Time of origin of neurons of the rat superior colliculus in relation to other components of the visual and visuomotor pathways

Summary Groups of pregnant rats were injected with two successive daily doses of ³H-thymidine from gestational day 12 and 13 (E12+E13) until the day before parturition (E21+22) in order to label all the multiplying precursors of neurons. At 60 days of age the proportion of neurons generated (or no longer labelled) on specific days was determined in the separate layers of the superior colliculus. Neurogenesis begins with the production of a few large multipolar neurons in layers V and IV on day E12; the bulk (87%) of these cells are generated on day E13. This early-produced band of large neurons, the intermediate magnocellular zone, divides the superior colliculus into two cytogenetically distinct regions. In both the deep and the superficial superior colliculus neuron production is relatively protracted. In the deep superior colliculus neuron production peaks on day E15 in layer VII, on day E15 and E16 in layer VI, and on day E16 (the large neurons excluded) in layer V, indicating an inside-out sequence. In the superficial superior colliculus peak production time of layer III cells is on day E15 and of layer IV cells on day E16; peak production time of both layer I and II is on day E16 but in the latter region neuron production is more prolonged and ends on day El8. One interpretation of these results is that the two pairs of superficial layers are produced in an outside-in sequence. These three cytogenetic subdivisions of the superior colliculus may be correlated with its structural-functional parcellation into an efferent spinotectal, a deep somatomotor and a superficial visual component.A comparison of neurogenesis in different components of the visuomotor and visual pathways of the rat indicates that the motor neurons of the extraocular muscles, the abducens, trochlear and oculomotor nuclei, and neurons of the nucleus of Darkschewitsch are produced first. Next in line are source neurons of efferents to the bulb and the spinal cord: those of the Edinger-Westphal nucleus and the intermediate magnocellular zone of the superior colliculus. These are followed by the relay neurons of the dorsal nucleus of the lateral geniculate body. The neurons of the superficial superior colliculus and of the visual cortex implicated in visual sensori-motor integrations are produced last.Abbreviations A aqueduct - ap stratum album profundum (layer VII) - bi brachium of the inferior colliculus - c caudal - CGd central gray, pars dorsalis - CGl central gray, pars lateralis - CGv central gray, pars ventralis - dm deep magnocellular zone - EW Edinger-Westphal nucleus - gi stratum griseum intermediale (layer IV) - gp stratum griseum profundum (layer VI) - gs stratum griseum superficiale (layer II) - IC inferior colliculus - im intermediate magnocellular zone - LGd lateral geniculate nucleus, pars dorsalis - ll lateral lemniscus - lm stratum lemnisci (layer V) - MG medial geniculate nucleus - ND nucleus of Darkschewitsch - NO nucleus of the optic tract - op stratum opticum (layer III) - ot optic tract - r rostral - SC superior colliculus - vIII third ventricle - ZO stratum zonale (layer I) - III oculomotor nucleus - IV trochlear nucleus - Vm mesencephalic nucleus of the trigeminal - VI abducens nucleus     

Histogenesis of the inferior colliculus in rat

Summary The early histogenesis of the inferior colliculus from embryonic day 11 to 18 (E11 to E18) has been studied in the rat by analysis of Golgi impregnated material and plastic sections.This analysis has shown that the pseudo-stratified columnar neuroepithelium observed at E11 is followed by the appearance at E12 of three zones: marginal, intermediate and ventricular. Signs of cell differentiation are first observed in the intermediate zone. Secondary rearrangements occur within this zone, and by E16 a thin cortical plate (the cortex of the inferior colliculus) develops at the junction of the intermediate and marginal zones giving rise to the external and pericentral nuclei of this structure, which has a cortical organization in adults. The remainder of the intermediate zone (the nucleus of the inferior colliculus), invaded by axons, expands dramatically by E16–E17 and gives rise to the central and dorso-medial nuclei of the inferior colliculus which have a nuclear organization in adults. The morphogenetic events which take place in these two regions differ and can be identified by the study of cell migration and differentiation.In the nucleus of the inferior colliculus, neuronal migration begins with detachment of the ventricularly directed process, or trailing process, of the primitive epithelial cell from the ventricular surface. This is followed by the ascent of the cell nucleus through the pially directed, or leading, process by a mechanism identical to the perikaryal translocation already described in other regions of the nervous system. This mechanism of cell migration is characteristic of a first type of migratory young neuron (type I). Axons initiate from the leading process of these cells during migration and dendrites grow out in various directions giving these cells a bipolar or a multipolar appearance. Dendritic differentiation occurs first in the outermost cells of the nucleus and proceeds inwards.In the cortex of the inferior colliculus, neuronal migration also begins with detachment of the ventricular process, which occurs by E12, immediately followed by the detachment and retraction of the apical or leading process. Within the intermediate zone, migratory cells become rounded and sprout numerous processes. One of these processes is tipped by a growth cone and displays all the characteristics of an axon. It is directed tangentially in the intermediate layer. Dendritic growth and differentiation starts when the cells reach their final position in the cortical plate, and proceeds from the innermost cells outward. Due to the inadequacy of our methods for identifying radial glial fibers, the mechanism of migration of this type of cell (type II) remains unclear.Our results confirm that the inferior colliculus of the rat is organized as a central nuclear mass surrounded by a thin cortex. As previously observed in other regions of the nervous system, the modes of cell migration and differentiation in the cortical and non-cortical structures of the inferior colliculus appear different.     

