Depth relationships and measures of tissue thickness in dorsal midbrain

Dorsal human midbrain contains two nuclei with clear laminar organization, the superior and inferior colliculi. These nuclei extend in depth between the superficial dorsal surface of midbrain and a deep midbrain nucleus, the periaqueductal gray matter (PAG). The PAG, in turn, surrounds the cerebral aqueduct (CA). This study examined the use of two depth metrics to characterize depth and thickness relationships within dorsal midbrain using the superficial surface of midbrain and CA as references. The first utilized nearest-neighbor Euclidean distance from one reference surface, while the second used a level-set approach that combines signed distances from both reference surfaces. Both depth methods provided similar functional depth profiles generated by saccadic eye movements in a functional MRI task, confirming their efficacy for delineating depth for superficial functional activity. Next, the boundaries of the PAG were estimated using Euclidean distance together with elliptical fitting, indicating that the PAG can be readily characterized by a smooth surface surrounding PAG. Finally, we used the level-set approach to measure tissue depth between the superficial surface and the PAG, thus characterizing the variable thickness of the colliculi. Overall, this study demonstrates depth-mapping schemes for human midbrain that enables accurate segmentation of the PAG and consistent depth and thickness estimates of the superior and inferior colliculi.

Functional organization of the mammalian auditory midbrain

The inferior colliculus (IC) is a critical nexus between the auditory brainstem and the forebrain. Parallel auditory pathways that emerge from the brainstem are integrated in the IC. In this integration, de-novo auditory information processed as local and ascending inputs converge via the complex neural circuit of the IC. However, it is still unclear how information is processed within the neural circuit. The purpose of this review is to give an anatomical and physiological overview of the IC neural circuit. We address the functional organization of the IC where the excitatory and inhibitory synaptic inputs interact to shape the responses of IC neurons to sound.

Modification of a Colliculo-thalamocortical Mouse Brain Slice, Incorporating 3-D printing of Chamber Components and Multi-scale Optical Imaging

The ability of the brain to process sensory information relies on both ascending and descending sets of projections. Until recently, the only way to study these two systems and how they interact has been with the use of in vivo preparations. Major advances have been made with acute brain slices containing the thalamocortical and cortico-thalamic pathways in the somatosensory, visual, and auditory systems. With key refinements to our recent modification of the auditory thalamocortical slice(1), we are able to more reliably capture the projections between most of the major auditory midbrain and forebrain structures: the inferior colliculus (IC), medial geniculate body (MGB), thalamic reticular nucleus (TRN), and the auditory cortex (AC). With portions of all these connections retained, we are able to answer detailed questions that complement the questions that can be answered with in vivo preparations. The use of flavoprotein autofluorescence imaging enables us to rapidly assess connectivity in any given slice and guide the ensuing experiment. Using this slice in conjunction with recording and imaging techniques, we are now better equipped to understand how information processing occurs at each point in the auditory forebrain as information ascends to the cortex, and the impact of descending cortical modulation. 3-D printing to build slice chamber components permits double-sided perfusion and broad access to networks within the slice and maintains the widespread connections key to fully utilizing this preparation.

Divergence of dim-light vision among bats (order: Chiroptera) as estimated by molecular and electrophysiological methods

Dim-light vision is present in all bats, but is divergent among species. Old-World fruit bats (Pteropodidae) have fully developed eyes; the eyes of insectivorous bats are generally degraded, and these bats rely on well-developed echolocation. An exception is the Emballonuridae, which are capable of laryngeal echolocation but prefer to use vision for navigation and have normal eyes. In this study, integrated methods, comprising manganese-enhanced magnetic resonance imaging (MEMRI), f-VEP and RNA-seq, were utilized to verify the divergence. The results of MEMRI showed that Pteropodidae bats have a much larger superior colliculus (SC)/ inferior colliculus (IC) volume ratio (3:1) than insectivorous bats (1:7). Furthermore, the absolute visual thresholds (log cd/m(2)•s) of Pteropodidae (-6.30 and -6.37) and Emballonuridae (-3.71) bats were lower than those of other insectivorous bats (-1.90). Finally, genes related to the visual pathway showed signs of positive selection, convergent evolution, upregulation and similar gene expression patterns in Pteropodidae and Emballonuridae bats. Different results imply that Pteropodidae and Emballonuridae bats have more developed vision than the insectivorous bats and suggest that further research on bat behavior is warranted.

Regional and age-related differences in GAD67 expression of parvalbumin- and calbindin-expressing neurons in the rhesus macaque auditory midbrain and brainstem

Neurons expressing the calcium binding proteins (CaBPs) parvalbumin (PV) and calbindin (CB) have shown age-related density changes throughout the ascending auditory system of both rodents and macaque monkeys. In the cerebral cortex, neurons expressing these CaBPs express markers of γ-aminobutyric acidergic neurotransmission, such as GAD67, and have well-understood physiological response properties. Recent evidence suggests that, in the rodent auditory brainstem, CaBP-containing cells do not express GAD67. It is unknown whether PV- and CB-containing cells in subcortical auditory structures of macaques similarly do not express GAD67, and a better understanding of the neurotransmission of neurons expressing these proteins is necessary for understanding the age-related changes in their density throughout the macaque auditory system. This was investigated with immunofluorescent double-labeling techniques to coregister PV- and CB-expressing neurons with GAD67 in the superior olivary complex and the inferior colliculus of young and aged rhesus macaques. The proportions of GAD67-expressing PV- and CB-positive neurons were computed with unbiased sampling techniques. Our results indicate that between 42% and 62% of PV- and CB-positive neurons in the auditory brainstem and midbrain express GAD67, which is significantly less than in the cerebrum. In general, fewer PV(+) neurons and more CB(+) neurons expressed GAD67 as a function of age. These results demonstrate that the inhibitory molecular profile of PV- and CB-expressing neurons can change with age in subcortical auditory structures and that these neurons are distinct from the well-described inhibitory interneurons that express these proteins in the cerebral cortex.

Patterns of convergence in the central nucleus of the inferior colliculus of the Mongolian gerbil: organization of inputs from the superior olivary complex in the low frequency representation

Projections to the inferior colliculus (IC) from the lateral and medial superior olivary nuclei (LSO and MSO) were studied in the gerbil (Meriones unguiculatus) with neuroanatomical tract-tracing methods. The terminal fields of projecting axons were labeled via anterograde transport of biotinylated dextran amine (BDA) and were localized on series of horizontal sections through the IC. In addition, to make the results easier to visualize in three dimensions and to facilitate comparisons among cases, the data were also reconstructed into the transverse plane. The results show that the terminal fields from the low frequency parts of the LSO and MSO are concentrated in a dorsal, lateral, and rostral area that is referred to as the "pars lateralis" of the central nucleus by analogy with the cat. This region also receives substantial input from both the contralateral and ipsilateral cochlear nuclei (Cant and Benson, 2008) and presumably plays a major role in processing binaural, low frequency information. The basic pattern of organization in the gerbil IC is similar to that of other rodents, although the low frequency part of the central nucleus in gerbils appears to be relatively greater than in the rat, consistent with differences in the audiograms of the two species.

