Biomedical subjects

G D Pollak

Publications and source records attributed to G D Pollak.

At least 37 records · Page 2Linked to original sources

PubMed1994

Azimuthal receptive fields are shaped by GABAergic inhibition in the inferior colliculus of the mustache bat.

1. In this study we examine the effects of GABAergic inhibition on the response properties and the constructed azimuthal receptive fields of 54 excitatory/inhibitory (EI) neurons tuned to 60 kHz in the inferior colliculus of the mustache bat. The constructed azimuthal receptive fields predict the spike counts that would be evoked by different intensities of 60-kHz sounds presented from each of 13 azimuthal locations in the frontal sound field. 2. Action potentials were recorded with a micropipette attached to a multibarrel glass electrode. Bicuculline, an antagonist specific for gamma-aminobutyric acid-A (GABAA) receptors, was iontophoretically applied through the multibarrel electrode. Both monaural and binaural response properties were initially recorded at a variety of interaural intensity disparities (IIDs) and absolute intensities, and the same response properties were subsequently assessed while GABAergic inhibition was blocked by bicuculline. Azimuthal receptive fields both before and during the application of bicuculline were constructed from response properties obtained with earphones after correcting for the directional properties of the ear and the IIDs generated by 60-kHz sounds presented from a variety of azimuthal locations. 3. Bicuculline had virtually no effect on either the monaural or binaural properties of 19 cells (35%). The constructed azimuthal receptive fields of these cells were also unaffected by bicuculline. Presumably the properties of these cells were formed in a lower nucleus, most likely the contralateral lateral superior olive (LSO), and were imposed on the collicular cell via the crossed projection from the LSO to the inferior colliculus, which is known to be excitatory. 4. In more than half of the neurons (65%) GABAergic inhibition influenced one or more features of the cell's response properties and thus its azimuthal receptive field. Some response properties were formed in the colliculus through GABAergic inhibition, whereas others appear to have been shaped initially in a lower nucleus and then further modified by GABAergic inhibition in the inferior colliculus. Moreover, a number of features of GABAergic inhibition that acted on inferior collicular cells were evoked by stimulation of the contralateral (excitatory) ear, whereas other features were influenced by stimulation of the ipsilateral (inhibitory) ear. 5. In 20 cells (37%) blocking GABAergic inhibition reduced or abolished the inhibition evoked by the ipsilateral ear. The receptive fields of cells in which the ipsilaterally evoked inhibition was reduced by bicuculline expanded further into the ipsilateral sound field than they did before bicuculline.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1993

Some comments on the proposed perception of phase and nanosecond time disparities by echolocating bats.

In a series of recent reports, Simmons and his colleagues propose that bats are able to accurately encode the spectral, temporal and phase information of their emitted calls and echoes. The information so encoded is then extracted by the networks of the auditory system with specialized processing. They propose that bats use this information to determine the distance to their target by crosscorrelating the entire structure of the emitted call with the structure of the echo. The idea is that slight deviations in the correlation function can be detected by the bat and the degree of mismatch provides an accurate measure of temporal disparity and hence range. The data in the reports purport to show that bats perceive the phase of ultrasonic signals and that they can resolve temporal disparities of about 10 ns, and thus can distinguish range differences as small as 2 microns. The hypothesis also attempts to explain how a variety of acoustic cues are processed and represented in the auditory system and how they are combined to form a unitary percept of space and fine structure. The theory incorporates some time honored processes of extracting information, such as crosscorrelations. The implications of the hypothesis, however, go far beyond a theory of neural processing and representation of information by ensembles of cells. The hypothesis requires some remarkable abilities, such as the phase coding of ultrasonic signals and a temporal acuity on the order of 10 ns. These features have never been seen in any neurophysiological study of any animal nor has its existence been implied in behavioral studies of other animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

The dorsal nucleus of the lateral lemniscus in the mustache bat: monaural properties.

