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PubMed1984

[Primary inhibitory responses of neurons of a chronically isolated band of auditory cortex to intracortical stimulation].

The reactions of 579 neurons to intracortical stimulation of an auditory cortex slab (three weeks after its isolation) were studied intracellularly in three series of experiments on cats. The distances between stimulating and recording electrodes were 0.5, 1.0 and 2.0 mm. It was shown that 67.7% of neurons responded to intracortical stimulation by primary IPSP, which is by 10% more than that in acute isolated slab. The distribution of neurons reacting by primary IPSP was analysed according to the depth of location in the isolated slab and to the duration of IPSPs and their latent periods depending on the distance between the point of stimulating and recording microelectrodes. Latent periods of IPSPs did not exceed 10 ms like in an acute isolated slab. Amplitude and duration of IPSP were in the same range as in acute isolated slab and intact auditory cortex of cat brain. The histological structure of the chronically isolated auditory cortex slab is described.

Animals↗
PubMed1996

Stimulus-induced spike bursts in two fields of cat auditory cortex.

The sound-evoked responses of extracellularly recorded cat primary auditory cortical neurons usually consist of a single spike or a short-term burst of 2-4 spikes, irrespective of the nature of the acoustic signal. In the cat's auditory cortex, the properties of such responses have to date been described only for cells in the primary field (AI). The purpose of the present study was to describe the properties of stimulus-evoked spike-burst responses seen in neurons of the posterior auditory field (P) and to compare those properties with those of a sample of AI neurons studied under similar conditions. The data come from 80 field P and 31 AI neurons studied with tonal and noise-burst stimuli in barbiturate-anesthetized cats, using calibrated, sealed stimulus delivery systems and conventional extracellular recording techniques. The mean inter-spike intervals (ISI) seen in the transient burst responses of posterior field cells were typically short (2-5 ms) and, where it was possible to test them, independent of the rise time of tonal signals, suggesting that they were also independent of the onset spectrum of the stimulus. The mean ISIs were often independent of the stimulus amplitude, even though the signal level had profound effects on the number of spikes evoked and the latency and regularity with which the responses were initiated. Each neuron was assigned a 'characteristic ISI', i.e., the mean ISI seen in the most vigorous responses. The distribution of characteristic ISIs for AI and P neurons overlapped, but were significantly different, with the characteristic ISIs of field P neurons being longer. In both AI and P populations, characteristic ISI was significantly correlated with minimal first-spike latency. The slopes of the regression lines of characteristic ISI on minimal latency for AI and for P cells were not significantly different from each other. Since the minimal latencies of AI neurons were usually shorter than those of field P neurons, the shorter characteristic ISIs of AI cells may thus be interpreted as secondary to their shorter latent periods. The general properties of stimulus-evoked spike bursts seen in field P neurons were thus very similar those previously described for AI cells. These data are consistent with the view that the majority of extracellular recordings in the cat's auditory cortex come from pyramidal neurons and are appropriate as a specialization for transfer of information to nonpyramidal, inhibitory interneurons.

Acoustic Stimulation↗
PubMed2003

Tonotopic organization in human auditory cortex revealed by progressions of frequency sensitivity.

Functional neuroimaging experiments have revealed an organization of frequency-dependent responses in human auditory cortex suggestive of multiple tonotopically organized areas. Numerous studies have sampled cortical responses to isolated narrow-band stimuli, revealing multiple locations in auditory cortex at which the position of response varies systematically with frequency content. Because appropriate anatomical or functional grouping of these distinct frequency-dependent responses is uncertain, the number and location of tonotopic mappings within human auditory cortex remains unclear. Further, sampling does not address whether the observed mappings exhibit continuity as a function of position. This functional magnetic resonance imaging study used frequency-swept stimuli to identify progressions of frequency sensitivity across the cortical surface. The center-frequency of narrow-band, amplitude-modulated noise was slowly swept between 125 and 8,000 Hz. The latency of response relative to sweep onset was determined for each cortical surface location. Because frequency varied systematically with time, response latency indicated the frequency to which a location was maximally sensitive. Areas of cortex exhibiting a progressive change in response latency with position were considered tonotopically organized. There exist two main findings. First, six progressions of frequency sensitivity (i.e., tonotopic mappings) were repeatably observed in the superior temporal plane. Second, the locations of the higher- and lower-frequency endpoints of these progressions were approximately congruent with regions reported to be most responsive to discrete higher- and lower-frequency stimuli. Based on these findings and previous anatomical work, we propose a correspondence between these progressions and anatomically defined cortical areas, suggesting that five areas in human auditory cortex exhibit at least six tonotopic organizations.

