Free-field Stimulation Research Articles

Neuroethology of sound localization in anurans.

Albert Feng pioneered the study of neuroethology of sound localization in anurans by combining behavioral experiments on phonotaxis with detailed investigations of neural processing of sound direction from the periphery to the central nervous system. The main advantage of these studies is that many species of female frogs readily perform phonotaxis towards loudspeakers emitting the species-specific advertisement call. Behavioral studies using synthetic calls can identify which parameters are important for phonotaxis and also quantify localization accuracy. Feng was the first to investigate binaural processing using single-unit recordings in the first two auditory nuclei in the central auditory pathway and later investigated the directional properties of auditory nerve fibers with free-field stimulation. These studies showed not only that the frog ear is inherently directional by virtue of acoustical coupling or crosstalk between the two eardrums, but also confirmed that there are extratympanic pathways that affect directionality in the low-frequency region of the frog's hearing range. Feng's recordings in the midbrain also showed that directional information is enhanced by cross-midline inhibition. An important contribution toward the end of his career involved his participation in neuroethological research with a team of scientists working with frogs that produce ultrasonic calls.

Регистрация стационарных слуховых потенциалов в свободном звуковом поле у нормально слышащих лиц. Наш опыт

Background: Currently, for an objective assessment of the thresholds of auditory sensitivity, one of the most popular diagnostic methods is the method of recording of auditory steady state response - the ASSR test. One of the primary uses of ASSR is to assess sound thresholds correlated with tonal threshold audiometry. Actually, there are no reliable criteria for the norm for this study with free field stimulation, in conditions close to the natural listening environment. The identification of these criteria will allow this method to be applied as a routine in patients who cannot be tested in the usual way using headsets or in-ear phones (headphones). Objective: To determine the correlation of the ASSR thresholds with free field stimulation and the responses of insert-transducers stimulation and values of tonal threshold audiometry in the free field in normally hearing individuals. Design: Behavioral thresholds in a free field were compared with the results of ASSR in 20 normally hearing adults (40 ears) with stimulus delivery both via insert-transducers and using a loudspeaker. Conclusion: The ASSR thresholds in normal hearing adults obtained by free field were comparable to the thresholds obtained with a stimulus through insert- transducers and the results of behavioral thresholds. The correction coefficients for measuring ASSR thresholds and behavioral thresholds in free field in normal hearing adults have been obtained.

Search for normality criteria of auditory brain responses and auditory steady state response with free-field stimulation

ObjectiveTo discover the thresholds of normality of the tests of ABR and ASSR in free-field stimulation compared with headphones. MethodsDescriptive study of a series of cases of children without hearing loss carried out between April 2016 and January 2017. The ABR and conventional ASR were performed using insert headphones and free-field stimulation. ResultsFifty-four ears were evaluated with absolute criteria of normality. The mean age was 16.7 months (SD = 5.7). The evocation of the action potential at 70 dBs in the free-field showed a mean on wave I of 3.47 ms, on wave II of 4.01, on wave III of 5.97, on wave IV of 6.71 and on wave V of 8.22. There is a statistically significant difference (p < 0.05) between the value obtained with insert earphones and the free-field on wave I (mean difference 1.91), on wave III (1.88), on wave IV (1.69) and on wave V (1.95), their being higher in stimulus in free field. There were no statistically significant differences in the interlatencies. The responses in the ASSR with free-field showed an increase of 1.56 min compared to the headphones. ConclusionsThe thresholds obtained in ABR by free-field were differentiated from those obtained by headphones in the increase of latencies, caused by delayed stimulation of the ear due to the distance between the sound source and the eardrum. Both tests are feasible by free-field stimulation, which would allow them to be applied routinely to patients on whom conventional tests cannot be performed.