Neurons in the mustache bat dorsal nucleus of the lateral lemniscus (DNLL) were examined in response to monaural stimulation. There are six main findings of this study. First, the mustache bat DNLL is tonotopically organized. Second, EI cells are the predominant aural type and comprise about 80% of DNLL neurons, whereas monaural and EE cells are far less numerous. Third, the majority of DNLL neurons have either monotonic or weakly nonmonotonic rate-intensity functions. Fourth, a chopping pattern is evoked by contralateral stimulation in 58% of DNLL neurons and is the predominant temporal response pattern. Fifth, neurons with different aural properties tend to exhibit different temporal response patterns: EI cells are largely choppers, while EO and EE cells are more often primarylike or primarylike-with-notch. Sixth, the sustained responses of EE units to contralateral stimulation differs dramatically from their onset responses to ipsilateral stimulation. Here we have demonstrated that although a large proportion of mustache bat DNLL neurons are EI choppers, the DNLL nonetheless contains a heterogeneous population of neurons based on physiological responses to pure tones.

Acoustic Stimulation↗

PubMed1993

The effects of GABAergic inhibition on monaural response properties of neurons in the mustache bat's inferior colliculus.

The effects of GABAergic inhibition on response properties of neurons in the inferior colliculus were investigated. The experimental animals were mustache bats and responses were monitored from neurons in the hypertrophied 60 kHz isofrequency contour of the inferior colliculus. The features we report on here are: 1) the maximum discharge rates evoked by tone bursts at each unit's best frequency; 2) the forms of the rate-level functions; 3) the discharge patterns evoked by best frequency tone bursts; and 4) the changes in these response features that were observed when GABAergic inhibition was blocked with bicuculline, an antagonist specific for GABAA receptors. There were three main findings. The first is that bicuculline caused the discharge rate to increase in the majority of neurons. The maximum firing rates in more than half of the units increased by at least 100%, and in 15% of the cells the maximum spike-count increased 400% or more. Of particular interest were the 13 cells that were nearly unresponsive to any tone burst, but responded vigorously to the same stimuli after application of bicuculline. The second main finding is that the increased discharge rates were due either to a change from a phasic to a tonic discharge pattern, or to a change in overall excitability with no change in discharge pattern. The third main finding was that bicuculline changed the shape of the rate-level functions in almost half of the cells studied. The general trend was that units whose pre-drug rate-level functions were upper-threshold were most likely to be changed, followed by regular nonmonotonic and non-saturated monotonic. Units with saturated monotonic functions were the least likely to be affected by bicuculline. These results lead us to suggest that GABAergic inhibition acts on collicular cells in two principal ways. The first way is to modify the effects of the excitatory innervation and thereby shape the response features of collicular neurons. The formation of rate-level functions is but one illustration of the shaping action of GABAergic inhibition. Other features that are shaped by GABAergic inhibition include discharge patterns, thresholds, latencies and tuning curves. The second way is to provide a regulated suppression of evoked activity. We propose that the suppression is situation dependent and may act to enhance the operating range of collicular neurons in situations of particular importance to the animal, such as during periods of selective attention and perhaps in other situations as well.

Acoustic Stimulation↗

PubMed1993

GABA shapes a topographic organization of response latency in the mustache bat's inferior colliculus.

Many neurons in the auditory forebrain of the mustache bat act as coincidence detectors for signals separated in time by up to 20 msec. Differences in path lengths cannot adequately explain how the nervous system delays one signal relative to the other to such a large degree. Several researchers have proposed that an inhibitory mechanism might account for long delays, but it has not been known where these delays are created. Previous studies, using a variety of mammals, have reported that the inferior colliculus contains some cells with much longer latencies than those of cells in lower auditory centers, suggesting that the inferior colliculus might be the site where long delays are generated. We characterized the latencies of cells in the 60 kHz contour of the mustache bat inferior colliculus and examined how GABAergic inhibition affected the latencies of those cells. Evaluations of the influence of GABA were made by documenting changes in response latency that occurred when GABAergic inputs were reversibly blocked by iontophoretic application of the GABAA antagonist bicuculline. Prior to bicuculline application, latencies varied over a wide range among the population of cells and we observed a pattern of latency changes with dorsoventral location. The pattern was that the population of neurons in the dorsal regions of the inferior colliculus had a wide range of latencies while the population in more ventral regions had progressively narrower latency ranges. Thus, while some cells at each depth had comparably short latencies, the average latency of the population at a given depth was long in the dorsal inferior colliculus and became progressively shorter ventrally. The same characteristic distribution of latencies and pattern of latency changes with depth were observed for cells that had different aural preferences, different rate-intensity functions, and different discharge patterns, suggesting that latency is an important organizational feature of the inferior colliculus. Bicuculline substantially shortened latency in about half of the cells studied, and it dramatically altered the pattern of latency changes with depth. These results suggest that GABA normally lengthens response latencies and creates a dorsoventral grading of delays in the inferior colliculus. This wide range of latencies could provide the large latency differences necessary for the coincidence detectors in the medial geniculate body tuned to signals separated by up to 20 msec.