Acoustic Stimulation↗
PubMed2003

Functional asymmetry for auditory processing in human primary auditory cortex.

Structural asymmetries in the supratemporal plane of the human brain are often cited as the anatomical basis for the lateralization of language predominantly to the left hemisphere. However, similar asymmetries are found for structures mediating earlier events in the auditory processing stream, suggesting that functional lateralization may occur even at the level of primary auditory cortex. We tested this hypothesis using functional magnetic resonance imaging to evaluate human auditory cortex responses to monaurally presented tones. Relative to silence, tones presented separately to either ear produced greater activation in left than right Heschl's gyrus, the location of primary auditory cortex. This functional lateralization for primary auditory cortex is distinct from the contralateral dominance reported for other mammals, including nonhuman primates, and may have contributed to the evolution of a unique role for the left hemisphere in language processing.

Adult↗
PubMed2004

Hemispheric shifts of sound representation in auditory cortex with conceptual listening.

The weak field specificity and the heterogeneity of neuronal filters found in any given auditory cortex field does not substantiate the view that such fields are merely descriptive maps of sound features. But field mechanisms were previously shown to support behaviourally relevant classification of sounds. Here the prediction was tested in human auditory cortex (AC) that classification-tasks rather than the stimulus class per se determine which auditory cortex area is recruited. By presenting the same set of frequency-modulations we found that categorization of their pitch direction (rising versus falling) increased functional magnetic resonance imaging activation in right posterior AC compared with stimulus exposure and in contrast to left posterior AC dominance during categorization of their duration (short versus long). Thus, top-down influences appear to select not only auditory cortex areas but also the hemisphere for specific processing.

Adult↗
PubMed2001

Regional glucose metabolic increases in left auditory cortex in tinnitus patients: a preliminary study with positron emission tomography.

OBJECTIVE: To investigate the relationship between tinnitus and glucose metabolism in auditory cortex and whether positron emission tomography (PET) can be an objective tool in measuring tinnitus. METHODS: Eleven right-handed patients with severe tinnitus and ten right-handed control subjects participated in the 18F-FDG/PET study. Analysis with regions of interests was used to calculate asymmetry indices according to the formula: [(L-R) x 100/[(L + R) divided by 2]]. RESULTS: Glucose metabolism in the auditory cortex of tinnitus patients was asymmetric between the left and right auditory cortices, with that of the left being much higher than that of the right. The asymmetry indices of tinnitus patients was significantly higher than that of the control group (unpaired t test, P < 0.001). This revealed that the increased metabolic activity was present in the predominant left hemisphere with a significant focus on the superior and transverse temporal gyri (Brodmann areas 41 and 42, respectively corresponding to primary and secondary auditory cortex), and the results were independent of the subjective localization of the tinnitus sensation. CONCLUSION: It is suggested that the increased metabolism in the left auditory cortex is related to the tinnitus sensation. PET is capable of providing objective evidence for tinnitus and may be used as a potential tool in measuring tinnitus.

Adolescent↗
PubMed2004

Environmental enrichment improves response strength, threshold, selectivity, and latency of auditory cortex neurons.

Over the last 50 yr, environmental enrichment has been shown to generate more than a dozen changes in brain anatomy. The consequences of these physical changes on information processing have not been well studied. In this study, rats were housed in enriched or standard conditions either prior to or after reaching sexual maturity. Evoked potentials from awake rats and extracellular recordings from anesthetized rats were used to document responses of auditory cortex neurons. This report details several significant, new findings about the influence of housing conditions on the responses of rat auditory cortex neurons. First, enrichment dramatically increases the strength of auditory cortex responses. Tone-evoked potentials of enriched rats, for example, were more than twice the amplitude of rats raised in standard laboratory conditions. Second, cortical responses of both young and adult animals benefit from exposure to an enriched environment and are degraded by exposure to an impoverished environment. Third, housing condition resulted in rapid remodeling of cortical responses in <2 wk. Fourth, recordings made under anesthesia indicate that enrichment increases the number of neurons activated by any sound. This finding shows that the evoked potential plasticity documented in awake rats was not due to differences in behavioral state. Finally, enrichment made primary auditory cortex (A1) neurons more sensitive to quiet sounds, more selective for tone frequency, and altered their response latencies. These experiments provide the first evidence of physiologic changes in auditory cortex processing resulting from generalized environmental enrichment.

