Neuromagnetic auditory evoked responses after a stroke in the right temporal lobe

Longitudinal maturation of auditory cortical function during adolescence

Cross-sectional studies have demonstrated that the cortical auditory evoked potential (CAEP) changes substantially in amplitude and latency from childhood to adulthood, suggesting that these aspects of the CAEP continue to mature through adolescence. However, no study to date has longitudinally followed maturation of these CAEP measures through this developmental period. Additionally, no study has examined the trial-to-trial variability of the CAEP during adolescence. Therefore, we longitudinally tracked changes in the latency, amplitude, and variability of the P1, N1, P2, and N2 components of the CAEP in 68 adolescents from age 14 years to age 17 years. Latency decreased for N1 and N2, and did not change for P1 or P2. Amplitude decreased for P1 and N2, increased for N1, and did not change for P2. Variability decreased with age for all CAEP components. These findings provide longitudinal support for the view that the human auditory system continues to mature through adolescence. Continued auditory system maturation through adolescence suggests that CAEP neural generators remain plastic during this age range and potentially amenable to experience-based enhancement or deprivation.

Developmental and cross-modal plasticity in deafness: evidence from the P1 and N1 event related potentials in cochlear implanted children

Cortical development is dependent on extrinsic stimulation. As such, sensory deprivation, as in congenital deafness, can dramatically alter functional connectivity and growth in the auditory system. Cochlear implants ameliorate deprivation-induced delays in maturation by directly stimulating the central nervous system, and thereby restoring auditory input. The scenario in which hearing is lost due to deafness and then reestablished via a cochlear implant provides a window into the development of the central auditory system. Converging evidence from electrophysiologic and brain imaging studies of deaf animals and children fitted with cochlear implants has allowed us to elucidate the details of the time course for auditory cortical maturation under conditions of deprivation. Here, we review how the P1 cortical auditory evoked potential (CAEP) provides useful insight into sensitive period cut-offs for development of the primary auditory cortex in deaf children fitted with cochlear implants. Additionally, we present new data on similar sensitive period dynamics in higher-order auditory cortices, as measured by the N1 CAEP in cochlear implant recipients. Furthermore, cortical re-organization, secondary to sensory deprivation, may take the form of compensatory cross-modal plasticity. We provide new case-study evidence that cross-modal re-organization, in which intact sensory modalities (i.e., vision and somatosensation) recruit cortical regions associated with deficient sensory modalities (i.e., auditory) in cochlear implanted children may influence their behavioral outcomes with the implant. Improvements in our understanding of developmental neuroplasticity in the auditory system should lead to harnessing central auditory plasticity for superior clinical technique.

Deprivation-induced cortical reorganization in children with cochlear implants

A basic finding in developmental neurophysiology is that some areas of the cortex cortical areas will reorganize following a period of stimulus deprivation. In this review, we discuss mainly electroencephalography (EEG) studies of normal and deprivation-induced abnormal development of the central auditory pathways in children and in animal models. We describe age cut-off for sensitive periods for central auditory development in congenitally deaf children who are fitted with a cochlear implant. We speculate on mechanisms of decoupling and reorganization which may underlie the end of the sensitive period. Finally, we describe new magentoencephalography (MEG) evidence of somatosensory cross-modal plasticity following long-term auditory deprivation.

Cortical differentiation of speech and nonspeech sounds at 100 ms: implications for dyslexia