Búsqueda de criterios de normalidad de potenciales evocados auditivos de tronco cerebral y potenciales evocados auditivos de estado estable con estímulo en campo libre

ObjectiveTo discover the thresholds of normality of the tests of ABR and ASSR in free-field stimulation compared with headphones. MethodsDescriptive study of a series of cases of children without hearing loss carried out between April 2016 and January 2017. The ABR and conventional ASR were performed using insert headphones and free-field stimulation. ResultsFifty-four ears were evaluated with absolute criteria of normality. The mean age was 16.7 months (SD=5.7). The evocation of the action potential at 70 dBs in the free-field showed a mean on wave I of 3.47ms, on wave II of 4.01, on wave III of 5.97, on wave IV of 6.71 and on wave V of 8.22. There is a statistically significant difference (p<.05) between the value obtained with insert earphones and the free-field on wave I (mean difference 1.91), on wave III (1.88), on wave IV (1.69) and on wave V (1.95), their being higher in stimulus in free field. There were no statistically significant differences in the interlatencies. The responses in the ASSR with free-field showed an increase of 1.56minutes compared to the headphones. ConclusionsThe thresholds obtained in ABR by free-field were differentiated from those obtained by headphones in the increase of latencies, caused by delayed stimulation of the ear due to the distance between the sound source and the eardrum. Both tests are feasible by free-field stimulation, which would allow them to be applied routinely to patients on whom conventional tests cannot be performed.

Exploring binaural hearing in gerbils (Meriones unguiculatus) using virtual headphones

The Mongolian gerbil (Meriones unguiculatus) has become a key species in investigations of the neural processing of sound localization cues in mammals. While its sound localization has been tested extensively under free-field stimulation, many neurophysiological studies use headphones to present signals with binaural localization cues. The gerbil's behavioral sensitivity to binaural cues, however, is unknown for the lack of appropriate stimulation paradigms in awake behaving gerbils. We close this gap in knowledge by mimicking a headphone stimulation; we use free-field loudspeakers and apply cross-talk cancellation techniques to present pure tones with binaural cues via “virtual headphones” to gerbils trained in a sound localization task. All gerbils were able to lateralize sounds depending on the interaural time or level difference (ITD and ILD, respectively). For ITD stimuli, reliable responses were seen for frequencies ≤2.9 kHz, the highest frequency tested with ITD stimuli. ITD sensitivity was frequency-dependent with the highest sensitivity observed at 1 kHz. For stimuli with ITD outside the gerbil's physiological range, responses were cyclic indicating the use of phase information when lateralizing narrow-band sounds. For ILD stimuli, reliable responses were obtained for frequencies ≥2 kHz. The comparison of ITD and ILD thresholds with ITD and ILD thresholds derived from gerbils’ free-field performance suggests that ongoing ITD information is the main cue for sound localization at frequencies <2 kHz. At 2 kHz, ITD and ILD cues are likely used in a complementary way. Verification of the use of the virtual headphones suggests that they can serve as a suitable substitute for conventional headphones particularly at frequencies ≤2 kHz.

Local neuronal circuits that may shape the discharge patterns of inferior collicular neurons

The discharge patterns of neurons in auditory centers encode information about sounds. However, few studies have focused on the synaptic mechanisms underlying the shaping of discharge patterns using intracellular recording techniques. Here, we investigated the discharge patterns of inferior collicular (IC) neurons using intracellular recordings to further elucidate the mechanisms underlying the shaping of discharge patterns. Under in vivo intracellular recording conditions, recordings were obtained from 66 IC neurons in 18 healthy adult mice (Mus musculus, Km) under free field-stimulation. Fifty-eight of these neurons fi red bursts of action potentials (APs) to auditory stimuli and the remaining eight just generated local responses such as excitatory (n = 4) or inhibitory (n = 4) postsynaptic potentials. Based on the APs and subthreshold responses, the discharge patterns were classified into seven types: phasic (24/58, 41.4%), phasic burst (8/58,13.8%), pauser (4/58, 6.9%), phasic-pauser (1/58, 1.7%), chopper (2/58, 3.4%), primary-like tonic (14/58, 24.1%) and sound-induced inhibitory (5/58,8.6%). We concluded that (1) IC neurons exhibit at least seven distinct discharge patterns; (2) inhibition participates in shaping the discharge pattern of most IC neurons and plays a role in sculpting the pattern, except for the primary-like tonic pattern which was not shaped by inhibition; and (3) local neural circuits are the likely structural basis that shapes the discharge patterns of IC neurons and can be formed either in the IC or in lower-level auditory structures.

