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PubMed2001

Preoperative functional assessment of auditory cortex in adult cochlear implant users.

OBJECTIVES: To explore functional neuroanatomical responses to auditory stimulation before and after implantation. STUDY DESIGN: A prospective study of three cochlear implant candidates (pure-tone averages of 90 dB HL or greater bilaterally and hearing in noise test [HINT] performances of <40%) in which regional cerebral blood flow (rCBF) was assessed using single photon emission computed tomography (SPECT). METHODS: Candidates watched a 15-minute videotaped story under four conditions: audio presented monaurally in the right and left ears (aided), audio presented binaurally (aided), and visual-only presentation of the story. Five minutes into each story, 20 to 25 mCi of technetium 99m (99mTc) hexamethyl-propyleneamine-oxime (HMPAO) (Ceratec; Nycomed Amersham, Princeton, NJ, U.SA) was injected over a 30-second period to ensure that subjects were unaware of tracer administration. Subjects were scanned for 20 minutes using a PRISM 3000 gamma camera (Picker International, Cleveland, OH, U.S.A.). Data were normalized and co-registered, and subtraction images were compiled. Subtraction images contrasted activation patterns generated under the visual-only control condition to the auditory activation states acquired monaurally and binaurally. RESULTS: Right and left ear monaural stimulation in normal hearing subjects resulted in significant bilateral activation of Brodmann areas 41, 42, 21, 22, and 38. Although substantial intersubject response variability was noted, subjects generally failed to bilaterally activate these areas under monaural hearing aid presentations; however, bilateral activation of areas 41 and 22 was noted under binaural presentations. CONCLUSIONS: Despite relatively similar hearing losses in each ear, significant differences in preoperative auditory cortex activation were observed between ears. These data suggest that functional brain imaging provides a useful tool for exploring the responsiveness of the auditory cortex in cochlear implant candidates.

Acoustic Stimulation↗
PubMed1999

Influence of acoustic masking noise in fMRI of the auditory cortex during phonetic discrimination.

The application of functional magnetic resonance imaging (fMRI) to study activation of auditory cortex suffers from one significant confounding factor, namely, that of the acoustic noise generated by the gradient system, which is an integral part of the imaging process. Earlier work has shown that it is indeed possible to distinguish cortical activation resulting from presentation of auditory stimuli despite the presence of background noise from the gradient system. The influence of acoustic noise from the gradient system of the MRI scanner on the blood oxygen level-dependent (BOLD) response during functional activation of the auditory cortex has been investigated in six healthy subjects with no hearing difficulties. Experiments were performed using gradient-echo echoplanar imaging (EPI) and a verbal, auditory discrimination paradigm, presented in a block-wise manner, in which carefully aligned consonant-vowel syllables were presented at a rate of 1 Hz. For each volunteer the experiment was repeated three times with all parameters fixed, except slice number, which was 4, 16, or 64. The positioning of the central four slices in each experiment was common. Thus, the fraction of TR during which the stimulus is on but no imaging is being performed, varies from almost zero, in the case of 64 slices, to over 8 seconds, in the case of four slices. Only the central four slices were of interest; additional slices simply generated acoustic noise and were discarded. During the four-slice experiment, all subjects showed a robust BOLD response in the superior temporal gyrus covering the primary and secondary auditory cortex. The spatial extent and the z-scores of the activated regions decreased with longer duration of gradient noise from the scanner. For a phonetic discrimination task, the results indicate that presentation of the stimulus during periods free from scanner noise leads to a more pronounced BOLD response.

Acoustic Stimulation↗
PubMed1988

Effect of tone-pulse rise time on rate-level functions of cat auditory cortex neurons: excitatory and inhibitory processes shaping responses to tone onset.

