Auditory neuronal sizes after a unilateral conductive hearing loss

EXPERIMENTAL

NEUROLOGY

Auditory

79, 130- 140 (1983)

Neuronal Sizes after a Unilateral Conductive Hearing Loss DOUGLAS

B. WEBSTER’

Departments of Otorhinolaryngology and Anatomy, Kresge Hearing Research Laboratory of the South, Louisiana State University Medical Center, New Orleans, Louisiana 70119 Received May IO, 1982; revision received August 9, 1982 The left external auditory meatus was removed in 4-day-old CBA/J mice; after killing at 45 days, serial sections of the cochleae and brain stem were prepared. From these, the cross-sectional areas of spiral ganglion neurons and of 14 auditory brain stem neuronal types were measured, using a total of 2 10 neurons of each of the 15 types from both the right and left sides. Nine neuronal types were significantly smaller (P < 0.01) on the left side: spiral ganglion neurons; globular, small spherical, large spherical, octopus, multipolar, and granule cells of the ventral cochlear nucleus; Purkinje-like cells of the dorsal cochlear nucleus; and spindle cells of the lateral superior olivary nucleus. Two neuronal types were significantly smaller (P < 0.01) on the right: principal cells of the medial nucleus of the trapezoid body (superior olivary complex), and spindle-shape principal neurons of the central nucleus of the inferior colliculus. The let? ventral cochlear nucleus had significantly smaller volume (P < 0.0 1) than the right but right and left dorsal cochlear nuclear volumes did not differ significantly (P> 0.05). Right and left sides were not significantly different (P > 0.05) for the following neuronal types: fusiform cells and coarse- and fine-Nissl deep cells of the dorsal cochlear nucleus, and rostral bipolar cells of the medial superior olivary nucleus. Neurons a&ted by unilateral conductive loss were not significantly different (P > 0.05) from the same cells in mice with bilateral conductive losses; neurons not affected by unilateral conductive loss were not significantly different (P > 0.05) from the same cells in normal mice. ’ Outstanding technical assistance was cheerfully performed by Ms. Sandie Blanchard and Ms. Jennifer Powell and excellent secretarial work by Ms. Judy Knight and Ms. Mary Hagler. Ms. Molly Webster helped edit the manuscript. This research was supported by National Institutes of Health grant NS11647. Equipment important to this research was provided by Zenetron, Inc. and by the Louisiana Lions Eye Foundation. Please send correspondence to Douglas B. Webster, Ph.D., Kresge Hearing Research Laboratory of the South, Louisiana State University Medical Center, 1100 Florida Ave., Bldg. 124, New Orleans, LA 70119. 130 0014-4886/83/010130-11$03.00/0 Copyright Q 1983 by Academic Press Inc. All rigbu of reproduction in any form reserved

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INTRODUCTION Several studies have shown that neonatal auditory deprivation adversely ai&ts acoustically initiated competitive behavior (14, 35), acoustic durational discrimination (27), frequency pattern discrimination (28), sound localization (5), and frequency generalization (2 l), but have indicated that there may not be an adverse effect on frequency or intensity discriminations (27, 28). The physiologic bases for these behavioral deficits are not totally understood, however, the available data suggest that neonatal auditory deprivation results in elevated thresholds ( 1,6), broader tuning curves (6), and altered binaural interactions (7, 8, 25). Studies on the anatomic effects of neonatal auditory deprivation and conductive losses have shown reduced neuronal cell body sixes in auditory brain stem nuclei (10, 11, 32, 33) reduced dendritic branching in the superior olivary nuclei (13, 16), and reduced cochlear nuclear volume (9). We have shown that either bilateral external auditory meatus removal or auditory deprivation from postnatal day 4 to postnatal day 45 resulted in significantly smaller than normal neurons in several auditory brain stem nuclei of mice (33). Now that detailed descriptions of the cochlear nuclei (31) and the superior olivary complex (22) are available, it is possible to extend the analysis of these effects to yet more cell types. The present study does this, and also asks: Does unilateral external auditory meatus removal in 4-day-old mice result in predictable auditory brain stem asymmetries? MATERIALS

