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

Linguistic lateralization in adolescents with Down syndrome revealed by a dichotic monitoring test

Linguistic lateralization in 10 adolescents with Down syndrome (average age: 15.7 years), 15 adolescents with intellectual disabilities of unknown etiology (average age: 17.8 years), 2 groups of children without disabilities (11 children, average age: 4.7 years; 10 children, average age: 8.5 years), and 14 adolescents without disabilities (average age: 18.7 years) was examined, using a dichotic monitoring test (DMT). Different Japanese words with 2 consonant-vowel syllables were presented to each ear simultaneously. Participants pressed a button when they heard the target word. The younger children without disabilities and the adolescents with intellectual disabilities exhibited a right-ear advantage, whereas the adolescents with Down syndrome showed the reverse pattern, i.e., a left-ear advantage. These results suggest that there is atypical linguistic lateralization in adolescents with Down syndrome.

A dichotic listening study of attention control in older adults

The influence of age on attentional control of bottom-up processing was investigated with a dichotic-listening paradigm. The typical right-ear-advantage (REA) without specific attentional instructions was used as a measure of bottom-up processing (non-forced condition). Top-down attentional control was evaluated by instructing subjects to report only the right or left ear stimulus of the dichotic pair (forced-right, forced-left conditions). Both young and old participants showed a REA in the non-forced condition and an increased REA in the forced-right condition. The old group failed, however, to use attention to modulate the REA to the same degree as the younger participants in the forced-left condition. Only the young group showed a significant left ear advantage (LEA). The results are discussed in relation to cognitive decline in normal aging and in early stages of dementia.

Cortical and subcortical contributions to the attentive processing of speech

The neuroanatomical correlates of attentive listening were investigated with functional magnetic resonance imaging and an attention task in which listeners responded only to words that combined two specific attributes of voice and semantic content. This task was performed under two different attentive listening conditions: (i) diotically, with words presented sequentially, and (ii) dichotically, with male and female voices presented simultaneously but segregated to different ears. For both conditions, functional magnetic resonance imaging revealed bihemispheric but right-lateralized activity patterns in mid-prefrontal, anterior cingulate, and inferior parietal areas, as well as significant anterior insular and subcortical activation. Manipulating attentional demands under different listening conditions revealed an important role for right anterior insula, striatum, and thalamus in the regulation of attentive listening to spoken language.

Neuromagnetic functional coupling during dichotic listening of speech sounds

The present magnetoencephalography (MEG) study tested the hypothesis of a phase synchronization (functional coupling) of cortical alpha rhythms (about 6-12 Hz) within a "speech" cortical neural network comprising bilateral primary auditory cortex and Wernicke's areas, during dichotic listening (DL) of consonant-vowel (CV) syllables. Dichotic stimulation was done with the CV-syllable pairs /da/-/ba/ (true DL, yielded by stimuli having high spectral overlap) and /da/-/ka/ (sham DL, obtained with stimuli having poor spectral overlap). Whole-head MEG activity (165 sensors) was recorded from 10 healthy right-handed non-musicians showing right ear advantage in a speech DL task. Functional coupling of alpha rhythms was defined as the spectral coherence at the following bands: alpha 1 (about 6-8 Hz), alpha 2 (about 8-10 Hz), and alpha 3 (about 10-12) with respect to the peak of individual alpha frequency. Results showed an inverse pattern of functional coupling: during DL of speech sounds, spectral coherence of the high-band alpha rhythms increased between left auditory and Wernicke's areas with respect to sham DL, whereas it decreased between left and right auditory areas. The increase of functional coupling within the left hemisphere would underlie the processing of the syllable presented to the right ear, which arrives to the left auditory cortex without the interference of the other syllable presented to the left ear. Conversely, the decrease of inter-hemispherical coupling of the high-band alpha might be due to the fact that the two auditory cortices do not receive the same information from the ears during DL. These results suggest that functional coupling of alpha rhythms can constitute a neural substrate for the lateralization of auditory stimuli during DL.

