Activity In Primary Auditory Cortex Research Articles

Primary auditory cortex activation by visual speech: an fMRI study at 3???T

Recent studies have yielded contradictory evidence on whether visual speech perception (watching articulatory gestures) can activate the human primary auditory cortex. To circumvent confounds due to inter-individual anatomical variation, we defined our subjects' Heschl's gyri and assessed blood oxygenation-dependent signal changes at 3 T within this confined region during visual speech perception and observation of moving circles. Visual speech perception activated Heschl's gyri in nine subjects, with activation in seven of them extending to the area of primary auditory cortex. Activation was significantly stronger during visual speech perception than during observation of the moving circles. Further, a significant hemisphere by stimulus interaction occurred, suggesting left Heschl's gyrus specialization for visual speech processing.

Neuronavigated repetitive transcranial magnetic stimulation in patients with tinnitus: a short case series

Background Clinical as well as neurophysiological and neuroimaging data suggest that chronic tinnitus resembles neuropsychiatric syndromes characterized by focal brain activation. Low-frequency repetitive transcranial magnetic stimulation (rTMS) has been proposed as an efficient method in treating brain hyperexcitability disorders by reducing cortical excitability. Methods In three patients suffering from chronic tinnitus, the effect of magnetic resonance imaging and positron emission tomography guided neuronavigated 1 Hz rTMS on auditory cortex activity was evaluated, using a sham controlled double-blind crossover design. Results Two of three patients revealed clearly increased metabolic activity in circumscript areas of the primary auditory cortex (PAC), allowing a selective stimulation of these cortical areas with low-frequency rTMS. Considerable improvement in tinnitus was achieved in these patients. Conclusions Neuronavigated rTMS of increased PAC activity may help to better understand the neuronal basis of chronic tinnitus and might offer a new option for treating auditory phantom perceptions like chronic tinnitus.

Neuronavigated rTMS in a patient with chronic tinnitus. Effects of 4 weeks treatment

Clinical, neurophysiological and neuroimaging data suggest that chronic tinnitus resembles neuropsychiatric syndromes characterised by focal brain activation. Low frequency repetitive transcranial magnetic stimulation (rTMS) has been proposed as an efficient method in treating brain hyperexcitability disorders. In one patient suffering from chronic tinnitus, [18F]deoxyglucose PET revealed increased metabolic activity in a circumscript area of the left primary auditory cortex (PAC). The effect of MRI and PET guided neuronavigated IHz rTMS of this area was evaluated in a single-blind, sham-controlled, cross-over manner, followed by a 4-week open treatment. Following active stimulation there was a remarkable effect, enduring several weeks, on tinnitus sensation, which was paralleled by altered cortical excitability. These findings suggest that neuronavigated rTMS of increased PAC activity might offer a new option for treating auditory phantom perceptions like chronic tinnitus. NeuroReport 14:977–980 © 2003 Lippincott Williams & Wilkins.

Eye Position Affects Activity in Primary Auditory Cortex of Primates

Background: Neurons in primary auditory cortex are known to be sensitive to the locations of sounds in space, but the reference frame for this spatial sensitivity has not been investigated. Conventional wisdom holds that the auditory and visual pathways employ different reference frames, with the auditory pathway using a head-centered reference frame and the visual pathway using an eye-centered reference frame. Reconciling these discrepant reference frames is therefore a critical component of multisensory integration.Results: We tested the reference frame of neurons in the auditory cortex of primates trained to fixate visual stimuli at different orbital positions. We found that eye position altered the activity of about one third of the neurons in this region (35 of 113, or 31%). Eye position affected not only the responses to sounds (26 of 113, or 23%), but also the spontaneous activity (14 of 113, or 12%). Such effects were also evident when monkeys moved their eyes freely in the dark. Eye position and sound location interacted to produce a representation for auditory space that was neither head- nor eye-centered in reference frame.Conclusions: Taken together with emerging results in both visual and other auditory areas, these findings suggest that neurons whose responses reflect complex interactions between stimulus position and eye position set the stage for the eventual convergence of auditory and visual information.