AND

METHODS

Fourteen 4day-old CBA/J mice were immobilized by hypothermia, and under 25X magnification the skin just ventral to the left pinna was incised. Using No. 5 watchmaker’s forceps, the blastema of the external auditory meatus was defined from the base of the pinna to the tympanic membrane. The blastema was firmly grasped with the forceps, adjacent to the tympanic membrane, and pulled free of the anulus; its peripheral end was freed from the base of the pinna and the blastema was discarded. The wound was closed without sutures and the young mice regained normal body temperature before being returned to the mother. All surgeries were carried out under aseptic conditions. All 14 mice were killed at 45 days of age. After an overdose of chloral hydrate, seven animals were intracardially perfused with 20 ml Tyrode’s solution containing 1%NaNOz , followed by 40 ml kaformacet [85 ml water, 5 ml glacial acetic acid, 10 ml 40% formaldehyde, 2.6 g potassium dichromate (23)]. At autopsy it was determined that both middle ears appeared normal and that the left external auditory meatus was absent, its only visible remnant being a small amount of “pearly” material just lateral to but never penetrating the tympanic membrane. The brains were removed, fixed 3 h

132

DOUGLAS

B. WEBSTER

in kaformacet, washed, dehydrated in an alcohol series, cleared in toluene, embedded in paraffin, and serially sectioned at 15 pm in the transverse plane. The sections were mounted on glass slides and stained with cresyl violet. The other seven mice were killed similarly, except that they were perfused and postfixed with mixed aldehydes (2% paraformaldehyde, 3% glutaraldehyde in cacodylate buffer at pH 7.3). The periotic bones containing the cochleae were removed after transection of the VIII nerve; they were decalcified 4 days in 8% EDTA, dehydrated in an alcohol series, embedded in glycol methacrylate, and serially sectioned at 2 pm in the plane of the modiolus on a JB-4 microtome. Sections were mounted on glass slides and stained with toluidine blue. Different fixatives were used for these two groups because: (a) aldehydefixed brains do not section well in paraffin; (b) kaformacet-fixed cochleae do not section well in plastic; (c) aldehyde fixation is superior for cochleae. Spiral ganglion neurons from the upper part of the first turn and 14 neuronal cell types from the auditory brain stem were selected for quantitative study (Table 1). The following brain stem neuronal types were measured: within the cochlear nuclei, those defined by Webster and Trune (3 1); in the superior olivary complex, the spindle-shape cells of the lateral superior olivary nucleus, the rostral bipolar cells of the medial superior olivary nucleus, and the principal cells of the medial nucleus of the trapezoid body (22); and in the central nucleus of the inferior colliculus, the spindle-shape principal cells of the central nucleus. Great care was taken to avoid observer bias. First, blind procedures were used during the cell sampling, so that the observer was unaware of whether the normal or deprived side of the brain was being examined. Then, strict sampling techniques were used to ensure consistency and equal representation between right and left sides and between brains. In the spiral ganglion, all cells were from the upper first turn; neurons within Rosenthal’s canal were measured starting with those nearest the osseous spiral lamina and proceeding in a modiolar direction until 30 neurons had been measured. All neurons in this region were measured and no attempt was made to distinguish between type I and type II spiral ganglion neurons. In the cochlear nuclei the section midway between the rostra1 and caudal extent of each cell region was selected; neurons were measured starting with the ventralmost cells and proceeding dorsally until 30 had been measured. Because most neuronal regions of the cochlear nuclei contain more than one cell type, care was taken to measure only the correct type, and to measure each of these as encountered. In the lateral superior olivary nucleus, spindle-shape cells (22) were identified by their oval shape and orientation perpendicular to the curvature of the nucleus. These neurons were measured starting at the medial curvature