Relationship of auditory verbal hallucinations with cerebral asymmetry in patients with schizophrenia: an event-related fMRI study

The purpose of this study was to explore whether there are differences in cerebral asymmetry between subgroups of schizophrenic patients with or without auditory verbal hallucinations (AVHs) and normal controls by using event-related functional magnetic resonance imaging (efMRI). A total of 26 Chinese Han male patients with paranoid schizophrenia (diagnosed by DSM IV, including 13 patients with AVHs and 13 patients without) and 13 matched normal controls were recruited for the present study. The participants had been instructed to listen to short sentences from left or right side and to indicate laterality during efMRI scanning. Functional data were acquired using a 1.5T MR, and were analyzed using statistical parametric mapping. A random-effect model was employed to asses the difference in blood oxygen level dependent response between "left-sided" and "right-sided" conditions. The results from the present study have shown (1) within group comparisons: right precuneus and right superior parietal lobule were significantly activated showed significantly greater activation by left-sided voices than right-sided ones in controls. However, no significant difference in activation was found in any brain region between left and right-sided voices in either of the two patient subgroups, (2) between group comparisons: in comparison with AVHs patients, right middle frontal gyrus (MFG) was markedly activated when control subjects were differentiating right-sided voices. In comparison with patients without AVHs, right-side stimuli significantly activated bilateral MFG and left postcentral gyrus in control group. Furthermore, compared to the non-hallucination group, left Wernicke's area, including supramarginal gyrus, angular gyrus and superior temporal gyrus, was significantly activated by both left and right-sided voices in the hallucination group. In summary, auditory-related asymmetry in control subjects is attenuated in schizophrenic patients. The symptoms of AVHs in schizophrenia are possibly correlated with left hemispheric, particularly auditory and language-related areas dysfunction.

Left hemisphere lateralisation of auditory hallucinations in schizophrenia: a dichotic listening study

INTRODUCTION

We propose that auditory hallucinations are internally generated speech misrepresentations that are lateralised to the left temporal lobe. If hallucinations are misrepresentations involving the speech perception area of the left temporal lobe, then hallucinating patients should have problems identifying a simultaneously presented external speech sound, especially when the sound is lateralised to the left hemisphere. Lateralisation of speech perception can be experimentally studied with the dichotic listening task with consonant-vowel syllables. We predicted a negative relation between frequency of auditory hallucinations and performance on the dichotic listening task.

METHOD

We studied 87 right-handed patients with schizophrenia. Hallucination scores were taken from the BPRS symptom scale. Right and left ear scores in the dichotic listening task were recorded. A right ear advantage is expected in healthy individuals, indicating left temporal lobe processing superiority. The patients were compared with 36 right-handed healthy reference subjects.

RESULTS

A gradual decrease in the ability to process and report the right ear stimulus with increasing frequency of hallucinations was seen in the schizophrenia patients. No such relationship was found for processing and reporting of the left ear stimulus. There were no significant correlations with negative symptoms. Thus, the results were not the consequence of illness severity. There was however a significant correlation with unusual thought content symptom, pointing to a relationship also between delusions and auditory hallucinations.

CONCLUSION

The results support that auditory hallucinations may be internally generated speech misrepresentations, originating in the left temporal lobe.

A new verbal reports fMRI dichotic listening paradigm for studies of hemispheric asymmetry

Dichotic listening (DL) is one of the most frequently used paradigms to study hemispheric asymmetry and has been employed in several neuroimaging studies. The classic behavioral DL paradigm requires the subject to give a verbal response on each trial, which may cause image artifacts due to head movements when applied to an imaging environment. In order to avoid such artifacts most studies have used modified versions of the classic DL paradigm, where no verbal response is required. The purpose of the present study was to test a new DL paradigm, specifically developed for the fMRI environment, which is based on collecting verbal responses and thus being as close as possible to the classic DL behavioral task. By employing a sparse-sampling EPI acquisition schema we attempted to limit the negative impact of overt speech on image quality. A 5-s 'silent gap' allowed for stimulus presentation and collection of a verbal response to occur between subsequent image acquisitions and served as a high-pass filter that was optimized to detect activations of interest. Hence, the contribution of response-related activations to the measured signal was reduced. Twelve healthy volunteers (six males and six females) participated in the study. In order to obtain a measure of reliability, all participants went through the classic DL paradigm three times. The results, based on the estimation of the intraclass correlation coefficient, functional probability maps as well as on laterality maps, showed consistent activation in the right and left superior temporal gyrus, left middle temporal gyrus, and right inferior temporal gyrus, thus replicating previous results with visual display and motor response DL paradigms. It is concluded that an fMRI DL paradigm based on overt verbal responses is feasible and could have general implications for future fMRI studies of speech perception and product in general.

