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PubMed2003

Visual short-term memory load affects sensory processing of irrelevant sounds in human auditory cortex.

We used whole-head magnetoencephalography (MEG) to investigate neural activity in human auditory cortex elicited by irrelevant tones while the subjects were engaged in a short-term memory task presented in the visual modality. As compared to a no-memory-task condition, memory load enhanced the amplitude of the auditory N1m response. In addition, the N1m amplitude depended on the phase of the memory task, with larger response amplitudes observed during encoding than retention. Further, these amplitude modulations were accompanied by anterior-posterior shifts in N1m source locations. The results show that a memory task for visually presented stimuli alters sensory processing in human auditory cortex, even when subjects are explicitly instructed to ignore any auditory stimuli. Thus, it appears that task demands requiring attentional allocation and short-term memory result in interaction across visual and auditory brain areas carrying out the processing of stimulus features.

Acoustic Stimulation↗
PubMed1989

Extrathalamic ascending projections to physiologically identified fields of the cat auditory cortex.

The neurons of origin of ascending extrathalamic projections to the auditory cortex were labeled retrogradely with WGA-HRP injected in physiologically identified auditory cortical fields of the cat (anterior (AAF), primary (AI), posterior (PAF) and secondary (AII) fields). After injection in the tonotopically organized auditory cortical fields (AAF, AI and PAF), labeled neurons were distributed in 7 extrathalamic subcortical regions included in one or the other of 2 distinct systems of ascending projections to the neocortex. In the 'diffuse' system of projection, labeled neurons were observed bilaterally in the locus coeruleus, the nuclei of the raphe, the lateral hypothalamus, ipsilaterally in the ventromedial mesencephalic tegmentum and the basal forebrain; in the 'accessory sensory' system of projection, labeled neurons were found ipsilaterally in the nucleus of the brachium of the inferior colliculus and bilaterally in the claustrum. After injection in AII, labeled neurons were seen only in the 'diffuse' system of projection. For AAF and AI, the major contribution to the total extrathalamic ascending input originated from the lateral hypothalamus, whereas for AII it was the locus coeruleus. In contrast, PAF received extrathalamic ascending inputs mainly from the claustrum. Anterogradely labeled corticofugal terminal fields were found only in the nucleus of the brachium of the inferior colliculus and, after injection in PAF, in the claustrum.

Acetylcholine↗
PubMed1976

Foetal and neonatal development of evoked responses in guinea-pig auditory cortex.

Development of the response of the auditory cortex to unilateral acoustic stimulation by a chick was studied in guinea-pig foetuses from the 50th day to the end of gestation and in newborn animals. The first cortical response appeared on the 52nd to 53rd day of gestation. The maximum responses were concentrated in the temporal cortex, between the somatosensory (parietal) and optic (occipital) area. The progressive development of the latent period of the cortical response and of its various components distinctly slowed down on the last days of gestation. At the same time, the amplitude of the cortical response was temporarily augmented. The cortical response developed from a simple negative wave in the youngest embryos into an intricate complex with an initial positive component in newborn guinea-pigs. The basic components of this complex were already discernible on the 64th to 65th day of gestation. The ability to react to repeated peripheral stimulation of 0.1-2 c/s frequency increased with foetal age, with temporary deterioration on the last days of gestation. Resistance of the cortical auditory response to cerebral anoxia rose up to term, with a temporary drop from the 64th day of gestation. After the initiation of independent respiration, cerebral hypoxia and bilateral vagotomy chiefly influenced the stability of the more recent components of the cortical auditory response in mature foetuses.

Acoustic Stimulation↗
PubMed2005

Different effects of intracochlear sensory and neuronal injury stimulation on expression of synaptic N-methyl-D-aspartate receptors in the auditory cortex of rats in vivo.

CONCLUSIONS: The expression of synaptic N-methyl-D-aspartate (NMDA) receptors in the auditory cortex is dynamic and is bidirectionally regulated by auditory activity. Furthermore, the time course of changes in the level of NR2A protein differs after sensory and neuronal injury stimulation, which modulate different changes in synaptic plasticity. OBJECTIVE: To examine the effects of different types of auditory activity on the expression of synaptic NMDA receptors (NMDARs) in the auditory cortex of rats. MATERIAL AND METHODS: We prepared synaptosomes from the auditory cortices of postnatal Day 28 ototoxic-deafened Sprague-Dawley rats and postnatal Day 28 Sprague-Dawley rats subjected to noise trauma that were given various treatments and compared them to the synaptosomes of 1-6-week-old normal Sprague-Dawley rats. The expression of different NMDAR subunits in the synaptosomes was investigated by means of Western blotting. RESULTS: Changes in NR1 and NR2B proteins were not significant during different types of auditory activity. The level of NR2A protein increased remarkably during postnatal development and as a result of electrical intracochlear stimulation, auditory deprivation and noise trauma. Seventy-two h after a 2-h period of sensory electrical intracochlear stimulation, the expression of NR2A protein returned to the level caused by auditory deprivation. Seventy-two h after a 3-h period of noise trauma, elevation of the level of NR2A protein was unchanged. We also confirmed that elevation of the level of synaptic NR2A protein was sensitive to protein synthesis inhibitor and NMDAR antagonist. However, transcription inhibitor had no effect on NR2A protein expression.