Neurophysiological measures indicate cortical sensitivity to speech sounds by 150 ms after stimulus onset. In this time window dyslexic subjects start to show abnormal cortical processing. We investigated whether phonetic analysis is reflected in the robust auditory cortical activation at approximately 100 ms (N100m), and whether dyslexic subjects show abnormal N100m responses to speech or nonspeech sounds. We used magnetoencephalography to record auditory responses of 10 normally reading and 10 dyslexic adults. The speech stimuli were synthetic Finnish speech sounds (/a/, /u/, /pa/, /ka/). The nonspeech stimuli were complex nonspeech sounds and simple sine wave tones, composed of the F1+F2+F3 and F2 formant frequencies of the speech sounds, respectively. All sounds evoked a prominent N100m response in the bilateral auditory cortices. The N100m activation was stronger to speech than nonspeech sounds in the left but not in the right auditory cortex, in both subject groups. The leftward shift of hemispheric balance for speech sounds is likely to reflect analysis at the phonetic level. In dyslexic subjects the overall interhemispheric amplitude balance and timing were altered for all sound types alike. Dyslexic individuals thus seem to have an unusual cortical organization of general auditory processing in the time window of speech-sensitive analysis.

Altered auditory processing in acutely psychotic never-medicated first-episode patients

Individuals with psychosis fail to differentiate external impulses and suffer from distortions of reality testing. Schizophrenia group illnesses are also associated with deficits in working memory and perception. We examined the manifestations of a very early phase of psychotic illness to automatic auditory deviance detection to clarify the basic mechanisms underlying misinterpretations of perception.

METHODS

Twenty-five never-medicated patients admitted for hospital evaluation of acute psychosis were studied. Fifty-eight EEG channels were recorded during an auditory oddball paradigm. Event-related potentials (ERPs) time-locked to non-attended deviant auditory stimuli were studied in patients and compared with healthy controls. Auditory processing was examined both at the level of the measured biosignals (standard and deviant responses) and with subtraction waveforms. Topographical differences were characterized using global field power (GFP) and minimum norm estimates.

RESULTS

The maximum GFP amplitudes and mean amplitudes of the 58 channels within the time windows corresponding to the previously known 'N2b', 'P3a' and 'P3b' components were clearly reduced in patients when compared to healthy controls. However, the groups did not differ during attention-independent automatic processing corresponding to the 'N1' and 'MMN' components, or with respect to the peak latencies of the GFP maxima.

CONCLUSIONS

Impairment of the processing of a deviance in simple auditory input in acutely ill drug-naive first-episode psychotic patients only appears in attention-dependent processing after about 250 ms. The alterations in auditory processing differed between stimulus types, suggesting at least two mechanisms underlying the auditory discrimination impairments in acute psychosis. After 250 ms there was a linear and gradually increasing difference in magnitude between the groups in their responses to deviant stimuli, probably related to arousal. In addition, however, there was a striking difference between the groups in the processing of standard stimuli. The early processing was similar in patients and controls, but the striking difference appeared in later processing. The sensory memory deficits associated with psychosis may be explained by an abnormality in sensory model formation rather than by impaired deviant detection.

Magnetoencephalography in stroke: a 1-year follow-up study

Recovery after stroke is closely linked to cerebral plasticity. Magnetoencephalography (MEG) is a non-invasive technique, which allows location of cerebral cells activities. In the present work, a cohort of patients has been studied with MEG. Twelve patients with a recent ischemic or hemorragic stroke were included as soon as possible after onset of stroke. Neurologic assessment, including standard neurologic examination, functional independence measure (FIM) and Orgogozo's scale was performed for 1 year in addition to a study of the somatosensory evoked field (SEF) using a 37-channel Biomagnetometer system. No response could be recorded in five patients at the first SEF exploration. In three cases, no response was ever recorded during the study. All these patients had a bad recovery. The location of the SEF sources was always in the normal non-infarcted cortex of the postcentral gyrus. Sensory recovery seemed to be linked to the reorganization of the persistent functional cortex, which was a limiting factor for recovery. These observations confirm the experimental results obtained in animal models. After stroke it can be assumed that in the case of incomplete lesion, an intensive sensory peripheral stimulation could maximize the use of residual sensory function and then contribute to improve the sensory deficit. In case of total sensory loss other techniques have to be used, such as visual monitoring of hand activity in order to improve hand function.

