Anatomical markers for the subdivisions of the barn owl's inferior-collicular complex and adjacent peri- and subventricular structures

The anatomy of the inferior-collicular complex of the barn owl, situated below the fourth ventricle in the tectal lobe, was studied by determining the distribution of antigens with antibodies directed against tyrosine hydroxylase, gamma-aminobutyric acid (GABA)(Abeta), dopamine- and cyclic AMP-regulated phosphoprotein (DARPP-32), calretinin, and calbindin. Additionally, the somata were stained with cresyl violet, and fibers were marked according to the Gallyas procedure. These markers were chosen to allow for an easy delineation of the boundaries between the subnuclei of the inferior colliculus. We could discriminate eight structures that belong to the three subnuclei of the inferior colliculus [the central nucleus (ICC), the superficial nucleus (ICS), the external nucleus (ICX)] and to the optic tectum. Periventricular tectal layers 15a and 15b stained well with all the antibodies used. The ICS, embedded in tectal layer 15a, may be divided into a dorsal and a ventral lamina. It does not have direct contact with the other nuclei of the inferior colliculus. The border between tectal layer 15a and ICX was well marked by all antibodies, but less so in Gallyas and cresyl violet stains. The ICC consists of a core and a medial and lateral shell. The core was clearly demarcated with antibodies against calretinin and calbindin. The border between the lateral shell and the ICX was marked less well than the borders between ICX and 15a, but the somata were much more darkly labeled with the DARPP-32 antibody in ICX than in the lateral shell of ICC. None of the markers delineated the border between the medial and lateral shell of ICC.     

Encoding of amplitude-modulated tones by neurons of the inferior colliculus of the kitten

Responses of single neurons of the central nucleus of the inferior colliculus (ICC) of kittens 4–43 days of age were studied using sinusoidally amplitude-modulated (AM) tones delivered monaurally or binaurally via sealed and calibrated earphones. The carrier frequency of the AM signal was set to the CF of the neuron. CFs ranged from 2–26 kHz. During the about first 2 weeks of postnatal life, ICC neurons responded to sound with periodic bursts of activity. In response to AM tones, discharges of ICC neurons at all ages studied were phase-locked to the envelope of the modulation waveform over a wide range of stimulus level and modulation depth. A linear relationship, independent of SPL, was found between the average phase of discharge on the modulation cycle and modulation frequency. The slope of the line represents a time delay, which was highly correlated with the first-spike latency to tone onset, and hence with the age of the animal. The mean effective phase of the discharge remained relatively constant with age. There was little systematic change in average phase of discharge with changing stimulus level or modulation depth, although the number of spikes evoked and the temporal pattern of the spikes within a modulation cycle could vary. The sensitivity function relating spike synchrony or spike count to modulation was typically band-pass in nature. The most effective modulation frequency (MEMF) was, on average, 15 Hz, far below that reported for adult cat ICC cells. When AM tones were delivered binaurally, the discharge was a periodic function of the interaural phase difference of the stimulus envelopes. The results indicate that prior to the time the cochlea is able to respond to most environmental sounds, monaural and binaural circuits involving the ICC faithfully transmit information pertaining to amplitude-modulated signals in the rate and timing of their discharges. During the next several weeks, when neural thresholds fall to adult levels, ICC circuits are activated by amplitude modulated sounds at levels encountered in the normal acoustic environment even though they are restricted to modulation frequencies below those encoded by the adult.     

A topographic representation of auditory space in the external nucleus of the inferior colliculus of the guinea-pig

The possibility that the external nucleus of the inferior colliculus (ICX) of the pigmented guinea-pig contains a map of auditory space has been investigated. Auditory stimuli consisted of broad-band sound delivered under free-field anechoic conditions from a range of positions around the animal's azimuthal axis. The responses of clusters of neurons in the ICX to threshold and to near-threshold stimuli displayed sharp spatial tuning. The responses recorded from rostral ICX revealed a preference for auditory stimuli in the anterior field while more caudal neurons preferentially responded to sounds presented in the posterior field. Neurons at intermediate points, along the rostro-caudal axis of the nucleus, displayed preferences for sound stimuli in appropriately intermediate field positions along the contralateral azimuthal axis. At higher stimulus intensities the spatial tuning of the responses decreased, but the optimal direction of preference was usually retained. The contribution of binaural processing to auditory spatial tuning was evident, since unilateral cochlea ablation destroyed the spatial tuning at higher stimulus intensities. The results presented provide the first evidence that a topographically ordered representation of the contralateral auditory azimuth is present in the ICX of a mammal.     

