Biomedical subjects

L M Kitzes

Publications and source records attributed to L M Kitzes.

At least 19 recordsLinked to original sources

PubMed1996

Response properties of units in the posterior auditory field deprived of input from the ipsilateral primary auditory cortex.

The influence of the ipsilateral primary auditory field (AI) on the response properties of neurons in the posterior auditory field (Field P) was examined in three cats anesthetized with sodium pentobarbital. Rate/level functions were obtained, by extracellular recording, from single units in Field P before (n = 38) and after (n = 50) subpial aspiration of AI. The ablations were primarily confined to the medial ectosylvian gyrus, although in one case extended into the high-frequency portion of the anterior auditory field. Comparisons between the behavior of units isolated before and after AI ablation failed to demonstrate any changes in the response properties of neurons in Field P attributable to the ablation. Nonmonotonic response profiles, first spike latency, variability in latency, threshold and maximal discharge rates of the units to acoustic stimuli were not significantly altered by the AI ablation. These results indicate that the basic response properties of neurons in Field P do not depend on input from the ipsilateral AI. This suggests that these properties are most likely determined by thalamic input or by circuitry within Field P.

Acoustic Stimulation↗

PubMed1996

Stimulus-induced spike bursts in two fields of cat auditory cortex.

The sound-evoked responses of extracellularly recorded cat primary auditory cortical neurons usually consist of a single spike or a short-term burst of 2-4 spikes, irrespective of the nature of the acoustic signal. In the cat's auditory cortex, the properties of such responses have to date been described only for cells in the primary field (AI). The purpose of the present study was to describe the properties of stimulus-evoked spike-burst responses seen in neurons of the posterior auditory field (P) and to compare those properties with those of a sample of AI neurons studied under similar conditions. The data come from 80 field P and 31 AI neurons studied with tonal and noise-burst stimuli in barbiturate-anesthetized cats, using calibrated, sealed stimulus delivery systems and conventional extracellular recording techniques. The mean inter-spike intervals (ISI) seen in the transient burst responses of posterior field cells were typically short (2-5 ms) and, where it was possible to test them, independent of the rise time of tonal signals, suggesting that they were also independent of the onset spectrum of the stimulus. The mean ISIs were often independent of the stimulus amplitude, even though the signal level had profound effects on the number of spikes evoked and the latency and regularity with which the responses were initiated. Each neuron was assigned a 'characteristic ISI', i.e., the mean ISI seen in the most vigorous responses. The distribution of characteristic ISIs for AI and P neurons overlapped, but were significantly different, with the characteristic ISIs of field P neurons being longer. In both AI and P populations, characteristic ISI was significantly correlated with minimal first-spike latency. The slopes of the regression lines of characteristic ISI on minimal latency for AI and for P cells were not significantly different from each other. Since the minimal latencies of AI neurons were usually shorter than those of field P neurons, the shorter characteristic ISIs of AI cells may thus be interpreted as secondary to their shorter latent periods. The general properties of stimulus-evoked spike bursts seen in field P neurons were thus very similar those previously described for AI cells. These data are consistent with the view that the majority of extracellular recordings in the cat's auditory cortex come from pyramidal neurons and are appropriate as a specialization for transfer of information to nonpyramidal, inhibitory interneurons.

Acoustic Stimulation↗

PubMed1995

Development of ventral cochlear nucleus projections to the superior olivary complex in gerbil.

