Neonatal sensorineural hearing loss affects synaptic density in the auditory midbrain.

We examined the effect of neonatal sensorineural hearing loss on synaptic density in the central nucleus of the inferior colliculus (ICC) of adult cats to evaluate the role of auditory experience in synaptogenesis. Three groups of animals were used: bilaterally deafened, unilaterally deafened and normal hearing controls. Synaptic density in bilaterally deafened animals was significantly lower than in normal hearing animals. By contrast, there was no significant difference in synaptic density between normal hearing animals and unilaterally deaf animals. These results demonstrate, for the first time, that a sensorineural hearing loss during development can affect synaptogenesis in the auditory midbrain.

Features of the auditory development of the short-tailed Brazilian opossum, Monodelphis domestica: evoked responses, neonatal vocalizations and synapses in the inferior colliculus.

The onset of hearing in anesthetized South American opossums (Monodelphis domestica) was determined by the measurement of evoked potentials to click stimuli from the vertex of the skull immediately over the inferior colliculus. Evoked potentials were first recorded at postnatal day 24 at a threshold of 83 dB SPL; thresholds declined over subsequent weeks to below 58 dB at 40 days. Isolation calls emitted by the pups had stereotypic spectra with peaks at near 13 kHz and an octave higher. Such calls declined in frequency by day 32 and were not emitted at day 40. The peak frequency of the calls matched very closely the best frequency of hearing of adult Monodelphis. The number of synapses in the inferior colliculus increased at day 26; when plotted in relation to the number of cells, synaptic density increased steeply from day 27 after the animal had begun to hear. This suggests that environmental sound has a potent effect on the development of synapses in the auditory system.

Development of hearing and vocalization in a marsupial, the Northern Quoll, Dasyurus hallucatus.

The development of hearing was studied in the Northern Quoll, a nocturnal marsupial carnivore whose young are born into a pouch in an immature state after about 21 days in utero. Startle responses to noise bursts of 105 dB sound pressure level first appeared at 60 days after arrival in the pouch, but only to occasional stimuli; forelimb rather than whole body twitches were evoked. The latter were elicited regularly at 67 days onward. Auditory brainstem response (ABR) audiograms were measured during the period when startle responses first appeared. At 68 days responses were elicited between 1 and 16 kHz with thresholds in excess of 55 dB. At 81-88 days responses occurred over the adult range at lower thresholds than observed in the adult. The waveform of the ABR increased in complexity and decreased in latency during development. Pouch-young emitted acoustically well-defined isolation cells when separated from their mothers. Fourier analysis of these calls revealed peak energy at 8-11 kHz irrespective of the sex of the pouch-young. The isolation call first became evident at about 35 days, when the young were deliberately removed from the nipple, and ceased being emitted beyond about 80 days, some 5 or so days after the eyes open, when the young leave the pouch for substantial amounts of time. The spectrum of the call was similar at all ages within this 45 day period, and its peak frequency correlated closely with the best frequency in the adult quoll's ABR audiogram. At approximately 80 days of age the calls developed major low frequency components, similar to those measured in adult vocalizations, and the contributions of frequencies between 8-11 kHz waned in importance. The results indicate that the onset of hearing in quolls occurs at a relatively early time in the course of their development, but prior to this time the pouch-young emit characteristic isolation calls easily detectable by the mother.

Features of the structural development of the inferior colliculus in relation to the onset of hearing in a marsupial: the northern quoll, Dasyurus hallucatus.

