Auditory evoked fields to illusory sound source movements

Auditory motion can be simulated by presenting binaural sounds with time-varying interaural intensity differences. We studied the human cortical response to both the direction and the rate of illusory motion by recording the auditory evoked magnetic fields with a 122-channel whole-head neuromagnetometer. The illusion of motion from left to right, right to left, and towards and away from the subject was produced by varying a 6-dB intensity difference between the two ears in the middle of a 600-ms tone. Both the onset and the intensity transition within the stimulus elicited clear responses in auditory cortices of both hemispheres, with the strongest responses occurring about 100 ms after the stimulus and transition onsets. The transition responses were significantly earlier and larger for fast than slow shifts and larger in the hemisphere contralateral to the increase in stimulus intensity for azimuthal shifts. Transition response amplitude varied with the direction of the simulated motion, suggesting that these responses are mediated by directionally selective cells in auditory cortex.     

Topographic mapping and source localization of the pattern reversal visual evoked magnetic response

Summary The topography of the visual evoked magnetic response (VEMR) to pattern reversal stimulation was studied in four normal subjects using a single channel BTI magnetometer. VEMRs were recorded from 20 locations over the occipital scalp and the topographic distribution of the most consistent component (P100M) studied. A single dipole in a sphere model was fitted to the data. Topographic maps were similar when recorded two months apart on the same subject to the same stimulus. Half field (HF) stimulation elicited responses from sources on the medial surface of the calcarine fissure mainly in the contralateral hemisphere as predicted by the cruciform model. The full field (FF) responses to large checks were approximately the sum of the HF responses. However, with small checks, FF stimulation appeared to activate a different combination of sources than the two HFs. In addition, HF topography was more consistent between subjects than FF for small check sizes. Topographic studies of the VEMR may help to explain the analogous visual evoked electrical response and will be essential to define optimal recording positions for clinical applications.Source localizations were performed using the facilities of the Open University and the assistance of Dr. S. Swithenby is gratefully acknowledged.     

Auditory Detection of Motion Velocity in Humans: a Magnetoencephalographic Study

No HeadingSummary: To investigate the cerebral mechanisms of auditory detection of motion velocity in the human brain, neuromagnetic fields elicited by six moving sounds and one stationary sound were investigated with a whole-cortex magnetoencephalography (MEG) system. The stationary sound evoked only one clear response at a latency of 109±6 ms (first response, or M100), but the six moving sounds evoked two clear responses: an earlier response at a latency of 116±7 ms (M100) and a later response at a latency ranging from 180 to 760 ms (magnetic motion response, or MM). The latency and amplitude of the MM were inversely related to the velocity of the moving sounds (p<0.02). The magnetic source of MM was related to the velocity of the moving sounds (p<0.05). A dynamic neuromagnetic response, MM, was elicited by the moving sounds, which likely encoded the neural processing of auditory detection of motion velocity. A specific neural network that processes the motion velocity in the human brain probably includes the bilateral superior temporal cortices and the brainstem. The left posterior and lateral part of the auditory cortex may play a pivotal role in the auditory detection of motion velocity.We thank Dr. Paul Babyn for his help and suggestions in these experiments. This paper was prepared with the assistance of Prof. Sharon Nancekivell, medical editor, Guelph, Ontario, Canada. This study was partially supported by the Savoy Foundation (Research Grant 77227).     

Stimulus duration influences the dipole location shift within the auditory evoked field component N100m

The equivalent source of the neuromagnetic auditory evoked field (AEF) component N100m shifts systematically within its latency range. In the current study, possible effects of stimulus duration on this shift were analysed. 15 subjects were stimulated monaurally with tones of different duration (50, 100, 200 ms) and AEFs were recorded successively over both hemispheres. Dipoles were calculated in 5-ms-steps from 15 ms before to 15 ms after the N100m peak maximum. A dipole location shift within the N100m latency from posterior to anterior and from superior to inferior was observed. The shift in anterior-posterior direction was found to be larger in the right compared to the left hemisphere. Stimulus duration significantly affected the degree of dipole shift in this direction. It was found to be shorter the shorter the stimulus.     

Extraversion and Individual Differences in Auditory Evoked Response

The auditory evoked response to low (0.5K Hz) and high (8.0K Hz) frequency tones at three levels of intensity (40, 55, and 80 dB) were recorded for subjects differing in degree of extra-version. Introverts were observed to have greater N₁-P₂ amplitude than extraverts with the low frequency tones at 55 and 80 dB intensity. The results support Eysenck's general proposal relating introversion to higher levels of cortical activity.     

