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PubMed1996

Thalamic modulation of high-frequency oscillating potentials in auditory cortex.

Perhaps the most widely recognized but least understood electrophysiological activity of the cerebral cortex is its characteristic electrical oscillations. Recently, there have been efforts to understand the mechanisms underlying high-frequency gamma oscillations(approximately 40 Hz) because they may coordinate sensory processing between populations of cortical cells. High-resolution cortical recordings show the gamma oscillations are constrained to sensory cortex, that they occur independently in auditory and somatosensory cortex, and that they are phase-locked between primary and secondary sensory cortex. As yet, the mechanism of their neurogenesis is unknown. Whereas cortical neurons can produce gamma oscillations without subcortical input, they may also be modulated by the thalamus and basal forebrain. Here we report that the neural generator of gamma oscillations in auditory cortex seems to be intracortical, serving to synchronize interactions between the primary and secondary areas. The acoustic thalamus directly modulates these oscillations, which are inhibited by stimulation of the dorsal and ventral divisions of the medial geniculate nucleus (MGd and MGv) and evoked by stimulation of the adjacent posterior intralaminar nucleus (PIL).

Animals↗
PubMed1988

Organization of auditory cortex in the albino rat: binaural response properties.

1. The binaural response properties of neurons in the auditory cortex of the albino rat were examined using microelectrode mapping techniques. Characteristic frequencies, binaural response classes, and interaural intensity differences for binaural interaction were determined for multiple electrode penetrations across the cortical surface. The location of electrode penetrations was determined by reference to the cortical vascular pattern in individual animals. 2. When examined over a wide range of interaural intensities binaural responses could be classified as one of the following types: summation, i.e., excited by stimulation of either ear alone and facilitated by stimulation of both ears together (35.3%); suppression, i.e., excited by contralateral stimulation, unaffected by ipsilateral stimulation alone, but inhibited under binaural stimulus conditions (42.2%); mixed, i.e., facilitated by binaural stimulation at near threshold levels, but strongly inhibited by increased sound pressure levels in the ipsilateral ear (18.5%); or other, i.e., responses that could not be classified as any other type (4%). 3. Neurons of the summation and suppression class often exhibited binaural interaction when the intensities at both ears were approximately equal. The modal interaural intensity difference for both response types was between 0 and +5 dB. Neurons of the mixed interaction class were facilitate at near equal dichotic intensity but suppressed when the intensity in the ipsilateral ear was increased. The modal value was between 0 and +5 dB for summation and +20 dB for suppression. 4. Summation, suppression, and mixed binaural response types were found over a wide range of sound frequencies from 1 to 40 kHz. There was some tendency for summation responses to prevail at lower frequencies and suppression responses to prevail at higher frequencies but the differences were not large. Generally, responses from each of the three binaural classes were well represented over the rat's hearing range. 5. Cells of the same binaural response type were grouped together to form aggregates of summation, suppression, or mixed interaction patterns. Cortical areas with similar binaural response properties appeared in some cases to extend across isofrequency contours.

Acoustic Stimulation↗
PubMed1976

Evoked potential decrements in auditory cortex. I. Discrete-trial and continual stimulation.

In experiment 1 cats were exposed to sets of clicks (trials) with 1 min inter-trial-intervals to determine if the effects of repetitive stimulation on potentials evoked in the auditory cortex would be cumulative despite discrete-trial stimulation. Evoked potentials were averaged to give one average evoked potential (AEP) for each trial for each electrode; there were four cortical electrodes per subject. To test for dishabituation pawshocks were given between trials 60 and 61. Subjects were paralyzed to insure stimulus constancy. The latency and peak-to-peak amplitude of each component of each AEP was measured; significant amplitude decremented; and decrements were more frequent in components with latencies greater than 15 msec. A few amplitude increments and latency changes were also observed...

Animals↗
PubMedDate not supplied

[The correlations of the evoked activity of the auditory cortex and amygdala in cats during conditioned reflex activities].

By averaged summary activity of symmetric points of the cat auditory cortex and amygdala evoked by sound, the character of interaction (Spirman coefficient of correlation) was determined between the structures before, at elaboration, at extinction and restoration of instrumental food-procuring reflex and also at automatization of the reflex. Individual character of co-tuning of the cortex and amygdala was found at adaptation of the animal, changing into close interconnections at formation of the reflex and into disturbance of their correlation at its extinction. Stabilization of the reflex up to the stage of automatization was accompanied by the weakening of connections between the cortex and amygdala. The results testify to complex and dynamic character of interaction in the system cortex-amygdala in conditioned activity.

Acoustic Stimulation↗
PubMed2006

Contralateral white noise selectively changes left human auditory cortex activity in a lexical decision task.