The time course of synaptogenesis and the arrival and myelination of afferent connections were studied in the developing inferior colliculus (IC) of a marsupial, the Northern Quoll, and related to the onset of hearing and patency of peripheral auditory structures in that species. The quoll is born after 3 weeks of intrauterine growth and completes its development in a pouch for a further 80 days before weaning. Synaptic terminals in the IC at 9 days after arrival in the pouch were extremely rare and were associated with very low vesicle numbers. The number of synapses increased smoothly during pouch life, whereas the number of neurons with nucleoli fell over the same time period. The ratio of synapses to cells steadily increased from day 9 to day 63, then rapidly accelerated to day 73; a similar high ratio was observed in adults. Retrograde labeling from the IC of fibers projected from the medullary auditory nuclei, first observed on day 36, became progressively denser during pouch life. Myelination of lateral lemniscal fibers was absent on day 45, extremely sparse on day 54, and clear on day 63. Myelin sheaths were not observed within the IC electron microscopically until day 73. Examination of the peripheral auditory system revealed that until about day 40 the middle ear was fluid-filled, and middle ear structures were spongy. Between days 51 and 63 the middle ear cleared, the eardrum became shiny, and the ear canal became patent. The structural development of the IC is therefore very mature at the time hearing begins (67 days), and the last major anatomical change preceding hearing appears to be the opening of the external ear canal.

Sensitivity to interaural intensity differences of neurons in primary auditory cortex of the cat. I. types of sensitivity and effects of variations in sound pressure level.

1. Interaural intensity differences (IIDs) provide the major cue to the azimuthal location of high-frequency narrowband sounds. In recent studies of the azimuthal sensitivity of high-frequency neurons in the primary auditory cortex (field AI) of the cat, a number of different types of azimuthal sensitivity have been described and the azimuthal sensitivity of many neurons was found to vary as a function of changes in stimulus intensity. The extent to which the shape and the intensity dependence of the azimuthal sensitivity of AI neurons reflects features of their IID sensitivity was investigated by obtaining data on IID sensitivity from a large sample of neurons with a characteristic frequency (CF) > 5.5 kHz in AI of anesthetized cats. IID sensitivity functions were classified in a manner that facilitated comparison with previously obtained data on azimuthal sensitivity, and the effects of changes in the base intensity at which IIDs were introduced were examined. 2. IID sensitivity functions for CF tonal stimuli were obtained at one or more intensities for a total of 294 neurons, in most cases by a method of generating IIDs that kept the average binaural intensity (ABI) of the stimuli at the two ears constant. In the standard ABI range at which a function was obtained for each unit, five types of IID sensitivity were distinguished. Contra-max neurons (50% of the sample) had maximum response (a peak or a plateau) at IIDs corresponding to contralateral azimuths, whereas ipsi-max neurons (17%) had the mirror-image form of sensitivity. Near-zero-max neurons (18%) had a clearly defined maximum response (peak) in the range of +/- 10 dB IID, whereas a small group of tough neurons (2%) had a restricted range of minimal responsiveness with near-maximal responses at IIDs on either side. A final 18% of AI neurons were classified as insensitive to IIDs. The proportions of neurons exhibiting the various types of sensitivity corresponded closely to the proportions found to exhibit corresponding types of azimuthal sensitivity in a previous study. 3. There was a strong correlation between a neuron's binaural interaction characteristics and the form of its IID sensitivity function. Thus, neurons excited by monaural stimulation of only one ear but with either inhibitory, facilitatory, or mixed facilitatory-inhibitory effects of stimulation of the other ear had predominantly contra-max IID sensitivity (if contralateral monaural stimulation was excitatory) or ipsi-max sensitivity (if ipsilateral monaural stimulation was excitatory). Neurons driven weakly or not at all by monaural stimulation but facilitated binaurally almost all exhibited near-zero-max IID sensitivity. The exception to this tight association between binaural input and IID sensitivity was provided by neurons excited by monaural stimulation of either ear (EE neurons). Although EE neurons have frequently been considered to be insensitive to IIDs, our data were in agreement with two recent reports indicating that they can exhibit various forms of IID sensitivity: only 23 of 75 EE neurons were classified as insensitive and the remainder exhibited diverse types of sensitivity. 4. IID sensitivity was examined at two or more intensities (3-5 in most cases) for 84 neurons. The form of the IID sensitivity function (defined in terms of both shape and position along the IID axis) was invariant with changes in ABI for only a small proportion of IID-sensitive neurons (approximately 15% if a strict criterion of invariance was employed), and for many of these neurons the spike counts associated with a given IID varied with ABI, particularly at near-threshold levels. When the patterns of variation in the form of IID sensitivity produced by changes in ABI were classified in a manner equivalent to that used previously to classify the effects of intensity on azimuthal sensitivity, there was a close correspondence between the effects of intensity on corresponding types of azimuthal and IID sensitivity

The anatomy of the cochlear nuclei and superior olivary complex of arboreal Australian marsupials.

