Sharp Frequency Tuning Research Articles

Noninvasive in vivo imaging reveals differences between tectorial membrane and basilar membrane traveling waves in the mouse cochlea

Sound is encoded within the auditory portion of the inner ear, the cochlea, after propagating down its length as a traveling wave. For over half a century, vibratory measurements to study cochlear traveling waves have been made using invasive approaches such as laser Doppler vibrometry. Although these studies have provided critical information regarding the nonlinear processes within the living cochlea that increase the amplitude of vibration and sharpen frequency tuning, the data have typically been limited to point measurements of basilar membrane vibration. In addition, opening the cochlea may alter its function and affect the findings. Here we describe volumetric optical coherence tomography vibrometry, a technique that overcomes these limitations by providing depth-resolved displacement measurements at 200 kHz inside a 3D volume of tissue with picometer sensitivity. We studied the mouse cochlea by imaging noninvasively through the surrounding bone to measure sound-induced vibrations of the sensory structures in vivo, and report, to our knowledge, the first measures of tectorial membrane vibration within the unopened cochlea. We found that the tectorial membrane sustains traveling wave propagation. Compared with basilar membrane traveling waves, tectorial membrane traveling waves have larger dynamic ranges, sharper frequency tuning, and apically shifted positions of peak vibration. These findings explain discrepancies between previously published basilar membrane vibration and auditory nerve single unit data. Because the tectorial membrane directly overlies the inner hair cell stereociliary bundles, these data provide the most accurate characterization of the stimulus shaping the afferent auditory response available to date.

Parallel pathways at the auditory periphery

We should consider the possibility that Low- and High- spontaneous rate (SR) auditory nerve fibers (ANFs) [1] constitute two different parallel pathways at the auditory periphery. The present study used a computational model of the auditory periphery [2] to demonstrate that Low- and High- SR ANFs have contrasting response properties. Anatomical studies suggest that Low- and High- SR ANF types have separate innervation sites (Figure ​(Figure1A)1A) on the same inner hair cell; lower-SR fibers synapse on the modiolar side and high-SR fibers synapse on the pillar side [3]. My hypothesis, prior to modeling the tuning curves (Figure ​(Figure1B),1B), was that Low Spontaneous Rate (Low-SR) fibers have a higher threshold for simulation and thus will have demonstrably sharper frequency selectivity than High-SR fibers. The results of the simulation support this framework; Low-SR ANFs were shown to have sharper frequency tuning (Figure ​(Figure1B)1B) than High-SR ANFs throughout a range of characteristic frequencies (CFs). While Low-SR ANFs have sharper frequency selectivity (Figure ​(Figure1B),1B), High-SR ANFs have finer temporal resolution, as the rate of change of the mean firing rate in High-SR ANFs was well above that of Low-SR fibers in the simulation (Figure ​(Figure1C).1C). It would seem that Low-SR and Medium-SR fibers (i.e. Lower-SR fibers) are optimized for “place theory” frequency coding and High-SR fibers are optimized for “volley-theory” synchronous phase locking. Future modeling efforts might maintain the integrity of these two parallel pathways, optimized for fine spectral (Lower-SR) and fine temporal (High-SR) resolution, by separating rather than summing their respective outputs. Figure 1 Innervation sites (A), tuning curves (B) and temporal response properties for Low- and High- SR ANFs

The sharp frequency selectivity of low- and medium- spontaneous rate auditory nerve fibers might allow for rate-place coding up to 5 kilohertz

One feature that might differentiate the three types of spontaneous rate (SR) auditory nerve fibers (ANFs) is the sharpness of frequency tuning at comparable sound pressure levels. My hypothesis, prior to modeling the tuning curves for each fiber type, was that low-SR fibers have a higher threshold for stimulation and thus have sharper frequency selectivity than medium-SR fibers, which in turn have sharper frequency selectivity than high-SR fibers. The results of the simulation support this framework. I used the Zilany et al. (2014) model of the auditory periphery and the cochlear tuning parameters from Shera et al. (2002) to generate tuning curves and neurogram raster plots for each of the three fiber types. The results of the simulation suggest that the sharp frequency selectivity of low-SR and medium-SR ANFs might allow for resolved rate-place coding up to 5 kHz. At frequencies below 1500 Hz, high-SR fibers were seen to have very different response properties than low-SR and medium-SR fibers. It is conceivable that the different fiber types constitute parallel pathways and mediate two different coding schemes. We should consider whether these sharply tuned low-SR and medium-SR ANFs, while fewer in number than High-SR ANFs, might be especially important for rate-place frequency coding.