Localization of Primary Auditory Cortex in Humans by Magnetoencephalography

Brief auditory stimuli activate the primary auditory cortex (PAC) earlier than any other cortical area so, within a certain latency range, the PAC is the only cortical source contributing to the auditory evoked field (AEF). Nevertheless, there is no AEF component specific to PAC that can be reliably detected in all individuals. The present study suggests that a peak in the first temporal derivative of the magnetic field at about 20 ms (dP20m) is a genuine correlate of PAC activity. AEFs in response to clicks presented to the right ear were recorded with a 37-channel axial gradiometer system positioned over the left hemisphere in nine normal-hearing subjects. More than 8500 stimuli were presented in each of two independent sessions at a rate of approximately 3/s. The dipole coordinates for the dP20m derived from the two sessions typically differed by only a few millimeters. Coregistration of the dipoles with structural magnetic resonance images suggests that dP20m arises from an area close to the retroinsular origin of Heschl's gyrus. Although the dP20m is simply the point of steepest slope on the well-known middle-latency peak, P30m or Pam, it would appear that dP20m and P30m do not have the same cortical origin. Evidence is provided that P30m receives major contributions from at least two distinct cortical areas, only one of which is PAC.

The representation of peripheral neural activity in the middle-latency evoked field of primary auditory cortex in humans

Short sweeps with increasing instantaneous frequency (up-chirps) designed to compensate for the propagation delay along the human cochlea enhance the magnitude of wave V of the auditory brainstem responses, while time reversed sweeps (down-chirps) reduce the magnitude of wave V [Dau, T., Wegner, O., Mellert, V., Kollmeier, B., J. Acoust. Soc. Am. 107 (2000) 1530–1540]. This effect is due to synchronisation of frequency channels along the basilar membrane and it indicates that cochlear phase delays are preserved up to the input of the inferior colliculus. The present magnetoencephalography study was designed to investigate the influence of peripheral synchronisation on the activation in primary auditory cortex. Spatio-temporal source analysis of middle-latency auditory evoked fields (MAEFs) elicited by clicks and up- and down-chirps showed that up-chirps elicited significantly larger MAEF responses compared to clicks or down-chirps. Both N19m–P30m magnitude and its latency are influenced by peripheral cross-channel phase effects. Furthermore, deconvolution of the empirical source waveforms with spike probability functions simulated with a cochlear model indicated that the source waves for all stimulus conditions could be explained with the same unit-response function, i.e. a far field recorded cortical response of a very small cell assembly along the medio-lateral axis of Heschl’s gyrus that receives input from a small number of excitatory fibres. The conclusion is that (i) phase delays between channels in the auditory pathway are preserved up to primary auditory cortex, and (ii) MAEFs can be described by a convolution of a unit-response function with the summary neural activity pattern of the auditory nerve.

Comparison between local field potentials and unit cluster activity in primary auditory cortex and anterior auditory field in the cat

Multi-unit (MU) activity and local field potentials (LFP) were simultaneously recorded from 161 sites in the middle cortical layers of the primary auditory cortex (AI) and the anterior auditory field (AAF) in 51 cats. Responses were obtained for frequencies between 625 Hz and 40 kHz, at intensities from 75 dB SPL to threshold. We compared the response properties of MU activity and LFP triggers, in terms of characteristic frequency (CF), threshold at CF, minimum latency and frequency tuning-curve bandwidth 20 dB above threshold. On average, thresholds at CF were significantly lower for LFP events than those for MU spikes (4.6 dB for AI, and 3 dB for AAF). Minimum latencies were significantly shorter for LFP events than for MU spikes (1.5 ms in AI, and 1.7 ms in AAF). Frequency tuning curves were significantly broader for LFP events than those for MU spikes (1.0 octave in AI, and 1.3 octaves in AAF). In contrast, the CF was not significantly different between LFP events and MU spikes. The LFP results indicate that cortical neurons receive convergent sub-cortical inputs from a broad frequency range. The sharper tuning curves for MU activity compared to those of LFP events are likely the result of intracortical inhibitory processes.

Visual speech perception without primary auditory cortex activation.

