Statistical methods in functional magnetic resonance imaging with respect to nonstationary time-series: auditory cortex activity

The Profiles of Non-stationarity and Non-linearity in the Time Series of Resting-State Brain Networks

The linearity and stationarity of fMRI time series need to be understood due to their important roles in the choice of approach for brain network analysis. In this paper, we investigated the stationarity and linearity of resting-state fMRI (rs-fMRI) time-series data from the Midnight Scan Club datasets. The degree of stationarity (DS) and the degree of non-linearity (DN) were, respectively, estimated for the time series of all gray matter voxels. The similarity and difference between the DS and DN were assessed in terms of voxels and intrinsic brain networks, including the visual network, somatomotor network, dorsal attention network, ventral attention network, limbic network, frontoparietal network, and default-mode network. The test-retest scans were utilized to quantify the reliability of DS and DN. We found that DS and DN maps had overlapping spatial distribution. Meanwhile, the probability density estimate function of DS had a long tail, and that of DN had a more normal distribution. Specifically, stronger DS was present in the somatomotor, limbic, and ventral attention networks compared to other networks, and stronger DN was found in the somatomotor, visual, limbic, ventral attention, and default-mode networks. The percentage of overlapping voxels between DS and DN in different networks demonstrated a decreasing trend in the order default mode, ventral attention, somatomotor, frontoparietal, dorsal attention, visual, and limbic. Furthermore, the ICC values of DS were higher than those of DN. Our results suggest that different functional networks have distinct properties of non-stationarity and non-linearity owing to the complexity of rs-fMRI time series. Thus, caution should be taken when analyzing fMRI data (both resting-state and task-activation) using simplified models.

Cortical hot spots and labyrinths: why cortical neuromodulation for episodic migraine with aura should be personalized

Stimulation protocols for medical devices should be rationally designed. For episodic migraine with aura we outline model-based design strategies toward preventive and acute therapies using stereotactic cortical neuromodulation. To this end, we regard a localized spreading depression (SD) wave segment as a central element in migraine pathophysiology. To describe nucleation and propagation features of the SD wave segment, we define the new concepts of cortical hot spots and labyrinths, respectively. In particular, we firstly focus exclusively on curvature-induced dynamical properties by studying a generic reaction-diffusion model of SD on the folded cortical surface. This surface is described with increasing level of details, including finally personalized simulations using patient's magnetic resonance imaging (MRI) scanner readings. At this stage, the only relevant factor that can modulate nucleation and propagation paths is the Gaussian curvature, which has the advantage of being rather readily accessible by MRI. We conclude with discussing further anatomical factors, such as areal, laminar, and cellular heterogeneity, that in addition to and in relation to Gaussian curvature determine the generalized concept of cortical hot spots and labyrinths as target structures for neuromodulation. Our numerical simulations suggest that these target structures are like fingerprints, they are individual features of each migraine sufferer. The goal in the future will be to provide individualized neural tissue simulations. These simulations should predict the clinical data and therefore can also serve as a test bed for exploring stereotactic cortical neuromodulation.

Predicted selective increase of cortical magnification due to cortical folding

The cortical magnification matrix M is introduced founded on a notion similar to that of the scalar cortical magnification factor M. Unlike M, this matrix is suitable to describe anisotropy in cortical magnification, which is of particular interest in the highly gyrified human cerebral cortex. The advantage of our tensor method over other surface-based 3D methods to explore cortical morphometry is that M expresses cortical quantities in the corresponding sensory space. It allows us to investigate the spatial relation between sensory function and anatomical structure. To this end, we consider the calcarine sulcus (CS) as an anatomical landmark for the primary visual cortex (V1). We found that a stereotypically formed 3D model of V1 compared to a flat model explains an excess of cortical tissue for the representation of visual information coming from the horizon of the visual field. This suggests that the intrinsic geometry of this sulcus is adapted to encephalize a particular function along the horizon. Since visual functions are assumed to be M-scaled, cortical folding can serve as an anatomical basis for increased functionality on the horizon similar to a retinal specialization known as visual streak, which is found in animals with lower encephalization. Thus, the gain of surface area by cortical folding links anatomical structure to cortical function in a previously unrecognized way, which may guide sulci development.