Hemispheric specialization during discrimination of sound sources reflected by MMN

The present study reports the hemispheric specificity of spatial auditory processing in 15 healthy subjects by measuring location mismatch negativity (MMN) under free field stimulation. The aim was to decide between the partly inconsistent hypotheses of contralateral and/or right-hemispheric dominance in spatial processing in the auditory cortex. The laterality of deviant-standard positions were systematically varied covering the whole of the frontal hemifield from 90 degrees left to 90 degrees right, while the spatial separation of deviant and standard were fixed at 17 degrees. This enabled the evaluation of the specific location-MMNs relating to distinct cortical processing of acoustic space. The inter-hemispheric comparison of the amplitudes of MMNs showed that spatial deviation towards the periphery at -/+17 degrees (relative to 0 degrees -standard) and at -/+90 degrees (relative to -/+73 degrees -standard) elicited a salient contralateral activation. In contrast, positional changes towards front at -/+56 degrees -positions (relative to -/+73 degrees -standard) resulted equal bilateral MMNs. Further, MMN latencies became longer with increasing laterality of respective deviant-standard pairs. Thus, the present study suggests a contralateral pre-attentive cortical processing of acoustic space information in the free field. The direction of positional changes ('towards periphery' vs. 'towards front') seems to augment or reduce this contralateral effect. The sound source discrimination performance across space is mirrored by the location-MMN latency.

Hemispheric Specialization during Discrimination of Sound Sources reflected by MMN

Event Abstract Back to Event Hemispheric Specialization during Discrimination of Sound Sources reflected by MMN Nicole Richter1, 2*, E. Schroger1 and R. Rübsamen2 1 Institute for Psychology I, University of Leipzig, Germany 2 Institute for Biology II, University of Leipzig, Germany The present study reports the hemispheric specificity of spatial auditory processing in 15 healthy subjects by measuring location-mismatch negativity (MMN) under free field stimulation. The aim was to decide between the partly inconsistent hypotheses of contralateral and/or right-hemispheric dominance in spatial processing in the auditory cortex. The laterality of deviant-standard positions were systematically varied covering the whole of the frontal hemifield from 90° left to 90° right, while the spatial separations of deviants and standards were fixed at 17°. This enabled the evaluation of the specific location-MMNs relating to distinct cortical processing of acoustic space. The inter-hemispheric comparison of the amplitudes of MMNs showed that spatial deviation towards the periphery elicited a prominent contralateral activation: Positional changes in frontal positions at -/+17° (relative to 0°-standard) elicited contralaterally higher MMN amplitudes and the respective positional changes at -/+90° (relative to -/+ 73°-standard) elicited both contralaterally pronounced MMN amplitudes and shortened MMN latencies. In contrast, positional changes towards front at -/+56° positions (relative to -/+ 73°-standard) resulted equal bilateral MMNs. Further, MMN latencies became longer with increasing laterality of respective deviant-standard pairs. Thus, the present study provides evidence for a contralateral pre-attentive cortical processing of acoustic space information in the free field. The direction of positional changes (‘towards periphery’ vs. ‘towards front’) seems to augment or reduce this contralateral effect. The sound source discrimination performance across space is also mirrored by the location-MMN latency. Conference: MMN 09 Fifth Conference on Mismatch Negativity (MMN) and its Clinical and Scientific Applications, Budapest, Hungary, 4 Apr - 7 Apr, 2009. Presentation Type: Poster Presentation Topic: Poster Presentations Citation: Richter N, Schroger E and Rübsamen R (2009). Hemispheric Specialization during Discrimination of Sound Sources reflected by MMN. Conference Abstract: MMN 09 Fifth Conference on Mismatch Negativity (MMN) and its Clinical and Scientific Applications. doi: 10.3389/conf.neuro.09.2009.05.107 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 26 Mar 2009; Published Online: 26 Mar 2009. * Correspondence: Nicole Richter, Institute for Psychology I, University of Leipzig, Leipzig, Germany, colle@uni-leipzig.de Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Nicole Richter E. Schroger R. Rübsamen Google Nicole Richter E. Schroger R. Rübsamen Google Scholar Nicole Richter E. Schroger R. Rübsamen PubMed Nicole Richter E. Schroger R. Rübsamen Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Presentation rate of stimulus modulates neuronal responses to frequency modulated sweeps in the mouse inferior colliculus