Speech perception is conventionally thought to be an auditory function, but humans often use their eyes to perceive speech. We investigated whether visual speech perception depends on processing by the primary auditory cortex in hearing adults. In a functional magnetic resonance imaging experiment, a pulse-tone was presented contrasted with gradient noise. During the same session, a silent video of a talker saying isolated words was presented contrasted with a still face. Visual speech activated the superior temporal gyrus anterior, posterior, and lateral to the primary auditory cortex, but not the region of the primary auditory cortex. These results suggest that visual speech perception is not critically dependent on the region of primary auditory cortex.

The Preparation and Execution of Self-Initiated and Externally-Triggered Movement: A Study of Event-Related fMRI

Studies of functional brain imaging in humans and single cell recordings in monkeys have generally shown preferential involvement of the medially located supplementary motor area (SMA) in self-initiated movement and the lateral premotor cortex in externally cued movement. Studies of event-related cortical potentials recorded during movement preparation, however, generally show increased cortical activity prior to self-initiated movements but little activity at early stages prior to movements that are externally cued at unpredictable times. In this study, the spatial location and relative timing of activation for self-initiated and externally triggered movements were examined using rapid event-related functional MRI. Twelve healthy right-handed subjects were imaged while performing a brief finger sequence movement (three rapid alternating button presses: index–middle–index finger) made either in response to an unpredictably timed auditory cue (between 8 to 24 s after the previous movement) or at self-paced irregular intervals. Both movement conditions involved similar strong activation of medial motor areas including the pre-SMA, SMA proper, and rostral cingulate cortex, as well as activation within contralateral primary motor, superior parietal, and insula cortex. Activation within the basal ganglia was found for self-initiated movements only, while externally triggered movements involved additional bilateral activation of primary auditory cortex. Although the level of SMA and cingulate cortex activation did not differ significantly between movement conditions, the timing of the hemodynamic response within the pre-SMA was significantly earlier for self-initiated compared with externally triggered movements. This clearly reflects involvement of the pre-SMA in early processes associated with the preparation for voluntary movement.

Linking Sight and Sound: fMRI Evidence of Primary Auditory Cortex Activation during Visual Word Recognition

We describe two studies that used repetition priming paradigms to investigate brain activity during the reading of single words. Functional magnetic resonance images were collected during a visual lexical decision task in which nonword stimuli were manipulated with regard to phonological properties and compared to genuine English words. We observed a region in left-hemisphere primary auditory cortex linked to a repetition priming effect. The priming effect activity was observed only for stimuli that sound like known words; moreover, this region was sensitive to strategic task differences. Thus, a brain region involved in the most basic aspects of auditory processing appears to be engaged in reading even when there is no environmental oral or auditory component.

Complex tone processing in primary auditory cortex of the awake monkey. I. Neural ensemble correlates of roughness.

Previous physiological studies [e.g., Bieser and Muller-Preuss, Exp. Brain Res. 108, 273-284 (1996); Schulze and Langner, J. Comp. Physiol. A 181, 651-663 (1997); Steinschneider et al., J. Acoust. Soc. Am. 104, 2935-2955 (1998)] have suggested that neural activity in primary auditory cortex (A1) phase-locked to the waveform envelope of complex sounds with low (<300 Hz) periodicities may represent a neural correlate of roughness perception. However, a correspondence between these temporal response patterns and human psychophysical boundaries of roughness has not yet been demonstrated. The present study examined whether the degree of synchronized phase-locked activity of neuronal ensembles in A1 of the awake monkey evoked by complex tones parallels human psychoacoustic data defining the existence region and frequency dependence of roughness. Stimuli consisted of three consecutive harmonics of fundamental frequencies (f(0)s) ranging from 25 to 4000 Hz. The center frequency of the complex tones was fixed at the best frequency (BF) of the cortical sites, which ranged from 0.3 to 10 kHz. Neural ensemble activity in the thalamorecipient zone (lower lamina III) and supragranular cortical laminae (upper lamina III and lamina II) was measured using multiunit activity and current source density techniques and the degree of phase-locking to the f0 was quantified by spectral analysis. In the thalamorecipient zone, the stimulus f0 at which phase-locking was maximal increased with BF and reached an upper limit between 75 and 150 Hz for BFs greater than about 3 kHz. Estimates of limiting phase-locking rates also increased with BF and approximated psychoacoustic values for the disappearance of roughness. These physiological relationships parallel human perceptual data and therefore support the relevance of phase-locked activity of neuronal ensembles in A1 for the physiological representation of roughness.

