Objective response detection in the frequency domain

Several different and related measures have been proposed for objective response detection in the frequency domain. We compared magnitude-squared coherence (MSC) to phase coherence (PC) using simulations with specified signal-to-noise ratios (SNRs) and varying numbers of subaverages; the performance measure was area unde a receiver operating characteristic (ROC) curve. MSC was superior to PC; test time required for equivalent performance is about 3 times greater PC than for MSC. MSC performance for a given final SNR increased with the number of subaverages, but reached a plateau at 16 subaverages. Simulations of noise non-stationarity (high-amplitude noise in some subaverages compared to the others) led to decreased performance advantage for MSC over PC. However, weighted averaging restored this advantage. MSC is shown to be a simple algebraic transform of Victor and Mast's (1991) “circular T²” statistic and of two earlier statistics; all have identical statistical power.     

Optimal (‘Wiener’) digital filtering of auditory evoked potentials: use of coherence estimates

Auditory evoked potentials (AEPs) to 40 Hz clicks and amplitude-modulated 500 Hz tones in human subjects were digitally filtered using an optimal (‘Wiener’) filter uniquely determined for each AEP. Use of coherence functions to compute coefficients appropriate for filtering grand average AEPs or subsets such as split-half averages is described. Wiener-filtered AEPs correlated better than unfiltered AEPs with split-half replicates and with references AEPs (obtained with long data collection periods). Visual detection thresholds were lower (more sensitive) for the Wiener-filtered AEPs, but not as low as objectively determined thresholds using coherence values.     

Alterations in event-related potentials (ERPs) of MPTP-treated monkeys

Using an auditory ‘oddball’ paradigm and classical conditioning, we have studied auditory evoked potentials (AEPs) and P300-like potentials in monkeys pre- and post-MPTP treatment. Free-field acoustic stimuli were 500 Hz and 4000 Hz tones, which were designated as the ‘frequent’ and ‘rare’ conditions, respectively. The 4000 Hz stimuli were reinforced with mild somatosensory electrical stimulation. During the first few weeks following 1-methyl-4-phenyl-1,2,5,6-tetrahydropyridine (MPTP) administration, all monkeys gradually developed a parkinsonian syndrome, which partially, but not completely improved within 30–40 days in 2 animals. The amplitudes of the AEP were initially significantly decreased, but progressively returned to pretreatment magnitudes in the 2 monkeys which partially recovered. P300-like potentials were initially abolished in all animals; however, 30–40 days later P300 spontaneously re-emerged in the same 2 monkeys. Latencies of both of these signals were unaffected by MPTP. Acute administration of dopamine precursor during the first phase of neurotoxicity partially and temporarily improved depressed AEP amplitudes, but did not restore absent P300-like potentials. The relevance of these results for Parkinson's disease is discussed.     

Changes in EEG power, acoustic evoked potentials and heart rate after mildly arousing non-acoustic priming stimuli in carp (Cyprinus carpio).

1. Changes in EEG power spectrum of carp to a priming non-acoustic stimulus followed by acoustic clicks were compared to those due to acoustic clicks delivered alone. Recordings were made from the telencephalon, midbrain and medulla. Acoustic evoked potentials (AEPs) to the clicks were also recorded. 2. EEG power changes to non-acoustic stimuli occurred over the whole 1-40 Hz frequency range and were regionally specific and consistent. 3. The changes in the EEG midfrequency 12-24 Hz power spectrum to non-acoustic stimuli were significantly correlated with changes in the AEP to subsequent clicks. An elevated medullary AEP amplitude and reduced duration were correlated with increased medullary EEG power and increased midbrain AEP duration. 4. Telencephalic EEG power changes were inversely related to changes in medullary and midbrain AEP amplitude.     

