Interaction between excitation and inhibition affects frequency tuning curve, response size and latency of neurons in the auditory cortex of the big brown bat, Eptesicus fuscus

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.

Focal electrical stimulation of dorsal nucleus of the lateral lemniscus modulates auditory response properties of inferior collicular neurons in the albino mouse

The inferior colliculus (IC) receives and integrates excitatory and inhibitory inputs from many bilateral lower auditory nuclei, intrinsic projections within IC, contralateral IC through the commissure of IC and from the auditory cortex (AC). These excitatory and inhibitory inputs from both ascending and descending auditory pathways contribute significantly to auditory response properties and temporal signal processing in IC. The present study examines the contribution of gamma-aminobutyric acid-ergic (GABAergic) inhibition of dorsal nucleus of the lateral lemniscus (DNLL) in influencing the response properties and amplitude sensitivity of contralateral IC neurons using focal electrical stimulation of contralateral DNLL and by the application of bicuculline to the recording site of modulated IC neurons. Focal electrical stimulation of contralateral DNLL produces inhibition (78.1%), facilitation (7.1%) or no effect (14.8%) in the number of spikes, firing duration and the first-spike latency of modulated IC neurons. The degree of modulation is inversely correlated to the difference in best frequency (BF) between electrically stimulated DNLL neurons and modulated IC neurons (p < 0.01). The application of bicuculline to the recording site of modulated IC neurons abolishes the inhibitory effect of focal electrical stimulation of DNLL neurons. DNLL inhibition also modulates the amplitude sensitivity of IC neurons by changing the dynamic range (DR) and the slope of rate-amplitude function (RAF) of modulated IC neurons. Possible biological significance of these findings in relation to auditory signal processing is discussed.

Late maturation of backward masking in auditory cortex

Speech perception relies on the accurate resolution of brief, successive sounds that change rapidly over time. Deficits in the perception of such sounds, indicated by a reduced ability to detect signals during auditory backward masking, strongly relate to language processing difficulties in children. Backward masking during normal development has a longer maturational trajectory than many other auditory percepts, implicating the involvement of central auditory neural mechanisms with protracted developmental time courses. Despite the importance of this percept, its neural correlates are not well described at any developmental stage. We therefore measured auditory cortical responses to masked signals in juvenile and adult Mongolian gerbils and quantified the detection ability of individual neurons and neural populations in a manner comparable with psychoacoustic measurements. Perceptually, auditory backward masking manifests as higher thresholds for detection of a short signal followed by a masker than for the same signal in silence. Cortical masking was driven by a combination of suppressed responses to the signal and a reduced dynamic range available for signal detection in the presence of the masker. Both coding elements contributed to greater masked threshold shifts in juveniles compared with adults, but signal-evoked firing suppression was more pronounced in juveniles. Neural threshold shifts were a better match to human psychophysical threshold shifts when quantified with a longer temporal window that included the response to the delayed masker, suggesting that temporally selective listening may contribute to age-related differences in backward masking. NEW & NOTEWORTHY In children, auditory detection of backward masked signals is immature well into adolescence, and detection deficits correlate with problems in speech processing. Our auditory cortical recordings reveal immature backward masking in adolescent animals that mirrors the prolonged development seen in children. This is driven by both signal-evoked suppression and dynamic range reduction. An extended window of analysis suggests that differences in temporally focused listening may contribute to late maturing thresholds for backward masked signals.