Magnetic resonance imaging mesencephalic tectum dimensions according to age and gender

OBJECTIVE

To analyze and classify normal MRI tectum length and colliculus dimensions according to age and gender.

METHODS

Tectum length and colliculus diameters were measured on the T1 midsagittal and axial cranial MR images in the radiology archive of 532 (344 women, 188 men) patients aged 37.36+/-21.49 (range: 4-91) years old on average, and with no disorders affecting the mesencephalic tectum. All 532 patients underwent clinical MR imaging of the cranium at the MRI Unit of Sivas Numune Hospital and Sivas Cumhuriyet University Hospital, Sivas, Turkey between February and December 2011.

RESULTS

Although there was a positive linear correlation between tectum length and age, there was a negative correlation between the anteroposterior diameter of the colliculus superior and colliculus inferior and age (p<0.01). While tectum length (M3) increases with age, the anteroposterior diameter of the colliculus superior and inferior (M1 and M2) decreased (p<0.01). The colliculi were larger, and the tectum was longer in men. Although there was no difference in size between right and left superior colliculi, the left colliculus inferior was larger than the right one.

CONCLUSION

In addition to the fact that normal mesencephalic tectum dimensions provide information on the brain development of individuals, they may also be beneficial for the detection and treatment of related pathologies.

Neuronal morphology in subdivisions of the inferior colliculus of chicken (Gallus gallus)

The avian inferior colliculus (IC), also referred to as the nucleus mesencephalicus lateralis pars dorsalis (MLd), is an auditory midbrain nucleus that converges auditory cues from tonotopically organized brainstem nuclei. This information is relayed onto the optic tectum on the one hand and to nucleus ovoidalis on the other hand. Morphologically, there has been considerable debate about the number and nomenclature of the subnuclei within the IC. Here, we provide morphological characteristics of single cells in five IC subnuclei in chicken. The cellular structure within the IC was studied by whole-cell patch technique and biocytin iontophoresis. In addition, histological staining was performed, to delineate the borders between subnuclei of the IC. We were able to discriminate between 5 subnuclei: the core of the central nucleus (ICCc), the medial and lateral shell of the central nucleus (ICCms and ICCls), the external nucleus (ICX) and the superficial nucleus (ICS) of the IC. Our findings suggest the existence of at least two different morphologies of neurons with two subtypes each. The IC in chicken is a largely homogenous nucleus in terms of neuronal anatomy on a cellular level. However, its compartmentation into diversified subnuclei with different neurophysiological characteristics suggests a complex system to process auditory information. The auditory system in chicken is not as hypertrophied as in specialists such as the barn owl, but appears to have comparable connectivity and cellular morphology.

Orthogonal representation of sound dimensions in the primate midbrain

Natural sounds are characterized by their spectral content and the modulation of energy over time. Using functional magnetic resonance imaging in awake macaques, we observed topographical representations of these spectral and temporal dimensions in a single structure, the inferior colliculus, the principal auditory nucleus in the midbrain. These representations are organized as a map with two approximately perpendicular axes: one representing increasing temporal rate and the other increasing spectral frequency.

Differential patterns of inputs create functional zones in central nucleus of inferior colliculus

Distinct pathways carry monaural and binaural information from the lower auditory brainstem to the central nucleus of the inferior colliculus (ICC). Previous anatomical and physiological studies suggest that differential ascending inputs to regions of the ICC create functionally distinct zones. Here, we provide direct evidence of this relationship by combining recordings of single unit responses to sound in the ICC with focal, iontophoretic injections of the retrograde tracer Fluoro-Gold at the physiologically characterized sites. Three main patterns of anatomical inputs were observed. One pattern was identified by inputs from the cochlear nucleus and ventral nucleus of the lateral lemniscus in isolation, and these injection sites were correlated with monaural responses. The second pattern had inputs only from the ipsilateral medial and lateral superior olive, and these sites were correlated with interaural time difference (ITD)-sensitive responses to low frequency (<500 Hz). A third pattern had inputs from a variety of olivary and lemniscal sources, notably the contralateral lateral superior olive and dorsal nucleus of the lateral lemniscus. These were correlated with high-frequency ITD sensitivity to complex acoustic stimuli. These data support the notion of anatomical regions formed by specific patterns of anatomical inputs to the ICC. Such synaptic domains may represent functional zones in ICC.

Multiplicative auditory spatial receptive fields created by a hierarchy of population codes

A multiplicative combination of tuning to interaural time difference (ITD) and interaural level difference (ILD) contributes to the generation of spatially selective auditory neurons in the owl's midbrain. Previous analyses of multiplicative responses in the owl have not taken into consideration the frequency-dependence of ITD and ILD cues that occur under natural listening conditions. Here, we present a model for the responses of ITD- and ILD-sensitive neurons in the barn owl's inferior colliculus which satisfies constraints raised by experimental data on frequency convergence, multiplicative interaction of ITD and ILD, and response properties of afferent neurons. We propose that multiplication between ITD- and ILD-dependent signals occurs only within frequency channels and that frequency integration occurs using a linear-threshold mechanism. The model reproduces the experimentally observed nonlinear responses to ITD and ILD in the inferior colliculus, with greater accuracy than previous models. We show that linear-threshold frequency integration allows the system to represent multiple sound sources with natural sound localization cues, whereas multiplicative frequency integration does not. Nonlinear responses in the owl's inferior colliculus can thus be generated using a combination of cellular and network mechanisms, showing that multiple elements of previous theories can be combined in a single system.

[Relations of aqueduct with some structures of mesencephalon]

INTRODUCTION

Aqueductus mesencephali is the biggest part of the ventricular system and that is why it is the most common place of intraventricular obstruction of cerebrospinal fluid. This study was done in order to study topographic characteristics of aqueduct more thoroughly.

MATERIALS AND METHODS

Transversal sections of mesencephalon were made in three levels. The first section was made caudally immediately from the posterior commissure. The second section was made in the middle part of the superior colliculi, and the third section was made in the rostral parts of the caudal sections of the superior colliculi. Distances of the aqueduct from structures of mesencephalon, obtained on the second section, are: 1. The distance of the aqueduct from the superior colliculi - 6.96 mm; 2. The distance of the aqueduct from the red nucleus - 6.02 mm; 3. The distance of the aqueduct from the substantia nigra - 12.29 mm; 4. The distance of the aqueduct from the interpeduncular fossa - 10.22 mm.

CONCLUSION

Knowledge of the anatomy of the aqueductus mesencephali is very important because of interpretation of patogenesis of hidrocefalus as well as of other syndromes that occure in some pathological processes in the system of ventricles.