1. The responses of cat auditory cortex neurons are largely dominated by transient stimulus events, including tone-pulse onset. In addition, these neurons often receive sensitive inhibitory inputs in tone frequency-intensity domains flanking the excitatory one centered at characteristic frequency (CF). These observations suggest that auditory cortex neurons might be sensitive to the spectral splatter that occurs at tone onset due to the tone-pulse envelope shape. 2. To investigate this hypothesis, single neurons in the primary auditory cortex of anesthetized cats were studied for the form of their spike-rate versus tone-level functions using CF tone pulses of different rise times. Stimuli were presented to the contralateral ear using a calibrated, sealed stimulus delivery system. 3. Some neurons with monotonic rate-level functions for conventional (5-10 ms) rise-time tones were relatively insensitive to variations in tone-pulse rise time. Other monotonic neurons showed rate-level functions that became increasingly bell shaped for shorter rise-time stimuli. All neurons with bell-shaped, nonmonotonic rate-level functions for conventional rise-time tones became increasingly nonmonotonic for shorter rise-time signals. In the same neurons, lengthening of tone rise times typically reduced the slope of the high-intensity, descending limb of the rate-level function, in some cases to zero. 4. This pattern of rise-time effects is consistent with previous evidence on the association between rate-level function shape and the presence of inhibitory tone response areas flanking the excitatory one at CF. The present data suggest that cortical neurons are sensitive to the gross shape of the short-term stimulus spectrum at tone onset, and that for many neurons, the nonmonotonic form of CF tone rate level functions may be configured as much by the rate of tone onset as by the plateau amplitude of a tone pulse.

Acoustic Stimulation↗
PubMed1984

Identification and structure of neurons in the medial geniculate body projecting to primary auditory cortex (AI) in the cat.

The neuronal types in the ventral nucleus of the cat medial geniculate body projecting to the primary auditory cortex (AI) were investigated using the retrograde transport of horseradish peroxidase. These cells were compared with the morphology of neurons as revealed in Golgi and Nissl preparations, plastic-embedded tissue, and electron microscopic material. After large injections, more than 90% of the neurons in the ventral nucleus, the principal nucleus of the lemniscal auditory pathway, were labeled, and the population of labeled cells included both large and small neuronal somata. Since the ventral nucleus contains only two varieties of cells--large neurons with bushy dendrites and an unbranched axon, and smaller cells with thin dendrites and a locally projecting axon--it is concluded that at least some of the small cells, previously believed to be interneurons, may function both as local circuit and as projection neurons. These findings were confirmed in toluidine blue-stained, 1-2 micron thick sections, and in the electron microscope, where small cells with sparse cytoplasm and a deeply invaginated nuclear envelope often contained intracellular horseradish peroxidase granules, as well as the larger neurons. Besides the small, labeled neurons in the ventral nucleus, many labeled cells were seen in the interstitial nucleus of the brachium of the inferior colliculus. This hitherto poorly characterized group of cells is embedded among the fibers of the brachium of the inferior colliculus. Many of the morphologically distinct varieties of cells in the medial division of the medial geniculate body, including small neurons, were labeled. Thus, in addition to the route embodied by the large bushy neurons which project to primary auditory cortex, at least one other pathway--represented by certain of the small cells in the ventral nucleus, reaches the primary auditory cortex.

Animals↗
PubMed1988

Cortical connections of electrophysiologically and architectonically defined subdivisions of auditory cortex in squirrels.