Attentional and linguistic interactions in speech perception

The role of attention in speech comprehension is not well understood. We used fMRI to study the neural correlates of auditory word, pseudoword, and nonspeech (spectrally rotated speech) perception during a bimodal (auditory, visual) selective attention task. In three conditions, Attend Auditory (ignore visual), Ignore Auditory (attend visual), and Visual (no auditory stimulation), 28 subjects performed a one-back matching task in the assigned attended modality. The visual task, attending to rapidly presented Japanese characters, was designed to be highly demanding in order to prevent attention to the simultaneously presented auditory stimuli. Regardless of stimulus type, attention to the auditory channel enhanced activation by the auditory stimuli (Attend Auditory>Ignore Auditory) in bilateral posterior superior temporal regions and left inferior frontal cortex. Across attentional conditions, there were main effects of speech processing (word+pseudoword>rotated speech) in left orbitofrontal cortex and several posterior right hemisphere regions, though these areas also showed strong interactions with attention (larger speech effects in the Attend Auditory than in the Ignore Auditory condition) and no significant speech effects in the Ignore Auditory condition. Several other regions, including the postcentral gyri, left supramarginal gyrus, and temporal lobes bilaterally, showed similar interactions due to the presence of speech effects only in the Attend Auditory condition. Main effects of lexicality (word>pseudoword) were isolated to a small region of the left lateral prefrontal cortex. Examination of this region showed significant word>pseudoword activation only in the Attend Auditory condition. Several other brain regions, including left ventromedial frontal lobe, left dorsal prefrontal cortex, and left middle temporal gyrus, showed Attention x Lexicality interactions due to the presence of lexical activation only in the Attend Auditory condition. These results support a model in which neutral speech presented in an unattended sensory channel undergoes relatively little processing beyond the early perceptual level. Specifically, processing of phonetic and lexical-semantic information appears to be very limited in such circumstances, consistent with prior behavioral studies.

Spatial and cross-modal attention alter responses to unattended sensory information in early visual and auditory human cortex

Attending to a visual or auditory stimulus often requires irrelevant information to be filtered out, both within the modality attended and in other modalities. For example, attentively listening to a phone conversation can diminish our ability to detect visual events. We used functional magnetic resonance imaging (fMRI) to examine brain responses to visual and auditory stimuli while subjects attended visual or auditory information. Although early cortical areas are traditionally considered unimodal, we found that brain responses to the same ignored information depended on the modality attended. In early visual area V1, responses to ignored visual stimuli were weaker when attending to another visual stimulus, compared with attending to an auditory stimulus. The opposite was true in more central visual area MT+, where responses to ignored visual stimuli were weaker when attending to an auditory stimulus. Furthermore, fMRI responses to the same ignored visual information depended on the location of the auditory stimulus, with stronger responses when the attended auditory stimulus shared the same side of space as the ignored visual stimulus. In early auditory cortex, responses to ignored auditory stimuli were weaker when attending a visual stimulus. A simple parameterization of our data can describe the effects of redirecting attention across space within the same modality (spatial attention) or across modalities (cross-modal attention), and the influence of spatial attention across modalities (cross-modal spatial attention). Our results suggest that the representation of unattended information depends on whether attention is directed to another stimulus in the same modality or the same region of space.

Activation of human auditory cortex during speech perception: effects of monaural, binaural, and dichotic presentation

We used a rapid event-related functional magnetic resonance imaging (fMRI) paradigm to compare cortical activation to speech tokens presented monaurally to each ear, binaurally, and dichotically. Two forms of dichotic conditions were examined: one presented consonant-vowel (CV) syllables simultaneously to each ear while the other paired a CV syllable with a non-speech stimulus (band-limited noise). Right-handed adults were asked to differentially respond to serially presented target and distractor CV syllables. Activations were localized with reference to anatomic segmentation algorithms that allowed us to distinguish between activity in primary (PAC) and non-primary auditory cortex (NPAC). Monaural CV syllables presented to the right ear (CVR) produced highly asymmetric activations in left PAC and NPAC. A similar but reduced left hemisphere (LH) bias was evident in binaural presentation, when monaural syllables were paired with contra-aural noise, and in dichotic CV-CV presentations. However, LH activation was two times larger to CVR than any other condition, while RH activation to CVR was insubstantial. By contrast, a small rightward asymmetry in PAC activation was observed from monaural left ear (CVL) presentation. In all conditions except CVL, magnitude of response favored left PAC and NPAC. CV processing across different listening conditions disclosed complex interactions in activation. Our results confirm the superiority of left NPAC in speech processing and suggest comparable left lateralization in PAC. The findings suggest that monaural CV presentation may be more useful than previously anticipated. The paradigm developed here may hold some promise in investigations where abnormal hemispheric balance of speech processing is suspected.