Animals↗

PubMed1993

GABA shapes sensitivity to interaural intensity disparities in the mustache bat's inferior colliculus: implications for encoding sound location.

This study examined how GABAergic inhibition affected binaural properties of neurons in the mustache bat's inferior colliculus. Evaluations were made by documenting changes in acoustically evoked inhibition that occurred when GABAergic inputs were reversibly blocked by iontophoretic application of the GABAA antagonist bicuculline. We studied neurons sensitive to interaural intensity disparities (IIDs), since these are the principal cues animals use to localize high-frequency sounds. Neurons sensitive to these cues receive excitation from one ear and inhibition from the other ear, and are called EI neurons. Recordings focused on the EI region in the hypertrophied 60 kHz isofrequency contour, where the sensitivities of the EI cells to IIDs are systematically ordered, thereby creating a map of IID sensitivity. EI neurons were classified on the basis of their IID functions, of which there were two principal types. Seventy percent of the cells had conventional IID functions where the firing rate evoked by a fixed intensity at the contralateral (excitatory) ear remained constant with low intensities at the ipsilateral (inhibitory) ear and then declined progressively as the intensity at the ipsilateral ear increased. We refer to cells that had this type of IID function simply as EI neurons. The IID functions in the remaining 30% of the cells showed binaural facilitation and were classified as EI/f neurons. In these cells, increasing sound intensity at the ipsilateral (inhibitory) ear when the intensity at the contralateral (excitatory) ear was fixed, initially caused the firing rate to increase by at least 25% above the rate evoked by the sound at the contralateral ear alone. Additional intensity increases at the ipsilateral ear then resulted in a marked decline in response rate. We examined the effects of bicuculline on three binaural properties: (1) the degree of inhibition evoked by the ipsilateral ear (the maximum inhibition), (2) the IID at which the unit's discharge rate declined by 50% (the 50% point), and (3) binaural facilitation. There are three main findings. First, bicuculline substantially reduced or eliminated the inhibition evoked by the ipsilateral ear in about 40% of the cells. In the other 60% of the cells, bicuculline had little or no effect on the magnitude of the ipsilaterally evoked inhibition. The second finding is that in more than half of the cells in which there was little or no reduction in the magnitude of the ipsilaterally evoked inhibition, bicuculline changed the IID at which the ipsilaterally evoked inhibition caused the discharge rate to decline by 50%.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1992

GABAergic circuits sharpen tuning curves and modify response properties in the mustache bat inferior colliculus.