Animals↗
PubMed2002

Effects of cholinergic enhancement on conditioning-related responses in human auditory cortex.

It has previously been shown that cholinergic blockade attenuates conditioning-related neuronal responses in human auditory cortex. The present study was conducted to investigate the effect of cholinergic enhancement on such experience-dependent cortical responses. The cholinesterase inhibitor physostigmine, or a placebo control, were continuously infused into healthy young volunteers, during differential aversive conditioning whilst brain activity was measured using event-related functional magnetic resonance imaging (fMRI). Volunteers were presented with two tones, one of which (CS+) was conditioned by pairing with an electrical shock whereas the other was always presented without the shock (CS-). Conditioning-related activations, expressed as an enhanced blood oxygenation level dependent (BOLD) response to the salient CS+, were evident in left auditory cortex under placebo but not under physostigmine. This absence of conditioning-related activations under physostigmine was due to enhanced responses to the CS- under physostigmine as compared to placebo. We suggest that an overactive cholinergic system leads to increased processing of behaviourally irrelevant stimuli and thus attenuates differential conditioning-related cortical activations.

Acetylcholine↗
PubMed2000

Noradrenaline does not change the mode of discharge of auditory cortex neurons.

The mode of discharge of auditory cortex cells was studied during iontophoretic application of noradrenaline (NA). Only seven of 190 cells showed changes in interspike interval distribution during NA application. A similar conclusion was drawn when the analysis focused on 68 cells classified as bursting (n = 15), regular spiking (n = 49) or thin spike (n = 4) cells. Only two bursting cells showed changes in their ISI distribution. The effects on the mode of discharge were independent of the effect on the spike rate and were not a function of cortical depth. These results suggest that the changes in firing mode previously described in vitro occur for a limited percentage of cells and/or for cell types not very often recorded in vivo.

Anesthesia↗
PubMed1993

Effects of superior olivary complex lesions on binaural responses in rat auditory cortex.

Unilateral or bilateral lesions of the superior olivary complex (SOC) were made by local injection of kainic acid through a micropipette lowered stereotaxically into the rat's auditory brain stem. The lesions had the effect of destroying cell bodies in the superior olive without disrupting fibers of passage. After a recovery period of approximately one month physiological recordings were made with tungsten microelectrodes from the auditory cortex of normal animals and animals with SOC lesions. For animals with unilateral SOC lesions recordings were made either ipsilateral or contralateral to the site of damage. Monaural and binaural tone bursts (110 ms duration) were presented dichotically through a sealed sound delivery system and binaural interaction patterns were determined by comparison of monaural and binaural responses. Some cells were excited by stimulation of either ear and facilitated by binaural stimulation (binaural summation or EE cells), whereas others were excited by contralateral stimulation and inhibited by simultaneous ipsilateral stimulation (binaural suppression or EI cells). Both binaural summation and suppression responses were still present following unilateral SOC lesions. Interaural intensity difference thresholds were within the normal range and no differences were found between animals with lesions placed ipsilateral or contralateral to the recording site. Following a bilateral lesion in one case, both binaural summation and suppression responses were still present. The bilateral lesion had the effect of shifting the average interaural intensity difference thresholds in favor of the contralateral ear. These data suggest that binaural interactions above the level of the superior olive contribute to physiological responses in auditory cortex and that binaural responses in the rat's auditory cortex are shaped by both olivary and supraolivary interactions.

Animals↗
PubMedDate not supplied

Specialization of left auditory cortex for speech perception in man depends on temporal coding.