Patterning of multiple layered projections to the auditory midbrain prior to experience

The precise arrangement of patterned inputs into discrete functional domains is a common organizational feature of primary sensory structures. While the specific organization of patterned connections has been well documented in the visual and somatosensory systems, comparatively little is known about the arrangement of neighboring afferent patterns in the emerging auditory system. Here we report early projection specificity for multiple converging inputs to the rat central nucleus of the inferior colliculus (ICC). Afferents arising from the dorsal cochlear nucleus (DCN), the dorsal nucleus of the lateral lemniscus (DNLL), and the lateral superior olive (LSO) establish discernible axonal layers a week prior to experience. By hearing onset, contralateral DCN and contralateral LSO layers are clearly defined and segregated from contralateral DNLL terminal zones. Layering of the ipsilateral LSO projection, on the other hand, exhibits considerable spatial overlap with the contralateral DNLL pattern. This fine laminar structure of interdigitating and overlapping inputs likely underlies the complex signal processing performed in the auditory midbrain and may serve as a model system for examining competitive interactions between neighboring excitatory and inhibitory projections early in development.

Lateralization of stimuli with independent fine-structure and envelope-based temporal disparities

Psychoacoustic experiments were conducted to investigate the role and interaction of fine-structure and envelope-based interaural temporal disparities. A computational model for the lateralization of binaural stimuli, motivated by recent physiological findings, is suggested and evaluated against the psychoacoustic data. The model is based on the independent extraction of the interaural phase difference (IPD) from the stimulus fine-structure and envelope. Sinusoidally amplitude-modulated 1-kHz tones were used in the experiments. The lateralization from either carrier (fine-structure) or modulator (envelope) IPD was matched with an interaural level difference, revealing a nearly linear dependence for both IPD types up to 135 degrees , independent of the modulation frequency. However, if a carrier IPD was traded with an opposed modulator IPD to produce a centered sound image, a carrier IPD of 45 degrees required the largest opposed modulator IPD. The data could be modeled assuming a population of binaural neurons with a physiological distribution of the best IPDs clustered around 45 degrees -50 degrees . The model was also used to predict the perceived lateralization of previously published data. Subject-dependent differences in the perceptual salience of fine-structure and envelope cues, also reported previously, could be modeled by individual weighting coefficients for the two cues.

The cytoarchitecture of the inferior colliculus revisited: a common organization of the lateral cortex in rat and cat

The inferior colliculus (IC) is the major component of the auditory midbrain and contains three major subdivisions: a central nucleus, a dorsal cortex, and a lateral cortex (LC). Discrepancies in the nomenclature and parcellation of the LC in the rat and cat seem to imply different, species-specific functions for this region. To establish a comparable parcellation of the LC for both rat and cat, we investigated its histochemistry and inputs. In both species, the deep lateral cortex is marked by a transition between the nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) rich superficial cortex and a cytochrome oxidase (CO) rich central nucleus. In both species, focal injections of anterograde tracers in the cochlear nucleus at sites of known best frequency produced bands of labeled inputs in two different subdivisions of the IC. A medial band of axons terminated in the central nucleus, while shorter bands were located laterally and oriented nearly perpendicularly to the medial bands. In the rat, these lateral bands were located in the third, deepest layer of the lateral (external) cortex. In the cat, the bands were located in a region that was previously ascribed to the central nucleus, but now considered to belong to the third, deepest layer of the LC, the ventrolateral nucleus. In both species, the LC inputs had a tonotopic organization. In view of this parallel organization, we propose a common parcellation of the IC for rat and cat with a new nomenclature. The deep layer of the LC, previously referred to as layer 3 in the rat, is designated as the 'ventrolateral nucleus' of the LC, making it clear that this region is thought to be homologous with the ventrolateral nucleus in the cat. The similar organization of the LC implies that this subdivision of the IC has similar functions in cats and rats.

Transcriptome changes associated with instructed learning in the barn owl auditory localization pathway

Owls reared wearing prismatic spectacles learn to make adaptive orienting movements. This instructed learning depends on re-calibration of the midbrain auditory space map, which in turn involves the formation of new synapses. Here we investigated whether these processes are associated with differential gene expression, using longSAGE. Newly fledged owls were reared for 8-36 days with prism or control lenses at which time the extent of learning was quantified by electrophysiological mapping. Transciptome profiles were obtained from the inferior colliculus (IC), the major site of synaptic plasticity, and the optic tectum (OT), which provides an instructive signal that controls the direction and extent of plasticity. Twenty-two differentially expressed sequence tags were identified in IC and 36 in OT, out of more than 35,000 unique tags. Of these, only four were regulated in both structures. These results indicate that regulation of two largely independent gene clusters is associated with synaptic remodeling (in IC) and generation of the instructive signal (in OT). Real-time PCR data confirmed the changes for two transcripts, ubiquitin/polyubiquitin and tyrosine 3-monooxgenase/tryotophan 5-monooxygenase activation protein, theta subunit (YWHAQ; also referred to as 14-3-3 protein). Ubiquitin was downregulated in IC, consistent with a model in which protein degradation pathways act as an inhibitory constraint on synaptogenesis. YWHAQ was up-regulated in OT, indicating a role in the synthesis or delivery of instructive information. In total, our results provide a path towards unraveling molecular cascades that link naturalistic experience with synaptic remodeling and, ultimately, with the expression of learned behavior.

Characterization of neuronal subsets surrounded by perineuronal nets in the rhesus auditory brainstem

The distribution of perineuronal nets and the potassium channel subunit Kv3.1b was studied in the subdivisions of the cochlear nucleus, the medial nucleus of the trapezoid body, the medial and lateral superior olivary nuclei, the lateral lemniscal nucleus and the inferior colliculus of the rhesus monkey. Additional sections were used for receptor autoradiography to visualize the patterns of GABAA and GABAB receptor distribution. The Kv3.1b protein and perineuronal nets [visualized as Wisteria floribunda agglutinin (WFA) binding] were revealed, showing corresponding region-specific patterns of distribution. There was a gradient of labelled perineuronal nets which corresponded to that seen for the intensity of Kv3.1b expression. In the cochlear nucleus intensely and faintly stained perineuronal nets were intermingled, whereas in the medial nucleus of the trapezoid body the pattern changed to intensely stained perineuronal nets in the medial part and weakly labelled nets in its lateral part. In the inferior colliculus, intensely labelled perineuronal nets were arranged in clusters and faintly labelled nets were arranged in sheets. Using receptor autoradiography, GABAB receptor expression in the anterior ventral cochlear nucleus was revealed. The medial part of the medial nucleus of the trapezoid body showed a high number of GABAA binding sites whereas the lateral part exhibited more binding sites for GABAB. In the inferior colliculus, we found moderate GABAB receptor expression. In conclusion, intensely WFA-labelled structures are those known to be functionally involved in high-frequency processing.