1. The influence of bicuculline on the tuning curves of 65 neurons in the inferior colliculus of the mustache bat was investigated. Single units were recorded with multibarrel electrodes where one barrel contained bicuculline, an antagonist specific for gamma-amino-butyric acid (GABA)A receptors. Fifty-nine tuning curves were recorded from units that were sharply tuned to 60 kHz, the dominant frequency of the bat's orientation call, but six tuning curves were also recorded from units tuned to lower frequencies and whose tuning curves were broader than the 60 kHz cells. Tuning curves were constructed from peristimulus time (PST) histograms obtained over a wide range of frequency-intensity combinations. Thus tuning curves, PST histograms evoked by frequencies within the tuning curve, and rate-level functions at the best frequency were obtained before iontophoresis of bicuculline and compared with the tuning curves and response properties obtained during the administration of bicuculline. 2. Three general types of tuning curves were obtained: 1) open tuning curves that broadened on both the high- and low-frequency sides with increasing sound level; 2) level-tolerant tuning curves in which the width of the tuning curve remained uniformly narrow with increasing sound level; and 3) upper-threshold tuning curves, which did not discharge to high-intensity tone bursts at the best frequency, thereby creating closed or folded tuning curves. 3. One major finding is that GABAergic inhibition plays an important role in sharpening frequency tuning properties of many neurons in the mustache bat inferior colliculus. In response to blocking GABAergic inputs with bicuculline, the tuning curves broadened in 42% of the neurons that were sharply tuned to 60 kHz. The degree of change in most units varied with sound level: tuning curves were least affected, or not affected at all, within 10 dB of threshold and showed progressively greater changes at higher sound levels. These effects were seen in units that had open, level-tolerant, and upper-threshold tuning curves. 4. A second key result is that bicuculline affected rate-level functions and/or temporal discharge patterns in many units. Bicuculline transformed the rate-level functions of 13 cells that originally had nonmonotonic rate level functions, from strongly nonmonotonic into weakly nonmonotonic or monotonic functions. It also changed the temporal discharge patterns in 22 cells, and the changes were often frequency specific.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗

PubMed1990

Determinants of horizontal sound location selectivity of binaurally excited neurons in an isofrequency region of the mustache bat inferior colliculus.

1. The monaural and binaural response properties and the horizontal sound location sensitivity of 78 binaurally excited neurons from 26 bats were examined with a combined closed-field and free-field stimulation paradigm. The aim was to determine how the response properties of these neurons shape their selectivity for auditory space. All neurons were recorded within a single, greatly enlarged isofrequency (60 kHz) region of the mustache bat's central nucleus of the inferior colliculus (ICC). In this and two companion papers (Wenstrup et al. 1988a,b) that focused on binaurally inhibited neurons in this isofrequency region, our goal was to examine the neural mechanisms for binaural processing within a single frequency channel of the primary auditory system. 2. Binaurally excited neurons were defined as either excited by monaural input from both ears or, if excited by monaural stimulation of only one ear, facilitated by binaural stimulation. Some neurons also exhibited binaural inhibition. These neurons were divided into functional classes based on their responses to monaural and binaural stimulation presented under dichotic, closed-field conditions. The following notation was used: response to contralateral stimulation (E, excitatory; O, no effect), response to ipsilateral stimulation (E, excitatory; O, no effect)/response to binaural stimulation (F, facilitatory; I, inhibitory; O, no effect). Seven functional classes were observed: EE/O, EE/F, EE/I, EE/FI, EO/F, EO/FI, and OO/F. 3. Among EE neurons, thresholds for contralateral monaural stimulation were typically lower than for ipsilateral stimulation, and response magnitudes for contralateral stimulation were typically greater. Among EO/F and EO/FI neurons, only one eye, an OE/FI neuron, was excited by ipsilateral monaural stimulation. These results suggest that contralateral input provides the dominant excitatory influence. EE/FI and EO/FI neurons, which exhibited both binaural facilitation and inhibition, were typically inhibited at interaural intensity differences (IIDs) favoring the ipsilateral ear, suggesting that ipsilateral input provides the dominant inhibitory influence. 4. Neurons were tested over the range of naturally occurring IIDs (+/- 30 dB) at intensities of -20 to 30 dB relative to threshold. The IID functions of these neurons assumed three configurations: flat, with facilitation occurring at all IIDs; stepped, with facilitation occurring over part of the IID range, and peaked, with facilitation occurring over a limited band of IIDs. The majority of cells (90%) exhibited peaked IID functions, and most of these (73%), regardless of functional class, were maximally facilitated at an IID of 0 dB. 5. Neurons differed considerably in the strength of their binaural interactions.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗

PubMed1989

Differential ascending projections to aural regions in the 60 kHz contour of the mustache bat's inferior colliculus.

