Sound repetition rate in the human auditory pathway: representations in the waveshape and amplitude of fMRI activation

Functional imaging of the thalamus in language

Herein, the literature regarding functional imaging of the thalamus during language tasks is reviewed. Fifty studies met criteria for analysis. Two of the most common task paradigms associated with thalamic activation were generative tasks (e.g. word or sentence generation) and naming, though activation was also seen in tasks that involve lexical decision, reading and working memory. Typically, thalamic activation was seen bilaterally, left greater than right, along with activation in frontal and temporal cortical regions. Thalamic activation was seen with perceptually challenging tasks, though few studies rigorously correlated thalamic activation with measures of attention or task difficulty. The peaks of activation loci were seen in virtually all thalamic regions, with a bias towards left-sided and midline activation. These analyses suggest that the thalamus may be involved in processes that involve manipulations of lexical information, but point to the need for more systematic study of the thalamus using language tasks.

Spatiotemporal Segregation of Neural Response to Auditory Stimulation: An fMRI Study Using Independent Component Analysis and Frequency-Domain Analysis

Although auditory processing has been widely studied with conventional parametric methods, there have been a limited number of independent component analysis (ICA) applications in this area. The purpose of this study was to examine spatiotemporal behavior of brain networks in response to passive auditory stimulation using ICA. Continuous broadband noise was presented binaurally to 19 subjects with normal hearing. ICA was performed to segregate spatial networks, which were subsequently classified according to their temporal relation to the stimulus using power spectrum analysis. Classification of separated networks resulted in 3 stimulus-activated, 9 stimulus-deactivated, 2 stimulus-neutral (stimulus-dependent but not correlated with the stimulation timing), and 2 stimulus-unrelated (fluctuations that did not follow the stimulus cycles) components. As a result of such classification, spatiotemporal subdivisions were observed in a number of cortical structures, namely auditory, cingulate, and sensorimotor cortices, where parts of the same cortical network responded to the stimulus with different temporal patterns. The majority of the classified networks seemed to comprise subparts of the known resting-state networks (RSNs); however, they displayed different temporal behavior in response to the auditory stimulus, indicating stimulus-dependent temporal segregation of RSNs. Only one of nine deactivated networks coincided with the “classic” default-mode network, suggesting the existence of a stimulus-dependent default-mode network, different from that commonly accepted.

Mechanisms of adaptation in human auditory cortex

This study investigates the temporal properties of adaptation in the late auditory-evoked potentials in humans. The results are used to make inferences about the mechanisms of adaptation in human auditory cortex. The first experiment measured adaptation by single adapters as a combined function of the adapter duration and the stimulus onset asynchrony (SOA) and interstimulus interval (ISI) between the adapter and the adapted sound ("probe"). The results showed recovery from adaptation with increasing ISI, as would be expected, but buildup of adaptation with increasing adapter duration and thus SOA. This suggests that adaptation in auditory cortex is caused by the ongoing, rather than the onset, response to the adapter. Quantitative modeling indicated that the rate of buildup of adaptation is almost an order of magnitude faster than the recovery rate of adaptation. The recovery rate suggests that cortical adaptation is caused by synaptic depression and slow afterhyperpolarization. The P2 was more strongly affected by adaptation than the N1, suggesting that the two deflections originate from different cortical generators. In the second experiment, the single adapters were replaced by trains of two or four identical adapters. The results indicated that adaptation decays faster after repeated presentation of the adapter. This increase in the recovery rate of adaptation might contribute to the elicitation of the auditory mismatch negativity response. It may be caused by top-down feedback or by local processes such as the buildup of residual Ca(2+) within presynaptic neurons.

Sound envelope encoding in the auditory cortex revealed by neuromagnetic responses in the theta to gamma frequency bands

Amplitude fluctuations of natural sounds carry multiple types of information represented at different time scales, such as syllables and voice pitch in speech. However, it is not well understood how such amplitude fluctuations at different time scales are processed in the brain. In the present study we investigated the effect of the stimulus rate on the cortical evoked responses using magnetoencephalography (MEG). We used a two-tone complex sound, whose envelope fluctuated at the difference frequency and induced an acoustic beat sensation. When the beat rate was continuously swept between 3Hz and 60Hz, auditory evoked response showed distinct transient waves at slow rates, while at fast rates continuous sinusoidal oscillations similar to the auditory steady-state response (ASSR) were observed. We further derived temporal modulation transfer functions (TMTF) from amplitudes of the transient responses and from the ASSR. The results identified two critical rates of 12.5Hz and 25Hz, at which consecutive transient responses overlapped with each other. These stimulus rates roughly corresponded to the rates at which the perceptual quality of the sound envelope is known to change. Low rates (> 10Hz) are perceived as loudness fluctuation, medium rates as acoustical flutter, and rates above 25Hz as roughness. We conclude that these results reflect cortical processes that integrate successive acoustic events at different time scales for extracting complex features of natural sound.

Hearing without listening: functional connectivity reveals the engagement of multiple nonauditory networks during basic sound processing

The present functional magnetic resonance imaging (fMRI) study presents data challenging the traditional view that sound is processed almost exclusively in the classical auditory pathway unless imbued with behavioral significance. In a first experiment, subjects were presented with broadband noise in on/off fashion as they performed an unrelated visual task. A conventional analysis assuming predictable sound-evoked responses demonstrated a typical activation pattern that was confined to classical auditory centers. In contrast, spatial independent component analysis (sICA) disclosed multiple networks of acoustically responsive brain centers. One network comprised classical auditory centers, but four others included nominally "nonauditory" areas: cingulo-insular cortex, mediotemporal limbic lobe, basal ganglia, and posterior orbitofrontal cortex, respectively. Functional connectivity analyses confirmed the sICA results by demonstrating coordinated activity between the involved brain structures. In a second experiment, fMRI data obtained from unstimulated (i.e., resting) subjects revealed largely similar networks. Together, these two experiments suggest the existence of a coordinated system of multiple acoustically responsive intrinsic brain networks, comprising classical auditory centers but also other brain areas. Our results suggest that nonauditory centers play a role in sound processing at a very basic level, even when the sound is not intertwined with behaviors requiring the well-known functionality of these regions.