UNILATERAL

CONDUCMVE TABLE

HEARING 1

Neuronal txossedonal Neuronal group

spiral ganglion Ventral cochlear nucleus Globular cells Small spherical cells Large spherical cells octopus cells Multipolar cells Granule cells Dorsal cochlear nucleus Fusiform cells Purkinje-like cells Coarse-Nissl deep cells Fine-Nissl deep cells Superior olivary complex Lateral superior olive Medial superior olive Medial nucleus trapezoid body Inferior colliculus Central nucleus

133

LOSS

Right

Left

area (pm2) P

Left/right

123

f 20

102

+ 20


0.83

145 117 133 234 138 21.9

+ 26 * 15 + 17 2 48 zk29 + 3.6

121 98 109 192 113 18.6

+ 18 zt15 + 15 +41 +24 + 3.0

4.01
0.83 0.84 0.82 0.82 0.82 0.85

218 f41 133 + 22 234 + 47 100 + 17

214 111 241 101

+43 + 19 k40 f 18

0.56
0.98 0.83 1.03 1.01

117 101 124

k21 + 18 221

93 + 14 100 + 17 164 + 29


0.79 0.99 1.32

75

+ 12

95


1.27

+13

Volume (mm3 X 10m2) Nucleus Dorsal cochlear nucleus Ventral cochlear nucleus

Right 11.6 + 1.3 18.0 f 0.9

Lefi 11.7 + 16.1 +

P

1.1 1.4

0.88
LefQight 1.01 0.89

Note. Values aresmeans f 1 SD.

and proceeding laterally. In the medial superior olivary nucleus, only rostral bipolar neurons were selected because the caudal group is very difficult to identify with certainty (22). Because a single section did not contain 30 of these neurons, all cells in each of two or three consecutive sections were measured until 30 had been obtained. Principal cells of the medial nucleus of the trapezoid body were identified by their eccentric nucleus and characteristic Nissl pattern (22). The section half-way between the rostral and caudal ends of the nucleus was selected, and each principal cell was measured starting laterally and proceeding medially until 30 neurons had been measured. Spindle-shape principal cells of the central nucleus of the inferior colliculus were identified by their oval shape, fine Nissl substance, and small size. The section at the rostral-caudal half-way point of the nucleus was chosen and, starting ventromedially, all spindle-shape principal cells were measured until a sample of 30 neurons was achieved.

134

DOUGLAS

B. WEBSTER

When the appropriate section was selected in this manner, the nucleolus of each identified neuron was brought into focus and the neuron’s perimeter was traced at a total magnification of 1240X, using a drawing tube on a Zeiss Photomicroscope I. In each of the seven brains, 30 neurons were traced from the right side and 30 from the left, yielding a total of 210 examples of each neuronal type on each side. The perimeters were then retraced onto a digitizing tablet interfaced with a microprocessor programmed to calculate areas. Nuclear volumes were measured only for the dorsal and ventral cochlear nuclei, because their discrete borders make this possible, whereas other nuclear groups are not so clearly defined. Preliminary studies had shown that measuring every fourth section was as accurate as measuring all sections when determining volume of these nuclei. Therefore, starting with the caudal-most section that contained any cochlear nucleus, every fourth section was measured throughout its length. Each perimeter was traced at a magnification of 90X onto the digitizing tablet. The microprocessor was programmed to calculate the cross-sectional area of that section in micrometers squared, and then to multiply that figure by 60 (15pm-thick sections and every fourth section measured). The total volume of the nucleus was then calculated by summing these individual volumes. All statistical comparisons were made using a two-tailed t test for independent measures. RESULTS Although qualitative right/left asymmetries-such as loss of a cell type on one side of the brain stem-were not found, quantitative differences were abundant (Table 1). Spiral ganglion neurons were 17% smaller in the left (experimental) ear than in the right ear (P < 0.0 1). In the ventral cochlear nuclei, all six measured neuronal types were 15 to 18% smaller on the left than on the right side (P < 0.0 1). In the dorsal cochlear nuclei, the Purkinjelike.cells were 17% smaller on the left than on the right side (P < 0.01); the other three neuronal types measured in this nucleus showed no significant differences (P > 0.05). In the superior olivary complex, there were significant (P c 0.01) size asymmetries in the neurons of the lateral superior olivary nucleus and medial nucleus of the trapezoid body, but not of the medial superior olivary nucleus (P > 0.05). Spindle-shape neurons of the left lateral superior olivary nucleus, which receive their direct cochlear nuclear input from the left ventral cochlear nucleus (18, 20, 29), were 21% smaller than comparable neurons on the right. Principal neurons of the right medial nucleus of the trapezoid body, which receive their main input from the left ventral cochlear nucleus (18, 20, 30), were 24% smaller than comparable neurons on the