Complex sound analysis (frequency resolution, filtering and spectral integration) by single units of the inferior colliculus of the cat

The central nucleus of the inferior colliculus (ICC) is a center of convergence of brainstem input and is critical for auditory information processing. Here, the analysis of complex sound spectra by single neurons in the ICC is investigated. Several measures of frequency resolution (excitatory/inhibitory tuning curves, effective bandwidths, critical ratio bands, critical bands derived using narrowband masking and two-tone separation paradigms) have been obtained from the responses of these neurons at sound pressure levels (SPL) up to 80 dB above the units' response thresholds (nearly 110 dB SPL). Among our results are the following: (1) Narrowband masking measures of critical bands from ICC neurons closely parallel behavioral measures using the same stimulus paradigm. (2) Frequency resolution power as measured by critical bandwidths varies little as a function of stimulus intensity. (3) Tuning curves of ICC neurons provide no simple basis for predicting the frequency filtering of the same neurons excited by complex sound spectra. (4) There is a frequency dependence of all measures of frequency resolution similar to that found in psychophysical determinations of critical bandwidths. That is, spatial frequency resolution in the cochlea is the origin for the resolution found in the ICC and in behavioral tests. (5) Lateral inhibition at the level of the ICC clearly plays a role in frequency resolution. (6) Frequency resolution is encoded by response rate changes of ICC neurons and is independent of tone response threshold, response latency, spontaneous activity, tone response type, binaural response type. It is concluded that spectral analysis of sound is established by processes, ncluding lateral inhibition, independent of other basic response properties of neurons at the level of the ICC.     

Subdivisions of the inferior colliculus in the barn owl (Tyto alba)

The inferior colliculus in the barn owl contains three subdivisions: the central (ICC), external (1CX), and superficial (ICS) nuclei. The nuclei are distinguished on the basis of their cyto- and myeloarchitecture, connectivity, and physiological properties. The ICC may be further divided into dorsal (ICCd) and ventral (ICCv) parts. Auditory fibers ascending in the lateral lemniscus enter the ICCd and ICCv, but not the ICX or ICS. The ICX receives its auditory input from the ICC. The ICC and ICX in owls are similar in position, anatomy, connectivity, and physiology to the ICC and ICX in mammals, suggesting that these structures are homologous. Units in the ICC are organized to potopically, whereas units in the ICX are organized according to the locationns of their spatial receptive fields. This implies that a transformation from tonotopic to a spatiotopic organization takes place in the ICX of the owl.     

Effects of chronic descending tractotomy on the response patterns of neurons in the trigeminal nuclei principalis and oralis.

Unilateral, descending, trigeminal tractotomy was carried out on adult cats using aseptic technique. Unit activity, evoked by bipolar noxious stimulation of the tooth pulp and by innocuous mechanical stimulation of other receptive fields, was recorded from neurons in the trigeminal principalis and oralis nuclei, 7 to 13 weeks after tractotomy. Seventy neurons from the lesion side of the brain stem and 59 from the control side were studied. All units responded to pulpal stimuli and the great majority to mechanical stimuli. Three general response configurations were produced by each of the two forms of stimulation: brief bursts of 1 to 5 spikes, trains of 6 to 20 spikes, and multiple component discharges consisting of sequences of brief bursts. No statistically significant differences between the two populations were found for peripheral field size or location, for discharge characteristics such as latency, configuration, or mean density, or for responsiveness to the two types of stimuli. However, a statistically greater proportion of control units fired dense impulse trains (more than eight spikes) to intense pulpal stimuli. The results provide evidence that the response patterns of most somatosensory neurons in nuclei principalis and oralis are unaffected by descending trigeminal tractotomy and that stimulus modality is not coded by the response characteristics of polymodal units in these nuclei. Although some modulatory influence is exerted on polymodal neurons in the rostral nuclei by the trigeminal nucleus caudalis, it appears doubtful that this is a significant factor in determining whether a peripheral stimulus is perceived as noxious or innocuous.     