Presentation rate (PR) represents the number of sound stimuli within the unit time. It is one of the essential indexes reflecting acoustical energy and information received by auditory system in a given time. Our recent investigations have shown that the temporal properties of inferior collicular (IC) neurons responding to the pure tone bursts are closely correlated to PR. The frequency modulated sweeps (FMs) share major dynamic parts in both human languages and animal communicating sounds. The modulation range (MR) and sweep directions of FMs, which are important temporal features of FMs, play key roles in speech distinguish in human. Although neuronal selectivity of MRs and sweep directions has been intensively studied in echolocating bats, whether the PR could modulate the neuronal selectivity of the FM remains unclear. In present study we addressed it in the mouse inferior colliculus (IC). 20 mice ( Mus musculus KM) were used in the experiments. Under the condition of free field stimulation and by using normal electrophysiological techniques, we recorded the responses of 90 IC neurons to FMs with different modulation ranges (2, 5, 10, 15, 20 and 25 kHz; the center frequency was always set at the CF of each recorded) at different presentation rates (0.5, 1.0, 5.0, 10.0 or 20.0 Hz). The main results were as follows: 1) When the PR was fixed at a mediate value (2.0 Hz), the majority of neurons responded best to narrower MRs (narrow-pass, up-sweep: 63.33%; down-sweep: 60.00%). Other types such as band-pass (up-sweep, 17/90, 18.89%; down-sweep, 12/90, 13.33%), all-pass (up-sweep, 15/90, 16.67%; down-sweep, 17/90, 18.89%) and wide-pass (up-sweep, 4/90, 4.44%; down-sweep, 4/90, 4.44%) also be found; 2) When different PRs were used, from 0.2 Hz to 10 Hz, the percentage of the narrow-pass neurons decreased, while the percentage of band-pass and all-pass neurons increased slightly and that of wide-pass neurons changed little; 3) As the PR increased, the recorded neuron's firing rate decreased, the latency delayed and the firing duration shortened; 4) The directional selectivity of the IC neurons was modulated by the MR and this influence was PR-dependant. Also, the PRs could affect the directional selectivity of the neurons alone and displayed MR-dependant. Our data demonstrated that PR could modulate the neural coding of MR and directional selectivity of the IC neurons responding to FMs stimuli, and the PR and MR have the co-modulated effect. It reveals that the temporal representation of dynamic sounds in the central auditory system is correlated with the repetition rate of naturally occurring sound.

Sound azimuth selectivity of inferior collicular neurons in juvenile bats, Myotis chinensis

The directional selectivity of auditory neurons is one of the essential response properties that underlie sound localization, an important task performed by the mammalian auditory system. Here we evaluated the sound azimuth selectivity of inferior collicular neurons in juvenile bats, Myotis chinensis, at the age of 15 days under free-field stimulation conditions. Compared with those in adult bats, neurons in juvenile bats were broadly tuned to sound azimuth angles as indicated by both the type and width of the azimuth selectivity curves. Their best azimuth was distributed over a wide range and, moreover, the adult-like relationship between the best azimuth and the best frequency of these neurons was not developed. These data indicate that the directional selectivity of inferior collicular neurons, like other response properties examined previously, undergoes considerable change during postnatal maturation.

