Cerebral mechanisms underlying orienting of attention towards auditory frequency changes

Brain mechanisms underlying detection of auditory frequency changes were studied with event-related potentials (ERPs) in 14 human subjects discriminating visual stimuli. Scalp-current density mapping revealed bilateral components of mismatch negativity (MMN) in frontal and auditory cortices. Deviance-related activations in frontal and temporal cortex began to be significant at 94 ms and 154 ms in the right hemisphere, and at 128 ms and 132 ms in the left hemisphere. The magnitude of MMN-neuroelectric currents from the left temporal cortex correlated significantly (r = -0.56, p < 0.05) with distraction caused by MMN-eliciting deviant tones. These results suggest a complex cerebral circuitry involved in frequency change detection and strongly support the role of this circuitry in driving attention involuntarily towards potentially relevant frequency changes in the acoustic environment.     

Selective tuning of the left and right auditory cortices during spatially directed attention

Effects of spatially directed auditory attention on human brain activity, as indicated by changes in regional cerebral blood flow (rCBF), were measured with positron emission tomography (PET). Subjects attended to left-ear tones, right-ear tones, or foveal visual stimuli presented at rapid rates in three concurrent stimulus sequences. It was found that attending selectively to the right-ear input activated the auditory cortex predominantly in the left hemisphere and vice versa. This selective tuning of the left and right auditory cortices according to the direction of attention was presumably controlled by executive attention mechanisms of the frontal cortex, where enhanced activation during auditory attention was also observed.     

Late auditory evoked potentials asymmetry revisited.

OBJECTIVE: To investigate brain asymmetries of the auditory evoked potential (AEP) N100, T-complex, and P200 in response to monaural stimulation. METHODS: Electroencephalographic (EEG) recordings from 68 channels were used to record auditory cortex responses to monaural stimulation from normal hearing participants (N=16). White-noise stimuli and 1000Hz tones were repeatedly presented to either the left or right ear. Source localization of the AEP N100 response was carried out with two symmetric regional sources placed into left and right auditory cortex. Regional source waveform amplitude and latency asymmetries were analyzed for tangential and radial activity explaining the N100, T-complex and P200 AEP components. RESULTS: Regional source waveform analysis showed that early tangential activity in the N100 latency range exhibited larger contralateral amplitudes and shorter latencies for both tone and noise monaural stimuli. Lateralized activity was significantly greater when tones or noise was presented to the left compared to the right ear (p<.001). The ear difference in the degree of lateralization arose due to hemispheric asymmetry. Significantly more tangential activity in the N100 latency range was recorded in the right compared to the left hemisphere in response to stimulation by either tones or noise (p<.001). Neither the radial activity modelling the T-complex, nor activity modelling the P200, showed robust ear or hemisphere differences. CONCLUSIONS: Regional source waveform analysis revealed that the extent of auditory evoked potential asymmetries depends on the ear and hemisphere examined. These findings have implications for future studies utilizing AEP asymmetries to examine normal auditory function or experience-related changes in the auditory cortex. SIGNIFICANCE: The right compared to the left auditory cortex may be more involved in processing monaurally presented tone and noise stimuli.     

The effects of frontal cortex lesions on event-related potentials during auditory selective attention

We compared electrophysiological indices of auditory selective attention in control subjects and in patients with unilateral lesions of the dorsolateral frontal lobes. In control subjects, ERPs following attended tones showed an enhanced negativity from 80 to 500 msec post-stimulus which had a different topographic distribution than the N120. Lesions of the frontal lobes reduced the attention-related negativity and impaired behavioral performance. The ERP reductions were equivalent in recordings obtained from electrodes placed over the damaged and intact cortex. A difference was noted between left and right frontal groups as a function of ear of delivery of the stimuli. Patients with left frontal lesions showed reduced attention effects following tones presented to either ear. Patients with right frontal lesions showed intact attention effects to right ear tones, but no attention-related negativity to left ear tones. When the left and right frontal groups were considered together, tones in ignored channels produced larger responses when presented to the ear contralateral to damaged cortex. These results underline the important role of the frontal lobes in processes of selective attention. Although the endogenous negativity produced in selective attention tasks does not appear to originate in dorsolateral frontal cortex, the frontal lobes exhibit a modulating influence upon it. In addition, the endogenous attention related negativity and exogenous N120 components apparently arise from different neural generators.     