Firing of inferior colliculus auditory neurons is phase-locked to the hippocampus theta rhythm during paradoxical sleep and waking

The activity of 52 single auditory units in the central nucleus of the inferior colliculus (IC) was recorded along with cortical and hippocampal (CA1) electrograms and neck muscle electromyograms in behaving, head-restrained guinea pigs during paradoxical sleep (PS) and wakefulness. Sixteen (30%) of the IC auditory units showed positive correlation with the hippocampal theta () rhythm: 8 (15%) were rhythmic with phase-locking (type 1), 8 (15%) showed only phase-locking with no rhythmicity (type 2), while 70% did not show any correlation to hippocampal rhythm (type 3). During wakefulness IC neurons (4 of 13) showed a higher synchrony with hippocampal when sound-stimulated at the unit's characteristic frequency. During PS all IC auditory neurons recorded presented some hippocampal correlation: 40% were rhythmic and phase-locked to the frequency and 60% were nonrhythmic maintaining the phase-locking. Shifts in the angle of phase-locking to the rhythm were observed during PS. It is suggested that the hippocampal rhythm may play the part of an internal clock, adding a temporal dimension to the processing of auditory sensory information.     

Frequency response areas of neurons in the mouse inferior colliculus. I. Threshold and tuning characteristics

Excitatory and inhibitory frequency response areas of 130 neurons of the central nucleus of the mouse inferior colliculus (ICC) were mapped by extracellular single-unit recordings and quantitatively evaluated with regard to thresholds, steepness of slopes of excitatory tuning, characteristic frequencies of excitation (CF_E), inhibition (CF_I), and bandwidths of response areas (sharpness of tuning). Two-tone stimuli were used to determine the shapes of inhibitory response areas. Class I neurons (n=54) had asymmetrical (with regard to the CF_E) excitatory and inhibitory response areas, with inhibition above CF_E having lower thresholds and covering larger areas than inhibition below CF_E. Quantitative relationships between CF_E and CF_I thresholds, and sharpness of tuning showed that the receptive fields of about two-thirds of these neurons had properties similar to auditory nerve fibers. Class II neurons (n=36) had small symmetrical or tilted excitatory areas of rather constant bandwidths and broad inhibitory areas reaching far into and often through the excitatory area, leading to closed excitatory areas in ten neurons. Class III neurons (n=32) had higher excitatory thresholds and the highest proportions of unilateral inhibitory areas compared with neurons of the other classes. Their excitatory area often widened symmetrically with increasing sound level. Their inhibitory areas did not overlap with the excitatory area. Class IV neurons (n=8) had two branches of excitatory areas (two-CFs_E) and six of the neurons had a central inhibitory area in addition to the low- and high-frequency inhibitory areas. In most neurons, the shapes of excitatory response areas predicted the shapes of inhibitory areas. Altogether, 15 neurons from all 4 classes had areas of facilitation in addition to inhibitory areas. Facilitation in six class IV neurons occurred between the two branches of the excitatory area. All 130 neurons had large inhibitory areas, 106 of them on both sides of the excitatory area. That is, sound processing in the ICC shows strong inhibitory components. The close relationships between excitatory and inhibitory CFs found here indicate that inhibitory projections to and interactions within the ICC are tonotopically organized comparable to the excitatory ones. Electronic Publication     