The inferior colliculus of the mustache bat is similar in many respects to the inferior colliculus of more commonly studied mammals. However, the isofrequency contour devoted to processing 60 kHz, the dorsoposterior division (DPD) is greatly expanded, encompassing an area approximately equal to one-third of the central nucleus. Of particular significance is that monaural and binaural neurons are segregated in the DPD into 4 spatially distinct aural regions. In this study we exploit the great enlargement of the 60 kHz region in the central nucleus of the inferior colliculus (ICc) of the mustache bat to determine the source of ascending projections to the 4 different aural regions of the DPD. Small iontophoretic deposits of HRP were made within each of the physiologically defined aural regions, and the locations and numbers of retrogradely labeled cells in the auditory brainstem nuclei were determined. Two major features of collicular organization were found. The first is that each aural region receives a unique set of projections from a subset of lower auditory nuclei and thus is distinguished both by its neural response properties and by the pattern of ascending projections it receives. The dorsomedial EE region receives inputs primarily from the ipsilateral intermediate nucleus of the lateral lemniscus (INLL) and ventral nucleus of the lateral lemniscus (VNLL), and the contralateral ICc. In contrast, the ventrolateral EE region receives projections from the ipsilateral medial superior olivary nucleus (MSO), VNLL, and INLL. The inputs to the EI region originate primarily from the dorsal nucleus of the lateral lemniscus (DNLL) and lateral superior olivary nucleus (LSO) bilaterally and from the ipsilateral INLL. The afferents to the EO region include the contralateral cochlear nucleus, the ipsilateral VNLL and INLL and MSO. The second major organizational feature is that the binaural nuclei of the brain-stem project upon the DPD in a more restricted manner than do some of the lower monaural nuclei, such as the VNLL and INLL, which project in a more widespread manner. The unique set of projections terminating in each aural region of the DPD suggests that the neurons should have substantially different properties, even when neurons in different regions are of the same general aural type. Moreover, the elucidation of the micro-organization of the DPD provides insights into the different ways that binaural properties of DPD neurons are created by the convergence of inputs from particular subsets of lower auditory nuclei.(ABSTRACT TRUNCATED AT 400 WORDS)

Afferent Pathways↗

PubMed1988

Origin of ascending projections to an isofrequency region of the mustache bat's inferior colliculus.

The inferior colliculus of the mustache bat is similar in most respects to the inferior colliculus of more commonly studied mammals, but one isofrequency contour, the dorsoposterior division, is greatly overrepresented. The present study utilizes this specialization of the auditory system in the mustache bat to determine the total set of ascending projections to a single isofrequency contour of the inferior colliculus. Within the dorsoposterior division, neurons are all very narrowly tuned to 60 kHz, the major component of this bat's echolocation call. The afferent projections to this isofrequency contour were identified by making deposits of horseradish peroxidase (HRP) within the dorsoposterior division after physiologically defining its borders. Two other frequency representations are present in the central nucleus of the inferior colliculus of the mustache bat, the anterolateral division in which there is an orderly progression of frequencies from 59 down to 20 kHz, and the medial division in which frequencies from 63-120 kHz are represented. In additional experiments, the afferent projections to the medial and anterolateral divisions were examined, providing an anatomical description of the tonotopicity of the lower auditory nuclei. Deposits of HRP in the DPD labeled cells in each of the lower brainstem auditory nuclei that have previously been shown to project to the entire central nucleus of the inferior colliculus. The ascending projections to the dorsoposterior division include contralateral projections from the cochlear nucleus and inferior colliculus, ipsilateral projections from the medial superior olive, ventral and intermediate nuclei of the lateral lemniscus, and bilateral projections from the lateral superior olive and dorsal nucleus of the lateral lemniscus. In most of the nuclei, labeled cells were confined to specific portions of the nuclei, often forming "slabs" of labeled cells across the rostrocaudal extent of most nuclei. These slabs presumably represent the 60 kHz representation in each of the lower nuclei. When deposits of HRP were made into other frequency band representations of the inferior colliculus, in either the medial or anterolateral division, labeled cells again formed slabs in each lower nucleus. However, the location of the slab varied as a function of the best frequency of neurons at the deposit site, and labeled cells were not present within the 60 kHz representation. These results show the general tonotopy of the mustache bat's brainstem auditory nuclei, and with respect to the dorsoposterior division, clearly reveal the total set of projections to a single isofrequency region.(ABSTRACT TRUNCATED AT 400 WORDS)

Afferent Pathways↗

PubMed1988

Time is traded for intensity in the bat's auditory system.

