Binaural and frequency representation in the primary auditory cortex of the big brown bat, Eptesicus fuscus

Social Communication in Big Brown Bats

Bats are social mammals that display a wide array of social communication calls. Among them, it is common for most bats species to emit distress, agonistic, appeasement and infant isolation calls. Big brown bats (Eptesicus fuscus) are no different: They are gregarious animals living in colonies that can comprise hundreds of individuals. These bats live in North America and, typically found roosting in man-made structures like barns and attics, are considered common. They are insectivorous laryngeal echolocators, and while their calls and associated brain mechanisms in echolocation are well-documented, much less is known about their neural systems for analyzing social vocalizations. In this work we review what we know about the social lives of big brown bats and propose how to consolidate the nomenclature used to describe their social vocalizations. Furthermore, we discuss the next steps in the characterization of the social structure of this species and how these studies will advance both research in neuroethology and ecology of big brown bats.

Binaural Response Properties and Sensitivity to Interaural Difference of Neurons in the Auditory Cortex of the Big Brown Bat, Eptesicus fuscus

This study examines binaural response properties and sensitivity to interaural level difference of single neurons in the primary auditory cortex (AC) of the big brown bat, Eptesicus fuscus under earphone stimulation conditions. Contralateral sound stimulation always evoked response from all 306 AC neurons recorded but ipsilateral sound stimulation either excited, inhibited or did not affect their responses. High best frequency (BF) neurons typically had high minimum threshold (MT) and low BF neurons had low MT. However, both BF and MT did not correlate with their recording depth. The BF of these AC neurons progressively changed from high to low along the anteromedial-posterolateral axis of the AC. Their number of impulses and response latency varied with sound level and inter-aural level differences (ILD). Their number of impulses typically increased either monotonically or non-monotonically to a maximum and the latency shortened to a minimum at a specific sound level. Among 205 AC neurons studied at varied ILD, 178 (87%) and 127 (62%) neurons discharged maximally and responded with the shortest response latency at a specific ILD, respectively. Neurons sequentially isolated within an orthogonal electrode puncture shared similar BF, MT, binaurality and ILD curves. However, the response latency of these AC neurons progressively shortened with recording depth. Species-specific difference among this bat, the mustached bat and the pallid bat is discussed in terms of frequency and binaurality representation in the AC.

Functional segregation of monaural and binaural selectivity in the pallid bat auditory cortex

Different fields of the auditory cortex can be distinguished by the extent and level tolerance of spatial selectivity. The mechanisms underlying the range of spatial tuning properties observed across cortical fields are unclear. Here, this issue was addressed in the pallid bat because its auditory cortex contains two segregated regions of response selectivity that serve two different behaviors: echolocation for obstacle avoidance and localization of prey-generated noise. This provides the unique opportunity to examine mechanisms of spatial properties in two functionally distinct regions. Previous studies have shown that spatial selectivity of neurons in the region selective for noise (noise-selective region, NSR) is level tolerant and shaped by interaural level difference (ILD) selectivity. In contrast, spatial selectivity of neurons in the echolocation region ('FM sweep-selective region' or FMSR) is strongly level dependent with many neurons responding to multiple distinct spatial locations for louder sounds. To determine the mechanisms underlying such level dependence, frequency, azimuth, rate-level responses and ILD selectivity were measured from the same FMSR neurons. The majority (∼75%) of FMSR neurons were monaural (ILD insensitive). Azimuth tuning curves expanded or split into multiple peaks with increasing sound level in a manner that was predicted by the rate-level response of neurons. These data suggest that azimuth selectivity of FMSR neurons depends more on monaural ear directionality and rate-level responses. The pallid bat cortex utilizes segregated monaural and binaural regions to process echoes and prey-generated noise. Together the pallid bat FMSR/NSR data provide mechanistic explanations for a broad range of spatial tuning properties seen across species.