An electrophysiological study of neural pathways for corticofugally inhibited neurons in the central nucleus of the inferior colliculus of the big brown bat, Eptesicus fuscus

This electrophysiological study tests the hypothesis that one possible neural pathway for corticofugally inhibited neurons in the central nucleus of the inferior colliculus (ICc) of the big brown bat, Eptesicus fuscus, is mediated through excitatory projections from the auditory cortex (AC) to the external nucleus of the IC (ICx), which then sends inhibitory inputs to the ICc. This study shows that all neurons in the ICx are broadly tuned to stimulus frequency. Electrical stimulation in the AC typically increases the number of impulses, expands the auditory spatial response areas, and broadens the frequency tuning curves (FTCs) of neurons in the ICx. This corticofugal facilitation is mediated at least in part through NMDA receptors, since application of DL-2-amino-5-phosphonovaleric acid (APV), an antagonist for NMDA, decreases these response properties of neurons in the ICx. Electrical stimulation in the ICx typically decreases the number of impulses, reduces the auditory spatial response areas, and narrows the FTCs of neurons in the ICc. This inhibition is mediated at least in part through GABA_A receptors, since application of bicuculline, an antagonist for GABA, increases these response properties of neurons in the ICc. These data suggest that corticofugal facilitation of the ICx and the inhibition of the ICx to the ICc may be one of the polysynaptic pathways for corticofugal inhibition of neurons in the ICc. Possible functions of this polysynaptic pathway in acoustic orientation and signal processing are discussed. Electronic Publication     

Anatomy of the inferior colliculus in rat

Summary This paper defines the pattern of subdivision of the inferior colliculus in rat. It is based on serial sections of brains of albino and hooded rats cut in the frontal, sagittal and horizontal planes using Golgi, Nissl and a combined cell-myelin method. In rat, like in other mammals, the inferior colliculus consists of a central nucleus, an external cortex, and a dorsal cortex. The central nucleus is flattened in the frontal plane and confined to the caudomedial part of the inferior colliculus. It is characterized by a lamellar organization of disc-shaped neurons interspersed with multipolar cells. The cells are small to medium-sized. Although there is a dorsoventral gradient in size and packing density of cells within the nucleus, the overall size is smaller and the packing density larger than in adjacent subdivisions. The two cortices each consists of three layers. The outer-most layer is common to the two cortices, forming a fibro-cellular capsule continuous along most of the circumference of the inferior colliculus. The external cortex is located lateral, rostral, ventral and ventrocaudal to the central nucleus. Its second layer, deep to the superficial capsule, is characterized by clusters of many small and a few medium-sized neurons in a myelin-dense neuropil. Layer 3, which constitutes the major portion of the subdivision, consists of relatively scattered, small, medium and large cells, the most characteristic element being large multipolar neurons with coarse Nissl granules. The dorsal cortex is located dorsocaudal and dorsomedial to the central nucleus. Its second layer is composed of small neurons, while the third, deep layer in addition contains medium-sized neurons. The cell density is intermediate to that of the central nucleus and the deep part of the external cortex. We have tried to facilitate the parcellation by reference to easily recognizable, nearby structures and to standard stereotaxic coordinates.     

Anatomical and physiological investigation of auditory input to the superior colliculus of the echolocating megachiropteran bat Rousettus aegyptiacus

