IID sensitivity differs between two principal centers in the interaural intensity difference pathway: the LSO and the IC

Interaural intensity differences (IIDs) are the chief cues that animals use to localize high-frequency sounds. Neurons that are sensitive to IIDs are excited by sound at one ear and inhibited by sound at the other. Thus a given IID generates a combination of excitation and inhibition that is reflected in a cell's spike count. In mammals, the so-called "IID pathway" begins in the lateral superior olive (LSO), which is dominated by the type of IID-sensitive neurons just described. The LSO then sends a prominent projection to the inferior colliculus (IC), which also contains a substantial population of IID-sensitive cells. Recent pharmacological studies have suggested that the response properties of IID-sensitive neurons in the IC undergo considerable processing and thus should not simply reflect the output of the LSO. However, we have no direct evidence as to whether IID sensitivity, the defining response feature of these cells, differs at these two levels. The present study makes this direct comparison in the Mexican free-tailed bat, a species that relies greatly on high-frequency hearing and thus on IIDs for localizing sounds in space. Extracellular recording techniques were used to obtain IID functions from 50 IC neurons. Comparable data from 50 LSO cells were available from a previous study. The main result was that IID sensitivity significantly differed between cells in the LSO and the IC. Among LSO cells, sensitivity was centered approximately 0 dB (no intensity difference between the ears) whereas, in the IC, sensitivity was biased toward the inhibitory ear: on average, IC cells required a more intense signal at the inhibitory ear to reach the same degree of suppression as observed in LSO cells. Further analysis showed that the vast majority of IC cells (88%) exhibited a mismatch in the latencies of their inputs: inhibition arrived later when an equally strong excitation and inhibition were elicited; this reduced the effectiveness of the inhibition. Because latency shortens with increasing stimulus intensity, an IID with a more intense signal at the inhibitory ear could equate the latencies of excitation and inhibition, increasing the effectiveness of the inhibition. This result suggests that latency mismatches account, to a great extent, for the difference in sensitivity between the LSO and the IC; and when mismatches were negated by electronically time shifting the signals to the ears, sensitivity was no longer significantly different between the two nuclei.

Processing of interaural intensity differences in the LSO: role of interaural threshold differences

Cells in the lateral superior olive (LSO) are known to be sensitive to interaural intensity differences (IIDs) in that they are excited by IIDs that favor the ipsilateral ear and inhibited by IIDs that favor the contralateral ear. For each LSO neuron there is a particular IID that causes a complete inhibition of discharges, and the IID of complete inhibition varies from neuron to neuron. This variability in IID sensitivity among LSO neurons is a key factor that allows for the coding of a variety of IIDs among the population of cells. A fundamental question concerning the coding of IIDs is: how does each cell in the LSO derive its particular IID sensitivity? Although there have been a large number of neurophysiological studies on the LSO, this question has received little attention. Indeed, the only reports that have directly addressed this question are those of Reed and Blum, who modeled the binaural properties of LSO neurons and proposed that the IID at which discharges are completely suppressed should correspond to the difference in threshold between the excitatory, ipsilateral and inhibitory, contralateral inputs that innervate each LSO cell. The main purpose of this study was to test the threshold difference hypothesis proposed by Reed and Blum by recording responses to monaural stimulation and to IIDs from single cells in the LSO of the mustache bat. Our results show that although the IID sensitivities of some LSO cells correspond to the difference in threshold between the excitatory and inhibitory ears, in the majority of cells the difference in thresholds did not correspond to the cell's IID sensitivity. The results lead us to propose two models to account for IID sensitivities. One model is similar to that proposed by Reed and Blum and emphasizes differences in the thresholds of the excitatory and inhibitory inputs. This model accounts for the minority of cells in which the IID of complete inhibition corresponded to the difference in threshold of the inputs from the two ears. The other model, which accounts for the cells in which the IID of complete inhibition did not correspond to the difference in the thresholds of the inputs from the two ears (the majority of cells), places emphasis on differences in latencies of the excitatory and inhibitory inputs. The models incorporate features that are concordant with the known properties of the neurons that project to the LSO and together can account for the diversity of IID sensitivities among the population of LSO neurons.

