Doppler-shifted Constant Frequency Research Articles

Hemispheric and Sex Differences in Mustached Bat Primary Auditory Cortex Revealed by Neural Responses to Slow Frequency Modulations.

The mustached bat (Pteronotus parnellii) is a mammalian model of cortical hemispheric asymmetry. In this species, complex social vocalizations are processed preferentially in the left Doppler-shifted constant frequency (DSCF) subregion of primary auditory cortex. Like hemispheric specializations for speech and music, this bat brain asymmetry differs between sexes (i.e., males>females) and is linked to spectrotemporal processing based on selectivities to frequency modulations (FMs) with rapid rates (>0.5 kHz/ms). Analyzing responses to the long-duration (>10 ms), slow-rate (<0.5 kHz/ms) FMs to which most DSCF neurons respond may reveal additional neural substrates underlying this asymmetry. Here, we bilaterally recorded responses from 176 DSCF neurons in male and female bats that were elicited by upward and downward FMs fixed at 0.04 kHz/ms and presented at 0-90 dB SPL. In females, we found inter-hemispheric latency differences consistent with applying different temporal windows to precisely integrate spectrotemporal information. In males, we found a substrate for asymmetry less related to spectrotemporal processing than to acoustic energy (i.e., amplitude). These results suggest that in the DSCF area, (1) hemispheric differences in spectrotemporal processing manifest differently between sexes, and (2) cortical asymmetry for social communication is driven by spectrotemporal processing differences and neural selectivities for amplitude.

Neural mechanism of binding amplitude information of echo sound with its frequency one in echolocating bat

Most species of bats making echolocation use the sound pressure level (SPL) and Doppler-shifted frequency of ultrasonic echo pulse to measure the size and velocity of target. The neural circuits for detecting these target features are specialized for amplitude and frequency analysis of the second harmonic constant frequency (CF2) component of Doppler-shifted echoes. The neuronal circuits involved in detecting these echo features have been well known [1]. In natural situation, large natural objects in environment, like bushes or trees, produce complex stochastic echoes, which can be characterized by the echo roughness. The echo signal reflecting from a target insect is embedded in the complex signal. Even in such an environment, bats can detect accurately the detailed information of flying insect. We consider here two questions: how the neural circuits bind amplitude and frequency information of echo sound, and how bat distinguishes target information from information of background signal. To address these issues, we developed a neural network model for detecting SPL amplitude and Doppler-shifted frequency of echo sound. The model contains two hemispheres, in each of which, the network model consists of cochlea (Ch), inferior colliculus (IC), and Doppler-shifted constant frequency (DSCF) processing area. The Ch network has a frequency map by which sound frequency is encoded. The model for detecting frequency information of echo sound was based on the model previously presented by us [2].The SPL amplitude is encoded in to firing rate of IC neurons. The IC neurons encode SPL amplitude by means of a balance between excitatory connection from contralateral Ch neurons and inhibitory connection from ipsilateral ones, and then combine the amplitude and frequency information of echo sound. The DSCF network has two types of sub-networks detecting AC and DC components of echo sound, which represent the information of target and background signals, respectively. We showed that in IC, the amplitude information of echo sound is encoded by integrating the outputs of ipsi and contralateral Ch neurons and then combined with the Doppler-shifted frequency information encoded by tonotopical map of Ch neurons. The accuracy of the amplitude and frequency information was improved in the DSCF area. The model reproduced well several experimental results observed in IC and DSCF neurons. We also showed that AC and DC components of echo signal are discriminated in the two sub-networks of DSCF. The discrimination ability is due to the difference in time constant between DSCF neurons in the two sub-networks.

Sex-dependent hemispheric asymmetries for processing frequency-modulated sounds in the primary auditory cortex of the mustached bat.