Histaminergic modulation of nonspecific plasticity of the auditory system and differential gating

In the auditory system of the big brown bat (Eptesicus fuscus), paired conditioned tonal (CS) and unconditioned leg stimuli (US) for auditory fear conditioning elicit tone-specific plasticity represented by best-frequency (BF) shifts that are augmented by acetylcholine, whereas unpaired CS and US for pseudoconditioning elicit a small BF shift and prominent nonspecific plasticity at the same time. The latter represents the nonspecific augmentations of auditory responses accompanied by the broadening of frequency tuning and decrease in threshold. It is unknown which neuromodulators are important in evoking the nonspecific plasticity. We found that histamine (HA) and an HA3 receptor (HA3R) agonist (α-methyl-HA) decreased, but an HA3R antagonist (thioperamide) increased, cortical auditory responses; that the HA3R agonist applied to the primary auditory cortex before pseudoconditioning abolished the nonspecific augmentation in the cortex without affecting the small cortical BF shift; and that antagonists of acetylcholine, norepinephrine, dopamine, and serotonin receptors did not abolish the nonspecific augmentation elicited by pseudoconditioning. The histaminergic system plays an important role in eliciting the arousal and defensive behavior, possibly through nonspecific augmentation. Thus HA modulates the nonspecific augmentation, whereas acetylcholine amplifies the BF shifts. These two neuromodulators may mediate differential gating of cortical plasticity.

Modulation of amplitude sensitivity by bilateral collicular interaction among different frequency laminae

In the ascending auditory pathway, the commissure of the inferior colliculus (IC) interconnects the two ICs and may therefore mediate bilateral collicular interaction during sound processing. In this study, we show that electrically stimulates one IC produces facilitation or suppression of acoustically evoked response of neurons in the other IC. The facilitated IC neurons (14%) are located in bilateral corresponding frequency laminae while the suppressed IC neurons (86%) are widespreadly located in bilateral different frequency laminae. Whereas induced facilitation increases the dynamic range but decreases the slope of the rate-amplitude function of modulated IC neurons, induced suppression produces the opposite effect. As a result, bilateral collicular facilitation increases the sensitivity of modulated IC neurons to a wider range of sound amplitude while bilateral collicular suppression improves the sensitivity of modulated IC neurons to minor change in sound amplitude over a narrower range of sound amplitude. The degree of suppression is significantly greater for suppressed IC neurons located in bilateral corresponding frequency laminae than in non-corresponding frequency laminae. We suggest that bilateral collicular interaction through the commissure of the IC may play a role in modulation of amplitude sensitivity and in shaping the binaural property of IC neurons.

Comparison of auditory responses in the medial geniculate and pontine gray of the big brown bat, Eptesicus fuscus

The inferior colliculus has been well studied for its role of transmitting information from the brainstem to the thalamocortical system. However, it is also the source of a major pathway to the cerebellum, via the pontine gray (PG). We compared auditory responses from single neurons in the medial geniculate body (MGB) and PG of the awake big brown bat. MGB neurons were selective for a variety of stimulus types whereas PG neurons only responded to pure tones or simple FM sweeps. Best frequencies (BF) in MGB ranged from 8 kHz to > 80 kHz. BFs of PG neurons were all above 20 kHz with a high proportion above 60 kHz. The mean response latency was 19 ms for MGB neurons and 11 ms for PG neurons. MGB and PG contained neurons with a variety of discharge patterns but the most striking difference was the proportion of neurons with responses that lasted longer than the stimulus duration (MGB 13%, PG 58%). Both nuclei contained duration-sensitive neurons; the majority of those in MGB were band pass whereas in the PG they were long pass. Over half of the neurons in both nuclei were binaural. Differences between these nuclei are consistent with the idea that the thalamocortical pathway performs integration over time for cognitive analysis, thereby increasing selectivity and lengthening latency, while the colliculo-pontine pathway, which is more concerned with sensory-motor control, provides rapid input and a lasting trace of an auditory event.