A2A adenosine receptors inhibit ATP-induced Ca2+ influx in PC12 cells by involving protein kinase A

The regulatory role of A2A adenosine receptors in P2 purinoceptor-mediated calcium signaling was investigated in rat pheochromocytoma (PC12) cells. When PC12 cells were treated with 2-p-(2-carboxyethyl)-phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS-21680), a specific agonist of the A2A adenosine receptor, the extracellular ATP-evoked rise in cytosolic free Ca2+ concentration ([Ca2+]i) was inhibited by 20%. Both intracellular calcium release and inositol 1,4,5-trisphosphate production evoked by ATP were not affected by CGS-21680 treatment. However, ATP-evoked Ca2+ influx was inhibited following CGS-21680 stimulation. The CGS-21680-mediated inhibition occurred independently of nifedipine-induced inhibition of the [Ca2+]i rise. The CGS-21680-induced inhibition was completely blocked by reactive blue 2. The CGS-21680 effect was mimicked by forskolin and dibutyryl-cyclic AMP and blocked by Rp-adenosine 3',5'-cyclic monophosphothioate, a protein kinase A inhibitor, or by staurosporine, a general kinase inhibitor. The data suggest that in PC12 cells activation of A2A adenosine receptors leads to inhibition of P2 purinoceptor-mediated Ca2+ influx through ATP-gated cation channels and involves protein kinase A.

Medial superior olive in the free-tailed bat: response to pure tones and amplitude-modulated tones

In mammals with good low-frequency hearing and a moderate to large interear distance, neurons in the medial superior olive (MSO) are sensitive to interaural time differences (ITDs). Most small mammals, however, do not hear low frequencies and do not experience significant ITDs, suggesting that their MSOs participate in functions other than ITD coding. In one bat species, the mustached bat, the MSO is a functionally monaural nucleus, acting as a low-pass filter for the rate of sinusoidally amplitude-modulated (SAM) stimuli. We investigated whether the more typical binaural MSO of the MExican free-tailed bat also acts as an SAM filter. We recorded from 60 MSO neurons with their best frequencies covering the entire audiogram of this bat. The majority revealed bilateral excitation and indirect evidence for inhibition (EI/EI; 55%). The remaining neurons exhibited reduced inputs, mostly lacking ipsilateral inputs (28% I/EI; 12% O/EI; 5% EI/O). Most neurons (64%) responded with a phasic discharge to pure tones; the remaining neurons exhibited an additional sustained component. For stimulation with pure tones, two thirds of the cells exhibited monotonic rate-level functions for ipsilateral, contralateral, or binaural stimulation. In contrast, nearly all neurons exhibited nonmonotonic rate-level functions when tested with SAM stimuli. Eighty-eight percent of the neurons responded with a phase-locked discharge to SAM stimuli at low modulation rates and exhibited low-pass filter characteristics in the modulation transfer function (MTF) for ipsilateral, contralateral, and binaural stimulation. The MTF for ipsilateral stimulation usually did not match that for contralateral stimulation. Introducing interaural intensity differences (IIDs) changed the MTF in unpredictable ways. We also found that responses to SAMs depended on the carrier frequency. In some neurons we measured the time course of the ipsilaterally and contralaterally evoked inhibition by presenting brief frequency-modulated sweeps at different ITDs. The duration and timing of inhibition could be related to the SAM cutoff for binaural stimulation. We conclude that the response of the MSO in the free-tailed bat is created by a complex interaction of inhibition and excitation. The different time constants of inputs create a low-pass filter for SAM stimuli. However, the MSO output is an integrated response to the temporal structure of a stimulus as well as its azimuthal position, i.e., IIDs. There are no in vivo results concerning filter characteristics in a "classical" MSO, but our data confirm an earlier speculation about this interdependence based on data accessed from a gerbil brain slice preparation.

Synergistic activation of adenylyl cyclase is dependent upon phospholipase C-mediated processes in human neuroblastoma SK-N-BE(2)C cells