Does attention play a role in dynamic receptive field adaptation to changing acoustic salience in A1?

Acoustic filter properties of A1 neurons can dynamically adapt to stimulus statistics, classical conditioning, instrumental learning and the changing auditory attentional focus. We have recently developed an experimental paradigm that allows us to view cortical receptive field plasticity on-line as the animal meets different behavioral challenges by attending to salient acoustic cues and changing its cortical filters to enhance performance. We propose that attention is the key trigger that initiates a cascade of events leading to the dynamic receptive field changes that we observe. In our paradigm, ferrets were initially trained, using conditioned avoidance training techniques, to discriminate between background noise stimuli (temporally orthogonal ripple combinations) and foreground tonal target stimuli. They learned to generalize the task for a wide variety of distinct background and foreground target stimuli. We recorded cortical activity in the awake behaving animal and computed on-line spectrotemporal receptive fields (STRFs) of single neurons in A1. We observed clear, predictable task-related changes in STRF shape while the animal performed spectral tasks (including single tone and multi-tone detection, and two-tone discrimination) with different tonal targets. A different set of task-related changes occurred when the animal performed temporal tasks (including gap detection and click-rate discrimination). Distinctive cortical STRF changes may constitute a "task-specific signature". These spectral and temporal changes in cortical filters occur quite rapidly, within 2min of task onset, and fade just as quickly after task completion, or in some cases, persisted for hours. The same cell could multiplex by differentially changing its receptive field in different task conditions. On-line dynamic task-related changes, as well as persistent plastic changes, were observed at a single-unit, multi-unit and population level. Auditory attention is likely to be pivotal in mediating these task-related changes since the magnitude of STRF changes correlated with behavioral performance on tasks with novel targets. Overall, these results suggest the presence of an attention-triggered plasticity algorithm in A1 that can swiftly change STRF shape by transforming receptive fields to enhance figure/ground separation, by using a contrast matched filter to filter out the background, while simultaneously enhancing the salient acoustic target in the foreground. These results favor the view of a nimble, dynamic, attentive and adaptive brain that can quickly reshape its sensory filter properties and sensori-motor links on a moment-to-moment basis, depending upon the current challenges the animal faces. In this review, we summarize our results in the context of a broader survey of the field of auditory attention, and then consider neuronal networks that could give rise to this phenomenon of attention-driven receptive field plasticity in A1.

Determining language laterality by fMRI and dichotic listening

For imaging studies on hemispheric specialization of the human brain, data about known functional asymmetries other than handedness would be valuable for a reliable interpretation of lateralized activation in individuals or groups of subjects. As certain aspects of language processing are observed to be a function of primarily the left, it can be used as a reference for other asymmetric processes such as sensory or cognitive skills. For analyzing language laterality, there are a variety of methods, but these differ in application or accuracy. In this study, we tested the reliability of two widely used methods - dichotic listening and fMRI - to determine language dominance in 30 individual subjects. The German adaptation of a dichotic listening test (Hättig, H., Beier, M., 2000. FRWT: a dichotic listening test for clinical and scientific contexts, Zeitschr f Neuropsychologie 11. 233-245.) classified 54% of the 26 right-handed subjects as left hemispheric dominant. The results of the fMRI paradigm (Fernández, G., de Greiff, A., von Oertzen, J., et al., 2001. Language mapping in less than 15 min: real-time functional MRI during routine clinical investigation. Neuroimage 14, 585-594.) tested on the same subjects, however, classified 92% of the right-handed subjects as left dominant. The main reason for this discrepancy was that the ear dominance score of many subjects in the dichotic listening test was too low to determine a reliable ear advantage. As a consequence, this specific dichotic listening test cannot be used to determine language laterality in individual subjects. On the other hand, the fMRI results are consistent with numerous studies showing left dominant language processing in more than 90% of right-handers. In some subjects, however, language laterality critically depends on the areas used to determine the laterality index.

