Monitoring evoked potentials during surgery to assess the level of anaesthesia

The hypnotic and analgesic components of anaesthesia can be assessed using middle latency auditory evoked potentials (MLAEPs) and somatosensory evoked potentials (SEPs). To monitor these potentials rehab& during clinical anaesthesia, we have developed an evoked potential (EP) system based around a portable personal computer, a DSP board and an isolated pre-amplifier unit. Unlike many current& available systems, this amplifier is largely immune to diathermy interference due to excellent isolation via a digital fibre optical link, small size and RF screening and filtering. The pre-amplifier unit has integral auditoy and somatosensory stimulators, and automatic calibration and impedance checking. Stimulus intensity and profile are under software control and SEP stimulus level is constantly monitored. The unit is powered by two AA cells and battery status continuously monitored by the PC software. Up to eight channels of EEG may be recorded and displayed in a smoothly scrolling window and as moving average MLAEPs and SEPs.     

Stimulus intensity affects early sensory processing: sound intensity modulates auditory evoked gamma-band activity in human EEG.

We studied the effect of different sound intensities on the auditory evoked gamma-band response (GBR). Previous studies observed oscillatory gamma activity in the auditory cortex of animals and humans. For the visual modality, it has been demonstrated that the GBR can be modulated by top-down (attention, memory) as well as bottom-up factors (stimulus properties). Therefore, we expected to find a sound intensity modulation for the auditory GBR. 21 healthy participants without hearing deficits were investigated in a forced-choice discrimination task. Sinusoidal tones were presented at three systematically varied sound intensities (30, 45, 60 dB hearing level). The results of the auditory evoked potentials were predominantly consistent with previous studies. Furthermore, we observed an augmentation of the evoked GBR with increasing sound intensity. The analysis indicated that this intensity difference in the GBR amplitude most likely arises from increased phase-locking. The results demonstrate a distinct dependency between sound intensity and gamma-band oscillations. Future experiments that investigate the relationship between auditory evoked GBRs and higher cognitive processes should therefore select stimuli with an adequate sound intensity and control this variable to avoid confounding effects. In addition, it seems that gamma-band activity is more sensitive to exogenous stimulus parameters than evoked potentials.     

Click evoked neurogenic vestibular potentials (NVESTEPs): a method of assessing the function of the vestibular system

OBJECTIVES: To obtain neurogenic vestibular evoked potentials (NVESTEPs) with surface scalp recording using high intensity auditory clicks. The same stimulus is used in myogenic vestibular evoked potentials which has been shown to evoke potentials in the vestibular division of the vestibulocochlear nerve. METHODS: A whole head recording with surface EEG electrodes was performed using high intensity clicks in one normal volunteer to determine the best recording position for vestibular evoked potentials. The results were compared to responses at moderate click intensities used for brainstem auditory evoked potentials (BAEPs). The difference in the location of the two responses on the scalp was assumed to be from the vestibular system. RESULTS: Responses specific to the high intensity clicks were best obtained in the parietal areas, with no reproducible responses obtained in the same area with moderate intensity clicks normally used in BAEPs. Recordings in neurologically normal volunteers showed a consistent response with a negative polarity at around 3 ms, which we therefore called N3. Two case studies are presented. The first case is a patient with unilateral sensorineural hearing loss with NVESTEPs present, suggesting that NVESTEPs is not a cochlear response. The second case is a patient with multiple sclerosis with demyelinating lesions in the pons and an unobtainable NVESTEP response. CONCLUSION: NVESTEPs is a possible new diagnostic technique that may be specific for the vestibular pathway. It has potential use in patients with symptoms of dizziness, subclinical symptoms in multiple sclerosis, and in disorders specific for the vestibular nerve.     

Multiple time scales of adaptation in the auditory system as revealed by human evoked potentials

Single neurons in the primary auditory cortex of the cat show faster adaptation time constants to short- than long-term stimulus history. This ability to encode the complex past auditory stimulation in multiple time scales would enable the auditory system to generate expectations of the incoming stimuli. Here, we tested whether large neural populations exhibit this ability as well, by recording human auditory evoked potentials (AEP) to pure tones in a sequence embedding short- and long-term aspects of stimulus history. Our results yielded dynamic amplitude modulations of the P2 AEP to stimulus repetition spanning from milliseconds to tens of seconds concurrently, as well as amplitude modulations of the mismatch negativity AEP to regularity violations. A simple linear model of expectancy accounting for both short- and long-term stimulus history described our results, paralleling the behavior of neurons in the primary auditory cortex.     