Organization and trade-off of spectro-temporal tuning properties of duration-tuned neurons in the mammalian inferior colliculus

Neurons throughout the mammalian central auditory pathway respond selectively to stimulus frequency and amplitude, and some are also selective for stimulus duration. First found in the auditory midbrain or inferior colliculus (IC), these duration-tuned neurons (DTNs) provide a potential neural mechanism for encoding temporal features of sound. In this study, we investigated how having an additional neural response filter, one selective to the duration of an auditory stimulus, influences frequency tuning and neural organization by recording single-unit responses and measuring the dorsal-ventral position and spectral-temporal tuning properties of auditory DTNs from the IC of the awake big brown bat (Eptesicus fuscus). Like other IC neurons, DTNs were tonotopically organized and had either V-shaped, U-shaped, or O-shaped frequency tuning curves (excitatory frequency response areas). We hypothesized there would be an interaction between frequency and duration tuning in DTNs, as electrical engineering theory for resonant filters dictates a trade-off in spectral-temporal resolution: sharp tuning in the frequency domain results in poorer resolution in the time domain and vice versa. While the IC is a more complex signal analyzer than an electrical filter, a similar operational trade-off could exist in the responses of DTNs. Our data revealed two patterns of spectro-temporal sensitivity and spatial organization within the IC: DTNs with sharp frequency tuning and broad duration tuning were located in the dorsal IC, whereas cells with wide spectral tuning and narrow temporal tuning were found in the ventral IC.

Quantitative analysis of neuronal response properties in primary and higher-order auditory cortical fields of awake house mice (Mus musculus).

Because of its great genetic potential, the mouse (Mus musculus) has become a popular model species for studies on hearing and sound processing along the auditory pathways. Here, we present the first comparative study on the representation of neuronal response parameters to tones in primary and higher-order auditory cortical fields of awake mice. We quantified 12 neuronal properties of tone processing in order to estimate similarities and differences of function between the fields, and to discuss how far auditory cortex (AC) function in the mouse is comparable to that in awake monkeys and cats. Extracellular recordings were made from 1400 small clusters of neurons from cortical layers III/IV in the primary fields AI (primary auditory field) and AAF (anterior auditory field), and the higher-order fields AII (second auditory field) and DP (dorsoposterior field). Field specificity was shown with regard to spontaneous activity, correlation between spontaneous and evoked activity, tone response latency, sharpness of frequency tuning, temporal response patterns (occurrence of phasic responses, phasic-tonic responses, tonic responses, and off-responses), and degree of variation between the characteristic frequency (CF) and the best frequency (BF) (CF–BF relationship). Field similarities were noted as significant correlations between CFs and BFs, V-shaped frequency tuning curves, similar minimum response thresholds and non-monotonic rate-level functions in approximately two-thirds of the neurons. Comparative and quantitative analyses showed that the measured response characteristics were, to various degrees, susceptible to influences of anesthetics. Therefore, studies of neuronal responses in the awake AC are important in order to establish adequate relationships between neuronal data and auditory perception and acoustic response behavior.

Prestin Regulation and Function in Residual Outer Hair Cells after Noise-Induced Hearing Loss

The outer hair cell (OHC) motor protein prestin is necessary for electromotility, which drives cochlear amplification and produces exquisitely sharp frequency tuning. TectaC1509G transgenic mice have hearing loss, and surprisingly have increased OHC prestin levels. We hypothesized, therefore, that prestin up-regulation may represent a generalized response to compensate for a state of hearing loss. In the present study, we sought to determine the effects of noise-induced hearing loss on prestin expression. After noise exposure, we performed cytocochleograms and observed OHC loss only in the basal region of the cochlea. Next, we patch clamped OHCs from the apical turn (9–12 kHz region), where no OHCs were lost, in noise-exposed and age-matched control mice. The non-linear capacitance was significantly higher in noise-exposed mice, consistent with higher functional prestin levels. We then measured prestin protein and mRNA levels in whole-cochlea specimens. Both Western blot and qPCR studies demonstrated increased prestin expression after noise exposure. Finally, we examined the effect of the prestin increase in vivo following noise damage. Immediately after noise exposure, ABR and DPOAE thresholds were elevated by 30–40 dB. While most of the temporary threshold shifts recovered within 3 days, there were additional improvements over the next month. However, DPOAE magnitudes, basilar membrane vibration, and CAP tuning curve measurements from the 9–12 kHz cochlear region demonstrated no differences between noise-exposed mice and control mice. Taken together, these data indicate that prestin is up-regulated by 32–58% in residual OHCs after noise exposure and that the prestin is functional. These findings are consistent with the notion that prestin increases in an attempt to partially compensate for reduced force production because of missing OHCs. However, in regions where there is no OHC loss, the cochlea is able to compensate for the excess prestin in order to maintain stable auditory thresholds and frequency discrimination.