Multiunit recordings with microelectrodes were used to identify and delimit subdivision of auditory cortex in squirrels. In the same animals, cortical connections of subdivisions of auditory cortex were determined by placing injections of the tracer wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP) into electrophysiologically defined locations. The electrophysiological results and patterns of connections were later related to myeloarchitectonic distinctions in brain sections cut parallel to the surface of the artificially flattened cortex. As previously described (Merzenich et al.: J. Comp. Neurol. 166:387-402, '76), a primary auditory field, A-I, was characterized by (1) neurons narrowly tuned to tone frequency; (2) a tonotopic map with high frequencies, which represented caudal to low frequencies; and (3) dense myelination. A-I was reciprocally connected with a rostral field, R, a parietal ventral somatosensory representation, PV, cortex ventral to A-I, and other nearby regions of cortex of the same hemisphere. Callosal connections of A-I were with A-I, R, and two or more other regions of temporal cortex. The less densely myelinated rostral field, R, also had neurons that were frequency tuned, but the neurons were often less securely driven. R appeared to have a tonotopic organization that roughly mirrored that of A-I. Ipsilateral connections of R included A-I, PV, and cortex ventral and caudal to R. Callosal connections were with R, A-I, PV, and cortex ventral and caudal to R. Callosal connections were with R, A-I, PV, and other locations in temporal cortex. Cortex in caudal PV, ventral to A-I, and ventral to R was responsive to auditory stimuli, but responses to pure tones were weak and inconsistent, and habituation to a repeated stimulus was rapid. The cortex responsive to auditory stimuli included some but not all of the cortex connected with A-I and R. The results lead to the conclusion that auditory cortex of squirrels contains at least two tonotopically organized fields, possibly as many as five or more auditory fields, and at least two auditory-somatosensory fields.

Animals↗
PubMed1996

Alteration of auditory cortex activity with a visual stimulus through conditioning: a 2-deoxyglucose analysis.

In two experiments, the 2-deoxyglucose metabolic mapping technique was used to examine the hypothesis that a stimulus of one modality (a light) will begin to activate the sensory cortex of a stimulus of another modality (a tone) with which it has been repeatedly paired. Adult gerbils received repeated presentations of either a light or the light paired with a tone known to affect 2DG labeling patterns in the auditory cortex. Intermittent footshock was included on a pseudo-random basis to maintain arousal in the subjects. One day after training, each gerbil was injected with 2DG and either received repeated presentations of the light only or was simply exposed to the training context. Analysis of the auditory cortex revealed no differences in overall metabolic activity of the auditory cortex between the groups. However, in both experiments, the light that was previously paired with the tone changed the relative activity of the cortical subfields compared to the light not previously paired with the tone. Specifically, the results indicate greater activity in the anterior auditory field (AAF-Experiments 1 and 2) and the posterior fields (DPVP-Experiment 2) relative to the primary field AI in response to the light that was previously paired with the tone during training. Gerbils either only placed in the context during the 2DG session or that received unpaired presentations of the light and tone during training did not show this shift in relative labeling between the subfields. Because no differences in overall activity of the auditory cortex were found, we conclude that the shift in relative labeling between the subfields reflects, on average, both an increase in activity of fields AAF and DPVP and a concomitant decrease in AI activity in response to the light stimulus. The results have implications for our understanding both of brain learning mechanisms in general and the potential functions of auditory cortex subfields in particular.

Animals↗
PubMed1998

The posterior field P of cat auditory cortex: coding of envelope transients.

The posterior field (P) of the cat auditory cortex contains a very high proportion of neurons whose responses change non-monotonically with the sound pressure level (SPL) of tonal stimuli, leading to circumscribed frequency-SPL response areas, and it has therefore been suggested that field P may be specialized for processing of sound intensity. We demonstrate here a great diversity of response areas in field P. Furthermore, by varying tone SPL and rise time, we show that, as in primary auditory cortex (AI), the onset response of a field P neuron is better described as a function of the instantaneous peak pressure (envelope) at the time of response generation than of the steady-state SPL of the stimulus. Such responses could be used to track transients or represent envelopes in more general terms, rather than to code SPL. Compared with AI, field P neurons have relatively long minimum latencies along with a large jitter in spike timing. Tracking would therefore be most effective for slowly varying envelopes, and one function of the inhibition that generates non-monotonicity in field P may be to suppress temporally sluggish responses to rapid transients, such as the onsets of high-SPL, short rise time tones. Field P may thus be specialized for coding slowly varying signals.

Acoustic Stimulation↗
PubMed1986

Columnar organization and reciprocity of commissural connections in cat primary auditory cortex (AI).

