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At least 1,135 records · Page 63Linked to original sources
PubMed1995

Changes in density of brainstem afferents in ferret primary auditory cortex (AI) during postnatal development.

Histochemical methods were used to assess the distribution of 4 neurotransmitters thought to be involved in cortical plasticity. They were measured in the primary auditory cortex of the ferret from just before the onset of hearing. Acetylcholinesterase staining was strongest in layers I, IV and VI and there was a gradual increase in the amount of staining from postnatal day (PND) 21 through to adulthood. Serotonin fibres were located mainly in layers I-III and their density increased gradually over the same time period. Noradrenergic fibres were sparsely scattered throughout the cortex but their density and distribution showed little change over the age range studied. Dopaminergic fibres were densest in layers V and VI at all ages. However, there was a transient doubling in their density that started round about the onset of hearing at PND 28, peaked at PND 35 and had returned to baseline levels by 2 wk later. This transient peak in density did not occur in the adjacent suprasylvian gyrus and did not appear to be a general phenomenon. The local transient increase in dopaminergic fibres implies that they may have an important role during a short period in auditory cortical development. This role may involve modifying the cortical circuitry that is involved in analysing the input from the auditory periphery.

Acetylcholinesterase↗
PubMed2002

Temporal modulation transfer functions in cat primary auditory cortex: separating stimulus effects from neural mechanisms.

We present here a comparison between the local field potentials (LFP) and multiunit (MU) responses, comprising 401 single units, in primary auditory cortex (AI) of 31 cats to periodic click trains, gamma-tone and time-reversed gamma-tone trains, AM noise, AM tones, and frequency-modulated (FM) tones. In a large number of cases, the response to all six stimuli was obtained for the same neurons. We investigate whether cortical neurons are likely to respond to all types of repetitive transients and modulated stimuli and whether a dependence on modulating waveform, or tone or noise carrier, exists. In 97% of the recordings, a temporal modulation transfer function (tMTF) for MU activity was obtained for gamma-tone trains, in 92% for periodic click trains, in 83% for time-reversed gamma-tone trains, in 82% for AM noise, in 71% for FM tones, and only in 53% for AM tones. In 31% of the cases, the units responded to all six stimuli in an envelope-following way. These particular units had significantly larger onset responses to each stimulus compared with all other units. The overall response distribution shows the preference of AI units for stimuli with short rise times such as clicks and gamma tones. It also shows a clear asymmetry in the ability to respond to AM noise and AM tones and points to a strong effect of the frequency content of the carrier on the subcortical processing of AM stimuli. Yet all temporal response properties were independent of characteristic frequency and frequency-tuning curve bandwidth. We show that the observed differences in the tMTFs for different stimuli are to a large extent produced by the different degree of phase locking of the neuronal firings to the envelope of the first stimulus in the train or first modulation period. A normalization procedure, based on these synchronization differences, unified the tMTFs for all stimuli except clicks and allowed the identification of a largely stimulus-invariant, low-pass temporal filter function that most likely reflects the properties of synaptic depression and facilitation. For nonclick stimuli, the low-pass filter has a cutoff frequency of approximately 10 Hz and a slope of approximately 6 dB/octave. For nonclick stimuli, there was a systematic difference between the vector strength for LFPs and MU activity that can likely be attributed to postactivation suppression mechanisms.

Acoustic Stimulation↗
PubMed1991

Intrinsic inter- and intralaminar connections and their relationship to the tonotopic map in cat primary auditory cortex.