Interhemisphere Asymmetry of Auditory Evoked Potentials in Humans and Mismatch Negativity during Sound Source Localization

Results of studies in humans of long-latency auditory evoked potentials and mismatch negativity in conditions of dichotic stimulation during presentation of deviant stimuli producing instantaneous changes in stimulus azimuth from the null to +22.5° or movement at rates of 11.25–112.5°/sec from the midline of the head across the left and right hemispheres towards each ear are presented. These studies showed that the total amplitude of the components of the N1–P2 complex of auditory evoked potentials in the frontal lead of the right hemisphere was greater than that in the left hemisphere. Mismatch negativity parameters showed significant relationships with the spatial position of the sound source, namely, its displacement into the right hemisphere from the position of the sound image of the standard signal. Questions of the involvement of the right hemisphere in discriminating the spatial characteristics of sound sources are discussed.     

Auditory N1 as a change-related automatic response

To test the hypothesis that the N1 component of auditory evoked potentials (AEPs) is one form of the change-related response elicited by an abrupt change in sound pressure from a silent background, two AEP experiments were conducted. Change-N1 was evoked by a test stimulus at 70 dB following a 3-s conditioning stimulus of 0-69 dB. On-N1 was evoked by the test sound alone at various sound pressures. As the physical difference between stimuli increased, the amplitude of Change-N1 increased, and the latency shortened. The amplitude and latency of On-N1 showed a similar pattern to the Change-N1 response. These results support the idea that On-N1 is a change-related component elicited by a sound pressure change.     

Electrophysiological study of auditory development

Cortical auditory evoked potential (CAEP) testing, a non-invasive technique, is widely employed to study auditory brain development. The aim of this study was to investigate the development of the auditory electrophysiological signal without addressing specific abilities such as speech or music discrimination. We were interested in the temporal and spectral domains of conventional auditory evoked potentials. We analyzed cerebral responses to auditory stimulation (broadband noises) in 40 infants and children (1 month to 5 years 6 months) and 10 adults using high-density electrophysiological recording. We hypothesized that the adult auditory response has precursors that can be identified in infant and child responses. Results confirm that complex adult CAEP responses and spectral activity patterns appear after 5 years, showing decreased involvement of lower frequencies and increased involvement of higher frequencies. In addition, time-locked response to stimulus and event-related spectral pertubation across frequencies revealed alpha and beta band contributions to the CAEP of infants and toddlers before mutation to the beta and gamma band activity of the adult response. A detailed analysis of electrophysiological responses to a perceptual stimulation revealed general development patterns and developmental precursors of the adult response.     

Comparison of Binaural Release from Forward Masking in Animals and Humans. Electrophysiological Studies

Evoked potentials in the inferior colliculus and auditory areas of the cortex were studied in anesthetized guinea pigs and long-latency auditory evoked potentials (LAEP) were studied in waking humans using sequential binaural presentation of pairs of clicks – the masker and the masked signal – with a variable interval between them, to provide the conditions needed for the psychophysical phenomenon of direct forward masking. Introduction of phase differences between the masker and the masked signal led to decreases in suppression of responses to the masked signal and to faster recovery of the reaction types recorded. The greatest relative differences between response magnitudes to antiphase and synphase masked signals were seen at the beginning of the recovery process, and were 1.6, 1.5, and 1.4 respectively for responses from the inferior colliculus, auditory area of the cortex, and LAEP at stimulus intensities of 50–65 dB sound pressure level, differences subsequently decreasing to zero. There was a positive correlation between this measure and the stimulus intensity. The greatest differences between the time at which the recovery process ended for responses to antiphase and synphase masked signals were 4, 250, and about 2000 msec respectively for the inferior colliculus, auditory area of the cortex, and LAEP.     