Human cortical representation of virtual auditory space: differences between sound azimuth and elevation

Sounds convolved with individual head-related transfer functions and presented through headphones can give very natural percepts of the three-dimensional auditory space. We recorded whole-scalp neuromagnetic responses to such stimuli to compare reactivity of the human auditory cortex to sound azimuth and elevation. The results suggest that the human auditory cortex analyses sound azimuth, based on both binaural and monaural localization cues, mainly in the hemisphere contralateral to the sound, whereas elevation in the anterior space and in the lateral auditory space in general, both strongly relying on monaural spectral cues, are analyzed in more detail in the right auditory cortex. The binaural interaural time and interaural intensity difference cues were processed in the auditory cortex around 100-150 ms and the monaural spectral cues later around 200-250 ms.

Interhemispheric connection of auditory neural pathways assessed by auditory evoked magnetic fields in patients with fronto-temporal lobe infarction

In auditory evoked magnetic fields (AEFs), the latency of temporal N100m response elicited by the ipsilateral ear stimulation (Ipsi-Stim) is generally longer than that of N100m response elicited by the contralateral ear stimulation (Cont-Stim). The reason for this difference remains unclear. We measured AEFs in patients with fronto-temporal or frontal lobe infarction to clarify this question. In the patients with fronto-temporal lobe infarction, the N100m peak latencies in the healthy hemisphere by Ipsi-Stim measurements were significantly longer than the corresponding normal values. Such a latency prolongation was not observed in the patients with frontal lobe infarction. The results suggest that auditory impulses originated from the ear may first arrive at the contralateral temporal cortex and then return to the ipsilateral temporal cortex mediating through the corpus callosum. The disturbance of interhemispheric conduction by ischemic temporal lesions likely delays the N100m latency at the contralateral temporal cortex. The mediation of interhemispheric route may, thus, make the ipsilateral N100m latency generally longer than the contralateral N100m latency.

Abnormal auditory neural networks in patients with right hemispheric infarction, chronic dizziness, and moyamoya disease: a magnetoencephalogram study

The purpose of this study was to determine whether the auditory cortex is sensitive to cortical insults and to determine the specificity of the insults in three clinical situations with different cortical involvement. Auditory-evoked magnetic fields of ten normal subjects, 8 patients with right hemispheric infarction, 11 with chronic dizziness, and 2 with moyamoya disease were measured. To analyze the abnormality of auditory neural networks, the magnitude ratio and the angle difference (Deltatheta) between response vectors, which were determined from maximum current arrows corresponding to the N100m peak for contralateral and ipsilateral stimuli were used. A normal range of the parameters was defined so that abnormal values could be determined. Of the three parameters, Deltatheta was the most sensitive: 4 patients with right hemispheric infarction, 4 with chronic dizziness, and 1 with moyamoya disease had abnormal Deltatheta. The electrical activity in the patients with such abnormal Deltathetas had a circular current pattern. These findings suggest that right infarction lesions sometime affect the left auditory neural network, dizziness is caused by abnormal neural networks between the vestibular cortical area and the auditory cortex or by an imbalance between left and right auditory-cortex activities, and moyamoya-disease patients have almost normal auditory-electrical activity.

Maturation of human central auditory system activity: separating auditory evoked potentials by dipole source modeling

OBJECTIVES

Previous studies have shown that observed patterns of auditory evoked potential (AEP) maturation depend on the scalp location of the recording electrodes. Dipole source modeling incorporates the AEP information recorded at all electrode locations. This should provide a more robust description of auditory system maturation based on age-related changes in AEPs. Thus, the purpose of this study was to evaluate central auditory system maturation based dipole modeling of multi-electrode long-latency AEPs recordings.

METHODS

AEPs were recorded at 30 scalp-electrode locations from 118 subjects between 5 and 20 years of age. Regional dipole source analysis, using symmetrically located sources, was used to generate a spatio-temporal source model of age-related changes in AEP latency and magnitude.