Tonotopic cortical changes following stapes substitution in otosclerotic patients: a magnetoencephalographic study.

The aim of the study was to investigate and follow up the tonotopic organization of the primary auditory cortex in otosclerotic patients before and after corrective surgery. The characteristics of primary auditory cortex activation were studied in ten otosclerotic patients (i.e., subjects suffering from a conductive hearing loss, prior to and following stapes substitution). Magnetoencephalographic recordings of auditory evoked fields by tone-burst stimulation at octave frequencies between 250 and 2000 Hz were performed during monaural stimulation. The brain topography of the main cortical response (N100m) generators at different tones was studied in patients and compared with ten healthy controls; pre- post-surgical changes were also correlated to their clinical outcome following corrective surgery. A significant decrease of the tonotopic extension in the cortical region responsive to the four explored frequencies was found in patients before surgery with respect to the control population. At the time of postsurgical follow-up, the tonotopic representation had enlarged and was approaching the dimensions seen in normal subjects, although with higher variability. The extent of the enlargement of the postoperative tonotopically organized area was directly correlated with the postsurgery period duration. Our findings indicate that auditory cortical areas of human adults undergo functional reorganization following peripheral alteration of the sensory input entering the CNS. The restriction of the cortical tonotopic region caused by the long-term reduction of acoustic input is followed by a reorganization within the usual boundaries following the recovery of auditory function; this process is taking place in a time scale of a few weeks.

The N100 auditory cortical evoked potential indexes scanning of auditory short-term memory

Objective: To study activity of auditory cortex reflected by the N100 and P200 components of the auditory evoked potentials during memorization and scanning of short-term memory stores. Methods: In a MEMORY task subjects classified a probe digit either as a member or not a member of a previously presented list of digits that varied in size from one, 3, 5 and 7 items. For comparison, subjects in a NUMBER task listened to a list of digits as in the MEMORY task but determined only whether the probe digit as odd or even. Evoked potentials to the presentation list and to probes were recorded from scalp electrodes and separately averaged for both tasks. The components peaking at approximately 100 ms (N100) and 200 ms (P200) that reflect activity of primary auditory cortex were identified and peak amplitudes and latencies were measured. Results: For presentation set items, the amplitude of N100 was affected by set size in the MEMORY but not in the NUMBER task; N100 was larger for the one item set than for the 3, 5, and 7 item sets. P200 increased in amplitude in a linear manner for both the MEMORY and NUMBER tasks. For probe items, N100 but not P200 amplitude decreased in a linear manner as the number of items in the presentation set increased in the MEMORY but not in the NUMBER task. The linear change of N100 amplitude during memory scanning was particular to in-set but not to out-of-set probes. The amplitudes of both N100 and P200 were almost twice as large in probe digits than in the digits in the presentation set in both the MEMORY and NUMBER tasks. Conclusion: Auditory sensory cortical activity in humans during an auditory short-term memory task shows dynamic changes during both memorization and memory scanning.

Human auditory cortex electrophysiological correlates of the precedence effect: Binaural echo lateralization suppression

Echoes lagging shortly after a sound and originating from a different location blend with the sound source perceptually. The location of the fused “source+echoes” is dominated by the source, suggesting suppression of echo localization. This effect is diminished monaurally, implying involvement of binaural processing. The neural substrates underlying the echo localization suppression are still unclear. The electrophysiological indications of primary auditory cortex involvement in binaural suppression of echo lateralization are presented. Position judgment and auditory-evoked potentials (AEPs) were recorded to single- and pairs of binaural and monaural clicks. The pairs simulated a source and its echo. The binaural source+echo position judgment was dominated by the source at small echo lags. With lag increase, it shifted toward the echo. The AEPs were studied for binaural processes specific to a “real” echo, as opposed to an identical single sound (a “virtual” echo). A reduction in binaural peak amplitude and area and an increase in binaural peak latency were detected specifically in the real echo-evoked middle-latency component Pa. The binaural echo Pa suppression depended upon echo lag, although less strongly than the psychoacoustic position judgment. Pa is associated with primary auditory cortex activity, thus suggesting primary cortical involvement in echo-lateralization suppression.

