Intensity coding of auditory stimuli: an fMRI study

Acoustic noise concerns in functional magnetic resonance imaging

Magnetic resonance (MR) acoustic scanner noise may negatively affect the performance of functional magnetic resonance imaging (fMRI), a problem that worsens at the higher field strengths proposed to enhance fMRI. We present an overview of the current knowledge on the effects of confounding acoustic MR noise in fMRI experiments. The principles and effectiveness of various methods to reduce acoustic noise in fMRI are discussed, practical considerations are addressed and recommendations are made.

Functional Magnetic Resonance Imaging of Activation in Subcortical Auditory Pathway

OBJECTIVES

Functional magnetic resonance imaging (fMRI) has been used to investigate activation of the auditory cortex; however, assessment of activation in the subcortical auditory pathway has been challenging. The aim of this study was to examine neural correlates of cortical and subcortical auditory activation evoked by pure-tone stimulus using silent fMRI.

STUDY DESIGN

Prospective analysis.

METHODS

Seventeen normal-hearing volunteers (7 male, 10 female; age range, 14-37 yrs) underwent silent fMRI. An audiometer was used to deliver pure tones of 1000 Hz to the left ear. Pure tones were presented at hearing thresholds determined in the scanner. Brain regions showing increased activation during pure-tone stimulus presentation were mapped and auditory activations exceeding P <.001 were included in the analysis.

RESULTS

Pure-tone stimuli evoked bilateral activation in cortical regions of the transverse and superior temporal gyri and the planum temporale. Activation in subcortical structures included the medial geniculate body, inferior colliculus, lateral lemniscus, superior olivary complex, and cochlear nucleus.

CONCLUSIONS

Silent functional magnetic resonance imaging findings documented the feasibility of detecting activation elicited by pure tone along the cortical and subcortical auditory pathway. The use of this technique in the assessment of disorders with auditory dysfunction merits further investigation.

Spectrotemporal features of the auditory cortex: the activation in response to dynamic ripples

Functional MRI was used to investigate the characteristics of the human cerebral response to dynamic ripples. Dynamic ripples are sound stimuli containing regular spectrotemporal modulations, which are of major importance in speech processing; however, in contrast to speech, dynamic ripples can be characterized fully by a limited number of parameters. Extensive activation consisting of multiple separate regions was found bilaterally in the auditory cortex, particularly along the Heschl's gyri. This agrees with the presence of a structural cortical subdivision into functional fields. The level and the extent of activation were measured and correlated highly (R(2) = 0.97). Both measures depended strongly on the spectral density, temporal frequency, and amplitude of the modulations and matched the perceptual discernibility of the spectrotemporal modulations. The largest responses occurred for parameter values near the optimal human sensitivity. The drift direction of the modulations did not influence the activation. No quantitative differences were found between the two hemispheres. Average brain activation levels proved to be separable with regard to the spectral density and temporal frequency of the modulations. Topographic mappings of the modulation density and frequency onto the cortical surface were shown, approximately in posterolateral-to-anteromedial and lateral-to-medial directions, respectively. Posterolateral regions were most sensitive to spectrotemporal features at a scale similar to phonemes. Anteromedial regions, however, were also relatively sensitive to smaller scale acoustic features. This spatially dependent sensitivity suggests a functional topographic and hierarchical organization of the auditory cortex.

Evaluation of auditory cortex activation by using silent FMRI

PURPOSE

To evaluate auditory cortex activation evoked by pure-tone stimulus using silent functional magnetic resonance imaging (FMRI).

MATERIAL AND METHODS

Nine volunteers with normal hearing as determined with pure-tone audiometry were studied. Auditory cortex activation was evoked by pure-tone stimuli presented monaurally at 1,000, 2,000, and 4,000 Hz. Images of auditory cortex activation were obtained using silent and conventional FMRI techniques. Heschl's gyrus activation was evaluated by the number of voxels exceeding a predetermined significance level (P <.0001).

RESULTS

In both techniques, all subjects showed activation in the Heschl's gyrus. Silent FMRI detected more activation in all studied frequencies compared with the conventional FMRI. The observed difference in the Heschl's gyrus activation between the techniques reached statistical significance for 1,000 Hz frequency (P <.05).

CONCLUSIONS

The amount of Heschl's gyrus activation detected with silent FMRI is greater than that of conventional FMRI. Silent FMRI technique can be used to acquire functional images of the auditory cortex without the confounding effects of scanner noise.

Enhanced activation of the auditory cortex in patients with inner-ear hearing impairment: a magnetoencephalographic study

OBJECTIVE

Injury of peripheral auditory organ often induces abnormality of loudness sensation such as loudness recruitment. However, objective evaluation of this phenomenon has rarely been performed. To elucidate this abnormal loudness sensation, cortical mechanisms were investigated by recording auditory evoked magnetic fields (AEFs).