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135

60 g 40 & si 20 40

80

120

CROSS -SECTIONAL

160

am

AREAS (pm*,

FIG. 1. Distribution histogram of the cross-sectional areas of spiral ganglion neurons of 4% day-old mice that had their left external auditory meatus removed when 4 days old. A total of 2 10 cells were measured from both right and lefi co&leas.

left. Rostra1 bipolar cells of the medial superior olivary nuclei, which receive their major input from both ventral cochlear nuclei (l&20, 29), were only 1% different in right versus left neuronal size (P > 0.05). Principal neurons in the central nucleus of the inferior colliculus, which receive primarily from the contralateral ventral cochlear nuclei (2, 14, 29) were 21% smaller (P < 0.01) on the right than on the left side. 6.

io

SMALL

SPHERICAL

CELLS

ii0 150 GLOBULAR

CELLS OCTOPUS

MULTIPOLAR

CELLS

CELLS

60

GRANULE

CELLS

40

8

16

24

32

CROSS - SECTIONAL AREAS Iurn

2. Distribution histograms of the cross-sectional areas of neurons in the ventral cochlear nucleus of 45day-old mice that had their left external auditory meatus removed when 4 days old. A total of 210 neurons of each type were measured from both right and left nuclei. FIG.

136

DOUGLAS

60

es 2

PURKINJE-LIKE

60

B. WEBSTER

Right

CELLS

DIFFUSE-NISSL

MULTIPOLAR

COARSE-NISSL

0

MULW’OLAR

I 40

40

20

40

80

120

160

200

ml

210

290

370

CROSS-SECTIONALAREAS@m21

FIG. 3. Distribution histograms of the cross-sectional areas of neurons in the dorsal cochlear nucleus of 45-day-old mice that had their left external auditory meatus removed when 4 days old. A total of 210 neurons of each type were measured from both right and left nuclei.

As the distribution histograms show, there was considerable overlap in neuronal sizes between right and left populations in each neuronal group (Figs. l-4). The shapes of the histograms of right and left populations were similar, however, which suggested that the experimental treatment caused a uniform shift in cell size for the entire affected population, rather than causing an effect only on a subpopulation. There was a 1% difference between right and left dorsal cochlear nuclear volumes (P > 0.05); however, the left ventral cochlear nucleus was 10% smaller (P < 0.01) than the right ventral cochlear nucleus (Table 1). DISCUSSION In our previous report on the cross-sectional areas of brain stem auditory neurons in CBA/J mice with bilateral conductive losses, we measured 40 neurons of each of nine neuronal types (33). In the present unilateral conductive loss study, 2 10 neurons of each of 14 brain stem neuronal types were measured. In addition, spiral ganglion neurons were measured; those from the ear with the conductive loss were found to have somata 17% smaller than those from the normal ear. Because the spiral ganglion neurons were treated as one population rather than two, these data do not indicate whether neonatal conductive loss affects type I and type II neurons differently. What is clear, for the first time in the literature, is that neonatal conductive loss affects peripheral auditory neurons as well as brain stem auditory neurons.

