Intensity coding of auditory stimuli: an fMRI study

Reduced laterality as a trait marker of schizophrenia--evidence from structural and functional neuroimaging

Laterality is a characteristic principle of the organization of the brain systems for language, and reduced hemispheric asymmetry has been considered a risk factor for schizophrenia. Here we sought support for the risk factor hypothesis by investigating whether reduced asymmetry of temporal lobe structure and function is also present in unaffected relatives. Sixteen schizophrenia patients, 16 age-matched first-degree relatives, and 15 healthy controls underwent high-resolution three-dimensional anatomical imaging and functional magnetic resonance imaging during auditory stimulation. Both the overall auditory cortex and planum temporale volumes and the lateralization to the left hemisphere were markedly reduced in patients. The decrease of lateralization correlated with increased severity of symptoms. In addition, both the overall functional activation in response to auditory stimulation and its asymmetry were reduced in the patients. Relatives had intermediate values between patients and controls on both structural and functional measures. This study provides added support for the idea that reduced hemispheric asymmetry is a biological risk factor for schizophrenia.

Central auditory mechanisms associated with cochlear implantation: an overview of selected studies and comment

Recently, the evaluation of the role of the central auditory system in the utilization of cochlear implants has received much attention. In order to fully understand the results of cochlear implantation, one must have an understanding of how the brain is influenced by electrical stimulation of the auditory nerve. This review provides an overview of the structure and function of the central auditory nervous system and the changes that occur as a result of cochlear implant use. Evidence of central auditory plasticity is discussed highlighting key imaging and evoked potential studies.

Functional imaging of human auditory cortex

PURPOSE OF REVIEW

This review summarizes recent advances in functional magnetic resonance imaging that reveal similarities in the organization of human auditory cortex (HAC) and auditory cortex of nonhuman primates.

RECENT FINDINGS

Functional magnetic resonance imaging studies have shown that HAC is a compact region that covers less than 8% of the total cortical surface. HAC is subdivided into more than a dozen distinct auditory cortical fields (ACFs) that surround Heschl's gyri on the superior temporal plane. Recent advances that permit the visualization of the results of functional magnetic imaging experiments directly on the cortical surface have provided new insights into the organization of human ACFs. Evidence suggests that medial regions of HAC are organized in a manner similar to the auditory cortex of other primate species with a set of tonotopically organized core ACFs surrounded by belt ACFs that often share tonotopic organization with the core. Although influenced by attention, responses in HAC core and belt fields are largely determined by the acoustic properties of stimuli, including their frequency, intensity, and location. In contrast, lateral regions of HAC contain parabelt fields that are little influenced by simple acoustic features but rather respond to behaviorally relevant complex sounds such as speech and are strongly modulated by attention.

SUMMARY

HAC conserves the basic structural and functional organization of auditory cortex as seen in old world primate species. A central challenge to future research is to understand how this basic primate plan has evolved to support uniquely human abilities such as music and language.

A multisensory cortical network for understanding speech in noise

In noisy environments, listeners tend to hear a speaker's voice yet struggle to understand what is said. The most effective way to improve intelligibility in such conditions is to watch the speaker's mouth movements. Here we identify the neural networks that distinguish understanding from merely hearing speech, and determine how the brain applies visual information to improve intelligibility. Using functional magnetic resonance imaging, we show that understanding speech-in-noise is supported by a network of brain areas including the left superior parietal lobule, the motor/premotor cortex, and the left anterior superior temporal sulcus (STS), a likely apex of the acoustic processing hierarchy. Multisensory integration likely improves comprehension through improved communication between the left temporal-occipital boundary, the left medial-temporal lobe, and the left STS. This demonstrates how the brain uses information from multiple modalities to improve speech comprehension in naturalistic, acoustically adverse conditions.

Effect of sound intensity on tonotopic fMRI maps in the unanesthetized monkey

The monkey's auditory cortex includes a core region on the supratemporal plane (STP) made up of the tonotopically organized areas A1, R, and RT, together with a surrounding belt and a lateral parabelt region. The functional studies that yielded the tonotopic maps and corroborated the anatomical division into core, belt, and parabelt typically used low-amplitude pure tones that were often restricted to threshold-level intensities. Here we used functional magnetic resonance imaging in awake rhesus monkeys to determine whether, and if so how, the tonotopic maps and the pattern of activation in core, belt, and parabelt are affected by systematic changes in sound intensity. Blood oxygenation level-dependent (BOLD) responses to groups of low- and high-frequency pure tones 3-4 octaves apart were measured at multiple sound intensity levels. The results revealed tonotopic maps in the auditory core that reversed at the putative areal boundaries between A1 and R and between R and RT. Although these reversals of the tonotopic representations were present at all intensity levels, the lateral spread of activation depended on sound amplitude, with increasing recruitment of the adjacent belt areas as the intensities increased. Tonotopic organization along the STP was also evident in frequency-specific deactivation (i.e. "negative BOLD"), an effect that was intensity-specific as well. Regions of positive and negative BOLD were spatially interleaved, possibly reflecting lateral inhibition of high-frequency areas during activation of adjacent low-frequency areas, and vice versa. These results, which demonstrate the strong influence of tonal amplitude on activation levels, identify sound intensity as an important adjunct parameter for mapping the functional architecture of auditory cortex.

Auditory discrimination and auditory sensory behaviours in autism spectrum disorders

It has been hypothesised that auditory processing may be enhanced in autism spectrum disorders (ASD). We tested auditory discrimination ability in 72 adolescents with ASD (39 childhood autism; 33 other ASD) and 57 IQ and age-matched controls, assessing their capacity for successful discrimination of the frequency, intensity and duration differences in pairs of sounds. At the group level, auditory discrimination ability did not differ between the adolescents with and without ASD. However, we found a subgroup of 20% of individuals in the ASD group who showed 'exceptional' frequency discrimination skills (defined as 1.65 SDs above the control mean) and who were characterised by average intellectual ability and delayed language onset. Auditory sensory behaviours (i.e. behaviours in response to auditory sensory input) are common in ASD and we hypothesised that these would relate to auditory discrimination ability. For the ASD group, poor performers on the intensity discrimination task reported more auditory sensory behaviours associated with coping with loudness levels. Conversely, those who performed well on the duration discrimination task reported more auditory sensory behaviours across the full range measured. Frequency discrimination ability did not associate with auditory sensory behaviours. We therefore conclude that (i) enhanced frequency discrimination is present in around 1 in 5 individuals with ASD and may represent a specific phenotype; and (ii) individual differences in auditory discrimination ability in ASD may influence the expression of auditory sensory behaviours by modulating the degree to which sounds are detected or missed in the environment.