Response Properties and Location of Neurons Selective for Sinusoidal Frequency Modulations in the Inferior Colliculus of the Big Brown Bat

Most animal vocalizations, including echolocation signals used by bats, contain frequency-modulated (FM) components. Previous studies have described a class of neurons in the inferior colliculus (IC) of the big brown bat that respond exclusively to sinusoidally frequency modulated (SFM) signals and fail to respond to pure tones, noise, amplitude-modulated tones, or single FM sweeps. The aims of this study were to further characterize these neurons' response properties and to determine whether they are localized within a specific area of the IC. We recorded extracellularly from 214 neurons throughout the IC. Of these, 47 (22%) responded exclusively to SFM. SFM-selective cells were tuned to relatively low carrier frequencies (9–50 kHz), low modulation rates (20–210 Hz), and shallow modulation depths (3–10 kHz). Most had extremely low thresholds, with an average of 16.5 ± 7.6 dB SPL, and 89% had upper thresholds and closed response areas. For SFM-selective cells with spontaneous activity, the spontaneous activity was eliminated when sound amplitude exceeded their upper threshold and resumed after the stimulus was over. These findings suggest that SFM-selective cells receive low-threshold excitatory inputs and high-threshold inhibitory inputs. SFM-selective cells were clustered in the rostrodorsal part of the IC. Within this area, best modulation rate appeared to be correlated with best carrier frequency and depth within the IC.

Preceding weak noise sharpens the frequency tuning and elevates the response threshold of the mouse inferior collicular neurons through GABAergic inhibition

In acoustic communication, animals must extract biologically relevant signals that are embedded in noisy environment. The present study examines how weak noise may affect the auditory sensitivity of neurons in the central nucleus of the mouse inferior colliculus (IC) which receives convergent excitatory and inhibitory inputs from both lower and higher auditory centers. Specifically, we studied the frequency sensitivity and minimum threshold of IC neurons using a pure tone probe and a weak white noise masker under forward masking paradigm. For most IC neurons, probe-elicited response was decreased by a weak white noise that was presented at a specific gap (i.e. time window). When presented within this time window, weak noise masking sharpened the frequency tuning curve and increased the minimum threshold of IC neurons. The degree of weak noise masking of these two measurements increased with noise duration. Sharpening of the frequency tuning curve and increasing of the minimum threshold of IC neurons during weak noise masking were mostly mediated through GABAergic inhibition. In addition, sharpening of frequency tuning curve by the weak noise masker was more effective at the high than at low frequency limb. These data indicate that in the real world the ambient noise may improve frequency sensitivity of IC neurons through GABAergic inhibition while inevitably decrease the frequency response range and sensitivity of IC neurons.

Early segregation of layered projections from the lateral superior olivary nucleus to the central nucleus of the inferior colliculus in the neonatal cat

The central nucleus of the inferior colliculus (IC) is a laminated structure that receives multiple converging afferent projections. These projections terminate in a layered arrangement and are aligned with dendritic arbors of the predominant disc-shaped neurons, forming fibrodendritic laminae. Within this structural framework, inputs terminate in a precise manner, establishing a mosaic of partially overlapping domains that likely define functional compartments. Although several of these patterned inputs have been described in the adult, relatively little is known about their organization prior to hearing onset. The present study used the lipophilic carbocyanine dyes DiI and DiD to examine the ipsilateral and contralateral projections from the lateral superior olivary (LSO) nucleus to the IC in a developmental series of paraformaldehyde-fixed kitten tissue. By birth, the crossed and uncrossed projections had reached the IC and were distributed across the frequency axis of the central nucleus. At this earliest postnatal stage, projections already exhibited a characteristic banded arrangement similar to that described in the adult. The heaviest terminal fields of the two inputs were always complementary in nature, with the ipsilateral input appearing slightly denser. This early arrangement of interdigitating ipsilateral and contralateral LSO axonal bands that occupy adjacent sublayers supports the idea that the initial establishment of this highly organized mosaic of inputs that defines distinct synaptic domains within the IC occurs largely in the absence of auditory experience. Potential developmental mechanisms that may shape these highly ordered inputs prior to hearing onset are discussed.

Early myelination patterns in the brainstem auditory nuclei and pathway: MRI evaluation study

OBJECTIVE

The purpose of this study is to investigate the early myelination patterns of brainstem auditory nuclei and pathway on magnetic resonance imaging compared with past histological research. We aimed to identify the time course difference in myelination of the brainstem auditory nuclei and pathway between magnetic resonance imaging and histological research results.

METHODS

Subjects were 192 infants ranging in age from -4 to 224 corrected postnatal weeks. Images were obtained using a 1.5 T magnetic resonance unit. In four sites (cochlear nucleus, superior olivary nucleus, lateral lemniscus, inferior colliculus) of the brainstem auditory nuclei and pathway on four cross-sections obtained perpendicular to the long axis of the brainstem, signal changes of T1- and T2-weighted magnetic resonance images were analyzed using a region-of-interest methodology according to corrected postnatal age.

RESULTS

The cochlear nucleus and superior olivary nucleus showed myelinated intensity change from -3 to 13 corrected postnatal weeks on T2-weighted images. The lateral lemniscus showed myelinated intensity change from -3 to 8 corrected postnatal weeks on T1-weighted images and from -1 to 13 corrected postnatal weeks on T2-weighted images. The inferior colliculus showed myelinated intensity change from -2 to 39 corrected postnatal weeks on T2-weighted images.

CONCLUSIONS

Magnetic resonance imaging revealed the signal intensity change by myelination 11-18 weeks later than those reported in the histological literature. This time lag suggests that apart from histological research, the necessity for the milestones of auditory pathway maturation using MRI is suggested to evaluate the development of brainstem auditory pathway using MRI. This result suggests that myelination does not take place suddenly but happens gradually, so definite myelination, namely the complete change of myelin sheath ingredients, loss of water, and gain of lipids, is needed to be detected by magnetic resonance imaging.

Pathways from auditory cortex to the cochlear nucleus in guinea pigs

The inferior colliculus (IC) and superior olivary complex (SOC) are important sources of descending pathways to the cochlear nucleus. The IC and SOC are also targets of direct projections from the auditory cortex but it is not known if cortical axons contact the cells that project to the cochlear nucleus. Multi-labeling techniques were used to address this question in guinea pigs. A fluorescent anterograde tracer was injected into temporal cortex to label corticofugal axons. Different fluorescent tracers were injected into one or both cochlear nuclei to label olivary and collicular cells. The brain was subsequently processed for fluorescence microscopy and the IC and SOC were examined for apparent contacts between cortical axons and retrogradely labeled cells. The results suggest that cortical axons contact cochlear nucleus-projecting cells in both IC and SOC. In both regions, contacts were more numerous on the side ipsilateral to the injected cortex. In the IC, the contacted cells projected ipsilaterally or contralaterally to the CN. In the SOC, the contacted cells projected ipsilaterally, contralaterally or bilaterally to the CN. We conclude that auditory cortex is in a position to modulate descending pathways from both the IC and SOC to the cochlear nucleus.