Species-specific vocalizations of mammals, including humans, contain slow and fast frequency modulations (FMs) as well as tone and noise bursts. In this study, we established sex-specific hemispheric differences in the tonal and FM response characteristics of neurons in the Doppler-shifted constant-frequency processing area in the mustached bat's primary auditory cortex (A1). We recorded single-unit cortical activity from the right and left A1 in awake bats in response to the presentation of tone bursts and linear FM sweeps that are contained within their echolocation and/or communication sounds. Peak response latencies to neurons' preferred or best FMs were significantly longer on the right compared with the left in both sexes, and in males this right-left difference was also present for the most excitatory tone burst. Based on peak response magnitudes, right hemispheric A1 neurons in males preferred low-rate, narrowband FMs, whereas those on the left were less selective, responding to FMs with a variety of rates and bandwidths. The distributions of parameters for best FMs in females were similar on the two sides. Together, our data provide the first strong physiological support of a sex-specific, spectrotemporal hemispheric asymmetry for the representation of tones and FMs in a nonhuman mammal. Specifically, our results demonstrate a left hemispheric bias in males for the representation of a diverse array of FMs differing in rate and bandwidth. We propose that these asymmetries underlie lateralized processing of communication sounds and are common to species as divergent as bats and humans.

Neural Coding of Call Pitch and Syllable Type in the Auditory Cortex of Mustached Bats

Neural Coding of Call Pitch and Syllable Type in the Auditory Cortex of Mustached Bats

Right–left asymmetry in the cortical processing of sounds for social communication vs. navigation in mustached bats

In the Doppler-shifted constant frequency processing area in the primary auditory cortex of mustached bats, Pteronotus parnellii, neurons respond to both social calls and to echolocation signals. This multifunctional nature of cortical neurons creates a paradox for simultaneous processing of two behaviorally distinct categories of sound. To test the possibility of a stimulus-specific hemispheric bias, single-unit responses were obtained to both types of sounds, calls and pulse-echo tone pairs, from the right and left auditory cortex. Neurons on the left exhibited only slightly higher peak response magnitudes for their respective best calls, but they showed a significantly higher sensitivity (lower response thresholds) to calls than neurons on the right. On average, call-to-tone response ratios were significantly higher for neurons on the left than for those on the right. Neurons on the right responded significantly more strongly to pulse-echo tone pairs than those on the left. Overall, neurons in males responded to pulse-echo tone pairs with a much higher spike count compared to females, but this difference was less pronounced for calls. Multidimensional scaling of call responses yielded a segregated representation of call types only on the left. These data establish for the first time, a behaviorally directed right-left asymmetry at the level of single cortical neurons. It is proposed that a lateralized cortex emerges from multiparametric integration (e.g. combination-sensitivity) within a neuron and inhibitory interactions between neurons that come into play during the processing of complex sounds.

GABAergic organization of the auditory cortex in the mustached bat (Pteronotus p. parnellii)

The structure and distribution of neurons and axon terminals (puncta) immunostained for gamma-aminobutyric acid (GABA) in the parietotemporal neocortex of the mustached bat (Pteronotus p. parnellii) was studied. The types of GABAergic neurons and puncta (putative terminals) were analyzed, and the immunocytochemical patterns were compared to those in cat auditory cortex (AC). The classic map of mustached bat primary auditory cortex (AI) corresponds to a belt of granular six-layered cortex on the temporal convexity. This area encompasses the Doppler-shifted constant frequency 60 kHz domain (DSCF) described in physiological investigations, as well as its flanking, low-frequency, posterior field (AIp) and the anterior high-frequency region (AIa). Many types of GABAergic neurons correspond to those in cat primary AC. However, the bat had a significantly lower proportion of such cells in five of the six layers. The classes of GABAergic neurons in most layers were small, medium-sized, and large multipolar cells, and bipolar and bitufted neurons. Types found in only one or two layers included horizontal cells (layers I and VI) or extraverted multipolar neurons (layer II). Only layer IV had comparable percentages (∼26%), suggesting that the GABAergic influence on lemniscal thalamocortical input is conserved phylogenetically. While the cellular basis for GABAergic cortical processing may reflect shared neural circuits and common modes of inhibitory processing, laminar differences could underlie adaptations specific to microchioptera.