Coding of repetitive transients by auditory cortex on posterolateral superior temporal gyrus in humans: an intracranial electrophysiology study

Evidence regarding the functional subdivisions of human auditory cortex has been slow to converge on a definite model. In part, this reflects inadequacies of current understanding of how the cortex represents temporal information in acoustic signals. To address this, we investigated spatiotemporal properties of auditory responses in human posterolateral superior temporal (PLST) gyrus to acoustic click-train stimuli using intracranial recordings from neurosurgical patients. Subjects were patients undergoing chronic invasive monitoring for refractory epilepsy. The subjects listened passively to acoustic click-train stimuli of varying durations (160 or 1,000 ms) and rates (4-200 Hz), delivered diotically via insert earphones. Multicontact subdural grids placed over the perisylvian cortex recorded intracranial electrocorticographic responses from PLST and surrounding areas. Analyses focused on averaged evoked potentials (AEPs) and high gamma (70-150 Hz) event-related band power (ERBP). Responses to click trains featured prominent AEP waveforms and increases in ERBP. The magnitude of AEPs and ERBP typically increased with click rate. Superimposed on the AEPs were frequency-following responses (FFRs), most prominent at 50-Hz click rates but still detectable at stimulus rates up to 200 Hz. Loci with the largest high gamma responses on PLST were often different from those sites that exhibited the strongest FFRs. The data indicate that responses of non-core auditory cortex of PLST represent temporal stimulus features in multiple ways. These include an isomorphic representation of periodicity (as measured by the FFR), a representation based on increases in non-phase-locked activity (as measured by high gamma ERBP), and spatially distributed patterns of activity.

Sustained BOLD and theta activity in auditory cortex are related to slow stimulus fluctuations rather than to pitch

Human functional MRI (fMRI) and magnetoencephalography (MEG) studies indicate a pitch-specific area in lateral Heschl's gyrus. Single-cell recordings in monkey suggest that sustained-firing, pitch-specific neurons are located lateral to primary auditory cortex. We reevaluated whether pitch strength contrasts reveal sustained pitch-specific responses in human auditory cortex. Sustained BOLD activity in auditory cortex was found for iterated rippled noise (vs. noise or silence) but not for regular click trains (vs. jittered click trains or silence). In contrast, iterated rippled noise and click trains produced similar pitch responses in MEG. Subsequently performed time-frequency analysis of the MEG data suggested that the dissociation of cortical BOLD activity between iterated rippled noise and click trains is related to theta band activity. It appears that both sustained BOLD and theta activity are associated with slow non-pitch-specific stimulus fluctuations. BOLD activity in the inferior colliculus was sustained for both stimulus types and varied neither with pitch strength nor with the presence of slow stimulus fluctuations. These results suggest that BOLD activity in auditory cortex is much more sensitive to slow stimulus fluctuations than to constant pitch, compromising the accessibility of the latter. In contrast, pitch-related activity in MEG can easily be separated from theta band activity related to slow stimulus fluctuations.

Sensitivity to temporal modulation rate and spectral bandwidth in the human auditory system: fMRI evidence

Hierarchical models of auditory processing often posit that optimal stimuli, i.e., those eliciting a maximal neural response, will increase in bandwidth and decrease in modulation rate as one ascends the auditory neuraxis. Here, we tested how bandwidth and modulation rate interact at several loci along the human central auditory pathway using functional MRI in a cardiac-gated, sparse acquisition design. Participants listened passively to both narrowband (NB) and broadband (BB) carriers (1/4- or 4-octave pink noise), which were jittered about a mean sinusoidal amplitude modulation rate of 0, 3, 29, or 57 Hz. The jittering was introduced to minimize stimulus-specific adaptation. The results revealed a clear difference between spectral bandwidth and temporal modulation rate: sensitivity to bandwidth (BB > NB) decreased from subcortical structures to nonprimary auditory cortex, whereas sensitivity to slow modulation rates was largest in nonprimary auditory cortex and largely absent in subcortical structures. Furthermore, there was no parametric interaction between bandwidth and modulation rate. These results challenge simple hierarchical models, in that BB stimuli evoked stronger responses in primary auditory cortex (and subcortical structures) rather than nonprimary cortex. Furthermore, the strong preference for slow modulation rates in nonprimary cortex demonstrates the compelling global sensitivity of auditory cortex to modulation rates that are dominant in the principal signals that we process, e.g., speech.

Hemispheric asymmetries in speech perception: sense, nonsense and modulations

Background

The well-established left hemisphere specialisation for language processing has long been claimed to be based on a low-level auditory specialization for specific acoustic features in speech, particularly regarding ‘rapid temporal processing’.

Methodology

A novel analysis/synthesis technique was used to construct a variety of sounds based on simple sentences which could be manipulated in spectro-temporal complexity, and whether they were intelligible or not. All sounds consisted of two noise-excited spectral prominences (based on the lower two formants in the original speech) which could be static or varying in frequency and/or amplitude independently. Dynamically varying both acoustic features based on the same sentence led to intelligible speech but when either or both acoustic features were static, the stimuli were not intelligible. Using the frequency dynamics from one sentence with the amplitude dynamics of another led to unintelligible sounds of comparable spectro-temporal complexity to the intelligible ones. Positron emission tomography (PET) was used to compare which brain regions were active when participants listened to the different sounds.

Conclusions

Neural activity to spectral and amplitude modulations sufficient to support speech intelligibility (without actually being intelligible) was seen bilaterally, with a right temporal lobe dominance. A left dominant response was seen only to intelligible sounds. It thus appears that the left hemisphere specialisation for speech is based on the linguistic properties of utterances, not on particular acoustic features.

Assessment of temporal state-dependent interactions between auditory fMRI responses to desired and undesired acoustic sources

A confounding factor in auditory functional magnetic resonance imaging (fMRI) experiments is the presence of the acoustic noise inherently associated with the echo planar imaging acquisition technique. Previous studies have demonstrated that this noise can induce unwanted neuronal responses that can mask stimulus-induced responses. Similarly, activation accumulated over multiple stimuli has been demonstrated to elevate the baseline, thus reducing the dynamic range available for subsequent responses. To best evaluate responses to auditory stimuli, it is necessary to account for the presence of all recent acoustic stimulation, beginning with an understanding of the attenuating effects brought about by interaction between and among induced unwanted neuronal responses, and responses to desired auditory stimuli. This study focuses on the characterization of the duration of this temporal memory and qualitative assessment of the associated response attenuation. Two experimental parameters--inter-stimulus interval (ISI) and repetition time (TR)--were varied during an fMRI experiment in which participants were asked to passively attend to an auditory stimulus. Results present evidence of a state-dependent interaction between induced responses. As expected, attenuating effects of these interactions become less significant as TR and ISI increase and in contrast to previous work, persist up to 18s after a stimulus presentation.