Organization and origin of the connection from the inferior to the superior colliculi in the rat

The inferior colliculus (IC) is the main ascending auditory relay station prior to the superior colliculus (SC). The morphology and origin of the connection from inferior to superior colliculus (I-SC) was analyzed both by anterograde and retrograde tracing. Irrespective of the subregion of the IC in which they originate, the terminal fields of these connections formed two main tiers in the SC. While the dorsal one primarily involved the stratum opticum and the stratum griseum intermediale, the ventral one innervated the deep strata, although some fibers did connect these tiers. While the dorsal tier occupied almost the whole extension of the SC, the ventral one was mostly confined to its caudomedial quadrant. The fiber density in these tiers decreased gradually in a rostral gradient and the terminal fields became denser as the anterograde tracer at the injection site was distributed more externally in the cortex of the IC. Retrograde tracing confirmed this result, although it did not reveal any topographic ordering for the I-SC pathway. Most presynaptic boutons of the I-SC terminal field were located either inside or close to the patches of acetylcholinesterase activity. Together with previous anatomical and physiological studies, our results indicate that the I-SC connection relays behaviorally relevant information for sensory-motor processing. Our observation that this pathway terminates in regions of the superior colliculus, where neurons involved in fear-like responses are located, reinforce previous suggestions of a role for the IC in generating motor stereotypes that occur during audiogenic seizures.

Cerebral oxidative metabolism in rats with high and low exploratory activity

To reveal brain regions most significantly related to individual differences in exploratory behaviour, oxidative metabolism was measured by cytochrome c oxidase histochemistry in 2 months old Wistar rats with persistently high (HE) or low (LE) exploratory activity in a novel environment. LE-rats had significantly higher levels of oxidative metabolism in dorsal raphe and inferior colliculi. In contrast, HE-rats had higher metabolic activity in entorhinal cortex. In conclusion, rats with different exploratory styles differ in underlying cerebral activity as measured via oxidative metabolism in regions implicated in defensive behaviours and cognitive processing of sensory stimuli.

Midbrain pathways for prepulse inhibition and startle activation in rat

The midbrain is essential for prepulse inhibition (PPI) of the startle reflex, but the exact neural circuits for PPI are not yet determined. Electrical stimulation of the superior colliculus (SC) or pedunculopontine tegmentum was used to characterize the neurons and pathways that mediate PPI and the activation of startle that also occurs at higher currents in the same sites. Startle was inhibited by prepulses in most, but not all SC sites, with the lowest intensity sites in intermediate layers of SC. PPI latencies in SC sites were 4-6 ms longer than in inferior colliculus, intercollicular nucleus or pedunculopontine sites. Contrary to previous serial models, there must be two parallel midbrain pathways for PPI, a faster auditory pathway from inferior colliculus to pedunculopontine tegmentum, and a slower multimodal SC output for PPI. Double-pulse stimulation of SC sites shows that PPI results from direct stimulation of neurons with moderate refractory periods (0.4-1.0 ms), similar to SC neurons that mediate contraversive turning responses. By contrast, startle activation occurring at higher currents in all SC sites (even sites where PPI could not be elicited) results from stimulation of very short refractory period neurons (0.3-0.5 ms) and very long refractory period neurons (1.0-2.0 ms), with startle inhibition often found from 0.5-1.0 ms. Startle activation appears to result from stimulation of short refractory period neurons in deep SC layers that mediate fear-potentiated startle, plus long refractory period substrates in more dorsal SC sites.

Neuronal organization of the rat inferior colliculus participating in four major auditory pathways

The central nucleus of the inferior colliculus (CNIC) contains different types of neurons and is a source of ascending projection to the medial geniculate body (MGB), commissural projection to the contralateral IC, direct descending projection to the cochlea nucleus (CN) and indirect projection to the CN via the superior olivary complex (SOC). Using a retrograde tracing technique, we examined what kind of neurons and what percentage of neurons of each type recognized in the CNIC participated in the above-mentioned four projection pathways. We also examined whether the individual CNIC neurons send the collateral to the MGB, the contralateral IC, the CN and the SOC. In the present study, we demonstrated that the neurons participating in the four projections could be morphologically classified into two types of neurons with soma size variation. The percentages of neurons of each type differed among the four projection pathways. Using a double-labeling technique, we found very few double-labeled neurons, indicating the collateral projections to the ipsilateral MGB and the contralateral IC. There were no double-labeled neurons in the collateral projections between the other combinations of targets. Therefore, we conclude that the ascending projection, the commissural projection and the descending projection to these targets arise from separate populations of neurons.

Duration selectivity organization in the inferior colliculus of the big brown bat, Eptesicus fuscus

Duration selectivity of auditory neurons plays an important role in sound recognition. Previous studies show that GABA-mediated duration selectivity of neurons in the central nucleus of the inferior colliculus (IC) of many animal species behave as band-, short-, long- and all-pass filters to sound duration. The present study examines the organization of duration selectivity of IC neurons of the big brown bat, Eptesicus fuscus, in relation to graded spatial distribution of GABA(A) receptors, which are mostly distributed in the dorsomedial region of the IC but are sparsely distributed in the ventrolateral region. Duration selectivity of IC neuron is studied before and during iontophoretic application of GABA and its antagonist, bicuculline. Bicuculline application decreases and GABA application increases duration selectivity of IC neurons. Bicuculline application produces more pronounced broadening of the duration tuning curves of neurons at upper IC than at deeper IC but the opposite is observed during GABA application. The best duration of IC neurons progressively lengthens and duration selectivity decreases with recording depth both before and during drug application. As such, low best frequency neurons at upper IC have shorter best duration and sharper duration selectivity than high best frequency neurons in the deeper IC have. These data suggest that duration selectivity of IC neurons systematically varies with GABA(A) receptor distribution gradient within the IC.

