been documented regarding the hearing abilities of marine animals, with a number of authors purporting that fish and invertebrates are only responsive to strong vibrations and near field disturbances (e.g. Cohen and Dijkgraaf 1961; Larsel, 1967; Wever, 1976). This, however, is contrary to the findings of Parker (1903) and von Frisch (1938) on fish species, and Lovell et al. (2005) on the hearing abilities of crustaceans. Hearing thresholds from any animal possessing the appropriate receptor mechanism are illustrated in an audiogram (Myerberg, 1981), which presents the lowest level of sound that a species can hear as a function of frequency. Both the sensitivity of hearing and the frequency range over which sound can be heard varies greatly from species to species. For man, sound is ultrasonic above 18 to 20 kHz, whilst for many fish species, sounds above 1 kHz are ultrasonic and for a number of odontocetiforms, sounds above 150 kHz are ultrasonic. This diversity in hearing ability between organisms indicates the importance of being able to accurately define hearing thresholds, especially when evaluating the influence of intense underwater sounds on both the physiology and ecology of marine animals.
The hearing frequencies or audiograms for a number of odontocetiformes are well characterised and have been produced using both physiological and behavioural approaches (see Nachtigall et al., 1995; Kastelein, et al., 2003; Sauerland and Dehnhardt, 1998; Gerstein et al., 1999; Kastelein et al., 2002), though an audiogram for the common dolphin (D. delphis) has as yet to be produced. The bottlenose dolphin (T. truncates) hears frequencies from 100 Hz to 150 kHz (Johnson, 1966; 1967), and the striped dolphin (Stenella coeruleoalba) hears frequencies ranging from around 500 Hz to 150 kHz (Kastelein, 2003; Brill et al., 2001), with both producing broadband clicks for echolocation that range in frequency from 20 Hz to around 200 kHz. P. phocoena hears frequencies between 300 Hz (Kastelein et al., 2002), up to as high as 190 kHz (Bibikov, 1992; Popov, 1986; Kastelein et al., 2002), and utilises a narrow band high frequency sonar of around 120 to 140 kHz (Busnel and Dziedzic, 1966a). It is feasible that this difference in hearing ability between T. truncates and P. phocoena is explained by the larger cochlea in the bottlenose dolphin (Wever et al., 1971; Ketten, 1997). Physiological evidence (Lovell et al., in review) suggests that the audiogram for D. delphis may lie somewhere between the hearing range of T. truncates and P. phocoena. Therefore, the delineation of hearing ability is of considerable importance as part of an accurate assessment of the impact of anthropogenic sounds on the inner ear physiology of D. delphis.
Previous uses of ABR in cetacean audiometry Popov and Supin (1990) studied hearing in the beluga dolphin (Delphinapterus leucas), the bottlenose dolphin (Tursiops truncates), the Amazon River dolphin (Inia geoffrensis), tucuxi dolphin (Sotalia fluviatilis) and the Manatee (Trichechus inunquis) using the ABR technique. The hearing tests were conducted in either a 4 m x 0.6 m x 0.6 m bath, in a round pool, or in an enclosed sea bay. During the tests, the subject was supported on a stretcher positioned so only the top of the head with the blowhole and the back, as far as the dorsal fin was out of the water. The Auditory Evoked Potentials (AEP’s) were recorded using 0.4 mm to 0.6 mm diameter subcutaneous needle electrodes inserted into the skin at depths of between 3 mm to 5 mm (see also: Popov, Ladygina and Supin, 1986). The record electrode was placed on the dorsal head surface 60 mm to 90 mm caudal from the blowhole, and the reference electrode placed on the back near to the dorsal fin. The potential difference between the two electrodes was fed to a biological amplifier (gated between 5 Hz to 5000 Hz) and the signal averaged to reveal the AEP. The stimulus sounds used in the audiological tests were clicks, square enveloped noise or ramped tone bursts of frequencies of between 5 kHz to 160 kHz, generated using piezo-ceramic transducers with diameters of 20 mm, 30 mm and 50 mm. The array was stationed 300 mm below the water surface, at distances of between 1 m to 2 m anterior of the subject’s head.