Auditory cortex of the marmoset monkey - complex responses to tones and vocalizations under opiate anaesthesia in core and belt areas

Many anaesthetics commonly used in auditory research severely depress cortical responses, particularly in the supragranular layers of the primary auditory cortex and in non-primary areas. This is particularly true when stimuli other than simple tones are presented. Although awake preparations allow better preservation of the neuronal responses, there is an inherent limitation to this approach whenever the physiological data need to be combined with histological reconstruction or anatomical tracing. Here we tested the efficacy of an opiate-based anaesthetic regime to study physiological responses in the primary auditory cortex and middle lateral belt area. Adult marmosets were anaesthetized using a combination of sufentanil (8 μg/kg/h, i.v.) and N2 O (70%). Unit activity was recorded throughout the cortical layers, in response to auditory stimuli presented binaurally. Stimuli consisted of a battery of tones presented at different intensities, as well as two marmoset calls ('Tsik' and 'Twitter'). In addition to robust monotonic and non-monotonic responses to tones, we found that the neuronal activity reflected various aspects of the calls, including 'on' and 'off' components, and temporal fluctuations. Both phasic and tonic activities, as well as excitatory and inhibitory components, were observed. Furthermore, a late component (100-250 ms post-offset) was apparent. Our results indicate that the sufentanil/N2 O combination allows better preservation of response patterns in both the core and belt auditory cortex, in comparison with anaesthetics usually employed in auditory physiology. This anaesthetic regime holds promise in enabling the physiological study of complex auditory responses in acute preparations, combined with detailed anatomical and histological investigation.

Chronic reduction in inhibition reduces receptive field size in mouse auditory cortex

Inhibitory interneurons regulate the responses of cortical circuits. In auditory cortical areas, inhibition from these neurons narrows spectral tuning and shapes response dynamics. Acute disruptions of inhibition expand spectral receptive fields. However, the effects of long-term perturbations of inhibitory circuitry on auditory cortical responses are unknown. We ablated ~30% of dendrite-targeting cortical inhibitory interneurons after the critical period by studying mice with a conditional deletion of Dlx1. Following the loss of interneurons, baseline firing rates rose and tone-evoked responses became less sparse in auditory cortex. However, contrary to acute blockades of inhibition, the sizes of spectral receptive fields were reduced, demonstrating both higher thresholds and narrower bandwidths. Furthermore, long-latency responses at the edge of the receptive field were absent. On the basis of changes in response dynamics, the mechanism for the reduction in receptive field size appears to be a compensatory loss of cortico-cortically (CC) driven responses. Our findings suggest chronic conditions that feature changes in inhibitory circuitry are not likely to be well modeled by acute network manipulations, and compensation may be a critical component of chronic neuronal conditions.

Is the effect of tinnitus on auditory steady-state response amplitude mediated by attention?

Objectives: Auditory steady-state response (ASSR) amplitude enhancement effects have been reported in tinnitus patients. As ASSR amplitude is also enhanced by attention, the effect of tinnitus on ASSR amplitude could be interpreted as an effect of attention mediated by tinnitus. As N1 attention effects are significantly larger than those on the ASSR, if the effect of tinnitus on ASSR amplitude were due to attention, there should be similar amplitude enhancement effects in tinnitus for the N1 component of the auditory-evoked response. Methods: MEG recordings which were previously examined for the ASSR (Diesch et al., 2010a) were analyzed with respect to the N1m component. Like the ASSR previously, the N1m was analyzed in the source domain (source space projection). Stimuli were amplitude-modulated (AM) tones with one of three carrier frequencies matching the tinnitus frequency or a surrogate frequency 1½ octave above the audiometric edge frequency in controls, the audiometric edge frequency, and a frequency below the audiometric edge. Single AM-tones were presented in a single condition and superpositions of three AM-tones differing in carrier and modulation frequency in a composite condition. Results: In the earlier ASSR study (Diesch et al., 2010a), the ASSR amplitude in tinnitus patients, but not in controls, was significantly larger in the (surrogate) tinnitus condition than in the edge condition. Patients showed less evidence than controls of reciprocal inhibition of component ASSR responses in the composite condition. In the present study, N1m amplitudes elicited by stimuli located at the audiometric edge and at the (surrogate) tinnitus frequency were smaller than N1m amplitudes elicited by sub-edge tones both in patients and controls. The relationship of the N1m response in the composite condition to the N1m response in the single condition indicated that reciprocal inhibition among component N1m responses was reduced in patients compared against controls. Conclusions: In the present study, no evidence was found for an N1-amplitude enhancement effect in tinnitus. Compared to controls, reciprocal inhibition is reduced in tinnitus patients. Thus, as there is no effect on N1m that could potentially be attributed to attention, it seems unlikely that the enhancement effect of tinnitus on ASSR amplitude could be accounted for in terms of attention induced by tinnitus.