Human cortical organization for processing vocalizations indicates representation of harmonic structure as a signal attribute

The ability to detect and rapidly process harmonic sounds, which in nature are typical of animal vocalizations and speech, can be critical for communication among conspecifics and for survival. Single-unit studies have reported neurons in auditory cortex sensitive to specific combinations of frequencies (e.g., harmonics), theorized to rapidly abstract or filter for specific structures of incoming sounds, where large ensembles of such neurons may constitute spectral templates. We studied the contribution of harmonic structure to activation of putative spectral templates in human auditory cortex by using a wide variety of animal vocalizations, as well as artificially constructed iterated rippled noises (IRNs). Both the IRNs and vocalization sounds were quantitatively characterized by calculating a global harmonics-to-noise ratio (HNR). Using functional MRI, we identified HNR-sensitive regions when presenting either artificial IRNs and/or recordings of natural animal vocalizations. This activation included regions situated between functionally defined primary auditory cortices and regions preferential for processing human nonverbal vocalizations or speech sounds. These results demonstrate that the HNR of sound reflects an important second-order acoustic signal attribute that parametrically activates distinct pathways of human auditory cortex. Thus, these results provide novel support for the presence of spectral templates, which may subserve a major role in the hierarchical processing of vocalizations as a distinct category of behaviorally relevant sound.

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.

Auditory attention activates peripheral visual cortex

Background

Recent neuroimaging studies have revealed that putatively unimodal regions of visual cortex can be activated during auditory tasks in sighted as well as in blind subjects. However, the task determinants and functional significance of auditory occipital activations (AOAs) remains unclear.

Methodology/Principal Findings

We examined AOAs in an intermodal selective attention task to distinguish whether they were stimulus-bound or recruited by higher-level cognitive operations associated with auditory attention. Cortical surface mapping showed that auditory occipital activations were localized to retinotopic visual cortex subserving the far peripheral visual field. AOAs depended strictly on the sustained engagement of auditory attention and were enhanced in more difficult listening conditions. In contrast, unattended sounds produced no AOAs regardless of their intensity, spatial location, or frequency.

Conclusions/Significance

Auditory attention, but not passive exposure to sounds, routinely activated peripheral regions of visual cortex when subjects attended to sound sources outside the visual field. Functional connections between auditory cortex and visual cortex subserving the peripheral visual field appear to underlie the generation of AOAs, which may reflect the priming of visual regions to process soon-to-appear objects associated with unseen sound sources.

The human amygdala is sensitive to the valence of pictures and sounds irrespective of arousal: an fMRI study

With the advent of studies showing that amygdala responses are not limited to fear-related or highly unpleasant stimuli, studies began to focus on stimulus valence and stimulus-related arousal as predictors of amygdala activity. Recent studies in the chemosensory domain found amygdala activity to increase with the intensity of negative and positive chemosensory stimuli. This has led to the proposal that amygdala activity might be an indicator of emotional arousal, at least in the chemosensory domain. The present study investigated amygdala activity in response to visual and auditory stimuli. By selecting stimuli based on individual valence and arousal ratings, we were able to dissociate stimulus valence and stimulus-related arousal, both on the verbal and the peripheral physiological level. We found that the amygdala was sensitive to stimulus valence even when arousal was controlled for, and that increased amygdala activity was better explained by valence than by arousal. The proposed difference in the relation between amygdala activity and stimulus-related arousal between the chemosensory and the audiovisual domain is discussed in terms of the amygdala's embedding within these sensory systems and the processes by which emotional meaning is derived.

Cortical mechanisms of speech perception in noise

PURPOSE

The present study examines the brain basis of listening to spoken words in noise, which is a ubiquitous characteristic of communication, with the focus on the dorsal auditory pathway.

METHOD

English-speaking young adults identified single words in 3 listening conditions while their hemodynamic response was measured using fMRI: speech in quiet, speech in moderately loud noise (signal-to-noise ratio [SNR] 20 dB), and in loud noise (SNR -5 dB).

RESULTS

Behaviorally, participants' performance (both accuracy and reaction time) did not differ between the quiet and SNR 20 dB condition, whereas they were less accurate and responded slower in the SNR -5 dB condition compared with the other 2 conditions. In the superior temporal gyrus (STG), both left and right auditory cortex showed increased activation in the noise conditions relative to quiet, including the middle portion of STG (mSTG). Although the right posterior STG (pSTG) showed similar activation for the 2 noise conditions, the left pSTG showed increased activation in the SNR -5 dB condition relative to the SNR 20 dB condition.

CONCLUSION

We found cortical task-independent and noise-dependent effects concerning speech perception in noise involving bilateral mSTG and left pSTG. These results likely reflect demands in acoustic analysis, auditory-motor integration, and phonological memory, as well as auditory attention.

Sound level dependence of auditory evoked potentials: Simultaneous EEG recording and low-noise fMRI

The simultaneous recording of EEG and fMRI offers the advantage of combining precise spatial information about neuronal processing obtained by fMRI data with the high temporal resolution of EEG data. One problem for the analysis of auditory processing, however, is the noisy environment during fMRI measurements, especially when EPI sequences are employed. While EEG studies outside an MRI scanner repeatedly demonstrated a clear sound level-dependent increase of N1 amplitude, this finding was less obvious in simultaneous recordings inside a scanner. Based on the assumption that this inconsistency might be due to the confounding effect of the rather loud EPI noise, we employed a low-noise fMRI protocol. This method was previously used to reveal level-dependent fMRI activation in auditory cortex areas. We combined this method with simultaneous EEG recordings to investigate the effect of different sound intensities on the auditory evoked potentials. Eight participants without hearing deficits took part in our experiment. Frequency modulated tones (FM) were presented monaurally with two sound intensities (60 and 80 dB HL). The task of the participants was to categorize the FM-direction (rising vs. falling). Our results inside the scanner replicate the sound level dependence of AEPs from previous EEG studies outside the scanner. The data analysis revealed a significant shortening of N1 latency and an increase in the N1-P2 peak-to-peak amplitude for the higher sound intensity. On a descriptive level, the 80 dB HL stimulation yielded more activated voxels in fMRI and stronger activations. This effect was pronounced over the right hemisphere. Our results suggest that low-noise sequences might be advantageous for the examination of auditory processing in simultaneous EEG and fMRI recordings.