In a second series of experiments using the ABR technique on odontocetiforms, Bibikov (1992) studied hearing in the harbour porpoise (Phocoena phocoena) using both cutaneous and implanted electrodes. The porpoise was loosely restrained in a bath with dimensions of 2.5 m x 0.6 m x 0.65 m, which had been lined with sound absorbing rubber and filled with seawater. The record electrode used in the first experiment was a 10 mm diameter silver disc placed on the surface of the skin above the muscles overlying the vertex of the head, whilst the second experiment used an implanted electrode. In both experiments, the reference electrode was a subcutaneous needle electrode inserted into the skin close to the dorsal fin, and the AEP’s gated between 50 Hz and 4 kHz for the subcutaneous electrode and 200 Hz to 5 kHz for the surface electrode.
The non-invasive ABR technique The non-invasive ABR system and technique developed at the University of Plymouth, records far-field of synchronous neural activity in the eighth nerve and brainstem auditory nuclei elicited by acoustic stimuli (Jewett, 1970; Jewett and Williston, 1971; Jacobson, 1985; Kenyon et al., 1998). Similar protocols are used in hospitals to test for deafness in babies, so it is duly classified by the UK Home Office as being completely non-invasive. ABR measurements are used routinely in the clinical evaluation of human hearing (Jacobson, 1985) and allow for the determination of thresholds from uncooperative or inattentive subjects, in situations where behavioural methods cannot be readily applied. Figure 1 presents a set of AEP waveforms from a paddlefish (Polyodon spathula) in response to a four cycle 300 Hz tone burst, presented with the transducer and fish stationed 300 mm below the water surface. Waveforms clearly present with similarities between fish and higher vertebrates (Corwin, 1981) and between vertebrates and invertebrates (Lovell et al., 2005a).
Figure 1. AEP waveforms recorded from P. spathula in response to a 300 Hz tone burst attenuated in six steps. The X axis represents time and is termed the sweep velocity (from Lovell et al., in review)
Methodology In the proposed ABR audiological investigation of D. delphis, surface (cutaneous) electrodes are arranged with the reference electrode positioned at the vertex of the head, and the record electrodes positioned behind each ear. This configuration will enhance the recording of far field AEP’s, as the electrodes span the length of the nerve pathway between the cortex, brainstem and ear, thus negating the need for the implantation of subcutaneous electrodes. The advantage of using the three electrode setup is that each ear can be tested independently, a procedure commonly used in human audiological tests.
The ABR measurements of hearing thresholds is made using a control and analysis program, which both generates the stimulus signals and captures and analyses the response. The sound field is generated by a Laptop PC and presented through an array of free standing transducers positioned at least 1 m to 2 m from the head of the subject (see Figure 2 for equipment schematic). The array will need to generate tone bursts ranging in frequency from below 50 Hz to in excess of 150 KHz.
Figure 2. Schematic of the non-invasive ABR system
A low frequency transducer (20 Hz to 3 kHz) has been set up and calibrated, though the high frequency ceramic transducers have as yet to be incorporated in the system. The time it takes to identify thresholds is dependant on the frequency of the tone burst and the recording conditions; high frequencies (above 10 kHz) can be taken to threshold within 3 minutes, whilst lower frequencies take longer (up to 15 minutes at 100 Hz). Figure 3 shows the time taken to run two repeat ABR tests for single amplitude at a selected frequency (using a standard setup averaging 500 sweeps of 25 ms). The number of averaged sweeps required to determine threshold is dependant on the setup of the system and recording conditions, as in some instances threshold can be reached in as little as 200 sweeps, up to as many as 2000. The usual protocol is to attenuate the stimulus sound in six equal steps, until the EP waveform is no longer discernable above the averaged ambient electrical noise (about 0.1 µv in ideal recording conditions). In general, it takes just over an hour to complete a full audiological test, though there is no reason why this cannot be broken up into more than one session.
Figure 3. Graph showing the time taken to repeat test single amplitudes at a given frequency, averaged from 500 sweeps of 25 ms (using a 4 cycle tone burst)
The interface between the electrode and test subject When recording Auditory Evoked Potentials (AEP’s) in seawater, using cutaneous electrodes with the subject stationed below the surface, it was found that substantial attenuation of the evoked potential signal occurred at the tip and had a profound effect on the quality of the ABR trace. Previous ABR investigations of odontocetiform animals have mainly used sub-cutaneous electrodes that are out of the water during the test, thus reducing AEP attenuation through conduction. AEP responses have been examined in respect to the levels of attenuation in marine, brackish and fresh water (Lovell, unpublished data). It was found that in order to record high quality AEP’s in seawater, a modification needed to be made at the electrode tip (see Figure 3 for schematic). The modification involved fitting a silicone rubber cap to the upper portion of the exposed electrode tip, thus providing an insulated seal between the ambient seawater and the contact point between the electrode tip and fish. Figure 4 presents a schematic of the electrode configuration, which uses a rubber sucker disk to provide electrical insulation from the conductive properties of the ambient seawater.