Masking effect of different durations of forward masker sound on acoustical responses of mouse inferior collicular neurons to probe sound

To study the effects of different durations of forward masker sound on neuronal firing and rate-intensity function (RIF) of mouse inferior collicular (IC) neurons, a tone relative to 5 dB above the minimum threshold (re MT+5 dB) of the best frequency of recorded neurons was used as forward masker sound under free field stimulation condition. The masker durations used were 40, 60, 80, and 100 ms. Results showed that as masker duration was increased, inhibition in neuronal firing was enhanced (P < 0.000 1, n = 41) and the latency of neurons was lengthened (P<0.01, n = 41). In addition, among 41 inhibited IC neurons, 90.2% (37/41) exhibited narrowed dynamic range (DR) when masker sound duration was increased (P < 0.000 1), whereas the DR of 9.8%(4/41) became wider. These data suggest that masking effects of different durations of forward masker sound might be correlated with the amplitude and duration of inhibitory input to IC neurons elicited by the masker sound.

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 ± 4.4 kHz but sharpened the directional sensitivity of other 49 (55%) neurons with BF IC–AC differences of 2.3 ± 1.8 kHz. 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.

Azimuth-dependent recovery cycle affects directional selectivity of bat inferior collicular neurons determined with sound pulses within a pulse train

In our previous study, we have shown that the recovery cycle of most neurons in the inferior colliculus (IC) of the big brown bat, Eptesicus fuscus, is typically longer at ipsilateral azimuth than at contralateral azimuth under free-field stimulation conditions. The present study is to test the hypothesis that this azimuth-dependent recovery cycle may contribute to the variation of directional selectivity of IC neurons with sequential presented sound pulses within a pulse train. A 300-ms pulse train containing nine sound pulses of 4-ms with an inter-pulse interval of 33.3 ms was delivered at several selected azimuthal angles between ±80° lateral in the frontal auditory space of a bat. A family of nine directional selectivity curves was plotted with a neuron's number of impulses in response to each individual pulse against the azimuthal angles. The type and sharpness of these directional selectivity curves were then compared in relation to pulse position within the pulse train. All 675 directional selectivity curves obtained from 75 IC neurons could be described as directionally selective (423, 63%), hemifield (220, 32%), or non-directional (32, 5%). The directional selectivity curves of 45 (60%) neurons did not vary with pulse position. However, those of the remaining neurons (30, 40%) changed from one type to another such that the number of neurons with directionally selective curves progressively increased and the number of neurons with hemifield and non-directional selectivity curves decreased with increasing pulse position within the pulse train. Among 68 IC neurons whose directional selectivity curves were compared quantitatively, directional selectivity determined with sequentially presented sound pulses significantly increased in 38 (56%) neurons; decreased in 18 (26%) neurons but did not change in 12 (18%) neurons. This change of directional selectivity was due to the variation in recovery cycle of these IC neurons with azimuthal angle as we hypothesized.

Neural responses to free field and virtual acoustic stimulation in the inferior colliculus of the guinea pig.