METHODS

We recorded AEFs in 8 patients suffering from inner-ear hearing impairment with loudness recruitment and in 14 healthy hearing controls using a 122-channel whole-head neuromagnetometer. Tone bursts of 1 kHz were presented monaurally at 4 different intensities (40, 50, 60, 70 dB HL) with a constant interstimulus interval of 1 s.

RESULTS

In both groups, the 100 ms response (N100m) increased in amplitude and decreased in latency as a function of stimulus intensity in both hemispheres. Concerning the source strength, increment of dipole moment of N100m was more rapid according to the stimulus intensity in patients compared with that in healthy subjects. Source strength of N100m was enhanced at high stimulus intensity in patients, and its ratio to healthy subjects was 1.08 at 50 dB, 1.69 at 60 dB and 2.04 at 70 dB.

CONCLUSIONS

In patients with inner-ear hearing impairment, enhanced activation of the auditory cortex was observed, and may help explain loudness recruitment.

Assessment of reliability in functional imaging studies

PURPOSE

To investigate the reliability of functional magnetic resonance imaging (fMRI), an approach for mapping and quantifying reliably activated voxels was developed.

MATERIALS AND METHODS

First, a SPM99 analysis was performed, and the resulting statistical maps were taken as the basis for subsequent analyses of reliability. Several approaches were demonstrated using 1). a voxel-wise intraclass correlation coefficient (ICC); 2). an analysis of scatter plots, calculating the correlation of contrast t-values for pairs of activation maps; and 3). the ratio of overlapping volumes as suggested in the literature. The methods were applied to an fMRI study in which subjects were asked to vary their attentional effort during watching a flickering checkerboard pattern with varying letters in the center. The subjects had to ignore or attend to the presentation, or they had to detect a target letter within the checkerboard.

RESULTS

The imaging data showed good reliability in terms of ICC for regions of visual processing, as well as for frontal areas, especially in the letter detection task. Furthermore, the size of reliable clusters depended on the presumed attentional effort of the subjects.

CONCLUSION

Application of the method demonstrated that the activation due to visual stimulation could also be detected very consistently during a no-attend condition, but the reliability of the activations were best during the attended tasks.

Focused attention in a simple dichotic listening task: an fMRI experiment

Whole-head functional magnetic resonance imaging (fMRI) was used in nine neurologically intact subjects to measure the hemodynamic responses in the context of dichotic listening (DL). In order to eliminate the influence of verbal information processing, tones of different frequencies were used as stimuli. Three different dichotic listening tasks were used: the subjects were instructed to either concentrate on the stimuli presented in both ears (DIV), or only in the left (FL) or right (FR) ear and to monitor the auditory input for a specific target tone. When the target tone was detected, the subjects were required to indicate this by pressing a response button. Compared to the resting state, all dichotic listening tasks evoked strong hemodynamic responses within a distributed network comprising of temporal, parietal, and frontal brain areas. Thus, it is clear that dichotic listening makes use of various cognitive functions located within the dorsal and ventral stream of auditory information processing (i.e., the 'what' and 'where' streams). Comparing the three different dichotic listening conditions with each other only revealed a significant difference in the pre-SMA and within the left planum temporale area. The pre-SMA was generally more strongly activated during the DIV condition than during the FR and FL conditions. Within the planum temporale, the strongest activation was found during the FR condition and the weakest during the DIV condition. These findings were taken as evidence that even a simple dichotic listening task such as the one used here, makes use of a distributed neural network comprising of the dorsal and ventral stream of auditory information processing. In addition, these results support the previously made assumption that planum temporale activation is modulated by attentional strategies. Finally, the present findings uncovered that the pre-SMA, which is mostly thought to be involved in higher-order motor control processes, is also involved in cognitive processes operative during dichotic listening.