Analysis of auditory information in the brains of cetaceans

A characteristic feature of the brains of toothed cetaceans is the exclusive development of the auditory neural centers. The location of the projection sensory zones, including the auditory zones, in the cetacean cortex is significantly different from that in other mammals. The characteristics of evoked potentials demonstrate the existence of several functional subdivisions in the auditory cortex. Physiological studies of the auditory neural centers of cetaceans have been performed predominantly using the evoked potentials method. Of the several types of evoked potentials available for non-invasive recording, the most detailed studies have been performed using short-latency auditory evoked potentials (SLAEP). SLAEP in cetaceans are characterized by exclusively high time resolution, with integration times of about 0.3 msec, which on the frequency scale corresponds to a cut-off frequency of 1700 Hz. This is more than an order of magnitude greater than the time resolution of hearing in terrestrial mammals. The frequency selectivity of hearing in cetaceans has been measured using several versions of the masking method. The acuity of frequency selectivity in cetaceans is several times greater than that in most terrestrial mammals (except bats). The acute frequency selectivity allows the discrimination of very fine spectral patterns of sound signals. __________ Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 92, No. 1, pp. 73–83, January, 2006.     

Sensory stimulus evokes inhibition rather than excitation in cerebellar Purkinje cells in vivo in mice

Cerebellar Purkinje cells (PC) response precisely to tactile stimulus via granule cells, however, the interaction between sensory evoked synaptic input and the resulting pattern of output spikes in cerebellar cortex is unclear. In this study, we used electrophysiological recording and pharmacological methods to investigate the cerebellar PC in response to natural stimulus on ipsilateral whisker pad in urethane-anesthetized mice. We found that air-puff stimulus on ipsilateral whisker pad evoked neither complex spikes nor simple spike firing, but indeed evoked a strong GABA(A) receptor-mediated inhibition in PCs in cerebellar cortex folium Crus II. Field potential recordings from both molecular layer and PC layer showed that air-puff stimulus evoked a sequence of parallel fiber volley followed by a GABA(A) receptor-mediated inhibition, which completely blocked by AMPA receptor antagonist, NBQX. Cell-attached recordings showed that air-puff stimulus evoked a pause of simple spike firing, GABA(A) receptor antagonist abolished the pause, revealed the tactile stimulus-evoked spike firing in PCs. These results indicated that natural stimulus of whisker pad neither evoked complex spikes, nor fired simple spikes, but induced inhibition in PCs, suggesting that the interneuron network are rapid activated and involved in controlling the spread of sensory information processing in mouse cerebellar cortex folium Crus II.     

Functional connections between auditory cortex on Heschl's gyrus and on the lateral superior temporal gyrus in humans

Functional connections between auditory fields on Heschl's gyrus (HG) and the acoustically responsive posterior lateral superior temporal gyrus (field PLST) were studied using electrical stimulation and recording methods in patients undergoing diagnosis and treatment of intractable epilepsy. Averaged auditory (click-train) evoked potentials were recorded from multicontact subdural recording arrays chronically implanted over the lateral surface of the superior temporal gyrus (STG) and from modified depth electrodes inserted into HG. Biphasic electrical pulses (bipolar, constant current, 0.2 ms) were delivered to HG sites while recording from the electrode array over acoustically responsive STG cortex. Stimulation of sites along the mediolateral extent of HG resulted in complex waveforms distributed over posterolateral STG. These areas overlapped each other and field PLST. For any given HG stimulus site, the morphology of the electrically evoked waveform varied across the STG map. A characteristic waveform was recorded at the site of maximal amplitude of response to stimulation of mesial HG [presumed primary auditory field (AI)]. Latency measurements suggest that the earliest evoked wave resulted from activation of connections within the cortex. Waveforms changed with changes in rate of electrical HG stimulation or with shifts in the HG stimulus site. Data suggest widespread convergence and divergence of input from HG to posterior STG. Evidence is presented for a reciprocal functional projection, from posterolateral STG to HG. Results indicate that in humans there is a processing stream from AI on mesial HG to an associational auditory field (PLST) on the lateral surface of the superior temporal gyrus.     