Virtual auditory space (VAS) stimuli based on outer ear transfer functions became increasingly important in spatial hearing research. However, few studies have investigated the match between responses of auditory neurons to VAS and free-field (FF) stimulation. This study validates acoustic spatial receptive fields (SRFs) of 183 individual midbrain units using both VAS and FF stimuli. The first-spike latency, which varied systematically across SRFs, was 14.9 +/- 8.3 (SD) ms in FF, and 15.1 +/- 8.3 ms in VAS. Spike-count-based SRFs measured 0-20 dB above the neural threshold covered on average 44.5 +/- 18.0% of the recorded sphere in FF and 45.5 +/- 18.7% in VAS. The average deviation of the centroid position of SRFs using FF and VAS stimuli was 7.4 degrees azimuth and 3.3 degrees elevation. The average spike rate remained unchanged. The SRF overlap recorded using FF and VAS stimuli (mean: 71.3 +/- 12.6%) or repeated FF stimuli (70.2 +/- 14.2%) was high and strongly correlated (r = 0.96; P < 0.05). The SRF match observed with FF and VAS stimuli was not significantly altered over a range of stimulus levels (paired t-test P = 0.51; n = 6). Randomized VAS barely affected SRF sizes, centroids, or maximum spike count but decreased the average minimum response to 59% compared with sequential stimulation (paired t-test; P = 0.05; n = 26). SRF recordings in VAS excluding the acoustic distortions of the recording equipment differed from those in VAS incorporating the equipment (paired t-test P = 0.01; n = 5). In conclusion, neurophysiological recordings demonstrate that individualized VAS stimuli provided a good simulation of a FF environment.

The effect of bicuculline application on azimuth-dependent recovery cycle of inferior collicular neurons of the big brown bat, Eptesicus fuscus

The recovery cycle of auditory neurons is an important neuronal property, which determines a neuron’s ability to respond to pairs of sounds presented at short inter-sound intervals. This property is particularly important for bats, which rely upon analysis of returning echoes to extract the information about targets after emission of intense orientation sounds. Because target direction often changes throughout the course of hunting, the changing echo direction may affect the recovery cycle and thus temporal processing of auditory neurons. In this study, we examined the effect of sound azimuth on the recovery cycle of inferior collicular (IC) neurons in the big brown bat, Eptesicus fuscus, under free-field stimulation conditions. Our study showed that the recovery cycle of most IC neurons (42/49, 86%) was longer when determined with sounds delivered at 40° ipsilateral (i40°) than at 40° contralateral (c40°) to the recording site. To study the contribution of GABAergic inhibition to sound azimuth-dependent recovery cycle, we compared the recovery cycle of IC neurons determined at two sound azimuths before and during iontophoretic application of bicuculline, an antagonist for GABA A receptors. Bicuculline application produced a greater decrease of the recovery cycle of these neurons at i40° than at c40°. As a result, the azimuth-dependent recovery cycle of these neurons was abolished or greatly reduced. Possible mechanisms underlying these observations and biological relevance to bat echolocation are discussed.

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 γ-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° than at 30° contralateral. This direction-dependent frequency tuning was effectively abolished by occlusion of the ipsilateral ear. Possible mechanisms underlying these observations are discussed.

Binaurality and azimuth tuning of neurons in the auditory cortex of the big brown bat

By using a combined closed and free-field stimulation system, binaurality and azimuth tuning of the neurons in the auditory cortex of the big brown bat,Eptesicus fuscus, were studied. A variety of azimuth-tuning functions were demonstrated for the binaural neurons. The large majority of EE (contralateral and ipsilateral excitatory) neurons exhibited azimuth selectivity with the best azimuths (BA) at contralateral 30°–40°, some at ipsilateral 20°-40° and preferred azimuth ranges (PAR, response amplitude ⩾75% of maximum) between 8° and 40°. Sound source azimuths strongly modulate spike counts with a mean modulation depth of 83.8% for EE neurons. EI (contralateral excitatory and ipsilateral inhibitory) neurons have simple azimuth tuning with BA located at contralateral 20°-40° and a broad PAR ranged from 30° to 55°. The present results suggest that azimuth-tuning characteristics of binaural neurons in the auditory cortex of the bat are of significance for acoustic behaviour.