Age differences in FMRI adaptation for sound identity and location

We explored age differences in auditory perception by measuring fMRI adaptation of brain activity to repetitions of sound identity (what) and location (where), using meaningful environmental sounds. In one condition, both sound identity and location were repeated allowing us to assess non-specific adaptation. In other conditions, only one feature was repeated (identity or location) to assess domain-specific adaptation. Both young and older adults showed comparable non-specific adaptation (identity and location) in bilateral temporal lobes, medial parietal cortex, and subcortical regions. However, older adults showed reduced domain-specific adaptation to location repetitions in a distributed set of regions, including frontal and parietal areas, and to identity repetition in anterior temporal cortex. We also re-analyzed data from a previously published 1-back fMRI study, in which participants responded to infrequent repetition of the identity or location of meaningful sounds. This analysis revealed age differences in domain-specific adaptation in a set of brain regions that overlapped substantially with those identified in the adaptation experiment. This converging evidence of reductions in the degree of auditory fMRI adaptation in older adults suggests that the processing of specific auditory “what” and “where” information is altered with age, which may influence cognitive functions that depend on this processing.

Transient bold activity locked to perceptual reversals of auditory streaming in human auditory cortex and inferior colliculus

Our auditory system separates and tracks temporally interleaved sound sources by organizing them into distinct auditory streams. This streaming phenomenon is partly determined by physical stimulus properties but additionally depends on the internal state of the listener. As a consequence, streaming perception is often bistable and reversals between one- and two-stream percepts may occur spontaneously or be induced by a change of the stimulus. Here, we used functional MRI to investigate perceptual reversals in streaming based on interaural time differences (ITD) that produce a lateralized stimulus perception. Listeners were continuously presented with two interleaved streams, which slowly moved apart and together again. This paradigm produced longer intervals between reversals than stationary bistable stimuli but preserved temporal independence between perceptual reversals and physical stimulus transitions. Results showed prominent transient activity synchronized with the perceptual reversals in and around the auditory cortex. Sustained activity in the auditory cortex was observed during intervals where the ΔITD could potentially produce streaming, similar to previous studies. A localizer-based analysis additionally revealed transient activity time locked to perceptual reversals in the inferior colliculus. These data suggest that neural activity associated with streaming reversals is not limited to the thalamo-cortical system but involves early binaural processing in the auditory midbrain, already.

Functional dissociation of transient and sustained fMRI BOLD components in human auditory cortex revealed with a streaming paradigm based on interaural time differences

A number of physiological studies suggest that feature-selective adaptation is relevant to the pre-processing for auditory streaming, the perceptual separation of overlapping sound sources. Most of these studies are focused on spectral differences between streams, which are considered most important for streaming. However, spatial cues also support streaming, alone or in combination with spectral cues, but physiological studies of spatial cues for streaming remain scarce. Here, we investigate whether the tuning of selective adaptation for interaural time differences (ITD) coincides with the range where streaming perception is observed. FMRI activation that has been shown to adapt depending on the repetition rate was studied with a streaming paradigm where two tones were differently lateralized by ITD. Listeners were presented with five different ΔITD conditions (62.5, 125, 187.5, 343.75, or 687.5 μs) out of an active baseline with no ΔITD during fMRI. The results showed reduced adaptation for conditions with ΔITD ≥ 125 μs, reflected by enhanced sustained BOLD activity. The percentage of streaming perception for these stimuli increased from approximately 20% for ΔITD = 62.5 μs to > 60% for ΔITD = 125 μs. No further sustained BOLD enhancement was observed when the ΔITD was increased beyond ΔITD = 125 μs, whereas the streaming probability continued to increase up to 90% for ΔITD = 687.5 μs. Conversely, the transient BOLD response, at the transition from baseline to ΔITD blocks, increased most prominently as ΔITD was increased from 187.5 to 343.75 μs. These results demonstrate a clear dissociation of transient and sustained components of the BOLD activity in auditory cortex.

Tinnitus, diminished sound-level tolerance, and elevated auditory activity in humans with clinically normal hearing sensitivity

Phantom sensations and sensory hypersensitivity are disordered perceptions that characterize a variety of intractable conditions involving the somatosensory, visual, and auditory modalities. We report physiological correlates of two perceptual abnormalities in the auditory domain: tinnitus, the phantom perception of sound, and hyperacusis, a decreased tolerance of sound based on loudness. Here, subjects with and without tinnitus, all with clinically normal hearing thresholds, underwent 1) behavioral testing to assess sound-level tolerance and 2) functional MRI to measure sound-evoked activation of central auditory centers. Despite receiving identical sound stimulation levels, subjects with diminished sound-level tolerance (i.e., hyperacusis) showed elevated activation in the auditory midbrain, thalamus, and primary auditory cortex compared with subjects with normal tolerance. Primary auditory cortex, but not subcortical centers, showed elevated activation specifically related to tinnitus. The results directly link hyperacusis and tinnitus to hyperactivity within the central auditory system. We hypothesize that the tinnitus-related elevations in cortical activation may reflect undue attention drawn to the auditory domain, an interpretation consistent with the lack of tinnitus-related effects subcortically where activation is less potently modulated by attentional state. The data strengthen, at a mechanistic level, analogies drawn previously between tinnitus/hyperacusis and other, nonauditory disordered perceptions thought to arise from neural hyperactivity such as chronic neuropathic pain and photophobia.