Spatial frequency response functions obtained from cat visual evoked potentials

Visual evoked potentials (VEPs) were obtained from the surface of teh cat visual cortex in response to contrast reversing sinusoidal gratings. Gratings of different spatial frequency were presented either separately, using signal averaging to increase the signal-to-noise ratio, or as a spatial frequency sweep, in which spatial frequency was sequentially increased every 5 sec during a 40 sec trial (3.99 Hz) or every 3 sec during a 24 sec trial (6.65 Hz). The second harmonic amplitude- and phase-spatial frequency functions derived from averaging or from sweep trials were similar, indicating that the swept stimulus method can be used to provide a rapid and reliable measure of the VEP-spatial frequency function. Intravenous administration of physostigmine, an acetylcholinesterase inhibitor, evoked a spatial frequency-dependent change in VEP amplitude. At 3.99 Hz, responses to low spatial frequencies were enhanced to a greater extent than were responses to high spatial frequency stimuli. At 6.65 Hz, responses to mid-range spatial frequencies were enhanced to a greater extent than were responses to low and high spatial frequency stimuli. VEP phase at both 3.99 and 6.65 Hz was advanced to a greater degree at the higher spatial frequencies. These results indicate that the swept spatial frequency method may be useful in studying spatial frequency-dependent pharmacological effects on the VEP and support the possibility that pharmacological disruption of the cholinergic visual system can produce such changes.     

Changes in components of the auditory long-latency evoked potentials at different stages of the slow-wave sleep

In accordance with the present views, during sleep, analysis of external stimuli continues at the subconscious level, because the need to estimate the biological significance of external stimuli in order to maintain a flexible contact of a sleeping subject with the environment persists during sleep. It is known that new components of the auditory evoked potentials (AEP) appear as sleep deepens. However, the common procedure of analysis of event-related potentials averaged for a group of subjects has some drawbacks because of the interindividual variability of the event-related potentials. Therefore, an additional analysis of the interindividual variability of the AEP shape and component structure can simplify the detection of individual components of group-averaged AEP at different stages of the slow-wave sleep. The AEPs were recorded in healthy volunteers (n = 26) during falling asleep in the evening from eight EEG derivations (F3, F4, C3, C4, P3, P4, O1, O2) in reference to a linked mastoid electrode. Computer-generated sound stimuli (50 ms-pulses with the frequency of 1000 Hz, 60 dB HL) were presented binaurally through earphones with interstimulus intervals of 20-40 s. Selective summation of AEPs for all the subjects was performed for each stage of the slow-wave sleep individually for each of the eight derivations. It was shown that the account made for interindividual variability of the AEP shape facilitated the identification of individual components of the group-averaged AEP typical of wakefulness (P1, N1, P300) and those which appeared during sleep onset and at different stages of the slow-wave sleep (P2, N350, P450, N550, N900).     

Method for predicting an EEG waveform as an aid to the accurate recording of evoked potentials

Evoked potentials are the brain's responses to incoming stimuli and are usually recorded under noisy circumstances. To improve the signal-to-noise ratio, signal averaging has been widely applied to the recorded data, but an improved method is required. A method of EEG waveform prediction for the accurate recording of evoked potentials is proposed, and an electroencephalographic waveform predicted by using an EEG model and a nonlinear exponentially weighted least squares method, subtracting the predicted EEG waveform from the raw data and extracting the evoked waveform. By the use of this method, we have successfully predicted the EEG waveform and detected the evoked potential with only a small number of averages.     

Auditory evoked potentials for the assessment of depth of anaesthesia: different configurations of artefact detection algorithms.

Monitoring the depth of anaesthesia has become an important research topic in the field of biosignal processing. Auditory evoked potentials (AEPs) have been shown to be a promising tool for this purpose. Signals recorded in the noisy environment of an operating theatre are often contaminated by artefacts. Thus, artefact detection and elimination in the underlying electroencephalogram (EEG) are mandatory before AEP extraction. Determination of a suitable artefact detection configuration based on EEG data from a clinical study is described. Artefact detection algorithms and an AEP extraction procedure encompassing the artefact detection results are presented. Different configurations of artefact detection algorithms are evaluated using an AEP verification procedure and support vector machines to determine a suitable configuration for the assessment of depth of anaesthesia using AEPs.     

睡眠剥夺对脑电活动相位相干性的影响研究

将小波变换和相位相干分析应用到事件相关电位实验的脑电信号中。在正常状态和一夜睡眠剥夺状态下提取12名受试者的视觉ERP,进行30～60Hz的小波变换,以此计算前额叶区域的导联内相位相干,以及枕叶和前额叶之间的相位相干性。发现睡眠剥夺引起前额叶的导联内相位相干活动减少和延迟,表明大脑维持完成任务的能力下降;枕叶与前额叶之间的gamma波段相位相干活动减少,表明功能区域之间的电活动传递效应减弱。基于小波变换的相位相干分析可以得到脑电的同步活动,为更好地理解睡眠的机制和评价睡眠剥夺对认知的影响提供了一条思路。     