Dynamic Granger causality based on Kalman filter for evaluation of functional network connectivity in fMRI data

Increasing interest in understanding dynamic interactions of brain neural networks leads to formulation of sophisticated connectivity analysis methods. Recent studies have applied Granger causality based on standard multivariate autoregressive (MAR) modeling to assess the brain connectivity. Nevertheless, one important flaw of this commonly proposed method is that it requires the analyzed time series to be stationary, whereas such assumption is mostly violated due to the weakly nonstationary nature of functional magnetic resonance imaging (fMRI) time series. Therefore, we propose an approach to dynamic Granger causality in the frequency domain for evaluating functional network connectivity in fMRI data. The effectiveness and robustness of the dynamic approach was significantly improved by combining a forward and backward Kalman filter that improved estimates compared to the standard time-invariant MAR modeling. In our method, the functional networks were first detected by independent component analysis (ICA), a computational method for separating a multivariate signal into maximally independent components. Then the measure of Granger causality was evaluated using generalized partial directed coherence that is suitable for bivariate as well as multivariate data. Moreover, this metric provides identification of causal relation in frequency domain, which allows one to distinguish the frequency components related to the experimental paradigm. The procedure of evaluating Granger causality via dynamic MAR was demonstrated on simulated time series as well as on two sets of group fMRI data collected during an auditory sensorimotor (SM) or auditory oddball discrimination (AOD) tasks. Finally, a comparison with the results obtained from a standard time-invariant MAR model was provided.

Extended time-frequency Granger causality for evaluation of functional network connectivity in event-related FMRI data

In this article, we show that adaptive multivariate autoregressive (AMVAR) modeling accompanied by proper estimation of the delay and the width of hemodynamic response function is an effective technique for evaluation of spectral Granger causality among different functional brain networks identified by independent component analysis from event-related fMRI data. The entire concept is demonstrated on 28 subjects auditory oddball fMRI data.

Lorazepam modulates orbitofrontal signal changes during emotional processing in catatonia

OBJECTIVE

Catatonia is a psychomotor syndrome characterized by concomitant emotional, behavioural and motor symptoms. In many cases clinical symptoms disappear almost immediately with administration of lorazepam, which acts on GABA(A) receptors.

METHODS

Using functional magnetic resonance imaging (fMRI) we investigated prefrontal activation patterns during emotion processing in catatonic patients with and without lorazepam in a double-blind study design. For emotional stimulation the International Affective Picture System (IAPS) was used. BOLD-signals were determined using regions of interest (ROI) and were statistically compared between groups.

RESULTS

For negative emotional pictures lorazepam induced higher signal decreases in the orbitofrontal cortex (OFC) in catatonic patients than in healthy subjects resulting in a regularization of activity patterns comparable to healthy subjects with placebo.

CONCLUSIONS

Results indicate disturbances in the functioning of OFC in catatonia. GABAergic modified emotion regulation with decreased inhibition of affective stimuli could lead to the intense emotions reported by many catatonic patients.

DISSECTING NONVERBAL AUDITORY CORTEX ASYMMETRY: AN fMRI STUDY

The purpose of this study was to find a robust nonverbal paradigm to obtain reliable, reproducible auditory activation and characterize the nonverbal activation of the auditory cortex in regard to the Brodmann regions. The extent of localization and lateralization of activation was investigated utilizing functional magnetic resonance (fMR). Two tasks were used: monotonous repetitive stimuli of "double octaves" (DO) consisting of alternating four A with four C piano notes and a variated string of "sequential notes" (SN), which was a fast nonrepetitive sequence of piano notes. Eleven volunteers were investigated. The activation periods had a duration of 30 s, and presented every 30 s. All subjects demonstrated fMRI signal activity in the superior temporal gyrus (STG) involving the primary and secondary auditory cortex except one subject who showed no activation with the DO stimulus. SN elicited more activation than DO (p =<.03). The bulk activation for SN and DO was slightly greater in the right hemisphere, although the primary auditory area (Brodmann's 41) was better activated on the left p =<.001. Brodmann's area 22 was most frequently right-side dominant (p =.015, p =.017 for DO and SN, respectively). These findings appear to have implications in the examination of preverbal subjects.