Modeling GABA alterations in schizophrenia: a link between impaired inhibition and altered gamma and beta range auditory entrainment

The disorganized symptoms of schizophrenia, including severely disordered thought patterns, may be indicative of a problem with the construction and maintenance of cell assemblies during sensory processing and attention. The gamma and beta frequency bands (15-70 Hz) are believed relevant to such processing. This paper addresses the results of an experimental examination of the cortical response of 12 schizophrenia patients and 12 control subjects when presented with auditory click-train stimuli in the gamma/beta frequency band during measurement using magnetoencephalography (MEG), as well as earlier work by Kwon et al. These data indicate that control subjects show an increased 40-Hz response to both 20- and 40-Hz stimulation as compared with patients, whereas schizophrenic subjects show a preference for 20-Hz response to the same driving frequencies. In this work, two computational models of the auditory cortex are constructed based on postmortem studies that indicate cortical interneurons in schizophrenic subjects have decreased GAT-1 (a GABA transporter) and GAD(67) (1 of 2 enzymes responsible for GABA synthesis). The models transition from control to schizophrenic frequency response when an extended inhibitory decay time is introduced; this change captures a possible effect of these GABA alterations. Modeling gamma/beta range auditory entrainment in schizophrenia provides insight into how biophysical mechanisms can impact cognitive function. In addition, the study of dynamics that underlie auditory entrainment in schizophrenia may contribute to the understanding of how gamma and beta rhythms impact cognition in general.

Functional Magnetic Resonance Imaging in Zebra Finch Discerns the Neural Substrate Involved in Segregation of Conspecific Song From Background Noise

Recently, fMRI was introduced in a well-documented animal model for vocal learning, the songbird. Using fMRI and conspecific signals mixed with different levels of broadband noise, we now demonstrate auditory-induced activation representing discriminatory properties of auditory forebrain regions in anesthetized male zebra finches ( Taeniopygia guttata). Earlier behavioral tests showed comparable calling responses to the original conspecific song stimulus heard outside and inside the magnet. A significant fMRI response was elicited by conspecific song in the primary auditory thalamo-recipient subfield L2a; in neighboring subareas L2b, L3, and L; and in the rostral part of the higher-order auditory area NCM (caudomedial nidopallium). Temporal BOLD response clustering revealed rostral and caudal clusters that we defined as “cluster Field L” and “cluster NCM”, respectively. However, because the actual border between caudal Field L subregions and NCM cannot be seen in the structural MR image and is not precisely reported elsewhere, the cluster NCM might also contain subregion L and the medial extremes of the subregions L2b and L3. Our results show that whereas in cluster Field L the response was not reduced by added noise, in cluster NCM the response was reduced and finally disappeared with increasing levels of noise added to the song stimulus. The activation in cluster NCM was significant for only two experimental stimuli that showed significantly more behavioral responses than the more degraded stimuli, suggesting that the first area within the auditory system where the ability to discern song from masking noise emerges is located in cluster NCM.

fMRI reveals altered auditory processing in manifest and premanifest Huntington's disease

Structural alterations of the basal ganglia occur in patients with Huntington's disease (HD). The aim of this exploratory study was to assess auditory processing mechanisms by functional MRI (fMRI) in patients with premanifest (pHD) and manifest HD to gain more insight in possible alterations in basal ganglia-thalamic circuits. Sixteen HD and 18 pHD as well as corresponding age- and gender-matched controls were included. The pHD group was divided into two subgroups close (cpHD; <10 years) and far pHD (fcHP; >10 years), according to their estimated age of disease onset (eAO). Tone perception and processing were visualized by 3T fMRI by employing repeated tone stimulation through digitally generated pulsed (nu=5Hz) 800-Hz sine tones. We found altered activation in basal ganglia-thalamic circuits in HD and/or pHD compared to controls. (i) The cpHD group presented predominantly down-regulated processes compared to fpHD and HD. (ii) HD presented stronger bilateral activation of the putamen and (iii) fpHD presented stronger bilateral activation of the thalamus and also right caudatum. (iv) Depending on the progress of the disease, a shift towards the activation of more right hemispherical areas can be observed. Our findings seem to reflect an altered activation pattern to auditory stimulation depending on the progression of neuronal dysfunction in HD and pHD. They also stress the involvement of the basal ganglia-thalamic circuits in the processing of sensory auditory stimuli.

Assessing the influence of scanner background noise on auditory processing. II. An fMRI study comparing auditory processing in the absence and presence of recorded scanner noise using a sparse design

Several studies reported decreased signal intensities within auditory areas for experimental designs employing continuous scanner background noise (SBN) in comparison to designs with less or no SBN. This study examined the source for this SBN-induced masking effect of the blood oxygenation level-dependent (BOLD) response by directly comparing two experimental sessions with the same auditory stimulation, which was presented either with or without recorded scanner background noise (RecSBN). Ten subjects listened to a series of four one-syllable words and had to decide whether two of the words were identical. The words were either presented with a silent background or with added RecSBN. This was then contrasted with either silence or RecSBN. A sparse temporal sampling method was used in both sessions, which enabled us to directly assess the influence of RecSBN without varying scanning parameters, acquisition quantities, or auditory stimulations. Our results suggest that previously reported SBN-induced masking of the BOLD response in experimental designs with SBN might be caused by an interaction between increased baseline levels and nonlinearity effects within auditory cortices. Adding SBN to an experimental condition does not enhance signal intensities to the same degree that SBN does when presented with a silent background, and therefore contrasting an experimental and baseline condition that both have SBN may lead to signal decreases. In addition, our study shows this effect is greatest in Heschl's gyrus, but can also be observed in higher-order auditory areas.