Contribution of inhibition to stimulus selectivity in primary auditory cortex of awake primates

Recent studies have demonstrated the high selectivity of neurons in primary auditory cortex (A1) and a highly sparse representation of sounds by the population of A1 neurons in awake animals. However, the underlying receptive field structures that confer high selectivity on A1 neurons are poorly understood. The sharp tuning of A1 neurons' excitatory receptive fields (RFs) provides a partial explanation of the above properties. However, it remains unclear how inhibitory components of RFs contribute to the selectivity of A1 neurons observed in awake animals. To examine the role of the inhibition in sharpening stimulus selectivity, we have quantitatively analyzed stimulus-induced suppressive effects over populations of single neurons in frequency, amplitude, and time in A1 of awake marmosets. In addition to the well documented short-latency side-band suppression elicited by masking tones around the best frequency (BF) of a neuron, we uncovered long-latency suppressions caused by single-tone stimulation. Such long-latency suppressions also included monotonically increasing suppression with sound level both on-BF and off-BF, and persistent suppression lasting up to 100 ms after stimulus offset in a substantial proportion of A1 neurons. The extent of the suppression depended on the shape of a neuron's frequency-response area ("O" or "V" shaped). These findings suggest that the excitatory RF of A1 neurons is cocooned by wide-ranging inhibition that contributes to the high selectivity in A1 neurons' responses to complex stimuli. Population sparseness of the tone-responsive A1 neuron population may also be a consequence of this pervasive inhibition.

Interaction among the components of multiple auditory steady-state responses: enhancement in tinnitus patients, inhibition in controls

Amplitude and phase of steady-state signals recorded in response to amplitude-modulated (AM) sine tones vary over time, suggesting that the steady-state response (SSR) reflects not only stimulus input but also its interaction with other input streams or internally generated signals. Alterations of the interaction between simultaneous SSRs associated with tinnitus were studied by recording the magnetic field evoked by AM-tones with one of three carrier and one of three modulation frequencies. Single AM-tones were presented in single presentation mode and superpositions of three AM-tones differing in carrier and modulation frequency in multiple presentation mode. Modulation frequency-specific SSR components were recovered by bandpass filtering. Compared with single mode, in multiple mode SSR amplitude was reduced in healthy controls, but increased in tinnitus patients. Thus, while in controls multiple response components seem to reciprocally inhibit one another, in tinnitus reciprocal facilitation seems to predominate. Reciprocal inhibition was unrelated to the phase coherence among SSR components, but was correlated with the frequency of phase slips, indicating that the lateral interaction among SSR components acts in a quasi-paroxysmal manner and manifests itself in terms of a random train of phase reset events. Phase slips were more frequent in patients than controls both in single and multiple mode. Together, these findings indicate that lateral or surround inhibition of single units in auditory cortex is reduced and suggest that in-field inhibition is increased in tinnitus.

Linking the response properties of cells in auditory cortex with network architecture: cotuning versus lateral inhibition

The frequency-intensity receptive fields (RF) of neurons in primary auditory cortex (AI) are heterogeneous. Some neurons have V-shaped RFs, whereas others have enclosed ovoid RFs. Moreover, there is a wide range of temporal response profiles ranging from phasic to tonic firing. The mechanisms underlying this diversity of receptive field properties are yet unknown. Here we study the characteristics of thalamocortical (TC) and intracortical connectivity that give rise to the individual cell responses. Using a mouse auditory TC slice preparation, we found that the amplitude of synaptic responses in AI varies non-monotonically with the intensity of the stimulation in the medial geniculate nucleus (MGv). We constructed a network model of MGv and AI that was simulated using either rate model cells or in vitro neurons through an iterative procedure that used the recorded neural responses to reconstruct network activity. We compared the receptive fields and firing profiles obtained with networks configured to have either cotuned excitatory and inhibitory inputs or relatively broad, lateral inhibitory inputs. Each of these networks yielded distinct response properties consistent with those documented in vivo with natural stimuli. The cotuned network produced V-shaped RFs, phasic-tonic firing profiles, and predominantly monotonic rate-level functions. The lateral inhibitory network produced enclosed RFs with narrow frequency tuning, a variety of firing profiles, and robust non-monotonic rate-level functions. We conclude that both types of circuits must be present to account for the wide variety of responses observed in vivo.