Cytoarchitectural and morphometric analyses were carried out on the cochlear nuclear and superior olivary complexes of nine representative possums and gliders, members of a large group of nocturnal, arboreal Australian marsupials, many of which have well developed vocalizations. The cochlear nuclear complex was displaced medial to the restiform body in all species; this has previously been reported in other marsupials. The dorsal cochlear nucleus was generally very much larger than any other nucleus in this complex. A small cochlear nerve root nucleus was present in all species, a feature shared with rodents. The anteroventral cochlear nucleus was present throughout almost the entire rostrocaudal extent of the complex. The component nuclei of the superior olivary complex had similar positional relationships to those in eutherians. The lateral superior olive was the largest nucleus, having a volume usually greater than the sum of the volumes of the medial superior olive and medial nucleus of the trapezoid body. The smaller species had a very much larger number of neurons in the superior olive relative to brain size than did the larger species. A similar disproportion was demonstrated between cochlear nucleus volume and brain weight.

The auditory neurobiology of marsupials: a review.

The marsupials, the large group of mammals which develop during fetal life in an externalized pouch, have been given little attention by auditory neurobiologists. In this review the structure of the auditory systems of the handful of marsupials which have been studied is described, the course of auditory development mapped, and the behavioral and electrophysiological manifestations of hearing examined. It is argued that research on the highly accessible developing marsupial will provide information about the development of hearing difficult to obtain from, but applicable to all, mammalian species.

Hearing, vocalization and the external ear of a marsupial, the northern Quoll, Dasyurus hallucatus.

As part of a continuing study of the development of the marsupial auditory system, auditory brainstem responses (ABR) were recorded and an ABR audiogram was constructed for five female Northern Quolls (Dasyurus hallucatus), which are nocturnal carnivores. The best frequency for hearing lies between 8 and 10 kHz, and at 50 dB SPL there is a range from about 0.5 to 40 kHz. Vocalizations of adult quolls and pouch-young were recorded with a digital audio tape recorder, and the power spectra of representative calls were compared with the ABR audiogram. The common adult vocalizations have most energy at the lower end of the hearing range, whereas frequencies that are dominant in the isolation calls of the pouch-young lie close to the best frequency of hearing. Samples of nocturnal sounds of the habitat of the quoll were also recorded and analyzed. Power spectra have peak energy at frequencies between 2 and 5 kHz, with a smaller contribution above 10 kHz. The spectrum contains relatively little power at the best frequency of hearing. Measurements of the sound pressure level at the external ear canal as a function of stimulus frequency and location in space suggest that the directional amplifying properties of the pinna will operate most effectively on sound frequencies at the upper end of the quoll's hearing range, a region that may be important in prey detection. Comparisons are made with other mammalian nocturnal carnivores and with other marsupials. We speculate that, for nocturnal carnivores, one role of the low-frequency part of the hearing range concerns the recognition of adult conspecifics, the mid-frequency range is important for the detection of pouch-young, and the upper range may be particularly concerned with prey/predator detection.

The morphological development of the inferior colliculus in a marsupial, the Northern quoll (Dasyurus hallucatus).