1-[6-[17 beta-3-Methoxyestra-1,3.5(10)-trien-17-yl]amino]hexyl]-1 H-pyrrole-2,5-dione (U-73122), an inhibitor of processes involved in the activation of phospholipase C, was used to assess the role of phospholipase C activation in the synergistic elevation of cAMP induced by carbachol and prostaglandin E2 in human neuroblastoma (SK-N-BE(2)C cells. Pre-treatment of the cells with U-73122 resulted in inhibition of carbachol-induced intracellular Ca2+ ([Ca2+]i) rise and inositol 1,4,5-trisphosphate (InsP3) generation, with maximal and half maximal inhibition (IC50) occurring at approximately 15 microM and 3.2 microM, respectively. U-731222 also inhibited the synergistic enhancement of cAMP accumulation induced by carbachol and prostaglandin E2 in a concentration-dependent manner with maximum and IC50 at 12 +/- 4 microM and 3.4 +/- 0.3 microM, respectively. However, U-73122 did significantly inhibit prostaglandin E2-induced production. While 1,2-bis(o-aminophenoxy)ethane-N,N,N,'N'-tetraacetic acid (BAPTA/AM) treatment decreased the synergistic cAMP accumulation by 28%m addition U-73122 further decreased it down to complete inhibition. Furthermore, GTP gamma S- and A1F4(-)-induced InsP3 generation in digitonin-mediated permeabilized cells was also inhibited by U-73122 treatment. Pre-treatment of the cells with neomycin, another blocker of the phospholipase C pathway, also resulted in inhibition of the carbachol-induced [Ca2+]i rise, InsP3 generation, and the enhancing effect on cAMP accumulation, to a comparable extent. But, Ca2+ chelation by BAPTA/AM in addition to neomycin treatment further decreased the cAMP accumulation. These results suggest that the increase in cytosolic Ca2+ and the coupling process between muscarinic receptor-like G-protein and phospholipase C are important for the synergistic activation of adenylyl cyclase in SK-N-BE(2)C cells.

Neural delays shape selectivity to interaural intensity differences in the lateral superior olive

Neurons in the lateral superior olive (LSO) respond selectively to interaural intensity differences (IIDs), one of the chief cues used to localize sounds in space. LSO cells are innervated in a characteristic pattern: they receive an excitatory input from the ipsilateral ear and an inhibitory input from the contralateral ear. Consistent with this pattern, LSO cells generally are excited by sounds that are more intense at the ipsilateral ear and inhibited by sounds that are more intense at the contralateral ear. Despite their relatively homogeneous pattern of innervation, IID selectivity varies substantially from cell to cell, such that selectivities are distributed over the range of IIDs that would be encountered in nature. For some time, researchers have speculated that the relative timing of the excitatory and inhibitory inputs to an LSO cell might shape IID selectivity. To test this hypothesis, we recorded from 50 LSO cells in the free-tailed bat while presenting stimuli that varied in interaural intensity and in interaural time of arrival. The results suggest that, for more than half of the cells, the latency of inhibition was several hundred microseconds longer than the latency of excitation. Increasing the intensity to the inhibitory ear shortened the latency of inhibition and brought the timing of the inputs from the two ears into register. Thus, a neural delay of the inhibition helped to define the IID selectivity of these cells, accounting for a significant part of the variation in selectivity among LSO cells.

Anatomical basis of a congenital hearing impairment: basilar papilla dysplasia in the Belgian Waterslager canary

Recent investigations into the anatomy of the inner ear of Belgian Waterslager canaries (BWC) have demonstrated myriad malformations associated with dysgenesis of the pars inferior of the otocyst. In those studies, the surface anatomy of BWC's basilar papilla and sacculus was examined utilizing scanning electron microscopy. In the present investigation, we utilized both light microscopy and transmission electron microscopy to describe the cross sectional anatomical pathology of the BWC's basilar papilla. Examination of the BWC's organ of Corti revealed numerous dysmorphologies: 1) hair cells from the tall hair cell region appeared broad and stunted, with deformed cuticular plates, abnormal stereocilia, and recognizable microvilli; 2) quantitative analysis of these hair cells revealed disproportionately large nuclei and abnormally short stereocilia; 3) hair cells from the short hair cell region of the papilla were absent, replaced by a population of large cells with electron-lucent cytoplasm; and 4) the tectorial membrane in the BWC papilla was narrow, covering only the area where the deformed tall hair cells were found. The malformations appeared to be more severe at the apex and midsection of the basilar papilla than at the base. These observations allow us to suggest a hypothesis to account for the distinct anatomofunctional hearing deficit observed in these birds. In addition, they further support our hypothesis that the inner ear of BWC is afflicted by a disorder similar to Scheibe's dysplasia, the most common inner ear defect associated with congenital hearing loss in humans.