Disparities in time and intensity are the two chief cues animals use for localizing a sound source in space. Echolocating bats belonging to the family Molossidae emit brief, ultrasonic signals for orientation that sweep downward about an octave over the duration of the pulse. Due to acoustic shadowing and the directional properties of the ears, pronounced interaural intensity disparities are created that vary as a function of azimuth. However, due to the small headwidth of these animals, azimuthal changes create small interaural time disparities that are at most 30 microseconds. The experiments in this report are concerned with the binaural processing of time and intensity disparities using brief FM signals that simulate the animal's natural echolocation calls. Binaural neurons receiving excitation from one ear and inhibition from the other (E-I neurons) were recorded from the inferior colliculus of Mexican free-tailed bats. The majority of units sampled were highly sensitive for temporal disparities of 100-300 microseconds, and a few had significant changes in discharge probability when interaural time was changed by 10-20 microseconds. However, all E-I neurons were also sensitive to intensity disparities. With only one exception, all E-I neurons traded time for intensity. On the average, each decibel difference in intensity could be compensated for by advancing or delaying the inhibitory sound by 47 microseconds. The main conclusion is that the auditory system processes interaural disparities by transforming level differences at the two ears into latency differences. Thus the discharge probability of each binaural neuron is determined largely by the arrival times of the discharges from the excitatory and inhibitory ears. In view of the substantial time-intensity trading ratios, the small interaural time disparities produced by azimuthal locations off the midline play no role in shaping the response properties of these neurons. Specific examples of how time-intensity trades can translate into a high spatial selectivity are presented.

Animals↗

PubMed1988

Binaural neurons in the mustache bat's inferior colliculus. I. Responses of 60-kHz EI units to dichotic sound stimulation.

1. Single-unit responses to closed-field, dichotic sound stimuli were obtained from EI neurons in the mustache bat's inferior colliculus; these neurons are excited by sound to the contralateral ear and inhibited by sound to the ipsilateral ear. All units were tuned to the 60-kHz component of the bat's sonar signal. The goal of the study was to describe basic features of the sensitivity to interaural intensity differences (IIDs) and sound intensity among an isofrequency population of EI neurons. The following paper describes how these features of IID sensitivity shape the response to free-field sounds. 2. Three features of IID sensitivity were considered. The inhibitory threshold (Figs. 1 and 2) described the IID at which inhibitory effects became pronounced; it was defined as the IID at which the excitatory response to contralateral sound was suppressed by 50%. Most units (68%) were inhibited at positive IID values, for which the ipsilateral (inhibitory) sound was more intense. The maximum inhibition (Figs. 1 and 3) described the strength of ipsilateral inhibition; it was defined as the percent that each unit was inhibited below its response to monaural stimulation of the contralateral ear. The majority of units (58%) were almost totally suppressed by a sufficiently intense ipsilateral sound. The IID range (Figs. 1 and 4) described the sharpness or slope of the IID cutoff; it was defined as the IID range over which the response changed from nearly unsuppressed (80% of maximum response) to near maximum suppression (20% of maximum response). Most units (71%) had IID ranges of less than or equal to 15 dB. 3. A significant correlation between the inhibitory threshold and the maximum inhibition (Fig. 5) among the sample of EI units suggests that some common neural mechanisms underlie these features of IID sensitivity. 4. The response of EI neurons to binaural stimuli was a function of sound intensity as well as IID (Fig. 7). In part, this resulted from intensity-dependent changes in properties of binaural inhibition. For most units, basic measures of IID sensitivity changed to a limited extent as the sound intensity changed.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗

PubMed1988

Binaural neurons in the mustache bat's inferior colliculus. II. Determinants of spatial responses among 60-kHz EI units.