As a part of a project concerning the development of hearing, some features of the morphological development of the inferior colliculus were studied in a marsupial, the Northern quoll or native cat (Dasyurus hallucatus). Marsupials are of particular interest in developmental studies because much embryonic development occurs outside the uterus, in the pouch. Nissl-stained material was prepared from pouch-young at various ages between 11 and 81 days, and for a number of adults. Four pouch-young were injected with tritiated thymidine and killed later during pouch life. The inferior colliculus is first recognizable in pouch-young aged 23 days, when it is bordered by a cell-sparse ring of tissue. By this time, the labelling patterns following injections of tritiated thymidine made on days 7-9 suggest that migration of cells to the inferior colliculus from the ventricular germinal zone has been largely completed. At 81 days, close to the time when the young move out of the pouch, the adult cytoarchitecture--a central nucleus flanked by dorsal and lateral cortical regions--is clear. Cell areas expand monotonically as a function of age. The period of days 45-50 is associated with a large expansion of cell volume and a concomitant decrease in packing density. It is likely that functional connections are forming during this period, which may herald the onset of hearing in the quoll. Total cell numbers increase to a peak at day 36, fall to a minimum at day 50, and rise again to the adult value. The second increase is likely to be a phase of glial proliferation, in part associated with the onset of myelination. This increase correlates with departure of the young from the pouch.

The responses of neurons in subdivisions of the inferior colliculus of cats to tonal, noise and vocal stimuli.

The aim of this study was to gain information from anesthetized cats about the differential coding properties of neurons in the three major subdivisions of the inferior colliculus: the central (CNIC) and external (EN) nuclei and dorsal cortex (DC). Stimuli were presented in the free field from a speaker facing the contralateral pinna. For each unit, the characteristic frequency (CF, where threshold was lowest) was determined, and impulse rates to CF tone bursts, noise bursts and four feline vocal stimuli were measured as a function of increasing sound pressure level (rate/level functions). Peristimulus-time histograms were computed for responses to all stimuli. Sustained firing patterns to CF stimuli were observed for 81% of units in CNIC, for 50% of units in EN and 27% of units in DC. Sustained discharges were evoked by noise in 78-100% of units in all regions, and by at least one vocal stimulus in 86% of units in CNIC, 82% in EN and 55% in DC. In the CNIC, non-monotonic rate/level functions to CF stimuli were more common (41%) than either monotonic or plateau functions, whereas the reverse was the case with noise and vocal stimuli. Non-monotonic functions were uncommon to any stimulus in EN and DC (21-24%). Vocal stimuli were more effective in terms of higher firing rates than noise or CF stimuli in 27% of units in CNIC, 82% in EN and 72% in DC. There were no units that responded exclusively to one vocal stimulus, but a high proportion of units in EN responded strongly to broad band stimuli, and some of these showed clear preferences for one vocal stimulus over others.

Plasticity of auditory cortex associated with sensorineural hearing loss in adult C57BL/6J mice.

The representation of frequency was mapped in the primary auditory cortex (AI) of C57BL/6J (C57) mice during young adulthood (1.5-2 months) when hearing is optimal, and at 3, 6, and 12 months of age, a period during which progressive, high frequency, sensorineural hearing loss occurs in this strain. Maps were also obtained from CBA/CaJ mice which retain good hearing as they age. In AI of young adult C57 mice and CBA mice, characteristic frequencies (CFs) of multiple-unit clusters were easily identified with extracellular recordings, and a general tonotopic organization was observed from dorsal (high frequency) to ventral and caudal (low frequency). In individual cases there appeared to be deviations from the above tonotopic organization, despite the fact that inbred mice are genetically invariant. As progressive loss of high frequency sensitivity ensued peripherally, a substantially increased representation of middle frequencies was observed in AI. There was no apparent change in the surface area of the auditory cortex despite the elimination of high frequencies, and virtually the entire auditory cortex became devoted to the middle frequencies (especially 10-13 kHz) for which sensitivity remained high. Similar age-related changes were not observed in normal-hearing CBA mice. These findings indicate that plasticity in the representation of frequency in AI is associated with high frequency hearing loss in C57 mice.

Azimuthal processing in the posterior auditory thalamus of cats.