Migraine aura: retracting particle-like waves in weakly susceptible cortex

Cortical spreading depression (SD) has been suggested to underlie migraine aura. Despite a precise match in speed, the spatio-temporal patterns of SD observed in animal cortex and aura symptoms mapped to the cortical surface ordinarily differ in aspects of size and shape. We show that this mismatch is reconciled by utilizing that both pattern types bifurcate from an instability point of generic reaction-diffusion models. To classify these spatio-temporal pattern we suggest a susceptibility scale having the value sigma = 1 at the instability point. We predict that human cortex is only weakly susceptible to SD (sigma<1), and support this prediction by directly matching visual aura symptoms with anatomical landmarks using fMRI retinotopic mapping. Moreover, we use retinal SD to give a proof of concept of the existence of this instability point and describe how cortical susceptibility to SD must be adjusted for migraine drug testing. Close to the instability point at sigma = 1 the dynamical repertoire of cortical tissue is increased. As a consequence, the picture of an engulfing SD that became paradigmatic for migraine with aura needs to be modified in most cases towards a more spatially confined pattern that remains within the originating major gyrus or sulcus. Furthermore, we discuss the resulting implications on migraine pharmacology that is hitherto tested in the regime (sigma>1), and potentially silent aura occurring below a second bifurcation point at sigma = 0 on the susceptible scale.

FMRI activations of amygdala, cingulate cortex, and auditory cortex by infant laughing and crying

One of the functions of emotional vocalizations is the regulation of social relationships like those between adults and children. Listening to infant vocalizations is known to engage amygdala as well as anterior and posterior cingulate cortices. But, the functional relationships between these structures still need further clarification. Here, nonparental women and men listened to laughing and crying of preverbal infants and to vocalization-derived control stimuli, while performing a pure tone detection task during low-noise functional magnetic resonance imaging. Infant vocalizations elicited stronger activation in amygdala and anterior cingulate cortex (ACC) of women, whereas the alienated control stimuli elicited stronger activation in men. Independent of listeners' gender, auditory cortex (AC) and posterior cingulate cortex (PCC) were more strongly activated by the control stimuli than by infant laughing or crying. The gender-dependent correlates of neural activity in amygdala and ACC may reflect neural predispositions in women for responses to preverbal infant vocalizations, whereas the gender-independent similarity of activation patterns in PCC and AC may reflect more sensory-based and cognitive levels of neural processing. In comparison to our previous work on adult laughing and crying, the infant vocalizations elicited manifold higher amygdala activation.

Left auditory cortex and amygdala, but right insula dominance for human laughing and crying

Evidence suggests that in animals their own species-specific communication sounds are processed predominantly in the left hemisphere. In contrast, processing linguistic aspects of human speech involves the left hemisphere, whereas processing some prosodic aspects of speech as well as other not yet well-defined attributes of human voices predominantly involves the right hemisphere. This leaves open the question of hemispheric processing of universal (species-specific) human vocalizations that are more directly comparable to animal vocalizations. The present functional magnetic resonance imaging study addresses this question. Twenty subjects listened to human laughing and crying presented either in an original or time-reversed version while performing a pitch-shift detection task to control attention. Time-reversed presentation of these sounds is a suitable auditory control because it does not change the overall spectral content. The auditory cortex, amygdala, and insula in the left hemisphere were more strongly activated by original than by time-reversed laughing and crying. Thus, similar to speech, these nonspeech vocalizations involve predominantly left-hemisphere auditory processing. Functional data suggest that this lateralization effect is more likely based on acoustical similarities between speech and laughing or crying than on similarities with respect to communicative functions. Both the original and time-reversed laughing and crying activated more strongly the right insula, which may be compatible with its assumed function in emotional self-awareness.

