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Tonotopic organization in the midbrain of a teleost fish
Brain Research, 338 (1985) 387-391 Elsevier
387
BRE 20936
Tonotopic organization in the midbrainof a teleost fish STEPHEN M. ECHTELER Department of Neurosciences, School of Medicine and Neurobiology Unit, Scripps Institution of Oceanography, University of California at San Diego, La Jolla CA 92093 ( U.S.A.) (Accepted February 25th, 1985) Key words: teleost - - tonotopy- - midbrain - - hearing - - carp
Recordings from small clusters of units within the auditory midbrain of the Japanese carp, Cyprinus carpio, provide the first evidence for tonotopy within the teleost brain. These findingssuggestthat place mechanismsof frequencycodingand tonotopy, previously thought to have evolved first in amphibians, may be general and ancient features of the vertebrate auditory system.
In the cochleae of birds and mammals, sounds of different frequencies produce maximal mechanical displacements at different places along the basilar membrane 3. Auditory hair cells coupled to different portions of this membrane are thus 'tuned' to different frequencies, and nerve fibers innervating these receptors preserve this frequency specific information in an ordered 'tonotopic' projection onto neurons within the cochlear nuclei. The place principle of frequency coding and tonotopy are believed to be important mechanisms by which the auditory system performs fine frequency discriminations 3, and these features were once thought to be restricted to animals possessing an ear with a basilar membrane 38. Recently, this concept has changed because of findings that both place mechanisms and tonotopy are present in anurans, which lack a basilar membrane 14,21.26.28, and in reptiles which possess cochleae with untuned basilar membranest3, 23,24,37. Many species of teleost fish are also capable of frequency discrimination12, although the teleost ear lacks both a basilar and tectorial membrane. In these animals sound is processed by otolithic organs such as the lagena and especially the sacculusII,IS,19, organs which in terrestrial vertebrates function as equilibrial detectors 30. Temporal coding mechanisms are well developed in teleosts and may suffice for frequency
discriminationlO,34. Some reports, however, provide evidence for a crude place mechanism operating within the teleost sacculus 2.9A1.15.32. If these reports are accurate it would be reasonable to expect that tonotopy should also exist within the teleost brain. Yet previous electrophysiological studies of teleost medullary and midbrain auditory centers have not reported any evidence for tonotopic organization 16.20.27.33. Since most of these studies did not provide a detailed account of recording locations, it is not clear how thoroughly these regions were sampled. In the present study, a stereotaxic atlas was constructed of the torus semicircularis (TS, a midbrain structure thought to be homologous to the mammalian inferior colliculus) in the Japanese carp, Cyprinus carpio, an ostariophysan teleost with well developed auditory capacities29. Subdivisions of the TS, and their relation to overlying surface structures, were charted at 100/~m intervals in a series of camera lucida drawings of both horizontal and transverse sections of the carp brain. The TS was then systematically explored, with reference to this atlas, to test for tonotopic organization. Carp were anesthetized by immersion in a 0.01% solution of tricaine methanesulfonate (MS-222) and the right optic tectum exposed. Following surgery, animals were given an intramuscular injection of
Correspondence: S. M. Echteler. Present address: Auditory Research Laboratory, Northwestern University, Frances Searle Bldg., 2299 Sheridan Rd., Evanston, IL 60201, U.S.A. 0006-8993/85/$03,30© 1985 Elsevier Science Publishers B.V. (Biomedical Division)
388 Flaxedil (1.0/~g/g b.wt.) and placed in a head holder, with a ventilation tube. Animals were completely submerged except for the dorsal surface of the skull. The aquarium rested on a vibration isolation table within a sound isolation booth. Auditory unit clusters, consisting of 2-4 well isolated units, were recorded with indium-filled microelectrodes7 from the TS in response to isointensity tonebursts. Tones were produced by a waveform generator, attenuated, shaped into tonebursts of 100 ms duration with rise/fall times of 5 ms, and presented at 3 s intervals. Certain stimulus frequencies could not be employed since they were greatly distorted by the acoustics of the water-filled recording chamber. Eleven standard