Pre-frontal executive committee for perception, working memory, attention, long-term memory, motor control, and thinking: a tutorial review

As an explicit organizing metaphor, memory aid, and conceptual framework, the prefrontal cortex may be viewed as a five-member 'Executive Committee,' as the prefrontal-control extensions of five sub-and-posterior-cortical systems: (1) the 'Perceiver' (dominant-right-hemisphere ventral-lateral prefrontal cortex--VL/PERC-PFC) is the frontal extension of the ventral perceptual stream (the VL/PERC system) which represents the world and self in object coordinates; (2) the 'Verbalizer' (dominant-left-hemisphere ventral-lateral prefrontal cortex system--VL/VERB-PFC) is the frontal extension of the language stream (the VL/VERB system) which represents the world and self in language coordinates; (3) the 'Motivator' (ventral/medial-orbital pre-frontal cortex--VMO-PFC) is the frontal cortical extension of a subcortical extended-amygdala stream (the VMO system) which represents the world and self in motivational/emotional coordinates; (4) the 'Attender' (dorsal-medial/anterior cingulate--DM/AC-PFC) is the frontal cortical extension of a subcortical extended-hippocampal stream (the DM/AC system) which represents the world and self in spatiotemporal coordinates and directs attention to internal and external events; and (5) the 'Coordinator' (the dorsolateral prefrontal cortex--DL-PFC) is the frontal extension of the dorsal perceptual stream (the DL system) which represents the world and self in body- and eye-coordinates and controls willed action and working memory. This tutorial review examines the interacting roles of these five systems in perception, working memory, attention, long-term memory, motor control, and thinking.

Intensity-dependent activation of the primary auditory cortex in functional magnetic resonance imaging

PURPOSE

The purpose of this study was to investigate the activation patterns of the primary auditory cortex in response to varying intensities of pure tone stimuli.

METHOD

A 1,000-Hz pure tone stimulus was delivered monaurally to the right ear of 12 normal-hearing right-handed volunteers in 20-second on-off cycles. Stimuli were applied at 20 and 50 dB hearing level (HL) above threshold in 12 subjects and at 0, 20, 40, and 50 dB HL above threshold in 6 subjects. Functional magnetic resonance imaging (fMRI) data were obtained using a 1.5-T scanner and echoplanar imaging. Activated pixels were identified in the transverse temporal gyrus (TTG) of both hemispheres in response to pure tone stimuli at each intensity level using cross-correlation analysis (0.6; P < 0.0001).

RESULTS

Of the 24 right and left TTGs imaged (n = 12), activation to pure tone stimuli at 20 and 50 dB HL above threshold was seen in 46% and 79% of TTGs, respectively, with bilateral hemispheric activation in 27% and 64% of subjects, respectively. The mean numbers of activated voxels were 4.0 and 13.0, respectively. Of the 12 right and left TTGs imaged at 0, 20, 40, and 50 dB HL above threshold, activation was seen in 33%, 42%, 58%, and 75% of TTGs, respectively. The mean numbers of activated voxels were 5.8, 3.2, 9.8, and 15.3, respectively. There was a nonsignificant trend toward contralateral (left) dominant TTG activation with increased tone intensity.

CONCLUSION

Our results show an increased likelihood of TTG activation, increased TTG activation volume, and increased bilateral hemisphere TTG activation with increasing pure tone intensity. Our results suggest that the primary auditory cortex reflects or is directly involved in the central processing of sound intensity and that varying the intensity of even simple stimuli can alter the patterns of fMRI activation in auditory cortex.

Relationships between human auditory cortical structure and function

The human auditory cortex comprises multiple areas, largely distributed across the supratemporal plane, but the precise number and configuration of auditory areas and their functional significance have not yet been clearly established. In this paper, we discuss recent research concerning architectonic and functional organisation within the human auditory cortex, as well as architectonic and neurophysiological studies in non-human species, which can provide a broad conceptual framework for interpreting functional specialisation in humans. We review the pattern in human auditory cortex of the functional responses to various acoustic cues, such as frequency, pitch, sound level, temporal variation, motion and spatial location, and we discuss their correspondence to what is known about the organisation of the auditory cortex in other primates. There is some neuroimaging evidence of multiple tonotopically organised fields in humans and of functional specialisations of the fields in the processing of different sound features. It is thought that the primary area, on Heschl's gyrus, may have a larger involvement in processing basic sound features, such as frequency and level, and that posterior non-primary areas on the planum temporale may play a larger role in processing more spectrotemporally complex sounds. Ways in which current knowledge of auditory cortical organisation and different data analysis approaches may benefit future functional neuroimaging studies which seek to link auditory cortical structure and function are discussed.

Acoustic noise and functional magnetic resonance imaging: current strategies and future prospects

Functional magnetic resonance imaging (fMRI) has become the method of choice for studying the neural correlates of cognitive tasks. Nevertheless, the scanner produces acoustic noise during the image acquisition process, which is a problem in the study of auditory pathway and language generally. The scanner acoustic noise not only produces activation in brain regions involved in auditory processing, but also interferes with the stimulus presentation. Several strategies can be used to address this problem, including modifications of hardware and software. Although reduction of the source of the acoustic noise would be ideal, substantial hardware modifications to the current base of installed MRI systems would be required. Therefore, the most common strategy employed to minimize the problem involves software modifications. In this work we consider three main types of acquisitions: compressed, partially silent, and silent. For each implementation, paradigms using block and event-related designs are assessed. We also provide new data, using a silent event-related (SER) design, which demonstrate higher blood oxygen level-dependent (BOLD) response to a simple auditory cue when compared to a conventional image acquisition.