Evaluating an acoustically quiet EPI sequence for use in fMRI studies of speech and auditory processing

Echoplanar MRI is associated with significant acoustic noise, which can interfere with the presentation of auditory stimuli, create a more challenging listening environment, and increase discomfort felt by participants. Here we investigate a scanning sequence that significantly reduces the amplitude of acoustic noise associated with echoplanar imaging (EPI). This is accomplished using a constant phase encoding gradient and a sinusoidal readout echo train to produce a narrow-band acoustic frequency spectrum, which is adapted to the scanner's frequency response function by choosing an optimum gradient switching frequency. To evaluate the effect of these nonstandard parameters we conducted a speech experiment comparing four different EPI sequences: Quiet, Sparse, Standard, and Matched Standard (using the same readout duration as Quiet). For each sequence participants listened to sentences and signal-correlated noise (SCN), which provides an unintelligible amplitude-matched control condition. We used BOLD sensitivity maps to quantify sensitivity loss caused by the longer EPI readout duration used in the Quiet and Matched Standard EPI sequences. We found that the Quiet sequence provided more robust activation for SCN in primary auditory areas and comparable activation in frontal and temporal regions for Sentences>SCN, but less sentence-related activity in inferotemporal cortex. The increased listening effort associated with the louder Standard sequence relative to the Quiet sequence resulted in increased activation in the left temporal and inferior parietal cortices. Together, these results suggest that the Quiet sequence is suitable, and perhaps preferable, for many auditory studies. However, its applicability depends on the specific brain regions of interest.

The effect of slice orientation on auditory FMRI at the level of the brainstem

Although auditory information is processed in several subcortical nuclei, most fMRI studies focus solely on the auditory cortex and do not take brainstem responses into account. One common difficulty in obtaining clear functional brainstem recordings is due to heartbeat related motion, manifested in the rostro-caudal and in the ventro-dorsal directions in the contraction phase of the heart. The aim of this study was to investigate the effect of slice orientation on auditory functional magnetic resonance imagining (fMRI) measurements with respect to the pattern of brainstem oscillation. Fourteen healthy volunteers listened monaurally to modulated pink noise. Blood oxygenation level dependent (BOLD) contrast was performed with an echo-planar image (EPI) sequence using a 3T MRI system. Three different slice orientations were compared: approximately parallel, at 45 degrees , and orthogonal to the brainstem. The standard deviation of the residuals, the effect size, the median t-values, and the number of activated voxels were calculated to quantify variability in activation between orientations. The data for the inferior colliculi indicated that a slice orientation with a 45 degrees angle to the brainstem yielded the lowest sensitivity to motion (reflected in the standard deviation of the residuals). By contrast, the results did not suggest differences between the three imaging planes on the scanning of the auditory cortex. Findings indicate that the 45 degrees slice orientation is the optimum orientation for accurate measurement at the upper brainstem level.

Activity associated with stream segregation in human auditory cortex is similar for spatial and pitch cues

Streaming is a perceptual mechanism by which the brain segregates information from multiple sound sources in our environment and assigns them to distinct auditory streams. Examples for streaming cues are differences in frequency spectrum, pitch, or space, and potential neural correlates for streaming based on spectral and pitch cues have been identified in the auditory cortex. Here, magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI) were used to evaluate if response enhancement in auditory cortex associated with streaming represents a general pattern that is independent of the stimulus cue. Interaural time differences (ITDs) were used as a spatial streaming cue and were compared with streaming based on fundamental frequency (f(0)) differences. The MEG results showed enhancement of the P(1)m after 60-90 ms that was similar during streaming based on ITD and pitch. Sustained fMRI activity was enhanced at identical sites in Heschl's gyrus and planum temporale for both cues; no topographical specificity for space or pitch was found for the streaming-associated enhancement. These results support the hypothesis of an early convergence of the neural representation for auditory streams that is independent of the acoustic cue that the streaming is based on.

The effect of stimulus context on pitch representations in the human auditory cortex

Neuroimaging studies of pitch coding seek to identify pitch-related responses separate from responses to other properties of the stimulus, such as its energy onset, and other general aspects of the listening context. The current study reports the first attempt to evaluate these modulatory influences using functional magnetic resonance imaging (fMRI) measures of cortical pitch representations. Stimulus context was manipulated using a 'classical stimulation paradigm' (whereby successive pitch stimuli were separated by gaps of silence) and a 'continuous stimulation paradigm' (whereby successive pitch stimuli were interspersed with noise to maintain a stable envelope). Pitch responses were measured for two types of pitch-evoking stimuli; a harmonic-complex tone and a complex Huggins pitch. Results for a group of 15 normally hearing listeners revealed that context effects were mostly observed in primary auditory regions, while the most significant pitch responses were localized to posterior nonprimary auditory cortex, specifically planum temporale. Sensitivity to pitch was greater for the continuous stimulation conditions perhaps because they better controlled for concurrent responses to the noise energy onset and reduced the potential problem of a non-linear fMRI response becoming saturated. These results provide support for hierarchical processing within human auditory cortex, with some parts of primary auditory cortex engaged by general auditory energy, some parts of planum temporale specifically responsible for representing pitch information and adjacent regions that are responsible for complex higher-level auditory processing such as representing pitch information as a function of listening context.

BOLD responses in human auditory cortex are more closely related to transient MEG responses than to sustained ones

Blood oxygen level dependent-functional magnetic resonance imaging (BOLD-fMRI) and magnetoencephalographic (MEG) signals are both coupled to postsynaptic potentials, although their relationship is incompletely understood. Here, the wide range of BOLD-fMRI and MEG responses produced by auditory cortex was exploited to better understand the BOLD-fMRI/MEG relationship. Measurements of BOLD and MEG responses were made in the same subjects using the same stimuli for both modalities. The stimuli, 24-s sequences of click trains, had duty cycles of 2.5, 25, 72, and 100%. For the 2.5% sequence, the BOLD response was elevated throughout the sequence, whereas for 100%, it peaked after sequence onset and offset and showed a diminished elevation in between. On the finer timescale of MEG, responses at 2.5% consisted of a complex of transients, including N(1)m, to each click train of the sequence, whereas for 100% the only transients occurred at sequence onset and offset between which there was a sustained elevation in the MEG signal (a sustained field). A model that separately estimated the contributions of transient and sustained MEG signals to the BOLD response best fit BOLD measurements when the transient contribution was weighted 8- to 10-fold more than the sustained one. The findings suggest that BOLD responses in the auditory cortex are tightly coupled to the neural activity underlying transient, not sustained, MEG signals.