Long-latency auditory evoked potentials in humans and the localization of a sound image

In the article, we discuss data from an investigation concerning how boundary conditions for the creation of sound-image movement are reflected in long-latency auditory evoked potentials and discuss how an important feature associated with the human localizing function (resistance to interference during the localization of both a stationary and a moving sound image) appears in long-latency auditory evoked potentials. We establish that a change in the parameters of a signal creating a sensation of sound-image movement results in an exhaltation of the amplitudes of the N₁ and P₂ components. The effect of binaural freedom from masking is reflected in these same components of long-latency auditory evoked potentials during movement of spatially shifting signals.Translated from Fiziologicheskii Zhurnal SSSR imeni I. M. Sechenova, Vol. 73, No. 2, pp. 260–268, February, 1987.     

Theta rhythmicities following expected visual and auditory targets.

Evoked (EPs) as well as event-related potentials (ERPs) were recorded from two groups of 10 healthy, voluntary subjects in auditory and visual modalities. For ERP recordings 'the omitted stimulus paradigm' was employed, in which the subjects were expected to mark mentally the onset time (time prediction task) of the omitted stimulus (target). The standard auditory (AEP) and visual (VEP) evoked potentials and auditory and visual ERPs to the preceding stimuli of the omitted ones were analyzed in time and frequency domains. In the time domain the time prediction task induced increases of the amplitudes of waves existing in standard EPs; however, an additional wave or component could not be detected. Analysis of amplitude frequency characteristics (AFCs) revealed, however, selective, significant increases of the theta (3-6 Hz) frequency components of the responses concerned. These theta increases were especially evident in the frontal and parietal recording sites. Our findings suggest an association between the theta frequency components of transient evoked responses, the association areas of the brain and cognitive performance. The neurophysiological basis of scalp recorded ERPs are discussed in relation to the findings of animal studies with EEG and single unit recordings from cortical and subcortical structures.     

THE INTERACTION OF RESPONSES IN THE BRAIN TO SEMANTIC STIMULI

Long time-constant EEG recording during paired stimuli has led to the discovery of the contingent negative variation or expectancy wave (Walter, 1964). This effect is produced when a conditional stimulus signals that an imperative stimulus demanding action, decision, or attention will follow at a short, constant time interval. Symbolic and meaningful stimuli were presented to subjects tachistoscopically, and the evoked responses in the brain were electronically averaged. The cerebral evoked responses to such psychological stimuli are more complex than to flashes. A slow negative DC potential shift (CNV) was seen during the interval between an auditory ready signal and the visual exposure if recognition of the stimulus was required, or if it was interesting. Following the visual exposure, a slow positive DC shift occurred. The method has been developed to study the brain responses to psychological stimuli. The amplitude of the responses relates to the information content and subjective factors rather than to the physical strength of the stimulus.     

The Role of Phase Changes in Sound Signals in Localization of Sound Sources

The auditory system in humans and animals makes virtually no discrimination of phase changes in the structure of monaurally presented sound signals. However, electrophysiological studies have demonstrated marked changes in the responses of the central parts of the auditory system when the phase structure of the signal changes during presentation of the same type of stimulation. We have suggested that this inconsistency is due to the preparative role of phase effects during monaural stimulation for subsequent operations in the auditory system involved in determining the location of a sound source in space. This report presents experimental data on defined changes (increases in amplitude) in the electrical responses of the midbrain center of the auditory system (inferior colliculus) in antiphase binaural presentation of series of sound impulses (comparison with synphase presentation). These changes may be part of the mechanism underlying the interference resistance of the auditory system during determination of the location of a sound source (binaural release from masking). Neuronal cortical activity is sensitive and selective to dynamic interaural changes in the phase spectrum of the signal, which may provide the basis of the mechanism for locating a moving sound source. Auditory evoked potentials in humans demonstrate memorizing of the direction of movement of a sound image, as shown by the changes in parameters on presentation of stimuli of different locations (deviant stimuli) differing from the standard parameters of mismatch negativity.     