Heschl's gyrus is more sensitive to tone level than non-primary auditory cortex

Previous neuroimaging studies generally demonstrate a growth in the cortical response with an increase in sound level. However, the details of the shape and topographic location of such growth remain largely unknown. One limiting methodological factor has been the relatively sparse sampling of sound intensities. Additionally, most studies have either analysed the entire auditory cortex without differentiating primary and non-primary regions or have limited their analyses to Heschl's gyrus (HG). Here, we characterise the pattern of responses to a 300-Hz tone presented in 6-dB steps from 42 to 96 dB sound pressure level as a function of its sound level, within three anatomically defined auditory areas; the primary area, on HG, and two non-primary areas, consisting of a small area lateral to the axis of HG (the anterior lateral area, ALA) and the posterior part of auditory cortex (the planum temporale, PT). Extent and magnitude of auditory activation increased non-linearly with sound level. In HG, the extent and magnitude were more sensitive to increasing level than in ALA and PT. Thus, HG appears to have a larger involvement in sound-level processing than does ALA or PT.

Anatomical correlates of learning novel speech sounds

We examined the relationship between brain anatomy and the ability to learn nonnative speech sounds, as well as rapidly changing and steady-state nonlinguistic sounds, using voxel-based morphometry in 59 healthy adults. Faster phonetic learners appeared to have more white matter in parietal regions, especially in the left hemisphere. The pattern of results was similar for the rapidly changing but not for the steady-state nonlinguistic stimuli, suggesting that morphological correlates of phonetic learning are related to the ability to process rapid temporal variation. Greater asymmetry in the amount of white matter in faster learners may be related to greater myelination allowing more efficient neural processing, which is critical for the ability to process certain speech sounds.

Asymmetric hemodynamic responses of the human auditory cortex to monaural and binaural stimulation

Applying whole-head functional magnetic resonance imaging (fMRI) in 11 neurologically intact subjects, hemodynamic responses to mon- or binaurally presented auditory stimuli were measured. To expand on previous studies in this research area, we used tones and consonant-vowel (CV) syllables. In one group of subjects (n=6) the perceived loudness of the monaurally presented stimuli were adjusted so that they matched the loudness of the binaurally presented stimuli. In a second group (n=5) no loudness adjustment was performed, thus the monaural stimuli were perceived less loud ( approximately 10 dB) than the binaural stimuli. These extensions allowed us to test whether CV syllables and tones produce different contralaterality effects (stronger hemodynamic responses in the auditory cortex contralateral to the stimulated ear) and whether binaural stimulation results in stronger activations in the auditory areas than during both monaural stimulation conditions (binaural summation) independent of loudness influences. In summary, we obtained the following findings: (1) strong contralaterality effects during monaural acoustic stimulation in the posterior superior temporal gyrus (STG) comprising the planum temporale and the dorsal bank of the superior temporal sulcus to CV syllables and tones; (2) the hemodynamic responses to contralaterally presented stimuli (during the monaural conditions) were mostly stronger than those to binaurally presented CV syllables; (3) there was no interaction between stimulus type and the size of the contralaterality effect; (4) there was no indication of binaural summation, rather we found stronger hemodynamic responses to the sum of both monaural stimulations (right and left ear) than to binaural stimulation in all auditory areas; (5) there were generally stronger hemodynamic responses to CV syllables than to tones in the posterior STG, while the hemodynamic responses to tones were stronger in the anterior part of the STG (temporal pole); and finally (6) there was no general difference in terms of hemodynamic response in the auditory cortex between the two groups when receiving either loudness-matched or non-loudness-matched monaural stimulation. These findings are discussed in the context of the underlying neurophysiological mechanisms, the peculiarities of functional fMRI, and the direct access and callosal relay models of hemispheric lateralization.