Cyclic AMP-independent inhibition of voltage-sensitive calcium channels by forskolin in PC12 cells

Forskolin has been used to stimulate adenylyl cyclase. However, we found that forskolin inhibited voltage-sensitive Ca2+ channels (VSCCs) in a cyclic AMP (cAMP)-independent manner in PC12 cells. Ca2+ influx induced by membrane depolarization with 70 mM K+ was inhibited when cells were preincubated with 10 microM forskolin. Almost maximum inhibitory effect on Ca2+ influx without any significant increase in cellular cAMP level was observed in PC12 cells exposed to forskolin for 1 min. In addition, the forskolin effect on Ca2+ influx was not affected by the presence of 2',5'-dideoxyadenosine, an inhibitor of adenylyl cyclase that reduces dramatically forskolin-induced cAMP production. 1,9-Dideoxyforskolin, an inactive analogue of forskolin, also inhibited approximately 80% of Ca2+ influx induced by 70 mM K+ without any increase in cAMP. The data suggest that forskolin and its analogue inhibit VSCCs in PC12 cells and that the inhibition is independent of cAMP generation.

Glycine and GABA influence binaural processing in the inferior colliculus of the mustache bat

1. The mammalian inferior colliculus contains large populations of binaural cells that are excited by stimulation of the contralateral ear and are inhibited by stimulation of the ipsilateral ear, and are called excitatory/inhibitory (EI) cells. Neurons with EI properties are initially created in the lateral superior olive (LSO), which, in turn, sends strong bilateral projections to the inferior colliculus. The questions that we address in this report are 1) whether the inhibition evoked by stimulation of the ipsilateral ear occurs at the inferior colliculus or whether it occurs in a lower nucleus, presumably the LSO; and 2) if the ipsilaterally evoked inhibition occurs at the inferior colliculus, is the inhibition a consequence of glycinergic innervation or is it a consequence of GABAergic innervation. To study these questions, we recorded from 61 EI neurons in the inferior colliculus of the mustache bat before and during the iontophoretic application of the glycine receptor antagonist, strychnine. We also tested the effects of the gamma-aminobutyric acid-A (GABAA) receptor antagonist, bicuculline, on 38 of the 61 neurons that were tested with strychnine. The main finding is that glycinergic or GABAergic inhibition, or both, contribute to the ipsilaterally evoked inhibition in approximately 50% of the EI neurons in the inferior colliculus. 2. Strychnine and bicuculline had different effects on the magnitude of the spike counts evoked by stimulation of the contralateral (excitatory) ear. On average, strychnine caused the maximum spike count evoked by contralateral stimulation to increase by only 23%. The relatively small effects of strychnine on response magnitude are in marked contrast to the effects of bicuculline, which usually caused much larger increases in spike counts. For example, although strychnine caused spike counts to more than double in approximately 25% of the collicular neurons, bicuculline caused a doubling of the spike count in approximately 60% of the cells. 3. The inhibitory influences of ipsilateral stimulation were evaluated by driving the neurons with a fixed intensity at the contralateral ear and then documenting the reductions in spike counts due to the presentation of progressively higher intensities at the ipsilateral ear. In 64% of the neurons sampled, blocking glycinergic inhibition with strychnine had little or no effect on the ipsilaterally evoked inhibition. These cells remained as strongly inhibited during the application of strychnine as they did before its application. In addition, the ipsilateral intensity that produced complete or nearly complete spike suppression in the predrug condition was also unchanged by strychnine. 4. In 36% of the neurons, strychnine markedly reduced the degree of ipsilaterally evoked spike suppression. In five of these neurons, there was a complete elimination of the ipsilateral inhibition: these neurons were transformed from strongly inhibited EI neurons into monaural neurons. 5. The influence of both strychnine and bicuculline was tested sequentially in 38 neurons. In about one-half of these cells, (53%, 20/38) the ipsilaterally evoked inhibition was unaffected by either drug. In 10 other units (26%), both drugs substantially reduced or eliminated the ipsilaterally evoked inhibition. In most of these cells, both bicuculline and strychnine reduced the ipsilaterally evoked inhibition to a similar degree. In the remaining eight cells studied with both drugs (21%), the ipsilaterally evoked inhibition was reduced or eliminated by one of the drugs, but not by both. 6. These results show that both glycinergic and GABAergic projections influence the ipsilaterally evoked inhibition in about one-half of the EI neurons in the inferior colliculus. The glycinergic inhibition elicited by ipsilateral stimulation is most likely due to projections from the ipsilateral lateral superior olive, whereas the GABAergic inhibition evoked by ipsilateral stimulation is most likely caused b