UNILATERAL

CONDUCTIVE

HEARING

LOSS

137

Right fi YEDIAL

SUPERIOR

OLIVE

60 40

8

6il

loo

l&l

CENTRAL

NUCLEUS

INFERIOR

60

40

CROSS -SECTIONAL

so

120

160

AREAS lpmZl

FIG. 4. Distribution histograms of the cross-sectional areas of neurons in the superior olivary complex and inferior colliculus of 4Sday-old mice that had their left external auditory meatus removed when 4 days old. A total of 2 10 neurons of each type were measured from both right and letI nuclei.

In the ventral cochlear nucleus, all six measured neuronal types,including the granule cells,were significantly smaller on the conductive lossside. This is the first time granule cells have been analyzed after conductive loss. Of the four neuronal types analyzed in the dorsal co&ear nucleus, only the recently identified Purkinje-like cells (3 1)were al&ted. Becauseneither the functions nor the neuronal connections of these cells is known, we cannot suggestthe significance of this effect. The fact that the other three neuronal types showed no effect, however, is not surprising considering the strong efferent connections and relatively sparseaBerent connections of the dorsal cochlear nucleus.The significantly smaller volume of the lefi ventral cochlear nucleus compared with the right is consistent with the parallel effect on neuronal cell sizes,as is the fact that there was no signScant difference between volumes of the dorsal cochlear nuclei of right and left sides. Considering the alferent connections of the superior olivary nuclei, their reaction to the conductive loss was predictable. The medial superior olive receives bilateral input from the cochlear nuclei ( l&20,29), and no asymmetry was found following unilateral conductive loss. The lateral superior olive, which receives projections from the ipsilateral cochlear nuclei and @lateral medial nucleus of the trapezoid body, was alEcted only on the @lateral side, whereas the medial nucleus of the trapezoid body, which receives only from the contralateral cochlear nuclei (3, 18, 20, 29, 30), was affected only on the contralateral side.

138

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B. WEBSTER

The central nucleus of the inferior colliculus receives input primarily, but not entirely, from the contralateral side (2-4, 29), and, predictably, the neurons of this nucleus contralateral to the conductive loss were smaller than those on the ipsilateral side. Therefore, all neuronal groups of the superior olivaty complex and inferior colliculus which receive information primarily from the conductive loss ear had smaller somata than the comparable groups which receive primarily from the normal ear. Coleman and O’Connor ( 10) reported that a unilateral conductive loss had a greater effect on ipsilateral large spherical cells of rats than did a bilateral conductive loss. Our data do not confirm this in CBA/J mice. There are nine neuronal types that we studied with both bilateral (33) and unilateral conductive losses (present study); comparisons show that each of these types was not significantly different in size (P > 0.05) when it was affected by bilateral compared with unilateral conductive losses. Furthermore, data from our two studies show no significant differences (P > 0.05) between the neurons of normal mice and the unaffected neurons of unilateral conductive loss mice. Differences between my procedures and those of Coleman and O’Connor (10) include the fact that they unilaterally deprived their rats by surgical removal of the auditory ossicles at 10 or 16 days after birth, and I deprived the mice by surgical removal of the external auditory meatus at 4 days after birth. In normal development in the mouse the auditory neurons have attained adult soma size by 12 days of age (34), when hearing begins (12). The smaller auditory brain stem neurons of animals with neonatal conductive losses may therefore be the result of an actual reduction of neuronal size between days 12 and 45, due to inadequate sound stimulation. We are currently testing this hypothesis. The literature now contains enough information on the morphologic effects of auditory deprivation to permit comparisons with the much better understood results of visual deprivation. Neonatal visual deprivation, by either eyelid suturing or dark-rearing, results in a 35 to 40% decrease in cross-sectional areas of affected dorsal lateral geniculate neurons that normally receive binocular input, and about a 10% decrease in cross-sectional area of those normally receiving monocular input [see reviews (15, 24,26)]. The less dramatic effects of auditory deprivation on cochlear nuclear neurons (15 to 18%) may be due to the fact that all cochlear nuclear neurons receive only monaural input. The slightly greater decrease in soma areas in the superior olivary complex and inferior colliculus (19 to 24%) may be due to the binaural input to those nuclei. It is also true that some auditory stimulation occurs during both auditory behavioral deprivation, as done by Webster and Webster (32,33), and experimentally produced conductive losses, as done by Webster and Webster (32, 33, present study), Coleman and O’Connor (lo), and Conlee and Parks (11).