FMRI reveals brain regions mediating slow prosodic modulations in spoken sentences

By means of fMRI measurements, the present study identifies brain regions in left and right peri-sylvian areas that subserve grammatical or prosodic processing. Normal volunteers heard 1) normal sentences; 2) so-called syntactic sentences comprising syntactic, but no lexical-semantic information; and 3) manipulated speech signals comprising only prosodic information, i.e., speech melody. For all conditions, significant blood oxygenation signals were recorded from the supratemporal plane bilaterally. Left hemisphere areas that surround Heschl gyrus responded more strongly during the two sentence conditions than to speech melody. This finding suggests that the anterior and posterior portions of the superior temporal region (STR) support lexical-semantic and syntactic aspects of sentence processing. In contrast, the right superior temporal region, in especially the planum temporale, responded more strongly to speech melody. Significant brain activation in the fronto-opercular cortices was observed when participants heard pseudo sentences and was strongest during the speech melody condition. In contrast, the fronto-opercular area is not prominently involved in listening to normal sentences. Thus, the functional activation in fronto-opercular regions increases as the grammatical information available in the sentence decreases. Generally, brain responses to speech melody were stronger in right than left hemisphere sites, suggesting a particular role of right cortical areas in the processing of slow prosodic modulations.

The neural correlates of aversive auditory stimulation

Previous neuroimaging studies indicate that the human amygdala activates during exposure to aversive visual, olfactory and gustatory stimuli. To examine amygdala responses to aversive auditory stimuli, we exposed healthy human subjects to unpleasant sounds while regional cerebral blood flow (rCBF) was assayed with O-15 PET. Eight subjects, all of whom described themselves as reactive to aversive sounds, participated in the study. Relative to white noise, the aversive sounds produced significant rCBF increases in the lateral amygdala/claustrum region. Significant activations also localized to the dorsal brainstem, medial temporal pole, basal forebrain (nucleus accumbens), insula, right auditory association cortices, putamen, thalamus and cerebellum. These data indicate that the amygdala responds to aversive auditory stimuli in a manner similar to its response to unpleasant stimuli in other sensory modalities. The data further highlight a widely distributed network of cortical and subcortical areas activated during exposure to aversive sounds.

Phonetic perception and the temporal cortex

Recent functional neuroimaging studies have emphasized the role of the different areas within the left superior temporal sulcus (STS) for the perception of various speech stimuli. We report here the results of three independent studies additionally demonstrating hemodynamic responses in the vicinity of the planum temporale (PT). In these studies we used consonant-vowel (CV) syllables, tones, white noise, and vowels as acoustic stimuli in the context of whole-head functional magnetic resonance imaging, applying a long TR to attenuate possible masking effects by the scanner noise. To summarize, we obtained the following results for the contrasts comparing hemodynamic responses obtained during the perception of CV syllables compared to tones or white noise: (i) stronger activation in the vicinity of the left PT with two distinct foci of activation, one in a lateral position and the other more medial in the vicinity of Heschl's sulcus; (ii) stronger activation in the vicinity of the right PT; and (iii) stronger bilateral activation within the mid-STS. Further contrasts revealed the following findings: (iv) stronger bilateral activation to CV syllables than to vowels in the medial PT, (v) stronger left-sided activation to CV syllables than to vowels in the mid-STS, and (vi) stronger activation to CV syllables with voiceless initial consonants than to CV syllables with voiced initial consonants in the left medial PT. The results are compatible with the hypothesis that the STS contains neurons specialized for speech perception. However, these results also emphasize the role of the PT in the analysis of phonetic features, namely the voice-onset-time. Yet this does not mean that the PT is solely specialized for phonetic analysis. We hypothesize rather that the PT contains neurons specialized for the analysis of rapidly changing cues as was suggested by P. Tallal et al. (1993, Ann. N. Y. Acad. Sci. 682: 27-47).

Microelectrode and neuroimaging studies of central auditory function

Imaging studies in humans are revealing parallels with the functional organisation of the auditory brain discovered in microelectrode studies in animals: the rate of amplitude modulation generating the strongest response declines systematically from the lower brain stem to the cortex; an increase in sound level induces a higher level and a greater extent of activity; spectra are represented tonotopically in multiple cortical areas. There are also differences: evidence of organisation reflecting the sound level of the stimulus is absent in animals, but has been found in humans. Additionally, imaging has revealed functional specialisations which have not (yet) been located in animals: areas that respond more strongly to sounds with stronger pitches and to sounds that move in space. Microelectrode studies suggest that vocalisations are represented by spatially distributed populations of neurones in secondary auditory areas. In humans, likewise, activation progressively more specific to speech is found as the search moves from primary to secondary to accessory areas.