Characterisation of the BOLD response time course at different levels of the auditory pathway in non-human primates

Non-human-primate fMRI is becoming increasingly recognised as the missing link between the widely applied methods of human imaging and intracortical animal electrophysiology. A crucial requirement for the optimal application of this method is the precise knowledge of the time course of the Blood Oxygenation Level Dependent (BOLD) signal. We mapped the BOLD signal time course in the inferior colliculus (IC), medial geniculate body (MGB) and in tonotopically defined fields in the auditory cortex of two macaques. The results show little differences in the BOLD-signal time courses within the auditory pathway. However, we observed systematic differences in the magnitude of the change in the BOLD signal with significantly stronger signal changes in field A1 of the auditory cortex compared to field R. The measured time course of the signal was in good agreement with similar studies in human auditory cortex but showed considerable differences to data reported from macaque visual cortex. Consistent with the studies in humans we measured a peak in the BOLD response around 4 s after the onset of 2-s broadband noise stimuli while previous studies recorded from the primary visual cortex of the same species reported the earliest peaks to short visual stimuli several seconds later. The comparison of our results with previous studies does not support differences in haemodynamic responses within the auditory system between human and non-human primates. Furthermore, the data will aid optimal design of future auditory fMRI studies in non-human primates.

The auditory midbrain of people with tinnitus: abnormal sound-evoked activity revisited

Sound-evoked fMRI activation of the inferior colliculi (IC) was compared between tinnitus and non-tinnitus subjects matched in threshold (normal), age, depression, and anxiety. Subjects were stimulated with broadband sound in an "on/off" fMRI paradigm with and without on-going sound from the scanner coolant pump. (1) With pump sounds off, the tinnitus group showed greater stimulus-evoked activation of the IC than the non-tinnitus group, suggesting abnormal gain within the auditory pathway of tinnitus subjects. (2) Having pump sounds on reduced activation in the tinnitus, but not the non-tinnitus group. This result suggests response saturation in tinnitus subjects, possibly occurring because abnormal gain increased response amplitude to an upper limit. (3) In contrast to Melcher et al. (2000), the ratio of activation between right and left IC did not differ significantly between tinnitus and non-tinnitus subjects or in a manner dependent on tinnitus laterality. However, new data from subjects imaged previously by Melcher et al. suggest a possible tinnitus subgroup with abnormally asymmetric function of the IC. The present and previous data together suggest elevated responses to sound in the IC are common among those with tinnitus and normal thresholds, while abnormally asymmetric activation is not, even among those with lateralized tinnitus.

Brain responses to auditory and visual stimulus offset: shared representations of temporal edges

Edges are crucial for the formation of coherent objects from sequential sensory inputs within a single modality. Moreover, temporally coincident boundaries of perceptual objects across different sensory modalities facilitate crossmodal integration. Here, we used functional magnetic resonance imaging in order to examine the neural basis of temporal edge detection across modalities. Onsets of sensory inputs are not only related to the detection of an edge but also to the processing of novel sensory inputs. Thus, we used transitions from input to rest (offsets) as convenient stimuli for studying the neural underpinnings of visual and acoustic edge detection per se. We found, besides modality-specific patterns, shared visual and auditory offset-related activity in the superior temporal sulcus and insula of the right hemisphere. Our data suggest that right hemispheric regions known to be involved in multisensory processing are crucial for detection of edges in the temporal domain across both visual and auditory modalities. This operation is likely to facilitate cross-modal object feature binding based on temporal coincidence.

Simultaneous electroencephalography and functional magnetic resonance imaging of general anesthesia

It has been long appreciated that anesthetic drugs induce stereotyped changes in electroencephalogram (EEG), but the relationships between the EEG and underlying brain function remain poorly understood. Functional imaging methods including positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), have become important tools for studying how anesthetic drugs act in the human brain to induce the state of general anesthesia. To date, no investigation has combined functional MRI with EEG to study general anesthesia. We report here a paradigm for conducting combined fMRI and EEG studies of human subjects under general anesthesia. We discuss the several technical and safety problems that must be solved to undertake this type of multimodal functional imaging and show combined recordings from a human subject. Combined fMRI and EEG exploits simultaneously the high spatial resolution of fMRI and the high temporal resolution of EEG. In addition, combined fMRI and EEG offers a direct way to relate established EEG patterns induced by general anesthesia to changes in neural activity in specific brain regions as measured by changes in fMRI blood oxygen level dependent (BOLD) signals.

Functional maps of human auditory cortex: effects of acoustic features and attention

Background

While human auditory cortex is known to contain tonotopically organized auditory cortical fields (ACFs), little is known about how processing in these fields is modulated by other acoustic features or by attention.

Methodology/Principal Findings

We used functional magnetic resonance imaging (fMRI) and population-based cortical surface analysis to characterize the tonotopic organization of human auditory cortex and analyze the influence of tone intensity, ear of delivery, scanner background noise, and intermodal selective attention on auditory cortex activations. Medial auditory cortex surrounding Heschl's gyrus showed large sensory (unattended) activations with two mirror-symmetric tonotopic fields similar to those observed in non-human primates. Sensory responses in medial regions had symmetrical distributions with respect to the left and right hemispheres, were enlarged for tones of increased intensity, and were enhanced when sparse image acquisition reduced scanner acoustic noise. Spatial distribution analysis suggested that changes in tone intensity shifted activation within isofrequency bands. Activations to monaural tones were enhanced over the hemisphere contralateral to stimulation, where they produced activations similar to those produced by binaural sounds. Lateral regions of auditory cortex showed small sensory responses that were larger in the right than left hemisphere, lacked tonotopic organization, and were uninfluenced by acoustic parameters. Sensory responses in both medial and lateral auditory cortex decreased in magnitude throughout stimulus blocks. Attention-related modulations (ARMs) were larger in lateral than medial regions of auditory cortex and appeared to arise primarily in belt and parabelt auditory fields. ARMs lacked tonotopic organization, were unaffected by acoustic parameters, and had distributions that were distinct from those of sensory responses. Unlike the gradual adaptation seen for sensory responses, ARMs increased in amplitude throughout stimulus blocks.

Conclusions/Significance

The results are consistent with the view that medial regions of human auditory cortex contain tonotopically organized core and belt fields that map the basic acoustic features of sounds while surrounding higher-order parabelt regions are tuned to more abstract stimulus attributes. Intermodal selective attention enhances processing in neuronal populations that are partially distinct from those activated by unattended stimuli.