RESULTS

The regional dipole source model separated the AEPs into distinct groups depending on the orientation of the component dipoles. The sagittally oriented dipole sources contained two AEP peaks, comparable in latency to Pa and Pb of the middle latency response (MLR). Although some magnitude changes were noted, latencies of Pa and Pb showed no evidence of age-related change. The tangentially oriented sources contained activity comparable to P1, N1b, and P2. There were various age-related changes in the latency and magnitude of the AEPs represented in the tangential sources. The radially oriented sources contained activity comparable to the T-complex, including Ta, and Tb, that showed only small latency changes with age. In addition, a long-latency component labeled TP200 was observed.

CONCLUSIONS

It is possible to distinguish 3 maturation groups: one group reaching maturity at age 6 and comprising the MLR components Pa and Pb, P2, and the T-complex. A second group that was relatively fast to mature (50%/year) was represented by N2. A third group was characterized by a slower pattern of maturation with a rate of 11-17%/year and included the AEP peaks P1, N1b, and TP200. The observed latency differences combined with the differences in maturation rate indicate that P2 is not identical to TP200. The results also demonstrated the independence of the T-complex components, represented in the radial dipoles, from the P1, N1b, and P2 components, contained in the tangentially oriented dipole sources.

Functional role of auditory cortex in frequency processing and pitch perception

Microelectrode studies in nonhuman primates and other mammals have demonstrated that many neurons in auditory cortex are excited by pure tone stimulation only when the tone's frequency lies within a narrow range of the audible spectrum. However, the effects of auditory cortex lesions in animals and humans have been interpreted as evidence against the notion that neuronal frequency selectivity is functionally relevant to frequency discrimination. Here we report psychophysical and anatomical evidence in favor of the hypothesis that fine-grained frequency resolution at the perceptual level relies on neuronal frequency selectivity in auditory cortex. An adaptive procedure was used to measure difference thresholds for pure tone frequency discrimination in five humans with focal brain lesions and eight normal controls. Only the patient with bilateral lesions of primary auditory cortex and surrounding areas showed markedly elevated frequency difference thresholds: Weber fractions for frequency direction discrimination ("higher"-"lower" pitch judgments) were about eightfold higher than Weber fractions measured in patients with unilateral lesions of auditory cortex, auditory midbrain, or dorsolateral frontal cortex; Weber fractions for frequency change discrimination ("same"-"different" pitch judgments) were about seven times higher. In contrast, pure-tone detection thresholds, difference thresholds for pure tone duration discrimination centered at 500 ms, difference thresholds for vibrotactile intensity discrimination, and judgments of visual line orientation were within normal limits or only mildly impaired following bilateral auditory cortex lesions. In light of current knowledge about the physiology and anatomy of primate auditory cortex and a review of previous lesion studies, we interpret the present results as evidence that fine-grained frequency processing at the perceptual level relies on the integrity of finely tuned neurons in auditory cortex.

Plasticity in the adult human central auditory system: evidence from late-onset profound unilateral deafness

Experience-related changes in central nervous system (CNS) activity have been observed in the adult brain of many mammalian species, including humans. In humans, late-onset profound unilateral deafness creates an opportunity to study plasticity in the adult CNS consequent to monaural auditory deprivation. CNS activity was assessed by measuring long-latency auditory evoked potentials (AEPs) recorded from teens and adults with late-onset (post-childhood) profound unilateral deafness. Compared to monaurally stimulated normal-hearing subjects, the AEPs recorded from central electrode sites located over auditory cortical areas showed significant increases in inter-hemispheric waveform cross-correlation coefficients, and in inter-hemispheric AEP peak amplitude correlations. These increases provide evidence of substantial changes from the normal pattern of asymmetrical (contralateral > ipsilateral amplitude) and asynchronous (contralateral earlier than ipsilateral) central auditory system activation in the normal-hearing population to a much more symmetrical and synchronous activation in the unilaterally deaf. These cross-sectional analyses of AEP data recorded from the unilaterally deaf also suggest that the changes in cortical activity occur gradually and continue for at least 2 years after the onset of hearing loss. Analyses of peak amplitude correlations suggest that the increased inter-hemispheric symmetry may be a consequence of changes in the generators producing the N (approximately 100 ms peak latency) potential. These experience-related changes in central auditory system activity following late-onset profound unilateral deafness thus provide evidence of the presence and the time course of auditory system plasticity in the adult brain.