Sound-level-dependent representation of frequency modulations in human auditory cortex: a low-noise fMRI study

Recognition of sound patterns must be largely independent of level and of masking or jamming background sounds. Auditory patterns of relevance in numerous environmental sounds, species-specific vocalizations and speech are frequency modulations (FM). Level-dependent activation of the human auditory cortex (AC) in response to a large set of upward and downward FM tones was studied with low-noise (48 dB) functional magnetic resonance imaging at 3 Tesla. Separate analysis in four territories of AC was performed in each individual brain using a combination of anatomical landmarks and spatial activation criteria for their distinction. Activation of territory T1b (including primary AC) showed the most robust level dependence over the large range of 48-102 dB in terms of activated volume and blood oxygen level dependent contrast (BOLD) signal intensity. The left nonprimary territory T2 also showed a good correlation of level with activated volume but, in contrast to T1b, not with BOLD signal intensity. These findings are compatible with level coding mechanisms observed in animal AC. A systematic increase of activation with level was not observed for T1a (anterior of Heschl's gyrus) and T3 (on the planum temporale). Thus these areas might not be specifically involved in processing of the overall intensity of FM. The rostral territory T1a of the left hemisphere exhibited highest activation when the FM sound level fell 12 dB below scanner noise. This supports the previously suggested special involvement of this territory in foreground-background decomposition tasks. Overall, AC of the left hemisphere showed a stronger level-dependence of signal intensity and activated volume than the right hemisphere. But any side differences of signal intensity at given levels were lateralized to right AC. This might point to an involvement of the right hemisphere in more specific aspects of FM processing than level coding.

Short-term functional plasticity in the human auditory cortex: an fMRI study

Applying functional magnetic resonance imaging (fMRI) techniques, hemodynamic responses elicited by sequences of pure tones of 950 Hz (standard) and deviant tones of 952, 954, and 958 Hz were measured before and 1 week after subjects had been trained at frequency discrimination for five sessions (over 1 week) using an oddball procedure. The task of the subject was to detect deviants differing from the standard stimulus. Frequency discrimination improved during the training session for three subjects (performance gain: T+) but not for three other subjects (no performance gain: T-). Hemodynamic responses in the auditory cortex comprising the planum temporale, planum polare and sulcus temporalis superior significantly decreased during training only for the T+ group. These activation changes were strongest for those stimuli accompanied by the strongest performance gain (958 and 954 Hz). There was no difference with respect to the hemodynamic responses in the auditory cortex for the T- group and the control group (CO) who did not received any pitch discrimination training. The results suggest a plastic reorganization of the cortical representation for the trained frequencies which can be best explained on the basis of 'fast learning' theories.

Focused and nonfocused attention in verbal and emotional dichotic listening: an FMRI study

Functional magnetic resonance imaging (fMRI) was used to identify cortical regions which are involved in two dichotic listening tasks. During one task the subjects were required to allocate attention to both ears and to detect a specific target word (phonetic task), while during a second task the subjects were required to detect a specific emotional tone (emotional task). During three attentional conditions of each task, the subjects were required to focus attention to the right (FR) or left ear (FL), while during a third condition subjects were required to allocate attention to both ears simultaneously. In 11 right-handed male subjects, these dichotic listening tasks evoked strong activations in a temporofrontal network involving auditory cortices located in the temporal lobe and prefrontal brain regions. Hemodynamic responses were measured in the following regions of interest: Heschl's gyrus (HG), the planum polare (PP), the planum temporale (PT), the anterior superior temporal sulcus (aSTS), the posterior superior temporal sulcus (pSTS), and the inferior frontal gyrus region (IFG) of both hemispheres. The following findings were obtained: (1) the degree of activation in HG and PP depends on the direction of attention. In particular it was found that selectively attending to right-ear input led to increased activity specifically in the left HG and PP and attention to left ear input increased right-sided activity in these structures; (2) hemodynamic responses in the PT, aSTS, pSTS, and IFG were not modulated by the different focused-attention conditions; (3) hemodynamic responses in HG and PP in the nonforced conditions were the sum activation of the forced conditions; (4) there was no general difference between the phonetic and emotion tasks in terms of hemodynamic responses; (5) hemodynamic responses in the PT and pSTS were strongly left-lateralized, reflecting the specialization of these brain regions for language processing. These findings are discussed in the context of current theories of hemispheric specialization.

Functional magnetic resonance imaging measurements of sound-level encoding in the absence of background scanner noise