Small iontophoretic injections of the lectin, Phaseolus vulgaris leucoagglutinin (PHA-L), were made into different layers of the primary auditory cortex (AI) of cats. Injections in layer I labeled two types of morphologically distinct fibers in layer I as well as a smaller number of axons in layers II and III. Layer II injections labeled descending axons that produced a dense plexus of terminal fibers in layers I-III of both AI and adjacent auditory fields. Injections in layer III also labeled a dense plexus of axon collaterals at the junction of layers V and VI and labeled patches of terminal fibers in both AI and adjacent auditory fields. These were densest in layers I-III but usually extended into layers IV and V as well. The patches were partly formed by axon collaterals of layer III pyramidal cells that traveled for over 4 mm in the gray matter. Injections confined to layer IV labeled axons in all layers of the cortex but none of these axons appeared to reach the white matter. The axons spread laterally in layer IV and up into the superficial layers and ramified especially layer I. Injections in layers V and VI labeled axons in all layers' of the cortex but these were densest in the deep layers where labeling was fairly homogeneous. In the upper layers the labeling was arranged in semi-discrete patches. Large injections involving layers I-III were studied in tangential sections. Between 3 and 8 patches of terminal labeling were observed in AI and these were mainly arranged in a band with its long axis aligned approximately in the dorsoventral direction. However dense patches of terminal labeling also occurred both anterior and posterior to the injection site. In selected experiments portions of the tonotopic map in AI were mapped by single unit recording and subsequently the map was related to patches of anterogradely labeled fibers that surrounded injections of PHA-L. Rows of dorsoventrally oriented patches were among cells with a similar best frequency to those in the injection site. However patches located anterior or posterior to the injection site were among cells with higher or lower best frequencies. Two injections of PHA-L close together produce different patterns of labeling. One of the injections usually produces one or more patches that has no correlate among the patches of fibers labeled by the adjacent injection. This is clearest when one of the injections is made with biotinylated PHA-L that can be visualized directly without the use of primary antibodies.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗
PubMed2005

Propofol attenuates responses of the auditory cortex to acoustic stimulation in a dose-dependent manner: a FMRI study.

BACKGROUND: Functional magnetic resonance imaging (fMRI) using blood-oxygen-level-dependent (BOLD) contrasts is a common method for studying sensory or cognitive brain functions. The aim of the present study was to assess the effect of the intravenous anaesthetic propofol on auditory-induced brain activation using BOLD contrast fMRI. METHODS: In eight neurosurgical patients, musical stimuli were presented binaurally in a block design. Imaging was performed under five conditions: no propofol (or wakefulness) and propofol plasma target concentrations of 0.5, 1.0, 1.5, and 2.0 microg ml(-1). RESULTS: During wakefulness we found activations in the superior temporal gyrus (STG) corresponding to the primary and secondary auditory cortex as well as in regions of higher functions of auditory information processing. The BOLD response decreased with increasing concentrations of propofol but remained partially preserved in areas of basic auditory processing in the STG during propofol 2.0 microg ml(-1). CONCLUSIONS: Our results suggest a dose-dependent impairment of central processing of auditory information after propofol administration. These results are consistent with electrophysiological findings measuring neuronal activity directly, thus suggesting a dose-dependent impairment of central processing of auditory information after propofol administration. However, propofol did not totally blunt primary cortical responses to acoustic stimulation, indicating that patients may process auditory information under general anaesthesia.

Acoustic Stimulation↗
PubMedDate not supplied

Spatial representation of frequency-modulated tones in gerbil auditory cortex revealed by epidural electrocorticography.

The present study investigated the topography of epidurally recorded middle latency components P1 and N1 evoked by spectrally dynamic stimuli (linearly frequency-modulated (FM) tones) with respect to the tonotopic structure of the right primary auditory cortex, field AI. Whereas the gross topography corresponded to the spectral content of the FM tones, specific tonotopic offsets were found between the potential distributions evoked by FM tones of different modulation direction (i.e. 'rising' vs. 'falling'). Potentials evoked by rising FM tones were located at tonotopic positions corresponding to higher frequencies compared with potentials evoked by falling FM tones. Data indicated that the magnitude of these offsets can be attributed to the local tonotopic resolution in AI and are not dependent on the modulation rate.

Acoustic Stimulation↗
PubMedDate not supplied

[Visual evoked responses on the primary auditory cortex in the cat after an early suppression of cochlear receptors (author's transl)].

In the adult cat, after an early degeneration (hereditary deaf white cats) or surgical removal of auditory receptors, it is possible to record visual evoked responses on the primary auditory cortex. The appearance of such potentials depends on two conditions: a complete (bilateral) and early (first postnatal month) deafferentation. This can be explained by an early multi-afferentation of the primary cortical areas. The early suppression of the primary modality may keep the other modalities still working in the adult.