Sustained blood oxygenation and volume response to repetition rate-modulated sound in human auditory cortex

The blood oxygen level-dependent (BOLD) signal time course in the auditory cortex is characterized by two components, an initial transient peak and a subsequent sustained plateau with smaller amplitude. Because the T(2)(*) signal detected by functional magnetic resonance imaging (fMRI) depends on at least two counteracting factors, blood oxygenation and volume, we examined whether the reduction in the sustained BOLD signal results from decreased levels of oxygenation or from increased levels of blood volume. We used conventional fMRI to quantify the BOLD signal and fMRI in combination with superparamagnetic contrast agent to quantify blood volume and employed repetition rate-modulated sounds in a silent background to manipulate the response amplitude in the auditory cortex. In the BOLD signal, the initial peak reached 3.3% with pulsed sound and 1.9% with continuous sound, whereas the sustained BOLD signal fell to 2.2% with pulsed sound and to 0.5% with continuous sound, respectively. The repetition rate-dependent reduction in the sustained BOLD amplitude was accompanied by concordant changes in sustained blood volume levels, which, compared to silence, increased by approximately 30% with pulsed and by approximately 10% with continuous sound. Thus, our data suggest that the reduced amplitude of the sustained BOLD signal reflects stimulus-dependent modulation of blood oxygenation rather than blood volume-related effects.

Left-lateralized fMRI activation in the temporal lobe of high repressive women during the identification of sad prosodies

We investigated with fMRI whether different lateralization types of cortical activation in prosodic tasks are caused by individually different stress-related coping strategies. After healthy women had been classified as high or low repressive they performed four different identification tasks with acoustically presented speech material while being in the MR scanner. The two materials presented in blocks were emotionally irrelevant CV syllables and adjectives with a mix of different prosodic intonations. Sad and happy intonations had to be targeted by two affective identification tasks in the same adjective mixtures. For testing stimulus-material effects the phoneme /a/ had to be identified both in the syllables and the adjectives. This design allowed us to test influences of coping strategies and affective tasks on cortical activation in both hemispheres. Results showed no differences in global cortical lateralization as a function of high or low repressiveness and no global support for either the valence hypothesis or the right-hemisphere hypothesis of emotional processing. However, we observed differences in auditory and speech cortex. In accordance to the construct of repression/sensitization, high repressive women showed larger left, low repressive women larger right hemisphere activation during the identification of sad intonations. Thus, differences in stress-related coping strategies may not lead to general differences in cortical lateralization, but may depend on specific elicitors and task-relevant brain areas. In contrast, the identification of happy intonations led to strong and right-lateralized global cortical activation independent of coping strategies which complies with the right-hemisphere hypothesis of emotional processing. In addition, this may reflect general cognitive and arousal effects of task difficulty as well as auditory cue-specific attentional effects.

Audition of laughing and crying leads to right amygdala activation in a low-noise fMRI setting

Adequate behavioral responses to socially relevant stimuli are often impaired after lesions of the amygdala. These impaired behavioral responses in particular concern the recognition of facial, and sometimes vocal, expressions of fear. Using low-noise functional magnetic resonance imaging (fMRI) in combination with controlled sound delivery, we investigated how the amygdala, insula and auditory cortex are involved in the processing of affective non-verbal vocalizations (laughing, crying) in healthy humans. The same samples of male and female laughing and crying were presented in two different experimental conditions: self-induction of the corresponding emotions while listening, and detection of artificial pitch shifts in the same stimuli. Both conditions led to bilateral activation of the amygdala, insula and auditory cortex with a right-hemisphere advantage in the amygdala, and larger activation during laughing than crying in the auditory cortex with a slight right-hemisphere advantage for laughing, both likely due to acoustic stimulus features. The results show that amygdala activation by emotionally meaningful sounds like laughing and crying is independent of the emotional involvement, suggesting the pattern recognition aspect of these sounds is crucial for this activation. This aspect was revealed by a low-noise fMRI protocol which presumably minimized confounding effects of stressful high-noise fMRI.