Auditory evoked responses upon awakening from sleep in human subjects

The hypothesis that a state of hypoarousal upon awakening should lead to a decrease in amplitude and an increase in latency of the N1-P2 components of the Auditory Evoked Potentials (AEPs) as compared to presleep wakefulness levels, was evaluated after two nocturnal awakenings and after the final morning awakening from a 7.5-h night of sleep. The amplitude of the N1-P2 complex was reduced upon awakening as compared to presleep wakefulness levels, but only following the first nocturnal awakening, scheduled after the first 2 h of sleep. This result is interpreted as indicating a link between slow wave sleep amount, mainly present during the first part of the night, and lowered levels of brain activation upon awakening. The reaction times, recorded concomitantly to AEPs, were more sensitive to the negative effects of sleep inertia.     

Effects of voluntary hyperventilation on cortical sensory responses Electroencephalographic and magnetoencephalographic studies

 It is well established that voluntary hyperventilation (HV) slows down electroencephalographic (EEG) rhythms. Little information is available, however, on the effects of HV on cortical responses elicited by sensory stimulation. In the present study, we recorded auditory evoked potentials (AEPs) and magnetic fields (AEFs), and somatosensory evoked magnetic fields (SEFs) from healthy subjects before, during, and after a 3- to 5-min period of voluntary HV. The effectiveness of HV was verified by measuring the end-tidal CO₂ levels. Long-latency (100–200 ms) AEPs and long-latency AEFs originating at the supratemporal auditory cortex, as well as long-latency SEFs from the primary somatosensory cortex (SI) and from the opercular somatosensory cortex (OC), were all reduced during HV. The short-latency SEFs from SI were clearly less modified, there being, however, a slight reduction of the earliest cortical excitatory response, the N20m deflection. A middle-latency SEF deflection from SI at about 60 ms (P60 m) was slightly increased. For AEFs and SEFs, the center-of-gravity locations of the activated neuronal populations were not changed during HV. All amplitude changes returned to baseline levels within 10 min after the end of HV. The AEPs were not altered when the subjects breathed 5% CO₂ in air in a hyperventilation-like manner, which prevented the development of hypocapnia. We conclude that moderate HV suppresses long-latency evoked responses from the primary projection cortices, while the early responses are less reduced. The reduction of long-latency responses is probably mediated by hypocapnia rather than by other nonspecific effects of HV. It is suggested that increased neuronal excitability caused by HV-induced hypocapnia leads to spontaneous and/or asynchronous firing of cortical neurones, which in turn reduces stimulus-locked synaptic events. Received: 14 October 1997 / Accepted: 28 October 1998     

Intersession replicability of dipole parameters from three components of the auditory evoked magnetic field

Summary The replicability of dipole localizations between sessions in an unselected group of subjects was studied. Auditory evoked magnetic fields (AEMFs) in response to contralaterally and ipsilaterally presented 1 kHz tone bursts were recorded from the right hemisphere of 12 subjects with normal hearing in two replicate sessions several days apart. Three long-latency components of the AEMF were studied, occurring at latencies near 50 msec (P1_m), near 100 msec (N1_m) and near 165 msec (P2_m). A spherical model of the head was used to fit equivalent-current dipoles to the data. Statistical analysis of dipole parameters revealed virtually no differences between the two testing sessions. The variability between sessions had a mean absolute difference of 3 to 10 mm for the spatial parameters. Comparison of dipole parameters between components showed that there was a replicable, but nonsignificant, trend for a difference in the location of the N1_m from contralateral vs. ipsilateral stimulation, and a statistically significant confirmation that the P2_m is located anterior to the N1_m for contralateral stimulation. Magnetic resonance images from each subject were used to locate the dipoles near the primary auditory cortex in the Sylvian fissure.This research was supported by grants from The Moody Foundation of Galveston, #87-45 and from the Mobility Foundation.     

儿童感音神经性聋听觉稳态诱发反应阈与听性脑干反应阈比较

目的研究听觉稳态诱发反应(ASSR)和听性脑干反应(ABR)阈与纯音听阈的差别和相关性。方法选择74例儿童感音神经性聋患者(118耳)分别进行ASSR、ABR和电测听检查,比较ASSR、ABR反应阈及纯音听阈,同时就ASSR、ABR反应阈与纯音听阈进行相关性分析。结果 ASSR和ABR反应阈与纯音听阈均有良好的相关性。ABR的反应阈与纯音听阈阈值接近,而ASSR反应阈与纯音听阈间差值较大。ASSR反应阈与纯音听阈间的相关性要优于ABR反应阈与纯音听阈间的相关性。结论 ASSR和ABR均为较好的评估行为听阈的客观测听方法。     