frequencies were chosen, spanning the range of maximum frequency sensitivity in carp 29, which could be reliably produced without significant distortion: 52, 106, 205, 306, 406, 503, 603, 700, 797, 894 and 991 Hz. Stimulus frequencies were not presented in any fixed order. At each test frequency, the attenuator was calibrated to produce a toneburst at an intensity of 10 dB re 1.0/~bar, as monitored by a hydrophone (Clevite CH-17), in place of the animal. During experiments, toneburst intensity was monitored by a hydrophone next to the animal. Sounds were generated in air by a loudspeaker positioned 0.6 m behind and above the animal's head. Most auditory unit clusters within the carp TS showed no spontaneous activity and responded phasically to toneburst stimuli. Evoked neuronal activity was amplified, band-passed filtered, and units crossing an arbitrary level set by a window discriminator were counted with a digital signal averager. At each recording location the total number of spikes produced by 5 sequential tonebursts was calculated for each test frequency. From these data isointensity frequency response profiles were constructed. Absolute threshold frequency tuning curves were also occasionally collected to test the reliability of the isointensity response profiles for measuring frequency selectivity. Tuning curves obtained by these two methods were frequently mirror images of one another. Test frequencies producing the largest number of spikes also produced responses at the lowest absolute sound intensities. At the end of many electrode tracks, lesions were produced to mark TS recording sites and check the accuracy of the stereotaxic coordinates used for electrode placement.
Results confirm an earlier report that auditory and lateral line sensory modalities are strictly segregated within medial and lateral subdivisions of the carp TS throughout its entire rostrocaudal extentS. The data presented here pertain only to the medial (auditory) subdivision. The tonotopic organization described in this paper is derived from frequency maps obtained in 6 carp. At least 20 recording locations were sampled in each animal. In several cases an animal remained in good physiological condition for two days allowing 50-70 recording locations to be sampled. Most unit clusters respond best to a single test frequency. Best frequencies are grouped here into three ranges: high (500-1000 Hz), middle (300-400 Hz) and low (50-200 Hz) (Fig. 1A). Neurons within each of these frequency ranges are found together within well defined torsi regions. Within the upper layers of the TS, high, middle and low frequencies are represented, sequentially, along an oblique line extending from the rostromedial to caudolateral TS (Fig. 1B). High frequency neurons, located along the medial border of the rostral TS, respond best to frequencies between 500 and 700 Hz; unit clusters preferring higher frequencies (800-1000 Hz) are rare. Middle frequency neurons form a lateral band within the central TS, which begins and extends caudal to this high frequency region. Low frequency neurons are most frequently encountered within the caudal TS, but they are also present within the ventral layers of the rostral and central TS. Electrode penetrations into these regions reveal high or middle frequency neurons within the upper 200-300/~m of the TS followed by an abrupt transition to low frequency neurons within the lower 200-300/~m of the TS (Fig. 1C, 1-2). In the caudal TS low frequency neurons are present at all recording depths (Fig. 1C, 3). Tonotopy is not precise within either the high or middle frequency TS regions. In the rostromedial TS, for example, neurons responding best to 700 Hz tonebursts could be located either rostrai or caudal to units with a frequency preference of 500 or 600 Hz. In low frequency regions of the TS, however, neurons responding best to the higher part of the range, such as 200 Hz, are found dorsal to those preferring 50-100 Hz. It is likely that this tonotopy does not arise in the carp midbrain but rather reflects some form of place
389