Speech perception requires cortical mechanisms capable of analysing and encoding successive spectral (frequency) changes in the acoustic signal. To study temporal speech processing in the human auditory cortex, we recorded intracerebral evoked potentials to syllables in right and left human auditory cortices including Heschl's gyrus (HG), planum temporale (PT) and the posterior part of superior temporal gyrus (area 22). Natural voiced /ba/, /da/, /ga/) and voiceless (/pa/, /ta/, /ka/) syllables, spoken by a native French speaker, were used to study the processing of a specific temporally based acoustico-phonetic feature, the voice onset time (VOT). This acoustic feature is present in nearly all languages, and it is the VOT that provides the basis for the perceptual distinction between voiced and voiceless consonants. The present results show a lateralized processing of acoustic elements of syllables. First, processing of voiced and voiceless syllables is distinct in the left, but not in the right HG and PT. Second, only the evoked potentials in the left HG, and to a lesser extent in PT, reflect a sequential processing of the different components of the syllables. Third, we show that this acoustic temporal processing is not limited to speech sounds but applies also to non-verbal sounds mimicking the temporal structure of the syllable. Fourth, there was no difference between responses to voiced and voiceless syllables in either left or right areas 22. Our data suggest that a single mechanism in the auditory cortex, involved in general (not only speech-specific) temporal processing, may underlie the further processing of verbal (and non-verbal) stimuli. This coding, bilaterally localized in auditory cortex in animals, takes place specifically in the left HG in man. A defect of this mechanism could account for hearing discrimination impairments associated with language disorders.

Acoustic Stimulation↗
PubMed2003

The projection from auditory cortex to cochlear nucleus in guinea pigs: an in vivo anatomical and in vitro electrophysiological study.

Previous anatomical experiments have demonstrated the existence of a direct, bilateral projection from the auditory cortex (AC) to the cochlear nucleus (CN). However, the precise relationship between the origin of the projection in the AC and the distribution of axon terminals in the CN is not known. Moreover, the influence of this projection on CN principal cells has not been studied before. The aim of the present study was two-fold. First, to extend the anatomical data by tracing anterogradely the distribution of cortical axons in the CN by means of restricted injections of biotinylated dextran amine (BDA) in physiologically characterized sites in the AC. Second, in an in vitro isolated whole brain preparation (IWB), to assess the effect of electrical stimulation of the AC on CN principal cells from which intracellular recordings were derived. BDA injections in the tonotopically organized primary auditory cortex and dorsocaudal auditory field at high and low best frequency (BF) sites resulted in a consistent axonal labeling in the ipsilateral CN of all injected animals. In addition, fewer labeled terminals were observed in the contralateral CN, but only in the animals subjected to injections in low BF region. The axon terminal fields consisting of boutons en passant or terminaux were found in the superficial granule cell layer and, to a smaller extent, in the three CN subdivisions. No axonal labeling was seen in the CN as result of BDA injection in the secondary auditory area (dorsocaudal belt). In the IWB, the effects of ipsilateral AC stimulation were tested in a population of 52 intracellulary recorded and stained CN principal neurons, distributed in the three CN subdivisions. Stimulation of the AC evoked slow late excitatory postsynaptic potentials (EPSPs) in only two cells located in the dorsal CN. The EPSPs were induced in a giant and a pyramidal cell at latencies of 20 ms and 33 ms, respectively, suggesting involvement of polysynaptic circuits. These findings are consistent with anatomical data showing sparse projections from the AC to the CN and indicate a limited modulatory action of the AC on CN principal cells.

Animals↗
PubMed1988

Modification of neuromagnetic responses of the human auditory cortex by masking sounds.

We have studied the effects of masking sounds on auditory evoked magnetic fields (AEFs) of healthy humans. The AEFs were elicited by 25-ms tones presented randomly to the left or to the right ear, and the responses were recorded over the right auditory cortex. Without masking, the 100-ms deflection (N100m) was of somewhat higher amplitude and of shorter latency for contra- than ipsilateral stimuli. Continuous speech, music, or intermittent noise, delivered to the left ear, dampened N100m to stimulation of both ears without correlated changes in sensation. Intermittent noise had a weaker effect on N100m than speech or music. Continuous noise fed to the left ear dampened both the sensation of and the responses to the left-ear stimuli, with no significant effect on the responses to the right-ear stimuli. The results suggest that the masking effects of continuous noise, seen at the auditory cortex, derive mainly from the periphery whereas the effects of sounds with intensity and frequency modulations take place at more central auditory pathways.

Acoustic Stimulation↗
PubMed1994

Binaural organization of primary auditory cortex in the ferret (Mustela putorius).