Pre-Attentive, Context-Specific Representation of Fear Memory in the Auditory Cortex of Rat

Neural representation in the auditory cortex is rapidly modulated by both top-down attention and bottom-up stimulus properties, in order to improve perception in a given context. Learning-induced, pre-attentive, map plasticity has been also studied in the anesthetized cortex; however, little attention has been paid to rapid, context-dependent modulation. We hypothesize that context-specific learning leads to pre-attentively modulated, multiplex representation in the auditory cortex. Here, we investigate map plasticity in the auditory cortices of anesthetized rats conditioned in a context-dependent manner, such that a conditioned stimulus (CS) of a 20-kHz tone and an unconditioned stimulus (US) of a mild electrical shock were associated only under a noisy auditory context, but not in silence. After the conditioning, although no distinct plasticity was found in the tonotopic map, tone-evoked responses were more noise-resistive than pre-conditioning. Yet, the conditioned group showed a reduced spread of activation to each tone with noise, but not with silence, associated with a sharpening of frequency tuning. The encoding accuracy index of neurons showed that conditioning deteriorated the accuracy of tone-frequency representations in noisy condition at off-CS regions, but not at CS regions, suggesting that arbitrary tones around the frequency of the CS were more likely perceived as the CS in a specific context, where CS was associated with US. These results together demonstrate that learning-induced plasticity in the auditory cortex occurs in a context-dependent manner.

Synaptic Mechanisms Underlying Functional Dichotomy between Intrinsic-Bursting and Regular-Spiking Neurons in Auditory Cortical Layer 5

Corticofugal projections from the primary auditory cortex (A1) have been shown to play a role in modulating subcortical processing. However, functional properties of the corticofugal neurons and their synaptic circuitry mechanisms remain unclear. In this study, we performed in vivo whole-cell recordings from layer 5 (L5) pyramidal neurons in the rat A1 and found two distinct neuronal classes according to their functional properties. Intrinsic-bursting (IB) neurons, the L5 corticofugal neurons, exhibited early and rather unselective spike responses to a wide range of frequencies. The exceptionally broad spectral tuning of IB neurons was attributable to their broad excitatory inputs with long temporal durations and inhibitory inputs being more narrowly tuned than excitatory inputs. This uncommon pattern of excitatory-inhibitory interplay was attributed initially to a broad thalamocortical convergence onto IB neurons, which also receive temporally prolonged intracortical excitatory input as well as feedforward inhibitory input at least partially from more narrowly tuned fast-spiking inhibitory neurons. In contrast, regular-spiking neurons, which are mainly corticocortical, exhibited sharp frequency tuning similar to L4 pyramidal cells, underlying which are well-matched purely intracortical excitation and inhibition. The functional dichotomy among L5 pyramidal neurons suggests two distinct processing streams. The spectrally and temporally broad synaptic integration in IB neurons may ensure robust feedback signals to facilitate subcortical function and plasticity in a general manner.

An Active Mechanism for Signal Detection in the Mammalian Ear

The ear's exquisite sensitivity, sharp frequency tuning, and broad dynamical range result from an active process known as the cochlear amplifier. Although outer hair cells play a central role in cochlear amplification, their mechanism of action remains uncertain. In non-mammalian ears hair bundles, the sensory organelles of hair cells, can perform mechanical work and account for the active process in vitro. Although active hair-bundle motility also occurs in the mammalian cochlea, a membrane-based piezoelectric effect known as electromotility is required for amplification in vivo.We present a physical description of a segment of the mammalian cochlea that, by amplifying the power of an external input, acts as a sensitive, sharply tuned, and nonlinear signal detector. This model, which is based on the known physiology and morphology of the inner ear, couples active hair-bundle motility and electromotility through the geometric arrangement of hair cells in the organ of Corti. The model displays quantitative agreement with in vivo measurements of basilar-membrane movements. The model's predictions of the vibration pattern at different input levels and of the differences between wild-type and electromotility-deficient mice accord qualitatively with experimental observations.We demonstrate how internal sources of energy in this system enhance the signal-detection properties of the mammalian ear in comparison with those of a passive cochlea. Although the hair bundle provides nonlinearity and feedback sufficient for detecting low-frequency signals in non-mammals, electromotility produces additional feedback that allows the mammalian cochlea to detect high-frequency inputs. This work supports an evolutionary scenario in which high-frequency hearing in mammals arose by supplementing a preexisting auditory amplifier with an additional source of mechanical energy.