Afferent Pathways↗
PubMed1995

Comparison of noise and tone azimuth tuning of neurons in cat primary auditory cortex and medical geniculate body.

1. A comparison of the azimuth tuning of single neurons to broadband noise and to best frequency (BF) tone bursts was made in primary auditory cortex (AI: n = 173) and the medial geniculate body (MGB: n = 52) of barbiturate-anesthetized cats. Observations were largely restricted to cells located within the tonotopically organized divisions of the MGB (i.e., the ventral nucleus and the lateral division of the posterior nuclear group) and the middle layers of AI. All cells studied had BFs > or = 4 kHz. 2. The responses of each cell to sounds presented from seven frontal azimuthal locations (-90 to +90 degrees in 30 degrees steps; 0 degree elevation) and at five sound pressure levels (SPLs: 0-80 dB or 5-85 dB in 20-dB steps) provided an azimuth-level data set. Responses were averaged over SPL to obtain an azimuth function, and a number of features of this function were used to describe azimuth tuning to noise and to tone stimulation. Azimuth function modulation was used to assess azimuth sensitivity, and cells were categorized as sensitive or insensitive depending on whether modulation was > or = 75% or < 75% of maximum, respectively. The majority (88%) of cells in the sample were azimuth sensitive to noise stimulation, and statistical analyses were restricted to these cells, which are presumably best suited to encode sound source azimuth. Azimuth selectivity was assessed by a preferred azimuth range (PAR) over which azimuth function values exceeded 75% (PAR75) or 50% of maximum response. Cells were categorized according to the location and extent of their noise PARs. Unbounded cells had laterally located PARs that extended to the lateral pole (+/- 90 degrees); bounded cells had PARs that were contained entirely within the frontal hemifield, and a subset of these had PARs centered on the midline (+/- 15 degrees). A final group of cells exhibited multipeaked azimuth functions to noise stimulation. 3. Azimuth functions to noise were generally more selective and/or more sensitive than those to tones. Statistical analyses showed that these differences were significant for cells in each azimuth function category, and for the thalamic and cortical samples. With the exception of multipeaked cells, responsiveness to noise was significantly lower than that to tones in all categories, and for the thalamic and cortical samples.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗
PubMedDate not supplied

Comparison of electrically and acoustically evoked responses in the auditory cortex of the guinea pig: implications for a cochlear prosthesis.

Guinea pigs have been used to develop an animal model for evaluation of electric stimulation of the cochlea. Techniques were developed for recording from single neural units in the auditory cortex. Acoustic responses exhibit similar patterns of neural discharge ("on", "off", sustained excitation and inhibition, etc) to those reported for auditory neurons in other species. Similar response patterns are obtained when electric stimulation is used. However, cortical units display characteristic frequency tuning with acoustic stimulation, whereas it becomes increasingly difficult to evoke electric responses as stimulus frequency increases. Recording of gross evoked responses from the same cortical area indicates that the intensity function for electric stimulation is much steeper than that seen for acoustic stimulation.

Acoustic Stimulation↗
PubMed1995

Auditory cortex neurons: primary culture and ion channel activity in rat.

We have developed a primary dissociated cell culture of the fetal (E17) and post-natal (P0-P10) rat auditory cortex. Pyramidal and non-pyramidal cells had a mean cross-sectional diameter of 12.73 +/- 1.80 microns (mean +/- S.D., n = 25) and 17.58 +/- 1.67 microns (mean +/- S.D., n = 10), respectively, measured at 6 days in culture. These cells were viable for as long as 18-21 days. They expressed voltage-gated sodium and potassium channel currents as early as one day in culture, and at various phases in cell culture. Sodium current, activated at membrane potentials more positive than -60 mV, displayed fast activation and inactivation kinetics. Fifty percent inactivation of sodium channels occurred at a pre-pulse potential of -63 mV. Delayed rectifier potassium channels were activated at potentials positive to -40 mV. Large hyperpolarizing constant current pulses elicited anode break action potentials, and large depolarizing constant current pulses exhibited rectification indicative of the delayed rectifying potassium channel activity.