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Fig. 1. Tonotopic organization within the carp midbrain. A: isointensity frequency response profiles of three neuron clusters recorded from the torus semicircularis (TS) in one animal. Depending upon their location within the TS, auditory neurons produce maximum responses within one of three test frequency ranges: low (50-200 Hz, solid circles), middle (300-400 Hz, open circles), high (500-1000 Hz, X's). Response was measured by counting multiunit spikes produced following 5 isointensity tonebursts at each test frequency. For the frequency profiles illustrated here the total number of spikes produced at 100% response was 60 (low), 93 (mid), and 73 (high). B: the regional distribution of high, mid, and low frequency neurons within the upper layers of the TS. The outline of the TS, indicated by the dotted lines, is projected onto the surface of the optic tectum. Forty-five electrode penetrations, representing approximately 180 recording sites, were made in 6 animals. Twenty penetrations into the lateral and extreme caudal TS yielded no responses to acoustic stimuli, and are not plotted here. Of the remaining 25 electrode penetrations, 18 were histologically confirmed and are plotted above. Each symbol represents one electrode track, along which 3-5 unit clusters were recorded within the upper TS at increments of 50-100/~m. Calibration bar equals 500/~m. Abbreviations: CER, cerebellum; OT, optic tectum; TEL, telencephalon; TS, torus semicircularis; VL, vagal lobe. C: camera lucida drawings of three transverse sections through the TS in a single animal at the levels indicated in B, showing the distribution of high, mid and low frequency neurons with depth. Calibration arrows equal 300/tm and indicate dorsal (D) and medial (M) orientation.
mechanism operating within the auditory periphery, although a c o m p u t e d map is a possibility. F u r u k a w a and Ishi05, recording from the saccular nerve in gold-
fish, provided the first evidence suggesting that a place mechanism might be operating within a teleost ear. Neurons innervating hair cells within the anterior portion of the sacculus were found to r e s p o n d best to higher frequencies (600-800 Hz), whereas neurons contacting the posterior sacculus respond best to lower frequencies (200-400 Hz). Subsequent recordings of auditory hair cell r e c e p t o r potentials (microphonics) from different regions of the sacculus in another teleost with a more restricted hearing range, the sculpin, also showed that responses to higher frequencies (up to 500 Hz) could be o b t a i n e d only from the anterior portion of the sacculus 2. In addition, a recent r e p o r t on patterns of hair cell d a m a g e in the sculpin sacculus, following exposure to intense sounds of different frequencies, revealed that higher frequencies cause more destruction within the anterior sacculus, lower frequencies within the posterior sacculus 9. The potential mechanisms underlying frequency selectivity within the teleost sacculus are not well understood. Previous workers have suggested that saccular hair cells might be passively tuned by their mechanical linkages to supporting structures, such as the otolith 3~ or the macular sensory epithelium 5, which themselves could display frequency selective m o v e m e n t patterns. Subsequent studies of otolith movements in the perch sacculus, however, suggested only a very crude d e p e n d e n c e of m o v e m e n t pattern on frequency 32. Recently, however, intrinsic mechanisms of frequency tuning, first suggested by studies on electroreceptors4,1s,25,36, have been proposed for auditory hair cells in several reptilian species. These mechanisms do not d e p e n d upon hair cell coupling to accessory structures but rather relate either to micromechanical properties of the ciliary bundles capping the hair cells 17,35 or to the presence of electrically tuned filters within the hair cell membrane 6,t3. Most intriguing are reports of electrical tuning mechanisms within hair cells of the amphibian sacculust,22; it is conceivable that similar mechanisms may be operating within the otolithic organs of fishes. Finally, it is important to note that although studies suggest a rostrocaudal distribution of high to low frequencies within both the sculpin and goldfish sacculus, comparable data are not yet available for the carp. Therefore, at the present time, it is not possible
390 to relate the tonotopic organization found within the carp midbrain to frequency representation within the
found in carp to other teleosts.
carp sacculus. Moreover, given the marked speciesspecific diversity in the anatomical organization of the teleost sacculus 12, it seems unlikely that the pat-
I thank Dr. Theodore H. Bullock for his advice and encouragement. Drs. J. T. Enright, T. H. Bullock
tern of frequency representation will be the same in all teleost species. Consequently, caution must be advised against generalizing the pattern of tonotopy
manuscript. F u n d i n g for this study was provided by H I H and NSF grants to T. H. Bullock and an N I M H predoctoral fellowship to S . M . E .