Characterizing response to elemental unit of acoustic imaging noise: an FMRI study

Acoustic imaging noise produced during functional magnetic resonance imaging (fMRI) studies can hinder auditory fMRI research analysis by altering the properties of the acquired time-series data. Acoustic imaging noise can be especially confounding when estimating the time course of the hemodynamic response (HDR) in auditory event-related fMRI (fMRI) experiments. This study is motivated by the desire to establish a baseline function that can serve not only as a comparison to other quantities of acoustic imaging noise for determining how detrimental is one's experimental noise, but also as a foundation for a model that compensates for the response to acoustic imaging noise. Therefore, the amplitude and spatial extent of the HDR to the elemental unit of acoustic imaging noise (i.e., a single ping) associated with echoplanar acquisition were characterized and modeled. Results from this fMRI study at 1.5 T indicate that the group-averaged HDR in left and right auditory cortex to acoustic imaging noise (duration of 46 ms) has an estimated peak magnitude of 0.29% (right) to 0.48% (left) signal change from baseline, peaks between 3 and 5 s after stimulus presentation, and returns to baseline and remains within the noise range approximately 8 s after stimulus presentation.

Contributions of the basal ganglia and functionally related brain structures to motor learning

This review discusses the cerebral plasticity, and the role of the cortico-striatal system in particular, observed as one is learning or planning to execute a newly learned motor behavior up to when the skill is consolidated or has become highly automatized. A special emphasis is given to imaging work describing the neural substrate mediating motor sequence learning and motor adaptation paradigms. These results are then put into a plausible neurobiological model of motor skill learning, which proposes an integrated view of the brain plasticity mediating this form of memory at different stages of the acquisition process.

Auditory selective attention modulates activation of human inferior colliculus

Selective auditory attention powerfully modulates neural activity in the human auditory cortex (AC). In contrast, the role of attention in subcortical auditory processing is not well established. Here, we used functional MRI (fMRI) to examine activation of the human inferior colliculus (IC) during strictly controlled auditory attention tasks. The IC is an obligatory midbrain nucleus of the ascending auditory pathway with diverse internal and external connections. The IC also receives a massive descending projection from the AC, suggesting that cortical processes affect IC operations. In this study, 21 subjects selectively attended to left-ear or right-ear sounds and ignored sounds delivered to the other ear. IC activations depended on the direction of attention, indicating that auditory processing in the human IC is not only determined by acoustic input but also by the current behavioral goals.

Illusory Vowels Resulting from Perceptual Continuity: A Functional Magnetic Resonance Imaging Study

We used functional magnetic resonance imaging to study the neural processing of vowels whose perception depends on the continuity illusion. Participants heard sequences of two-formant vowels under a number of listening conditions. In the “vowel conditions,” both formants were always present simultaneously and the stimuli were perceived as speech-like. Contrasted with a range of nonspeech sounds, these vowels elicited activity in the posterior middle temporal gyrus (MTG) and superior temporal sulcus (STS). When the two formants alternated in time, the “speech-likeness” of the sounds was reduced. It could be partially restored by filling the silent gaps in each formant with bands of noise (the “Illusion” condition) because the noise induced an illusion of continuity in each formant region, causing the two formants to be perceived as simultaneous. However, this manipulation was only effective at low formant-to-noise ratios (FNRs). When the FNR was increased, the illusion broke down (the “illusion-break” condition). Activation in vowel-sensitive regions of the MTG was greater in the illusion than in the illusion-break condition, consistent with the perception of Illusion stimuli as vowels. Activity in Heschl's gyri (HG), the approximate location of the primary auditory cortex, showed the opposite pattern, and may depend instead on the number of perceptual onsets in a sound. Our results demonstrate that speech-sensitive regions of the MTG are sensitive not to the physical characteristics of the stimulus but to the perception of the stimulus as speech, and also provide an anatomically distinct, objective physiological correlate of the continuity illusion in human listeners.

Motor sequence learning and movement disorders

PURPOSE OF REVIEW

New insights into the psychophysiological determinants of performance changes and brain plasticity associated with motor sequence learning have recently been gained through behavioral and imaging studies in healthy individuals. In addition, using a variety of motor sequential paradigms in groups of patients affected by a movement disorder, major advances have been achieved in our understanding of the pathophysiological mechanisms underlying Parkinson's and Huntington's diseases, as well as primary forms of dystonia.

RECENT FINDINGS

This review begins by describing the latest findings in normal participants with regards to the dynamic alterations in neural networks observed across the different phases of motor sequence learning. It then focuses on the hotly debated issue of motor memory consolidation, highlighting the results of novel studies that investigated the role of both day and night sleep, the neural substrates and the developmental evolution mediating this process. Finally, this paper addresses current work looking at motor sequence learning in movement disorders that helps to better comprehend the functional contribution of basal ganglia structures to this type of memory, to assess the impact of such diseases on related patterns of brain activation, as well as to identify the neuronal compensatory mechanisms educed by these basal ganglia disorders.

SUMMARY

Such advances have major implications, not only for optimizing ways to learn new skilled behaviors in real-life situations, but also for guiding therapeutic approaches in patients with movement disorders.

Auditory stimulus repetition effects on cortical hemoglobin oxygenation: a near-infrared spectroscopy investigation

The cortical response to repeated sensory stimuli plateaus (or declines) as repetition frequencies increase beyond 2-8 Hz. This study examined the underlying changes in cortical oxygenated and deoxygenated hemoglobin associated with this phenomenon using near-infrared spectroscopy. The optical signal was measured from 11 healthy volunteers listening to noise-burst trains presented at 2, 10, and 35 Hz. In a bilateral region consistent with the posterior superior temporal gyrus there was an inverse relationship between deoxyhemoglobin concentration change and stimulus frequency: greatest at 2 Hz, intermediate at 10 Hz, and smallest at 35 Hz. These findings provide preliminary support for a relationship between the perceptual characteristics of auditory stimuli and modulation of cortical oxygenation as measured via an emerging neuromonitoring technique.