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In the normal cat visual system, the somata of the dorsal lateral geniculate neurons grow rapidly until about 28 days of age, by which time they have attained adult size (17, 19). In monocularly deprived kittens, Kalil (19) reported that these neurons grow more slowly initially and never attain normal adult size; Hickey (17), on the other hand, reported that they grow normally to adult size at 28 days but then undergo a reduction in size during the next few weeks. Our data on neonatal conductive loss in mice show that the auditory brain stem neurons have already attained adult size by the time hearing begins at 12 days after birth. Their later small size, after the auditory deprivation or conductive loss, may be due to shrinkage similar to that described by Hickey in the visual system (17). REFERENCES 1. BATKIN, S., H. GROTH, J. R. WATSON, AND M. ANSBERRY. 1970. Effects of auditory deprivation on the development of auditory sensitivity in albino rats. Ekctroenceph. Clin. Neurophysiol. 28: 351-359. 2. BEYERL, B. D. 1978. Afferent projections to the central nucleus of the inferior colliculus in the rat. Bruin Res. 145: 209-223. 3. BROWNER, R. H., AND D. B. WEBSTER. 1975. Projections of the trapezoid body and superior olivary complex of the kangaroo rat (Dipodomys merriami). Brain Behav. Evol. 11: 322-354. 4. BRUNSO-BECHTOLD, J. K., G. C. THOMPSON, AND R. B. MASTERTON. 1981. HRP study of the organization of auditory alferents ascending to central nucleus of inferior colliculus in cat. J. Comp. Neural. 197: 705-722. 5. CLEMENTS, M., AND J. B. KELLY. 1978. Auditory spatial responses of young guinea pigs (Cavia porcellus) during and after ear blocking. J. Comp. Physiol. Psychol. 92: 34-44. 6. CLIFTON, B. M. 1980. Neurophysiology of auditory deprivation. Pages 27 l-288 in R. J. GORLIN, Ed., Morphogenesis and Mabrmation of the Ear. Alan R. Liss, New York. 7. CLO~ON, B. M., AND M. S. SILVERMAN. 1977. Plasticity of binaural interaction. II. Critical period and changes in midline response. J. Neurophysiol. 40: 1275-1280. 8. CLOFTON, B. M., AND M. S. SILVERMAN. 1978. Changes in latency and duration of neural responding following developmental auditory deprivation. Exp. Brain Res. 32: 39-47. 9. COLEMAN, J., B. J. BLATCHLEY, AND J. E. WILLIAMS. 1982. Development of the dorsal and ventral cochlear nuclei in rat and effects of acoustic deprivation. Dev. Brain Res. 4: 119-123. 10. COLEMAN, J. R., AND P. O’CONNOR. 1979. Effects of monaural and binaural sound deprivation on cell development in the anteroventral cochlear nucleus of rats. Exp. Neural. 64: 553-566. 11. CONLEE, J. W., AND T. N. PARKS. 198 1. Age- and position-dependent effects of monaural acoustic deprivation in nucleus magnocellularis of the chicken. J. Comp. Neural. 202: 273-284. 12. EHRET, G. 1976. Development of absolute thresholds in the house mouse (Mus musculus). J. Am. Aud. Sot. 1: 179-184. 13. FENG, A. S., AND B. A. ROGOWSKI. 1980. Effects of monaural and binaural occlusion on the morphology of neurons in the medial superior olivary nucleus of the rat. Brain Res. 189: 530-534. 14. GAURON, E. F., AND W. C. BECKER. 1959. The effects of early sensory deprivation on

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15. 16. 17. 18. 19.