Thalamic connections of the auditory cortex in marmoset monkeys: core and medial belt regions

In this study and its companion, the cortical and subcortical connections of the medial belt region of the marmoset monkey auditory cortex were compared with the core region. The main objective was to document anatomical features that account for functional differences observed between areas. Injections of retrograde and bi-directional anatomical tracers targeted two core areas (A1 and R), and two medial belt areas (rostromedial [RM] and caudomedial [CM]). Topographically distinct patterns of connections were revealed among subdivisions of the medial geniculate complex (MGC) and multisensory thalamic nuclei, including the suprageniculate (Sg), limitans (Lim), medial pulvinar (PM), and posterior nucleus (Po). The dominant thalamic projection to the CM was the anterior dorsal division (MGad) of the MGC, whereas the posterior dorsal division (MGpd) targeted RM. CM also had substantial input from multisensory nuclei, especially the magnocellular division (MGm) of the MGC. RM had weak multisensory connections. Corticotectal projections of both RM and CM targeted the dorsomedial quadrant of the inferior colliculus, whereas the CM projection also included a pericentral extension around the ventromedial and lateral portion of the central nucleus. Areas A1 and R were characterized by focal topographic connections within the ventral division (MGv) of the MGC, reflecting the tonotopic organization of both core areas. The results indicate that parallel subcortical pathways target the core and medial belt regions and that RM and CM represent functionally distinct areas within the medial belt auditory cortex.

Potential output pathways for agonistic-like responses resulting from the GABAA blockade of the torus semicircularis dorsalis in weakly electric fish, Gymnotus carapo

The purpose of this study is to examine the pathways involved in the electromotor (electric organ discharge interruptions) and skeletomotor responses (defense-like) observed by blockade of GABAergic control of the torus semicircularis dorsalis (TSd) of the awake weakly electric fish Gymnotus carapo, described in a former study. Microinjection of NMDA (5 mM) into the pacemaker nucleus (PM) through a guide cannula previously implanted caused a prolonged interruption of the electric organ discharge (EOD) intermingled with reduction in frequency, similar to that described for TSd GABA(A) blockade, but without noticeable skeletomotor effects. The EOD alterations elicited by bicuculline microinjections (0.245 mM) into the TSd could be blocked or attenuated by a previous microinjection of AP-5 (0.5 mM), an NMDA antagonist, into the PM. Labeled terminals are found in the nucleus electrosensorius (nE) after injection of the biotinylated dextran amine (BDA) tracer into the TSd and into the sublemniscal prepacemaker nucleus (SPPn) subsequent to the tracer injection into the nE. Defense-like responses but not EOD interruptions are observed after microinjections of NMDA (5 mM) into the rhombencephalic reticular formation (RF), where labeled terminals are seen after BDA injection into the TSd and somata are filled after injection of the tracer into the spinal cord. In this last structure, marked fibers are seen subsequent to injection of BDA into the RF. These results suggest that two distinct pathways originate from the torus: one for EOD control, reaching PM through nE and SPPn, and the other one for skeletomotor control reaching premotor reticular neurons. Both paths could be activated by toral GABA(A) blockade.

An atlas of the inferior colliculus of the gerbil in three dimensions

An atlas of the inferior colliculus of the gerbil is presented in three dimensions. Sections were cut in the transverse (coronal), horizontal or saggital planes and fit to a common cartesian coordinate grid. The sections used for the atlas were reacted for cytochrome oxidase activity, a functional marker that can be used to distinguish different areas in the brainstem. The atlas can be used for representation, comparison and correlation of neuroanatomical, neurophysiological, neurochemical and other data that can be spatially mapped in the inferior colliculus.

Projections from auditory cortex contact cells in the cochlear nucleus that project to the inferior colliculus

Anterograde and retrograde tracing techniques were combined to determine whether auditory cortical axons contact cells in the cochlear nucleus that project to the inferior colliculus. FluoroRuby or fluorescein dextran was injected into auditory cortex to label cortical axons by anterograde transport. Different fluorescent tracers (Fast Blue, FluoroGold, FluoroRuby or fluorescein dextran) were injected into one or both inferior colliculi to label cells in the cochlear nucleus. After 12-15 days, the brain was processed for fluorescence microscopy and the cochlear nuclei were examined for apparent contacts between cortical axons and retrogradely labeled cochlear nucleus cells. The results suggest that axons from the ipsilateral or contralateral cortex contact fusiform and giant cells in the dorsal cochlear nucleus and multipolar cells in the ventral cochlear nucleus that project directly to the inferior colliculus. The contacts occur on cell bodies and dendrites. The target cells in the cochlear nucleus include cells that project ipsilaterally, contralaterally or bilaterally to the inferior colliculus. The results suggest that auditory cortex is in a position to exert direct effects on the monaural pathways that ascend from the cochlear nucleus.

Echolocation, vocal learning, auditory localization and the relative size of the avian auditory midbrain nucleus (MLd)

The avian nucleus mesencephalicus lateralis, pars dorsalis (MLd) is an auditory midbrain nucleus that plays a significant role in a variety of acoustically mediated behaviours. We tested whether MLd is hypertrophied in species with auditory specializations: owls, the vocal learners and echolocaters. Using both conventional and phylogenetically corrected statistics, we find that the echolocating species have a marginally enlarged MLd, but it does not differ significantly from auditory generalists, such as pigeons, raptors and chickens. Similarly, all of the vocal learners tend to have relatively small MLds. Finally, MLd is significantly larger in owls compared to all other birds regardless of how the size of MLd is scaled. This enlargement is far more marked in asymmetrically eared owls than symmetrically eared owls. Variation in MLd size therefore appears to be correlated with some auditory specializations, but not others. Whether an auditory specialist possesses a hypertrophied MLd appears to be depend upon their hearing range and sensitivity as well as the ability to resolve small azimuthal and elevational angles when determining the location of a sound. As a result, the only group to possess a significantly large MLd consistently across our analyses is the owls. Unlike other birds surveyed, owls have a battery of peripheral and other central auditory system specializations that correlate well with their hearing abilities. The lack of differences among the generalists, vocal learners and echolocaters therefore reflects an overall similarity in hearing abilities, despite the specific life history requirements of each specialization and species. This correlation between the size of a neural structure and the sensitivity of a perceptual domain parallels a similar pattern in mammals.

Separate projections from the inferior colliculus to the cochlear nucleus and thalamus in guinea pigs

We used multiple-labeling techniques with retrograde fluorescent tracers to determine whether individual cells in the inferior colliculus project to the medial geniculate body (MG) and the cochlear nucleus (CN) in guinea pigs. Four possible projection patterns were examined: (1) to ipsilateral MG and ipsilateral CN; (2) to ipsilateral MG and contralateral CN; (3) to contralateral MG and ipsilateral CN; and, (4) to contralateral MG and contralateral CN. Following injections of different tracers into two or more sites, no inferior collicular cells were double-labeled from the two contralateral targets and only a few cells were double-labeled from each of the other pairs of targets. The double-labeled cells always totaled < 1% of the single-labeled populations. We conclude that collateral projections from the inferior colliculus to the MG and CN are virtually non-existent. Therefore, the ascending and descending projections to these targets arise from different cells. These cells could potentially receive different inputs and send different information to higher or lower centers of the auditory pathway.