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

Sound repetition rate plays an important role in stream segregation, temporal pattern recognition, and the perception of successive sounds as either distinct or fused. This study was aimed at elucidating the neural coding of repetition rate and its perceptual correlates. We investigated the representations of rate in the auditory pathway of human listeners using functional magnetic resonance imaging (fMRI), an indicator of population neural activity. Stimuli were trains of noise bursts presented at rates ranging from low (1-2/s; each burst is perceptually distinct) to high (35/s; individual bursts are not distinguishable). There was a systematic change in the form of fMRI response rate-dependencies from midbrain to thalamus to cortex. In the inferior colliculus, response amplitude increased with increasing rate while response waveshape remained unchanged and sustained. In the medial geniculate body, increasing rate produced an increase in amplitude and a moderate change in waveshape at higher rates (from sustained to one showing a moderate peak just after train onset). In auditory cortex (Heschl's gyrus and the superior temporal gyrus), amplitude changed somewhat with rate, but a far more striking change occurred in response waveshape-low rates elicited a sustained response, whereas high rates elicited an unusual phasic response that included prominent peaks just after train onset and offset. The shift in cortical response waveshape from sustained to phasic with increasing rate corresponds to a perceptual shift from individually resolved bursts to fused bursts forming a continuous (but modulated) percept. Thus at high rates, a train forms a single perceptual "event," the onset and offset of which are delimited by the on and off peaks of phasic cortical responses. While auditory cortex showed a clear, qualitative correlation between perception and response waveshape, the medial geniculate body showed less correlation (since there was less change in waveshape with rate), and the inferior colliculus showed no correlation at all. Overall, our results suggest a population neural representation of the beginning and the end of distinct perceptual events that is weak or absent in the inferior colliculus, begins to emerge in the medial geniculate body, and is robust in auditory cortex.

GABA-ergic modulation of prefrontal spatio-temporal activation pattern during emotional processing: a combined fMRI/MEG study with placebo and lorazepam

Various prefrontal cortical regions have been shown to be activated during emotional stimulation, whereas neurochemical mechanisms underlying emotional processing in the prefrontal cortex remain unclear. We therefore investigated the influence of the GABA-A potentiator lorazepam on prefrontal cortical emotional-motor spatio-temporal activation pattern in a combined functional magnetic resonance imaging/magnetoencephalography study. Lorazepam led to the reversal in orbito-frontal activation pattern, a shift of the early magnetic field dipole from the orbito-frontal to medial prefrontal cortex, and alterations in premotor/motor cortical function during negative and positive emotional stimulation. It is concluded that negative emotional processing in the orbito-frontal cortex may be modulated either directly or indirectly by GABA-A receptors. Such a modulation of orbito-frontal cortical emotional function by lorazepam has to be distinguished from its effects on cortical motor function as being independent from the kind of processing either emotional or nonemotional.

Characterizing instantaneous phase relationships in whole-brain fMRI activation data

Typically, fMRI data is processed in the time domain with linear methods such as regression and correlation analysis. We propose that the theory of phase synchronization may be used to more completely understand the dynamics of interacting systems, and can be applied to fMRI data as a novel method of detecting activation. Generalized synchronization is a phenomenon that occurs when there is a nonlinear functional relationship present between two or more coupled, oscillatory systems, whereas phase synchronization is defined as the locking of the phases while the amplitudes may vary. In this study, we developed an application of phase synchronization analysis that is appropriate for fMRI data, in which the phase locking condition is investigated between a voxel time series and the reference function of the task performed. A synchronization index is calculated to quantify the level of phase locking, and a nonparametric permutation test is used to determine the statistical significance of the results. We performed the phase synchronization analysis on the data from five volunteers for an event-related finger-tapping task. Functional maps were created that provide information on the interrelations between the instantaneous phases of the reference function and the voxel time series in a whole-brain fMRI activation data set. We conclude that this method of analysis is useful for revealing additional information on the complex nature of the fMRI time series.