The objective of this study was to investigate whether a representation of auditory space in the superior colliculus (SC) of the echolocating megachiropteran bat (Rousettus aegyptiacus) exists. Additionally the subcortical auditory connectivity of the SC was investigated. A total of 207 units were recorded in five awake animals while presenting acoustic stimuli (white noise, clicks, and pure tones) at different positions in space. Six units responded to acoustic stimulation. Three of these located within the superficial layers and one located in the intermediate layers were classified as omnidirectional units. Two units were located within the deep layers. One was classified as a hemifield unit, and the other as a frontal unit. All units responded phasically to acoustic stimulation with a latency of 4–150 ms. None of them could be activated by visual stimuli. We further examined the interaction of paired auditory and visual stimulation in 116 visually responsive units. Responses to visual stimulation were markedly altered by acoustic stimulation in 5 units. The influence of the acoustic stimuli was temporally and spatially restricted, and resulted either in a reduction or an elevation of unit responsiveness. Horseradish peroxidase was injected into the SC of eight animals to investigate the auditory subcortical connectivity of the SC. Retrograde labeling in auditory structures was rare compared with labeling found in nonauditory structures (e.g., retina, substantia nigra, parabigeminal nucleus). In auditory structures retrograde labeling was found mainly in the external nucleus of the inferior colliculus and in the nucleus of the brachium of the inferior colliculus. To a lesser extent it was found in the nucleus sagulum and in the area medial to the lemniscal nuclei. In one case the dorsal nucleus of the lateral lemniscus and the anterolateral periolivary nucleus were labeled. Our results reveal only a sparse auditory input into the SC of the flying fox, R. aegyptiacus. On the basis of single-unit recordings, we did not find an elaborate representation of auditory space as it is described for several other species. The existence of auditory and bimodal neurones, in combination with their response properties, nonetheless indicate that there might be a representation of auditory space in the SC of R. aegyptiacus.     

Age-related decrease in GABAB receptor binding in the Fischer 344 rat i inferior colliculus

Quantitative receptor autoradiography was used to asses GABA_B receptor binding in three primary subdivisions of the inferior colliculus (IC): dorsal cortex (DCIC), external cortex (ECIC), and the central nucleus (CIC) of 3-, 18–20- and 26-month-old Fischer 344 rats. GABA_B binding sites were localized using [³H]GABA in the presence of a saturating concentration of isoguvacine, a selective GABA_A receptor agonist, to displace [³H]GABA bound to GABA_A receptor sites. In the three IC subdivisions examined, GABA_B receptor binding was significantly reduced in 26-month-old rats when compared to 3-month-old rats (DCIC, −44%; ECIC, −36%; CIC, −32%; p .05 For comparison, GABA_B binding was determined in the portion of cerebellum located in the recess of the IC. In the molecular layer of this region, there were no statistically significant differences in receptor binding between 3, 18–20- and 26-month-old rats. In addition, there was not a significant age-related change in the cross-sectional area of the IC. These findings provide additional evidence to support the existence of selective age-related changes in GABA neurotransmitter function in the rat IC.     

Physiology of pathway from dorsal cochlear nucleus to inferior colliculus revealed by electrical and auditory stimulation

Summary The dorsal cochlear nucleus (DCN) projects axons to the contralateral central nucleus of the inferior colliculus (ICC) via the dorsal acoustic stria (DAS). In the anaesthetised cat, when brief electrical stimuli are applied to the caudal surface of DCN, single unit and field activity is evoked preferentially in the ventro-lateral region of ICC. Most single ICC units judged by electrical stimulation to have received a direct input from DCN are excited by contralateral tonal stimulation and inhibited or uninfluenced by ipsilateral tones. The sharp non-monotonic intensity functions of most of these units are similar to those of units in the dorsal cochlear nucleus, but ipsilateral inhibition is likely to be provided by a source other than DCN. Thus, although it is suggested that axons of DAS terminate preferentially in the ventro-lateral aspect of the contralateral ICC, it is likely that at least some neurones in this latter region receive additional input from another source — possibly the lateral superior olivary nucleus.This study was supported by grants from the Australian Research Grants Committee     

Diverse responses of single auditory afferent fibres to electrical stimulation of the inferior colliculus in guinea-pig

Medial olivocochlear (MOC) neurons in the auditory brainstem project to the cochlea and inhibit cochlear neural output by their action on the cochlear outer hair cells. The function of the lateral olivocochlear (LOC) neurons, projecting to the auditory primary afferents is still under debate. Recent studies have suggested that the olivocochlear system can have frequency-specific, spatially restricted effects within the cochlea. It has been shown that the inferior colliculus (IC) projects to the MOC neurons in a tonotopic manner and that electrical stimulation of the IC can activate the MOC system, suppressing cochlear gross potentials. In addition, it has been shown that stimulation of the IC may be able to activate the LOC neurons. We investigated the effect of IC stimulation on single units in the cochlea of guinea-pigs and searched for evidence of spatially restricted effects of the MOC system and effects of the LOC system. We found a variety of effects on single units. About 40% of units were unchanged whereas others (53%) showed inhibitory effects, reflected in a rightward shift of their rate-level function, sometimes accompanied by a suppression of the spontaneous rate. About 18% of the inhibited neurons showed an increased spontaneous rate. In 5% of the units we observed an excitatory effect of IC stimulation, resulting in a leftward shift of the rate-level functions. We also found that the effect could vary greatly between units of the same and adjacent frequencies within a single animal. These results imply an involvement of another regulatory system besides the MOC system, possibly the LOC system, which acts directly on the primary afferents. These data also demonstrate that the olivocochlear system is capable of eliciting highly localized effects on different frequency regions in the cochlea.     