1. The distribution of binaural responses within the ferret's primary auditory cortex was determined by standard microelectrode mapping techniques. Single and multiple unit responses were recorded from the middle ectosylvian gyrus of barbiturate-anesthetized animals with tungsten microelectrodes (1.2-1.8 M omega) inserted into the brain perpendicular to the cortical surface. The characteristic frequency (CF) and binaural response characteristics were determined for each point sampled. 2. Gated pure tones were delivered independently to the two ears through a sealed sound system, and binaural response types were determined by comparison of evoked activity for monaural and binaural stimulation. Most binaural responses fell into one of three major groups: binaural summation (EE/F), binaural suppression (EO/I), or mixed binaural summation and suppression. 3. The neurons tended to be grouped together on the basis of their binaural response properties. Zones of binaural summation and suppression extended across the surface of the middle ectosylvian gyrus and intersected with regions of sound frequency representation. 4. Particular attention was paid to the distribution of binaural responses within isofrequency contours in the ferret's primary auditory cortex (AI). Along the length of each isofrequency contour, areas of EE/F alternated with areas of EO/I. The summation areas were typically between 0.5 and 0.7 mm wide. Single neurons with mixed binaural response properties were frequently found between groups of EE/F and EO/I cells. The mixed responses appeared to mark a transition in location between zones of summation and suppression responses. 5. The distribution of interaural intensity difference (IID) thresholds was also examined along the length of isofrequency contours. No systematic relation was found between IID threshold and the distance along an isofrequency contour.

Animals↗
PubMed2002

Sustained magnetic fields reveal separate sites for sound level and temporal regularity in human auditory cortex.

Magnetoencephalography was used to investigate the relationship between the sustained magnetic field in auditory cortex and the perception of periodic sounds. The response to regular and irregular click trains was measured at three sound intensities. Two separate sources were isolated adjacent to primary auditory cortex: One, located in lateral Heschl's gyrus, was particularly sensitive to regularity and largely insensitive to sound level. The second, located just posterior to the first in planum temporale, was particularly sensitive to sound level and largely insensitive to regularity. This double dissociation to the same stimuli indicates that the two sources represent separate mechanisms; the first would appear to be involved with pitch perception and the second with loudness. The delay of the offset of the sustained field was found to increase with interclick interval up to 200 ms at least, which suggests that the sustained field offset represents a sophisticated offset-monitoring mechanism rather than simply the cessation of stimulation.

Acoustic Stimulation↗
PubMed2002

Cholinergic modulation of experience-dependent plasticity in human auditory cortex.

The factors that influence experience-dependent plasticity in the human brain are unknown. We used event-related functional magnetic resonance imaging (fMRI) and a pharmacological manipulation to measure cholinergic modulation of experience-dependent plasticity in human auditory cortex. In a differential aversive conditioning paradigm, subjects were presented with high (1600 Hz) and low tones (400 Hz), one of which was conditioned by pairing with an electrical shock. Prior to presentation, subjects were given either a placebo or an anticholinergic drug (0.4 mg iv scopolamine). Experience-dependent plasticity, expressed as a conditioning-specific enhanced BOLD response, was evident in auditory cortex in the placebo group, but not with scopolamine. This study provides in vivo evidence that experience-dependent plasticity, evident in hemodynamic changes in human auditory cortex, is modulated by acetylcholine.

Acetylcholine↗
PubMed1997

Optical imaging of neural activity in auditory cortex induced by intracochlear electrical stimulation.

Little is known about the representation of electrically evoked activity in the auditory cortex. We observed evoked activity in guinea pig auditory cortex evoked by acoustical and electrical stimulation to the cochlea by optical imaging with the aid of a voltage-sensitive dye. Light signals from the cortex were recorded with a 12 x 12 array of photodiodes, and transferred to the spatio-temporal images by every 0.57 ms. The activity by pure tones was shown spatio-temporally through tonotopical organization in the cortex according to the sound frequencies. The tonotopic responses were dynamically changed. When the cochlea was stimulated with single electrical pulses, focal activities were observed in the cortex as spatio-temporal patterns. Activated cortical regions were not sharply localized, but varied with stimulating positions of the cochlea. The curves of response magnitude versus stimulus intensity showed the narrow dynamic range, and that of latency was almost constant. These results were significantly different from those for normal sound stimulation.

Acoustic Stimulation↗
PubMed2000

Binaural interactions in primary auditory cortex of the awake macaque.