Assessing the influence of scanner background noise on auditory processing. I. An fMRI study comparing three experimental designs with varying degrees of scanner noise

We compared two experimental designs aimed at minimizing the influence of scanner background noise (SBN) on functional MRI (fMRI) of auditory processes with one conventional fMRI design. Ten subjects listened to a series of four one-syllable words and had to decide whether two of the words were identical. This was contrasted with a no-stimulus control condition. All three experimental designs had a duration of approximately 17 min: 1) a behavior interleaved gradients (BIG; Eden et al. [1999] J Magn Reson Imaging 41:13-20) design (repetition time, TR, = 6 s), where stimuli were presented during the SBN-free periods between clustered volume acquisitions (CVA); 2) a sparse temporal sampling technique (STsamp; e.g., Gaab et al., [2003] Neuroimage 19:1417-1426) acquiring only one set of slices following each of the stimulations with a 16-s TR and jittered delay times between stimulus offset and image acquisition; and 3) an event-related design with continuous scanning (ERcont) using the stimulation design of STsamp but with a 2-s TR. The results demonstrated increased signal within Heschl's gyrus for the STsamp and BIG-CVA design in comparison to ERcont as well as differences in the overall functional anatomy among the designs. The possibility to obtain a time course of activation as well as the full recovery of the stimulus- and SBN-induced hemodynamic response function signal and lack of signal suppression from SBN during the STsamp design makes this technique a powerful approach for conducting auditory experiments using fMRI. Practical strengths and limitations of the three auditory acquisition paradigms are discussed.

fMRI during natural sleep as a method to study brain function during early childhood

Many techniques to study early functional brain development lack the whole-brain spatial resolution that is available with fMRI. We utilized a relatively novel method in which fMRI data were collected from children during natural sleep. Stimulus-evoked responses to auditory and visual stimuli as well as stimulus-independent functional networks were examined in typically developing 2-4-year-old children. Reliable fMRI data were collected from 13 children during presentation of auditory stimuli (tones, vocal sounds, and nonvocal sounds) in a block design. Twelve children were presented with visual flashing lights at 2.5 Hz. When analyses combined all three types of auditory stimulus conditions as compared to rest, activation included bilateral superior temporal gyri/sulci (STG/S) and right cerebellum. Direct comparisons between conditions revealed significantly greater responses to nonvocal sounds and tones than to vocal sounds in a number of brain regions including superior temporal gyrus/sulcus, medial frontal cortex and right lateral cerebellum. The response to visual stimuli was localized to occipital cortex. Furthermore, stimulus-independent functional connectivity MRI analyses (fcMRI) revealed functional connectivity between STG and other temporal regions (including contralateral STG) and medial and lateral prefrontal regions. Functional connectivity with an occipital seed was localized to occipital and parietal cortex. In sum, 2-4 year olds showed a differential fMRI response both between stimulus modalities and between stimuli in the auditory modality. Furthermore, superior temporal regions showed functional connectivity with numerous higher-order regions during sleep. We conclude that the use of sleep fMRI may be a valuable tool for examining functional brain organization in young children.

fMRI activation in relation to sound intensity and loudness

The aim of this fMRI study was to relate cortical fMRI responses to both physical and perceptual sound level characteristics. Besides subjects with normal hearing, subjects with high-frequency sensorineural hearing loss were included, as distortion of loudness perception is a characteristic of such impairment. Cortical responses in both subject groups were analyzed as a function of the physical intensity and the perceived loudness of low and high-frequency stimuli. For the low-frequency stimuli, intensity levels ranged from 0 to 70 dB SL; for the high-frequency stimuli, intensity levels were set such that the corresponding loudness levels matched those of the low-frequency stimuli. Responses were found to increase significantly and predominantly linearly with intensity level and with loudness level. Response saturation at the highest levels was not apparent, but activation exhibited a steep rise between 0 and 10 dB for the low-frequency stimuli. The activation in the subjects with hearing loss increased significantly more strongly with stimulus intensity than that in the normally hearing subjects. This reflects loudness recruitment, characterized by a disproportionate increase in loudness with stimulus intensity. In contrast, the rate of activation increase as a function of loudness level did not differ between both subject groups. This demonstrates that fMRI activation at the level of the auditory cortex is more closely related to the percept of a stimulus (i.e., loudness) rather than to its physical characteristics (i.e., intensity).

Prediction of treatment response in major depression: integration of concepts

BACKGROUND

Two promising approaches have been introduced for the prediction of treatment response in major depression: one concept is based on the activity in the rostral anterior cingulate cortex (rACC). Subjects with higher metabolic rates respond better to sleep deprivation or antidepressive medication. Another approach is the investigation of the loudness dependence of the auditory evoked potential (LDAEP). Here, a high LDAEP is supposed to reflect low central serotonergic activity. We present the first study comparing both approaches in the same group of patients.

METHODS

Patients with major depression (n=20) were investigated using both resting EEG and LDAEP before treatment with either citalopram or reboxetine.

RESULTS

We found significant differences between responders and non-responders in the rACC in the theta-frequency range (6.5-8 Hz, p<0.05). In the subgroup of patients, treated with citalopram we found higher LDAEP-values in responders versus non-responders (p<0.05) and a significant correlation between pre-treatment-LDAEP and improvement in the Hamilton score after treatment (r=0.71, p<0.05).

CONCLUSIONS

In combining both methods a prediction whether a patient with major depression might be at risk for non-response to a standard therapy as well as a suggestion for a pharmacological approach of choice seems to be possible.

Lateralization of dichotic speech stimuli is based on specific auditory pathway interactions: neuromagnetic evidence

Dichotic listening (DL) is a neuropsychological technique for the study of functional laterality. Based on behavioral patient studies, the "structural theory" states that lateralization of the auditory input during DL is allowed by an inhibition of the ipsilateral pathways. We aimed here at extending this theory to provide a neurophysiological basis of verbal DL. We investigated the magnetic responses of the primary auditory cortices elicited by dichotic consonant-vowel syllables. Dichotic stimuli consisted of 2 syllables pairs, a "competing" one composed by syllables with high spectral overlap (/da/ and /ba/) and a "noncompeting" pair (/da/ and /ka/). One of the syllables in each pair was delivered at 2 intensities, whereas the other did not change. A reduced increase of source intensity in response to dichotic pairs at the 2 levels was assumed to indicate pathway inhibition effects. We obtained that the left ipsilateral pathway (i.e., the left ipsilateral signal) was strongly inhibited by the right contralateral one. Conversely, the right ipsilateral pathway did not show an inhibition larger than the left contralateral one. These results extend the notion of auditory functional asymmetries by showing that beyond hemispheric functional specialization there is an asymmetry within the ascending auditory system, which is based on a competition mechanism. The larger the competition between the left and right ear stimuli, the larger are the inhibition effects, which determine the pathway asymmetry. These findings represent as well a neurophysiological basis for the "structural theory" explaining the right ear preference usually found in verbal DL tasks.

Functional imaging of the central auditory system using PET

In the last few decades functional neuroimaging tools have emerged to study the function of the human brain in vivo. These techniques have increased the knowledge of how the brain processes stimuli of different sensory modalities, including auditory processing. Positron emission tomography (PET) has been used for nearly 20 years to study changes in cerebral blood flow associated with auditory stimulation in normal and hearing impaired subjects. PET studies gave insight into the neural base of processing basic sound features such as frequency and intensity, but complex stimuli such as speech and music have also been investigated extensively. Knowledge of the normal auditory function of the brain helps us to understand the neural base of hearing deficits and provides ideas for possible treatments. Although functional magnetic resonance imaging (fMRI) is replacing PET in many neuroimaging studies nowadays, PET still holds unique advantages and can give us valuable knowledge about the auditory cortex and auditory perception.