Differences between the N1 waves of the responses to interaural time and intensity disparities: scalp topography and dipole sources

OBJECTIVES

Being the two complementary cues to directional hearing, interaural time and intensity disparities (ITD and IID, respectively), are known to be separately encoded in the brain stem. We address the question as to whether their codes are collapsed into a single lateralization code subcortically or they reach the cortex via separate channels and are processed there in different areas.

METHODS

Two continuous trains of 100/s clicks were dichotically presented. At 2 s intervals either an interaural time delay of 1ms or an interaural level difference of 20 dB (HL) was introduced for 50 ms, shifting the intracranial sound image laterally for this brief period of time. Long-latency responses to these directional stimuli, which had been tested to evoke no potentials under monotic or diotic conditions, as well as to sound pips of 50 ms duration were recorded from 124 scalp electrodes. Scalp potential and current density maps at N1 latency were obtained from thirteen normal subjects. A 4-sphere head model with bilaterally symmetrical dipoles was used for source analysis and a simplex algorithm preceded by a genetic algorithm was employed for solving the inverse problem.

RESULTS

Inter- and intra-subject comparisons showed that the N1 responses evoked by IID and ITD as well as by sound pip stimuli had significantly different scalp topographies and interhemispheric dominance patterns. Significant location and orientation differences between their estimated dipole sources were also noted.

CONCLUSIONS

We conclude that interaural time and intensity disparities (thus the lateral shifts of a sound image caused by these two cues) are processed in different ways and/or in different areas in auditory cortex.

Somatosensory, auditory, and visual evoked magnetic fields in patients with brain diseases

The features of somatosensory (SEFs), auditory (AEFs), and visual evoked fields (VEFs) in healthy subjects and patients with brain diseases provide the basis for clinical investigations using magnetoencephalography (MEG). The SEFs provide clinically useful information to identify the central sulcus and somatotopic organization of the primary somatosensory cortex. Localization accuracy of the SEFs can be tested by cortical stimulation during surgery. Functional reorganization suggested by SEF studies must be verified by other modalities. The AEFs can localize the auditory cortex in the bilateral temporal lobes. Separation of bilateral activities is much clearer in AEFs than in auditory evoked potentials. Modulation of the interhemispheric differences of latency, amplitude, and source localization of AEFs can be used to evaluate auditory function in patients with intracranial lesions. Pattern reversal VEFs provide stable localization of the primary visual function. Separation of bihemispherical activities is the advantage of VEFs over visual evoked potentials. Investigation of VEFs provides objective evaluation of visual field deficits such as homonymous or bitemporal hemianopsia in patients with intracranial lesions. Evoked magnetic fields can provide useful diagnostic information. Such clinical findings, in turn, provides the opportunity to test the source estimation accuracy of MEG.

Maturation of human central auditory system activity: evidence from multi-channel evoked potentials

OBJECTIVE

The purpose of this study was to evaluate central auditory system maturation based on detailed data from multi-electrode recordings of long-latency auditory evoked potentials (AEPs).

METHODS

AEPs were measured at 30 scalp-electrode locations from 118 subjects between 5 and 20 years of age. Analyses focused on age-related latency and amplitude changes in the P1, N1b, P2, and N2 peaks of the AEPs generated by a brief train of clicks presented to the left ear.