Animals↗
PubMed1990

Classical conditioning induces CS-specific receptive field plasticity in the auditory cortex of the guinea pig.

To determine if classical conditioning produces general or specific modification of responses to acoustic conditioned stimuli (CS), frequency receptive fields (RF) of neurons in guinea pig auditory cortex were determined before and up to 24 h after fear conditioning. Highly specific RF plasticity characterized by maximal increased responses to the CS frequency and decreased responses to the pretraining best frequency (BF) and other frequencies was observed in 70% of conditioning cases. These opposing changes were often sufficient to produce a shift in tuning such that the frequency of the CS became the new BF. CS frequency specific plasticity was maintained as long as 24 h. Sensitization training produced general increased responses across the RF without CS specificity. The findings indicate that associative processes produce systematic modification of the auditory system's processing of frequency information and exemplify the advantages of combining receptive field analysis with behavioral training in the study of the neural bases of learning and memory.

Acoustic Stimulation↗
PubMed2004

Unique responses of auditory cortex networks in vitro to low concentrations of quinine.

The anti-malarial drug quinine has several side effects including tinnitus. The aim of the study was to determine if cultured auditory networks growing on microelectrode arrays exhibited unique dynamic states when exposed to quinine. Eight auditory cortex networks (ACN), eight frontal cortex networks (FCN), and five inferior colliculus networks (ICN) were used in this study. Response of ACNs to quinine was biphasic, with an excitatory phase followed by inhibition. FCNs and ICNs revealed only inhibitory responses. The concentrations at which the spike rate was inhibited by 50% (IC50 mean +/- SE) were 42.5 +/- 3.9, 28.7 +/- 4.8 and 23.9 +/- 2.1 microM for ACNs, FCNs, and ICNs, respectively. Quinine increased the regularity and coordination of bursting in all three tissues. The increased burst pattern regularity of ICNs coupled with the excitatory responses seen only in ACNs between 1 and 10 microM show a unique susceptibility of auditory tissues to quinine that may be related to the underlying mechanism that triggers tinnitus-like activity.

Action Potentials↗
PubMed2002

Order-sensitive plasticity in adult primary auditory cortex.

The neural response to a stimulus presented as part of a rapid sequence is often quite different from the response to the same stimulus presented in isolation. In primary auditory cortex (A1), although the most common effect of preceding stimuli is inhibitory, most neurons can also exhibit response facilitation if the appropriate spectral and temporal separation of sequence elements is presented. In this study, we investigated whether A1 neurons in adult animals can develop context-dependent facilitation to a novel acoustic sequence. After repeatedly pairing electrical stimulation of the basal forebrain with a three-element sequence (high frequency tone--low frequency tone-- noise burst), 25% of A1 neurons exhibited facilitation to the low tone when preceded by the high tone, compared with only 5% in controls. In contrast, there was no increase in the percent of sites that showed facilitation for the reversed tone order (low preceding high). Nearly 60% of sites exhibited a facilitated response to the noise burst when preceded by the two tones. Although facilitation was greatest in response to the paired sequence, facilitation also generalized to related sequences that were either temporally distorted or missing one of the tones. Pairing basal forebrain stimulation with the acoustic sequence also caused a decrease in the time to peak response and an increase in population discharge synchrony, which was not seen after pairing simple tones, tone trains, or broadband stimuli. These results indicate that context-dependent facilitation and response synchronization can be substantially altered in an experience-dependent fashion and provide a potential mechanism for learning spectrotemporal patterns.

Animals↗
PubMed2004

Bimodal speech: early suppressive visual effects in human auditory cortex.

While everyone has experienced that seeing lip movements may improve speech perception, little is known about the neural mechanisms by which audiovisual speech information is combined. Event-related potentials (ERPs) were recorded while subjects performed an auditory recognition task among four different natural syllables randomly presented in the auditory (A), visual (V) or congruent bimodal (AV) condition. We found that: (i) bimodal syllables were identified more rapidly than auditory alone stimuli; (ii) this behavioural facilitation was associated with cross-modal [AV-(A+V)] ERP effects around 120-190 ms latency, expressed mainly as a decrease of unimodal N1 generator activities in the auditory cortex. This finding provides evidence for suppressive, speech-specific audiovisual integration mechanisms, which are likely to be related to the dominance of the auditory modality for speech perception. Furthermore, the latency of the effect indicates that integration operates at pre-representational stages of stimulus analysis, probably via feedback projections from visual and/or polymodal areas.