The responses to free-field acoustic stimuli of 157 units in the auditory thalamus of anesthetized cats were studied in relation to the localization of pure tone stimuli in the azimuthal plane. Units were classified as 'directional' if their firing rates at sound levels in excess of 20 dB above threshold varied by more than 50% as a function of azimuth. Sixty-five % of the units in the nucleus of the brachium of the inferior colliculus and 30% in the ventral division of the medial geniculate body were found to be directional, suggesting different processing channels for sound localization between colliculus and cortex.

Neurogenesis in the brain auditory pathway of a marsupial, the northern native cat (Dasyurus hallucatus).

Neurogenesis in the auditory pathway of the marsupial Dasyurus hallucatus was studied. Intraperitoneal injections of tritiated thymidine (20-40 microCi) were made into pouch-young varying from 1 to 56 days pouch-life. Animals were killed as adults and brain sections were prepared for autoradiography and counterstained with a Nissl stain. Neurons in the ventral cochlear nucleus were generated prior to 3 days pouch-life, in the superior olive at 5-7 days, and in the dorsal cochlear nucleus over a prolonged period. Inferior collicular neurogenesis lagged behind that in the medial geniculate, the latter taking place between days 3 and 9 and the former between days 7 and 22. Neurogenesis began in the auditory cortex on day 9 and was completed by about day 42. Thus neurogenesis was complete in the medullary auditory nuclei before that in the midbrain commenced, and in the medial geniculate before that in the auditory cortex commenced. The time course of neurogenesis in the auditory pathway of the native cat was very similar to that in another marsupial, the brushtail possum. For both, neurogenesis occurred earlier than in eutherian mammals of a similar size but was more protracted.

Rate-level functions of neurons in the inferior colliculus of cats measured with the use of free-field sound stimuli.

1. The responses as a function of stimulus level of 125 single units in the inferior colliculus of anesthetized cats were studied with the use of free-field acoustic stimuli. 2. The characteristic frequency (CF; frequency at which threshold was lowest) of each unit was determined, and stimuli were presented from one of three speaker positions: 45 degrees contralateral to the midline, midline, and 45 degrees ipsilateral to the midline. 3. For each unit a variety of stimulus levels was presented at CF, and the total spike count was summed for 20 stimuli at each level. If time permitted, a similar series of levels of noise was presented. 4. Four classes of rate-level (RL) functions were observed. Monotonic increases in firing rate were observed in 10% of units stimulated with CF stimuli and 57% of units studied with noise. Nonmonotonic RL functions, for which firing first increased and then declined to less than 50% of the peak level, were observed in 61% of units responding to CF tones and in 10% responding to noise. Plateau functions, with shapes lying between these, accounted for 19% of CF responses and the remaining units excited by noise. Some very complex shapes that could not be categorized into the above groups were seen in the remaining 10% of the units responding to CF stimuli. 5. The RL functions of units studied with both noise and CF tones could belong to different classes; commonly, nonmonotonic RL functions to tones were associated with monotonic RL functions to noise. The noise thresholds averaged 10 dB, some 10-20 dB less sensitive than those to CF stimuli. 6. For the vast majority of both noise and tone responses, stimuli from the contralateral location were more effective than those from the other two positions in terms of a lower threshold, higher peak discharge rate, and, for nonmonotonic units, a lower sound level at which the function became nonmonotonic (turnover point). 7. The turnover points of nonmonotonic functions at any given CF could be spread broadly but, overall, tended to be concentrated between -6 and 44 dB. 8. The dynamic ranges (range of levels over which firing rate increased) were larger for monotonic and plateau functions than for nonmonotonic functions, which had dynamic ranges less than 45 dB. The median dynamic range for units stimulated with CF tones was 20 dB and for noise stimuli, 40 dB.(ABSTRACT TRUNCATED AT 400 WORDS)

Neurons in the inferior colliculus of cats sensitive to sound-source elevation.