In a previous study, we hypothesized that the approach of presenting information-bearing stimuli to one ear and noise to the other ear may be a general strategy to determine hemispheric specialization in auditory cortex (AC). In that study, we confirmed the dominant role of the right AC in directional categorization of frequency modulations by showing that fMRI activation of right but not left AC was sharply emphasized when masking noise was presented to the contralateral ear. Here, we tested this hypothesis using a lexical decision task supposed to be mainly processed in the left hemisphere. Subjects had to distinguish between pseudowords and natural words presented monaurally to the left or right ear either with or without white noise to the other ear. According to our hypothesis, we expected a strong effect of contralateral noise on fMRI activity in left AC. For the control conditions without noise, we found that activation in both auditory cortices was stronger on contralateral than on ipsilateral word stimulation consistent with a more influential contralateral than ipsilateral auditory pathway. Additional presentation of contralateral noise did not significantly change activation in right AC, whereas it led to a significant increase of activation in left AC compared with the condition without noise. This is consistent with a left hemispheric specialization for lexical decisions. Thus our results support the hypothesis that activation by ipsilateral information-bearing stimuli is upregulated mainly in the hemisphere specialized for a given task when noise is presented to the more influential contralateral ear.

Acoustic Stimulation↗
PubMed1980

A vertical stereotaxic approach to auditory cortex in the unanesthetized monkey.

A procedure is described for chronic single-unit recording from monkey auditory cortex. The cortex is approached in a vertical stereotaxic plane and subsequent histology is performed on tissue sections made in the same plane. The procedure maximizes the probability of locating and identifying auditory cortical areas during the chronic unit recording sessions and of identifying individual electrode tracks in subsequent histologic examination. Average stereotaxic coordinates for the center of area A1 are A-P +5, M-L 17.5 and D-V +20, but variation of +/-5 mm can occur across subjects. The superior temporal plane is sloped at an angle of about 30 degrees in the A-P dimension but is relatively flat in the M-L dimension. Microelectrode penetrations made with this procedure were found, on the average, to terminate within 0.7 mm of the intended site. Procedures for improving this accuracy and for identifying closely-spaced penetrations are discussed.

Animals↗
PubMed1976

Auditory cortex in the grey squirrel: tonotopic organization and architectonic fields.

The representation of sound frequency within auditory cortex has been investigated with microelectrode mapping techniques in grey squirrels. The cyto- and myeloarchitecture of mapped cortical surfaces was examined and related to recording data. Among the results were the following: (1) A primary field can be defined on the basis of both physiological and anatomical criteria.

Animals↗
PubMed1991

[Spread of excitation in the upper layers of the auditory cortex with the participation of intracortical interneuronal connections].

In the chronically isolated slab of the cat auditory cortex when lower layers were additionally undercut and one, two or three cortical layers under the pial surface remained structurally intact, impulse reactions of slab neurons to the intracortical stimulation applied on the other side of this additional undercut were investigated. High effectiveness of axo-dendritic and axo-spinal excitatory contacts formed by nerve elements of intracortical origin in the upper cortical layers was shown. Participation of geniculo-cortical fibres in the spread of excitation in the cortex through synaptic contacts in layer I with dendrites of pyramidal neurons of the lower layers is discussed. The possibility to generate polysynaptic excitatory reactions by neurons shows that in the isolated cortical slab with an undercut of the lower layers the complex interneuronal relations are retained.

Animals↗
PubMed1992

Sensitivity of neurons in cat primary auditory cortex to tones and frequency-modulated stimuli. II: Organization of response properties along the 'isofrequency' dimension.

The spatial distribution of neuronal responses to tones and frequency-modulated (FM) stimuli was mapped along the 'isofrequency' dimension of the primary auditory cortex (AI) of barbiturate-anesthetized cats. In each cat, electrode penetrations roughly orthogonal to the cortical surface were closely spaced (average separation approximately 130 microns) along the dorsoventral extent of a single 'isofrequency' strip in high frequency parts of AI (> 15 kHz). Characteristic frequency (CF), minimum threshold, sharpness of frequency tuning (Q10 and Q20), the dynamic range of the spike count-intensity function at CF, sensitivity to the rate of change of frequency (RCF) and to the direction of frequency-modulation (DS) were determined for contralaterally-presented tone and FM stimuli. Sharpness of tuning attained maximum values at central loci along the dorsoventral 'isofrequency' axis and values declined towards more dorsal and more ventral locations. Minimum threshold and dynamic range varied between high and low values in a similar and correlated periodic fashion. Their combined organization yielded an orderly spatial representation of response strength, relative to maximum, as a function of stimulus amplitude. The distributions of the most common forms of FM rate sensitivity (RCF response categories) and best RCF along 'isofrequency' strips were significantly non-random although there was a considerable degree of variability between cats. FM directional preference and sensitivity appeared to be randomly distributed. Sharpness of tuning may be related to the analysis of the spectral content of an acoustic stimulus, both minimum threshold and dynamic range are related to the encoding of stimulus intensity, and measures of FM rate and directional sensitivity assess the coding of temporal changes of stimulus spectra. The independent, or for minimum threshold and dynamic range dependent, topographic organizations of these neuronal parameters therefore suggest parallel and independent processing of these aspects of acoustic signals in AI.