A biophysically detailed model of the primary auditory cortex explains physiological forward masking, co-tuning of excitation and inhibition and cortical signal amplification

For a long time, studies argued for inhibition as the main mechanism responsible for two-tone suppression (a.k.a. forward masking) seen in primary auditory cortex (A1) neurons [1,2]. However, both computational [3] and experimental [4] papers afford a significant role to thalamocortical (ThC) synaptic depression in shaping the temporal response properties of A1. Also, the duration of inhibitory currents in A1 has been an issue of contention [6,7]. Another study of forward masking [5] used noise click stimuli to show that while responses to the probe were not fully recovered even 512 ms after presentation of the masker, inhibitory currents evoked by the masker lasted at most 100 ms, coinciding in duration with the complete suppression of probe responses. The authors proposed that a longer-lasting mechanism such as ThC or intracortical (IC) synaptic depression could complement inhibition by accounting for the lingering effect seen. They also demonstrated that pentobarbital anesthesia significantly prolongs inhibition, thus calling into question results such as [1,2]. Here we present a biophysically detailed, tonotopically organized network model of A1, which employs Hodgkin and Huxley neurons with stochastic synaptic depression in ThC synapses. Our model accounts for forward masking seen with both single tones [1,2] and noise stimuli [5], while showing approximately balanced excitation and inhibition [7-9]. Inspired by [10], we propose a plausible IC connectivity for the layers III and IV of A1, which selectively amplifies the broad input from the thalamus to yield the sharp frequency tuning seen in many studies of A1. We conclude that a combination of IC currents and ThC synaptic depression is imperative for accounting for the wealth of data seen in the neurophysiological literature, such as the phenomena we study here.

Low‐voltage activated Kv1.1 subunits are crucial for the processing of sound source location in the lateral superior olive in mice

Voltage-gated potassium (Kv) channels containing Kv1.1 subunits are strongly expressed in neurons that fire temporally precise action potentials (APs). In the auditory system, AP timing is used to localize sound sources by integrating interaural differences in time (ITD) and intensity (IID) using sound arriving at both cochleae. In mammals, the first nucleus to encode IIDs is the lateral superior olive (LSO), which integrates excitation from the ipsilateral ventral cochlear nucleus and contralateral inhibition mediated via the medial nucleus of the trapezoid body. Previously we reported that neurons in this pathway show reduced firing rates, longer latencies and increased jitter in Kv1.1 knockout (Kcna1−/−) mice. Here, we investigate whether these differences have direct impact on IID processing by LSO neurons. Single-unit recordings were made from LSO neurons of wild-type (Kcna1+/+) and from Kcna1−/− mice. IID functions were measured to evaluate genotype-specific changes in integrating excitatory and inhibitory inputs. In Kcna1+/+ mice, IID sensitivity ranged from +27 dB (excitatory ear more intense) to −20 dB (inhibitory ear more intense), thus covering the physiologically relevant range of IIDs. However, the distribution of IID functions in Kcna1−/− mice was skewed towards positive IIDs, favouring ipsilateral sound positions. Our computational model revealed that the reduced performance of IID encoding in the LSO of Kcna1−/− mice is mainly caused by a decrease in temporal fidelity along the inhibitory pathway. These results imply a fundamental role for Kv1.1 in temporal integration of excitation and inhibition during sound source localization.