The laminar distribution and reciprocity of commissural axon terminals and cells of origin in cat primary auditory cortex (AI) were studied after injections of tritiated proline combined with horseradish peroxidase in the middle ectosylvian gyrus. Terminal fields were found in every cortical layer in the contralateral AI, and they were characterized quantitatively. The largest concentration of silver grains was in layer III (about 25% of the total number of silver grains) and, to a lesser extent, in layers V, VI, and I (some 18% of the total in each layer). The labeling in layer I was concentrated in its deeper half, while the labeling in the other layers was more homogeneous. Layer IV had the least labeling, followed by layer II, each receiving about 10% of the total. The labeling was always heaviest over the neuropil and lightest over neuronal perikarya. Commissural terminal fields formed radial patches oriented perpendicularly to the pia, and averaging 543 micron in width. There was consistently three times more silver grains in a patch than in an inter-patch area. However, the number of silver grains in an inter-patch area was always significantly above background, indicating a possible commissural projection to these zones as well. The patches of commissural terminal fields formed bands oriented across AI and running in a caudoventral to rostrodorsal direction. Strict reciprocity between the commissural cells of origin and terminal fields was not found at the light microscopic level when adjacent sections, corrected for differential shrinkage, were compared. Often, patches of terminal fields were free of retrogradely labeled cells and, conversely, there were patches of labeled cells without an overlying commissural terminal field. The terminal fields connected homotopic regions of the contralateral AI, and every region of AI received commissural innervation, unlike the primary somatic sensory and visual cortex, where large zones receive only a few commissural afferents. The more complete pattern of interhemispheric connectivity in auditory cortex is in contrast to the less continuous commissural representation in other sensory neocortical fields. Perhaps this pattern contributes to the anatomical representation of binaurality in auditory cortex.

Animals↗
PubMed1985

Morphology and laminar organization of electrophysiologically identified neurons in the primary auditory cortex in the cat.

The morphology of electrophysiologically identified neurons was examined in the primary auditory cortex (AI) of the cat. After stimulation of the medial geniculate nucleus (MG), second auditory cortex, posterior ectosylvian gyrus, contralateral AI, or corpus callosum, intracellular potentials were recorded from AI neurons, which were then injected intracellularly with horseradish peroxidase and recovered. Layer IV neurons, which receive MG fibers monosynaptically, are spiny and nonspiny stellate cells, small and medium-sized nonspiny tufted cells, and fusiform cells. They send their axons to layer III of the AI. Corticocortical AI neurons are medium-sized pyramidal cells in layer III. They receive axons from layer IV neurons of the AI and send their axons to layers I, II, IV, and V of the AI. Horizontal cells in layer I receive slow-conducting MG fibers monosynaptically, and send their axons to layer II of the AI. Stellate cells and small pyramidal cells in layer II receive afferent inputs polysynaptically from the MG. Layer II pyramidal cells receive afferent inputs from the MG via AI neurons in layers I and III, and send their axons to layers V and VI. The axons of layer II stellate cells were distributed within layer II. Pyramidal cells which send their axons to the MG are located in layers V and VI, distributing their axon collaterals to layers III-VI of the AI.

Animals↗
PubMed2001

Modular organization of intrinsic connections associated with spectral tuning in cat auditory cortex.

Many response properties in primary auditory cortex (AI) are segregated spatially and organized topographically as those in primary visual cortex. Intensive study has not revealed an intrinsic, anatomical organizing principle related to an AI functional topography. We used retrograde anatomic tracing and topographic physiologic mapping of acoustic response properties to reveal long-range (> or = 1.5 mm) convergent intrinsic horizontal connections between AI subregions with similar bandwidth and characteristic frequency selectivity. This suggests a modular organization for processing spectral bandwidth in AI.

Animals↗
PubMed1998

Plasticity of temporal information processing in the primary auditory cortex.