Midbrain pathways of audiogenic seizures in DBA2 mice

The effects on audiogenic seizures of bilateral electrolytic lesions at levels of the inferior colliculus (IC), superior colliculus (SC), or medial geniculate body were determined for inbred $\text{DBA}\text{2}$ mice. Of 88 control mice (surgical and nonsurgical), 82% displayed the full seizure syndrome when exposed to intense sound: wildrunning attacks progressed to clonic convulsions and then tonic convulsions. All remaining control animals displayed at least wild-running attacks. In 27 of 49 mice with IC-level lesions, either no seizure activity was observed or the syndrome terminated at the wild-running stage. In these mice, lesions consistently damaged the central IC nucleus (ICC) and sometimes adjacent midbrain tegmentum. In the 12 IC-lesion mice that displayed the full seizure syndrome, IC damage was generally lateral, primarily involving the external IC nucleus (ICX). In SC-lesion mice, absence of wild-running was observed in only 3 of 23 mice, but termination of seizures at wild-running or clonic stages was often seen (14 of 23 mice). Attenuation of seizure activity was correlated with damage to the deep SC and tegmentum, including the central gray. Little or no effect on audiogenic seizures was found with lesions of the dorsal SC or medial geniculate body. Analysis of the time course of components of the seizure syndrome revealed that IC-lesion mice attaining only the wild-running stage had increased latencies to onset of this behavior. In SC-lesion mice, time to onset of wild-running did not differ significantly from controls. Progression from wild-running attacks to onset of clonic convulsions was slowed by lesions at both IC and SC levels. The data indicate that the primary pathway of audiogenic seizure activity includes the ICC, deep SC, and adjacent tegmentum. Wild-running attacks appear to be mediated at the IC level, whereas progression to clonic and tonic convulsions may require more rostral levels.     

Inferior colliculus stimulation causes similar efferent effects on ipsilateral and contralateral cochlear potentials in the guinea pig

The inferior colliculus (IC) is a processing center in both the ascending and descending auditory pathways. It has been demonstrated anatomically to send descending projections to the region of the medial olivocochlear (MOC) neurons in the auditory brainstem. Activation of MOC system produces reductions in cochlear neural activity. Individual MOC fibers innervate relatively restricted regions of the cochlea. Recent studies have shown that selective electrical stimulation within the IC central nucleus (ICC) produces frequency-specific reductions of neural activity in the contralateral cochlea (Ota, Y., Oliver, D.L., Dolan, D.F., 2004. Frequency-specific effects on cochlear responses during activation of the inferior colliculus in the guinea pig. J. Neurophysiol. 91, 2185-2193). This efferent effect is likely mediated through selective activation of MOC cells. In this study, we investigated the effects of selective stimulation of one ICC on cochlear output in both ears in anesthetized and paralyzed guinea pigs to explore possible differences in the effective efferent innervation of the two ears. ICC stimulation had a similar tonotopically tuned effect on the distortion product otoacoustic emission (DPOAE) and the cochlear whole-nerve action potential (CAP) in each cochlea. The bandwidth of the efferent effect in each ear was measured and compared at different stimulation levels. For a given ICC stimulation site, the efferent effects were larger for the CAP response. The effect on each response measure was greater in the contralateral than the ipsilateral ear. The effective bandwidth of the efferent effect on the CAP was current-level-dependent but less so for the DPOAE. The results of transections at various locations within the brainstem suggest that the effects were mediated by the MOC system. From the results presented here, the descending efferent system, which originates in the auditory cortex, has frequency-specific, spatially restricted, bilateral effects. The effects are greater in the contralateral ear.     

Convergence of spinal trigeminal and cochlear nucleus projections in the inferior colliculus of the guinea pig

In addition to ascending auditory inputs, the external cortex of the inferior colliculus (ICX) receives prominent somatosensory inputs. To elucidate the extent of interaction between auditory and somatosensory representations at the level of IC, we explored the dual projections from the cochlear nucleus (CN) and the spinal trigeminal nucleus (Sp5) to the inferior colliculus (IC) in the guinea pig, using both retrograde and anterograde tracing techniques. Injections of retrograde tracers into ICX resulted in cell-labeling primarily in the contralateral DCN and pars interpolaris and caudalis of Sp5. Labeled cells in DCN were either fusiform or multipolar cells, whereas those in Sp5 varied in size and shape. Injections of anterograde tracers into either CN or Sp5 resulted in terminal labeling in ICX primarily on the contralateral side. Most projection fibers from Sp5 terminated in a laminar pattern from ventromedial to dorsolateral within the ventrolateral ICX, the ventral border of IC, and the ventromedial edge of IC (collectively termed "the ventrolateral border region of IC," ICXV). Less dense anterograde labeling was observed in lateral and rostral ICX. Injecting different tracers into both Sp5 and CN confirmed the overlapping areas of convergent projections from Sp5 and CN in IC: The most intense dual labeling was seen in the ICXV, and less intense dual labeling was also observed in the rostral part of ICX. This convergence of projection fibers from CN and Sp5 provides an anatomical substrate for multimodal integration in the IC.     