Adaptation of brain regions to habitat complexity: a comparative analysis in bats (Chiroptera)

Vertebrate brains are organized in modules which process information from sensory inputs selectively. Therefore they are probably under different evolutionary pressures. We investigated the impact of environmental influences on specific brain centres in bats. We showed in a phylogenetically independent contrast analysis that the wing area of a species corrected for body size correlated with estimates of habitat complexity. We subsequently compared wing area, as an indirect measure of habitat complexity, with the size of regions associated with hearing, olfaction and spatial memory, while controlling for phylogeny and body mass. The inferior colliculi, the largest sub-cortical auditory centre, showed a strong positive correlation with wing area in echolocating bats. The size of the main olfactory bulb did not increase with wing area, suggesting that the need for olfaction may not increase during the localization of food and orientation in denser habitat. As expected, a larger wing area was linked to a larger hippocampus in all bats. Our results suggest that morphological adaptations related to flight and neuronal capabilities as reflected by the sizes of brain regions coevolved under similar ecological pressures. Thus, habitat complexity presumably influenced and shaped sensory abilities in this mammalian order independently of each other.

Fiber dissection technique for demonstrating the lateral lemniscus of the human brain

This report describes fiber dissection technique for tracing the auditory pathway from the cochlear nerve to the medial geniculate body via the lateral lemniscus, inferior colliculus and inferior brachium. Some fibers of the lateral leminiscus appear to reach the thalamus in conjunction with fibers of the medial lemniscus.

Frequency-Specific Effects on Cochlear Responses During Activation of the Inferior Colliculus in the Guinea Pig

The inferior colliculus (IC) is a major processing center in the ascending auditory pathway. The role of the IC in the descending efferent auditory system is less clear. Although the IC central nucleus (ICC) is the major relay station for the ascending auditory pathways, the IC's cortex receives its main input from the neocortex and nonauditory sources. The goal of this study was to determine if the IC subdivisions had different functions in the descending efferent auditory system. IC subdivisions were identified by their tuning curves evoked by tone stimulation, and the effects of localized electrical stimulation on the cochlear whole-nerve action potential (CAP). Sharp tuning curves were obtained from ICC in contrast to broad tuning curves from the lateral, external cortex. Electrical stimulation within the central nucleus had a sharply tuned effect on the CAP. The frequency region affected within the cochlea closely matched the best frequency of local cells within the central nucleus. The effect of electrical stimulation within the lateral, external cortex on the CAP was smaller in comparison to central nucleus stimulation. Similar to the broad tuning of cells within the lateral cortex, electrical stimulation had a broad frequency effect on CAP thresholds.

Organization of binaural excitatory and inhibitory inputs to the inferior colliculus from the superior olive

The major excitatory, binaural inputs to the central nucleus of the inferior colliculus (ICC) are from two groups of neurons with different functions-the ipsilateral medial superior olive (MSO) and the contralateral lateral superior olive (LSO). A major inhibitory, binaural input emerges from glycinergic neurons in the ipsilateral LSO. To determine whether these inputs converge on the same postsynaptic targets in the ICC, two different anterograde tracers were injected in tonotopically matched areas of the MSO and the LSO on the opposite side in the same animal. The main findings were that the boutons from MSO axons terminated primarily in the central and caudal parts of the ICC laminae but that contralateral LSO terminals were concentrated more rostrally and on the ventral margins of the MSO inputs. In contrast, the ipsilateral LSO projection converged with the MSO inputs and was denser than the contralateral LSO projection. Consistent with this finding, retrograde transport experiments showed that the very low-frequency areas of the ICC with dense MSO inputs also received inputs from greater numbers of ipsilateral LSO neurons than from contralateral LSO neurons. The results suggest that different binaural pathways through the low-frequency ICC may be formed by the segregation of excitatory inputs to ICC from the MSO and the contralateral LSO. At the same time, the ipsilateral LSO is a major inhibitory influence in the target region of the MSO. These data support the concept that each frequency-band lamina in the ICC may comprise several functional modules with different combinations of inputs.

Representation of species-specific vocalizations in the inferior colliculus of the guinea pig

The responses of individual neurons to 4 typical guinea pig vocalization calls (purr, chutter, chirp, and whistle) were recorded in the inferior colliculus (IC) of anesthetized guinea pigs. All calls elicited a response in about 80% of units. Unit selectivity for individual calls was low, given that a majority of neurons (55% of 124 units) responded to all vocalizations and only a small portion of neurons (3%) responded to only one call or did not respond to any of the calls (3%). In 15% of units, the response to one call was > or =25% stronger than the response to any other sound (tone, noise, and other calls); these neurons were selective for chirp or whistle, and no unit preferred chutter or purr. Neuronal activity provided information about the spectrotemporal patterns of the calls. Peristimulus time histograms (PSTHs) reflected the energy of the near-characteristic frequency band, and the population PSTH reliably matched the sound envelope for calls characterized by one or more short impulses (chirp, purr, and chutter) but did not exactly fit the envelope for whistle--a slow-modulated and relatively long call. Calculations based on firing rates indicated the approximate positions of the main spectral peaks but did not always reflect their relative magnitude. The time-reversed version of whistle elicited on average a weaker response than did the natural whistle (by 24%), but there were neurons with a significantly stronger response to the natural ("forward-selective," 30%) as well as to the time-reversed whistle ("reverse-selective," 15%). This study does not prove the existence of units selectively responding to animal calls, but it provides evidence for the encoding of the spectrotemporal acoustic patterns of vocalizations by IC units.

Quantitative measurement of afferent layers in the ferret inferior colliculus: DNLL projections to sublayers

In the central nucleus of the inferior colliculus (IC), afferent projections are aligned with dendritic arbors of disk-shaped cells, forming fibrodendritic layers. One feature that may serve as a guide for study of the intrinsic organization of the IC layers is the segregation of certain inputs to bands and patches within the layers of the central nucleus. In this study, we used Phaseolus leucoagglutinin as an anterograde tracer to examine the projections from the dorsal nucleus of the lateral lemniscus to the contralateral IC in adult ferrets. The labeled afferent projections distributed along the IC layers in a series of bands where there were dense endings and interband spaces where there were few if any endings. Branches of individual labeled axons that were reconstructed distributed within a single afferent band. Measurements of both the terminal density distribution and the optical density across the band were similar indicating that afferent bands were approximately 85 microm thick. Quantitative measurements of the labeled afferent bands will enhance comparison with other afferent projections and analysis of afferent development and plasticity.

Anatomophysiology of the central auditory nervous system: basic concepts

The purpose of this review is to provide an overview of the central auditory nervous system (CANS). Three main relay nuclei are located between the auditory nerve and the primary auditory cerebral cortex: 1- the cochlear nucleus, 2- the contralateral inferior colliculus and 3- the contralateral medial geniculate body. Some fibers of this main ascending pathway branch off to other nuclei such as the nuclei of the superior olivary complex and the nucleus of the lateral lemniscus. Fiber tracts connect the two sides of the ascending pathway at several levels, in such a manner that each ear projects heavier to the contralateral temporal cortex. As sensory information travels within the CANS, its processing occurs not only in a serial order but also in a parallel manner resulting a highly efficient and redundant system. Finally, this review put forth a model of central auditory processing that can serve as the basis for evaluating and addressing the needs of eventual revalidation.