The auditory cortex of the bat Phyllostomus discolor: Localization and organization of basic response properties

BACKGROUND

The mammalian auditory cortex can be subdivided into various fields characterized by neurophysiological and neuroarchitectural properties and by connections with different nuclei of the thalamus. Besides the primary auditory cortex, echolocating bats have cortical fields for the processing of temporal and spectral features of the echolocation pulses. This paper reports on location, neuroarchitecture and basic functional organization of the auditory cortex of the microchiropteran bat Phyllostomus discolor (family: Phyllostomidae).

RESULTS

The auditory cortical area of P. discolor is located at parieto-temporal portions of the neocortex. It covers a rostro-caudal range of about 4800 mum and a medio-lateral distance of about 7000 mum on the flattened cortical surface. The auditory cortices of ten adult P. discolor were electrophysiologically mapped in detail. Responses of 849 units (single neurons and neuronal clusters up to three neurons) to pure tone stimulation were recorded extracellularly. Cortical units were characterized and classified depending on their response properties such as best frequency, auditory threshold, first spike latency, response duration, width and shape of the frequency response area and binaural interactions. Based on neurophysiological and neuroanatomical criteria, the auditory cortex of P. discolor could be subdivided into anterior and posterior ventral fields and anterior and posterior dorsal fields. The representation of response properties within the different auditory cortical fields was analyzed in detail. The two ventral fields were distinguished by their tonotopic organization with opposing frequency gradients. The dorsal cortical fields were not tonotopically organized but contained neurons that were responsive to high frequencies only.

CONCLUSION

The auditory cortex of P. discolor resembles the auditory cortex of other phyllostomid bats in size and basic functional organization. The tonotopically organized posterior ventral field might represent the primary auditory cortex and the tonotopically organized anterior ventral field seems to be similar to the anterior auditory field of other mammals. As most energy of the echolocation pulse of P. discolor is contained in the high-frequency range, the non-tonotopically organized high-frequency dorsal region seems to be particularly important for echolocation.

Corticofugal modulation of the paradoxical latency shifts of inferior collicular neurons

The central auditory system creates various types of neurons tuned to different acoustic parameters other than a specific frequency. The response latency of auditory neurons typically shortens with an increase in stimulus intensity. However, approximately 10% of collicular neurons of the little brown bat show a "paradoxical latency-shift (PLS)": long latencies to intense sounds but short latencies to weak sounds. These neurons presumably are involved in the processing of target distance information carried by a pair of an intense biosonar pulse and its weak echo. Our current studies show that collicular PLS neurons of the big brown bat are modulated by the corticofugal (descending) system. Electric stimulation of cortical auditory neurons evoked two types of changes in the PLS neurons, depending on the relationship in the best frequency (BF) between the stimulated cortical and recorded collicular neurons. When the BF was matched between them, the cortical stimulation did not shift the BFs of the collicular neurons and shortened their response latencies at intense sounds so that the PLS became smaller. When the BF was unmatched, however, the cortical stimulation shifted the BFs of the collicular neurons and lengthened their response latencies at intense sounds, so that the PLS became larger. Cortical electric stimulation also modulated the response latencies of non-PLS neurons. It produced an inhibitory frequency tuning curve or curves. Our findings indicate that corticofugal feedback is involved in shaping the spectrotemporal patterns of responses of subcortical auditory neurons presumably through inhibition.