Tone-specific and nonspecific plasticity of inferior colliculus elicited by pseudo-conditioning: role of acetylcholine and auditory and somatosensory cortices

Experience-dependent plasticity in the central sensory systems depends on activation of both the sensory and neuromodulatory systems. Sensitization or nonspecific augmentation of central auditory neurons elicited by pseudo-conditioning with unpaired conditioning tonal (CS) and unconditioned electric leg (US) stimuli is quite different from tone-specific plasticity, called best frequency (BF) shifts, of the neurons elicited by auditory fear conditioning with paired CS and US. Therefore the neural circuits eliciting the nonspecific augmentation must be different from that eliciting the BF shifts. We first examined plastic changes in the response properties of collicular neurons of the big brown bat elicited by pseudo-conditioning and found that it elicited prominent nonspecific augmentation-an auditory response increase, a frequency-tuning broadening, and a threshold decreas-and that, in addition, it elicited a small short-lasting BF shift only when the CS frequency was 5 kHz lower than the BF of a recorded neuron. We examined the role of acetylcholine and the auditory and somatosensory cortices in these collicular changes. The development of the nonspecific augmentation was affected little by a muscarinic acetylcholine receptor antagonist applied to the inferior colliculus and by a GABA(A) receptor agonist applied to the auditory or somatosensory cortex. However, these drugs abolished the small short-lasting BF shift as they abolished the large long-lasting cortical and short-lasting collicular BF shifts elicited by the conditioning. These results indicate that, different from the BF shift, the nonspecific augmentation of the inferior colliculus depends on neither the cholinergic neuromodulator nor the auditory and somatosensory cortices.

Tone-specific and nonspecific plasticity of the auditory cortex elicited by pseudoconditioning: role of acetylcholine receptors and the somatosensory cortex

Experience-dependent plastic changes in the central sensory systems are due to activation of both the sensory and neuromodulatory systems. Nonspecific changes of cortical auditory neurons elicited by pseudoconditioning are quite different from tone-specific changes of the neurons elicited by auditory fear conditioning. Therefore the neural circuit evoking the nonspecific changes must also be different from that evoking the tone-specific changes. We first examined changes in the response properties of cortical auditory neurons of the big brown bat elicited by pseudoconditioning with unpaired tonal (CS(u)) and electric leg (US(u)) stimuli and found that it elicited nonspecific changes to CS(u) (a heart-rate decrease, an auditory response increase, a broadening of frequency tuning, and a decrease in threshold) and, in addition, a small tone-specific change to CS(u) (a small short-lasting best-frequency shift) only when CS(u) frequency was 5 kHz lower than the best frequency of a recorded neuron. We then examined the effects of drugs on the cortical changes elicited by the pseudoconditioning. The development of the nonspecific changes was scarcely affected by atropine (a muscarinic cholinergic receptor antagonist) and mecamylamine (a nicotinic cholinergic receptor antagonist) applied to the auditory cortex and by muscimol (a GABAA-receptor agonist) applied to the somatosensory cortex. However, these drugs abolished the small short-lasting tone-specific change as they abolished the large long-lasting tone-specific change elicited by auditory fear conditioning. Our current results indicate that, different from the tone-specific change, the nonspecific changes depend on neither the cholinergic neuromodulator nor the somatosensory cortex.

Parallel thalamocortical pathways for echolocation and passive sound localization in a gleaning bat, Antrozous pallidus

We present evidence for parallel auditory thalamocortical pathways that serve two different behaviors. The pallid bat listens for prey-generated noise (5-35 kHz) to localize prey, while reserving echolocation [downward frequency-modulated (FM) sweeps, 60-30 kHz] for obstacle avoidance. Its auditory cortex contains a tonotopic map representing frequencies from 6 to 70 kHz. The high-frequency (BF > 30 kHz) representation is dominated by FM sweep-selective neurons, whereas most neurons tuned to lower frequencies prefer noise. Retrograde tracer injections into these physiologically distinct cortical regions revealed that the high-frequency region receives input from the suprageniculate (SG) nucleus, but not the ventral division of the medial geniculate body (MGBv), in all experiments (n = 9). In contrast, the low-frequency region receives tonotopically organized input from the MGBv in all experiments (n = 16). Labeling in the SG was observed in only two of these experiments. Both cortical regions also receive sparse inputs from medial (MGBm) and parts of the dorsal division (MGBd) outside the SG. These results show that the low- and high-frequency regions of a single tonotopic map receive dominant inputs from different thalamic divisions. Within the low-frequency region, most neurons are binaurally inhibited, and an orderly map of interaural intensity difference (IID) sensitivity is present. We show that the input to the IID map arises from topographically organized projections from the MGBv. As observed in other species, a frequency-dependent organization is observed in the lateromedial direction in the MGBv. These data demonstrate that MGBv-to-auditory cortex connections are organized with respect to both frequency and binaural selectivity.