Analysis of temporal structure in sound by the human brain.

For over a century, models of pitch perception have been based on the frequency composition of the sound. Pitch phenomena can also be explained, however, in terms of the time structure, or temporal regularity, of sounds. To locate the mechanism for the detection of temporal regularity in humans, we used functional imaging and a 'delay-and-add' noise, which activates all frequency regions uniformly, like noise, but which nevertheless produces strong pitch perceptions and tuneful melodies. This stimulus has temporal regularity that can be systematically altered. We found that the activity of primary auditory cortex increased with the regularity of the sound. Moreover, a melody composed of delay-and-add 'notes' produced a distinct pattern of activation in two areas of the temporal lobe distinct from primary auditory cortex. These results suggest a hierarchical analysis of time structure in the human brain.

Salicylate and quinine selectively increase spontaneous firing rates in secondary auditory cortex

This study presents firing rates for simultaneously recorded spontaneous and stimulus driven multi-unit activity in primary auditory cortex (AI), anterior auditory field (AAF) and secondary auditory cortex (AII) in cats before and after application of salicylate or quinine. From 21 cats, in three cortical areas simultaneously, a total of 1533 multi-unit files were obtained. The data suggest (1) that both salicylate and quinine significantly increase spontaneous firing rates in AII, whereas in AI and AAF both quinine and salicylate reduced the spontaneous rate; (2) the effect of both drugs was to increase spontaneous rates for recording sites with high characteristic frequency (CF) and a tendency to decrease them for low CF sites; (3) the mean stimulus driven firing rates were not affected by either drug except for a decrease produced by quinine in AI; (4) changes in driven firing rate were positively correlated with changes in spontaneous firing rates. This suggests that tinnitus inducing agents selectively increase spontaneous firing rates in the extralemniscal pathway.

Effects of quinine on neural activity in cat primary auditory cortex

The effect of systemically applied quinine on single-unit firing activity in primary auditory cortex was investigated in seven cats. A dose of 100 or 200 mg/kg of quinine hydrochloride was administered intramuscularly and recordings from the same units were performed prior to application and continuously up to on average 5.5 h after administration. All animals showed 10–40 dB of threshold shift about 30 min after administration and some animals showed recovery during the course of the investigation. Significant increases were found in spontaneous firing rates for low-firing-rate units (initial firing rate < 1 spike/s). For high-firing-rate units (initial firing rate > 1 spike/s) no significant changes were observed. There were no significant changes in modal and mean interspike interval. The time-to-rebound peak in the autocorrelation function for spontaneous firings was not altered significantly. The rate of burst occurrence showed no significant change. The best modulation frequency in response to stimulation with periodic click trains decreased after administration, but the limiting rate did not change. Peak cross-correlation coefficients for the spontaneous firings of simultaneously recorded cells showed a significant increase and the correlogram's central peak was significantly narrower after quinine application. Dose effects were only present for cross-correlation results and temporal modulation transfer functions. The results for both spontaneous firing rate, peak width in the cross-correlogram and click stimulation were similar to those observed in salicylate-treated cats (Ochi and Eggermont, 1996). The other findings were different from those observed after salicylate. It is obvious that the effects of quinine on the auditory system are not the same as those of salicylate. The increased synchronization of the spontaneous firings across different neurons observed after application of both drugs may be related to tinnitus.