RESULTS

Substantial and unexpected changes that extend well into adolescence were found for both the amplitude and latency of the AEP components. While the maturational changes in latency followed a pattern of gradual change, amplitude changes tended to be more abrupt and step-like. Age-related latency decreases were largest for the P1 and N1b peaks. In contrast, P2 latency did not change significantly and the N2 peak increased in latency as a function of age. Abrupt changes in P1, P1-N1b, and N2 peak amplitude (also RMS amplitude) were observed around age 10 at the lateral electrode locations C3 and C4, but not at the midline electrodes Cz and Fz. These changes in amplitude coincided with a sharp increase and plateau in AEP peak and RMS amplitude variability from 9 to 11 years of age.

CONCLUSIONS

These analyses demonstrated that the observed pattern of AEP maturation depends on the scalp location at which the responses are recorded. The distinct maturational time courses observed for individual AEP peaks support a model of AEP generation in which activity originates from two or more at least partly independent central nervous system pathways. A striking parallel was observed between previously reported maturational changes in auditory cortex synaptic density and, in particular, the age-related changes in P1 amplitude. The results indicate that some areas of the brain activated by sound stimulation have a maturational time course that extends into adolescence. Maturation of certain auditory processing skills such as speech recognition in noise also has a prolonged time course. This raises the possibility that the emergence of adult-like auditory processing skills may be governed by the same maturing neural processes that affect AEP latency and amplitude.

Native language, gender, and functional organization of the auditory cortex

Whole-head magnetoencephalography was employed in 40 normal subjects to investigate whether the basic functional organization of the auditory cortex varies with linguistic environment. Robust activations of the bilateral supratemporal auditory cortices to 1-kHz pure tones, maximum at about 100 ms after stimulus onset, were studied in Finnish and German female and male subject groups with monolingual background. Activations elicited by the tones were mutually indistinguishable in German and Finnish women. In contrast, German men showed significantly stronger auditory responses to pure tones in the left, language-dominant hemisphere than Finnish men. We discuss the possibility that the prominent left-hemisphere activation in German males reflects higher frequency resolution required for distinguishing between German than Finnish vowels and that the clear effect of native language in male but not in female auditory cortex derives from more pronounced functional lateralization in men. The present data suggest that the influence of native language can extend to auditory cortical processing of pure-tone stimuli with no linguistic content and that this effect is conspicuous in the male brain.

Subject's own speech reduces reactivity of the human auditory cortex

Previous studies on monkeys have shown that uttering-related cortical areas exert an inhibitory effect on the auditory cortex, and cerebral blood-flow analyses on humans have revealed modulation of the activity of the auditory cortex during own speech. To study this modulation on a millisecond time scale, we recorded neuromagnetic evoked responses to short 1-kHz tones while the subjects were reading silently and aloud. The 100-ms response (M100) of the auditory cortex was delayed by 10-21 ms and its amplitude was dampened by 44-71% during reading aloud compared with reading silently. This effect was more prominent in responses to ipsilateral than contralateral tones, possibly due to a sum effect of diminished ipsilateral input to the cortex and decreased transcallosal excitation.

Auditory cortical responses in humans with profound unilateral sensorineural hearing loss from early childhood

We recorded auditory evoked magnetic fields from five patients with profound unilateral sensorineural hearing loss from early childhood, using a 122-channel whole-scalp neuromagnetometer. The stimuli were 50-ms 1-kHz tone bursts delivered to the healthy ear at interstimulus intervals (ISI) of 1, 2, and 4 s. As the normal-hearing controls, four patients had shorter latencies of N100m, the 100-ms response, over the hemisphere contralateral to the stimulation than over the ipsilateral hemisphere. With 1-s ISI, three patients had, instead of N100m, a deflection of opposite polarity at about 100 ms (P100m) after the stimulus onset. A 10-year-old patient had a prominent P100m response, did not produce a clear N100m at any ISI, but had a clear N50m at the 4-s ISI. Four patients had bilateral N200m deflections peaking about 200 ms after the stimulus onset; the fifth patient showed N200m over the right hemisphere. N200m was also observed in the three youngest controls in both hemispheres. The ISI dependence of N100m amplitude and latency was similar in controls and patients. The amplitudes and latencies of N200m did not show any ISI dependence. In patients, the appearance of P100m-N200m deflections of auditory evoked fields, normally present in children, is more pronounced than in controls. The defect apparently delays the development of N100m, possibly by interfering with function of callosal connections.