Neurotransmitter and neuromodulator systems of the rat inferior colliculus and auditory brainstem studied by in situ hybridization

This study was concerned with the distribution of a variety of putative neuromodulator and neurotransmitter systems in auditory regions of the rat brainstem using in situ hybridization histochemistry. Serial brain sections were screened for the presence of mRNAs for (i) precursors of the neuroactive substances cholecystokinin, somatostatin, proenkephalin and substance P (preprotachykinin), (ii) glutamic acid decarboxylase, the key synthesizing enzyme for GABA, or (iii) subunits l, 2 and 3 of the GABA_A receptor. Detectable message for all of these probes was found in at least one auditory brainstem area. There were clear differences in the distribution of the various mRNAs in subregions of the inferior colliculus, superior olivary complex, lateral lemniscus and cochlear nucleus. Cells expressing mRNA for glutamic acid decarboxylase were most prominent in the inferior colliculus, but were also present in all lower auditory brainstem nuclei, except the medial superior olivary nucleus and medial nucleus of trapezoid body. The mRNA for GABA_A1 receptor subunits was detectable in all auditory regions investigated, although at different levels of expression. GABA_A2 and 3 mRNA signals were seen in inferior colliculus, lateral lemniscus and in almost all superior olivary complex regions, but in fewer cells and at lower levels than the GABA_A1 subtype. Moderate to high levels of preprocholecystokinin mRNA expression were seen in all subregions of the inferior colliculus. In other auditory brainstem areas, preprocholecystokinin mRNA levels were either low or absent. With regard to mRNAs for the neuroactive peptides somatostatin, preprotachykinin and preproenkephalin, all were expressed in the inferior colliculus but there were differences in their cellular distribution. For example, there were almost no preprotachykinin mRNA expressing cells in the central nucleus of inferior colliculus and levels of somatostatin mRNA were especially high in the dorsal cortex and in layer 3 of the external cortex of inferior colliculus. There were also differences in the pattern of expression of these mRNAs in the various brainstem auditory nuclei; there was no preprotachykinin mRNA in any part of the superior olivary complex, only somatostatin mRNA was found in the ventral cochlear nucleus, and expression of preproenkephalin mRNA was pronounced in the ventral nucleus of the trapezoid body and the rostral periolivary zone. The data are considered in light of the connectivity and functional organization of the auditory brainstem.     

Pallidotectal projection to the inferior colliculus of the rat

Summary After injection of fluorescent tracer into the inferior colliculus (IC), retrogradely labeled cells were observed not only in the temporoauditory cortex (ACx) and the substantia nigra pars lateralis, but also in the globus pallidus (GP). These labeled GP cells were localized exclusively in the caudal portion of the GP, which has been known to project to the ACx. Employing a retrograde fluorescent double labeling technique, the GP-IC neurons were found to be distributed in a separate manner from the GP-ACx neurons within the caudal GP. The present study provides further anatomical evidence that the caudal GP has a functional role in auditory processing.Abbreviations ACx temporoauditory cortex - BC Brachium conjunctivum - CP cerebral peduncle - CPu caudate putamen - DY Diamidino Yellow - EP entopeduncular nucleus - FG Fluoro-Gold - GP globus pallidus - I internal capsule - IC inferior colliculus - OT optic tract - SC superior colliculus - SN1 substantia nigra pars lateralis - T thalamus - TB True Blue - TPC nucleus tegmenti pedunculopontinus pars compacta     

Frequency and space representation in the inferior colliculus of the FM bat,Eptesicus fuscus