Diminished auditory mismatch fields in dyslexic adults

Electroencephalographic studies have demonstrated smaller auditory responses to infrequent deviances of speech and nonspeech sounds in dyslexic than normal-reading subjects. We used a whole-scalp neuromagnetometer to study selectively reactivity of the auditory cortices to sound deviances in 8 dyslexic and 11 normal-reading adults. Within a monotonous sequence of 50-millisecond 1000 Hz binaural tones, tones of 920 and 1080 Hz occurred with 7% probability each. Magnetic mismatch fields, elicited by the stimulus deviances, were diminished in the left hemisphere of the dyslexic subjects. The results indicate deficient change detection in the left auditory cortex of right-handed dyslexic adults.     

Auditory agnosia and auditory spatial deficits following left hemispheric lesions: evidence for distinct processing pathways

Auditory recognition and auditory spatial functions were studied in four patients with circumscribed left hemispheric lesions. Patient FD was severely deficient in recognition of environmental sounds but normal in auditory localisation and auditory motion perception. The lesion included the left superior, middle and inferior temporal gyri and lateral auditory areas (as identified in previous anatomical studies), but spared Heschl's gyrus, the acoustic radiation and the thalamus. Patient SD had the same profile as FD, with deficient recognition of environmental sounds but normal auditory localisation and motion perception. The lesion comprised the postero-inferior part of the frontal convexity and the anterior third of the temporal lobe; data from non-human primates indicate that the latter are interconnected with lateral auditory areas. Patient MA was deficient in recognition of environmental sounds, auditory localisation and auditory motion perception, confirming that auditory spatial functions can be disturbed by left unilateral damage; the lesion involved the supratemporal region as well as the temporal, postero-inferior frontal and antero-inferior parietal convexities. Patient CZ was severely deficient in auditory motion perception and partially deficient in auditory localisation, but normal in recognition of environmental sounds; the lesion involved large parts of the parieto-frontal convexity and the supratemporal region. We propose that auditory information is processed in the human auditory cortex along two distinct pathways, one lateral devoted to auditory recognition and one medial and posterior devoted to auditory spatial functions.     

Neural correlates of auditory repetition priming: reduced fMRI activation in the auditory cortex

Repetition priming refers to enhanced or biased performance with repeatedly presented stimuli. Modality-specific perceptual repetition priming has been demonstrated behaviorally for both visually and auditorily presented stimuli. In functional neuroimaging studies, repetition of visual stimuli has resulted in reduced activation in the visual cortex, as well as in multimodal frontal and temporal regions. The reductions in sensory cortices are thought to reflect plasticity in modality-specific neocortex. Unexpectedly, repetition of auditory stimuli has resulted in reduced activation in multimodal and visual regions, but not in the auditory temporal lobe cortex. This finding puts the coupling of perceptual priming and modality-specific cortical plasticity into question. Here, functional magnetic resonance imaging was used with environmental sounds to reexamine whether auditory priming is associated with reduced activation in the auditory cortex. Participants heard environmental sounds (e.g., animals, machines, musical instruments, etc.) in blocks, alternating between initial and repeated presentations, and decided whether or not each sound was produced by an animal. Repeated versus initial presentations of sounds resulted in repetition priming (faster responses) and reduced activation in the right superior temporal gyrus, bilateral superior temporal sulci, and right inferior prefrontal cortex. The magnitude of behavioral priming correlated positively with reduced activation in these regions. This indicates that priming for environmental sounds is associated with modification of neural activation in modality-specific auditory cortex, as well as in multimodal areas.     

Several attention-related wave forms in auditory areas: a topographic study

The purpose of this study was to progress in the understanding of the electrogenesis of attention-related wave forms in order to highlight some of the underlying attentional processes. ERPs were recorded from 16 electrodes, from 12 subjects who attended selectively to either high or low pitch tones delivered at a constant inter-stimulus interval of 800 msec to either right or left ear, while ignoring a concurrent sequence of tones of the other pitch delivered to the other ear. The attention-related wave forms were obtained by subtracting ERPs to unattended tones from ERPs to the same tones when they were attended. These wave forms were topographically displayed by both potential maps and scalp current density maps and compared with the corresponding maps of the N1 component of the ERPs, to determine the similarity of their generators. It has been shown that the attention effect is expressed by at least two components in specific auditory areas, one of small amplitude, occurring during the ascending slope of the N1 component, sensitive to the pitch of the attended stimulus, and possibly originating in the supratemporal plane of the auditory cortex; another of large amplitude, peaking symmetrically over both hemispheres and having a different topography from that of the N1 component. As described by other authors, a third, later, component appears over frontal areas, but probably originates from deeper sources of the brain. Models of selective attention processes, particularly the 'attentional trace' concept, are discussed in the light of these results.     