Neurons in the rat primary auditory cortex (A1) generally cannot respond to tone sequences faster than 12 pulses per second (pps). To test whether experience can modify this maximum following rate in adult rats, trains of brief tones with random carrier frequency but fixed repetition rate were paired with electrical stimulation of the nucleus basalis (NB) 300 to 400 times per day for 20-25 days. Pairing NB stimulation with 5-pps stimuli markedly decreased the cortical response to rapidly presented stimuli, whereas pairing with 15-pps stimuli significantly increased the maximum cortical following rate. In contrast, pairing with fixed carrier frequency 15-pps trains did not significantly increase the mean maximum following rate. Thus this protocol elicits extensive cortical remodeling of temporal response properties and demonstrates that simple differences in spectral and temporal features of the sensory input can drive very different cortical reorganizations.

Acoustic Stimulation↗
PubMed2005

Responses to species-specific vocalizations in the auditory cortex of awake and anesthetized guinea pigs.

Species-specific vocalizations represent an important acoustical signal that must be decoded in the auditory system of the listener. We were interested in examining to what extent anesthesia may change the process of signal decoding in neurons of the auditory cortex in the guinea pig. With this aim, the multiple-unit activity, either spontaneous or acoustically evoked, was recorded in the auditory cortex of guinea pigs, at first in the awake state and then after the injection of anesthetics (33 mg/kg ketamine with 6.6 mg/kg xylazine). Acoustical stimuli, presented in free-field conditions, consisted of four typical guinea pig calls (purr, chutter, chirp and whistle), a time-reversed version of the whistle and a broad-band noise burst. The administration of anesthesia typically resulted in a decrease in the level of spontaneous activity and in changes in the strength of the neuronal response to acoustical stimuli. The effect of anesthesia was mostly, but not exclusively, suppressive. Diversity in the effects of anesthesia led in some recordings to an enhanced response to one call accompanied by a suppressed response to another call. The temporal pattern of the response to vocalizations was changed in some cases under anesthesia, which may indicate a change in the synaptic input of the recorded neurons. In summary, our results suggest that anesthesia must be considered as an important factor when investigating the processing of complex sounds such as species-specific vocalizations in the auditory cortex.

Acoustic Stimulation↗
PubMed2003

Visual stimuli activate auditory cortex in deaf subjects: evidence from MEG.

Studies using fMRI have demonstrated that visual stimuli activate auditory cortex in deaf subjects. Given the low temporal resolution of fMRI, it is uncertain whether this activation is associated with initial stimulus processing. Here, we used MEG in deaf and hearing subjects to evaluate whether auditory cortex, devoid of its normal input, comes to serve the visual modality early in the course of stimulus processing. In line with previous findings, visual activity was observed in the auditory cortex of deaf, but not hearing, subjects. This activity occurred within 100-400 ms of stimulus presentation and was primarily over the right hemisphere. These results add to the mounting evidence that removal of one sensory modality in humans leads to neural reorganization of the remaining modalities.

Adult↗
PubMed1991

Primary auditory cortex in the rat: transient expression of acetylcholinesterase activity in developing geniculocortical projections.

A characteristic pattern of acetylcholinesterase (AChE) activity is expressed transiently in primary auditory cortex (cortical area 41) of developing laboratory rats during early postnatal life. This AChE activity occurs as a dense plexus in cortical layer IV and the deep part of layer III. This transient band of AChE activity is first detected by histochemical techniques on postnatal day (P) 3, reaches peak intensity at approximately P8-10, and declines to form the adult pattern by P23. The ventral nucleus of the medial geniculate body of the thalamus also displays prominent, and transient, staining for AChE. This intense staining for AChE, found within neuronal somata and neuropil, is detected at the time of birth, reaches peak intensity around P8, and declines to adult levels by P16. The areal and laminar patterns of the transient band of AChE activity in temporal cortex correspond to the patterns of anterograde transneuronal labeling of geniculocortical terminals following injection of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the inferior colliculus. Placement of lesions that include the medial geniculate nucleus or the geniculocortical axons results in a marked decrease in AChE staining in thalamorecipient layers of auditory cortex. Placement of lesions that include the medial globus pallidus reduce AChE staining of some axons in temporal cortex of developing rats, but the dense band of AChE in layers III and IV remains. Placement of lesions in the inferior colliculus in newborn animals results in marked decrease in AChE staining in cells of the ipsilateral ventral medial geniculate nucleus and in ipsilateral auditory cortex of developing pups. These data indicate that transiently expressed AChE activity is characteristic of geniculocortical neurons, including their somata in the medial geniculate body and their terminal axons in primary auditory cortex. This AChE activity is expressed early in postnatal development, probably during the time when thalamocortical axons are proliferating in cortical layer IV and forming synaptic contacts with cortical neurons.