The sensitivity to variations in sound-source elevation was studied in 48 units, previously examined as to their azimuthal sensitivity, of the inferior colliculi of cats. Of these units, 36 were directionally-sensitive (firing rate varied by more than 50% across the range of positions studied) to both azimuthal and elevational changes. Elevation sensitivity was common to noise stimuli (19/25 units) and pure tones in excess of 6 kHz (17/17 units). Not one of the 8 azimuth-sensitive units with CFs below 6 kHz was directionally-sensitive to the elevation of CF stimuli. The 4 units omnidirectional to azimuthal variation were similarly insensitive to elevation. The shapes of functions relating sound-source elevation to spike count (elevation functions) varied across an apparent continuum, with some very sharply-peaked functions being observed. Peak spike counts almost invariably occurred at stimulus elevations above the horizontal plane. Comparisons of the widths of elevation and azimuth functions at the same sound pressure level were made for 36 units. The relative sharpness of elevation and azimuthal tuning varied across the population. The common association of sensitivity to both azimuth and elevation suggests that elevation sensitivity may be mediated partly by binaural comparisons.

Azimuthal sensitivity of neurons in primary auditory cortex of cats. I. Types of sensitivity and the effects of variations in stimulus parameters.

1. Preliminary to studying the organization of azimuthal sensitivity of neurons along frequency-band strips in the primary auditory cortex (AI) of cat (see companion paper), this study examined the sensitivity of 251 units in cat AI to variations in the azimuthal location of sound sources in the frontal hemifield. Most units (231) were tested with tones at the characteristic frequency (CF; frequency to which the unit had the lowest threshold). Unit CFs ranged from 5 to 36 kHz. A large number of units (91) were tested with broadband noise stimuli, and a few units were also tested at other frequencies within the cell's tuning response area. 2. When tested at stimulus intensities 20-30 dB above CF or noise threshold, the different forms of azimuthal sensitivity exhibited by AI neurons could be divided into (1) contra-field azimuth functions; (2) ipsi-field functions; (3) central-field functions; (4) omnidirectional functions, and (5) multipeaked functions. Contra-field azimuth functions were the most prevalent, with 45.9% of units tested with CF tones and 42.9% of units tested with noise exhibiting this type of azimuthal sensitivity. Ipsi-field azimuthal sensitivity was found in 16.9% of units tested with CF tones and 19.8% of units tested with noise. Central-field azimuthal sensitivity was seen in 10.8% of units tested with CF tones and 17.6% of units tested with noise. Omnidirectional azimuthal sensitivity was seen in 19.9% of units tested with CF tones and 17.6% of units tested with noise, whereas multipeaked azimuthal sensitivity was found in 6.5% of units tested with CF tones and 5.5% of units tested with noise. 3. The effects of increasing stimulus intensity on azimuthal sensitivity were examined in 185 units tested with CF tones and 67 units tested with noise. For four major classes of azimuthal sensitivity (contra-field, ipsi-field, central-field and omnidirectional), the most common effect (approximately 60% of each class) was for the azimuth function to remain constant in form by the defining criteria for these classes. The next most common effect for all classes except omnidirectional azimuth functions was for an expansion of the azimuthal range eliciting responses. (The definition of omnidirectionality precluded any expansion of the response range in this class of azimuth function). A smaller number of units in some classes showed a compression of the azimuth function to a smaller response range, and others showed more complex expansive and compressive effects with increasing stimulus intensity.(ABSTRACT TRUNCATED AT 400 WORDS)

Azimuthal sensitivity of neurons in primary auditory cortex of cats. II. Organization along frequency-band strips.