Auditory perception of laughing and crying activates human amygdala regardless of attentional state

Adequate behavioral responses to socially relevant stimuli are often impaired after lesions of the amygdala. Such lesions concern especially the recognition of facial and sometimes of vocal expression of emotions. Using low-noise functional magnetic resonance imaging (fMRI), we investigated in which way the amygdala, auditory cortex and insula are involved in the processing of affective nonverbal vocalizations (Laughing and Crying) in healthy humans. The same samples of male and female Laughing and Crying were presented in different experimental conditions: Simply listening to the stimuli, self-induction of the corresponding emotions while listening, and detection of artificial pitch shifts in the same stimuli. All conditions activated the amygdala similarly and bilaterally, whereby the amount of activation was larger in the right amygdala. The auditory cortex was more strongly activated by Laughing than by Crying with a slight right-hemisphere advantage for Laughing, both likely due to acoustic stimulus features. The insula was bilaterally activated in all conditions. The mean signal intensity change with stimulation was much larger in the amygdala than in auditory cortex and insula. The amygdala results seem to be in accordance with the right-hemisphere hypothesis of emotion processing which may not be applicable as strongly to the level of auditory cortex or insula.

Functional fields in human auditory cortex revealed by time-resolved fMRI without interference of EPI noise

The gradient switching during fast echoplanar functional magnetic resonance imaging (EPI-fMRI) produces loud noises that may interact with the functional activation of the central auditory system induced by experimental acoustic stimuli. This interaction is unpredictable and is likely to confound the interpretation of functional maps of the auditory cortex. In the present study we used an experimental design which does not require the presentation of stimuli during EPI acquisitions and allows for mapping of the auditory cortex without the interference of scanner noise. The design relies on the physiological delays between the onset, or the end, of stimulation and the corresponding hemodynamic response. Owing to these delays and through a time-resolved acquisition protocol it is possible to analyze the decay of the stimulus-specific signal changes after the cessation of the stimulus itself and before the onset of the EPI-acoustic noise related activation (decay-sampling technique). This experimental design, which might permit a more detailed insight in the auditory cortex, has been applied to the study of the cortical responses to pulsed 1000 Hz sine tones. Distinct activation clusters were detected in the Heschl's gyri and the planum temporale, with an increased extension compared to a conventional block-design paradigm. Furthermore, the comparison of the hemodynamic response of the most anterior and the posterior clusters of activation highlighted differential response patterns to the sound stimulation and to the EPI-noise. These differences, attributable to reciprocal saturation effects unevenly distributed over the superior temporal cortex, provided evidence for functionally distinct auditory fields.

A new approach to measure single-event related brain activity using real-time fMRI: feasibility of sensory, motor, and higher cognitive tasks

Real-time fMRI is a rapidly emerging methodology that enables monitoring changes in brain activity during an ongoing experiment. In this article we demonstrate the feasibility of performing single-event sensory, motor, and higher cognitive tasks in real-time on a clinical whole-body scanner. This approach requires sensitivity optimized fMRI methods: Using statistical parametric mapping we quantified the spatial extent of BOLD contrast signal changes as a function of voxel size and demonstrate that sacrificing spatial resolution and readout bandwidth improves the detection of signal changes in real time. Further increases in BOLD contrast sensitivity were obtained by using real-time multi-echo EPI. Real-time image analysis was performed using our previously described Functional Imaging in REal time (FIRE) software package, which features real-time motion compensation, sliding window correlation analysis, and automatic reference vector optimization. This new fMRI methodology was validated using single-block design paradigms of standard visual, motor, and auditory tasks. Further, we demonstrate the sensitivity of this method for online detection of higher cognitive functions during a language task using single-block design paradigms. Finally, we used single-event fMRI to characterize the variability of the hemodynamic impulse response in primary and supplementary motor cortex in consecutive trials using single movements. Real-time fMRI can improve reliability of clinical and research studies and offers new opportunities for studying higher cognitive functions.