The postnatal development of the projection from the ventral cochlear nucleus to the principal nuclei in the superior olivary complex in gerbil (Meriones unguiculatus) was studied in an age-graded series of pups ranging from 0 to 18 days old. Small crystals of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) were inserted into the ventral cochlear nucleus of aldehyde-fixed brains, and the labeled projections were examined with epifluorescence microscopy. Selected sections were photooxidized in a solution of diaminobenzidine and subsequently processed for electron microscopy to examine the development of labeled synapses in the target nuclei. Horseradish peroxidase was injected into the ventral cochlear nucleus of adult gerbils to assess the form and persistence of projections observed in the neonatal animals. In addition, electrophysiological responses to acoustic stimuli of single units in the adult auditory brainstem were analyzed to confirm the functionality of the novel projection from the ventral cochlear nucleus to the contralateral lateral superior olive. By the day of birth (P0), developing axons from the ventral cochlear nucleus have already established highly ordered pathways to the three primary nuclei of the superior olivary complex: the ipsilateral lateral superior olive, the contralateral medial nucleus of the trapezoid body, and at the lateral and medial dendrites of the ipsilateral and contralateral medial superior olive, respectively. Developing axons from the ventral cochlear nucleus that innervated the contralateral medial nucleus of the trapezoid body lacked the terminal morphology characteristic of the calyx of Held, but began to adopt a more characteristic form on P5. The mature calyx appeared around P14-16. Exuberant developmental projections to topographically inappropriate areas of the superior olivary complex were not observed at the postnatal ages studied. In addition to the projections of the ventral cochlear nucleus to the superior olivary complex described in other species, we observed the development and maintenance of a major direct projection from the ventral cochlear nucleus to the contralateral lateral superior olive. On P0, ventral cochlear nucleus axons decussate in the dorsal trapezoid body, form a plexus at the dorsal edge of the contralateral medial superior olive, and enter the ventrolateral limb of the contralateral lateral superior olive. Over the next 2 weeks, fascicles of fibers form on the hilar and ventral aspects of the ventrolateral limb. Fibers arising from these fascicles form converging, but nonoverlapping, arborizations within the ventrolateral limb at right angles to the curvature of the nucleus. The medial region was devoid of labeled axons.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1995

Development of ectopic projections from the ventral cochlear nucleus to the superior olivary complex induced by neonatal ablation of the contralateral cochlea.

The ability of an animal to localize a sound in space requires the precise innervation of the superior olivary complex by the ventral cochlear nuclei on each side of the lower brainstem. This precise pattern of innervation could require an immutable recognition of appropriate targets by afferent processes arising from these nuclei. This possibility was investigated by destroying one cochlea of gerbil pups (Meriones unguiculatus) on the second postnatal day and assessing the projections from the ventral cochlear nucleus (VCN) on the unablated side to the superior olivary complex during the subsequent 2 weeks and after the animals had reached maturity. A crystal of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) was inserted into VCN on the unablated side in animals ranging in age from 3 to 14 days. To assess the permanence of any altered pattern of innervation, horseradish peroxidase was injected into VCN on the unablated side in adult, neonatally ablated animals. Finally, electrophysiological responses to acoustic stimuli delivered to the ear on the unablated side were recorded in the superior olivary complex of adult animals to assess whether altered innervation patterns were functional. Normative data were derived from our accompanying study of the development of VCN projections to the superior olivary complex in normal gerbils (Kil et al., this issue). Whereas VCN normally projects to the lateral aspect of the ipsilateral medial superior olive and to the medial aspect of the contralateral medial superior olive in control animals, in experimental animals VCN on the unablated side projects to both sides of these nuclei. Whereas in the gerbil, VCN normally projects only to the hilar area and to the ventrolateral limb of the contralateral lateral superior olive, in experimental animals VCN on the unablated side projects throughout this nucleus. This induced projection is specific in that the efferents to each limb of the contralateral nucleus are linked to the normal projection to the homotopic region of the ipsilateral nucleus. Whereas VCN innervates the contralateral medial nucleus of the trapezoid body in control animals, in experimental animals VCN on the unablated side provides calyces of Held in the ipsilateral nucleus as well. The induced projections to these three major subnuclei of the superior olivary complex first appear within 24 hours of the cochlear ablation and continue to develop over at least the subsequent 11 days. Thus, prior to the day when the cochlea becomes functional, VCN has established specific ectopic projections to loci normally innervated by VCN on the ablated side.(ABSTRACT TRUNCATED AT 400 WORDS)

Afferent Pathways↗

PubMed1995

Factors shaping the tone level sensitivity of single neurons in posterior field of cat auditory cortex.