Neuromagnetic study of the auditory responses in right and left hemispheres of the human brain evoked by pure tones and speech sounds

Summary Neuromagnetic responses in the human auditory cortex evoked by various burst stimuli of pure tones and monosyllable speech sounds were measured separately from two hemispheres. Both the pure tones and speech sounds elicited clear responses with two main peaks. The field patterns over the scalp at the peak latencies indicated a single current dipole as an equivalent field generator. The depth of the current dipoles computed from the mapped field data was deeper from the scalp for a tone stimulus of higher frequency, which confirms the tonotopic organization in the auditory cortex. A difference was found in the dipole locations in the horizontal plane for the speech stimuli of a vowel /a/ and a consonant-vowel /ka/. It suggests the sensitivity of the magnetic responses to the acoustic structure of the speech sound.     

Auditory brainstem responses in the aged cat

Auditory brainstem responses (ABRs) were compared in young adult and aged cats. Mean thresholds for click-evoked ABRs were greater in the aged cats. Clicks normalized to 15 and 30 dB above individual thresholds at rates of 10, 20, 50 and 100/sec evoked ABRs with similar latencies and central conduction times in both groups. Background noise at equal intensity for all cats completely suppressed ABRs evoked by clicks 30 dB above threshold in 2/3 of the young but none of the old cats. As rise time of a 25 msec noise burst at equal intensity for all cats increased 1, 2, 5, and 10 msec, latency of wave 4 increased more for the old cats than for the young. Summed monaural ABRs from both ears were greater than binaural ABRs for waves 4 and 5 in both groups. These data indicate peripheral auditory dysfunction in aged cats but little abnormality in auditory brainstem transmission with click intensity normalized for ABR threshold.     

Auditory Analgesia: Somatosensory Evoked Response and Subjective Pain Rating

Subjective rating responses and averaged evoked responses (AERs) to shock stimuli of varying intensity were recorded in 20 subjects to examine the possible analgesic effects of sound stimulation (music) and suggested analgesia. Subjects (10 men, 10 women, ages 19–31) were divided into two groups of 10, each receiving the sound-suggestion condition and the no-sound, no-suggestion condition in different order. Sound and suggestion produced the following significant (p<.05) effects: 1) increased electrical stimulus levels required to elicit discomfort ratings; 2) decreased slope of somatosensory AER amplitudes plotted against stimulus intensity; and 3) decreased mean AER amplitudes. These AER effects were greatest in time bands centered on the P100 component. Prior exposure to the electrical stimuli also reduced AER slopes and mean amplitudes, but mostly in time bands centered on the P200 component.     

Mismatch negativity in dichotic listening: Evidence for interhemispheric differences and multiple generators

The characteristics of mismatch negativity (MMN) elicited by dichotic stimulation were examined using frequency-deviant stimuli presented to the right, to the left, or to both sides. The experiment was run twice, once using earphones and once using loudspeakers in free field. With both modes of stimulation, deviants presented in the left, right, or both ears, or tones that were switched between ears, elicited comparable MMNs, with a peak latency of about 180 ms. With earphones, the amplitude of the MMN was bigger at the frontal-lateral right hemisphere sites than at the homologous left-hemisphere sites for all deviance conditions. Scalp current density analysis revealed that deviance in the right side elicited bilaterally equivalent frontal current sinks and a trend towards stronger contralateral current sources at the mastoid sites. In contrast, left side deviance elicited frontal sinks and temporal current sources stronger over the right hemiscalp. These results are compatible with the multiple-generator model of MMN. The attention-related role of the MMN is discussed, suggesting comparable attention mechanisms for vision and audition.     

Extraction of the constituent components of brain stem auditory evoked responses by a filtering and optimisation procedure

A cascaded filtering and optimisation procedure is used to break down the human brain stem auditory evoked respose into what are believed to be its basic constituent components. The three main components are a slow background wave, a repetitive wave and a delayed component Peaks I, II, III, IV and VI are identified with the crests of the repetitive wave. This suggests very strongly that they originate from a common generator source. Peak V is formed by the superposition of the delayed component on the repetitive wave. It is the most stable feature of the response. In addition to the main components the scalp-recorded response is inferred to have a smaller signal from the auditory nerve, the polarity of which depends on the location of the reference electrode.     