Task-modulated "what" and "where" pathways in human auditory cortex

Human neuroimaging studies suggest that localization and identification of relevant auditory objects are accomplished via parallel parietal-to-lateral-prefrontal "where" and anterior-temporal-to-inferior-frontal "what" pathways, respectively. Using combined hemodynamic (functional MRI) and electromagnetic (magnetoencephalography) measurements, we investigated whether such dual pathways exist already in the human nonprimary auditory cortex, as suggested by animal models, and whether selective attention facilitates sound localization and identification by modulating these pathways in a feature-specific fashion. We found a double dissociation in response adaptation to sound pairs with phonetic vs. spatial sound changes, demonstrating that the human nonprimary auditory cortex indeed processes speech-sound identity and location in parallel anterior "what" (in anterolateral Heschl's gyrus, anterior superior temporal gyrus, and posterior planum polare) and posterior "where" (in planum temporale and posterior superior temporal gyrus) pathways as early as approximately 70-150 ms from stimulus onset. Our data further show that the "where" pathway is activated approximately 30 ms earlier than the "what" pathway, possibly enabling the brain to use top-down spatial information in auditory object perception. Notably, selectively attending to phonetic content modulated response adaptation in the "what" pathway, whereas attending to sound location produced analogous effects in the "where" pathway. This finding suggests that selective-attention effects are feature-specific in the human nonprimary auditory cortex and that they arise from enhanced tuning of receptive fields of task-relevant neuronal populations.

Effect of unpleasant loud noise on hippocampal activities during picture encoding: An fMRI study

The functional link between the amygdala and hippocampus in humans has not been well documented. We examined the effect of unpleasant loud noise on hippocampal and amygdaloid activities during picture encoding by means of fMRI, and on the correct response in humans. The noise reduced activity in the hippocampus during picture encoding, decreased the correct response and increased the activity of the amygdala. A path diagram using structural equation modeling suggested that hippocampus activity might be depressed by high amygdala activity. Therefore, noise should diminish memory by reducing hippocampal activity, which might be depressed by high amygdala activity.

Top-down and bottom-up processes in speech comprehension

Speech comprehension includes both bottom-up and top-down processes, and imaging studies have isolated a frontal-temporal network of brain areas active during speech perception. However, the precise role of the various areas in this network during normal speech comprehension is not yet fully understood. In the present fMRI study, the signal-to-noise ratio (SNR) of spoken sentences was varied in 144 steps, and speech intelligibility was measured independently in order to study in detail its effect on the activation of brain areas involved in speech perception. Relative to noise alone, intelligible speech in noise evoked spatially extended activation in left frontal, bilateral temporal, and medial occipital brain regions. Increasing SNR led to a sigmoid-shaped increase of activation in all areas of the frontal-temporal network. The onset of the activation with respect to SNR was similar in temporal and frontal regions, but frontal activation was found to be smaller than temporal activation at the highest SNRs. Finally, only Broca's area (BA44) showed activation to unintelligible speech presented at low SNRs. These findings demonstrate distinct roles of frontal and temporal areas in speech comprehension in that temporal regions subserve bottom-up processing of speech, whereas frontal areas are more involved in top-down supplementary mechanisms.

Effects of sound level on fMRI activation in human brainstem, thalamic and cortical centers

The dependence of fMRI activation on sound level was examined throughout the auditory pathway of normal human listeners using continuous broadband noise, a stimulus widely used in neuroscientific investigations of auditory processing, but largely neglected in neuro-imaging. Several specialized techniques were combined here for the first time to enhance detection of brainstem activation, mitigate scanner noise, and recover temporal resolution lost by the mitigation technique. The main finding was increased activation with increasing level in cochlear nucleus, superior olive, inferior colliculus, medial geniculate body and auditory cortical areas. We suggest that these increases reflect monotonically increasing activity in a preponderance of individual auditory neurons responsive to broadband noise. While the time-course of activation changed with level, the change was subtle and only significant in a part of the cortex. To our knowledge, these are the first fMRI data showing the effects of sound level in subcortical centers or for a non-tonal, non-speech stimulus at any stage of the pathway. The present results add to the body of parametric data in normal human listeners and are fundamental to the design of any fMRI experiment employing continuous noise.

Differential representation of speech sounds in the human cerebral hemispheres

Various methods in auditory neuroscience have been used to gain knowledge about the structure and function of the human auditory cortical system. Regardless of method, hemispheric differences are evident in the normal processing of speech sounds. This review article, augmented by the authors' own work, provides evidence that asymmetries exist in both cortical and subcortical structures of the human auditory system. Asymmetries are affected by stimulus type, for example, hemispheric activation patterns have been shown to change from right to left cortex as stimuli change from speech to nonspeech. In addition, the presence of noise has differential effects on the contribution of the two hemispheres. Modifications of typical asymmetric cortical patterns occur when pathology is present, as in hearing loss or tinnitus. We show that in response to speech sounds, individuals with unilateral hearing loss lose the normal asymmetric pattern due to both a decrease in contralateral hemispheric activity and an increase in the ipsilateral hemisphere. These studies demonstrate the utility of modern neuroimaging techniques in functional investigations of the human auditory system. Neuroimaging techniques may provide additional insight as to how the cortical auditory pathways change with experience, including sound deprivation (e.g., hearing loss) and sound experience (e.g., training). Such investigations may explain why some populations appear to be more vulnerable to changes in hemispheric symmetry such as children with learning problems and the elderly.