Temporal resolution of the Risso’s dolphin, Grampus griseus, auditory system

Toothed whales and dolphins (Odontocetes) are known to echolocate, producing short, broadband clicks and receiving the corresponding echoes, at extremely rapid rates. Auditory evoked potentials (AEP) and broadband click stimuli were used to determine the modulation rate transfer function (MRTF) of a neonate Risso’s dolphin, Grampus griseus, thus estimating the dolphin’s temporal resolution, and quantifying its physiological delay to sound stimuli. The Risso’s dolphin followed sound stimuli up to 1,000 Hz with a second peak response at 500 Hz. A weighted MRTF reflected that the animal followed a broad range of rates from 100 to 1,000 Hz, but beyond 1,250 Hz the animal’s hearing response was simply an onset/offset response. Similar to other mammals, the dolphin’s AEP response to a single stimulus was a series of waves. The delay of the first wave, PI, was 2.76 ms and the duration of the multi-peaked response was 4.13 ms. The MRTF was similar in shape to other marine mammals except that the response delay was among the fastest measured. Results predicted that the Risso’s dolphin should have the ability to follow clicks and echoes while foraging at close range.     

Dynamics of potentials evoked in the auditory pathway and reticular formation of the cat. Studies during waking and sleep stages

Averaged evoked potentials in the inferior colliculus (IC), medial geniculate nucleus (MG) and reticular formation (RF) of chronically implanted and freely moving cats were measured using auditory step functions in the form of tone bursts of 2000 Hz. The most prominent components of the AEP of the inferior colliculus were a positive wave of 13 msec and a negative wave of 40–55 msec latency. The AEP of the medial geniculate nucleus was characterized by a large negative wave peaking at 35–40 msec. During spindle sleep and slow wave sleep stages changes in the AEPs of both nuclei occured.Transient evoked responses of the inferior colliculus, medial geniculate nucleus and reticular formation were transformed to the frequency domain using the Laplace transform (one sided Fourier transform) in order to obtain frequency characteristics of the systems under study. The amplitude characteristics of IC, MG. and RF obtained in this way revealed maxima in alpha (8–13 Hz), beta (18–35 Hz) and higher frequency (50–80 Hz) ranges. During spindle sleep stage a maximum in the theta frequency range (3–8 Hz) and during slow wave sleep maximum in the delta (1–3 Hz) frequency range appeared in the amplitude characteristics of these nuclei.The amplitude characteristics of the inferior colliculus and medial geniculate nucleus were compared with the amplitude characteristics of other brain structures. The comparison of AEPs and amplitude frequency characteristics obtained using these AEPs reveals that the existence of a number of peaks (waves) with different latencies in the time course does not necessarily indicate the existence of different functional structures or neural groups giving rise to these waves. The entire time course of evoked potentials and not the number and latencies of the waves, carries, the whole information concerning different activities and frequency selectivities of brain structures.Supported by Turkish Scientific and Technical Research Council Grant TAG-266.Presented in Part at the VIIIth International Congress of Electroencephalography and Clinical Neurophysiology in Marseilles, September 1–7, 1973.     

Use of wavelet coherence to assess two-joint coordination during quiet upright stance

Joint coordination plays a critical role in maintaining postural stability, yet there is limited existing work describing joint coordination patterns in the time–frequency domain. Here, two-joint coordination was examined during quiet upright stance. A wavelet coherence method was applied to quantify the coherence between ankle–trunk and ankle–head angles in the sagittal and frontal planes. Wavelet coherence results indicated intermittent joint coordination particularly for frequencies of 2.5–4.0 Hz. Coherence results were further processed to estimate mean time intervals between coherence instances, coherence burst frequency, and the ratio of in-phase versus anti-phase behaviors. Time intervals between intermittent coherence were 1.3–1.5 sec, coherence burst frequency was ~0.4 Hz, and phase ratios were ~1.0. Intermittent “bursting” of postural muscles may account for the finding of intermittent coherence in the noted frequency band. Some age and/or gender differences in coherence were found, and may be related to comparable differences in postural control ability or strategies. Results from application of this new method support earlier evidence that kinematic coordination is achieved intermittently rather than continuously during quiet upright stance. This method may provide richer information regarding such coordination, and could be a useful approach in future studies.     