Effects of the binaural auditory filter in the human brain

The effects of the binaural auditory filter in the human auditory cortex were examined by auditory-evoked magnetic fields. Two tones with different frequency separations, which were presented dichotically to the left and right ears, were used as the sound stimuli, with all signals presented at sound pressure level of 60 dB. The results indicated that the N1m amplitudes were approximately constant when the frequency separation was less than 100 Hz; however, the N1m amplitudes increased with increasing frequency separation when the frequency separation was greater than 200 Hz. These findings indicate that binaural auditory filter-like behavior might be reflected in N1m amplitudes.     

Auditory lexical decision, categorical perception, and FM direction discrimination differentially engage left and right auditory cortex

Recent neuroimaging and neuropsychological data suggest that speech perception is supported in bilaterally auditory areas. We evaluate this issue building on well-known behavioral effects. While undergoing positron emission tomography (PET), subjects performed standard auditory tasks: direction discrimination of frequency-modulated (FM) tones, categorical perception (CP) of consonant-vowel (CV) syllables, and word/non-word judgments (lexical decision, LD). Compared to rest, the three conditions led to bilateral activation of the auditory cortices. However, lateralization patterns differed as a function of stimulus type: the LD task generated stronger responses in the left, the FM task a stronger response in the right hemisphere. Contrasts between either words or syllables versus FM were associated with significantly greater activity bilaterally in superior temporal gyrus (STG) ventro-lateral to Heschl's gyrus. These activations extended into the superior temporal sulcus (STS) and the middle temporal gyrus (MTG) and were greater in the left. The same areas were more active in the LD than the CP task. In contrast, the FM task was associated with significantly greater activity in the right lateral-posterior STG and lateral MTG. The findings argue for a view in which speech perception is mediated bilaterally in the auditory cortices and that the well-documented lateralization is likely associated with processes subsequent to the auditory analysis of speech.     

EEG coherence measures during auditory hallucinations in schizophrenia

We studied the change in EEG alpha-band average coherence between auditory hallucination (AH) and non-auditory hallucination (non-AH) states in seven auditory hallucinating schizophrenia patients. Four cortical regions were considered based on the existing dominant models for auditory hallucinations, the inner speech model and the central auditory processing deficit (CAPD) model. Coherences between electrodes located over Broca's area (BA 44/45) and Wernicke's area (BA 22/42) and between electrodes located over left-right temporal cortices were examined. There was no significant change observed in the coherence between Broca's and Wernicke's areas, but a significant increase was observed in coherence between the left and right superior temporal cortices during AHs compared with non-AHs, suggesting increased bilateral coherence between auditory cortical areas. Since coherence is a pairwise measure of functional correlation between regions, our findings suggest abnormally increased synchrony between the left and right auditory cortices during AHs in schizophrenia. Further, a significant increase in relative power was observed in the left, but not in the right auditory cortex during AHs. Thus our findings support the CAPD model and are consistent with that which postulate reduced prosodic processing during AHs.     

Neural bases of categorization of simple speech and nonspeech sounds

Categorization is fundamental to our perception and understanding of the environment. However, little is known about the neural bases underlying the categorization of sounds. Using human functional magnetic resonance imaging (fMRI) we compared the brain responses to a category discrimination task with an auditory discrimination task using identical sets of sounds. Our stimuli differed along two dimensions: a speech-nonspeech dimension and a fast-slow temporal dynamics dimension. All stimuli activated regions in the primary and nonprimary auditory cortices in the temporal cortex and in the parietal and frontal cortices for the two tasks. When comparing the activation patterns for the category discrimination task to those for the auditory discrimination task, the results show that a core group of regions beyond the auditory cortices, including inferior and middle frontal gyri, dorsomedial frontal gyrus, and intraparietal sulcus, were preferentially activated for familiar speech categories and for novel nonspeech categories. These regions have been shown to play a role in working memory tasks by a number of studies. Additionally, the categorization of nonspeech sounds activated left middle frontal gyrus and right parietal cortex to a greater extent than did the categorization of speech sounds. Processing the temporal aspects of the stimuli had a greater impact on the left lateralization of the categorization network than did other factors, particularly in the inferior frontal gyrus, suggesting that there is no inherent left hemisphere advantage in the categorical processing of speech stimuli, or for the categorization task itself.     