Auditory spatial sensitivity of inferior collicular neurons of echolocating bats

The sensitivity of 94 inferior collicular (IC) neurons of Eptesicus fuscus and Myotis lucifugus to spatial location of the acoustic stimulus were studied under free-field stimulus conditions. The best frequency (BF) and minimum threshold (MT) of each neuron were determined with sound delivered in front of the bat. Then the variation in discharge rate of the neuron was measured with a BF sound broadcast from a moving loudspeaker at different angular positions along the horizontal, vertical or diagonal plane of the frontal auditory space. A wide range of stimulus intensities above the MT of the neuron was used. Neurons were classified into 3 classes on the basis of their spatial sensitivity: (1) omnisensitive neurons (15%) were broadly tuned to sound delivered in the frontal auditory space and their responses did not show any correlation with sound location; (2) stimulus intensity-dependent neurons (28%) varied their discharge rates with sound location and intensity so that the peak of their spatial response profiles also varied with stimulus intensity; and (3) stimulus intensity-independent neurons (57%) varied their discharge rates only with sound location over a wide range of stimulus intensities so that their peak discharge always appeared at the same or a small range of angle. In most cases, the medial limbs of the spatial sensitivity curve for these neurons were extremely sharp and congruent. By moving the loudspeaker along the horizontal, vertical and diagonal planes, it was possible to approximate the boundary of the spatial response area of a neuron. Most IC neurons responded to sound delivered within 20 degrees ipsilateral, 60 degrees contralateral, 45 degrees up and 40 degrees down of the frontal auditory space, confirming previous similar studies. In general, an increasing stimulus repetition rate appeared to sharpen the spatial sensitivity curve of a neuron. Conversely, an increasing moving velocity of the stimulus decreased its response. The possible role of these 3 classes of neurons in echolocation and neural mechanisms underlying the spatial sensitivity of these neurons is discussed.     

Local excitability in the superior colliculus influences evoked responses of lateral geniculate cells in rabbits

In anesthetized, immobilized rabbits recordings were made simultaneously from cells in the Lateral Geniculate Nucleus (CGL) and Superior Colliculus (CS), in order to study how the CS influences the CGL. The experimental protocol consisted of three steps. In the initial step (first control) the light stimulus was triggered electronically. In the second step (Test), the same stimulus was triggered by a spontaneous spike arising from a collicular cell. Thus the stimulus presentation was time-locked to collicular endogenous activity. The third step was the same as the first and constituted a second control. The frequencies of stimulus application were gated to be approximately the same. The results indicated that the CS exerts two separate effects on CGL units. In 37 pairs (26%), conditioning the stimulus presentation to collicular firing produced a significant enhancement of geniculate responses. In 24 pairs (17%), the geniculate responses declined. In 82 pairs (57%), no significant influence was noted. The colliculo-geniculate inlfuence is transient. The effects peaked between 100 to 200 msec after the collicular spike and returned to their control levels within 300 msec. Collicular cells producing a decline were encountered mostly in the ventral part of the stratum griseum superficiale, and the stratum opticum, whereas collicular cells that were related to an increased geniculate response were more frequently found dorsally. Increments were more pronounced if the distance (D) between receptive fields was short (0° < D < 40°) or if the collicular and geniculate fields were far apart (120° < D < 180°). The decrement effect was attenuated as the distance separating the two receptive fields. This study suggests that the superior colliculus is capable of generating an internal signal powerful enough to modulate at the geniculate nucleus the visual message conveyed toward the visual cortex. A possible role of the CS in the initiation of the corollary discharge is briefly discussed.     