用独立分量分析和小波变换方法研究实验过程中脑诱发电位信号强度的变化

诱发电位的提取通常依靠相干平均方法,需要进行多次的重复刺激,实验时间较长。随着实验时间的增加,受试者生理因素及环境因素的变化,会影响诱发电位的正常形态(波形、强度和相位)。利用独立分量分析和小波变换方法,通过时域信息和空域信息的综合应用,可成功提取到听觉诱发电位晚成分的强度在实验过程中的变化,对由于实验时间增加对晚成分的影响做出定量评价。结果表明,在10min左右的实验过程中,听觉诱发电位晚成分的幅度会下降约40％。     

Is the somatosensory N250 related to deviance discrimination or conscious target detection?

Effects of attention to, and probability of sudden changes in, repetitive stimuli on somatosensory evoked potentials (SEP) were studied. Low- (30 Hz) and high-frequency (140 Hz) vibratory stimuli were delivered in random order to the middle finger of the left hand with different presentation probabilities in different blocks. Also ignore conditions were administered.In the ignore conditions, the probability had no effect on SEPs. However, when the standard stimuli were omitted, the “deviants” elicited small N140 and P300 deflections not observed in response to deviants when standards were also present. In the attention conditions, deviant stimuli (targets) elicited large N250 and P300 deflections which increased in amplitude with a decreased target probability. However, when subjects counted infrequently presented “deviants” alone (standards omitted) the enhanced N140 and the P300 with shortened latency were elicited, but no N250 wave could be found. At the ipsilateral side, a distinct N200 deflection was seen which could be the N250 with a shorter latency because of an easier task (detection instead of discrimination). The results might be interpreted as suggesting that the somatosensory N250 is related to conscious detection of target stimuli.     

Speech-evoked activity in primary auditory cortex: effects of voice onset time

Neural encoding of temporal speech features is a key component of acoustic and phonetic analyses. We examined the temporal encoding of the syllables /da/ and /ta/, which differ along the temporally based, phonetic parameter of voice onset time (VOT), in primary auditory cortex (A1) of awake monkeys using concurrent multilaminar recordings of auditory evoked potentials (AEP), the derived current source density, and multiunit activity. A general sequence of A1 activation consisting of a lamina-specific profile of parallel and sequential excitatory and inhibitory processes is described. VOT is encoded in the temporal response patterns of phase-locked activity to the periodic speech segments and by “on” responses to stimulus and voicing onset. A transformation occurs between responses in the thalamocortical (TC) fiber input and A1 cells. TC fibers are more likely to encode VOT with “on” responses to stimulus onset followed by phase-locked responses during the voiced segment, whereas A1 responses are more likely to exhibit transient responses both to stimulus and voicing onset. Relevance to subcortical speech processing, the human AEP and speech psychoacoustics are discussed. A mechanism for categorical differentiation of voiced and unvoiced consonants is proposed.     

Tibial nerve somatosensory evoked response detection using uni and multivariate coherence

This work aims at comparing the capability of two Objective Response Detection techniques, the Magnitude-Squared Coherence (MSC or Ordinary Coherence) and its multivariate extension, the Multiple Coherence (MC), of detecting the somatosensory evoked response. Electroencephalographic (EEG) signals were collected during somatosensory stimulation from forty adult volunteers without history of neurological disease and with normal somatosensory evoked potential (SEP), using the 10-20 International System. All leads were referenced to the earlobe average. Current pulses with 200 μs of duration were applied to the right posterior tibial nerve at the motor threshold intensity level (the lowest intensity able to produce hallux oscillations) at the rate of 5 Hz. The MSC was applied to the derivations [Cz], [Fz], [C3] and [C4] – commonly used for tibial nerve SEP recordings with bipolar derivations – and the MC was applied to the pairs [Cz][Fz] and [C3][C4]. Both estimates (MC and MSC) were calculated with M = 100 and 500 epochs and the response detection was based on rejecting the null hypothesis of response absence, which is achieved when the estimates exceed the critical value (detection threshold) calculated for a given significance level (α = 0.05). The results showed that if two leads are available, the application of the MC is better than the MSC applied to each lead individually.     

Scalp potentials following sudden coherence and discoherence of binaural noise and change in the inter-aural time difference: a specific binaural evoked potential or a “mismatch” response?