Acetylcholinesterase↗
PubMed2002

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

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

Animals↗
PubMed2006

Effects of stimulus azimuth and intensity on the single-neuron activity in the auditory cortex of the alert macaque monkey.

It has been hypothesized that the primate auditory cortex is composed of at least two processing streams, one of which is believed to selectively process spatial information. To test whether spatial information is differentially encoded in different auditory cortical fields, we recorded the responses of single neurons in the auditory cortex of alert macaque monkeys to broadband noise stimuli presented from 360 degrees in azimuth at four different absolute intensities. Cortical areas tested were core areas A1 and rostral (R), caudal belt fields caudomedial and caudolateral, and more rostral belt fields middle lateral and middle medial (MM). We found that almost all neurons encountered showed some spatial tuning. However, spatial selectivity measures showed that the caudal belt fields had the sharpest spatial tuning, A1 had intermediate spatial tuning, and areas R and MM had the least spatial tuning. Although most neurons showed their best responses to contralateral space, best azimuths were observed across the entire 360 degrees of tested space. We also noted that although the responses of many neurons were significantly influenced by eye position, eye position did not systematically influence any of the spatially dependent responses that we measured. These data are consistent with the hypothesis that caudal auditory cortical fields in the primate process spatial features more accurately than the core and more rostral belt fields.

Acoustic Stimulation↗
PubMedDate not supplied

Suprathreshold auditory cortex activation visualized by intrinsic signal optical imaging.

The suprathreshold tonotopic organization of rat and guinea pig auditory cortex was investigated using intrinsic signal optical imaging through a thinned skull. Optical imaging revealed that suprathreshold pure sine wave tone stimulation (25-80 dB) evoked activity over large cortical areas that were tonotopically organized. Three-dimensional surface plots of the activated areas revealed "patchy' auditory-evoked activity consisting of numerous local peaks and valleys building to a maximum. Subsequent detailed electrophysiological mapping in the same subjects confirmed the localization of auditory-evoked activity based on optical imaging, including responses to a test frequency at cortical loci more than 2 octaves away from the threshold-defined isofrequency contour. The success of this technique in visualizing auditory cortex functional organization at suprathreshold stimulus levels will allow for future investigations of auditory cortex frequency representation, including representational plasticity induced by a variety of experimental manipulations.

Acoustic Stimulation↗
PubMed1996

SPET monitoring of perfusion changes in auditory cortex following mono- and multi-frequency stimuli.

AIM: In order to assess the relationship between auditory cortex perfusion and the frequency of acoustic stimuli, twenty normally-hearing subjects underwent cerebral SPET. METHODS: In 10 patients a multi-frequency stimulus (250-4000 Hz at 40 dB SL) was delivered, while 10 subjects were stimulated with a 500 Hz pure tone at 40 dB SL. The prestimulation SPET was subtracted from poststimulation study and auditory cortex activation was expressed as percent increments. RESULTS: Contralateral cortex was the most active area with multi-frequency and monofrequency stimuli as well. A clear demonstration of a tonotopic distribution of acoustic stimuli in the auditory cortex was achieved. In addition, the accessory role played by homolateral acoustic areas was confirmed. CONCLUSION: The results of the present research support the hypothesis that brain SPET may be useful to obtain semi-quantitative reliable information on low frequency auditory level in profoundly deaf patients. This may be achieved comparing the extension of the cortical areas activated by high-intensity multifrequency stimuli.

Acoustic Stimulation↗