1. The posterior field (field P) of the cat's auditory cortex contains a higher proportion of neurons whose response/level functions for characteristic frequency (CF) tones are nonmonotonic than does the primary field (AI). The general purpose of the present study is to assess whether the response/level functions of field P neurons are generated by the same mechanisms as those of cells in AI. All of the data came from single neurons in the cortices of barbiturate-anesthetized cats, to which we presented tonal stimuli through sealed, calibrated stimulating systems. 2. We obtained quantitative data from 123 neurons, of which 108 were located in field P. Of the 108 field P cells, 70% had nonmonotonic response/level functions for 5-ms rise time tones of CF. For cells of any given CF, both CF thresholds and best SPLs (i.e., SPLs associated with maximal responses) varied widely. A correlation analysis revealed that a linear relation between best SPL and CF threshold accounted for 73% of the data variance in the association between those response variables. An analysis of data from 83 nonmonotonic cells in AI revealed a similar relation. 3. Field P neurons whose response/level functions were non-monotonic for 5-ms rise time CF tones became even more narrowly tuned to SPL when the rise time of the tone bursts was reduced to 1 ms. Lengthening the rise time to 20 ms reduced or eliminated the SPL tuning in almost all of these neurons. The general form of monotonic tone response/level functions was commonly unaffected by variation in signal rise time. In a few instances, cells with monotonic response/level functions for 5- and 20-ms rise time tones developed nonmonotonic functions for 1-ms rise time tones. 4. Field P neurons with nonmonotonic response/level functions for CF tones usually failed to respond to wideband noise pulses, or, less commonly, responded to noise only at low SPLs. In contrast, field P cells with a monotonic response to CF tones usually responded monotonically to noise. 5. The minimal mean first-spike latencies of field P neurons were generally longer than those of AI cells studied under similar conditions. The precision of first-spike timing, measured using the SD of the mean first-spike latency, was commonly poorer than that of AI cells. 6. The properties of field P cells followed the same rules as those seen in AI.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗

PubMed1994

Level-dependent representation of stimulus frequency in cat primary auditory cortex.

The tonotopicity of the cat's primary auditory cortex (AI) is thought to provide the framework for frequency-specific processing in that field. This study was designed to assess this postulate by examining the spatial distribution of neurons within AI that are activated by a single tonal frequency delivered to the contralateral ear. Distributions obtained at each of several stimulus levels were then compared to assess the influence of stimulus amplitude on the spatial representation of a given stimulus frequency in AI. Data were obtained from 308 single units in AI of four adult, barbiturate-anesthetized cats, using extracellular recording methods. Stimuli were 40-ms tone pulses presented through calibrated, sealed stimulating systems. In each animal, the CF (stimulus frequency to which the unit is most sensitive), threshold at CF, response/level function at CF, and binaural interactions were determined for isolated neurons (usually one per track) in 60-90 electrode tracks. For each unit, regardless of its CF, responses to 40 repetitions of contralateral tones of a single frequency, presented at each of four or five sound pressure levels (SPLs) in the range from 10 to 80 dB were obtained. Different test frequencies were used in each of four cats (1.6, 8.0, 11.0, and 16.0 kHz). For tones of each SPL, we generated maps of the response rates across the cortical surface. These maps were then superimposed on the more traditional maps of threshold CF. All units whose CF was equal to the test frequency could be driven at some SPL, given an appropriate monaural or binaural configuration of the stimulus. There was a clear spatial segregation of neurons according to the shapes of their CF tone response/level functions. Patches of cortex, often occupying more than 2 mm2, seemed to contain only monotonic or only nonmonotonic units. In three cortices, a patch of nonmonotonic cells was bounded ventrally by a patch of monotonic cells, and in one of these cases, a second patch of monotonic cells was found dorsal to the nonmonotonic patch. Contralateral tones of any given SPL evoked excitatory responses in discontinuous cortical territories. At low SPLs (10, 20 dB), small foci of activity occurred along the isofrequency line representing the test frequency. Many of these cells had nonmonotonic response/level functions. (ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1994

Influence of callosal activity on units in the auditory cortex of ferret (Mustela putorius).