Acoustic Stimulation↗
PubMed2003

Rapid task-related plasticity of spectrotemporal receptive fields in primary auditory cortex.

We investigated the hypothesis that task performance can rapidly and adaptively reshape cortical receptive field properties in accord with specific task demands and salient sensory cues. We recorded neuronal responses in the primary auditory cortex of behaving ferrets that were trained to detect a target tone of any frequency. Cortical plasticity was quantified by measuring focal changes in each cell's spectrotemporal response field (STRF) in a series of passive and active behavioral conditions. STRF measurements were made simultaneously with task performance, providing multiple snapshots of the dynamic STRF during ongoing behavior. Attending to a specific target frequency during the detection task consistently induced localized facilitative changes in STRF shape, which were swift in onset. Such modulatory changes may enhance overall cortical responsiveness to the target tone and increase the likelihood of 'capturing' the attended target during the detection task. Some receptive field changes persisted for hours after the task was over and hence may contribute to long-term sensory memory.

Acoustic Stimulation↗
PubMedDate not supplied

Plasticity of the human auditory cortex induced by discrimination learning of non-native, mora-timed contrasts of the Japanese language.

In this magnetoencephalographic (MEG) study, we examined with high temporal resolution the traces of learning in the speech-dominant left-hemispheric auditory cortex as a function of newly trained mora-timing. In Japanese, the "mora" is a temporal unit that divides words into almost isochronous segments (e.g., na-ka-mu-ra and to-o-kyo-o each comprises four mora). Changes in the brain responses of a group of German and Japanese subjects to differences in the mora structure of Japanese words were compared. German subjects performed a discrimination training in 10 sessions of 1.5 h each day. They learned to discriminate Japanese pairs of words (in a consonant, anni-ani; and a vowel, kiyo-kyo, condition), where the second word was shortened by one mora in eight steps of 15 msec each. A significant increase in learning performance, as reflected by behavioral measures, was observed, accompanied by a significant increase of the amplitude of the Mismatch Negativity Field (MMF). The German subjects' hit rate for detecting durational deviants increased by up to 35%. Reaction times and MMF latencies decreased significantly across training sessions. Japanese subjects showed a more sensitive MMF to smaller differences. Thus, even in young adults, perceptual learning of non-native mora-timing occurs rapidly and deeply. The enhanced behavioral and neurophysiological sensitivity found after training indicates a strong relationship between learning and (plastic) changes in the cortical substrate.

Adult↗
PubMed1989

Cholinergic modulation of frequency receptive fields in auditory cortex: II. Frequency-specific effects of anticholinesterases provide evidence for a modulatory action of endogenous ACh.

Exogenously applied muscarinic agonists--for example, acetylcholine (ACh) and acetyl-beta-methacholine (MCh)--modify frequency receptive fields in auditory cortex of unanesthetized animals in a frequency-specific rather than global manner. The present study sought to relate these findings to endogenous actions of ACh by using the anticholinesterase agents eserine sulphate and soman (0-1,2,2-trimethylpropylmethylphosphonofluoridate) to facilitate the effects of endogenous ACh. Frequency receptive fields (FRF) were determined by presenting sequences of different isointensity tones before, during, and after application of ACh, MCh, eserine, or soman; also the cholinesterase blockers were applied between applications of ACh or MCh. The major effects produced by the inhibitors were similar to those of the agonists. Predominant effects were frequency-specific changes in FRF. Further, eserine and soman, similar to ACh and MCh, produced shifts in the best frequency (BF) of FRF due mainly to coordinated depression of responses to the BF and increased responses to adjacent, non-BF. The results indicate that exogenous and endogenous ACh, acting via muscarinic receptors, can significantly influence the physiological functioning of cortical neurons and consequently their processing of sensory information.

Acetylcholine↗