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adult rat behavior under competition stress: an attempt at replication of a study by Alexander Wolf. J. Comp. Physiol. Psychol. 52: 689-693. GLOBUS, A. 1975. Brain morphology as a function of presynaptic morphology and activity. Pages 9-91 in A. H. RIESEN, Ed., The Developmental Neuropsychology of Sensory Deprivation. Academic Press, New York. GRAY, L., Z. SMITH, AND E. W. RUBEL. 1982. Developmental and experiential changes in dendritic symmetry in N. laminaris of the chick. Brain Res. 244: 360-364. HICKEY, T. L. 1980. Development of the dorsal lateral geniculate nucleus in normal and visually deprived cats. J. Comp. Neural. 189: 467-48 I. JONES, D. R., AND J. H. CASSEDAY. 1979. Projections of auditory nerve in the cat as seen by anterograde transport methods. Neuroscience 4: 1299-13 13. KALIL, R. 1980. A quantitative study of the effects of monocular enucleation and deprivation on cell growth in the dorsal lateral geniculate nucleus of the cat. J. Comp. Neural.

189: 483-524. 20. KELLY, J. P., A.

21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35.

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J. HUDSPETH, AND S. KENNEDY. 1978. Transneuronal transport in the auditory system of the cat. Brain Res. 158: 207-212. KERR, L. M., E. M. GSTAFQFF, AND E. W. RUBEL. 1979. Influence of acoustic experience on the ontogeny of frequency generalization gradients in the chicken. J. Exp. Psychol. Anim. Behav. Proc. 5: 97-l 15. OLLO, C., AND I. R. SCHWARTZ. 1979. The superior olivary complex in C57BL/6 mice. Am. J. Anat. 155: 349-374. ROMEIS, B. 1948. Mikroskopische Technik. Leibniz Verlag, Munich. SHERMAN, S. M., AND P. D. SPEAR. 1982. Organization of visual pathways in normal and visually deprived cats. Physiol. Rev. 62: 738-855. SILVERMAN, M. S., AND B. M. CLOFTON. 1977. Plasticity of binaural interaction. I. Effect of early deprivation. J. Neurophysiol. 40: 1266- 1274. SMITH, D. E. 1977. The effect of deafferentation on the development of brain and spinal nuclei. Prog. Neurobiol. 8: 349-367. TEES, R. C. 1967. The effects of early auditory restriction in the rat on adult duration discrimination, J. Aud. Res. 7: 195-207. TEES, R. C. 1967. Effects of early auditory restriction in the rat on adult pattern discrimination. J. Comp. Physiol. Psychol. 63: 389-393. WARR, W. B. 1966. Fiber degeneration following lesions in the anterior ventral cochlear nucleus of the cat. Exp. Neural. 14: 453-474. WARR, W. B. 1972. Fiber degeneration following lesions in the multipolar and globular cell areas in the ventral cochlear nucleus of the cat. Brain Res. 40: 247-270. WEBSTER, D. B., AND D. R. TRUNE. 1982. Cochlear nuclear complex of mice. Am. J. Anat. 163: 103-130. WEBSTER, D. B., AND M. WEBSTER. 1977. Neonatal sound deprivation affects brainstem auditory nuclei. Arch. Otolatyngol. 103: 392-396. WEBSTER, D. B., AND M. WEBSTER. 1979. Effects of neonatal conductive hearing loss on brain stem auditory nuclei. Ann. Otol. Rhinol. Laryngol. 88: 684-688. WEBSTER, D. B., AND M. WEBSTER. 1980. Mouse brainstem auditory nuclei development. Ann. Otol. Rhinol. Laryngol. 89, Suppl. 68: 254-256. WOLF, A. 1943. The dynamics of selective inhibition of specific functions in neurosis. Psychosom. Med. 5: 27-38.