Neuronal Modifications During Visuomotor Association Learning Assessed by Electric Brain Tomography

In everyday life specific situations need specific reactions. Through repetitive practice, such stimulus-response associations can be learned and performed automatically. The aim of the present EEG study was the illustration of learning dependent modifications in neuronal pathways during short-term practice of visuomotor associations. Participants performed a visuomotor association task including four visual stimuli, which should be associated with four keys, learned by trial and error. We assumed that distinct cognitive processes might be dominant during early learning e.g., visual perception and decision making. Advanced learning, however, might be indicated by increased neuronal activation in integration- and memory-related regions. For assessment of learning progress, visual- and movement-related brain potentials were measured and compared between three learning stages (early, intermediate, and late). The results have revealed significant differences between the learning stages during distinct time intervals. Related to visual stimulus presentation, Low Resolution Electromagnetic Brain Tomography (LORETA) revealed strong neuronal activation in a parieto-prefrontal network in time intervals between 100-400 ms post event and during early learning. In relation to the motor response neuronal activation was significantly increased during intermediate compared to early learning. Prior to the motor response (120-360 ms pre event), neuronal activation was detected in the cingulate motor area and the right dorsal premotor cortex. Subsequent to the motor response (68-430 ms post event) there was an increase in neuronal activation in visuomotor- and memory-related areas including parietal cortex, SMA, premotor, dorsolateral prefrontal, and parahippocampal cortex. The present study has shown specific time elements of a visuomotor-memory-related network, which might support learning progress during visuomotor association learning.

Neural control of playing a reversed piano: empirical evidence for an unusual cortical organization of musical functions

Using functional magnetic imaging techniques and neuropsychological tests, we studied a young male musician (C.S.) who performs at a professional level both on a regular piano keyboard and on a reverse keyboard (reversed right to left). The participant was left-handed, had left dominance for language but, remarkably, right dominance for the control of piano playing on both keyboards. With respect to music perception, C.S. showed left-sided activation dominance within the left superior temporal sulcus, which is normally associated with higher order auditory processing and right-sided activations in the secondary sensory cortex extending into the supramarginal gyrus. We suggest that C.S.'s pattern of functional asymmetry, characterized by audio-motor control using a right-sided network, could be a factor in his exceptional piano-playing ability on both the standard and reversed keyboard.

Listening in silence activates auditory areas: a functional magnetic resonance imaging study

Directing attention to some acoustic features of a sound has been shown repeatedly to modulate the stimulus-induced neural responses. On the contrary, little is known about the neurophysiological impact of auditory attention when the auditory scene remains empty. We performed an experiment in which subjects had to detect a sound emerging from silence (the sound was detectable after different durations of silence). Two frontal activations (right dorsolateral prefrontal and inferior frontal) were found, regardless of the side where sound was searched for, consistent with the well established role of these regions in attentional control. The main result was that the superior temporal cortex showed activations contralateral to the side where sound was expected to be present. The area extended from the vicinity of Heschl's gyrus to the surrounding areas (planum temporale/anterior lateral areas). The effect consisted of both an increase in the response to a sound delivered after attention was directed to detect its emergence and a baseline shift during the silent period. Thus, in absence of any acoustic stimulus, the search for an auditory input was found to activate the auditory cortex.

Dynamics of a temporo-fronto-parietal network during sustained spatial or spectral auditory processing

Animal and human studies have suggested that posterior temporal, parietal, and frontal regions are specifically involved in auditory spatial (location and motion) processing, forming a putative dorsal "where" pathway. We used scalp EEG and current density mapping to investigate the dynamics of this network in human subjects presented with a varying acoustic stream in a two-factor paradigm: spatial versus pitch variations, focused versus diverted attention. The main findings were: (i) a temporo-parieto-frontal network was activated during the whole duration of the stream in all conditions and modulated by attention; (ii) the left superior temporal cortex was the only region showing different activations for pitch and spatial variations. Therefore, parietal and frontal regions would be involved in task-related processes (attention and motor preparation), whereas the differential processing of acoustic spatial and object-related features seems to take place at the temporal level.