Postnatal Maturation of Primary Auditory Cortex in the Mustached Bat, Pteronotus parnellii

The primary auditory cortex (AI) of adult Pteronotus parnellii features a foveal representation of the second harmonic constant frequency (CF2) echolocation call component. In the corresponding Doppler-shifted constant frequency (DSCF) area, the 61 kHz range is over-represented for extraction of frequency-shift information in CF2 echoes. To assess to which degree AI postnatal maturation depends on active echolocation or/and reflects ongoing cochlear maturation, cortical neurons were recorded in juveniles up to postnatal day P29, before the bats are capable of active foraging. At P1-2, neurons in posterior AI are tuned sensitively to low frequencies (22-45 dB SPL, 28-35 kHz). Within the prospective DSCF area, neurons had insensitive responses (>60 dB SPL) to frequencies <40 kHz and lacked sensitive tuning curve tips. Up to P10, when bats do not yet actively echolocate, tonotopy is further developed and DSCF neurons respond to frequencies of 51-57 kHz with maximum tuning sharpness (Q(10dB)) of 57. Between P11 and 20, the frequency representation in AI includes higher frequencies anterior and dorsal to the DSCF area. More multipeaked neurons (33%) are found than at older age. In the oldest group, DSCF neurons are tuned to frequencies close to 61 kHz with Q(10dB) values < or =212, and threshold sensitivity, tuning sharpness and cortical latencies are adult-like. The data show that basic aspects of cortical tonotopy are established before the bats actively echolocate. Maturation of tonotopy, increase of tuning sharpness, and upward shift in the characteristic frequency of DSCF neurons appear to strongly reflect cochlear maturation.

DSCF neurons within the primary auditory cortex of the mustached bat process frequency modulations present within social calls.

Neurons in the Doppler-shifted constant frequency processing (DSCF) area in the primary auditory cortex of mustached bats, Pteronotus parnellii, are multifunctional, responding both to echolocation and communication sounds. Simultaneous presentation of a DSCF neuron's best low and high frequencies (BF(low) and BF(high), respectively) facilitates its response. BF(low) corresponds to a frequency in the frequency-modulated (FM) component of the first harmonic in the echolocation pulse, and BF(high) corresponds to the constant frequency (CF) component in the second harmonic of the echo. We systematically varied the slopes, bandwidths, and central frequencies of FMs traversing the BF(high) region to arrive at the "best FM" for single DSCF neurons. We report that nearly half (46%) of DSCF neurons preferred linear FMs to CFs and average response magnitude to FMs was not significantly less (P = 0.08) than that to CFs at BF(high) when each test stimulus was paired with a CF at BF(low). For linear FMs ranging in slope from 0.04 to 4.0 kHz/ms and in bandwidth from 0.44 to 7.88 kHz, the majority of DSCF neurons preferred upward (55%) to downward (21%) FMs. Central frequencies of the best FMs were typically close to but did not always match a neuron's BF(high). Neurons exhibited combination-sensitivity to "call fragments" (calls that were band-pass filtered in the BF(high) region) paired with their BF(low). Our data show a close match between the modulation direction of a neuron's best FM and that of its preferred call fragment. These response properties show that DSCF neurons extract multiple parameters of FMs and are specialized for processing both FMs for communication and CFs for echolocation.

Asymmetry in corticofugal modulation of frequency-tuning in mustached bat auditory system.

Focal electric stimulation of the auditory cortex is well suited for exploration of the function of the corticofugal (descending) system and the neural mechanism of plasticity in the central auditory system, because it evokes changes in frequency-tuning, called best frequency (BF) shifts, as does auditory fear conditioning. The Doppler-shifted constant frequency (DSCF) area of the primary auditory cortex of the mustached bat is highly specialized for fine frequency analysis. Focal electric stimulation of the DSCF area evokes the BF shifts of ipsilateral cortical and collicular neurons away from the BF of stimulated neurons, whereas the stimulation evokes the BF shifts of contralateral cortical and collicular neurons either toward or away from the stimulated BF. The direction of contralateral BF shifts shows a flip-flop, depending on the spatial relationship between the stimulated and recorded neurons. This asymmetry in corticofugal modulation is mostly, if not totally, created by two subdivisions of the stimulated DSCF area that transmit signals to the contralateral DSCF area, presumably through the corpus callosum. This intriguing asymmetry in corticofugal modulation presumably functions for equalization of the reorganization of the frequency maps of the DSCF areas and subcortical auditory nuclei on both sides.