Effect of ethanol on BOLD response to acoustic stimulation: implications for neuropharmacological fMRI

The effects of ethanol on acoustically stimulated blood oxygenation level-dependent (BOLD) signal response in healthy humans was examined with echo planar functional magnetic resonance imaging (fMRI). An acquisition mode minimizing neuronal activation by scanner noise in combination with acoustic excitation by a pulsed 1000-Hz sine tone was used. Paradigms were repeated three times before and after the ingestion of 0.7 g of ethanol/kg(body weight). Linear correlation analyses (r>/=0.40) revealed bilateral BOLD responses in the auditory cortex. Significant voxels covered a cortical volume of approximately 3 ml that was reduced by approximately 40% after ethanol. The BOLD signal change initially reaching approximately 3% was reduced by 12-27%, depending on the definition of the region of interest for signal quantitation. Because ethanol produces vasodilation, the hemodynamic contribution to the BOLD signal change was estimated by modeling the relationship between regional cerebral blood flow (rCBF) and BOLD signal changes. Assuming a baseline flow increase by 10% after ethanol intake, the resulting 'Flow-BOLD-Dependence' (FBD) curve suggested that the ethanol-related BOLD signal reduction was approximately 7-12% greater than the reduction contributed purely by vasodilation. However, simultaneous determination of rCBF and regional cerebral blood volume would be required for an exact quantitation of the neuronally induced BOLD response. Although the FBD model needs empirical validation, its cautious implementation appears to be helpful if fMRI is used in combination with vasoactive drugs.

New insights into the hemodynamic blood oxygenation level-dependent response through combination of functional magnetic resonance imaging and optical recording in gerbil barrel cortex

Fast, low-angle shoot functional magnetic resonance imaging (fMRI), based on the blood oxygenation level-dependent (BOLD) effect, was combined with optical recording of intrinsic signals (ORIS) and 2-deoxyglucose labeling in gerbil barrel cortex. We observed over the activated barrel a positive BOLD signal and increased levels of deoxyhemoglobin and total hemoglobin during each period of prolonged (30 sec) D2 vibrissal stimulation. These data show that the hemodynamic basis of this fMRI signal is not necessarily a washout of deoxyhemoglobin, as generally assumed. Instead, they suggest that a positive BOLD signal can also be caused by a local increase of blood volume, even if deoxyhemoglobin levels are persistently elevated. We also show that this alternative interpretation is consistent with theoretical models of the BOLD signal. The changes in BOLD signal and blood volume, which are most tightly correlated with the periodic stimulation, peak at the site of neuronal activation. These results contribute to the understanding of the hemodynamic mechanisms underlying the BOLD signal and also suggest analysis methods, which improve the spatial localization of neuronal activation with both fMRI and ORIS.

Assessment of auditory cortex activation with functional magnetic resonance imaging

OBJECTIVE

The goal was to assess auditory cortex activation evoked by pure-tone stimulus with functional MRI.

METHODS

Five healthy children, aged 7 to 10 years, were studied. Hearing evaluation was performed by pure-tone audiometry in a sound-treated room and in the MRI scanner with the scanner noise in the background. Subjects were asked to listen to pure tones (500, 1000, 2000, and 4000 Hz) at thresholds determined in the MRI scanner. Functional image processing was performed with a cross-correlation technique with a correlation coefficient of 0.5 (P < 0.0001). Auditory cortex activation was assessed by observing activated pixels in functional images.

RESULTS

Functional images of auditory cortex activation were obtained in 3 children. All children showed activation in Heschl's gyrus, middle temporal gyrus, superior temporal gyrus, and planum temporale. The number of activated pixels in auditory cortexes ranged from 4 to 33.

CONCLUSIONS

Functional images of auditory cortex activation evoked by pure-tone stimuli are obtained in healthy children with the functional MRI technique.