1 Ashmore, J. F., Frequency tuning in a frog vestibular organ, Nature (Lond.), 304 (1983) 536-538. 2 Anderson, R. A. and Enger, P. S., Microphonic potentials from the sacculus of a teleost fish, Comp. Biochem. Physiol., 27 (1968) 879-881. 3 von B6k6sy, G., Experiments in Hearing, Wiley, New York, 1960, 745 pp. 4 Bennett, M. V. L., Mechanisms of electroreception. In P. Cahn (Ed.), Lateral Line Detectors, Indiana University Press, Bloomington, 1967, pp. 313-393. 5 van Bergeijk, W. A., Directional and nondirectional hearing in fish. In W. N. Tavolga (Ed.), Marine Bio-Acoustics, Pergamon Press, Oxford, 1964, pp. 281-350. 6 Crawford, A. C. and Fettiplace, R., The frequency selectivity of auditory nerve fibers and hair cells in the cochlea of the turtle, J. Physiol. (Lond.), 306 (1980) 79-126. 7 Dowben, R. M. and Rose, J. E., A metal-filled microelectrode, Science, 118 (1953) 22-23. 8 Echteler, S. M., Organization of the central auditory pathways in a teleost fish Cyprinus carpio, J. comp. Physiol., 156 (1985) 267-280. 9 Enger, P. S., Frequency discrimination in teleosts-peripheral or central? In W. N. Tavolga, A. N. Popper and R. R. Fay (Eds.), Hearing and Sound Communication in Fishes, Springer-Verlag, New York, 1981, pp. 243-253. 10 Fay, R. R., Phase-locking in goldfish saccular nerve fibers account for frequency discrimination capacities, Nature (Lond.), 275 (1978) 320-322. 11 Fay, R. R., Coding of information in single auditory nerve fibers of the goldfish, J. Acoust. Soc. Arner., 63 (1978) 136-146. 12 Fay, R. R. and Popper, A. N., Structure and function in teleost auditory systems. In A. N. Popper and R. R. Fay (Eds.), Comparative Studies of Hearing in Vertebrates, Springer-Verlag, New York, 1980, pp. 3-42. 13 Fettiplace, R. and Crawford, A. C., The origin of tuning in turtle cochlear hair cells, Hearing Res., 2 (1980) 447-454. 14 Fuzessery, Z. M. and Feng, A. S., Frequency representation in the dorsal medullary nucleus of the leopard frog, Rana p. pipiens, J. comp. Physiol., 143 (1981) 339-347, 15 Furukawa, T. and Ishii, Y., Neurophysiological studies of hearing in the goldfish, J. Neurophysiol., 30 (1967) 1377-1403. 16 GrOzinger, B., Elektro-physiologische Untersuchungen an der HOrbahn der Schleie (Tinca tinca L.), Z. vergl. Physiol., 57 (1967) 44-76. 17 Holton, T. and Hudspeth, A. J., A micromechanical contribution to cochlear tuning and tonotopic organization, Science, 222 (1983) 508-510. 18 Hopkins, C. D., Stimulus filtering and electroreception: tuberous electroreceptors in three species of gymnotid fish,