In vivo intracellular characteristics of inferior colliculus neurons in guinea pigs

Intracellular in vivo recordings of physiologically identified inferior colliculus central nucleus (ICc) auditory neurons (n=71) were carried out in anesthetized guinea pigs. The neuronal membrane characteristics are described showing mainly quantitative differences with a previous report [Nelson, P.G. and Erulkar, S.D., J. Neurophysiol., 26 (1963) 908–923]. The spontaneous spike activity was consistent with the discharge pattern of most extracellularly recorded units. The action potentials showed different spike durations, short and long, and some of them exhibited hyperpolarizing post-potentials. There were also differences in firing rate. The ICc neurons exhibited irregular activity producing spike trains as well as long silent periods (without spikes). Intracellular current injection revealed membrane potential adaptation and shifts that outlasted the electrical stimuli by 20–30 ms. Both evoked synaptic potentials and the spike activity in response to click and tone-burst stimulation were analyzed. Depolarizing-hyperpolarizing synaptic potentials were found in response to contralateral and binaural sound stimulation that far outlasted the stimulus (up to 90 ms). When ipsilaterally stimulated, inhibitory responses and no-responses were also recorded. Although few cells were studied, a similar phenomenon was observed using tone-burst stimulation; moreover, a good correlation was obtained between membrane potential shifts and the triggered spikes (input–output relationship). These in vivo results demonstrate the synaptic activity underlying many of the extracellularly recorded discharge patterns. The data are consistent with the known multi-synaptic ascending pathway by which signals arrive at the ICc as well as the descending corticofugal input that may contribute to the generation of long duration post-synaptic potentials.     

A microiontophoretic study of acetylcholine effects in the inferior colliculus of horseshoe bats: implications for a modulatory role

The effects of acetylcholine (ACh) in processing acoustical information in the inferior colliculus (IC) of awake horseshoe bats (Rhinolophus rouxi) were examined with single cell recordings and microiontophoresis. Cholinergic agonists, acetylcholine and carbachol raised the stimulus evoked discharge in 37% and suppressed responses in 16% of the sample. They did not alter the shapes of tuning curves and rate-intensity functions but the latter showed parallel shifting. The nicotinic antagonist, hexamethonium raised neuronal activity in 52% of neurons without affecting discharge patterns. The nonspecific muscarinic antagonist atropine was mostly inhibitory (62% of units) and caused changes in temporal discharge patterns by affecting the tonic response component. The selective muscarinic ml antagonist pirenzepine, also had an inhibitory effect (37% of units) and predominantly influenced the tonic response component. The selective m2 antagonist, gallamine however produced mainly excitatory effects (64% of units) and changed temporal discharge patterns by adding tonic response components. These findings may indicate a differential pre- and postsynaptic synaptic distribution of m1/m2 receptors in the inferior colliculus as reported for other brain structures. The results indicate that ACh plays a neuromodulatory transmitter role in the auditory midbrain by setting the level of neuronal activity. Its exact function in particular behavioral contexts remains to be determined, since the origin of cholinergic innervation of the mammalian IC is still unclear.     

Topographic projection from the optic tectum to the auditory space map in the inferior colliculus of the barn owl

In the barn owl (Tyto alba), the external nucleus of the inferior colliculus (ICX) contains a map of auditory space that is calibrated by visual experience. The source of the visually based instructive signal to the ICX is unknown. Injections of biotinylated dextran amine and Fluoro-Gold in the ICX retrogradely labelled neurons in layers 8-15 of the ipsilateral optic tectum (OT) that could carry this instructive signal. This projection was point-to-point and in register with the feed-forward, auditory projection from the ICX to the OT. Most labelled neurons were in layers 10-11, and most were bipolar. Tripolar, multipolar, and unipolar neurons were also observed. Multipolar neurons had dendrites that were oriented parallel to the tectal laminae. In contrast, most labelled bipolar and tripolar neurons had dendrites oriented perpendicular to the tectal laminae, extending superficially into the retino-recipient laminae and deep into the auditory recipient laminae. Therefore, these neurons were positioned to receive both visual and auditory information from particular locations in space. Biocytin injected into the superficial layers of the OT labelled bouton-laden axons in the ICX. These axons were generally finer than, but had similar bouton densities as, feed-forward auditory fibers in the ICX, labelled by injections of biocytin into the central nucleus of the inferior colliculus (ICC). These data demonstrate a point-to-point projection from the OT to the ICX that could provide a spatial template for calibrating the auditory space map in the ICX.     