Development of functional organization of the pallid bat auditory cortex

The primary auditory cortex is characterized by a tonotopic map and a clustered organization of binaural properties. The factors involved in the development of overlain representation of these two properties are unclear. We addressed this issue in the auditory cortex of the pallid bat. The adult pallid bat cortex contains a systematic relationship between best frequency (BF) and binaural properties. Most neurons with BF<30 kHz are binaurally inhibited (EO/I), while most neurons with BF>30 kHz are monaural (EO). As in other species, binaural properties are clustered. The EO/I cluster contains a systematic map of interaural intensity difference (IID) sensitivity. We asked if these properties are present at the time the bat acquires its full audible range (postnatal day [P] 15). Tonotopy, relationship between BF and binaural properties, and the map of IID sensitivity are adult-like at P15. However, binaural facilitation is only observed in pups older than P25. Frequency selectivity shows a BF-dependent sharpening during development. Thus, overlain representation of binaural properties and tonotopy in the pallid bat cortex is remarkably adult-like at an age when the full audible range is first present, suggesting an experience-independent development of overlapping feature maps.

Auditory cortical neurons in the mouse have salient best azimuths

Azimuth sensitivity of the primary auditory cortex in mammals is not well understood, and there is a debate as to whether the primary auditory cortex plays role in spatial hearing. We show for the first time, using single-unit recordings in the mouse primary auditory cortex, that auditory cortical neurons demonstrate a variety of azimuth-tuning functions and there are a majority of neurons showing salient best azimuths. The findings differ from those in the cat, ferret and monkey, and imply that there is some representation of auditory space in the mouse primary auditory cortex.

Corticofugal modulation of directional sensitivity in the midbrain of the big brown bat, Eptesicus fuscus

In our recent study of corticofugal modulation of collicular amplitude sensitivity of the big brown bat, Eptesicus fuscus, we suggested that the corticofugal modulation is based upon the best frequency (BF) differences and the relative amplitude sensitivity difference between collicular (IC) and cortical (AC) neurons but not the absolute amplitude sensitivity of IC and AC neurons. To show that corticofugal modulation is systematic and multiparametric, we studied corticofugal modulation of directional sensitivity in 89 corticofugally inhibited IC neurons in the same bat species under free field stimulation conditions. A neuron's directional sensitivity was expressed with the azimuthal range (AR) at 50% below the maximum of each directional sensitivity curve and the best azimuth (BAZ) at which the neuron discharged maximally. Cortical electrical stimulation did not affect the directional sensitivity of 40 (45%) neurons with BF(IC-AC) differences of 7.3+/-4.4kHz but sharpened the directional sensitivity of other 49 (55%) neurons with BF(IC-AC) differences of 2.3+/-1.8kHz. Corticofugal modulation sharpened directional sensitivity curves of IC neurons by decreasing the AR and shifting collicular BAZ toward cortical BAZ. The decrease in AR and the shift in BAZ increased significantly with AR(IC-AC) and BAZ(IC-AC) differences but not with absolute AR and BAZ of IC and AC neurons or BF(IC-AC) differences. Corticofual modulation also shifted collicular BF toward cortical BF. The shift in BF increased significantly with BF(IC-AC) differences but not with the BF of IC and AC neurons or BAZ shift. Consonant with our previous study, these data indicate that corticofugal modulation of collicular directional sensitivity is based on topographic projections between the IC and the AC and the difference in directional sensitivity but not the absolute directional sensitivity of IC or AC neurons.