Local shaping of function in the motor cortex: motor contrast, directional tuning

In this review we bring together three different lines of evidence to bear on the issue of local shaping of function in the motor cortex. The first line of evidence comes from the description by Cajal (1904) of the recurrent collaterals of pyramidal cell axons in the precentral gyrus. The second line of evidence comes from the electrophysiological study of the functional effects of these collaterals [Stefanis, C., Jasper, H. 1964a. Intracellular microelectrode studies of antidromic responses in cortical pyramidal tract neurons. J. Neurophysiol. 27, 828-854.; Stefanis, C., Jasper, H. 1964b. Recurrent collateral inhibition in pyramidal tract neurons. J. Neurophysiol. 27, 855-877.] and associated interneurons [Stefanis, C. 1969. Interneuronal mechanisms in the cortex. In: The Interneuron, Brazier, M.A.B. (ed.), Berkeley, CA: University of California Press, pp. 497-526.] using intracellular recordings. And third came the discovery of directional tuning in the motor cortex [Georgopoulos, A.P., Kalaska, J.F., Caminiti, R., Massey, J.T. 1982. On the relations between the direction of two-dimensional arm movements and cell discharge in primate motor cortex. J. Neurosci. 2, 1527-1537.] in the behaving monkey. We hazard the hypothesis that the bell-shaped directional tuning curve is the outcome of orderly, local neuronal interactions in the motor cortex in which the recurrent pyramidal cell collaterals play a crucial role. Specifically, we propose that these collaterals and the intercalated interneurons they impinge upon serve to spatially sharpen the motor cortical activation to a locus corresponding to the direction of the intended movement. Thus, the originally proposed role of the pyramidal cell collaterals in enhancing "motor contrast" [Stefanis, C. 1969. Interneuronal mechanisms in the cortex. In: The Interneuron, Brazier, M.A.B. (ed.), Berkeley, CA: University of California Press, pp. 497-526.] would translate to creating a "directional tuning field" on the motor cortical surface, where the enhanced motor contrast would correspond to high activity at the center of directional field, and the suppression of the fringe would correspond to lower activity at the periphery of the field, resulting, together in spatial tuning.

Synaptic mechanisms underlying auditory processing

In vivo voltage clamp recordings have provided new insights into the synaptic mechanisms that underlie processing in the primary auditory cortex. Of particular importance are the discoveries that excitatory and inhibitory inputs have similar frequency and intensity tuning, that excitation is followed by inhibition with a short delay, and that the duration of inhibition is briefer than expected. These findings challenge existing models of auditory processing in which broadly tuned lateral inhibition is used to limit excitatory receptive fields and suggest new mechanisms by which inhibition and short term plasticity shape neural responses.

Fast inhibition alters first spike timing in auditory brainstem neurons

Within the first processing site of the central auditory pathway, inhibitory neurons (D stellate cells) broadly tuned to tonal frequency project on narrowly tuned, excitatory output neurons (T stellate cells). The latter is thought to provide a topographic representation of sound spectrum, whereas the former is thought to provide lateral inhibition that improves spectral contrast, particularly in noise. In response to pure tones, the overall discharge rate in T stellate cells is unlikely to be suppressed dramatically by D stellate cells because they respond primarily to stimulus onset and provide fast, short-duration inhibition. In vivo intracellular recordings from the ventral cochlear nucleus (VCN) showed that, when tones were presented above or below the characteristic frequency (CF) of a T stellate neuron, they were inhibited during depolarization. This resulted in a delay in the initial action potential produced by T stellate cells. This ability of fast inhibition to alter the first spike timing of a T stellate neuron was confirmed by electrically activating the D stellate cell pathway that arises in the contralateral cochlear nucleus. Delay was also induced when two tones were presented: one at CF and one outside the frequency response area of the T stellate neuron. These findings suggest that the traditional view of lateral inhibition within the VCN should incorporate delay as one of its principle outcomes.