The effect of sound duration on rate–amplitude functions of inferior collicular neurons in the big brown bat, Eptesicus fuscus

During echolocation, the amplitude and duration of echo pulses of the big brown bat, Eptesicus fuscus, covary throughout the entire course of hunting. The purpose of this study was to examine if variation in sound duration might affect the amplitude selectivity of inferior collicular (IC) neurons of this bat species under free-field stimulation conditions. A family of rate–amplitude functions of each IC neuron was obtained with different sound durations. The effect of sound duration on the neuron’s amplitude selectivity was then studied by examining the type, best amplitude, dynamic range and slope of each rate–amplitude function. The rate–amplitude functions of 83 IC neurons determined with different sound durations were either monotonic, saturated or non-monotonic. Neurons with monotonic rate–amplitude functions had the highest best amplitude, largest dynamic range but smallest slope. Neurons with non-monotonic rate–amplitude functions had the lowest best amplitude, smallest dynamic range but largest slope. The best amplitude, dynamic range and slope of neurons with saturated rate–amplitude functions were intermediate between these two types. Rate–amplitude functions of one group (47, 57%) of IC neurons changed from one type to another with sound duration and one-third of these neurons were tuned to sound duration. As a result, the best amplitude, dynamic range, and slope also varied with sound duration. However, rate–amplitude functions of the other group (36, 43%) of IC neurons were hardly affected by sound duration and two-thirds of these neurons were tuned to sound duration. Biological relevance of these findings in relation to bat echolocation is discussed.

Representation of sound source direction in the superior colliculus of the guinea pig in a virtual auditory environment.

The deep layers of the superior colliculus (SC) receive visual, auditory, and somatosensory input. A major function of the SC is the control of orientation movements of the eye, head, and pinna. While a topographical map for sound source direction remains elusive in primary auditory structures of mammals, such a map for azimuthal sound source directions has been reported in the deep layers of the SC. Moreover, a gradient of elevation tuning has been also seen in the SC of ferrets and cats. Here we demonstrate that a virtual auditory environment can be used to reveal azimuthal and elevational topography for auditory spatial receptive fields in neurons in the SC of guinea pigs. Individual, head-related transfer functions (HRTF) were measured in ten guinea pigs for 122 directions in the upper hemispheric field and convolved with white noise. Many neurons (39%) in the deep layers showed robust responses to these virtual sounds, and the majority of these neurons had small spatial receptive fields that were restricted to the contralateral hemifield. Best directions varied from 0 degree to 135 degree azimuth along the contralateral side and from --10 degree to 60 degree elevation. Like previous studies using free-field stimulation, a gradient of best azimuth direction was found along the rostral-caudal axis, with rear directions represented caudally and front directions rostrally. The topographical organization for best elevations had not been studied previously in the guinea pig. We found that it roughly followed the mediolateral axis, with preference for high elevations represented medially and low elevations laterally. A similar organization using free-field stimulation has been reported in the ferret.

Involuntary attention in children as a function of sound source location: evidence from event-related potentials

Objectives: The present study addressed the question of whether location of the auditory stimulation source affects an involuntary attention triggering to the deviant sounds in a passive oddball paradigm in 8–10-year-old children.Methods: Using free-field stimulation two late event-related potentials components were examined: the mismatch negativity (MMN), indexing preconscious sound change detection and the P3a, indexing involuntary attention switch. Data were registered to frequency changes in sounds of different complexities in two experimental conditions. In the ‘in-front’ condition, the sound sequences were presented through the loudspeakers situated in front of a participant on both sides of the video display. In the ‘on-sides’ condition, the sources of auditory and visual stimuli were separated by moving the loudspeakers to the sides of the participant.Results: The MMN amplitude or the MMN and P3a latencies varied in neither stimulus class significantly as a function of sound location. However, significantly larger P3 amplitude was found in the ‘in-front’, as compared to the ‘on-sides’ condition.Conclusions: The present results indicate enhanced involuntary attention switching in children when unattended auditory events occur within the space attended actively for visual modality. Such study design favouring cross-modal integration can be advantageous when studying involuntary auditory attention and its impairment in children.