The time course of the BOLD response in the human auditory cortex to acoustic stimuli of different duration

The relationship between activity within the human auditory cortices and the duration of heard tones was investigated by measuring the hemodynamic response with functional magnetic resonance imaging. We demonstrate that there is no significant influence of stimulus duration as used here on the intensity and spatial extent of the hemodynamic response in the auditory cortices. We found however, that the time course of the hemodynamic response to the repeated stimulus presentation exhibited a characteristic decline after the first stimulus exposure during the activation period. The possible reasons for this time course are currently unknown, however, several factors may be involved, including top-down mechanisms and/or the interplay of tissue perfusion and oxygen consumption.

Exploring brain function with magnetic resonance imaging

Since its invention in the early 1990s, functional magnetic resonance imaging (fMRI) has rapidly assumed a leading role among the techniques used to localize brain activity. The spatial and temporal resolution provided by state-of-the-art MR technology and its non-invasive character, which allows multiple studies of the same subject, are some of the main advantages of fMRI over the other functional neuroimaging modalities that are based on changes in blood flow and cortical metabolism. This paper describes the basic principles and methodology of fMRI and some aspects of its application to functional activation studies. Attention is focused on the physiology of the blood oxygenation level-dependent (BOLD) contrast mechanism and on the acquisition of functional time-series with echo planar imaging (EPI). We also provide an introduction to the current strategies for the correction of signal artefacts and other image processing techniques. In order to convey an idea of the numerous applications of fMRI, we will review some of the recent results in the fields of cognitive and sensorimotor psychology and physiology.

Functional magnetic resonance imaging of a human auditory cortex area involved in foreground-background decomposition

Auditory foreground-background decomposition is a pattern recognition process which combines simultaneous and sequential grouping in complex sound sequences. Using functional magnetic resonance imaging with reduced scanner noise and stimulation through a new type of earphones, we investigated the possibility that this process activates topographically distinct areas of human auditory cortex. A basic matching-to-sample task with variable tones (sequential grouping) caused significant activity in three separate landmark-related territories on the supratemporal plane. A similar task in the presence of a strongly masking acoustic background pattern to challenge simultaneous grouping led to the distinction of the subterritory in which foreground signal-related or task-related signal properties were exclusively seen. In contrast to the remainder of territories the level of activity and the periodicity of the signal time-course was resistant to the masking influence of the background. This suggests that auditory foreground-background decomposition involves a specialized non-primary auditory cortex field. Generally, the findings demonstrate functional parcellation of auditory cortex for which the evidence in humans, in contrast to other primates, is only indirect to date.

Lateralized processing of speech prosodies in the temporal cortex: a 3-T functional magnetic resonance imaging study

Prosodic modulation of speech provides information about emotional states of speakers (affective prosodies) or serves as syntactic markers to change linguistic aspects of speech (linguistic prosodies). Previous electrophysiological investigations and studies on patients with right or left hemisphere damage showed nonuniform results with respect to lateralization of prosodic processing. In this study 20 healthy right-handed volunteers were investigated with functional magnetic resonance imaging of the acoustically responsive areas on the supratemporal plane while detecting phonemes as control targets or prosodies in strings of nonsense syllables and adjectives, the latter randomly intonated in a declarative, interrogative, commanding, happy, or sad fashion. In control task A the phoneme /a/ was detected in the syllables. In control task B the phoneme /a/ was detected in the adjectives, and in the experimental task C the sad intonations (affective) and in the experimental task D the interrogative intonations (linguistic) had to be detected in the same material. In task A intensity-weighted volumes of activated voxels were not different in the two hemispheres (laterality index 0). In task B with an irrelevant phoneme detection with respect to prosodic material, the population split into two subgroups with similar right or left hemispheric lateralization of activity leading to an absolute laterality index of 26.8 across all subjects. During detection of affective prosodies (task C), lateralization was maintained yet the absolute laterality index reduced to 14.5, while there was no lateralization during detection of linguistic prosodies. The sum of activations in the two hemispheres was the same across all tasks and subgroups, which suggests that the lateralizations occurring with presentation and detection of prosodic material depend on a redistribution of activity between hemispheres.