1. The responses to closed-field and free-field sound stimuli were obtained from EI neurons in the 60-kHz isofrequency representation of the mustache bat's inferior colliculus. The goals of the study were 1) to examine how binaural interactions and the directional properties of the ear shape the spatial response of 60-kHz EI neurons and 2) to examine features of the spatial responses of these isofrequency neurons which may encode the location of a sound. 2. The free-field response of monaural neurons was used to describe directional effects of the head and external ears at 60 kHz. Monaural neurons were most sensitive to sounds originating 26 degrees into the contralateral field, with sharply reduced sensitivity in the ipsilateral field and the lateral extreme of the contralateral field. The interaural intensity difference (IID), computed by assuming that the ears had mirror-image directional sensitivities, was greatest for sounds between 26 and 52 degrees off the vertical midline, where IIDs typically ranged from 20 to 30 dB. These values agree well with previous measures of the directional sensitivity of the ear obtained from cochlear microphonic recordings. 3. The response of a monaural unit to closed-field stimulation was used to predict its response to free-field stimulation. This prediction agreed closely with the observed free-field response, indicating that the monaural unit's free-field response could be explained by the directional sensitivity of the contralateral ear and the unit's response as a function of contralateral sound intensity. 4. EI neurons differed in basic features of their binaural response properties, as described in a companion paper. To examine how these features of binaural inhibition shaped spatial responses, we obtained responses to free-field sounds among neurons whose binaural properties had been thoroughly characterized. We then compared the spatial response to a prediction based upon the directional properties of the ear and the sensitivity of the EI neurons to IIDs and intensity. The close agreement between the observed and predicted spatial responses in 11 of 12 EI units indicated that the spatial responses were determined largely by sensitivity to IIDs and intensity and by the directional properties of the ear. 5. EI neurons in our sample were most sensitive, and responded best, to sounds originating 26-39 degrees into the contralateral field. These spatial response features were independent of the binaural properties of EI neurons. Instead, they depended upon the directional properties of the ears at 60 kHz.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗

PubMed1986

Binaural response organization within a frequency-band representation of the inferior colliculus: implications for sound localization.

The auditory system of the mustache bat (Pteronotus parnellii) contains a disproportionately large representation of a narrow frequency band, corresponding to the dominant, 60 kHz component of its echolocation signal. In the inferior colliculus (IC), the 60 kHz representation comprises an architectonically distinct region called the dorsoposterior division (DPD), which is accessible for detailed physiological study. We examined the topographic distribution of binaural responses within this one frequency-band representation of the inferior colliculus. We describe two primary results. First, neurons with different binaural response properties are spatially segregated into one of four binaural response-specific regions of the DPD: a large region of monaural (EO) responses; two regions containing neurons excited by sound from both ears (EE); and a region containing neurons excited by one ear and inhibited by the other (EI). Regions dominated by 60 kHz EI responses are also found in the lateral extremity of the IC, probably within the external nucleus. These results demonstrate functionally defined subdivisions in a single frequency-band representation of the IC. Moreover, they suggest that brain stem auditory projections to the DPD and/or intrinsic connections within the DPD are highly organized. Second, within the EI region of the DPD, there is a systematic shift in the sensitivity of EI multiunit responses to interaural intensity disparities (IIDs). Dorsally, EI neurons are suppressed only by relatively loud ipsilateral sounds, and there is a systematic decrease in the relative ipsilateral intensity required for suppression at more ventral recording sites. This result demonstrates that neurons sensitive to a sound localization cue are systematically organized within a frequency-band representation of the inferior colliculus. It has implications for the manner in which the location of a sound source is encoded within the primary auditory pathway.

Animals↗

PubMed1985

Topology of the central nucleus of the mustache bat's inferior colliculus: correlation of single unit properties and neuronal architecture.