1. Callosal neurons in primary auditory cortex project heavily to regions of the contralateral homotopic field that are excited by stimulation of one or both ears and minimally to regions that are excited by stimulation of one ear and inhibited by stimulation of the other ear. The intent of this study is to begin assessing the function of this extensive intercortical projection in determining the response properties of single units in the recipient auditory cortex. 2. Callosal neurons in the right auditory cortex of barbiturate-anesthetized ferrets were stimulated electrically through a pair of monopolar microelectrodes that were separated by 2-3 mm and whose uninsulated tips were located 500 microns below the pial surface. Electrical stimuli consisted of a single 100-microseconds pulse delivered at a fixed intensity and temporal relationship to the onset of each of a set of monaural or binaural acoustic stimuli. Pulse amplitude was almost always < 350 microA. Acoustic stimuli consisted of 100-ms best-frequency tones delivered monaurally or binaurally at 1-s intervals. The influence exerted by electrical stimulation of the right auditory cortex on responses of single units isolated in the left auditory cortex was assessed by comparing responses evoked by the same acoustic stimuli delivered alone and when accompanied by the electrical stimulation. In addition, the latencies of discharges evoked by the electrical stimuli delivered alone were assessed. Three categories of influence were observed: suppression, excitation, and a mixed class consisting of both suppression and excitation. 3. Suppression was the most commonly observed influence of callosal input on acoustically evoked responses. Depending on the strength of the acoustic stimuli and electrical pulse and the temporal relationship between the two kinds of stimulation, responses to acoustic stimuli could be completely suppressed or reduced to a smaller version of the control responses. The duration of the inhibitory influence was often > 100 ms and the suppression was frequently effective at latencies of 2-4 ms. 4. Although unit activity was commonly excited by electrical stimulation of the right auditory cortex, facilitation of acoustically evoked responses was infrequently observed. Apparent facilitation usually was the result of the summation of acoustically and electrically evoked discharges.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1993

Binaural processing of sound pressure level in cat primary auditory cortex: evidence for a representation based on absolute levels rather than interaural level differences.

1. Single-neuron responses were recorded in high-frequency regions of primary auditory cortex (AI) of anesthetized cats. Best-frequency tone pips were presented to each ear independently via sealed stimulus delivery systems, and the sound pressure level (SPL) at each ear was independently manipulated. Each neuron was studied with many dichotic combinations of SPL, chosen to incorporate a broad range of the two synthetic interaural level variables, interaural level difference (ILD) and average binaural level (ABL). This paper illustrates the common forms of binaural SPL selectivity observed in a sample of 204 single neurons located in AI. 2. Most neurons (> 90%) were jointly influenced by ILD and ABL. A small proportion of bilaterally excitable (EE) neurons responded to ABL rather independently of ILD. Only one neuron was determined to respond to ILD independently of ABL. 3. Nonmonotonic selectivity for one or both of the binaural level cues was evident in > 60% of our sample. Within the most effective range of ILD values, response strength was usually related nonmonotonically to related both to ILD and ABL. We have described units exhibiting this kind of dual nonmonotonic selectivity for the two binaural variables as being influenced by a Two-Way Intensity Network (TWIN). 4. Each of the response forms identified in an earlier study of the gerbil inferior colliculus were found in this study of cat auditory cortex. However the classes were evident in markedly different proportions. In particular, TWIN responses alone accounted for 36.2% of the sample, nearly four times the proportion found in the inferior colliculus in a previous study. 5. Units with similar binaural responses do not necessarily have similar monaural properties. For example, the typically nonmonotonic relation between response strength and ABL was often observed in the absence of a monaurally demonstrable nonmonotonicity. There is no simple relation between a neuron's classification according to the sign of monaural influence and its response to ILD and ABL. In particular, EE neurons exhibited remarkably diverse binaural properties. 6. Since responses of nearly all AI neurons are influenced jointly by ABL and ILD, we contend that single neurons in primary auditory cortex are not specifically tuned to either cue. ILD and ABL are mathematical expressions relating the SPLs at the two ears to each other (as the difference and average, respectively) and any such combination is expressed most simply as a particular combination of SPL at each ear.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗

PubMed1993

Focal selectivity for binaural sound pressure level in cat primary auditory cortex: two-way intensity network tuning.