Effects of sound level on auditory cortical activation are seen in neuroimaging data. However, factors such as the cortical response to the intense ambient scanner noise and to the bandwidth of the acoustic stimuli will both confound precise quantification and interpretation of such sound-level effects. The present study used temporally "sparse" imaging to reduce effects of scanner noise. To achieve control for stimulus bandwidth, three schemes were compared for sound-level matching across bandwidth: component level, root-mean-square power and loudness. The calculation of the loudness match was based on the model reported by Moore and Glasberg [Acta Acust. 82, 335-345 (1996)]. Ten normally hearing volunteers were scanned using functional magnetic resonance imaging (tMRI) while listening to a 300-Hz tone presented at six different sound levels between 66 and 91 dB SPL and a harmonic-complex tone (F0= 186 Hz) presented at 65 and 85 dB SPL. This range of sound levels encompassed all three bases of sound-level matching. Activation in the superior temporal gyrus, induced by each of the eight tone conditions relative to a quiet baseline condition, was quantified as to extent and magnitude. Sound level had a small, but significant, effect on the extent of activation for the pure tone, but not for the harmonic-complex tone, while it had a significant effect on the response magnitude for both types of stimulus. Response magnitude increased linearly as a function of sound level for the full range of levels for the pure tone. The harmonic-complex tone produced greater activation than the pure tone, irrespective of the matching scheme for sound level, indicating that bandwidth had a greater effect on the pattern of auditory activation than sound level. Nevertheless, when the data were collapsed across stimulus class, extent and magnitude were significantly correlated with the loudness scale (measured in phons), but not with the intensity scale (measured in SPL). We therefore recommend the loudness formula as the most appropriate basis of matching sound level to control for loudness effects when cortical responses to other stimulus attributes, such as stimulus class, are the principal concern.

Broca's area and the discrimination of frequency transitions: a functional MRI study

The left inferior frontal lobe has been traditionally viewed as a "language area," although its involvement in the discrimination of rapid nonverbal frequency changes has been also shown. Using functional MRI, we studied seven healthy adults during discrimination of relatively slow (200 ms) tonal frequency glides. Compared to a control task, in which subjects indiscriminately responded to white noise bursts, tonal discrimination was associated with bilateral superior and middle temporal and medial frontal activations. Inferior frontal activations were bilateral, but stronger on the left. Contrary to previous studies comparing discrimination of slow frequency changes to rest, our results suggest that such discriminations-when compared to an auditory control task-activate the left inferior frontal gyrus. Our findings are consistent with a participation of Broca's area in nonlinguistic processes besides its known roles in semantic, syntactic, and phonological functions.

Cortical activations during paced finger-tapping applying visual and auditory pacing stimuli

In order to study neural systems which are involved in motor timing we used whole-brain functional resonance imaging while subjects performed a paced finger-tapping task (PFT) with their right index finger. During one condition, subjects were imaged while tapping in synchrony with tones separated by a constant interval (auditory synchronisation, AS), followed by tapping without the pacing stimulus (auditory continuation, AC). In another condition, subjects were imaged while tapping in synchrony with a visual stimulus presented at the same frequency as the tones (visual synchronisation, VS) followed by a tapping sequence without visual pacing (visual continuation, VC). The following main results were obtained: (1) tapping in the context of visual pacing was generally more variable than tapping in the context of auditory stimuli; (2) during all conditions, a fronto-parietal network was active including the dorsal lateral premotor cortex (dPMC), M1, S1, inferior parietal lobule (LPi), supplementary motor cortex (SMA), the right cerebellar hemisphere, and the paravermial region; (3) stronger activation in the bilateral ventral premotor cortex (vPMC), the left LPi, the SMA, the right inferior cerebellum, and the left thalamus during both auditory conditions (AS and AC) compared to the visual conditions (VS and VC); (4) stronger activation in the right superior cerebellum, the vermis, and the right LPi during the visual conditions (VS and VC); (5) similar activations for the AS and AC conditions; but (6) marked differences between the VS and VC conditions especially in the dorsal premotor cortex (dPMC) and LPi areas; and (7) finally, there were no activations in the auditory and visual cortices when the pacing stimuli were absent. These findings were taken as evidence for a general difference between the motor control modes operative during the auditory and visual conditions. Paced finger tapping in the context of auditory pacing stimuli relies more on brain structures subserving internal motor control while paced finger-tapping in the context of visual pacing stimuli relies on brain structures relying on the subserving processing or imagination of visual pacing stimuli.

The effect of sequence repeat time on auditory cortex stimulation during phonetic discrimination

Acoustic noise generated by the MR scanner gradient system during fMRI studies of auditory function is a very significant potential confound. Despite these deleterious effects, fMRI of the auditory cortex has been successful and numerous practitioners have circumvented the problem of acoustic masking noise. In the context of auditory cortex fMRI, the sequence repeat time (TR) has the effect of altering the length of time during which the scanner is quiet. Indeed, the move to whole-brain fMRI makes the problem of acoustic noise more acute and points to the need to examine the role of TR and its influence on the BOLD signal. The aim of this study was to examine the effect of varying the TR time on activation of auditory cortex during presentation and performance of a phonetic discrimination task. The results presented here demonstrate that the influence of sequence repeat time is considerable. For a short repeat time it is likely that the noise from the scanner is a significant mask and hinders accurate task performance. At the other extreme, a repeat time of 9 s is also suboptimal, probably due to attentional effects and lack of concentration and not least because of the longer overall measurement times. The results of this study point to a complicated interplay between psychophysical factors as well as physical parameters; attention, acoustic noise, total duration of the experiment, consideration of the volume of acquisition, and overall difficulty of the task have to be assessed and balanced. For the paradigm used here, the results suggest an optimal TR of around 6 s for a 16-slice acquisition.