Acoustic Stimulation↗
PubMed1984

Thalamocortical projections to layer I of the primary auditory cortex in the cat: a horseradish peroxidase study.

HRP injected into layer I of the primary auditory cortex (AI) in the cat labeled neuronal cell bodies ipsilaterally in the medial, dorsal and ventrolateral divisions of the medial geniculate nucleus (MGN), suprageniculate nucleus, and nucleus of the brachium of the inferior colliculus. MGN neurons labeled after HRP injected into layer I were statistically smaller than those labeled after HRP injected into layer IV.

Animals↗
PubMed1988

Connections of the primary auditory cortex in the common marmoset, Callithrix jacchus jacchus.

The afferent and efferent connections of the primary auditory cortex (AI) of common marmosets were traced following small injections of wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP) made at best frequency (BF)-defined sites in the AI. After the injections the animals remained anesthetized for 15-23 hours; they were then perfused transcardially with fixative and the brains were processed for WGA-HRP reaction product. Examination of the disposition of labelled material revealed the following results. First, patches of terminal labelling, and to a lesser extent retrograde labelling, were found outside the injection site in the ipsilateral cortex rostral and caudal to the AI. Second, the region of the contralateral cortex corresponding to the injection site contained labelled terminals throughout the depth of the cortex; labelled neurons were found in the middle layers. Third, in each experiment a discrete region of the medial geniculate body (MG) contained retrogradely labelled neurons interspersed with anterogradely labelled terminals. These regions had a banded appearance, were found in the dorsal and rostral half of the MG, and shifted in location progressively dorsalward as the injection site BF increased. The presence of projection zones rostral and caudal to the AI of marmosets, and the disposition of the MG sources of projection in relation to BF, are similar to observations made on other New World monkeys. The ipsilateral corticocortical projections confirm electrophysiological evidence suggesting the existence of auditory fields rostral and caudal to the AI. The thalamocortical auditory system of the marmoset appears relatively simple, with a comparatively undifferentiated MG projecting to a cortical auditory system dominated by a large AI.

Animals↗
PubMed2001

Effect of auditory cortex lesions on NADPH-diaphorase staining in the inferior colliculus of rat.

Projections from the auditory cortex (AC) in the rat terminate in the dorsal cortex (DC) and in the external cortex (EC) of the inferior colliculus (IC), areas which exhibit a moderate number of nicotinamide-adenine dinucleotide phosphate-diaphorase (NADPH-d) positive neurons. NADPH-d co-localizes with nitric oxide synthase, which is responsible for the production of the transcellular messenger, nitric oxide. Changes in NADPH-d staining in the IC were found after unilateral lesions of the AC. Lesions resulted in a reduction in NADPH-d staining in neurons and neuropil within the ipsilateral DC and EC with the maximum reduction occurring 3-4 days after lesion. The reduction in NADPH-d staining in the contralateral IC was less pronounced. Lesions affecting auditory areas Te 1 and Te 3 produced the largest decrease in NADPH-d staining in neurons and neuropil. This finding may be related to the abolition of the influence of glutamatergic corticocollicular and commissural pathways.

Animals↗
PubMed1996

Induction of receptive field plasticity in the auditory cortex of the guinea pig during instrumental avoidance conditioning.

Classical tone conditioning shifts frequency tuning in the auditory cortex to favor processing of the conditioned stimulus (CS) frequency versus other frequencies. This receptive field (RF) plasticity is associative, highly specific, rapidly acquired, and indefinitely retained-all important characteristics of memory. The investigators determined whether RF plasticity also develops during instrumental learning. RFs were obtained before and up to 24 hr after 1 session of successful 1-tone avoidance conditioning in guinea pigs. Long-term RF plasticity developed in all subjects (N = 6). Two-tone discrimination training also produced RF plasticity, like classical conditioning. Because avoidance responses prevent full elicitation of fear by the CS, long-term RF plasticity does not require the continual evocation of fear, suggesting that neural substrates of fear expression are not essential to RF plasticity.