Analysis of auditory evoked potential parameters in the presence of radiofrequency fields using a support vector machines method

The paper presents a study of global system for mobile (GSM) phone radio-frequency effects on human cerebral activity. The work was based on the study of auditory evoked potentials (AEPs) recorded from healthy humans and epileptic patients. The protocol allowed the comparison of AEPs recorded with or without exposure to electrical fields. Ten variables measured from AEPs were employed in the design of a supervised support vector machines classifier. The classification performance measured the classifier′s ability to discriminate features performed with or without radiofrequency exposure. Most significant features were chosen by a backward sequential selection that ranked the variables according to their pertinence for the discrimination. Finally, the most discriminating features were analysed statistically by a Wilcoxon signed rank test. For both populations, the N100 amplitudes were reduced under the influence of GSM radiofrequency (mean attenuation of −0.36μV for healthy subjects and −0.6OμV for epileptic patients). Healthy subjects showed a NIOO latency decrease (−5.23ms in mean), which could be consistent with mild, localised heating. The auditory cortical activity in humans was modified by GSM phone radio-frequencies, but an effect on brain functionality has not been proven. MBEC online number: 20043906     

Auditory memory for backward masking signals in children with language impairment

This study was designed to investigate early auditory memory and its possible contribution to an auditory processing deficit shown by some children with language impairment. Ten children with language impairment and 10 age-matched controls participated in a series of simultaneous and backward masking tasks. The same backward masking stimulus was then used to elicit a mismatch negativity response. In the behavioral conditions, children in the language impairment group had significantly higher (poorer) signal thresholds than their nonimpaired controls in backward masking, but their thresholds in simultaneous masking were not significantly different. In the mismatch-negativity conditions, latency was prolonged and the amplitude was diminished in the children with language impairment. Taken together, these psychoacoustic and electrophysiological data suggest that in a group of children with language impairment, underlying the nonsensory language disorder, there is a neurophysiological impairment in auditory memory for complex, nonlinguistic sounds.     

Proprioceptive effects on evoked responses to sounds in the cat auditory cortex

Summary With a view to analyse the influence of neck proprioceptors on directional hearing, evoked potentials (EPs) to dichotically or monaurally presented clicks were recorded from the auditory cortex of cats under deep Nembutal anaesthesia with their head pointing to the front, and then to the right or to the left side at 45°.The change in the head position produced considerable changes in the amplitude of the two primary EP components and in their thresholds. The changes were of two kinds: either decrease or increase of the amplitude. At symmetrical points of the auditory cortex they went in the same direction. The also appeared in the associative zone with the same sign.With monaurally presented clicks, the change of the side of stimulation for the most part resulted in a reversal of the sign of the proprioceptive effect.Similar proprioceptive influences were recorded when the clicks were presented not through earphones but in an open acoustic field.     

Reduction of somatosensory evoked fields in the primary somatosensory cortex in a one-back task

In the present study, responses of the somatosensory cortex to sensory input of ten human volunteers were investigated during a one-back task with different conditions of attention. During an condition of attention subjects were requested to detect a predefined sequence of tactile stimuli applied to two different fingers of the dominant hand while a series of visual stimuli was presented simultaneously with an asynchronous stimulus-onset to the tactile stimuli. During an condition of distraction subjects received the identical series of visual and tactile stimuli like in the condition of attention but were now requested to detect a predefined stimulus sequence within the visual stimulus domain. In both conditions, somatosensory evoked magnetic fields (SEFs) to the tactile stimuli were recorded by means of a 31-channel magnetoencephalograph (MEG) from subjects‘ contralateral primary somatosensory cortex. The mean global field power, the dipole strength, the maximum current density, and the first component of the singular value decomposition (SVD) of magnetic fields were used to compare early components of the SEF in the conditions of attention versus distraction. Surprisingly, results revealed significant decreases of measures of all four parameters during the condition of attention as compared to the condition of distraction indicating that early responses of the primary somatosensory cortex became significantly reduced in the condition of attention. We hypothesize that changes in the centre-periphery-relationship of receptive fields in the primary somatosensory cortex may account for this unexpected result.     

Temporal characteristics of auditory sensory memory: Neuromagnetic evidence

We investigated the temporal dependencies of N100m, the most prominent deflection of the auditory evoked response, using whole-head neuromagnetic recordings. Stimuli were presented singly or in pairs (tones in the pair were separated by 210 ms) at interstimulus intervals (ISIs) of 0.6–8.1 s. N100m to single stimuli and to the first tone of the pair had similar temporal recovery functions, plateauing at ISIs of 6 s. N100m to the second tone in the pair, which was smaller than that to the first except with short ISIs, plateaued with ISIs of about 4 s. Source analysis revealed that the N100m could be decomposed into two sources separated by about 1 cm on the supratemporal plane. The recovery function of the posterior source was not affected by stimulus presentation, whereas that of the anterior source was. Activity in the anterior area appears to reflect the effects of temporal integration. We relate these results to auditory sensory memory.