精神分裂症幻听患者初级听觉皮质的脑磁图研究

目的 探讨在不同频率纯音刺激下男性精神分裂症幻听患者初级听觉皮质的脑磁图(MEG)定位.方法 对均为右利手的10例男性精神分裂症幻听患者(研究组)和11名男性健康受试者(对照组),分别给予频率为0.5,2,4,8 kHz的纯音刺激,强度90 dB,持续200ms,刺激声间隔1 s.用脑磁图设备记录刺激后产生的听觉诱发磁场,并将MEG资料叠加到核磁共振成像以获得磁源性影像.结果 (1)对照组初级听觉皮质均定位于双侧颞横回;与对照组比较,研究组右侧初级听觉皮质位置更靠近颞横回外部,左侧明显偏向颞上回后外下部(P<0.05).(2)在分别给予2 kHz和4kHz纯音刺激时,研究组大脑双侧M100潜伏期[2 kHz:左(97±16)ms,右(97±10)ms,4 kHz:左(93±13)ms,右(99±14)ms]均短于对照组[2 kHz:左(121±15)ms,右(113±6)ms,4 kHz:左(113±13)ms,右(114±6)ms](均P<0.01),而波幅[2 kHz:左(89±10)fT,右(118±37)fT,4 kHz:左(81±9)fT,右(108±14)fT]高于对照组[2 kHz:左(73±12)fT,右(79±13)fT,4 kHz:左(69±14)fT,右(81±20)fT](均P<0.05～0.01).结论 男性精神分裂症幻听患者的初级听觉皮质位置与正常人不同,其M100波幅高,潜伏期短,这些功能及解剖结构的异常可能是精神分裂症幻听产生的病理生理机制之一.     

Effects of center frequency on binaural auditory filter bandwidth in the human brain

Effects of center frequency on the binaural auditory filter in the human auditory cortex were examined using auditory-evoked magnetic fields. Two tones with different frequency separations, which were presented dichotically to the left and right ears, were used as the sound stimuli. Eight normal-hearing participants took part in the study. The amplitudes of the N1m components of auditory-evoked magnetic fields were approximately constant when the frequency separation was less than 10-20% of the center frequency; however, the N1m amplitudes increased with increasing frequency separation when the frequency separation was greater than 10-20% of the center frequency. This indicates that binaural auditory filter bandwidth is approximately 10-20% of the center frequency.     

Functional magnetic resonance imaging measure of automatic and controlled auditory processing

Activity within fronto-striato-temporal regions during processing of unattended auditory deviant tones and an auditory target detection task was investigated using event-related functional magnetic resonance imaging. Activation within the middle frontal gyrus, inferior frontal gyrus, anterior cingulate gyrus, superior temporal gyrus, thalamus, and basal ganglia were analyzed for differences in activity patterns between the two stimulus conditions. Unattended deviant tones elicited robust activation in the superior temporal gyrus; by contrast, attended tones evoked stronger superior temporal gyrus activation and greater frontal and striatal activation. The results suggest that attention enhances neural activation evoked by auditory pitch deviance in auditory brain regions, possibly through top-down control from the dorsolateral prefrontal cortex involved in goal-directed selection and response generation.     

The auditory C-process of spectral profile analysis.

OBJECTIVES: We here review the findings of several experiments, aimed at clarifying the functional role of the human auditory cortex in the processing of complex sound mixtures.METHODS: Long-latency auditory evoked potentials were recorded to abrupt changes in the pitch or timbre of continuous complex tones (synthesized musical instrument sounds). Changes were made at intervals of 0.5-4.5 s while the subjects read a magazine.RESULTS: The main response was a P1/N1/P2 complex which was maximal at the vertex and symmetrically distributed, consistent with origin in the supratemporal cortices of both hemispheres. To distinguish them from the conventional responses to brief pure tones, the potentials were named CP1 (c. 55 ms), CN1 (90 ms) and CP2 (165 ms). Responses to changes of pitch, where all the spectral components changed frequency, and to changes of timbre, where the frequencies remained the same but their energy levels changed, were very similar to one another. The response amplitudes were little affected by the magnitude of frequency changes in the range 6-100%, but were strongly influenced by the rate at which changes occurred (requiring at least 4 s for full recovery) and by the breadth of the changing frequency spectrum (the upper partials of the tone in sum contributing more than the fundamental). When the C-potentials were made refractory by a high rate of pitch changes (16/s) within a narrow frequency range, responses could still be elicited by infrequently interspersed changes of timbre. When the tones were split into their high and low partials, the responses to change in the two frequency bands combined roughly algebraically.CONCLUSIONS: The responses appear to represent a cortical process concerned with analysing the distribution of sound energy across the frequency spectrum ('spectral profile analysis'). This may be an important stage in the analysis of complex sound mixtures and in the perception of sound quality.     