J. comp. Physiol., 111 (1976) 171-207. 19 Horner, K., Hawkins, A. D. and Fraser, P. J., Frequency characteristics of primary auditory neurons from the ear in the cod, Gadus morhua L. In W. N. Tavolga, A. N. Popper and R, R. Fay (Eds.), Hearing and Sound Communication in Fishes, Springer-Verlag, New York, 1981, pp. 223-241. 20 Knudsen, E. I., Distinct auditory and lateral line nuclei in the midbrain of catfishes, J. comp. Neurol., 173 (1977) 417-432. 21 Lewis, E. R., Leverenz, E. L. and Koyama, H., The tonotopic organization of the bullfrog amphibian papilla, an auditory organ lacking a basilar membrane, J. comp. Physiol., 145 (1982) 437-445. 22 Lewis, R. S. and Hudspeth, A. J., Voltage- and ion-dependent conductances in solitary vertebrate hair cells, Nature (Lond.), 304 (1983) 538-541. 23 Manley, G,, Frequency selectivity of auditory neurons in the caiman cochlear nerve, Z. vergl. Physiol., 66 (1970) 251-256. 24 Manley, J. A., Single unit studies in the midbrain auditory area of caiman, Z. vergl. Physiol., 71 (1971) 255-261. 25 Meyer, J. H. and Zakon, H. H., Androgens alter the tuning of electroreceptors, Science, 2l 7 (1982) 635-637. 26 Mohneke, R., Tonotopic organisation of the auditory midbrain of the midwife toad, Alytes obstetricans, Hearing Res., 9 (1983) 91-102. 27 Page, C. H,, Electrophysiological study of auditory responses in the goldfish brain, J. Neurophysiol., 33 (1970) 116-127. 28 Pettigrew, A. G., Anson, M. and Chung, S. H., Hearing in the frog: a neurophysiological study of the auditory response in the midbrain, Proc. roy. Soc. Lond. B, 212 (1981) 433-457. 29 Popper, A. N., Pure-tone auditory thresholds for the carp, Cyprinus carpio. J. Acoust, Soc. Amer., 52 (1972) 1714-1717. 30 Precht, W., Neuronal Operations in the Vestibular System, Springer Verlag, Berlin, 1978, pp. 29-40. 31 Sand, O. and Enger, P. S., Possible mechanisms for directional hearing and pitch discrimination in fish. In J. Schwartzkopff (Ed.), Mechanoreception, Westdeutscher Verlag, Opladen, 1974, pp. 223-242. 32 Sand, O. and Michelsen, A., Vibration measurements of the perch sacculus, J. comp. Physiol., 123 (1978) 85-89. 33 Sawa, M., Auditory responses from single neurons of the medulla oblongata in the goldfish, Bull. Jap. Soc. Sci. Fish., 42 (1976) 141-152. 34 Schwartzkopff, J., Comparative-physiological problems of hearing in fish. In A. Schuijf and A. D. Hawkins (Eds.), Sound Reception in Fish, Elsevier, Amsterdam, 1976, pp. 3-18.
and J. Schweitzer provided valuable criticism of the
391 35 Turner, R. G., Muraski, A. A. and Nielsen, D. W., Cilium length: influence on neural tonotopic organization, Science, 213 (1981) 1519-1521. 36 Viancour, T. A., Electroreceptors of a weakly electric fish. II. Individually tuned receptor oscillations, J. comp. Physiol., 133 (1979) 327-338. 37 Weiss, T. F., Peake, W. T., Ling, A. Jr. and Holton, T., Which structures determine frequency selectivity and tonotopic organization of vertebrate cochlear nerve fibers? Evi-
dence from the alligator lizard. In R. Naunton and C. Fernandez (Eds.), Evoked Electrical Activity in the Auditory Nervous System, Academic Press, New York, 1978, pp. 91-112. 38 Wever, E. G., The evolution of vertebrate hearing. In W. D. Keidel and W. D. Neff (Eds.), Handbook of Sensory Physiology, Vol. V Part 1, Springer Verlag, Berlin, 1974, pp. 423-454.