Acoustic Stimulation↗
PubMedDate not supplied

Bilateral ablation of auditory cortex in Mongolian gerbil affects discrimination of frequency modulated tones but not of pure tones.

This study examines the role of auditory cortex in the Mongolian gerbil in differential conditioning to pure tones and to linearly frequency-modulated (FM) tones by analyzing the effects of bilateral auditory cortex ablation. Learning behavior and performance were studied in a GO/NO-GO task aiming at avoidance of a mild foot shock by crossing a hurdle in a two-way shuttle box. Hurdle crossing as the conditioned response to the reinforced stimulus (CR+), as false alarm in response to the unreinforced stimulus (CR-), intertrial activity, and reaction times were monitored. The analysis revealed no effects of lesion on pure tone discrimination but impairment of FM tone discrimination. In the latter case lesion effects were dependent on timing of lesion relative to FM tone discrimination training. Lesions before training in naive animals led to a reduced CR+ rate and had no effect on CR- rate. Lesions in pretrained animals led to an increased CR- rate without effects on the CR+ rate. The results suggest that auditory cortex plays a more critical role in discrimination of FM tones than in discrimination of pure tones. The different lesion effects on FM tone discrimination before and after training are compatible with both the hypothesis of a purely sensory deficit in FM tone processing and the hypothesis of a differential involvement of auditory cortex in acquisition and retention, respectively.

Animals↗
PubMed1990

Auditory cortex of the long-eared hedgehog (Hemiechinus auritus). I. Boundaries and frequency representation.

The boundaries of the primary auditory cortex of the long-eared hedgehog, Hemiechinus auritus, were determined by single-cell recordings, myeloarchitecture and retrograde horseradish peroxidase labeling in the medial geniculate, using anesthetized animals. The auditory cortex is located on the lateral surface of the temporal cortex, medial to the rhinal fissure. Responses to pure tones revealed an orderly representation of best frequencies in the primary auditory cortex, with low frequencies represented rostrally and high frequencies caudally. A second auditory field caudal to the primary one was indicated.

Animals↗
PubMed2004

Magnetic and electrical stimulation of the auditory cortex for intractable tinnitus. Case report.

Tinnitus is a distressing symptom that affects up to 15% of the population for whom no satisfactory treatment exists. The authors present a novel surgical approach for the treatment of intractable tinnitus, based on cortical stimulation of the auditory cortex. Tinnitus can be considered an auditory phantom phenomenon similar to deafferentation pain, which is observed in the somatosensory system. Tinnitus is accompanied by a change in the tonotopic map of the auditory cortex. Furthermore, there is a highly positive association between the subjective intensity of the tinnitus and the amount of shift in tinnitus frequency in the auditory cortex, that is, the amount of cortical reorganization. This cortical reorganization can be demonstrated by functional magnetic resonance (fMR) imaging. Transcranial magnetic stimulation (TMS) is a noninvasive method of activating or deactivating focal areas of the human brain. Linked to a navigation system that is guided by fMR images of the auditory system, TMS can suppress areas of cortical plasticity. If it is successful in suppressing a patient's tinnitus, this focal and temporary effect can be perpetualized by implanting a cortical electrode. A neuronavigation-based auditory fMR imaging-guided TMS session was performed in a patient who suffered from tinnitus due to a cochlear nerve lesion. Complete suppression of the tinnitus was obtained. At a later time an extradural electrode was implanted with the guidance of auditory fMR imaging navigation. Postoperatively, the patient's tinnitus disappeared and remains absent 10 months later. Focal extradural electrical stimulation of the primary auditory cortex at the area of cortical plasticity is capable of suppressing contralateral tinnitus completely. Transcranial magnetic stimulation may be an ideal method for noninvasive studies of surgical candidates in whom stimulating electrodes might be implanted for tinnitus suppression.

Adult↗
PubMed1997

Optical recording of azimuth representation in guinea pig auditory cortex.