Belgian Waterslager canaries are afflicted by Scheibe's-like dysplasia

Behavioral investigations of Belgian Waterslager canaries (BWCs) have demonstrated a congenital hearing impairment that primarily affects high frequencies. Research into the surface anatomy of the basilar papilla of these birds has pointed to the hair cells as the site of the lesion. Given that the basilar papilla and the vestibular organs both develop from the otocyst, we were interested in ascertaining whether the vestibular sensory epithelia also displayed abnormal hair cells. The inner ear of adult BWCs was examined by scanning electron microscopy. As expected, hair cells in the basilar papilla of BWCs were abnormal. As for the vestibular parenchyma, abnormal hair cells were detected in only one structure: the sacculus. Morphological abnormalities of the cochlea and sacculus are pathognomonic signs of Scheibe's dysplasia, the most common inner ear defect associated with congenital hearing loss. Our results suggest that BWCs are afflicted by this genetic disorder.

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)

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.

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)

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.

Sound localization in small birds: absolute localization in azimuth

Nine small birds of 3 species (Melopsittacus undulatus, Serinus canarius, and Poephila guttata) were trained in an operant procedure to fly to sound sources for food reward. The angle between the 2 sound sources was varied on a session-by-session basis, and threshold (i.e., minimum resolvable angle) was taken as the angle that corresponded to a performance level of 75% correct. In all, thresholds were calculated for pure tones of 5 different frequencies, noise bands of 3 different spectral compositions, and species-specific contact or distance calls recorded from each of the 3 species. Thresholds for both simple and complex stimuli were larger than 25 degrees. There were statistically significant species differences for each stimulus set, but these differences were not correlated with species differences in head size. Birds with 1 ear plugged performed as well as binaural birds in this task. Birds deafened in 1 ear, however, performed at chance.

Perceptual organization of acoustic stimuli by budgerigars (Melopsittacus undulatus): II. Vocal signals

Operant conditioning and multidimensional scaling procedures were used to study auditory perception of complex sounds in the budgerigar. In a same-different discrimination task, budgerigars learned to discriminate among natural vocal signals. Multidimensional scaling procedures were used to arrange these complex acoustic stimuli in a two-dimensional space reflecting perceptual organization. Results show that budgerigars group vocal stimuli according to functional and acoustical categories. Studies with only contact calls show that birds also make within-category discriminations. The acoustic cues in contact calls most salient to budgerigars appear to be quite complex. There is a suggestion that the sex of the signaler may also be encoded in these calls. The results from budgerigars were compared with the results from humans tested on some of the same sets of complex sounds.

Perceptual organization of acoustic stimuli by budgerigars (Melopsittacus undulatus): I. Pure tones

A new combination of operant conditioning and psychophysical scaling procedures was used to study auditory perception in a small bird. In a same-different discrimination task, budgerigars learned to discriminate among pure tones that varied along one or more acoustic dimensions. Response latencies were used to generate a matrix of interstimulus similarities. Multidimensional scaling procedures were used to arrange these acoustic stimuli in a multidimensional space that supposedly reflects the bird's perceptual organization. For tones that varied in intensity, duration, and frequency simultaneously, budgerigars were much more sensitive to frequency changes. From a set of tones that varied only in intensity, it was possible to calculate the growth of loudness with intensity for the budgerigar. For tones that varied only in frequency, budgerigars showed evidence of an "acoustic fovea" for frequency change in the spectral region of 2-4 kHz. Budgerigars and humans also differed in their perceptual grouping of tone sequences that rise, fall, or remain constant in pitch. Surprisingly, budgerigars were much less responsive to pitch contour than were humans.

Perception of species-specific contact calls by budgerigars (Melopsittacus undulatus)

Three budgerigars were trained with operant techniques to discriminate examples of species-specific contact calls. Budgerigars were also tested on vocalizations from another avian species--the canary (Serinus canarius). Budgerigars showed an equivalent ability to discriminate and remember both budgerigar and canary calls. Additional tests showed that both temporal and spectral cues were important in the discrimination of species-specific calls by the budgerigar. However, spectral cues occurring in the region of 2.0-4.0 kHz appear to be critical for the discrimination. These experiments support the notion of a generalized, but highly sophisticated, perceptual learning system in the budgerigar for the processing of vocal signals.