Dual Mechanism of Neuronal Ensemble Inhibition in Primary Auditory Cortex

Inhibition plays an essential role in shaping and refining the brain's representation of sensory stimulus attributes. In primary auditory cortex (A1), so-called "sideband" inhibition helps to sharpen the tuning of local neuronal responses. Several distinct types of anatomical circuitry could underlie sideband inhibition, including direct thalamocortical (TC) afferents, as well as indirect intracortical mechanisms. The goal of the present study was to characterize sideband inhibition in A1 and to determine its mechanism by analyzing laminar profiles of neuronal ensemble activity. Our results indicate that both lemniscal and nonlemniscal TC afferents play a role in inhibitory responses via feedforward inhibition and oscillatory phase reset, respectively. We propose that the dynamic modulation of excitability in A1 due to the phase reset of ongoing oscillations may alter the tuning of local neuronal ensembles and can be regarded as a flexible overlay on the more obligatory system of lemniscal feedforward type responses.

The interplay between active hair bundle motility and electromotility in the cochlea

The cochlear amplifier is a nonlinear active process providing the mammalian ear with its extraordinary sensitivity, large dynamic range and sharp frequency tuning. While there is much evidence that amplification results from active force generation by mechanosensory hair cells, there is debate about the cellular processes behind nonlinear amplification. Outer hair cell electromotility has been suggested to underlie the cochlear amplifier. However, it has been shown in frog and turtle that spontaneous movements of hair bundles endow them with a nonlinear response with increased sensitivity that could be the basis of amplification. The present work shows that the properties of the cochlear amplifier could be understood as resulting from the combination of both hair bundle motility and electromotility in an integrated system that couples these processes through the geometric arrangement of hair cells embedded in the cochlear partition. In this scenario, the cochlear partition can become a dynamic oscillator which in the vicinity of a Hopf bifurcation exhibits all the key properties of the cochlear amplifier. The oscillatory behavior and the nonlinearity are provided by active hair bundles. Electromotility is largely linear but produces an additional feedback that allows hair bundle movements to couple to basilar membrane vibrations.

Maintenance of enriched environment-induced changes of auditory spatial sensitivity and expression of GABA A, NMDA, and AMPA receptor subunits in rat auditory cortex

Enriched environment (EE) has an important role in the development and plasticity of the brain. In this study, we investigated the maintenance of early EE exposure-induced changes of spatial sensitivity, and the possible underlying mechanisms of this maintenance. We found that, compared with the age-matched control, the spatial sensitivity of A1 neurons was still enhanced after EE rats had been returned to the normal condition for 2 months. The enhancement was expressed by a sharper frequency tuning curve, smaller spatial receptive field, and a more selective directional curve of the early EE-exposed rats. Simultaneously, we detected significant increases in GABA A receptor α1, β3 subunits; NMDA receptor NR2A, NR2B subunits; AMPA receptor GluR2 subunit protein expression; and in the ratios of GABA Aα1/GABA Aα3 and NR2A/NR2B. In particular, the expression ratio change of the GABA Aα1/GABA Aα3 was significant greater than that of NR2A/NR2B in early EE-exposed rats. These observations indicate that the persistent higher expression levels of the GABAergic and glutamatergic receptors expression induced by early EE exposure, especially enhancement of GABAergic inhibition in the auditory cortex, might be responsible for the maintenance of improved effects in auditory spatial sensitivity after the rats had been returned to the normal condition.

WITHDRAWN: Coupled hair bundles could endow the cochlear amplifier with sharp frequency tuning and nonlinear compression

This article has been withdrawn at the request of the author(s) and/or editor. The Publisher apologizes for any inconvenience this may cause. The full Elsevier Policy on Article Withdrawal can be found at http://www.elsevier.com/locate/withdrawalpolicy.

The auditory response properties of single-on and double-on responders in the inferior colliculus of the leaf-nosed bat, Hipposideros armiger

The present study examines the response properties of neurons in the central nucleus of the inferior colliculus (IC) of the CF–FM (constant frequency–frequency-modulated) bat, Hipposideros armiger using CF, FM and CF–FM sounds as stimuli. All 169 IC neurons recorded are tonotopically organized along the dorsoventral axis of the IC. Collicular neurons have V-shaped or upper-threshold frequency tuning curves. Those neurons tuned at the predominant second harmonic have extremely sharp frequency tuning curves and low minimum thresholds. Collicular neurons typically discharge impulses to both CF and FM sounds. However, when stimulated with CF–FM sounds, most (76%) neurons only discharge impulses to the onset of CF–FM sounds (single-on responders). The remaining neurons (24%) discharge impulses to both CF and FM components (double-on responders) of CF–FM sounds. The double-on responders have higher minimum threshold and longer latency to the FM component than to the CF component of CF–FM sounds. Our data show that the FM component of the CF–FM sounds contributes significantly in shaping the discharge pattern, latency and number of impulses of IC neurons. The present study suggests that using CF–FM sounds to study auditory response properties of the CF–FM bat may be essential for a better understanding of echo analysis by the CF–FM in the real world. Because the double-on responders have shorter response latency than single-on responders, we speculate that these two types of responders may be best suited for echo analysis during different phases of hunting.