387
BRE 20936
Tonotopic organization in the midbrainof a teleost fish STEPHEN M. ECHTELER Department of Neurosciences, School of Medicine and Neurobiology Unit, Scripps Institution of Oceanography, University of California at San Diego, La Jolla CA 92093 ( U.S.A.) (Accepted February 25th, 1985) Key words: teleost - - tonotopy- - midbrain - - hearing - - carp
Recordings from small clusters of units within the auditory midbrain of the Japanese carp, Cyprinus carpio, provide the first evidence for tonotopy within the teleost brain. These findingssuggestthat place mechanismsof frequencycodingand tonotopy, previously thought to have evolved first in amphibians, may be general and ancient features of the vertebrate auditory system.
In the cochleae of birds and mammals, sounds of different frequencies produce maximal mechanical displacements at different places along the basilar membrane 3. Auditory hair cells coupled to different portions of this membrane are thus 'tuned' to different frequencies, and nerve fibers innervating these receptors preserve this frequency specific information in an ordered 'tonotopic' projection onto neurons within the cochlear nuclei. The place principle of frequency coding and tonotopy are believed to be important mechanisms by which the auditory system performs fine frequency discriminations 3, and these features were once thought to be restricted to animals possessing an ear with a basilar membrane 38. Recently, this concept has changed because of findings that both place mechanisms and tonotopy are present in anurans, which lack a basilar membrane 14,21.26.28, and in reptiles which possess cochleae with untuned basilar membranest3, 23,24,37. Many species of teleost fish are also capable of frequency discrimination12, although the teleost ear lacks both a basilar and tectorial membrane. In these animals sound is processed by otolithic organs such as the lagena and especially the sacculusII,IS,19, organs which in terrestrial vertebrates function as equilibrial detectors 30. Temporal coding mechanisms are well developed in teleosts and may suffice for frequency
discriminationlO,34. Some reports, however, provide evidence for a crude place mechanism operating within the teleost sacculus 2.9A1.15.32. If these reports are accurate it would be reasonable to expect that tonotopy should also exist within the teleost brain. Yet previous electrophysiological studies of teleost medullary and midbrain auditory centers have not reported any evidence for tonotopic organization 16.20.27.33. Since most of these studies did not provide a detailed account of recording locations, it is not clear how thoroughly these regions were sampled. In the present study, a stereotaxic atlas was constructed of the torus semicircularis (TS, a midbrain structure thought to be homologous to the mammalian inferior colliculus) in the Japanese carp, Cyprinus carpio, an ostariophysan teleost with well developed auditory capacities29. Subdivisions of the TS, and their relation to overlying surface structures, were charted at 100/~m intervals in a series of camera lucida drawings of both horizontal and transverse sections of the carp brain. The TS was then systematically explored, with reference to this atlas, to test for tonotopic organization. Carp were anesthetized by immersion in a 0.01% solution of tricaine methanesulfonate (MS-222) and the right optic tectum exposed. Following surgery, animals were given an intramuscular injection of
Correspondence: S. M. Echteler. Present address: Auditory Research Laboratory, Northwestern University, Frances Searle Bldg., 2299 Sheridan Rd., Evanston, IL 60201, U.S.A. 0006-8993/85/$03,30© 1985 Elsevier Science Publishers B.V. (Biomedical Division)
388 Flaxedil (1.0/~g/g b.wt.) and placed in a head holder, with a ventilation tube. Animals were completely submerged except for the dorsal surface of the skull. The aquarium rested on a vibration isolation table within a sound isolation booth. Auditory unit clusters, consisting of 2-4 well isolated units, were recorded with indium-filled microelectrodes7 from the TS in response to isointensity tonebursts. Tones were produced by a waveform generator, attenuated, shaped into tonebursts of 100 ms duration with rise/fall times of 5 ms, and presented at 3 s intervals. Certain stimulus frequencies could not be employed since they were greatly distorted by the acoustics of the water-filled recording chamber. Eleven standard frequencies were chosen, spanning the range of maximum frequency sensitivity in carp 29, which could be reliably produced without significant distortion: 52, 106, 205, 306, 406, 503, 603, 700, 797, 894 and 991 Hz. Stimulus frequencies were not presented in any fixed order. At each test frequency, the attenuator was calibrated to produce a toneburst at an intensity of 10 dB re 1.0/~bar, as monitored by a hydrophone (Clevite CH-17), in place of the animal. During experiments, toneburst intensity was monitored by a hydrophone next to the animal. Sounds were generated in air by a loudspeaker positioned 0.6 m behind and above the animal's head. Most auditory unit clusters within the carp TS showed no spontaneous