Event related potentials and fMRI in neuropsychopharmacology

Event related potentials (ERP) are important clinical and research instruments in neuropsychiatry, particularly due to their strategic role for the investigation of brain function. These techniques are often underutilized in the evaluation of neurological and psychiatric disorders, but nevertheless they can be most useful and highly effective in the diagnostic workup of a wide range of neuropsychiatric disorders as well as in monitoring the course of the disorders and the prediction of treatment responses. ERP are noninvasive instruments that directly reflect cortical neuronal activity. Cortical neuronal dysfunction plays a major role in variable neuropsychiatric disorders, and a change in cortical activity under medication might reflect treatment response and could be useful for monitoring drug effects. ERP are the only methods with a sufficiently high time resolution for the analysis of the dynamic patterns of neuronal brain activity, e.g., synchronization and desynchronization, oscillations, coherence, gamma band activity, latency of event related activity, etc., which are crucial for a deeper understanding of functional (neurophysiological) correlates of cognitive, emotional and behavioral disturbances in neuropsychiatric patients. Methodological advances have further improved and strengthened the position of ERP concerning research and clinical application. The usefulness and applicability of ERP in determining and monitoring clinico-pharmacological effects will be summarized mainly by focussing on the auditory evoked P300 and the N1/P2 component of auditory evoked potentials. Owing to important recent developments in the field of brain functional diagnostics the combination of neurophysiological techniques and functional magnetic resonance imaging (fMRI) will be included.

Effects of the critical band on auditory-evoked magnetic fields

Changes in the bandwidth affect the perceived loudness of a stimulus even when the level of the stimulus remains fixed. If the bandwidth of a sound is varied while maintaining the overall intensity, the loudness remains constant as long as the bandwidth is less than the critical bandwidth. If the bandwidth is increased beyond the critical bandwidth, the loudness increases with increasing bandwidth. Human cortical responses as a function of stimulus bandwidth were examined by recording auditory-evoked magnetic fields. The results showed that the N1m magnitudes, that is, the estimated equivalent current dipole moments, increased with increasing bandwidth when the bandwidth was increased beyond the critical bandwidth.

fMRI studies of sensitivity and habituation effects within the auditory cortex at 1.5 T and 3 T

PURPOSE

To assess habituation effects in relation to field strength by fMRI at 1.5 vs. 3.0 T within the auditory cortex of healthy subjects.

MATERIALS AND METHODS

fMRI experiments were performed on 19 healthy subjects at 1.5 T (N = 12) and 3 T (N = 12). The auditory cortex was stimulated binaurally by digitally generated pulsed (nu = 5 Hz) 800 Hz sine tones with three alternating on and off periods.

RESULTS

The mean activation after stimulation (4.4% +/- 1.2% (1.5 T) and 5.3% +/- 2.3% (3 T)) and number of activated pixels (96.7 +/- 49.8 (1.5 T) and 139.9 +/- 101 (3 T)) were higher at 3 T compared to 1.5 T; however, that difference did not reach statistical significance. A characteristic signal decay with repeated stimuli was revealed at both 1.5 and 3 T, and the response to the second and third stimulation blocks was significantly lower compared to the first. The habituation pattern was the same, independently of field strength and age.

CONCLUSION

The mean activation and number of pixels were only modestly higher at 3 T, probably due to higher physiologic noise and higher local macroscopic susceptibility gradients within the temporal lobes at 3 T. Our data reveal that measured auditory habituation is independent of field strength, and data obtained at two different field strengths do not differ fundamentally in this context.

Sound level dependence of the primary auditory cortex: Simultaneous measurement with 61-channel EEG and fMRI

Sound level dependence has been investigated for years with event-related potentials (ERP). A serotonergic modulation of the sound level dependence only of the primary auditory cortex but not of the auditory association cortex has been suggested by a number of clinical and preclinical studies. Therefore, a precise covering of the activity of the primary auditory cortex seems necessary if sound level dependence is used as an indicator of the central serotonergic system. Recent fMRI studies described a pronounced sound level dependence only in the Heschl gyrus/primary auditory cortex but not in auditory association areas. In the present simultaneous 61-channel EEG and fMRI study investigating fourteen healthy subjects, we found a high correlation between the loudness-dependent change of the extent of fMRI activation (number of activated voxels) and the corresponding changes of the mean current source density within the same region of interest covering the primary auditory cortex (r = 0.84, P < 0.001). Our findings suggest a close relationship between the fMRI signal and event-related potential activity. In addition, the correspondence of the ERP-based data and the fMRI results further supports the validity of the ERP localization approach.

Distinct fMRI responses to laughter, speech, and sounds along the human peri-sylvian cortex

In this event-related fMRI study, 12 right-handed volunteers heard human laughter, sentential speech, and nonvocal sounds in which global temporal and harmonic information were varied whilst they were performing a simple auditory target detection. This study aimed to delineate distinct peri-auditory regions which preferentially respond to laughter, speech, and nonvocal sounds. Results show that all three types of stimuli evoked blood-oxygen-level-dependent responses along the left and right peri-sylvian cortex. However, we observed differences in regional strength and lateralization in that (i) hearing human laughter preferentially involves auditory and somatosensory fields primarily in the right hemisphere, (ii) hearing spoken sentences activates left anterior and posterior lateral temporal regions, (iii) hearing nonvocal sounds recruits bilateral areas in the medial portion of Heschl's gyrus and at the medial wall of the posterior Sylvian Fissure (planum parietale and parietal operculum). Generally, the data imply a differential regional sensitivity of peri-sylvian areas to different auditory stimuli with the left hemisphere responding more strongly to speech and with the right hemisphere being more amenable to nonspeech stimuli. Interestingly, passive perception of human laughter activates brain regions which control motor (larynx) functions. This observation may speak to the issue of a dense intertwining of expressive and receptive mechanisms in the auditory domain. Furthermore, the present study provides evidence for a functional role of inferior parietal areas in auditory processing. Finally, a post hoc conjunction analysis meant to reveal the neural substrates of human vocal timbre demonstrates a particular preference of left and right lateral parts of the superior temporal lobes for stimuli which are made up of human voices relative to nonvocal sounds.

Spatiotemporal properties of the BOLD response in the songbirds' auditory circuit during a variety of listening tasks

Auditory fMRI in humans has recently received increasing attention from cognitive neuroscientists as a tool to understand mental processing of learned acoustic sequences and analyzing speech recognition and development of musical skills. The present study introduces this tool in a well-documented animal model for vocal learning, the songbird, and provides fundamental insight in the main technical issues associated with auditory fMRI in these songbirds. Stimulation protocols with various listening tasks lead to appropriate activation of successive relays in the songbirds' auditory pathway. The elicited BOLD response is also region and stimulus specific, and its temporal aspects provide accurate measures of the changes in brain physiology induced by the acoustic stimuli. Extensive repetition of an identical stimulus does not lead to habituation of the response in the primary or secondary telencephalic auditory regions of anesthetized subjects. The BOLD signal intensity changes during a stimulation and subsequent rest period have a very specific time course which shows a remarkable resemblance to auditory evoked BOLD responses commonly observed in human subjects. This observation indicates that auditory fMRI in the songbird may establish a link between auditory related neuro-imaging studies done in humans and the large body of neuro-ethological research on song learning and neuro-plasticity performed in songbirds.