1. The organization of azimuthal sensitivity of units across the dorsoventral extent of primary auditory cortex (AI) was studied in electrode penetrations made along frequency-band strips of AI. Azimuthal sensitivity for each unit was represented by a mean azimuth function (MF) calculated from all azimuth functions obtained to characteristic frequency (CF) stimuli at intensities 20 dB or more greater than threshold. MFs were classified as contrafield, ipsi-field, central-field, omnidirectional, or multipeaked, according to the criteria established in the companion paper (Rajan et al. 1990). 2. The spatial distribution of three types of MFs was not random across frequency-band strips: for contra-field, ipsi-field, and central-field MFs there was a significant tendency for clustering of functions of the same type in sequentially encountered units. Occasionally, repeated clusters of a particular MF type could be found along a frequency-band strip. In contrast, the spatial distribution of omnidirectional MFs along frequency-band strips appeared to be random. 3. Apart from the clustering of MF types, there were also regions along a frequency-band strip in which there were rapid changes in the type of MF encountered in units isolated over short distances. Most often such changes took the form of irregular, rapid juxtapositions of MF types. Less frequently such changes appeared to show more systematic changes from one type of MF to another type. In contrast to these changes in azimuthal sensitivity seen in electrode penetrations oblique to the cortical surface, much less change in azimuthal sensitivity was seen in the form of azimuthal sensitivity displayed by successively isolated units in penetrations made normal to the cortical surface. 4. To determine whether some significant feature or features of azimuthal sensitivity shifted in a more continuous and/or systematic manner along frequency-band strips, azimuthal sensitivity was quantified in terms of the peak-response azimuth (PRA) of the MFs of successive units and of the azimuthal range over which the peaks occurred in the individual azimuth functions contributing to each MF (the peak-response range). In different experiments shifts in these measures of the peaks in successively isolated units along a frequency-band strip were found generally to fall into one of four categories: 1) shifts across the entire frontal hemifield; 2) clustering in the contralateral quadrant; 3) clustering in the ipsilateral quadrant; and 4) clustering about the midline. In two cases more than one of these four patterns were found along a frequency-band strip.(ABSTRACT TRUNCATED AT 400 WORDS)

Neurogenesis in a marsupial: the brush-tailed possum (Trichosurus vulpecula). I. Visual and auditory pathways.

The times of origin of neurons in the visual and auditory systems were studied in a marsupial, the brush-tailed possum, using tritiated thymidine autoradiography. Within the subcortical visual pathways, most neurons are generated between postnatal days 5 and 21, and the neurons of the primary visual cortex up to postnatal day 68. In the subcortical auditory pathways, most neurons are generated between postnatal days 5 and 28, and all auditory cortex neurons have appeared by postnatal day 46. Neurons in a single layer of cerebral cortex are generated during a period of about 2 weeks. Thus cortical neurogenesis in marsupials extends over a period similar to that seen in primates.

Auditory forebrain organization of an Australian marsupial, the northern native cat (Dasyurus hallucatus).

Structures and connections of auditory forebrain regions of the Northern native cat, a member of one of the most primitive families among Australian marsupials, have been examined anatomically by using anterograde and retrograde tracing techniques with wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) after defining the acoustically responsive neocortical area physiologically. The structure of the medial geniculate body (MG) was similar to that described in other species. The results obtained from a case with a WGA-HRP injection into the MG showed that the MG strongly projects to the lateral amygdaloid nucleus (LAmy) and the putamen as well as the auditory neocortex (ACx). Results obtained from other cases with WGA-HRP injections into the physiologically defined ACx show also that the ACx is connected not only with the ipsilateral MG and the contralateral ACx but also with the LAmy both bilaterally and reciprocally. The regions within the LAmy in which the MG-LAmy projection fibers terminate largely overlap with those in which the ACx-LAmy fibers terminate and the LAmy-ACx pathway originates. The connectional relationships revealed in the present study--that the LAmy receives auditory information from the MG and reciprocates auditory information with the ACx bilaterally--strongly suggest that, in some primitive mammals with small neocortical areas, a specific portion of noncortical telencephalon functions as an auditory center and occupies a relatively large volume of space in the forebrain. It is possible that the auditory sector of noncortical telencephalon in some primitive mammals such as the American Didelphidae and the Australian Dasyuridae is homologous with part of the auditory sector of the dorsal ventricular ridge (DVR) in reptiles and birds and also may have functions shared with the auditory primary and association neocortex in advanced mammals such as the domestic cat and the monkey (Kudo et al., '86a).