Comparison of neuronal response patterns in the external and central nuclei of inferior colliculus during ethanol administration and ethanol withdrawal

Convulsive seizures during ethanol withdrawal (ETX) in rodents can be precipitated by acoustic stimulation. The inferior colliculus (IC) is strongly implicated in the neuronal network for these audiogenic seizures (AGS) in animals undergoing ETX. Previous evidence indicates that the central nucleus of IC (ICc) is important in AGS initiation in ETX, but the ICc does not project directly to motor pathways. The external nucleus of IC (ICx) receives convergent output from a broad range of ICc neurons, which is not tonotopically organized, and projects to several nuclei with major motor connections. Lesion, neuroanatomical, and stimulation experiments suggest the involvement of the ICx in the AGS network in several forms of AGS, including ETX. The present study examined ICx neuronal firing patterns in awake behaving rats during ethanol administration and during ETX to examine the role of this structure directly. ICx neuronal responses during both ethanol intoxication and ETX were significantly suppressed as compared to pre-ethanol responses. ICx neuronal responsiveness was reduced (habituated) at faster (>0.25 Hz) rates of stimulus presentation. However, immediately prior to the onset of AGS, there was an increase in ICx neuronal responses that continued into the wild running phase of AGS. This increase in neuronal responses temporally corresponded to the sustained ICc neuronal responses during ETX just prior to AGS. The enhanced ICx neuronal responsiveness may be mediated, in part, by changes in GABA and glutamate receptor regulation previously observed during ETX. The net result of these changes involves a functional reversal of response habituation normally observed in ICx neurons. These data illuminate the nature of the changes in ICx neuronal function that serves to transmit the sensory input that originates in the ICc and propagates seizure to the brainstem AGS network nuclei responsible for the convulsive motor behaviors of ETX seizures.     

Response of rat paraventricular neurones with central projections to suckling, haemorrhage or osmotic stimuli

In lactating, urethane-anaesthetized female rats extracellular recordings were made from paraventricular nucleus (PVN) neurones that were antidromically activated following electrical stimulation of the neurohypophysis, amygdala or nucleus tractus solitarius/vagal complex (NTS/VC). Of the PVN units, 98 projected to the neurohypophysis but none of these neurosecretory neurones were found to simultaneously project to extrahypothalamic areas. From the firing patterns and the response of these neurons to suckling, haemorrhage or osmotic stimuli both 'vasopressinergic' and 'oxytocinergic' neurones were identified. We found 43 PVN units to project to the NTS/VC and 22% of tested neurones were activated by osmotic or haemorrhage stimuli; no phasic activity was associated with this activation. The suckling stimulus failed to elicit any response from these units. Upon testing the PVN units that projected to the amygdala (n = 35), it was found that haemorrhage and suckling stimuli were without effect, while the osmotic stimulus activated one of 6 units tested. Thus, the extrahypothalamic PVN projections examined in this study were not associated with the suckling reflex response, although there is evidence for their limited involvement in neural response to osmotic or haemorrhage stimuli.     

Thermally evoked tail avoidance reflex: Input-output relationships and their modulation

Latency to onset and magnitude (angular displacement) of thermally evoked tail avoidance reflexes (TETAR) to graded thermal stimuli were measured in pentobarbital-treated and untreated rats. The latency to onset was inversely and the magnitude was directly proportional to the thermal stimulus intensity. Pentobarbital prolonged the latency to onset of the TETAR to low by not high stimulus intensities. Activation of (-)-nicotine sensitive brainstem hyperalgesic processes shortened the latency and increased the magnitude of the TETAR. The hyperalgesic actions of (-)-nicotine were most demonstrable at lower thermal stimulus intensities. These studies suggest that the TETAR is graded in intensity as the stimulus intensity is increased and that the response evoked by different intensities of stimulus probably involve common neurophysiologic mechanisms.     