Recognition of emotional prosody and verbal components of spoken language: an fMRI study

This study examined the neural areas involved in the recognition of both emotional prosody and phonemic components of words expressed in spoken language using echo-planar, functional magnetic resonance imaging (fMRI). Ten right-handed males were asked to discriminate words based on either expressed emotional tone (angry, happy, sad, or neutral) or phonemic characteristics, specifically, initial consonant sound (bower, dower, power, or tower). Significant bilateral activity was observed in the detection of both emotional and verbal aspects of language when compared to baseline activity. We found that the detection of emotion compared with verbal detection resulted in significant activity in the right inferior frontal lobe. Conversely, the detection of verbal stimuli compared with the detection of emotion activated left inferior frontal lobe regions most significantly. Specific analysis of the anterior auditory cortex revealed increased right hemisphere activity during the detection of emotion compared to activity during verbal detection. These findings illustrate bilateral involvement in the detection of emotion in language while concomitantly showing significantly lateralized activity in both emotional and verbal detection, in both the temporal and frontal lobes.

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.

Motivation effects in a dichotic listening task as evident from functional magnetic resonance imaging in human subjects

The present study addresses the effect of motivation on cerebral activity using functional magnetic resonance imaging. Five healthy volunteers performed a dichotic listening task in two sets of three trials during which high or low levels of achievement motivation were introduced. They were told that the first set would be used for calculation of intellectual capacity (high achievement motivation) and the second set for scanner calibration (neutral motivation). In three volunteers, high compared with neutral motivation produced activation in the right prefrontal cortex and the dorsal cingulate. We conclude that motivational effects may lead to significant activations and should be controlled in future cognitive imaging studies. We present preliminary evidence that right prefrontal and dorsal cingulate regions might be involved in motivational processes.

Attention modulates activity in the primary and the secondary auditory cortex: a functional magnetic resonance imaging study in human subjects

Using functional magnetic resonance imaging, ten healthy subjects were scanned whilst listening to consonant-vowel syllables under three different conditions: (i) a 'no-attention' condition required subjects to ignore the stimuli; (ii) an 'attend' condition requiring attentive listening to stimuli; (iii) a 'detect' condition requiring detection of a specific target syllable. Hemodynamic responses were measured in the primary and secondary auditory cortex. These three conditions were associated with significantly different activations in the primary and secondary auditory cortex. The strongest activations were found for the 'detect' condition, followed by the 'attend' condition. The weakest activation was evident during the 'no-attention' condition. There were also stronger activations in the left hemisphere and within the primary auditory cortex. These results suggest that the primary and secondary auditory cortex play a main role in the selective attention.

Influence of acoustic masking noise in fMRI of the auditory cortex during phonetic discrimination

The application of functional magnetic resonance imaging (fMRI) to study activation of auditory cortex suffers from one significant confounding factor, namely, that of the acoustic noise generated by the gradient system, which is an integral part of the imaging process. Earlier work has shown that it is indeed possible to distinguish cortical activation resulting from presentation of auditory stimuli despite the presence of background noise from the gradient system. The influence of acoustic noise from the gradient system of the MRI scanner on the blood oxygen level-dependent (BOLD) response during functional activation of the auditory cortex has been investigated in six healthy subjects with no hearing difficulties. Experiments were performed using gradient-echo echoplanar imaging (EPI) and a verbal, auditory discrimination paradigm, presented in a block-wise manner, in which carefully aligned consonant-vowel syllables were presented at a rate of 1 Hz. For each volunteer the experiment was repeated three times with all parameters fixed, except slice number, which was 4, 16, or 64. The positioning of the central four slices in each experiment was common. Thus, the fraction of TR during which the stimulus is on but no imaging is being performed, varies from almost zero, in the case of 64 slices, to over 8 seconds, in the case of four slices. Only the central four slices were of interest; additional slices simply generated acoustic noise and were discarded. During the four-slice experiment, all subjects showed a robust BOLD response in the superior temporal gyrus covering the primary and secondary auditory cortex. The spatial extent and the z-scores of the activated regions decreased with longer duration of gradient noise from the scanner. For a phonetic discrimination task, the results indicate that presentation of the stimulus during periods free from scanner noise leads to a more pronounced BOLD response.