Cortical plasticity associated with stuttering therapy

Neuroimaging studies have indicated that persistent developmental stuttering (PDS) may be associated both with an abnormality in white matter of left-hemispheric speech areas and a right-hemispheric hyperactivity. The latter may compensate for the deficient structural connectivity in the left hemisphere. To investigate the effects of stuttering therapy on brain activity nine male adults with PDS underwent functional magnetic resonance imaging (fMRI) before and within 12 weeks after fluency shaping therapy. Brain response differences during overt sentence reading before and after therapy were assessed by utilizing random effects analyses. After therapy, a more widespread activation was observed in frontal speech and language regions and temporal areas of both hemispheres, particularly and more pronounced on the left side. Interestingly, distinct posttreatment left-sided activation increases were located directly adjacent to a recently detected area of white matter anomaly [M. Sommer, M.A. Koch, W. Paulus, C. Weiller, C. Buchel (2002). Disconnection of speech-relevant brain areas in persistent developmental stuttering. The Lancet, 360, 380-383] suggesting that fluency shaping techniques reorganize neuronal communication between left-sided speech motor planning, motor execution, and temporal areas. Hence, a therapeutic mechanism can be assumed to remodel brain circuitry close to the source of the dysfunction instead of reinforcing compensation via homologous contralateral brain networks.

EDUCATIONAL OBJECTIVES

The reader will learn about and be able to: (1) describe brain activation changes detected shortly after fluency-shaping therapy; (2) identify left-hemispheric regions where a (re)functionalization after fluency-shaping therapy seems to occur adjacent to a recently described abnormal white matter region in PDS subjects; and (3) discuss how an effective cerebral compensation mechanism for stuttering could work.

Right hemisphere specialization for intensity discrimination of musical and speech sounds

Sound intensity is the primary and most elementary feature of auditory signals. Its discrimination plays a fundamental role in different behaviours related to auditory perception such as sound source localization, motion detection, and recognition of speech sounds. This study was aimed at investigating hemispheric asymmetries for processing intensity of complex tones and consonant-vowel syllables. Forty-four right-handed non-musicians were presented with two dichotic matching-to-sample tests with focused attention: one with complex tones of different intensities (musical test) and the other with consonant-vowel syllables of different intensities (speech test). Intensity differences (60, 70, and 80 dBA) were obtained by altering the gain of a synthesized harmonic tone (260 Hz fundamental frequency) and of a consonant-vowel syllable (/ba/) recorded from a natural voice. Dependent variables were accuracy and reaction time. Results showed a significant clear-cut left ear advantage in both tests for both dependent variables. A monaural control experiment ruled out possible attentional biases. This study provides behavioural evidence of a right hemisphere specialization for the perception of the intensity of musical and speech sounds in healthy subjects.

Reliability estimation of grouped functional imaging data using penalized maximum likelihood

We analyzed grouped fMRI data and developed a reliability analysis for such data using the method of penalized maximum likelihood (ML). Specifically, this technique was applied to a somatosensory paradigm that used a mechanical probe to provide noxious stimuli to the foot, and a paradigm consisting of four levels of graded peripheral neuromuscular electrical stimulation (NMES). In each case, reliability maps of activation were generated. Receiver operating characteristic (ROC) curves were constructed in the case of the graded NMES paradigm for each level of stimulation, which revealed an increase in the specificity of activation with increasing stimulation levels. In addition, penalized ML was used to determine whether the grouped reliability maps obtained from one stimulus level were significantly different from those obtained at other levels. The results show a significant difference (P < 0.01) in the reliability of activation from one stimulation level to the next. These results are in agreement with those obtained using generalized linear modeling (GLM). While the reliability maps generated are not directly comparable, they are qualitatively similar to those obtained by controlling the expected false discovery rate (FDR). The proposed methodology can be used to objectively compare activation maps between groups, as well as to perform reliability assessments. Furthermore, this method potentially can be used to assess the longitudinal effect of treatment therapies within a group.

Functional MRI of the immediate impact of transcranial magnetic stimulation on cortical and subcortical motor circuits

Recent studies indicate that the cortical effects of transcranial magnetic stimulation (TMS) may not be localized to the site of stimulation, but spread to other distant areas. Using echo-planar imaging with blood-oxygenation-level-dependent (BOLD) contrast at 3 Tesla, we measured MRI signal changes in cortical and subcortical motor regions during high-frequency (3.125 Hz) repetitive TMS (rTMS) of the left sensorimotor cortex (M1/S1) at intensities above and below the active motor threshold in healthy humans. The supra- and subthreshold nature of the TMS pulses was confirmed by simultaneous electromyographic monitoring of a hand muscle. Suprathreshold rTMS activated a network of primary and secondary cortical motor regions including M1/S1, supplementary motor area, dorsal premotor cortex, cingulate motor area, the putamen and thalamus. Subthreshold rTMS elicited no MRI-detectable activity in the stimulated M1/S1, but otherwise led to a similar activation pattern as obtained for suprathreshold stimulation though at reduced intensity. In addition, we observed activations within the auditory system, including the transverse and superior temporal gyrus, inferior colliculus and medial geniculate nucleus. The present findings support the notion that re-afferent feedback from evoked movements represents the dominant input to the motor system via M1 during suprathreshold stimulation. The BOLD MRI changes in motor areas distant from the site of subthreshold stimulation are likely to originate from altered synaptic transmissions due to induced excitability changes in M1/S1. They reflect the capability of rTMS to target both local and remote brain regions as tightly connected constituents of a cortical and subcortical network.