Selective influences of cross-modal spatial-cues on preattentive auditory processing: a whole-head magnetoencephalography study

The processing streams of the various sensory modalities are known to interact within the central nervous system. These interactions differ depending on the level of stimulus representation and attention. The current study focused on cross-sensory influences on stimulus change detection during unattended auditory processing. We employed an oddball paradigm to assess cortical processing using whole-head magnetoencephalography (MEG) in 20 volunteers. While subjects performed distraction tasks of varying difficulties, auditory duration deviants were applied randomly to the left or the right ear preceded (200-400 ms) by oculomotor, static visual, or flow field co-stimulation at either side. Mismatch fields were recorded over both hemispheres. Changes in gaze direction and static visual stimuli elicited the most reliable enhancement of deviance detection at the same side (most prominent at the right auditory cortex). Under both conditions, the lateralized unattended and unpredictive pre-cues acted analogously to shifts in selective attention, but were not reduced by attentional load. Thus, the early cognitive representation of sounds seems to reflect automatic cross-modal interference. Preattentive multisensory integration may provide the neuronal basis for orienting reactions to objects in space and thus for voluntary control of selective attention.

“Soundmorphing”: A new approach to studying speech perception in humans

A problem in current studies of brain activation in speech perception is that most studies use stimuli that are sampled from different categories. This study presents a new approach in creating acoustic stimuli by filtering real words in different ways, thus this enables one to 'morph' the sounds gradually from something like tones to real words while preserving temporal characteristics ("soundmorphing"). This will make it possible to explore steps between tone and speech processing on a gradual scale. This study aimed to investigate more closely the network, involved in the perception of speech and the decoding of auditory (speech-)stimuli. Sets of auditory stimuli were created by dividing real words into several frequency-bands and creating a randomised rearrangement of these components. Four different sets of stimuli were used, containing 1, 2, 3 or 4 frequency bands (total range: 0-2500 Hz). Only the latter one contained intelligible words. During the fMRI session, these four trial types were presented pseudo-randomised. In all conditions, significant activations of auditory cortex were observed. The number of activated voxels and their significance increased over the four trial types. In addition, Broca's area, SMA, left thalamus, and right cerebellum were activated in the most complex and most speech-like condition. Comparing these complexes with the tone-like trials, only the left auditory cortex, left SMA, and cingulate gyrus became significant. These activations reflect the bilateral processing of the auditory stimuli, but only the left temporal areas demonstrated an increasing BOLD response with an increasing number of presented frequency components.

Decrease of functional coupling between left and right auditory cortices during dichotic listening: An electroencephalography study

The present study focused on functional coupling between human bilateral auditory cortices and on possible influence of right over left auditory cortex during dichotic listening of complex non-verbal tones having near (competing) compared with distant non-competing fundamental frequencies. It was hypothesized that dichotic stimulation with competing tones would induce a decline of functional coupling between the two auditory cortices, as revealed by a decrease of electroencephalography coherence and an increase of directed transfer function from right (specialized for the present stimulus material) to left auditory cortex. Electroencephalograph was recorded from T3 and T4 scalp sites, overlying respectively left and right auditory cortices, and from Cz scalp site (vertex) for control purposes. Event-related coherence between T3 and T4 scalp sites was significantly lower for all electroencephalography bands of interest during dichotic listening of competing than non-competing tone pairs. This was a specific effect, since event-related coherence did not differ in a monotic control condition. Furthermore, event-related coherence between T3 and Cz and between T4 and Cz scalp sites showed no significant effects. Conversely, the directed transfer function results showed negligible influence at group level of right over left auditory cortex during dichotic listening. These results suggest a decrease of functional coupling between bilateral auditory cortices during competing dichotic stimuli as a possible neural substrate for the lateralization of auditory stimuli during dichotic listening.

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.

Bimanual versus unimanual coordination: what makes the difference?