The frequency dependency of directional sensitivity of the auditory cortex of barbiturate-anesthetized guinea pigs in response to tone bursts was studied by an optical equipment and a voltage sensitive dye (RH795). Tone bursts at 4, 8 and 14 kHz were presented from loudspeakers placed at the azimuths from contralateral 90 degrees to ipsilateral 90 degrees in 30 degrees steps on a front-horizontal semi-circular board (100 cm in radius). The maximum responses were observed at around contralateral 30 degrees (best azimuth). The onset and peak latencies were the shortest at the best azimuth. The best azimuth was the same for all these frequencies, but the azimuth selectivity became broader for the lower frequencies. These results indicate that the azimuth selectivity observed in the guinea pig auditory cortex is mainly associated with pinna characteristics.

Animals↗
PubMed1995

Patchy and laminar terminations of medial geniculate axons in monkey auditory cortex.

The object of this study was to identify the terminal distributions of thalamocortical axons arising in chemically characterized subdivisions of the medial geniculate complex. Large injections of wheat germ agglutinin-conjugated horseradish peroxidase or small injections of Phaseolus vulgaris leucoagglutinin were made in the medial geniculate complex of Macaca fuscata. The terminal distributions of labeled axons in the cortex were correlated with auditory cortical fields demonstrable by different intensities of immunoreactivity for parvalbumin. Fibers from the ventral nucleus terminated mainly in layer IV and deep portion of layer III (IIIB), with additional terminations in layers I-IIIA and in layer VI. In layers IIIB-IV, a major terminal plexus was formed by a small number of dense patches, 300-500 microns in diameter, surrounded by smaller satellite patches. The patches conformed to a similarly lobulated pattern of parvalbumin fiber immunoreactivity. Terminations of some individually labeled thalamocortical fibers were restricted to a single patch, whereas others innervated more than one patch by collateral branches. Fibers from the dorsal nuclei ending in areas of less dense parvalbumin immunoreactivity surrounding the primary auditory cortex formed much larger terminal patches centered largely in layer IIIB. Fibers from the magnocellular nucleus had relatively few terminal branches but innervated extremely wide areas by collaterals of single axons. Two types of axons arose from the magnocellular nucleus, one terminating preferentially in middle cortical layers and the other exclusively in layer I. These may arise respectively from parvalbumin- and calbindin-immunoreactive cell populations in the magnocellular nucleus.

Animals↗
PubMed1977

Response plasticity of neurons in auditory cortex of the rhesus monkey.

Auditory-evoked responses in single neurons from rhesus monkey auditory cortex were measured under four relatively well defined behavioral and physiological conditions: (1) monkey awake and performing a simple auditory reaction time task; (2) monkey awake but not performing a task (Stage A); (3) monkey in a drowsy or Stage 1 sleep state (State B); and (4) monkey anesthetized with a short-acting nonbarbiturate anesthetic. For most units studied the response evoked by the auditory stimulus was greater in the performance condition than in the nonperformance condition. Similarly, evoked activity was usually greater in State A than in State B. Finally, evoked responses under anesthesia were usually weaker than those obtained in the unanesthetized animal. Some exceptions were noted in each case. Differences in response patterns and in rate versus intensity functions of neurons were also found to be associated with the behavioral and physiological state of the preparation. No significant changes in unit spontaneous activity associated with changes in behavioral or physiological condition were observed.

Acoustic Stimulation↗
PubMed2000

A critical period for nicotine-induced disruption of synaptic development in rat auditory cortex.

Cholinergic markers in the middle layers of rat auditory cortex are transiently upregulated during the second postnatal week, at which time alpha 7 nicotinic acetylcholine receptors (nAChRs) selectively regulate NMDA receptor (NMDAR)-mediated EPSPs. To investigate the developmental role of this regulation, we determined whether manipulating nAChR function at specific times during the first 4 weeks after birth could alter subsequent neuronal function. Rat pups were injected twice daily with nicotine (1 or 2 mg/kg) or saline during approximately the first, second, or fourth postnatal week (i. e., before, during, or after the peak upregulation of nAChRs). Glutamate EPSPs and intrinsic membrane properties were measured during whole-cell recordings from visually identified pyramidal neurons in layers II-IV of brain slices prepared at least 15 hr after the last injection. Chronic nicotine exposure (CNE) had little effect on intrinsic membrane properties and during week 1 or 4 did not affect synaptic function. However, CNE during week 2 resulted in EPSPs with long durations, multiple peaks, and enhanced NMDAR components. These changes remained significant even 10 d after CNE. Rapid application of nicotine, which in control neurons selectively enhances NMDAR EPSPs during week 2, produced only weak effects after CNE. Receptor binding studies showed that CNE-induced EPSP alterations occurred in the absence of altered alpha 7 nAChR numbers or agonist binding affinity. Thus, altered stimulation of nAChRs by CNE during week 2, but not before or after, disrupts the development of glutamate synapses in rat auditory cortex.