When uncorrelated random noise signals presented to the two ears suddenly become identical (coherent), a centrally located sound image is abruptly perceived and long latency scalp potentials are evoked. When the same signals are presented monaurally there is no perceived change and no potentials are evoked: hence the response must be purely a function of the binaural interaction.P70, N130 and P220 components were consistently recorded to both coherence and discoherence. N130 was usually largest at Fz and P220 at Cz. No potentials of shorter latency were identified, even after averaging 5000 or more sweeps. When the noise became coherent with an inter-aural time difference (δT) of ±0.5 msec (giving rise to an off-centre sound image), the responses were of slightly longer latency and showed no significant asymmetries between C3 and C4. In binaurally coherent noise, δT changes of ±0.5 or ±1.0 msec evoked similar responses which showed no significant asymmetries on the scalp. N130 was of longer latency when δT was changed from ±0.5 msec to zero, as compared with the converse change.In view of the similarity of all these responses it is considered unlikely that they were due to specific populations of binaurally responsive cortical neurones. The N130 and P220 components are thought to be non-specific potentials which are elicited by amy perceptible change in steady auditory stimulus conditions, due to a “mismatch” between the stimulus and the contents of a short-term auditory memory.     

NeuMonD: a tool for the development of new indicators of anaesthetic effect.

Electroencephalogram (EEG) signals and auditory evoked potentials (AEPs) have been suggested as a measure of depth of anaesthesia, because they reflect activity of the main target organ of anaesthesia, the brain. The online signal processing module NeuMonD is part of a PC-based development platform for monitoring "depth" of anaesthesia using EEG and AEP data. NeuMonD allows collection of signals from different clinical monitors, and calculation and simultaneous visualisation of several potentially useful parameters indicating "depth" of anaesthesia using different signal processing methods. The main advantage of NeuMonD is the possibility of early evaluation of the performance of parameters or indicators by the anaesthetist in the clinical environment which may accelerate the process of developing new, multiparametric indicators of anaesthetic "depth".     

Temporal resolution of the Florida manatee (<Emphasis Type="Italic">Trichechus manatus latirostris</Emphasis>) auditory system

Auditory evoked potential (AEP) measurements of two Florida manatees (Trichechus manatus latirostris) were measured in response to amplitude modulated tones. The AEP measurements showed weak responses to test stimuli from 4 kHz to 40 kHz. The manatee modulation rate transfer function (MRTF) is maximally sensitive to 150 and 600 Hz amplitude modulation (AM) rates. The 600 Hz AM rate is midway between the AM sensitivities of terrestrial mammals (chinchillas, gerbils, and humans) (80–150 Hz) and dolphins (1,000–1,200 Hz). Audiograms estimated from the input–output functions of the EPs greatly underestimate behavioral hearing thresholds measured in two other manatees. This underestimation is probably due to the electrodes being located several centimeters from the brain.     

KILLER WHALE (ORCINUS ORCA) AUDITORY EVOKED POTENTIALS TO RHYTHMIC CLICKS

Temporal auditory mechanisms were measured in killer whales ( Orcinus orca ) by recording auditory evoked potentials (AEPs) to clicks. Clicks were presented at rates from 10/sec to 1,600/sec. At low rates, clicks evoked an AEP similar to the auditory brainstem response (ABR) of other odontocetes; however, peak latencies of the main waves were 3–3.7 msec longer than in bottlenose dolphins. Fourier analysis of the ABR showed a prominent peak at 300–400 Hz and a smaller one at 800–1,200 Hz. High-rate click presentation (more than 100/sec) evoked a rate-following response (RFR). The RFR amplitude depended little on rate up to 400/sec, decreased at higher rates and became undetectable at 1,120/sec. Fourier analysis showed that RFR fundamental amplitude dependence on frequency closely resembled the ABR spectrum. The fundamental could follow clicks to around 1,000/sec, although higher harmonics of lower rates could arise at frequencies as high as 1,200 Hz. Both RFR fundamental phase dependence on frequency and the response lag after a click train indicated an RFR group delay of around 7.5 msec. This corresponds to the latency of ABR waves PIII-NIV, which indicates the RFR originates as a rhythmic, overlapping ABR sequence. The data suggest the killer whale auditory system can follow high click rates, an ability that may have been selected for as a function of high-frequency hearing and the use of rapid clicks in echolocation.