Corticofugal feedback for collicular plasticity evoked by electric stimulation of the inferior colliculus

Focal electric stimulation of the auditory cortex, 30-min repetitive acoustic stimulation, and auditory fear conditioning each evoke shifts of the frequency-tuning curves [hereafter, best frequency (BF) shifts] of cortical and collicular neurons. The short-term collicular BF shift is produced by the corticofugal system and primarily depends on the relationship in BF between a recorded collicular and a stimulated cortical neuron or between the BF of a recorded collicular neuron and the frequency of an acoustic stimulus. However, it has been unknown whether focal electric stimulation of the inferior colliculus evokes the collicular BF shift and whether the collicular BF shift, if evoked, depends on corticofugal feedback. In our present research with the awake big brown bat, we found that focal electric stimulation of collicular neurons evoked the BF shifts of collicular neurons located near the stimulated ones; that there were two types of BF shifts: centripetal and centrifugal BF shifts, i.e., shifts toward and shifts away from the BF of stimulated neurons, respectively; and that the development of these collicular BF shifts was blocked by inactivation of the auditory cortex. Our data indicate that the collicular BF shifts (plasticity) evoked by collicular electric stimulation depended on corticofugal feedback. It should be noted that collicular BF shifts also depend on acetylcholine because it has been demonstrated that atropine (an antagonist of muscarinic acetylcholine receptors) applied to the IC blocks the development of collicular BF shifts.

Optical imaging of binaural interaction in multiple fields of the guinea pig auditory cortex

Locating the source of a sound is an important function of the auditory system and interaural intensity differences are one of the most important cues. To study the functional pathways of sound localisation processing in the auditory cortex, activity in multiple fields of the guinea pig auditory cortex during stimulation with interaural intensity differences was studied using optical imaging with a voltage-sensitive dye. Of the auditory core (primary and dorsocaudal) and the belt fields which surround them, the posterior and ventroposterior belt fields were the most sensitive to interaural intensity differences. This suggests that the caudal pathway of the auditory cortex is involved in sound localisation.

Corticofugal modulation of amplitude domain processing in the midbrain of the big brown bat, Eptesicus fuscus

Recent studies have shown that the corticofugal system systematically modulates and improves subcortical signal processing in the frequency, time and spatial domains. The present study examined corticofugal modulation of amplitude sensitivity of 113 corticofugally inhibited neurons in the central nucleus of the inferior colliculus (IC) of the big brown bat, Eptesicus fuscus. Cortical electrical stimulation decreased the number of impulses and increased the response latency of these neurons. They had an average of 5.9+/-4.4 kHz best frequency (BF) differences between collicular and electrically stimulated cortical neurons. Cortical electrical stimulation synchronized with sound stimulation for 30 min compressed the rate-amplitude functions of half (56, 49.6%) of these collicular neurons and shifted their minimum thresholds (MT) and dynamic ranges (DR) toward that of electrically stimulated cortical neurons for as long as 40 min. These collicular neurons had an average of 1.6+/-1.4 kHz BF differences. The shift in collicular MT and DR significantly increased with differences in MT and DR between collicular and cortical neurons. Cortical electrical stimulation also shifted the BF and best amplitude (BA) of collicular neurons toward that of cortical neurons. The BF shift increased with BF differences and the BA shift increased with BA differences. These data suggest that the corticofugal system modulates collicular responses on the basis of topographic projections between the IC and auditory cortex. However, corticofugal modulation of collicular amplitude sensitivity is primarily dependent upon the difference but not the absolute amplitude sensitivity between collicular and cortical neurons.