WITHDRAWN: Coupled hair bundles could endow the cochlear amplifier with sharp frequency tuning and nonlinear compression

This article has been withdrawn at the request of the author(s) and/or editor. The Publisher apologizes for any inconvenience this may cause. The full Elsevier Policy on Article Withdrawal can be found at http://www.elsevier.com/locate/withdrawalpolicy.

Adaptation reveals multiple levels of representation in auditory stream segregation.

When presented with alternating low and high tones, listeners are more likely to perceive 2 separate streams of tones ("streaming") than a single coherent stream when the frequency separation (Deltaf) between tones is greater and the number of tone presentations is greater ("buildup"). However, the same large-Deltaf sequence reduces streaming for subsequent patterns presented after a gap of up to several seconds. Buildup occurs at a level of neural representation with sharp frequency tuning. The authors used adaptation to demonstrate that the contextual effect of prior Deltaf arose from a representation with broad frequency tuning, unlike buildup. Separate adaptation did not occur in a representation of Deltaf independent of frequency range, suggesting that any frequency-shift detectors undergoing adaptation are also frequency specific. A separate effect of prior perception was observed, dissociating stimulus-related (i.e., Deltaf) and perception-related (i.e., 1 stream vs. 2 streams) adaptation. Viewing a visual analogue to auditory streaming had no effect on subsequent perception of streaming, suggesting adaptation in auditory-specific brain circuits. These results, along with previous findings on buildup, suggest that processing in at least 3 levels of auditory neural representation underlies segregation and formation of auditory streams.

Brief exposure of juvenile rats to noise impairs the development of the response properties of inferior colliculus neurons

Temporary impairment of the auditory periphery during the sensitive period of postnatal development of rats may result in a deterioration of neuronal responsiveness in the central auditory nuclei of adult animals. In this study, juvenile rats (postnatal day 14) were exposed for 8 min to intense broad-band noise; at the age of 3-6 months, the excitatory and inhibitory response areas of neurons in the central nucleus of the inferior colliculus were recorded under ketamine-xylazine anaesthesia in these animals and compared with those of age-matched controls. The response thresholds were similar in the exposed and control animals. The frequency selectivity of low-frequency neurons was comparable in both groups; however, high-frequency neurons had significantly wider excitatory response areas in the exposed rats, indicating disrupted development of high-frequency hearing. Forty-one per cent and 25% of neurons in exposed animals and in controls, respectively, lacked a distinct inhibitory area; these neurons had similar frequency selectivity in the exposed and control rats. As the presence of an inhibitory sideband was associated with sharper frequency tuning in both groups, it appears that lateral inhibition substantially influences neuronal frequency selectivity. If present, the inhibitory areas had comparable bandwidths in both groups; however, they were shifted to the side in the exposed animals, allowing the expansion of the excitatory areas. The results indicate that a brief exposure of juvenile rats to noise leads to a significant worsening of the frequency selectivity of inferior colliculus neurons in adult animals; the poorer frequency selectivity may be due to missing or displaced inhibitory sidebands.

Environmental enrichment improves behavioral performance and auditory spatial representation of primary auditory cortical neurons in rat

Environmental enrichment (EE) has an important role in brain plasticity. Early research has shown that EE increases the response strength of neurons in the auditory cortex, but it remains unknown whether EE can influence the process of spatial localization in the auditory system. In this study, we raised rats in enriched and standard conditions from postnatal day 10 to day 56. By testing behavioral tasks via auditory cues, we have shown that EE improved the number of correct scores, but decreased the reaction time and azimuth deviation in behavioral performance of sound-azimuth discrimination. By in vivo extracellular recording, we have shown that EE enhanced the directional sensitivity of neurons in the primary auditory cortex. For example, EE rats had a smaller spatial receptive field, sharper frequency tuning curve and directional selective curve of auditory neurons compared with normal rats. Our findings indicate that early exposure to EE increases directional sensitivity. These results provide an insight into developmental plasticity in the auditory system.