Attentional modulation of human auditory cortex

Attention powerfully influences auditory perception, but little is understood about the mechanisms whereby attention sharpens responses to unattended sounds. We used high-resolution surface mapping techniques (using functional magnetic resonance imaging, fMRI) to examine activity in human auditory cortex during an intermodal selective attention task. Stimulus-dependent activations (SDAs), evoked by unattended sounds during demanding visual tasks, were maximal over mesial auditory cortex. They were tuned to sound frequency and location, and showed rapid adaptation to repeated sounds. Attention-related modulations (ARMs) were isolated as response enhancements that occurred when subjects performed pitch-discrimination tasks. In contrast to SDAs, ARMs were localized to lateral auditory cortex, showed broad frequency and location tuning, and increased in amplitude with sound repetition. The results suggest a functional dichotomy of auditory cortical fields: stimulus-determined mesial fields that faithfully transmit acoustic information, and attentionally labile lateral fields that analyze acoustic features of behaviorally relevant sounds.

fMRI of the auditory system: understanding the neural basis of auditory gestalt

Functional magnetic resonance imaging (fMRI) has rapidly become the most widely used imaging method for studying brain functions in humans. This is a result of its extreme flexibility of use and of the astonishingly detailed spatial and temporal information it provides. Nevertheless, until very recently, the study of the auditory system has progressed at a considerably slower pace compared to other functional systems. Several factors have limited fMRI research in the auditory field, including some intrinsic features of auditory functional anatomy and some peculiar interactions between fMRI technique and audition. A well known difficulty arises from the high intensity acoustic noise produced by gradient switching in echo-planar imaging (EPI), as well as in other fMRI sequences more similar to conventional MR sequences. The acoustic noise interacts in an unpredictable way with the experimental stimuli both from a perceptual point of view and in the evoked hemodynamics. To overcome this problem, different approaches have been proposed recently that generally require careful tailoring of the experimental design and the fMRI methodology to the specific requirements posed by the auditory research. The novel methodological approaches can make the fMRI exploration of auditory processing much easier and more reliable, and thus may permit filling the gap with other fields of neuroscience research. As a result, some fundamental neural underpinnings of audition are being clarified, and the way sound stimuli are integrated in the auditory gestalt are beginning to be understood.

Cortical bases of speech perception:evidence from functional lesion studies

Functional lesion studies have yielded new information about the cortical organization of speech perception in the human brain. We will review a number of recent findings, focusing on studies of speech perception that use the techniques of electrocortical mapping by cortical stimulation and hemispheric anesthetization by intracarotid amobarbital. Implications for recent developments in neuroimaging studies of speech perception will be discussed. This discussion will provide the framework for a developing model of the cortical circuitry critical for speech perception.

Stimulating music increases motor coordination in patients afflicted with Morbus Parkinson

The present study measured the short-term effect of special stimulating music on motor coordination in Parkinson patients. Eleven patients with a dominant akinetic Parkinson syndrome as well as ten healthy persons (age-matched control group) participated in this study. In the Parkinson group, the measurement of fine motor coordination with the 'Vienna Test System' showed an improvement in two (aiming, line tracking) of the four subtests after listening to the music. The patients improved their performance with the right arm significantly in the subtest aiming-error-time. No statistical differences were found in the other two subtests (steadiness, tapping) in both groups. There was also no improvement in frequency of tapping movement on the power-force-working-plate. Accordingly, music effects more the precision of a movement than the speediness. The measurements on the power-force-working-plate showed a significant improvement in two of five measured parameters: contact time, variability coefficient for total step and impact maximum changed significantly. This study gives evidence that specific music can improve the precision of arm and finger movements.

Silent Functional Magnetic Resonance Imaging (fMRI) of Tonotopicity and Stimulus Intensity Coding in Human Primary Auditory Cortex

OBJECTIVES

The aims of this study were to determine the feasibility of obtaining auditory cortex activation evoked by pure tones presented at threshold and suprathreshold hearing levels, to evaluate tonotopicity of the primary auditory cortex, and to determine the effect of stimulus intensity on auditory cortex activation using silent functional magnetic resonance imaging (fMRI).

METHODS

Sixteen subjects with normal hearing underwent silent fMRI. An audiometer was used to deliver pure tones of 1000, 2000, and 4000 Hz to the left ear. Two levels of acoustic stimulation were used: 1). threshold, hearing level determined in the scanner room and 2). suprathreshold, 70 dB hearing loss (HL). Tonotopicity and stimulus intensity coding was assessed on the basis of the location, extent, and amount of the auditory cortex activation.

RESULTS

The localization of activation moved to more medial and posterior regions of the primary auditory cortex as the frequency of the pure tone increased. Compared with a threshold stimulus, a suprathreshold stimulus evoked the same regions with increased spatial extent. The average increase in the right auditory cortex activation in response to suprathreshold stimulus was 57% at 1000, 51% at 2000, and 45% at 4000 Hz compared with that activated by the threshold stimulus.

CONCLUSIONS

Silent fMRI can be used to evaluate auditory cortex activation using low-intensity stimuli. The level of stimulus intensity increases the amount of auditory cortex activation and influences the fMRI mapping of the tonotopic organization of the primary auditory cortex.

Learning new sounds of speech: reallocation of neural substrates

Functional magnetic resonance imaging (fMRI) was used to investigate changes in brain activity related to phonetic learning. Ten monolingual English-speaking subjects were scanned while performing an identification task both before and after five sessions of training with a Hindi dental-retroflex nonnative contrast. Behaviorally, training resulted in an improvement in the ability to identify the nonnative contrast. Imaging results suggest that the successful learning of a nonnative phonetic contrast results in the recruitment of the same areas that are involved during the processing of native contrasts, including the left superior temporal gyrus, insula-frontal operculum, and inferior frontal gyrus. Additionally, results of correlational analyses between behavioral improvement and the blood-oxygenation-level-dependent (BOLD) signal obtained during the posttraining Hindi task suggest that the degree of success in learning is accompanied by more efficient neural processing in classical frontal speech regions, and by a reduction of deactivation relative to a noise baseline condition in left parietotemporal speech regions.

FMRI brain activation in a finnish family with specific language impairment compared with a normal control group

The aim of the present study was to investigate differences in brain activation in a family with SLI as compared to intact individuals with normally developed language during processing of language stimuli. Functional magnetic resonance imaging (fMRI) was used to monitor changes in neuronal activation in temporal and frontal lobe areas in 5 Finnish family members with specific language impairment (SLI) and 6 individuals in an intact control group. Magnetic resonance (MR) image acquisitions were made while the participants listened to series of isolated vowel sounds, pseudowords, and real words. The stimuli were digitized single Finnish vowel sounds, 3-phoneme pseudowords, and 3- and 4-phoneme real words. MR scanning was made with a 1.5 T Siemens Vision Plus scanner, and the auditory stimuli were presented according to an event-related fMRI design. The results showed significant differences between the family with SLI and the intact control group with regard to brain activation in areas in the temporal and frontal lobes. Temporal lobe activation differences were most pronounced in the middle temporal gyrus bordering the superior temporal sulcus. The control participants also activated an area in the inferior frontal lobe in BA 44. It is concluded that individuals with SLI showed reduced activation in brain areas that are critical for speech processing and phonological awareness. The present functional brain imaging data fit well with other recent imaging data that also showed structural abnormalities in the same and neighboring areas.