Lateral Inhibition for Center-Surround Reorganization of the Frequency Map of Bat Auditory Cortex

Repetitive acoustic stimulation, auditory fear conditioning, and focal electric stimulation of the auditory cortex (AC) each evoke the reorganization of the central auditory system. Our current study of the big brown bat indicates that focal electric stimulation of the AC evokes center-surround reorganization of the frequency map of the AC. In the center, the neuron's best frequencies (BFs), together with their frequency-tuning curves, shift toward the BFs of electrically stimulated cortical neurons (centripetal BF shifts). In the surround, BFs shift away from the stimulated cortical BF (centrifugal BF shifts). Centripetal BF shifts are much larger than centrifugal BF shifts. An antagonist (bicuculline methiodide) of inhibitory synaptic transmitter receptors changes centrifugal BF shifts into centripetal BF shifts, whereas its agonist (muscimol) changes centripetal BF shifts into centrifugal BF shifts. This reorganization of the AC thus depends on a balance between facilitation and inhibition evoked by focal cortical electric stimulation. Unlike neurons in the AC of the big brown bat, neurons in the Doppler-shifted constant-frequency (DSCF) area of the AC of the mustached bat are highly specialized for fine-frequency analysis and show almost exclusively centrifugal BF shifts for focal electric stimulation of the DSCF area. Our current data indicate that in the highly specialized area, lateral inhibition is strong compared with the less-specialized area and that the specialized and nonspecialized areas both share the same inhibitory mechanism for centrifugal BF shifts.

Reorganization of the cochleotopic map in the bat's auditory system by inhibition.

The central auditory system of the mustached bat shows two types of reorganization of cochleotopic (frequency) maps: expanded reorganization resulting from shifts in the best frequencies (BFs) of neurons toward the BF of repetitively stimulated cortical neurons (hereafter centripetal BF shifts) and compressed reorganization resulting from the BF shifts of neurons away from the BF of the stimulated cortical neurons (hereafter centrifugal BF shifts). Facilitation and inhibition evoked by the corticofugal system have been hypothesized to be respectively related to centripetal and centrifugal BF shifts. If this hypothesis is correct, bicuculline (an antagonist of inhibitory GABA-A receptors) applied to cortical neurons would change centrifugal BF shifts into centripetal BF shifts. In the mustached bat, electric stimulation of cortical Doppler-shifted constant-frequency neurons, which are highly specialized for frequency analysis, evokes the centrifugal BF shifts of ipsilateral collicular and cortical Doppler-shifted constant-frequency neurons and contralateral cochlear hair cells. Bicuculline applied to the stimulation site changed the centrifugal BF shifts into centripetal BF shifts. On the other hand, electric stimulation of neurons in the posterior division of the auditory cortex, which are not particularly specialized for frequency analysis, evokes centripetal BF shifts of cortical neurons located near the stimulated cortical neurons. Bicuculline applied to the stimulation site augmented centripetal BF shifts but did not change the direction of the shifts. These observations support the hypothesis and indicate that centripetal and centrifugal BF shifts are both based on a single mechanism consisting of two components: facilitation and inhibition.

Modulation of responses and frequency tuning of thalamic and collicular neurons by cortical activation in mustached bats.

In the Jamaican mustached bat, Pteronotus parnellii parnellii, one of the subdivisions of the primary auditory cortex is disproportionately large and over-represents sound at approximately 61 kHz. This area, called the Doppler-shifted constant frequency (DSCF) processing area, consists of neurons extremely sharply tuned to a sound at approximately 61 kHz. We found that a focal activation of the DSCF area evokes highly specific corticofugal modulation in the inferior colliculus and medial geniculate body. Namely a focal activation of cortical DSCF neurons tuned to, say, 61. 2 kHz with 0.2-ms-long, 100-nA electric pulses drastically increases the excitatory responses of thalamic and collicular neurons tuned to 61.2 kHz without shifting their best frequencies (BFs). However, it decreases the excitatory responses of subcortical neurons tuned to frequencies slightly higher or lower than 61.2 kHz and shifts their BFs away from 61.2 kHz. The BF shifts are symmetrical and centrifugal around 61.2 kHz. These corticofugal effects are larger on thalamic neurons than on collicular neurons. The cortical electrical stimulation sharpens the frequency-tuning curves of subcortical neurons. These corticofugal effects named "egocentric selection" last </=2.5 h after the cessation of a 7-min-long cortical electrical stimulation. In the mustached bat, corticofugal modulation serves to increase the contrast in neural representation of sound at approximately 61 kHz, which is an important component of an echo bearing velocity information. It is also most likely that the corticofugal system plays an important role in plasticity of the central auditory system. Another subdivision of the auditory cortex of the mustached bat is called the FM-FM area. This area consists of delay-tuned combination-sensitive neurons, called FM-FM neurons, and has the echo-delay axis for the systematic representation of target distances. A focal electric stimulation of the FM-FM area evokes changes in the responses of collicular and thalamic FM-FM neurons. These changes are basically the same as those described in the present paper. Therefore corticofugal modulation takes place for frequency domain analysis in exactly the same way as it does in time domain analysis.