The Role of the Thalamus in “Top Down” Modulation of Attention to Sound

By correlating rCBF with rate of presentation of tones we used PET to identify brain regions where auditory signals elicited a transient neural response. In one condition volunteers were asked to attend to the tones and ignore visual signals, while in the second condition they were asked to attend to the visual signals and ignore the tones. Activity in primary auditory cortex and adjacent areas was strongly correlated with rate of tone presentation, but this relationship was not affected by the direction of attention. In only one area, the right midthalamus, was the response to tones modulated by attention. In this area responses to tones occurred when attention was directed to sound, but not when attention was directed to visual stimuli. There is considerable evidence that the EEG evoked response to tones (N100/Nd response) is strongly modulated by attention and arises in auditory cortex. The ERP is the sum of activity from many sources. The amplitude of this response reflects not only the amount of activity in these sources, but also the degree of synchrony between them. The difference between these typical ERP results and our result from PET could be resolved if we assume that, in our paradigm, attention did not increase the amount of neural activity in auditory cortex, but rather the degree of synchrony between many sources. The signal in the thalamus, which we observed only when volunteers were attending to the tones, might provide the basis for this synchrony.

Role of cortical N-methyl-D-aspartate receptors in auditory sensory memory and mismatch negativity generation: implications for schizophrenia.

Working memory refers to the ability of the brain to store and manipulate information over brief time periods, ranging from seconds to minutes. As opposed to long-term memory, which is critically dependent upon hippocampal processing, critical substrates for working memory are distributed in a modality-specific fashion throughout cortex. N-methyl-D-aspartate (NMDA) receptors play a crucial role in the initiation of long-term memory. Neurochemical mechanisms underlying the transient memory storage required for working memory, however, remain obscure. Auditory sensory memory, which refers to the ability of the brain to retain transient representations of the physical features (e.g., pitch) of simple auditory stimuli for periods of up to approximately 30 sec, represents one of the simplest components of the brain working memory system. Functioning of the auditory sensory memory system is indexed by the generation of a well-defined event-related potential, termed mismatch negativity (MMN). MMN can thus be used as an objective index of auditory sensory memory functioning and a probe for investigating underlying neurochemical mechanisms. Monkeys generate cortical activity in response to deviant stimuli that closely resembles human MMN. This study uses a combination of intracortical recording and pharmacological micromanipulations in awake monkeys to demonstrate that both competitive and noncompetitive NMDA antagonists block the generation of MMN without affecting prior obligatory activity in primary auditory cortex. These findings suggest that, on a neurophysiological level, MMN represents selective current flow through open, unblocked NMDA channels. Furthermore, they suggest a crucial role of cortical NMDA receptors in the assessment of stimulus familiarity/unfamiliarity, which is a key process underlying working memory performance.

Effects of salicylate on neural activity in cat primary auditory cortex

The effect of systemically applied salicylate on single-unit firing activity in primary auditory cortex was investigated in six cats. A dose of 200 mg/kg sodium salicylate was administered intraperitoneally, and recordings from the same units were performed prior to application and continuously up to, on average, 6 h after administration. Local field potentials were used to track the threshold shifts and general input-output ( I O ) behavior following salicylate administration. All animals showed 20–30 dB of threshold shift about 2 h after administration and showed no recovery during the following 4 h. I O curves were invariably of the recruitment type. Significant changes were found in spontaneous firing rates for two groups of units separately. Low-spontaneous rate units (initial firing rate < 1 spike/s) showed an increase in spontaneous rate and high-spontaneous rate units (initial firing rate > 1 spike/s) showed a decrease in spontaneous firing rate. There were no significant changes in modal and mean values for interspike-interval (ISO histograms. The duration-to-rebound peak in the autocorrelation function for spontaneous firings was prolonged significantly after salicylate administration. Peak cross-correlation coefficients for the firing patterns of simultaneously recorded cells showed no significant change but the correlogram's central peak was significantly narrower after salicylate application. The percentage of firings occurring in bursts showed no significant change after administration of salicylate. The best modulation frequency in response to stimulation with periodic click trains decreased after administration. Both the changes in the spontaneous autocorrelogram and in the temporal modulation transfer function suggest a prolongation in the duration of the Ca 2+-activated K + conductance of the cortical pyramidal cells following salicylate. This suggests that salicylates affect both the auditory periphery and the auditory cortex.