Immunolocalization of PICK1 in the ascending auditory pathways of the adult rat

Protein that interacts with C-kinase alpha (PICK1) is a PDZ domain protein that interacts with many binding partners in the central nervous system (CNS), including activated protein kinase Calpha and subunits of the AMPA subtype of glutamate receptor. Almost nothing is known about the anatomic distribution of PICK1 in the intact adult CNS. By using PICK1 antisera and peroxidase immunocytochemistry, we report on the distribution of PICK1 in the ascending pathways of the central auditory system of the adult rat. PICK1-immunoreactivity (ir) was observed in many component nuclei of the central auditory system, including the dorsal cochlear nucleus, anteroventral cochlear nucleus, posteroventral cochlear nucleus, some divisions of the superior olivary complex, inferior colliculus, medial geniculate body, and primary auditory cortex. The general staining pattern for PICK1-immunoreactivity was somatodendritic with scattered puncta in neuropil and somatodendritic regions. The distribution of PICK1 partially overlaps with PKCalpha and glutamate receptor subunits such as GluR2. These data suggest that PICK1 may function in the regulation of PKCalpha and GluR2 localization in components of the rat auditory system, which may be a fundamental mechanism of synaptic transmission and/or plasticity. J. Comp. Neurol.

Temporal and spatial coding of periodicity information in the inferior colliculus of awake chinchilla (Chinchilla laniger)

Amplitude modulation responses and onset latencies of multi-unit recordings and evoked potentials were investigated in the central nucleus of inferior colliculus (ICC) in the awake chinchilla. Nine hundred and one recording sites with best frequencies between 60 and 30 kHz showed either phasic (18%), tonic (25%), or phasic-tonic (57%) responses. Of 554 sites tested for responses to modulation frequencies 73% were responsive and 57% showed clear preference for a narrow range of modulation frequencies. Well defined bandpass characteristics were found for 32% of rate modulation transfer functions (rate-MTFs) and 36% of synchronization MTFs (sync-MTFs). The highest best modulation frequency (BMF) of a bandpass rate-MTF was 600 Hz. Neurons with phasic responses to best-frequency tones showed strong phase coupling to modulation frequencies and were dominated by bandpass rate-MTFs and sync-MTFs. Most neurons with tonic responses showed bandpass tuning only for sync-MTFs. Both BMFs and onset latencies changed systematically across frequency-band laminae of the ICC. Low BMFs and long latencies were located medially and high BMFs and short latencies laterally. Latency distributions obtained with evoked potentials to clicks showed a similar gradient to the multi-unit data. These findings are in line with previous findings in different animals including humans and support the hypothesis that temporal processing results in a topographic arrangement orthogonal to the spectral processing axis, thus forming a second neural axis of the auditory system.

Descending projections to the inferior colliculus from the posterior thalamus and the auditory cortex in rat, cat, and monkey

Projections from the posterior thalamus and medial geniculate body were labeled retrogradely with wheat germ agglutinin conjugated to horseradish peroxidase injected into the rat, cat, and squirrel monkey inferior colliculus. Neurons were found ipsilaterally in the (1) medial division of the medial geniculate body, (2) central gray, (3) posterior limitans nucleus, and the (4) reticular part of the substantia nigra. Bilateral projections involved the (5) peripeduncular/suprapeduncular nucleus, (6) subparafascicular and posterior intralaminar nuclei, (7) nucleus of the brachium of the inferior colliculus, (8) lateral tegmental/lateral mesencephalic areas, and (9) deep layers of the superior colliculus. The medial geniculate projection was concentrated in the caudal one-third of the thalamus; in contrast, the labeling in the subparafascicular nucleus, substantia nigra, and central gray continued much further rostrally. Robust anterograde labeling corresponded to known patterns of tectothalamic projection. Biotinylated dextran amine deposits in the rat inferior colliculus revealed that (1) many thalamotectal cells were elongated multipolar neurons with long, sparsely branched dendrites, resembling neurons in the posterior intralaminar system, and that other labeled cells were more typical of thalamic relay neurons; (2) some cells have reciprocal projections. Similar results were seen in the cat and squirrel monkey. The widespread origins of descending thalamic influences on the inferior colliculus may represent a phylogenetically ancient feedback system onto the acoustic tectum, one that predates the corticocollicular system and modulates nonauditory centers and brainstem autonomic nuclei. Besides their role in normal hearing such pathways may influence behaviors ranging from the startle reflex to the genesis of sound-induced seizures.

Intracranial auditory pathways: anatomy correlated with evoked response data

The intracranial auditory pathways are complex, and they have been incompletely mapped. The purpose of the communication is to summarize what is known about these fibers onto clinically relevant imaging sections. Anatomic information is correlated with the clinical auditory evoked responses currently used to localize lesions in these pathways. This linkage will improve communication between imagers and clinicians. In addition, it can be used for teaching.

Responses of young and aged rat inferior colliculus neurons to sinusoidally amplitude modulated stimuli

The inferior colliculus (IC) is a processing center for monaural and binaural auditory signals. Many units in the central nucleus of the inferior colliculus (CIC) respond to amplitude and frequency modulated tones, features found in communication signals. The present study examined potential effects of age on responses to sinusoidally amplitude modulated (SAM) tones in CIC and external cortex of the inferior colliculus (ECIC) units in young and aged F344 rats. Extracellular recordings from 154 localized single units of aged (24 month) rats were compared to recordings from 135 IC units from young adult (3 month) animals. SAM tones were presented at 30 dB above threshold. Comparisons were made between CIC and ECIC regarding the percentage of units responding to SAM stimuli, the relationship between SAM responsiveness and temporal response patterns, maximum discharge rates and maximum modulation gains, shapes of rate transfer functions and synchronization modulation transfer functions (MTFs) in response to SAM tones. Sixty percent of units in young and aged rat IC were selectively responsive to SAM stimuli. Eighty-one percent of units classified as onset temporal response patterns were not tonically responsive to SAM stimuli. Median maximum discharge rate in response to SAM tones was 17.6/s in young F344 rats; median maximum modulation gain was 3.85 dB. These measurements did not change significantly with age. Thirty-seven percent of young rat units displayed bandpass MTFs and 53% had lowpass MTFs. There was a significant age-related shift in the distribution of MTF shapes in both the CIC and ECIC. Aged animals showed a lower percentage of bandpass functions and a higher percentage of lowpass functions. Age-related changes observed in SAM coding may reflect an altered balance between excitatory/inhibitory neurotransmitter efficacy in the aged rat IC, and/or possibly a change in the functional dynamic range of IC neurons.