1. The influence of sound pressure level (SPL) at the two ears was studied in single-neuron responses recorded in high-frequency regions of primary auditory cortex (AI) of anesthetized cats. For each unit, many binaural combinations of SPL were tested by using best-frequency tone pips presented to each ear independently via sealed stimulus delivery systems. In the preceding paper, we illustrated the different forms of response observed in our sample of units. Here we explore in more detail the mechanisms underlying the properties of the largest single class of binaural response, characterized by joint nonmonotonic tuning to the SPLs at the two ears. We have described such units as being influenced by a Two-Way Intensity Network (TWIN). 2. Under binaural conditions, 62% of our sample of well documented neurons (81/130) exhibited a nonmonotonic relation between response magnitude and the SPL at one or the other ear. Of these units, 47 displayed clear bilateral nonmonotonicity (TWIN tuning), 17 units displayed only unilateral nonmonotonicity, and an additional 17 units showed intermediate (or transitional) characteristics between unilateral and bilateral nonmonotonicity. These characteristics can also be described in terms of average binaural level (ABL) and interaural level difference (ILD). Thus there is commonly a nonmonotonic relation between response magnitude and ABL and sometimes a TWIN tuning to ABL and ILD. The distribution of best frequencies for TWIN neurons is broad. 3. Under monaural conditions, TWIN neurons exhibit diverse properties. Some are responsive only under binaural conditions [i.e., predominantly binaural (PB)]. Some monaurally responsive TWINs are contralaterally excitable and ipsilaterally unresponsive (EO), some are ipsilaterally excitable and contralaterally unresponsive (OE), and a few are bilaterally excitable (EE). Monaural rate/level functions are monotonic for some of these neurons and nonmonotonic for others. Neurons of the PB class have previously been found to have nonmonotonic selectivity for ILDs near zero. In this study we have found that virtually all PB neurons are also nonmonotonically selective for ABL with different PB neurons having different best ABLs. 4. For TWIN neurons that respond monaurally, it is possible to demonstrate a mixed binaural influence. The optimal stimulus (or best binaural combination) for a TWIN neuron is associated with binaural facilitation. Flanking the most effective combination of ABL and ILD are less effective combinations that generate lower response magnitudes, either through threshold effects (at low SPLs) or through binaural suppression (at higher SPLs).(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗

PubMed1991

Chemoarchitectonic organization of the cat primary auditory cortex.

Acetylcholinesterase (AChE) activity, demonstrated histochemically, defines an area of cortex on the middle ectosylvian gyrus that appears to correspond to the cytoarchitectonically defined area 41 and the physiologically defined primary auditory area (AI). In this area there are high levels of AChE in layers III, IV and VI while in the surrounding areas there are comparatively low levels of enzyme in these layers. The monoclonal antibody CAT 301, which was raised against a cell surface proteoglycan, also defines this area. There are high levels of CAT 301 immunoreactivity in cell bodies and the neuropil of layer III and an absence of very large immunoreactive neurons in layer V. Furthermore there are higher levels of the calcium binding protein, parvalbumin and the metabolic enzyme, cytochrome oxidase, in layers III and IV of AI, than in most of the surrounding cortex. By contrast the distribution of the calcium binding protein, calbindin and the distribution of myelinated fibers are similar in area 41 and the surrounding areas.

Acetylcholinesterase↗

PubMed1991

Intrinsic inter- and intralaminar connections and their relationship to the tonotopic map in cat primary auditory cortex.