The central nucleus of the mustache bat's inferior colliculus was studied in Golgi, Nissl, and fiber stained preparations; the neuronal organization and cytoarchitecture were correlated with the tonotopic maps revealed by single cell recordings. Three divisions of the central nucleus were defined by anatomical and physiological criteria: the anterolateral, medial, and dorsoposterior divisions. In horizontal sections, the anterolateral division has pronounced, semicircular fibrodendritic laminae. The dendritic fields of adjacent neurons form rostro-caudally-oriented laminae related to the tonotopic sequence. The neurons in the medial division are similar in size and arrangement, but here the laminar orientation follows the lateral-to-medial axis, with less curvature. The dorsoposterior division has many small disc-shaped and stellate neurons and a different, somewhat less fully expressed, laminar orientation. Each division has a unique frequency representation and tonotopic organization which conform to the pattern of dendritic orientation. In the anterolateral division, frequencies from about 10 kHz to about 59 kHz are represented, whereas the frequency representation in the medial division ranges from about 65 kHz to 110 kHz, and higher. The dorsoposterior division has an isofrequency organization in which the best frequency is characteristic for each bat, ranging from 60 to 64 kHz and varying by only +/- 300 Hz. This frequency corresponds to the dominant echo frequency of the bat's echolocation signals. We suggest that the dorsoposterior division is a hypertrophied isofrequency lamina, with many neurocytological features comparable to the isofrequency laminae in the central nucleus of other mammals.

Animals↗

PubMed1985

A functional organization of binaural responses in the inferior colliculus.

Binaural responses are segregated within an enlarged isofrequency region of the mustache bat's inferior colliculus. In the part of this region containing E-I responses, there is an orderly shift in the sensitivity of unit clusters to interaural intensity disparities, having implications for the representation of auditory space within isofrequency regions of the inferior colliculus.

Animals↗

PubMed1985

A representation of horizontal sound location in the inferior colliculus of the mustache bat (Pteronotus p. parnellii).

Binaurally sensitive E-I neurons in the inferior colliculus are topographically organized with respect to their sensitivity to interaural intensity disparities (IIDs). IID sensitivity dictates the azimuthal position of the medial border of a neuron's spatial receptive field. A sound moving along the azimuth will result in a change in the percentage of excited neurons as receptive field borders are crossed. Percent excitation within this population may provide a neural code for horizontal sound location.

Animals↗

PubMed1985

Determinants of sound location selectivity in bat inferior colliculus: a combined dichotic and free-field stimulation study.

This study of the neural representation of sound location in the bat Pteronotus parnellii describes how the peripheral and central components of its auditory system shape the horizontal and vertical spatial selectivity of single neurons in the inferior colliculus. Pteronotus extracts spatial information from the echoes of an emitted pulse composed of four constant-frequency harmonics (30, 60, 90, and 120 kHz), each terminated by a downward frequency sweep. To quantify the intensity cues available in the echo, cochlear microphonic response thresholds were used to measure the directional selectivity of the ear and the interaural intensity level disparities (IIDs) created between ears at standardized speaker positions in the bat's frontal sound field, at frequencies in the pulse spectrum. Speaker positions where thresholds were lowest were termed the sensitive area (SA) of the ear. Positions where IID values were greater than 10 dB were termed the difference area (DA). Ear directionality exhibited a pronounced frequency dependence, both in terms of the degree of directional selectivity and the position of the SA. At the 30-kHz harmonic of the pulse, the ear was broadly directional; the SA covered most of the lower half of the ipsilateral field. The ear was highly directional at the 60- and 90-kHz harmonics. Also, the vertical position of the SA changed dramatically between 60 and 90 kHz, from the horizontal midline at 60 kHz to 40 degrees below the midline at 90 kHz. The positions of the DAs also showed a pronounced frequency dependence. The 30-kHz DA was restricted to the extreme lateral part of the frontal sound field. The 60- and 90-kHz DAs were located in the same positions as the equivalent SAs and exhibited the same difference in vertical position. The DAs of the pulse harmonics differ in both their horizontal and vertical positions; the ears thus generate pronounced binaural spectral cues, which provide two-dimensional spatial information. In the inferior colliculus, a combined paradigm of closed-field dichotic stimulation, followed by free-field stimulation, was used to document the frequency tuning and binaural response properties of single neurons and to correlate these properties with the neuron's horizontal and vertical spatial selectivity in the frontal sound field. Where a neuron responded to free-field stimulation at the lowest intensity is termed its SA. A neuron's frequency tuning primarily influenced its degree of spatial selectivity and its sensitivity in the vertical plane, reflecting the directional properties of the external ears at the neuron's best frequency.(ABSTRACT TRUNCATED AT 400 WORDS)

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