Binaural and commissural organization of the primary auditory cortex of the mustached bat.

In the mustached bat, the primary auditory cortex (AI) can be divided into three subdivisions: the Doppler-shifted constant-frequency processing (DSCF) area, and the anterior (AIa) and posterior (AIp) regions. The DSCF area is composed of two subdivisions: excitatory-excitatory (E-E) and inhibitory-excitatory (I-E). The E-E division is located in the ventral portion of the DSCF area and mainly consists of neurons excited bilaterally, while the I-E division is located in the dorsal portion and mainly consists of neurons which are inhibited by ipsilateral ear stimuli, but excited by contralateral ear stimuli. The E-E division is bilaterally connected by commissural fibers, while the I-E division is not. The AIa and AIp regions have neither E-E neurons nor commissural connections. In the AI of the cat, E-E and I-E neurons form alternating bands which are parallel to the frequency axis. E-E bands are bilaterally connected by commissural fibers, but I-E bands are not. The DSCF area shares a similar functional organization with the AI of the cat.

Sharpening of Frequency Tuning by Inhibition in the Thalamic Auditory Nucleus of the Mustached Bat

Unlike the quasitriangular frequency-tuning curves of peripheral neurons, pencil- or spindle-shaped frequency-tuning curves (excitatory areas) have been found in the central auditory systems of many species of animals belonging to different classes. Inhibitory tuning curves (areas) are commonly found on both sides of such "level-tolerant" sharp frequency-tuning curves. However, it has not yet been examined whether sharpening of frequency tuning takes place in the medial geniculate body (MGB). We injected an inhibitory transmitter antagonist, bicuculline methiodide (BMI), into the MGB of the mustached bat to examine whether frequency tuning is sharpened by inhibition in the MGB and whether this sharpening, if any, occurs in addition to that performed in prethalamic auditory nuclei. Thirty-seven percent of thalamic Doppler-shifted constant frequency (DSCF) neurons mostly showing a level-tolerant frequency-tuning curve had an inhibitory area or areas. BMI changed the inhibitory areas of these neurons into excitatory areas, so that their excitatory frequency-tuning curves became broader. However, the BMI-broadened excitatory frequency-tuning curves were still much narrower than those of peripheral neurons. Our results indicate that level-tolerant frequency tuning of thalamic DSCF neurons is mostly created by prethalamic auditory nuclei and that it is further sharpened in 37% of thalamic DSCF neurons by lateral inhibition occurring in the MGB. The comparisons in sharpness (quality factors) of frequency-tuning curves between peripheral, thalamic, and cortical DSCF neurons indicate that the skirt portion of tuning curves is sharper in the above order, and that their tip portion is not significantly different between the peripheral and thalamic DSCF neurons, but significantly sharper in the cortical DSCF neurons than in the thalamic DSCF neurons. Therefore the central auditory system has inhibitory mechanisms for the progressive sharpening of frequency tuning. DSCF neurons in the primary auditory cortex were recently found to show facilitative responses to paired sounds. That is, they are combination sensitive. In the present studies, we found that thalamic DSCF neurons also showed facilitative responses to paired sounds. The responses of thalamic DSCF neurons to acoustic stimuli consisted of a slow and a fast component. BMI mainly increased the slow component and an excitatory transmitter antagonist, D-2-amino-5-phosphonovalerate mainly suppressed the slow component. Therefore the response pattern of these thalamic neurons is shaped by both gamma-aminobutyric acid-mediated inhibition and N-methyl-D-aspartate-mediated facilitation.