Summary The tonotopic organization and spatial sensitivity of 217 inferior collicular (IC) neurons of Eptesicus fuscus were studied under free field stimulation conditions. Acoustic stimuli were delivered from a loudspeaker placed 21 cm ahead of the bat to determine the best frequency (BF) and minimum threshold (MT) of isolated IC neurons. A BF stimulus was then delivered as the loudspeaker was moved horizontally across the frontal auditory space of the bat to locate the best azimuthal angle (BAZ) at which the neuron had its lowest MT. The stimulus was then raised 3 dB above the lowest MT to determine the horizontal extent of the auditory space within which a sound could elicit responses from the neuron. This was done by moving the loudspeaker laterally at every 5° or 10° until the neuron failed to respond. These measurements also allowed us to redetermine the BAZ at which the neuron fired maximal number of impulses. Electrodes were placed evenly across the whole IC surface and IC neurons were sampled as many as possible within each electrode penetration. Tonotopic organization and spatial sensitivity were examined among all 217 IC neurons as a whole as well as among IC neurons sequentially sampled within individual electrode penetrations. The whole population of 217 IC neurons is organized tonotopically along the dorsoventral axis of the IC. Thus, low frequency neurons are mostly located dorsally and high frequency neurons ventrally with median frequency neurons intervening in between. The BAZ of these 217 IC neurons tend to shift from lateral to medial portions of the contralateral frontal auditory space with increasing BF. Thus, the auditory space appears to have an orderly representation along the tonotopic axis of the IC. The lateral space is represented dorsally and the medial space ventrally. Nevertheless, tonotopic organization and spatial sensitivitty of sequentially isolated IC neurons within each electrode penetration may vary with the point of electrode penetration. This variation may be explained on the basis of the arrangement and thickness of each frequency lamina within the IC.     

Serotonin-immunoreactive neurons in the postnatal MAO-A KO mouse lateral superior olive project to the inferior colliculus

During development, serotonin (5-HT) accumulates in thalamic, noradrenergic, and auditory brainstem neurons that are non-serotonergic in the adult. As demonstrated in somatosensory thalamocortical projections, this accumulation of 5-HT is necessary for the precise organization of afferent terminal arborizations. Accumulation of 5-HT in the auditory brainstem appears to be most robust in the lateral superior olive (LSO) and as demonstrated in the MAO-A knockout mouse, is present at birth and begins to taper off at postnatal day 7 (P7). During the same developmental period, 5-HT-positive terminal endings in the inferior colliculus (IC) have been reported to be more numerous than in the adult [O. Cases, C. Lebrand, B. Giros, T. Vitalis, E. De Maeyer, M. Caron, D. Price, P. Gaspar, I. Seif, Plasma membrane transporters of serotonin, dopamine and norepinephrine mediate serotonin accumulation in atypical locations in the developing brain of monoamine oxidase A knock-outs, J. Neurosci. 18 (1998) 6914–6927]. It has been hypothesized that the serotonergic terminal fibers in the IC belong to neurons whose cell bodies reside in the LSO. Here, we provide evidence based on morphological and tract-tracing data that LSO neurons containing serotonin in the perinatal mouse, project to the IC. These data suggest that, similar to thalamocortical projections in other sensory systems, 5-HT may play a role in regulating development of LSO terminal arbors in the IC.     

Tonotopic organization in the inferior colliculus of the rat demonstrated with the 2-deoxyglucose method

Summary We studied the tonotopic organization in the inferior colliculus of the rat with the 2-deoxyglucose method. Isofrequency bands were observed in the central nucleus of the inferior colliculus. In coronal sections, higher sound frequencies elicited bands that were located more ventrally. At caudal levels of the inferior colliculus, isofrequency bands were relatively short and tilted slightly downward toward the midsagittal plane. As the plane of section moved more rostrally, isofrequency bands gradually lengthened, their orientation first turned horizontally and continued with a downward tilt toward the lateral aspect of the brainstem. At rostral levels, high-frequency (> 8000 Hz) bands showed more significant increases in length than low-frequency (< 4000 Hz) bands. Thus, the amount of tissue in the inferior colliculus devoted to higher frequencies was significantly more than that for lower frequencies. In sagittal sections, the isofrequency band moved from a dorsoposterior position to a ventroanterior one as the plane of section moved more laterally. A three-dimensional model of isofrequency planes was reconstructed from the above data.     