Binaural interaction revisited in the cat primary auditory cortex

The binaural interactions of neurons were studied in the primary auditory cortex (AI) of barbiturate-anesthetized cats with a matrix of binaural tonal stimuli varying in both interaural level differences (ILD) and average binaural level (ABL). The purpose of this study was to determine: 1) the distribution of preferred binaural combinations (PBCs) of a large population of neurons and its relationships with binaural interactions and binaural monotonicity; 2) whether monaural responses are predictive of binaural responses; and 3) whether there is a restricted set of representative binaural stimulus configurations that could effectively classify the binaural interactions. Binaural interactions were often diverse in the matrix and dependent on both ABL and ILD. Compared with previous studies, a higher proportion of mixed binaural interaction type and a lower proportion of EO/I type were found. No monaural neurons were found. Binaural responses often differed from monaural responses in the number of spikes and/or the form of the response functions. The PBCs of the majority of EO and PB neurons were in the contralateral field and midline, respectively. However, the PBCs of EE units were evenly distributed across the contralateral and ipsilateral fields. The majority of the nonmonotonic neurons responded most strongly to lower ABLs, whereas the majority of monotonic neurons responded most strongly to higher ABLs. This study demonstrated that in AI a restricted set of binaural stimulus configurations is not sufficient to reveal the binaural responses properties. Also, monaural responses are not predictive of binaural responses.

Characterisation of multiple physiological fields within the anatomical core of rat auditory cortex

The organisation and response properties of the rat auditory cortex were investigated with single and multi-unit electrophysiological recording. Two tonotopically organised 'core' fields, i.e. the primary (A1) and anterior (AAF) auditory fields, as well as three non-tonotopically organised 'belt' fields, i.e. the posterodorsal (PDB), dorsal (DB) and anterodorsal (ADB) belt fields, were identified. Compared to neurones in A1, units in AAF exhibited broader frequency tuning, as well as shorter minimum, modal and mean first spike latencies. In addition, units in AAF showed significantly higher thresholds and best SPLs, as well as broader dynamic ranges. Units in PDB, DB and ADB were characterised by strong responses to white noise and showed either poor or no responses to pure tones. The differences in response properties found between the core and belt fields may reflect a functional specificity in processing different features of auditory stimuli. The present study also combined microelectrode mapping with Nissl staining to determine if the physiological differences between A1 and AAF corresponded to cytoarchitectonically defined borders. Both A1 and AAF were located within temporal cortex 1 (Te1), with AAF occupying an anteroventral subdivision of Te1, indicating that the two neighbouring, physiologically distinct fields are cytoarchitectonically homogeneous.

Binaural interaction in the inferior colliculus of the big brown bat, Eptesicus fuscus

Binaural interaction plays an important role in shaping response properties of central auditory neurons. Using single unit recording and iontophoresis, we examined frequency tuning curves (FTCs), interaural intensity difference (IID) curves, and rate-intensity functions of inferior collicular (IC) neurons of the big brown bat, Eptesicus fuscus, under closed system or free field stimulation conditions. We isolated 46 EI (excitation-inhibition), 24 EO (monaural excitation) and 6 EE (excitation-excitation) neurons. Inhibitory FTCs of EI neurons plotted under ipsilateral sound stimulation fell within (n=10, 22%), partly overlapped (n=26, 56%), or almost entirely encompassed (n=10, 22%) excitatory FTCs plotted by contralateral sound stimulation. The discharge rate of EI neurons was a sigmoid function of IID. The peak discharge rate occurred at IIDs at which contralateral sound stimulation was stronger than ipsilateral sound stimulation. Application of bicuculline, an antagonist for gamma-aminobutyric acid A receptors, raised the IID curves and broadened the excitatory FTCs but partly or completely abolished the ipsilateral inhibitory FTCs. For EE neurons, excitatory FTCs and rate-intensity functions plotted by contralateral sound stimulation were always broader and higher than those plotted by ipsilateral sound stimulation. The sharpness of FTCs of EI neurons was significantly greater at ipsilateral 30 degrees than at 30 degrees contralateral. This direction-dependent frequency tuning was effectively abolished by occlusion of the ipsilateral ear. Possible mechanisms underlying these observations are discussed.