Small iontophoretic injections of the lectin, Phaseolus vulgaris leucoagglutinin (PHA-L), were made into different layers of the primary auditory cortex (AI) of cats. Injections in layer I labeled two types of morphologically distinct fibers in layer I as well as a smaller number of axons in layers II and III. Layer II injections labeled descending axons that produced a dense plexus of terminal fibers in layers I-III of both AI and adjacent auditory fields. Injections in layer III also labeled a dense plexus of axon collaterals at the junction of layers V and VI and labeled patches of terminal fibers in both AI and adjacent auditory fields. These were densest in layers I-III but usually extended into layers IV and V as well. The patches were partly formed by axon collaterals of layer III pyramidal cells that traveled for over 4 mm in the gray matter. Injections confined to layer IV labeled axons in all layers of the cortex but none of these axons appeared to reach the white matter. The axons spread laterally in layer IV and up into the superficial layers and ramified especially layer I. Injections in layers V and VI labeled axons in all layers' of the cortex but these were densest in the deep layers where labeling was fairly homogeneous. In the upper layers the labeling was arranged in semi-discrete patches. Large injections involving layers I-III were studied in tangential sections. Between 3 and 8 patches of terminal labeling were observed in AI and these were mainly arranged in a band with its long axis aligned approximately in the dorsoventral direction. However dense patches of terminal labeling also occurred both anterior and posterior to the injection site. In selected experiments portions of the tonotopic map in AI were mapped by single unit recording and subsequently the map was related to patches of anterogradely labeled fibers that surrounded injections of PHA-L. Rows of dorsoventrally oriented patches were among cells with a similar best frequency to those in the injection site. However patches located anterior or posterior to the injection site were among cells with higher or lower best frequencies. Two injections of PHA-L close together produce different patterns of labeling. One of the injections usually produces one or more patches that has no correlate among the patches of fibers labeled by the adjacent injection. This is clearest when one of the injections is made with biotinylated PHA-L that can be visualized directly without the use of primary antibodies.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗

PubMed1987

Binaural processing of sound pressure level in the inferior colliculus.

The central auditory system could encode information about the location of a high-frequency sound source by comparing the sound pressure levels at the ears. Two potential computations are the interaural intensity difference (IID) and the average binaural intensity (ABI). In this study of the central nucleus of the inferior colliculus (ICC) of the anesthetized gerbil, we demonstrate that responses of 85% of the 97 single units in our sample were jointly influenced by IID and ABI. For a given ABI, discharge rate of most units is a sigmoidal function of IID, and peak rates occur at IIDs favoring the contralateral ear. Most commonly, successive increments of ABI cause successive shifts of the IID functions toward IIDs favoring the ipsilateral ear. Neurons displaying this behavior include many that would conventionally be classified EI (receiving predominantly excitatory input arising from one ear and inhibitory input from the other), many that would be classified EE (receiving predominantly excitatory input arising from each ear), and all that are responsive only to contralateral stimulation. The IID sensitivity of a very few EI neurons is unaffected by ABI, except near threshold. Such units could provide directional information that is independent of source intensity. A few EE neurons are very sensitive to ABI, but are minimally sensitive to IID. Nevertheless, our data indicate that responses of most EE units in ICC are strongly dominated by excitation of contralateral origin. For some units, discharge rate is nonmonotonically related to IID and is maximal when the stimuli at the two ears are of comparable sound pressure. This preference for zero IID is common for all binaural levels. Many EI neurons respond nonmonotonically to ABI. Discharge rates are greater for IIDs representative of contralateral space and are maximal at a single best ABI. For a subset of these neurons, the influence arising from the ipsilateral ear is comprised of a mixture of excitation and inhibition. As a consequence, discharge rates are nonmonotonically related not only to ABI but also to IID. This dual nonmonotonicity creates a clear focus of peak response at a particular ABI/IID combination. Because of their mixed monaural influences, such units would be ascribed to different classes of the conventional (EE/EI) binaural classification scheme depending on the binaural level presented. Several response classes were identified in this study, and each might contribute differently to the encoding of spatial information.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗

PubMed1986

Cochlear nucleus lesions in the adult gerbil: effects on neurone responses in the contralateral inferior colliculus.