Connections of the superior olivary complex in the rufous horseshoe bat Rhinolophus rouxi

In the rufous horseshoe bat (Rhinolophus rouxi), the superior olivary complex contains four main divisions. In comparison with other species, the most lateral division is clearly homologous to the lateral superior olive (LSO); the most medial division is homologous to the medial nucleus of the trapezoid body (MNTB). Lying between these landmarks, in approximately the position of the medial superior olive (MSO) of other mammals, are two additional divisions that are cytoarchitecturally distinct from one another yet do not greatly resemble the MSO of nonecholocating mammals such as the cat. We refer to these nuclei as the dorsal medial superior olive (DMSO) and the ventral medial superior olive (VMSO). We examined the afferent and efferent connections of all of these cell groups with retrograde and anterograde transport of WGA-HRP from the superior olivary complex. In the same animals we recorded the binaural response properties of single units in the superior olivary complex. Virtually all units recorded in LSO were excitatory to the ipsilateral ear and inhibitory to the contralateral ear (EI); all of the units sampled in the MNTB and most of those sampled in the VMSO responded only to the contralateral ear (OE). In DMSO the binaural properties of units were varied: the number of units that were inhibitory to the ipsilateral ear and excitatory to the contralateral ear (IE) was about equal to the number of units excitatory to both ears (EE); a few units had OE responses; no units had EI responses. Connectional correlates for these binaural response properties are seen in the patterns of retrograde transport from WGA-HRP injections in the divisions of the superior olive. The LSO receives projections from the ipsilateral cochlear nucleus and MNTB; MNTB receives projections from the contralateral cochlear nucleus. The DMSO and VMSO both receive bilateral projections from the cochlear nuclei. The results of retrograde and anterograde transport suggest that VMSO, in addition, receives projections from the ipsilateral MNTB. The LSO, DMSO, and VMSO all project to the ventral two-thirds of the central nucleus of the inferior colliculus, and their targets are approximately coextensive. However, the LSO projects bilaterally to the inferior colliculus, whereas the medial cell groups project mainly ipsilaterally.     

Auditory space representation in the superior colliculus of the big brown bat, Eptesicus fuscus

The auditory response areas of 123 superior collicular (SC) units of Eptesicus fuscus were studied under free-field acoustic stimulus conditions. A stimulus was delivered from a loudspeaker placed 14 cm in front of a bat. The best frequency of a unit was determined by changing the stimulus frequency until the minimum threshold was measured. A best frequency stimulus was then delivered as the loudspeaker was moved across the auditory space to determine the response center of the auditory response area of each unit. The response center was defined as the direction at which the unit had its lowest minimum threshold. The stimulus intensity was then raised 2–20 dB above the lowest minimum threshold of the unit and the response area for each stimulus intensity was determined.The response area of a unit expands with stimulus intensity, but the expansion is not even in all directions. The size of the response area of a unit does not correlate with its minimum threshold, best frequency, or recording depth. Response centers of 7 units were located directly in front of the animal, but most response centers were located in a limited portion of the contralateral auditory space.Although each unit has a response center which is the point of maximal sensitivity, the point-to-point representation of the auditory space is not systematically organized. We suggest that an animal with highly mobile external pinnae may not need an orderly auditory space map in its neural tissue for accurate sound localization.     

Visuocortical epileptiform discharges in rabbits: Differential effects on neuronal development in the lateral geniculate nucleus and superior colliculus

We have studied the effects of interictal epileptiform discharges originating from the striate cortex on the development of the receptive field characteristics of neurons in the lateral geniculate nucleus (LGNd) and superior colliculus (SC) in neonatal rabbits. The paroxysmal discharges were generated by twice-daily injections of penicillin into an implanted cannula. Control injections of penicillin + penicillinase were given to the other striate cortex of the same animal. Similar experimental procedures were used to study the effect of such projected discharges on the LGNd neurons in adult rabbit.The results of the first experiment show that cortical epileptiform discharges, initiated in neonatal rabbits 7–9 days of age and continuing to 20–25 days of age, retard the normal development of LGNd cells. There was an abnormal increase of indefinite cells, cells failing to respond to any light stimulation, and a concurrent decrease of cells with concentric or uniform receptive fields. The second experiment shows that the effect of such interictal discharges is age-dependent; they did not cause any changes on the receptive fields of LGNd cells in adult rabbit. That these epileptiform discharges, occuring early in life, had long-lasting effects is demonstrated in the third experiment. An abnormal increase of uniform cells and a concurrent decrease of concentric cells was still present in adult rabbits which had interictal discharges in the striate cortex limited to the period of 7–9 dats to 21–25 days of age. The fourth experiment shows that the interictal discharges in neonatal rabbits do not affect the normal receptive field development of neurons in the SC.The present results demonstrate that asymptomatic interictal epileptiform discharges, produced without focal structural damages in immature brain, can affect the development of neuronal connectivity. These results may have some clinical implications in relation to our understanding about the learning and developmental disabilities exhibited in children who had episodic seizure discharges.