Using fMRI, we investigated the neuronal structures controlling bimanual coordination applying a visuomotor coordination task. Recent studies suggest the existence of a widespread network for the neuronal control of bimanual coordination including primary sensorimotor cortices (M1/S1), lateral and medial premotor cortices (PMC, SMA), cingulate motor area (CMA), and cerebellum (CB). In the present study, subjects performed bimanual and unimanual tasks requiring the coordination of two fingers at a time to navigate a cursor on a computer screen. Thus, in contrast to previous studies, we are using appropriate unimanual control (UNI) tasks. By using this new motor task, we identified a similar activation network for uni- and bimanual movements. Subjects exhibited bilateral activations in PMC, SMA, posterior-parietal cortex (PPC), occipital, and inferiotemporal cortex, as well as in the contralateral M1/S1 and ipsilateral CB. We did not find any additional activation when comparing bimanual with unimanual conditions. The lack of significant activation in the comparison "bimanual > unimanual" gives reason to suggest that this network is not limited to the control of bimanual motor actions, but responsible for unimanually coordinated movements as well. Interestingly, we found stronger activations for unimanual as compared to bimanual coordination. We hypothesize that task difficulty (degrees of freedom to control, e.g., number of limbs) is more important in determining which network components are activated and to what extent, compared to the factor of bimanuality. It even seemed to be less demanding for the motor system to control the cursor bimanually compared to the unimanual performance with two adjacent fingers.

Inhibition of auditory cortical responses to ipsilateral stimuli during dichotic listening: evidence from magnetoencephalography

The present magnetoencephalography (MEG) study on auditory evoked magnetic fields (AEFs) was aimed at verifying whether during dichotic listening the contralateral auditory pathway inhibits the ipsilateral one, as suggested by behavioural and patient studies. Ten healthy subjects were given a randomized series of three complex tones (261, 293 and 391 Hz, 500 ms duration), which were delivered monotically and dichotically with different intensities [60, 70 or 80 dBA (audio decibels)]. MEG data were recorded from the right auditory cortex. Results showed that the M100 amplitude over the right auditory cortex increased progressively when tones of increasing intensity were provided at the ipsilateral (right) ear. This effect on M100 was abolished when a concurrent tone of constant intensity was delivered dichotically at the contralateral (left) ear, suggesting that the contralateral pathway inhibited the ipsilateral one. The ipsilateral inhibition was present only when the contralateral tone fundamental frequency was similar to the ipsilateral tone. It was proposed that the occlusion mechanism would be exerted in cortical auditory areas as the dichotic effects were observed at M100 but not M50 component. This is the first evidence showing a neurophysiological inhibition driven by the contralateral auditory pathway over the ipsilateral one during dichotic listening.

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.

'Hearing' syllables by 'seeing' visual stimuli

Currently it is discussed whether the same cortical areas are activated during the imagination of as during the actual presentation of specific stimuli. Some argue that mostly the secondary but not the primary sensory areas are active during imagination. Using functional magnetic resonance imaging we explored whether auditory verbal imagery of syllables has sufficient power to evoke haemodynamic responses in the auditory cortex. To overcome the detrimental effects of scanner noise, one group of subjects was trained to vividly imagine hearing a syllable while a flashlight was presented. A control group did not receive this training. We found that only the trained group revealed haemodynamic responses in the auditory cortex during auditory imagination while the control group showed no activation within the auditory cortex. Peak activations during auditory verbal imagery are located bilaterally within the superior temporal gyrus region in the vicinity of the planum temporale. While these secondary auditory areas are active during auditory verbal imagery, there was no activation in Heschl's gyrus. We hypothesize that auditory verbal imagery is associated with haemodynamic responses in secondary auditory and not primary auditory areas.

Dichotic listening reveals functional specificity in prefrontal cortex: an fMRI study

The present study used fMRI to investigate the relationship between stimulus presentation mode and attentional instruction in a free-report dichotic listening (DL) task with consonant-vowel (CV) syllables. Binaural and dichotic CV syllables were randomly presented to the subjects during four different instructional conditions: a passive listening instruction and three active instructions where subjects listened to both ears, right ear and left ear, respectively. The results showed that dichotic presentations activated areas in the superior temporal gyrus, middle and inferior frontal gyrus and the cingulate cortex to a larger extent than binaural presentations. Moreover, the results showed that increase of activation in these areas was differentially dependent on presentation mode and attentional instruction. Thus, it seems that speech perception, as studied with the DL procedure, involves a cortical network extending beyond primary speech perception areas in the brain, also including prefrontal cortex.