Functional organization of the pallid bat auditory cortex: emphasis on binaural organization

This report maps the organization of the primary auditory cortex of the pallid bat in terms of frequency tuning, selectivity for behaviorally relevant sounds, and interaural intensity difference (IID) sensitivity. The pallid bat is unusual in that it localizes terrestrial prey by passively listening to prey-generated noise transients (1-20 kHz), while reserving high-frequency (<30 kHz) echolocation for obstacle avoidance. The functional organization of its auditory cortex reflects the need for specializations in echolocation and passive sound localization. Best frequencies were arranged tonotopically with a general increase in the caudolateral to rostromedial direction. Frequencies between 24 and 32 kHz were under-represented, resulting in hypertrophy of frequencies relevant for prey localization and echolocation. Most neurons (83%) tuned <30 kHz responded preferentially to broadband or band-pass noise over single tones. Most neurons (62%) tuned >30 kHz responded selectively or exclusively to the 60- to 30-kHz downward frequency-modulated (FM) sweep used for echolocation. Within the low-frequency region, neurons were placed in two groups that occurred in two separate clusters: those selective for low- or high-frequency band-pass noise and suppressed by broadband noise, and neurons that showed no preference for band-pass noise over broadband noise. Neurons were organized in homogeneous clusters with respect to their binaural response properties. The distribution of binaural properties differed in the noise- and FM sweep-preferring regions, suggesting task-dependent differences in binaural processing. The low-frequency region was dominated by a large cluster of binaurally inhibited neurons with a smaller cluster of neurons with mixed binaural interactions. The FM sweep-selective region was dominated by neurons with mixed binaural interactions or monaural neurons. Finally, this report describes a cortical substrate for systematic representation of a spatial cue, IIDs, in the low-frequency region. This substrate may underlie a population code for sound localization based on a systematic shift in the distribution of activity across the cortex with sound source location.

Plasticity of the cochleotopic (frequency) map in specialized and nonspecialized auditory cortices

Auditory conditioning (associative learning) causes reorganization of the cochleotopic (frequency) maps of the primary auditory cortex (AI) and the inferior colliculus. Focal electric stimulation of the AI also evokes basically the same cortical and collicular reorganization as that caused by conditioning. Therefore, part of the neural mechanism for the plasticity of the central auditory system caused by conditioning can be explored by focal electric stimulation of the AI. The reorganization is due to shifts in best frequencies (BFs) together with shifts in frequency-tuning curves of single neurons. In the AI of the Mongolian gerbil (Meriones unguiculatus) and the posterior division of the AI of the mustached bat (Pteronotus parnellii), focal electric stimulation evokes BF shifts of cortical auditory neurons located within a 0.7-mm distance along the frequency axis. The amount and direction of BF shift differ depending on the relationship in BF between stimulated and recorded neurons, and between the gerbil and mustached bat. Comparison in BF shift between different mammalian species and between different cortical areas of a single species indicates that BF shift toward the BF of electrically stimulated cortical neurons (centripetal BF shift) is common in the AI, whereas BF shift away from the BF of electrically stimulated cortical neurons (centrifugal BF shift) is special. Therefore, we propose a hypothesis that reorganization, and accordingly organization, of cortical auditory areas caused by associative learning can be quite different between specialized and nonspecialized (ordinary) areas of the auditory cortex.

Bicuculline application affects discharge patterns, rate-intensity functions, and frequency tuning characteristics of bat auditory cortical neurons

This study examined the effect of bicuculline application on the auditory response properties in the auditory cortex of the big brown bat, Eptesicus fuscus. All auditory cortical neurons studied discharged either 1-2 or 3-7 impulses to 4 ms sound stimuli. Cortical neurons with high best frequencies tended to have high minimum thresholds. Bicuculline application increased the number of impulses and shortened the response latencies of all cortical neurons as well as changing the discharge patterns of half of the cortical neurons studied. Bicuculline application raised the rate-intensity functions but lowered the latency-intensity functions to varying degrees. Threshold-frequency tuning curves (FTCs) were either V-shaped, upper threshold or double-peaked. Threshold-FTCs and impulse-FTCs were mirror-images of each other. Bicuculline application expanded and raised the impulse-FTCs but lowered the threshold-FTCs, resulting in significantly decreased Q(n) values. Threshold-FTCs of cortical neurons determined within an orthogonally inserted electrode were very similar and expanded FTCs during bicuculline application were also very similar. Possible mechanisms for the contribution of GABAergic inhibition to shaping these response properties of cortical neurons are discussed.