视听联合刺激诱发事件相关电位的时变源分析

目的 运用偶极子源分析方法分析视（V）、听（A）单独刺激和视听（VA）联合刺激早期阶段脑功能区的动态变化.方法 采用64道脑电采集系统,记录14例志愿者在视觉、听觉和视听联合刺激下诱发的事件相关电位（ERPs）,并将这3种任务的早期阶段分成不同子时间段进行偶极子源分析.结果 视觉、听觉刺激诱发的早期偶极子源定位在特定脑皮层功能区,并且偶极子的位置和强度随时间发生变化.结论 脑功能区在视听刺激的早期阶段（＜150 ms）对感觉信息存在动态的处理过程.     

Use of pseudorandom noise in studies of auditory evoked potentials

The extent to which sensorineural systems such as the auditory system are nonlinear depends on the type of stimulus that is used, and the part of the system from which recordings are made. An estimate of the first-order Wiener kernel of the evoked response from the inferior colliculus to amplitude-modulated tones and noise was obtained by cross-correlating the response with the same pseudorandom noise as was used to amplitude modulate the sounds that were used as stimuli, in order to characterize the linear portion of the system. The shape of these cross-correlograms resembled the potentials evoked to short bursts of the unmodulated tones and noise. The degree of nonlinearity in the response to amplitude-modulated tones and noise was determined, and information about the type of nonlinearity was obtained using the inverse-repeat feature of the pseudorandom noise. Recordings both from the surface and from deep in the nucleus of the inferior colliculus revealed nonlinearities that were predominantly of an even order, but the magnitude of the nonlinearities depended on what stimulus was used, the stimulus intensity, and from which neural structure the recording was made.     

Developments for recording lumbosacral evoked potentials

Detailed measurement of lumbosacral evoked potentials is usually only feasible using sophisticated, multichannel neurophysiological equipment. This paper describes the recording technique, and the additional equipment developed for use with a high-quality two-channel recording system, the Dantec Neuromatic 2000M, to facilitate the routine measurement of these potentials. The characteristic feature is a microprocessor based control unit, which allows switching of multiple input channels prior to preamplification and detects the electrocardiogram to trigger the stimulus with a variable time delay.     

视觉听觉同时刺激模式下ERP的同步性研究

目的：研究视觉、听觉脑区在认知过程中的同步性。方法：设计了视觉、听觉同时刺激模式下的脑电实验，采用128道高密度脑电采集系统，记录了14位年龄在18～29岁之间的在校男性大学生的诱发脑电信号数据，并运用希尔伯特(Hilbert)相位同步算法对视觉、听觉区域的事件相关电位(ERP)进行同步量化。结果：在视听觉同时利用模式下，视觉脑区(枕叶)和听觉脑区(颞叶)之间的同步指数明显大于它们与其他脑区间的同步指数。结论：人在感知和认知事物时，相关的脑区间自动产生了神经活动的同步化。     

Audio–vocal system regulation in children with autism spectrum disorders

Do children with autism spectrum disorders (ASD) respond similarly to perturbations in auditory feedback as typically developing (TD) children? Presentation of pitch-shifted voice auditory feedback to vocalizing participants reveals a close coupling between the processing of auditory feedback and vocal motor control. This paradigm was used to test the hypothesis that abnormalities in the audio–vocal system would negatively impact ASD compensatory responses to perturbed auditory feedback. Voice fundamental frequency (F ₀) was measured while children produced an /a/ sound into a microphone. The voice signal was fed back to the subjects in real time through headphones. During production, the feedback was pitch shifted (?100 cents, 200 ms) at random intervals for 80 trials. Averaged voice F ₀ responses to pitch-shifted stimuli were calculated and correlated with both mental and language abilities as tested via standardized tests. A subset of children with ASD produced larger responses to perturbed auditory feedback than TD children, while the other children with ASD produced significantly lower response magnitudes. Furthermore, robust relationships between language ability, response magnitude and time of peak magnitude were identified. Because auditory feedback helps to stabilize voice F ₀ (a major acoustic cue of prosody) and individuals with ASD have problems with prosody, this study identified potential mechanisms of dysfunction in the audio–vocal system for voice pitch regulation in some children with ASD. Objectively quantifying this deficit may inform both the assessment of a subgroup of ASD children with prosody deficits, as well as remediation strategies that incorporate pitch training.