activity and responded phasically to toneburst stimuli. Evoked neuronal activity was amplified, band-passed filtered, and units crossing an arbitrary level set by a window discriminator were counted with a digital signal averager. At each recording location the total number of spikes produced by 5 sequential tonebursts was calculated for each test frequency. From these data isointensity frequency response profiles were constructed. Absolute threshold frequency tuning curves were also occasionally collected to test the reliability of the isointensity response profiles for measuring frequency selectivity. Tuning curves obtained by these two methods were frequently mirror images of one another. Test frequencies producing the largest number of spikes also produced responses at the lowest absolute sound intensities. At the end of many electrode tracks, lesions were produced to mark TS recording sites and check the accuracy of the stereotaxic coordinates used for electrode placement.
Results confirm an earlier report that auditory and lateral line sensory modalities are strictly segregated within medial and lateral subdivisions of the carp TS throughout its entire rostrocaudal extentS. The data presented here pertain only to the medial (auditory) subdivision. The tonotopic organization described in this paper is derived from frequency maps obtained in 6 carp. At least 20 recording locations were sampled in each animal. In several cases an animal remained in good physiological condition for two days allowing 50-70 recording locations to be sampled. Most unit clusters respond best to a single test frequency. Best frequencies are grouped here into three ranges: high (500-1000 Hz), middle (300-400 Hz) and low (50-200 Hz) (Fig. 1A). Neurons within each of these frequency ranges are found together within well defined torsi regions. Within the upper layers of the TS, high, middle and low frequencies are represented, sequentially, along an oblique line extending from the rostromedial to caudolateral TS (Fig. 1B). High frequency neurons, located along the medial border of the rostral TS, respond best to frequencies between 500 and 700 Hz; unit clusters preferring higher frequencies (800-1000 Hz) are rare. Middle frequency neurons form a lateral band within the central TS, which begins and extends caudal to this high frequency region. Low frequency neurons are most frequently encountered within the caudal TS, but they are also present within the ventral layers of the rostral and central TS. Electrode penetrations into these regions reveal high or middle frequency neurons within the upper 200-300/~m of the TS followed by an abrupt transition to low frequency neurons within the lower 200-300/~m of the TS (Fig. 1C, 1-2). In the caudal TS low frequency neurons are present at all recording depths (Fig. 1C, 3). Tonotopy is not precise within either the high or middle frequency TS regions. In the rostromedial TS, for example, neurons responding best to 700 Hz tonebursts could be located either rostrai or caudal to units with a frequency preference of 500 or 600 Hz. In low frequency regions of the TS, however, neurons responding best to the higher part of the range, such as 200 Hz, are found dorsal to those preferring 50-100 Hz. It is likely that this tonotopy does not arise in the carp midbrain but rather reflects some form of place
389
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Fig. 1. Tonotopic organization within the carp midbrain. A: isointensity frequency response profiles of three neuron clusters recorded from the torus semicircularis (TS) in one animal. Depending upon their location within the TS, auditory neurons produce maximum responses within one of three test frequency ranges: low (50-200 Hz, solid circles), middle (300-400 Hz, open circles), high (500-1000 Hz, X's). Response was measured by counting multiunit spikes produced following 5 isointensity tonebursts at each test frequency. For the frequency profiles illustrated here the total number of spikes produced at 100% response was 60 (low), 93 (mid), and 73 (high). B: the regional distribution of high, mid, and low frequency neurons within the upper layers of the TS. The outline of the TS, indicated by the dotted lines, is projected onto the surface of the optic tectum. Forty-five electrode penetrations, representing approximately 180 recording sites, were made in 6 animals. Twenty penetrations into the lateral and extreme caudal TS yielded no responses to acoustic stimuli, and are not plotted here. Of the remaining 25 electrode penetrations, 18 were histologically confirmed and are plotted above. Each symbol represents one electrode track, along which 3-5 unit clusters were recorded within the upper TS at increments of 50-100/~m. Calibration bar equals 500/~m. Abbreviations: CER, cerebellum; OT, optic tectum; TEL, telencephalon; TS, torus semicircularis; VL, vagal lobe. C: camera lucida drawings of three transverse sections through the TS in a single animal at the levels indicated in B, showing the distribution of high, mid and low frequency neurons with depth. Calibration arrows equal 300/tm and indicate dorsal (D) and medial (M) orientation.