Human cortical activity during streaming without spectral cues suggests a general neural substrate for auditory stream segregation

The brain continuously disentangles competing sounds, such as two people speaking, and assigns them to distinct streams. Neural mechanisms have been proposed for streaming based on gross spectral differences between sounds, but not for streaming based on other nonspectral features. Here, human listeners were presented with sequences of harmonic complex tones that had identical spectral envelopes, and unresolved spectral fine structure, but one of two fundamental frequencies (f0) and pitches. As the f0 difference between tones increased, listeners perceived the tones as being segregated into two streams (one stream for each f0) and cortical activity measured with functional magnetic resonance imaging and magnetoencephalography increased. This trend was seen in primary cortex of Heschl's gyrus and in surrounding nonprimary areas. The results strongly resemble those for pure tones. Both the present and pure tone results may reflect neuronal forward suppression that diminishes as one or more features of successive sounds become increasingly different. We hypothesize that feature-specific forward suppression subserves streaming based on diverse perceptual cues and results in explicit neural representations for auditory streams within auditory cortex.

Concurrent Encoding of Frequency and Amplitude Modulation in Human Auditory Cortex: Encoding Transition

Complex natural sounds (e.g., animal vocalizations or speech) can be characterized by specific spectrotemporal patterns the components of which change in both frequency (FM) and amplitude (AM). The neural coding of AM and FM has been widely studied in humans and animals but typically with either pure AM or pure FM stimuli. The neural mechanisms employed to perceptually unify AM and FM acoustic features remain unclear. Using stimuli with simultaneous sinusoidal AM (at rate fAM = 37 Hz) and FM (with varying rates ƒFM), magnetoencephalography (MEG) is used to investigate the elicited auditory steady-state response (aSSR) at relevant frequencies (ƒAM, ƒFM, ƒAM + fFM). Previous work demonstrated that for sounds with slower FM dynamics ( fFM < 5 Hz), the phase of the aSSR at ƒAM tracked the FM; in other words, AM and FM features were co-tracked and co-represented by “phase modulation” encoding. This study explores the neural coding mechanism for stimuli with faster FM dynamics (≤30 Hz), demonstrating that at faster rates ( fFM > 5 Hz), there is a transition from pure phase modulation encoding to a single-upper-sideband (SSB) response (at frequency fAM + fFM) pattern. We propose that this unexpected SSB response can be explained by the additional involvement of subsidiary AM encoding responses simultaneously to, and in quadrature with, the ongoing phase modulation. These results, using MEG to reveal a possible neural encoding of specific acoustic properties, demonstrate more generally that physiological tests of encoding hypotheses can be performed noninvasively on human subjects, complementing invasive, single-unit recordings in animals.

Attention modulates sound processing in human auditory cortex but not the inferior colliculus

Auditory attention powerfully influences perception and modulates sound processing in auditory cortex, but the extent of attentional modulation in the subcortical auditory pathway remains poorly understood. We examined the effects of intermodal attention using functional magnetic resonance imaging of the inferior colliculus and auditory cortex in a demanding intermodal selective attention task using a silent imaging paradigm designed to optimize inferior colliculus activations. Both the inferior colliculus and auditory cortex showed strong activations to sound, but attentional modulations were restricted to auditory cortex.

BOLD correlates of edge detection in human auditory cortex

Edges are important cues defining coherent auditory objects. As a model of auditory edges, sound on- and offset are particularly suitable to study their neural underpinnings because they contrast a specific physical input against no physical input. Change from silence to sound, that is onset, has extensively been studied and elicits transient neural responses bilaterally in auditory cortex. However, neural activity associated with sound onset is not only related to edge detection but also to novel afferent inputs. Edges at the change from sound to silence, that is offset, are not confounded by novel physical input and thus allow to examine neural activity associated with sound edges per se. In the first experiment, we used silent acquisition functional magnetic resonance imaging and found that the offset of pulsed sound activates planum temporale, superior temporal sulcus and planum polare of the right hemisphere. In the planum temporale and the superior temporal sulcus, offset response amplitudes were related to the pulse repetition rate of the preceding stimulation. In the second experiment, we found that these offset-responsive regions were also activated by single sound pulses, onset of sound pulse sequences and single sound pulse omissions within sound pulse sequences. However, they were not active during sustained sound presentation. Thus, our data show that circumscribed areas in right temporal cortex are specifically involved in identifying auditory edges. This operation is crucial for translating acoustic signal time series into coherent auditory objects.

The role of auditory cortex in the formation of auditory streams

Auditory streaming refers to the perceptual parsing of acoustic sequences into "streams", which makes it possible for a listener to follow the sounds from a given source amidst other sounds. Streaming is currently regarded as an important function of the auditory system in both humans and animals, crucial for survival in environments that typically contain multiple sound sources. This article reviews recent findings concerning the possible neural mechanisms behind this perceptual phenomenon at the level of the auditory cortex. The first part is devoted to intra-cortical recordings, which provide insight into the neural "micromechanisms" of auditory streaming in the primary auditory cortex (A1). In the second part, recent results obtained using functional magnetic resonance imaging (fMRI) and magnetoencephalography (MEG) in humans, which suggest a contribution from cortical areas other than A1, are presented. Overall, the findings concur to demonstrate that many important features of sequential streaming can be explained relatively simply based on neural responses in the auditory cortex.

Cortical FMRI activation to sequences of tones alternating in frequency: relationship to perceived rate and streaming

Human listeners were functionally imaged while reporting their perception of sequences of alternating-frequency tone bursts separated by 0, 1/8, 1, or 20 semitones. Our goal was to determine whether functional magnetic resonance imaging (fMRI) activation of auditory cortex changes with frequency separation in a manner predictable from the perceived rate of the stimulus. At the null and small separations, the tones were generally heard as a single stream with a perceived rate equal to the physical tone presentation rate. fMRI activation in auditory cortex was appreciably phasic, showing prominent peaks at the sequence onset and offset. At larger-frequency separations, the higher- and lower-frequency tones perceptually separated into two streams, each with a rate equal to half the overall tone presentation rate. Under those conditions, fMRI activation in auditory cortex was more sustained throughout the sequence duration and was larger in magnitude and extent. Phasic to sustained changes in fMRI activation with changes in frequency separation and perceived rate are comparable to, and consistent with, those produced by changes in the physical rate of a sequence and are far greater than the effects produced by changing other physical stimulus variables, such as sound level or bandwidth. We suggest that the neural activity underlying the changes in fMRI activation with frequency separation contribute to the coding of the co-occurring changes in perceived rate and perceptual organization of the sound sequences into auditory streams.