Figure 4. The electrode tip configuration with the rubber sucker disk, providing insulation from the conductive properties of the ambient seawater (scale1.5:1)
The ABR response is readily dominated by myogenic noise caused by muscular movement, thus while undertaking the ABR measurements on fish, the subject is held in a cradle and supplied with oxygenated water so it has no need to swim. The ambient light levels are kept low during the assessment, as many species react to this by becoming passive, though the system we use can detect and reject ABR responses that are contaminated by myogenic noise. In order for an ABR assessment of hearing to be conducted on D. delphis, protocols will need to be developed that will minimise voluntary muscular activity during the test. Primarily, this will involve discussions with handlers and others involved in the animals day to day maintenance and welfare, as a psychological approach may be the most appropriate to achieve optimum results.
References
Bibikov, N.G. (1992). Auditory brainstem responses in the harbour porpoise (Phocoena phocoena). In: 'Marine Mammal Sensory Systems', 197-211. Thomas, J. et al (eds). Plenum Press, New York.
Brill, R.L., Moore, P.W.B. & Dankiewicz, L.A. (2001). Assessment of dolphin (Tursiops truncates) auditory sensitivity and hearing loss using jawphones. JASA, 109(4), 1717-1722.
Busnel, R.G., & Dziedzic, A (1966a). Acoustic signals of the pilot whale Globicephala melaena and of the porpoises Delphinus delphis and Phocoena phocoena. (in K.S. Norris, ed.) Whales, dolphins and porpoises. University of California Press, Berkeley. pp. 607-646
Corwin, J.T., (1981). Audition in elasmobranchs. In: Tavolga WN, Popper AN, Fay RR (eds) Hearing and sound communication in fishes. Springer, Berlin Heidelberg New York, pp 81 - 105
Corwin, J.T, Bullock T.H. & Schweitzer J. (1982). The auditory brainstem response in five vertebrate classes. Electroencephalogr. Clin. Neurophysiol. 54: 629-641
Costa, D.P, Crocker, D.E, Gedamke, J, Webb, P.M, Houser, D.S, Blackwell, S.B, Waples, D, Hayes, S.A. & Le Boeuf, B.J. (2003). The effect of a low-frequency sound source (acoustic thermometry of the ocean climate) on the diving behavior of juvenile northern elephant seals, Mirounga angustirostris. Journal of the Acoustical Society of America 113(2):1155-1165.
Engås, A., Løkkeborg, S., Ona, E., & Soldal, A.V. (1996). Effects of seismic shooting on local abundance and catch rates of cod (Gadus Morhua) and haddock (Melanogrammus aeglefinus). Canadian J. of Fisheries and Aquatic Sciences. 53, 2238 - 2249.
Frisch, K. von, (1936). UÈ ber den GehoÈ rsinn der Fische. Biol Rev 11: 210 - 246
Gerstein, E.R., Gerstein, L., Forsythe, S.E, & Blue, J.E. (1999). The underwater audiogram of the West Indian manatee (Trichechus manatus). JASA, 105(6), 3575-3583.
Jacobson, J.T., (1985). An overview of the auditory brainstem response. In: Jacobson JT (ed) The auditory brainstem response. College-Hill Press, San Diego, pp 3 – 12
Jewett, D.L., (1970) Volume conducted potentials in response to auditory stimuli as detected by averaging in the cat. Electroencephalogr Clin Neurophysiol 28: 609 – 618
Jewett, D.L., Williston, J.S., (1971). Auditory evoked far fields averaged from the scalp of humans. Brain 94: 681 - 696
Johnson, C.S. (1966). Auditory thresholds of the bottlenosed porpoise (Tursiops truncatus). U.S. Naval Ord. Test Stn., Tech. Oubl., 4178: 1-28.
Johnson, C.S, (1967). Sound detection thresholds in marine mammals. In W.N. Tavolga (ed), Marine bio-acoustics, vol. 2. Pergamon, Oxford, U.K.