Shapes and level tolerances of frequency tuning curves in primary auditory cortex: quantitative measures and population codes

The shape and level tolerance of the excitatory frequency/intensity tuning curves (eFTCs) of 160 cat primary auditory cortical (A1) neurons were investigated. Overall, A1 cells were characterized by tremendous variety in eFTC shapes and symmetries; eFTCs were U-shaped ( approximately 20%), V-shaped ( approximately 20%), lower-tail-upper-sharp ( approximately 15%), upper-tail-lower-sharp (<2%), slant-lower ( approximately 10%), slant-upper (<3%), multipeaked ( approximately 10%), and circumscribed ( approximately 20%). Quantitative analysis suggests that eFTC are best thought of as forming a continuum of shapes, rather than falling into discrete categories. A1 eFTCs tended to be more level tolerant than eFTCs from earlier stations in the ascending auditory system as inferred from other studies. While individual peaks of multipeaked eFTCs were similar to single peaked eFTCs, the overall eFTC of multipeaked neurons (spanning the range of all peaks) tended to have high-frequency tails. Measurements of shape and symmetry indicate that A1 eFTCs, on average, tended to have greater area on the low-frequency side of characteristic frequency (CF) than on the high-frequency side. A1 cells showed a relationship between CF and the inverse slope of low-frequency edges of eFTCs, but not for high-frequency edges. These data demonstrate that frequency tuning, particularly along the eFTC low-frequency border, sharpens along the lemniscal pathway to A1. The results are consistent with studies in mustached bats (Suga 1997) and support the idea that spectral decomposition along the ascending lemniscal pathway up to A1 is a general organizing principle of mammalian auditory systems. Altogether, these data suggest that A1 neurons' eFTCs are shaped by complex patterns of inhibition and excitation accumulating along the auditory pathways, implying that central rather than peripheral filtering properties are responsible for certain psychophysical phenomena.

Organisation of binaural interactions in the primary and dorsocaudal fields of the guinea pig auditory cortex

This study investigated the nature and topography of binaural interactions in the primary auditory field (AI) and dorsocaudal field (DC) of the urethane anaesthetised guinea pig auditory cortex. Single and multi-units were classified by their responses to monaural and binaural stimulation. In both AI and DC, units displayed binaural facilitation, binaural inhibition, or a level dependent mixture of facilitation and inhibition. There was a significant difference in the distribution of binaural response types between the two fields. Facilitated units predominated in DC (facilitated: 58%; inhibited: 24%; mixed: 6%; non-interacting: 12%), while inhibited units were the most common class in AI (facilitated: 15%; inhibited: 44%; mixed: 18%; non-interacting: 22%). It has previously been suggested that inhibited and facilitated units are concerned with processing different areas of space suggesting a possible separation of function between the two core fields. Topographically, the binaural response properties in AI and DC varied along isofrequency bands, with neurones displaying similar interactions aggregating in clusters. These clusters were similar in size for the two fields and often overlapped neighbouring isofrequency bands. However, their shape and position varied between different animals. This clustered organisation of binaural interactions is similar to that reported in recent studies of AI in other mammals.

Evidence for columnar organization in the auditory cortex of the mouse

Single cortical auditory neurons sequentially isolated within orthogonal electrode penetrations in the mouse were studied using tonal stimulation. They had common functional properties, such as firing pattern, best frequency, minimum threshold, sharpness of frequency tuning and onset latency. The finding suggests that there is columnar organization in the cortex.