Unilateral cochlea ablation in neonatal gerbils has previously been shown to result in transneuronal degeneration of the ventral cochlear nucleus (CN) and enhanced responses of neurones in the contralateral inferior colliculus (IC) to ipsilateral stimulation of the non-ablated ear. Neither effect occurs in adult-ablated animals. To determine whether the lack of physiological change in the adult is due to the persistence of the ventral CN we lesioned the left CN of adult gerbils and recorded neurone responses in the right IC to stimulation of the right ear. Neurone thresholds, latency and intensity/response functions were unaffected by the lesion. The proportion of recording loci in the IC at which excitatory responses were obtained was also unaffected. A transient increase was observed in the maximum unit discharge level. The findings suggest that CN lesions in adults do not produce either the range or degree of neuronal changes resulting from neonatal cochlea ablation and the subsequent transneuronal degeneration of the CN. Thus, the effects of cochlea ablation are age-dependent.

Animals↗

PubMed1985

Projections from the cochlear nucleus to the inferior colliculus in normal and neonatally cochlea-ablated gerbils.

The distribution of the projection from one cochlear nucleus (CN) within each inferior colliculus (IC) was studied in adult, normal gerbils and adult gerbils subjected to unilateral ablation of the contralateral cochlea at 2 days of age. The projection was studied by using the Fink-Heimer technique for impregnating degenerating axons and their terminal processes with silver. Following an extensive, unilateral lesion of the CN, degeneration was seen in both ICs of all animals. In normal animals, degeneration was both more widespread and heavier in the contralateral than in the ipsilateral central nucleus of IC (ICC). Degeneration was most widespread in the rostral and lateral parts of both ICCs and in the ventral part of the contralateral ICC. Degeneration was observed in 26% of the area examined in ipsilateral ICC and in 73% of the area examined in contralateral ICC. In cochlea-ablated animals there was a much greater similarity in the area of degeneration in the ICC ipsilateral (57%) and contralateral (67%) to the CN lesion. The same regional distributions of degeneration were observed as in the normal animals except that the distribution of degeneration in the ipsilateral ICC more closely resembled the normal contralateral than the normal ipsilateral profile. We conclude that the normal distribution of projections from the CN within the ipsilateral ICC is substantially modified by neonatal ablation of the contralateral cochlea.

Animals↗

PubMed1985

Single-unit responses in the inferior colliculus: different consequences of contralateral and ipsilateral auditory stimulation.

Monaural excitatory responses of 181 single units in the central nucleus of the inferior colliculus of 15 anesthetized gerbils (Meriones unguiculatus) were examined quantitatively. Pure-tone stimuli were presented monaurally through sealed, calibrated sound-delivery systems. Most units were excited only by contralateral stimulation (EO); 23% were bilaterally excitable (EE). The threshold frequency tuning curves for contralateral stimulation of EE units were significantly broader than those produced by ipsilateral stimulation of EE units and those produced by contralateral stimulation of EO units. The frequency at which threshold was lowest (best frequency), or BF) was very similar for ipsilateral and contralateral stimulation of individual EE units; however, ipsilateral BFs were slightly but significantly lower than contralateral BFs. For EE units, ipsilateral BF thresholds (mean: 29.2 dB SPL) were significantly higher than contralateral BF thresholds (mean: 14.9 dB SPL). Monotonic and nonmonotonic relationships between discharge rate and stimulus intensity at BF were observed in responses evoked both by contralateral and ipsilateral stimulation. Interestingly, for individual EE units it was not uncommon for the rate/intensity function for one monaural condition to be monotonic although the relationship for stimulation of the other ear was markedly nonmonotonic. There was no qualitative difference between rate/intensity functions evoked by contralateral stimulation in EO and EE units. Ipsilateral discharge rates were characteristically much lower than contralateral rates for a given stimulus intensity. For 50 BF tones of 100 ms duration, the median peak numbers of discharges for contralateral stimulation of EO and EE units were 361 and 339, respectively; the median for ipsilateral stimulation of EE units was 102. The dynamic range of each rate/intensity function was calculated by measuring the intensity range associated with an increase in spike count from 10 to 90% of the peak rate. No differences were detected between the distributions of dynamic range for contralateral stimulation in EO or EE units, or between contralateral and ipsilateral dynamic ranges within individual EE units. For all response types the distributions of dynamic range were approximately normal, with means near 20 dB. The minimum mean latency to the first spike at BF was generally longer for ipsilateral than for contralateral responses.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗

PubMed1985

Single-unit responses in the inferior colliculus: effects of neonatal unilateral cochlear ablation.