Age Factors↗
PubMed2004

Aberrant localization of synchronous hemodynamic activity in auditory cortex reliably characterizes schizophrenia.

BACKGROUND: Among the most prominent features of schizophrenic brains are abnormalities in auditory cortex structure and function, particularly in the superior temporal gyrus (STG). In this study, we attempted to examine auditory cortex function using an intrinsic, task-uncorrelated measure. METHODS: Using functional magnetic resonance imaging data, we calculated synchronous hemodynamic independent maps (SHIMs) of auditory cortex in patients with schizophrenia and matched healthy control subjects while they performed an auditory oddball task. RESULTS: Patient SHIMs revealed greater synchrony in ventral and medial STG regions (including auditory association Brodmann area [BA] 42); control SHIMs had greater synchrony in dorsal and lateral STG regions (which did not include BA 42). A within-participant subtractive comparison of these two sets of regions differentiated schizophrenic from healthy control subjects with 97% accuracy initially (further validated by a retest of the healthy control subjects) and performed with 94% accuracy in a confirmatory study of new subjects scanned at a different site. CONCLUSIONS: These results shed new light on STG functional differences in schizophrenia, suggest that aberrant patterns of coherence in temporal lobe cortical regions are a cardinal abnormality in schizophrenia, and have the potential to provide a powerful, quantitative clinical tool for the assessment of schizophrenia.

Acoustic Stimulation↗
PubMed2002

Stimulus-related gamma oscillations in primate auditory cortex.

With a multielectrode system, we explored neuronal activity in the gamma range (>40 Hz) in the primary and caudomedial auditory cortex of six anesthetized macaque monkeys. Stimuli were tone bursts of 100- to 500-ms duration that were presented at sound pressure levels of 40-60 dB and were varied over a wide range of frequencies. These stimuli induced gamma oscillations, not phase-locked to the onset of stimulation, in 465 of 616 multiunit clusters and at 321 of 422 sites at which field potentials were recorded. Occurrence of gamma activity was stimulus dependent. It was mostly seen when the stimulus was at the units' preferred frequency. The incidence of gamma activity decreased with increasing difference between stimulus frequency and preferred frequency. gamma activity emerged 100-900 ms after stimulus onset with highest incidence ~120 ms. Amplitudes of stimulus-induced gamma oscillations in field potentials were, on average, almost twice the amplitude of spontaneously occurring gamma oscillations. gamma activity at different sites within the primary and the caudomedial auditory field could be synchronized at near-zero phase. Synchrony depended on the spatial distance and on the receptive fields similarity of pairs of units. It decreased with increasing distance between recording sites and increased with similarity of preferred frequencies of the pairs of units. The results indicate that stimulus-induced gamma oscillations originate from sources in the auditory cortex. They further suggest that gamma oscillations may provide a mechanism utilized in many parts of the sensory cortex, including the auditory cortex, to integrate neurons according to the similarity of their receptive fields.

Acoustic Stimulation↗
PubMed1999

Differential effects of overt, covert and replayed speech on vowel-evoked responses of the human auditory cortex.

Recent magnetoencephalographic studies showed that speaking dampens voice-evoked activity in the human auditory cortex. To further characterize this audio-vocal interaction, neuromagnetic responses to short tape-recorded probe vowels were measured while subjects were vocalizing long (8 s) 'background' vowels either aloud or silently, or while both probe and background vowels were replayed from tape. Auditory cortex responses peaking at 100 ms (M100) were delayed and dampened bilaterally relative to a background-free control during both overtly spoken and replayed long vowels, identifying auditory interference as the main cause for these modifications. During covert speech M100 peaked later for matching than non-matching probe/background vowels in the speech-dominant left hemisphere. Thus, voiceless 'inner' speaking is sufficient to modify utterance-specific processing in the human auditory cortex.

Adult↗