mechanism operating within the auditory periphery, although a c o m p u t e d map is a possibility. F u r u k a w a and Ishi05, recording from the saccular nerve in gold-
fish, provided the first evidence suggesting that a place mechanism might be operating within a teleost ear. Neurons innervating hair cells within the anterior portion of the sacculus were found to r e s p o n d best to higher frequencies (600-800 Hz), whereas neurons contacting the posterior sacculus respond best to lower frequencies (200-400 Hz). Subsequent recordings of auditory hair cell r e c e p t o r potentials (microphonics) from different regions of the sacculus in another teleost with a more restricted hearing range, the sculpin, also showed that responses to higher frequencies (up to 500 Hz) could be o b t a i n e d only from the anterior portion of the sacculus 2. In addition, a recent r e p o r t on patterns of hair cell d a m a g e in the sculpin sacculus, following exposure to intense sounds of different frequencies, revealed that higher frequencies cause more destruction within the anterior sacculus, lower frequencies within the posterior sacculus 9. The potential mechanisms underlying frequency selectivity within the teleost sacculus are not well understood. Previous workers have suggested that saccular hair cells might be passively tuned by their mechanical linkages to supporting structures, such as the otolith 3~ or the macular sensory epithelium 5, which themselves could display frequency selective m o v e m e n t patterns. Subsequent studies of otolith movements in the perch sacculus, however, suggested only a very crude d e p e n d e n c e of m o v e m e n t pattern on frequency 32. Recently, however, intrinsic mechanisms of frequency tuning, first suggested by studies on electroreceptors4,1s,25,36, have been proposed for auditory hair cells in several reptilian species. These mechanisms do not d e p e n d upon hair cell coupling to accessory structures but rather relate either to micromechanical properties of the ciliary bundles capping the hair cells 17,35 or to the presence of electrically tuned filters within the hair cell membrane 6,t3. Most intriguing are reports of electrical tuning mechanisms within hair cells of the amphibian sacculust,22; it is conceivable that similar mechanisms may be operating within the otolithic organs of fishes. Finally, it is important to note that although studies suggest a rostrocaudal distribution of high to low frequencies within both the sculpin and goldfish sacculus, comparable data are not yet available for the carp. Therefore, at the present time, it is not possible
390 to relate the tonotopic organization found within the carp midbrain to frequency representation within the
found in carp to other teleosts.
carp sacculus. Moreover, given the marked speciesspecific diversity in the anatomical organization of the teleost sacculus 12, it seems unlikely that the pat-
I thank Dr. Theodore H. Bullock for his advice and encouragement. Drs. J. T. Enright, T. H. Bullock
tern of frequency representation will be the same in all teleost species. Consequently, caution must be advised against generalizing the pattern of tonotopy
manuscript. F u n d i n g for this study was provided by H I H and NSF grants to T. H. Bullock and an N I M H predoctoral fellowship to S . M . E .
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