Whole-head mapping of middle-latency auditory evoked magnetic fields

We recorded middle-latency auditory evoked magnetic fields from 9 healthy subjects with a 122-channel whole-head SQUID gradiometer. The stimuli were click triplets, 2.5 msec in total duration, delivered alternately to the two ears once every 333 msec. Contralateral clicks elicited P30m responses in 16 and P50m responses in 12 out of 18 hemispheres studied; ipsilateral clicks did so in 7 and 13 hemispheres, respectively. The field patterns were satisfactorily explained by current dipoles in 16 and 4 hemispheres for contra- and ipsilateral P30m, and in 4 and 10 hemispheres for contra- and ipsilateral P50m. The peak latencies of P30m and P50m were not affected by stimulation side. The results show that middle-latency auditory evoked responses receive a strong contribution from auditory cortical structures, and that differences of input latency to cortical auditory areas, evaluated from MLAEF latencies, do not explain the latency differences seen in late auditory evoked fields to contralateral vs. ipsilateral stimulation.

Anatomical substrates of auditory selective attention: behavioral and electrophysiological effects of posterior association cortex lesions

Event-related brain potentials (ERPs) and reaction times (RTs) were recorded in an auditory selective attention task in control subjects and two groups of patients with lesions centered in (1) the temporal/parietal junction (T/P, n = 9); and (2) the inferior parietal lobe (IPL, n = 7). High pitched tones were presented to one ear and low pitched tones to the other in random sequences that included infrequent longer-duration tones and occasional novel sounds. Subjects attended to a specified ear and pressed a button to the longer-duration tones in that ear. IPL and T/P lesions slowed reaction times (RTs) and increased error rates, but improved one aspect of performance--patients showed less distraction than controls when targets followed novel sounds. T/P lesions reduced the amplitude of early sensory ERPs, initially over the damaged hemisphere (N1a, 70-110 ms) and then bilaterally (N1b, 110-130 ms, and N1c 130-160 ms). The reduction was accentuated for tones presented contralateral to the lesion, suggesting that N1 generators receive excitatory input primarily from the contralateral ear. IPL lesions reduced N1 amplitudes to both low frequency tones and novel sounds. Nd components associated with attentional selection were diminished over both hemispheres in the T/P group and over the lesioned hemisphere in the IPL group independent of ear of stimulation. Target and novel N2s tended to be diminished by IPL lesions but were unaffected by T/P lesions. The mismatch negativity was unaffected by either T/P or IPL lesions. The results support different roles of T/P and IPL cortex in auditory selective attention.

Human auditory cortical mechanisms of sound lateralization: I. Interaural time differences within sound

Neuromagnetic responses to 600-ms binaural click trains, presented once every 1.1 s, were recorded with a 24-channel gradiometer from 6 healthy humans. During the first 300 ms, the left-ear stimulus led the right by 0.7 ms and the sound was lateralized to the left ear. At 300 ms, the interaural time difference (ITD) changed and the lateralization moved to one of 5 different locations between the ears. An N100m response peaked about 110 ms after the sound onset and an N130mc response (c to stress a response to the change) about 135 ms after the ITD change. The source locations of N100m and N130mc agreed with activity in the supratemporal auditory cortex; this was confirmed in one subject by superimposing MEG results on MR images. The sources of N100m and N130mc did not differ statistically significantly from each other, nor were there differences in N130mc sources to various lateralization changes. N130mc grew larger when the ITD change increased, in parallel with the increase in the change of the perceived location. We suggest that N130mc is analogous to N100m, but is delayed due to postmasking induced by the early part of the sound.