The functional organization of primary auditory cortex in non-primates is generally modeled as a tonotopic gradient with an orthogonal representation of independently mapped binaural interaction columns along the isofrequency contours. Little information is available regarding the validity of this model in the primate brain, despite the importance of binaural cues for sound localization and auditory scene analysis. Binaural and monaural responses of A1 to pure tone stimulation were studied using auditory evoked potentials, current source density and multiunit activity. Key findings include: (i) differential distribution of binaural responses with respect to best frequency, such that 74% of the sites exhibiting binaural summation had best frequencies below 2000 Hz; (ii) the pattern of binaural responses was variable with respect to cortical depth, with binaural summation often observed in the supragranular laminae of sites showing binaural suppression in thalamorecipient laminae; and (iii) dissociation of binaural responses between the initial and sustained action potential firing of neuronal ensembles in A1. These data support earlier findings regarding the temporal and spatial complexity of responses in A1 in the awake state, and are inconsistent with a simple orthogonal arrangement of binaural interaction columns and best frequency in A1 of the awake primate.

Animals↗
PubMed2004

Tonotopic and heterotopic projection systems in physiologically defined auditory cortex.

Combined physiological and connectional studies show significant non-topographic extrinsic projections to frequency-specific domains in the cat auditory cortex. These frequency-mismatched loci in the thalamus, ipsilateral cortex, and commissural system complement the predicted topographic and tonotopic projections. Two tonotopic areas, the primary auditory cortex (AI) and the anterior auditory field (AAF), were electrophysiologically characterized by their frequency organization. Next, either cholera toxin beta subunit or cholera toxin beta subunit gold conjugate was injected into frequency-matched locations in each area to reveal the projection pattern from the thalamus and cortex. Most retrograde labeling was found at tonotopically appropriate locations within a 1 mm-wide strip in the thalamus and a 2-3 mm-wide expanse of cortex (approximately 85%). However, approximately 13-30% of the neurons originated from frequency-mismatched locations far from their predicted positions in thalamic nuclei and cortical areas, respectively. We propose that these heterotopic projections satisfy at least three criteria that may be necessary to support the magnitude and character of plastic changes in physiological studies. First, they are found in the thalamus, ipsilateral and commissural cortex; since this reorganization could arise from any of these routes and may involve each, such projections ought to occur in them. Second, they originate from nuclei and areas with or without tonotopy; it is likely that plasticity is not exclusively shaped by spectral influences and not limited to cochleotopic regions. Finally, the projections are appropriate in magnitude and sign to plausibly support such rearrangements; given the rapidity of some aspects of plastic changes, they should be mediated by substantial existing connections. Alternative roles for these heterotopic projections are also considered.

Animals↗
PubMed1998

Reorganization of auditory cortex in tinnitus.

Magnetic source imaging was used to determine whether tonotopy in auditory cortex of individuals with tinnitus diverges from normative functional organization. Ten tinnitus subjects and 15 healthy controls were exposed to four sets of tones while magnetoencephalographic recordings were obtained from the two cortical hemispheres in sequence. A marked shift of the cortical representation of the tinnitus frequency into an area adjacent to the expected tonotopic location was observed. The Euclidean distance of the tinnitus frequency from the trajectory of the tonotopic map was 5.3 mm (SD = 3.1) compared with a distance of 2.5 mm (SD = 1.3) of a corresponding frequency in the healthy controls (t = 3.13, P < 0.01). In addition, a strong positive correlation was found between the subjective strength of the tinnitus and the amount of cortical reorganization (r = 0.82, P < 0.01). These results demonstrate that tinnitus is related to plastic alterations in auditory cortex. Similarities between these data and the previous demonstrations that phantom limb pain is highly correlated with cortical reorganization suggest that tinnitus may be an auditory phantom phenomenon.

Acoustic Stimulation↗
PubMed2002

Optical imaging of intrinsic signals in ferret auditory cortex: responses to narrowband sound stimuli.

This paper describes optical imaging of the auditory cortex in the anesthetized ferret, particularly addressing optimization of narrowband stimuli. The types of sound stimuli used were tone-pip trains and sinusoidal frequency and amplitude modulated (SFM and SAM) tones. By employing short illumination wavelengths (546 nm), we have successfully characterized the tonotopic arrangement, in agreement with the well-established electrophysiological tonotopic maps of the ferret auditory primary field (AI). The magnitude of the optical signal increased with sound level, was maximal for a modulation frequency (MF) of 2-4 Hz, and was larger for tone-pip trains and SFM sounds than for SAM sounds. Accordingly, an optimal narrowband stimulus was defined. Thus optical imaging can be used successfully to obtain frequency maps in auditory cortex by an appropriate choice of stimulus parameters. In addition, background noise consisting of 0.1-Hz oscillations could be reduced by introduction of blood pressure enhancing drugs. The optical maps were largely independent of 1) the type of narrowband stimulus, 2) the sound level, and 3) the MF. This stability of the optical maps was not predicted from the electrophysiological literature.

Acoustic Stimulation↗