Combination-sensitive neurons in the primary auditory cortex of the mustached bat

In the mustached bat, Pteronotus parnellii, neurons in the primary auditory cortex (AI) have been thought to respond primarily to single frequencies, as in other mammals. However, neurons in the Doppler-shifted constant-frequency (DSCF) area, a part of the mustached bat's AI that contains an overrepresentation of the prominent CF2 component of the biosonar signal, were found to show facilitative responses to combinations of different frequencies in the pulse and echo. The essential components for facilitation were the pulse FM1 and the echo CF2. The FM1-CF2 facilitation was sensitive to echo delays, indicating that DSCF neurons respond better to targets within particular ranges. On average, the longest discriminable echo delay, based on increased impulse counts due to facilitation, corresponded to a target range of 4.3 m, and the most discriminable delay corresponded to a target 3.6 m distant. Since mustached bats first show a behavioral response to a target at a distance of 3-4 m, DSCF neurons are suited to signal the presence of an insect within this behaviorally important range. DSCF neurons were broadly tuned to echo delay, with the average minimum discriminable echo delay corresponding to a target range of 1.9 m, and the delay tuning of the neurons followed (tracked) changes in pulse duration, indicating that facilitation occurs during much of the approach phase of insect pursuit when target characterization is presumably occurring. These results show that AI neurons in the mustached bat are specialized to respond to complex, behaviorally relevant stimuli during the search and approach phases of insect pursuit.

Cortical Computational Maps Control Auditory Perception

Mustached bats orient and find insects by emitting ultrasonic pulses and analyzing the returning echoes. Neurons in the Doppler-shifted constant-frequency (DSCF) and frequency-modulated (FM-FM) areas of the auditory cortex form maps of echo frequency (target velocity) and echo delay (target range), respectively. Bats were trained to discriminate changes in echo frequency or delay, and then these areas were selectively inactivated with muscimol. Inactivation of the DSCF area disrupted frequency but not delay discriminations; inactivation of the FM-FM area disrupted delay but not frequency discriminations. Thus, focal inactivation of specific cortical maps produces specific disruptions in the perception of biosonar signals.

Frequency and amplitude representations in anterior primary auditory cortex of the mustached bat.

The orientation sound emitted by the Panamanian mustached bat, Pteronotus parnellii rubiginosus, consists of four harmonics. The third harmonic is 6-12 dB weaker than the predominant second harmonic and consists of a long constant-frequency component (CF3) at about 92 kHz and a short frequency-modulated component (FM3) sweeping from about 92 to 74 kHz. Our primary aim is to examine how CF3 and FM3 are represented in a region of the primary auditory cortex anterior to the Doppler-shifted constant-frequency (DSCF) area. Extracellular recordings of neuronal responses from the unanesthetized animal were obtained during free-field stimulation of the ears with pure tones. FM sounds, and signals simulating their orientation sounds and echoes. Response properties of neurons and tonotopic and amplitopic representations were examined in the primary and the anteroventral nonprimary auditory cortex. In the anterior primary auditory cortex, neurons responded strongly to single pure tones but showed no facilitative responses to paired stimuli. Neurons with best frequencies from 110 to 90 kHz were tonotopically organized rostrocaudally, with higher frequencies located more rostrally. Neurons tuned to 92-94 kHz were overpresented, whereas neurons tuned to sound between 64 and 91 kHz were rarely found. Consequently a striking discontinuity in frequency representation from 91 to 64 kHz was found across the anterior DSCF border. Most neurons exhibited monotonic impulse-count functions and responded maximally to sound pressure level (SPL). There were also neurons that responded best to weak sounds but unlike the DSCF area, amplitopic representation was not found. Thus, the DSCF area is quite unique not only in its extensive representation of frequencies in the second harmonic CF component but also in its amplitopic representation. The anteroventral nonprimary auditory cortex consisted of neurons broadly tuned to pure tones between 88 and 99 kHz. Neither tonotopic nor amplitopic representation was observed. Caudal to this area and near the anteroventral border of the DSCF area, a small cluster of FM-FM neurons sensitive to particular echo delays was identified. The responses of these neurons fluctuated significantly during repetitive stimulation.