Animals↗
PubMed1982

Auditory cortex in the marsupial possum Trichosurus vulpecula.

A microelectrode mapping survey was made of the auditory cortex of the acallosal marsupial possum Trichosurus vulpecula. Single unit and unit cluster responses, as well as evoked potentials, were obtained from 64 tangential, cortical penetrations. Pontamine sky-blue marking of successful tracks aided in later reconstruction of tracks and identification of recording locations. Only one auditory field was identified. The 98 units sampled in this field covered a wide range of best frequencies from 330 Hz to 39 kHz with the most sensitive units (thresholds of best frequency of less than 10 dB SPL) being in the 17-19 kHz range. An orderly representation of cochlear place was found in the cortex with high frequencies located dorsally and low frequencies more ventrally. This orientation of frequencies is unlike that seen in other mammals.

Animals↗
PubMed1999

Functional specialization of the human auditory cortex in processing phonetic and musical sounds: A magnetoencephalographic (MEG) study.

Functional specialization of the human auditory cortex in processing phonetic vs musical sounds was investigated. While subjects watched a silent self-selected movie, they were presented with sequences consisting of frequent and infrequent phonemes (/e/ and /o/, respectively) or chords (A major and A minor, respectively). The subjects' brain responses to these sounds were recorded with a 122-channel whole-head magnetometer. The data indicated that within the right hemisphere, the magnetoencephalographic (MEG) counterpart MMNm of the mismatch negativity (MMN) elicited by an infrequent chord change was stronger than the MMNm elicited by a phoneme change. Within the left hemisphere, the MMNm strength for a chord vs phoneme change did not significantly differ. Furthermore, the MMNm sources for the phoneme and chord changes were posterior to the P1m sources generated at or near the primary auditory areas. In addition, the MMNm source for a phoneme change was superior to that for the chord change in both hemispheres. The data thus provide evidence for spatially distinct cortical areas in both hemispheres specialized in representing phonetic and musical sounds.

Acoustic Stimulation↗
PubMed1978

Aural representation in the Doppler-shifted-CF processing area of the auditory cortex of the mustache bat.

In the mustache bat (Pteronotus pamellii rubiginosus) the frequency and amplitude of an acoustic signal are represented in the coordinates parallel to the surface of the Doppler-shifted-CF (constant frequency) processing area ofthe primary auditory cortex. In this area all cortical neurons studied were excited by contralateral stimuli, and almost all of them were either excited or inhibited by ipsilateral stimuli. These are called E-E (ipsilateral and contralateral excitatory) and I-E (ipsilateral inhibitory and contralateral excitatory) neurons, respectively. The I-E neurons are directionally sensitive, while the E-E neurons are not. The E-E neurons are equally sensitive to echoes between 30 degrees contralateral and 30 degrees ipsilateral. Of the electrode penetrations orthogonal to the Doppler-shifted-CF processing area, 57 percent were characterized by either E-E or I-E neurons. Thus, there are at least two types of binaural columns: E-E columns, mainly located in a ventral part of the Doppler-shifted-CF processing area, where neurons are tuned to weak echoes; and IE columns, mainly distributed in a dorsal part, where neurons are tuned to moderate to intense echoes. Therefore, neurons tuned to weaker echoes integrate or even multiply faint signals from both ears for effective detection of a distant small target, while neurons tuned to moderate to intense echoes are suited for processing directional information and are stimulated when a bat approaches a target at short range. The Doppler-shifted-CF processing area may be considered to consist of two functional subdivisions.

Animals↗
PubMed2006

Hearing suppression induced by electrical stimulation of human auditory cortex.

In the course of performing electrical stimulation functional mapping (ESFM) in neurosurgery patients, we identified three subjects who experienced hearing suppression during stimulation of sites within the superior temporal gyrus (STG). One of these patients had long standing tinnitus that affected both ears. In all subjects, auditory event related potentials (ERPs) were recorded from chronically implanted intracranial electrodes and the results were used to localize auditory cortical fields within the STG. Hearing suppression sites were identified within anterior lateral Heschl's gyrus (HG) and posterior lateral STG, in what may be auditory belt and parabelt fields. Cortical stimulation suppressed hearing in both ears, which persisted beyond the period of electrical stimulation. Subjects experienced other stimulation-evoked perceptions at some of these same sites, including symptoms of vestibular activation and alteration of audio-visual speech processing. In contrast, stimulation of presumed core auditory cortex within posterior medial HG evoked sound perceptions, or in one case an increase in tinnitus intensity, that affected the contralateral ear and did not persist beyond the period of stimulation. The current results confirm a rarely reported experimental observation, and correlate the cortical sites associated with hearing suppression with physiologically identified auditory cortical fields.

Adult↗