Psychosocial stress attenuates general sound processing and duration change detection

An EEG-compatible adaptation of the Trier Social Stress Test was developed to induce psychosocial stress in healthy subjects while investigating their auditory processing of unattended sounds and salivary levels of the stress hormone cortisol. The mismatch negativity (MMN) and N1/P2 were assessed using a multifeature paradigm, while subjects were attending to visual tasks with high or low attentional workload. Only the responses to duration change were affected by the stress manipulation. Cortisol levels during stress were inversely related to the MMN amplitudes of duration deviants. During anticipatory stress, responses to the standard tones (general sound processing) increased, but their amplitude was not correlated with cortisol levels. We found that psychosocial stressor anticipation attenuates both general and deviance-specific sound processing, suggesting that cortisol interferes with cortical memory-trace formation.

Complex spatio-temporal dynamics of fMRI BOLD: A study of motor learning

Many studies have investigated the temporal properties of BOLD signal responses to task performance in regions of interest, often noting significant departures from the conventionally modelled response shape, and significant variation between regions. However, these investigations are rarely extended across the whole brain nor incorporated into the routine analysis of fMRI studies. As a result, little is known about the range of response shapes generated in the brain by common paradigms. The present study finds such temporal dynamics can be complex. We made a detailed investigation of BOLD signal responses across the whole brain during a two minute motor-sequence task, and tracked changes due to learning. The multi-component OSORU (Onset, Sustained, Offset, Ramp, Undershoot) linear model, developed by Harms and Melcher (J.Neurophysiology, 2003), was extended to characterise responses. In many regions, signal transients persisted for over thirty seconds, with large signal spikes at onset often followed by a dip in signal below the final sustained level of activation. Training altered certain features of the response shape, suggesting that different features of the response may reflect different aspects of neuro-vascular dynamics. Unmodelled, this may give rise to inconsistent results across paradigms of varying task durations. Few of the observed effects have been thoroughly addressed in physiological models of the BOLD response. The complex, extended dynamics generated by this simple, often employed task, suggests characterisation and modelling of temporal aspects of BOLD responses needs to be carried out routinely, informing experimental design and analysis, and physiological modelling.

Architectonic analysis of the auditory-related areas of the superior temporal region in human brain

Architecture of auditory areas of the superior temporal region (STR) in the human was analyzed in Nissl-stained material to see whether auditory cortex is organized according to principles that have been described in the rhesus monkey. Based on shared architectonic features, the auditory cortex in human and monkey is organized into three lines: areas in the cortex of the circular sulcus (root), areas on the supratemporal plane (core), and areas on the superior temporal gyrus (belt). The cytoarchitecture of the auditory area changes in a stepwise manner toward the koniocortical area, both from the direction of the temporal polar proisocortex as well as from the caudal temporal cortex. This architectonic dichotomy is consistent with differences in cortical and subcortical connections of STR and may be related to different functions of the rostral and caudal temporal cortices. There are some differences between rhesus monkey and human auditory anatomy. For instance, the koniocortex, root area PaI, and belt area PaA show further differentiation into subareas in the human brain. The relative volume of the core area is larger than that of the belt area in the human, but the reverse is true in the monkey. The functional significance of these differences across species is not known but may relate to speech and language functions.

Dissociated lateralization of transient and sustained blood oxygen level-dependent signal components in human primary auditory cortex

Among other auditory operations, the analysis of different sound levels received at both ears is fundamental for the localization of a sound source. These so-called interaural level differences, in animals, are coded by excitatory-inhibitory neurons yielding asymmetric hemispheric activity patterns with acoustic stimuli having maximal interaural level differences. In human auditory cortex, the temporal blood oxygen level-dependent (BOLD) response to auditory inputs, as measured by functional magnetic resonance imaging (fMRI), consists of at least two independent components: an initial transient and a subsequent sustained signal, which, on a different time scale, are consistent with electrophysiological human and animal response patterns. However, their specific functional role remains unclear. Animal studies suggest these temporal components being based on different neural networks and having specific roles in representing the external acoustic environment. Here we hypothesized that the transient and sustained response constituents are differentially involved in coding interaural level differences and therefore play different roles in spatial information processing. Healthy subjects underwent monaural and binaural acoustic stimulation and BOLD responses were measured using high signal-to-noise-ratio fMRI. In the anatomically segmented Heschl's gyrus the transient response was bilaterally balanced, independent of the side of stimulation, while in opposite the sustained response was contralateralized. This dissociation suggests a differential role at these two independent temporal response components, with an initial bilateral transient signal subserving rapid sound detection and a subsequent lateralized sustained signal subserving detailed sound characterization.

Periodicity and frequency coding in human auditory cortex

Understanding the neural coding of pitch and frequency is fundamental to the understanding of speech comprehension, music perception and the segregation of concurrent sound sources. Neuroimaging has made important contributions to defining the pattern of frequency sensitivity in humans. However, the precise way in which pitch sensitivity relates to these frequency-dependent regions remains unclear. Single-frequency tones also cannot be used to test this hypothesis as their pitch always equals their frequency. Here, temporal pitch (periodicity) and frequency coding were dissociated using stimuli that were bandpassed in different frequency spectra (centre frequencies 800 and 4500 Hz), yet were matched in their pitch characteristics. Cortical responses to both pitch-evoking stimuli typically occurred within a region that was also responsive to low frequencies. Its location extended across both primary and nonprimary auditory cortex. An additional control experiment demonstrated that this pitch-related effect was not simply caused by the generation of combination tones. Our findings support recent neurophysiological evidence for a cortical representation of pitch at the lateral border of the primary auditory cortex, while revealing new evidence that additional auditory fields are also likely to play a role in pitch coding.

Cortical response to auditory motion suggests an asymmetry in the reliance on inter-hemispheric connections between the left and right auditory cortices

The aim of the current study was to measure the brain's response to auditory motion using electroencephalography (EEG) to gain insight into the mechanisms by which hemispheric lateralization for auditory spatial processing is established in the human brain. The onset of left- or rightward motion in an otherwise continuous sound was found to elicit a large response, which appeared to arise from higher-level nonprimary auditory areas. This motion onset response was strongly lateralized to the hemisphere contralateral to the direction of motion. The response latencies suggest that the ipsilateral response to the leftward motion was produced by indirect callosal projections from the opposite hemisphere, whereas the ipsilateral response to the rightward motion seemed to receive contributions from direct thalamocortical projections. These results suggest an asymmetry in the reliance on inter-hemispheric projections between the left and right auditory cortices for auditory spatial processing.