Development of inferior colliculus response properties in C57BL/6J mouse pups

Summary Response properties of inferior colliculus (IC) neurons were studied in tranquilized C57BL/6J mice during a period of rapid auditory system development between 12 and 17 days of age. In IC units of the youngest mice, spontaneous activity was absent, a disproportionate number of onset responses was observed, and many units were not securely driven by sound. Frequency response ranges were restricted to relatively low frequencies, sharpness of tuning was poor, and thresholds at best frequencies (BFs) were quite high. Dynamic intensity ranges were restricted, but nonmonotonic functions were observed. By 15–17 days of age, spontaneous activity was appreciable, incidences of response patterns were near adult proportions, and most units in the ventrolateral nucleus were securely driven by tones. Response ranges had expanded markedly to include high frequencies, sharpness of tuning increased, and thresholds had decreased. Dynamic intensity ranges and intensity functions were similar to those observed in adult mice.     

Response properties of neurons in the central nucleus and external and dorsal cortices of the inferior colliculus in guinea pig

The inferior colliculus (IC) represents a mid-brain structure which integrates information from many ascending auditory pathways, descending corticotectal projections and intercollicular pathways. The processing of information is different in each of the three main subdivisions of the IC--the central nucleus (CNIC), the dorsal cortex (DCIC) and the external cortex (ECIC)--which may be distinguished morphologically as well as by different inputs and outputs. To assess the differences in information processing we compared the response properties of single neurons in individual subnuclei of the IC in anesthetized guinea pigs. In comparison with DCIC and ECIC neurons, the CNIC neurons as a group were characterized by a sharper frequency tuning (as expressed by Q10 values), a lower average threshold, a shorter average first-spike latency of response to tones at the characteristic frequency (CF), a higher occurrence of non-monotonic rate/level functions and a higher rate of spontaneous activity. CNIC neurons and DCIC neurons reacted to tones at the CF more frequently by a sustained type of response than did ECIC neurons. The difference between the parameters of DCIC neuronal activity and ECIC neuronal activity was found to be smaller. The frequency tuning (expressed in Q10 values), spontaneous activity and dominance of monotonic rate/level functions were very similar in both structures; ECIC neurons expressed a higher average threshold and a shorter average first-spike latency than did DCIC neurons. Responsiveness expressed as the average maximal firing rate to tones at the CF was significantly higher in the CNIC than in the ECIC. The results give additional support to the idea that the CNIC is a part of a fast, frequency-tuned, low threshold and intensity-sensitive ascending pathway, whereas the other two subdivisions are involved in additional processing of information that involves feedback loops and polysensory pathways.     

The neocortical projection to the inferior colliculus in the albino rat

Summary The purpose of the present study was to define the field of termination of the neocortical projection to the inferior colliculus in rat. The study was based on fiber degeneration following large lesions of the cerebral cortex, and anterograde transport of wheat germ agglutinin horseradish peroxidase ejected inotophoretically into more restricted neocortical loci. Neocortical fibers were found to supply the dorsal and external cortices of the inferior colliculus. The central nucleus, in contrast, did not receive such fibers. The results speak in favor of three separate projections, one partly bilateral to the deeper part of the dorsal collicular cortex, a second ipsilateral to the superficial part of this subdivision, and a third ipsilateral to the external collicular cortex.     

Discharge characteristics of neuronal pairs in the rabbit inferior colliculus

Summary Activity of neuronal pairs in the inferior colliculus of the rabbit was recorded with a single stainless-steel microelectrode. Seventy pairs were investigated with monaural and binaural tonal stimuli. The most common parameter of the response in neuronal pairs was the best frequency, which was similar in 100% of the pairs (n = 45). Q₁₀ values were indentical in 44% of pairs and threshold tuning curves in 27% of pairs. Units with a smaller spike amplitude usually had a shorter latency to both binaural and monaural stimuli, when measured 10–20 dB above the best frequency threshold. Most units discharged during the entire period of the 100 ms tone stimulation at their best frequency; large differences, however, were found in their firing pattern, when peristimulus histograms were compared. High correlation was found in pairs where both neurones exhibited the same type of binaural interaction. The following types of binaural interaction were found: binaural excitatory drive with occlusion, binaural excitatory drive with facilitation; monaural excitatory drive with inhibition from the other ear and pure monaural excitatory drive. In a significant number of neuronal pairs the influence of binaural stimulation was similar for both neurones. The results suggest that: (a) many adjacent neurones in the inferior colliculus convey parallel information concerning features of the auditory stimulus; (b) units with a similar type of binaural interaction may be organized in clusters within isofrequency layers.     

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 monotonie 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.