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First published online December 14, 2006
Journal of Experimental Biology 210, 75-81 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.02617

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Dipole hearing measurements in elasmobranch fishes

Brandon M. Casper^* and David A. Mann

College of Marine Science, University of South Florida, 140 7th Avenue South, St Petersburg, FL 33701, USA

View larger version (7K): [in this window] [in a new window]   Fig. 1. Diagram of the dipole hearing setup. Drawing not to scale.

View larger version (18K): [in this window] [in a new window]   Fig. 2. (A) Auditory evoked potentials (AEPs) from a bamboo shark in response to a 100 Hz signal at four signal levels. As the signal is decreased in level (particle acceleration, m s-2) the AEP signal also decreases until it is lost in the noise at 1.0-4 m s-2. (B) 2048 fast fourier transform (FFT) of the same AEP for the bamboo shark in response to a 100 Hz sound. The arrow indicates the frequency doubling peak, which occurs at 200 Hz. A positive detection is when the peak (at twice the frequency played) is at least 3 dB above the noise floor. The noise floor is estimated from the AEP power spectrum with a window of 100 Hz around the doubling frequency. (C) Pressure and particle velocity raw signals as recorded from the pressure/velocity probe. This example of particle velocity has been recorded in the z-axis. P, pressure; V, velocity.

View larger version (14K): [in this window] [in a new window]   Fig. 3. (A) Overhead view of a horn shark depicting the different locations that were stimulated by the dipole stimulus. (1) 5 cm in front of the anterior end of the shark, (2) right over the anterior end of the shark, (3) 2.5 cm posterior of the anterior end, (4) between the shark's eyes, (5) directly over the endolymphatic ducts, (6) 2.5 cm posterior of the endolymphatic duct pores, (7) 5 cm posterior of the endolymphatic duct pores, (8) 2.5 cm lateral of the endolymphatic duct pores, (9) 5 cm lateral of the endolymphatic duct pores, (10) 10 cm lateral of the endolymphatic duct pores, and (11) at the tail. The oval surrounding locations 5, 6 and 7 indicates the areas that yielded the strongest evoked potential from the dipole stimulus. Positions 5 and 6 are the location of the parietal fossa. (B,C) Evoked potential levels (mean ± s.d.) recorded from the horn shark and bamboo shark, respectively, at each location for 50, 100 and 200 Hz. Note that the closer the level obtained (in dBV) was to 0, the stronger the evoked potential that was recorded. 200 Hz yielded a weaker evoked potential in both species than 50 and 100 Hz, as it is the upper range of hearing in these species. Position numbers correspond to those in A.

View larger version (6K): [in this window] [in a new window]   Fig. 4. Dipole audiograms of the horn shark Heterodontus francisci (N=3) and the white-spotted bamboo shark Chiloscyllium plagiosum (N=5). Values are means ± s.e.m. Ambient noise levels found in a quiet, tidal-dominated shallow harbor are plotted for comparison (broken line). Broad-band background noise in the test tank was consistently below 10-7 m s-2.

View larger version (7K): [in this window] [in a new window]   Fig. 5. Comparison of shark dipole particle acceleration audiograms (white symbols) with dipole audiograms from the goldfish Carassius auratus (black squares) and the mottled sculpin Cottus bairdi (black diamonds), which were obtained using classical conditioning (Coombs, 1994).

View larger version (8K): [in this window] [in a new window]   Fig. 6. Particle acceleration audiograms of all tested species of elasmobranchs. Values for the nurse shark Ginglymostoma cirratum and yellow stingray Urobatis jamaicensis (Casper and Mann, 2006), lemon shark Negaprion brevirostris (Banner, 1967) and horn shark Heterodontus francisci monopole (Kelly and Nelson, 1975) were modified to compare to the particle acceleration data obtained in the dipole experiment. These four species were all tested with a monopole sound stimulus. The nurse shark and yellow stingray audiograms were obtained with auditory evoked potentials in terms of particle acceleration. The lemon shark and horn shark were obtained using classical conditioning methods with measurements in terms of particle displacement, which was converted to particle accelerations in this figure.

View larger version (9K): [in this window] [in a new window]   Fig. 7. Bamboo shark audiogram plotted in terms of (A) acceleration, (B) velocity and (C) displacement. These results support the proposal that the macula neglecta is a velocity detector, as there is a substantially flat response in terms of particle velocity irrespective of the change in frequency (B). For a velocity sensitive organ, if the thresholds are plotted in terms of acceleration (A) there is an increase in threshold with increase in frequency (approximately 6 dB per octave) and a decrease in threshold with increase in frequency when expressed in terms of particle displacement (C).