Kastelein, R.A., Bunskoek, P., Hagedoorn, M., Au, W.L.W. & de Haan, D. (2002). Audiogram of a harbor porpoise (Phocoena phocoena) measured with narrow-band frequency-modulated signals. JASA, 112(1), 334-344.
Kastelein, R.A., Hagedoorn, M., Au, W.W.L. & de Haan, D. (2003). Audiogram of a striped dolphin (Stenella coeruleoalba). JASA, 113(2), 1130-1137.
Kenyon, T.N., Ladich, F. & Yan, H.Y. (1998). A comparative study of the hearing ability in fishes: the auditory brainstem response approach”. J Comp Physiol A 182:307–318.
Ketten, D.R. (1995). Estimates of blast injury and acoustic trauma zones for marine mammals from underwater explosions. In: Sensory Systems of Aquatic Mammals, R. Kastelein, J. Thomas, and P. Nachtigall (eds.), DeSpil Publishers, pp. 391-408
Ketten, D.R. (1997). Structure and Function in Whale Ears, Bioacoustics, vol.8, no. 1, pp. 103-136.
Lovell, J.M., Findlay, M.M., Moate, R.M., & Yan, H.Y., 2005 A. The hearing abilities of the prawn Palaemon serratus. Comp. Biochem. Physiol. A Mol. Integr. Physiol. Jan;140(1):89-100.
J .M Lovell, M.M Findlay, R.M Moate, J.R Nedwell & M.A Pegg (in review). The inner ear morphology and hearing abilities of the Paddlefish (Polyodon spathula) and the Lake Sturgeon (Acipenser fulvescens). Comp. Biochem. Physiol. A Mol. Integr. Physiol.
Myrberg, A.A. (1981). Sound communication and interception in fishes. Hearing and Sound Communication in Fishes. Tavolga WN, Popper AN, and Fay RR, Eds. Springer-Verlag, New York.
Nachtigall, P.E., Au, W.W.L., Pawloski, J.L. & Moore, P.W.B. (1995). Risso’s dolphin (Grampus griseus) hearing thresholds in Kaneohe Bay, Hawaii. In ‘Sensory Systems of Aquatic Mammals’, 49-53. R.A. Kastelein et al (eds). De Spil Publ., Woerden, Netherlands.
Northrup, J. (1974). Detection of low-frequency underwater sounds from a submarine volcano in the western Pacific. J. Acoust. Soc. Am. 56(3), 837-841.
Parker, G.H., (1903). The sense of hearing in fishes. Am Nat 37: 185 – 204
Pilgrim, D.A. & Lovell, J.M., (2002). A review of current publications dealing with the impact of low frequency sounds upon fish. Report to Devon Sea Fishing Association.
Popov, V.V., Ladygina, T.F. & Supin, A.Ya. (1986). Evoked potentials of the auditory cortex of the porpoise, Phocoena phocoena. J. Comp. Physiol., 158:705-711.
Popov, V. & Supin, A. (1990). Electrophysiological studies of hearing in some cetaceans and a manatee. In ‘Sensory Abilities of Cetaceans’, 405-415. J. Thomas & R. Kastelein (eds). Plenum Press, N.Y.
Richardson, W. J., Greene, C. R., Jr., Malme, C. I., & Thomson, D. H. (1995). Effects of Noise on Marine Mammals. Academic, San Diego pp 576.
Sauerland, M. & Dehnhardt, G. (1998). Underwater audiogram of a tucuxi (Sotalia fluviatilis guianensis). JASA, 103(2): 1199-1204.
Todd, S., S. Stevick, J. Lien, F. Marques, & D. Ketten (1996). Behavioural effects of exposure to underwater explosions in humpback whales (Megaptera novaeangliae). Canadian Journal of Zoology, 74(9):1661-1672.
Wever, E. G., McCormick, J. G., Palin, J. & Ridway, S. H. (1971). The Cochlea of the Dolphin, Tursiops truncatus: Hair Cells and Ganglion Cells. Proceedings of the National Academy of Sciences of the United States of America 68: 2908-2912.
Whitlow, W., Au, W., Nachtigall, P. & Pawloski, J. (1997). Acoutic effects of the ATOC signal (75Hz, 195 dB) on dolphins and whales. Journal of the Acoustical Society of America, 101(5) Pt1
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