Cortical hemispheric asymmetries are present at young ages and further develop into adolescence

Specialization of the auditory cortices for pure tone listening may develop with age. In adults, the right hemisphere dominates when listening to pure tones and music; we thus hypothesized that (a) asymmetric function between auditory cortices increases with age and (b) this development is specific to tonal rather than broadband/non‐tonal stimuli. Cortical responses to tone‐bursts and broadband click‐trains were recorded by multichannel electroencephalography in young children (5.1 ± 0.8 years old) and adolescents (15.2 ± 1.7 years old) with normal hearing. Peak dipole moments indicating activity strength in right and left auditory cortices were calculated using the Time Restricted, Artefact and Coherence source Suppression (TRACS) beamformer. Monaural click‐trains and tone‐bursts in young children evoked a dominant response in the contralateral right cortex by left ear stimulation and, similarly, a contralateral left cortex response to click‐trains in the right ear. Responses to tone‐bursts in the right ear were more bilateral. In adolescents, peak activity dominated in the right cortex in most conditions (tone‐bursts from either ear and to clicks from the left ear). Bilateral activity was evoked by right ear click stimulation. Thus, right hemispheric specialization for monaural tonal stimuli begins in children as young as 5 years of age and becomes more prominent by adolescence. These changes were marked by consistent dipole moments in the right auditory cortex with age in contrast to decreases in dipole activity in all other stimulus conditions. Together, the findings reveal increasingly asymmetric function for the two auditory cortices, potentially to support greater cortical specialization with development into adolescence.     

Effect of attention on central auditory processing: an fMRI study

Functional magnetic resonance imaging was used to investigate preattentive and attentional processing of auditory stimuli in 18 right-handed normal volunteers. Responses to trains of 1000-Hz pure tones and infrequent (15%) deviant 1300-Hz tones were characterized while subjects ignored all tones; listened for deviants in the left ear; or listened for deviants in the right ear. Preattentive detection of deviants, associated with the mismatch negativity in electrophysiology, was associated with bilateral temporal lobe activation, with a rightward predominance. Processing of deviant stimuli while attending to either ear produced a more robust and widespread activation of these temporal regions, again with a rightward predominance. Thus, preattentive tone processing appears to be linked to asymmetric activation of a core set of temporal regions in which activity is significantly amplified by selective attention. Extratemporal regions activated by attending to targets in either ear included the anterior cingulate cortex, supramarginal gyrus, and dorsolateral prefrontal cortex.     

Neuromagnetic auditory steady-state responses to amplitude modulated sounds following dichotic or monaural presentation

ObjectiveThe goal of the present study was to further our understanding of how attention directed to carrier frequency changes in amplitude-modulated tones (AM) affects the auditory steady-state response (ASSR).MethodsASSR in the 40-Hz range were recorded in 15 adults using the frequency tagging method while subjects detected a carrier frequency change in amplitude-modulated tones. Spectral and temporal domain analyses were performed to examine the effect on response amplitudes during attending to the tones compared to ignoring them for both monaural and dichotic stimulations.ResultsLarger responses were found in the hemisphere contralateral to the stimulated ear. Binaural suppression for ipsilateral input was found in both hemispheres. Time series of the source waveforms were calculated from equivalent current dipoles and showed a 2 Hz beat for the dichotic presentation. Attention to carrier frequency change was significant only during dichotic presentation where larger right hemisphere responses were found at the onset of carrier change.ConclusionsAttending to carrier frequency change in stimulation enhances the right hemisphere ASSR amplitude for dichotic stimulation.SignificanceThe possibility of tagging frequency specific responses up to auditory cortex makes the ASSR approach interesting for studying hearing impairment mechanisms, integrity of auditory structures and attention.     

Evidence for the role of the right auditory cortex in fine pitch resolution

The neural basis of human pitch perception is not fully understood. It has been argued that the auditory cortices in the two hemispheres are specialized, such that certain right auditory cortical regions have a relatively finer resolution in the frequency domain than homologous regions in the left auditory cortex, but this concept has not been tested directly. Here, we used functional magnetic resonance imaging (fMRI) to test this specific prediction. Healthy volunteers were scanned while passively listening to pure-tone melodic-like sequences in which the pitch distance between consecutive tones was varied in a parametric fashion. As predicted, brain activation in a region of right lateral auditory cortex, corresponding to the planum temporale, was linearly responsive to increasing pitch distance, even across the fine changes in pitch. In contrast, the BOLD signal at the homologous left cortical region was relatively constant as a function of pitch distance, except at the largest pitch change. The results support the model of relative hemispheric specialization and indicate that the right secondary auditory cortex has a finer pitch resolution than the left.