Monaural responses of single units isolated in the inferior colliculus of adult gerbils that have developed postnatally with one cochlea were compared with monaural responses recorded in animals that have developed with both cochleas. One cochlea of 2-day-old gerbils was ablated, and at approximately 6 mo of age, excitatory responses to stimulation of the nonoperated ear were recorded in the ipsilateral inferior colliculus. These responses were compared quantitatively with responses evoked by ipsilateral and contralateral monaural stimulation in normal gerbils. Responses to ipsilateral stimulation in adult gerbils subjected at 2 days of age to ablation of the contralateral cochlea are significantly different from ipsilateral responses in nonoperated gerbils. In several respects they are very similar to contralateral responses in nonoperated gerbils. (Differences between monaural contralateral and ipsilateral responses in control animals are documented in the companion paper, Ref. 24.) These conclusions are based on analyses of response threshold, peak discharge rate, response pattern, and minimum response latency. The mean dynamic range of ipsilateral rate/intensity functions obtained in neonatally ablated gerbils is significantly larger than the mean ipsilateral and contralateral dynamic ranges in control animals. Analyses of threshold tuning curves indicate that the frequency tuning of units in the inferior colliculus of neonatally ablated animals does not differ significantly from the tuning of units in control animals in response to either ipsilateral or contralateral stimulation. These data reveal that in normal gerbils responses of single units in the inferior colliculus to stimulation of the ipsilateral ear result in part from interactions during postnatal development between pathways that convey information from the contralateral ear. The results are discussed in terms of the known anatomic consequences of a neonatal cochlear ablation and the competition for available synaptic space in the development of the retinotectal system.

Acoustic Stimulation↗

PubMed1984

Some physiological consequences of neonatal cochlear destruction in the inferior colliculus of the gerbil, Meriones unguiculatus.

Physiological consequences of neonatal cochlear ablation were assessed in the inferior colliculus of adult gerbils subjected at 2 days of age to ablation of one cochlea. Responses to tonal stimulation of the unoperated ear were evaluated at 100 microns intervals in the ipsilateral inferior colliculus. Compared with excitatory responses to ipsilateral stimulation in the inferior colliculus of unoperated animals, responses recorded in experimental animals are more widespread, stronger and occur at lower thresholds. The ubiquity of excitatory responses to ipsilateral stimulation in neonatally ablated animals indicates that responses to ipsilateral stimulation in normal gerbils are partly dependent upon interactions during development between pathways conveying information from each ear.

Animals↗

PubMed1983

Ascending auditory projections to the inferior colliculus in the adult gerbil, Meriones unguiculatus.

Ascending auditory projections to the inferior colliculus (IC) of the adult gerbil were studied using the retrograde transport of horseradish peroxidase. Our results indicate that in gerbils, the IC receives afferent projections from most brainstem auditory nuclei. A strong contralateral projection originates in the cochlear nuclear complex (CN). A smaller but consistent projection from all three divisions of ipsilateral CN is also present. The medial superior olive (MSO), superior parolivary nucleus, and ventral nucleus of the lateral lemniscus all maintain ipsilateral projections to the IC. Bilateral projections arise from the lateral superior olive, lateral nucleus of the trapezoid body, and dorsal nucleus of the lateral lemniscus. Previous investigations in other mammalian species provide conflicting data concerning the magnitude of a direct ipsilateral projection from CN to the IC. Our quantitative data indicate that the ipsilateral projection from CN in the gerbil is nearly one third as large as the projection from ipsilateral MSO. The projection from contralateral CN is six times larger than the MSO projection. The distribution of labeled cells across the rostrocaudal extent of MSO and the three divisions of the cochlear nuclear complex are presented.

Animals↗