A Speed Trap on the Auditory Pathway Investigation of Early Auditory Evoked Brainstem Activity

A Speed Trap on the Auditory Pathway Investigation of early auditory evoked brainstem activity

Masterarbeit

zur Erlangung des Mastergrades MSc. an der Naturwissenschaftlichen Fakultät der Paris-Lodron Universität Salzburg

eingereicht von Florian Geyer

Gutachter: Univ.-Prof. Dr. Nathan Weisz

Fachbereich: Psychologie

Salzburg, November 2018 A Speed Trap on the Auditory Pathway – Investigation of early auditory evoked brainstem activity

Abstract

In the first 10 ms after acoustic stimulation afferent information “travels” through the auditory pathway. Starting in cochlear nerve these early auditory evoked activity proceeds through the brainstem and parts of the thalamus into the auditory cortex. Early auditory evoked activity usually consists of five to seven positive vertex potentials and can be measured via

Electrocochleography or ABR. Those wave peaks can be defined and correlated to their source in the auditory pathway, e.g. wave V is located in the inferior colliculi of the brainstem and is known to elicit a high peak in ABR. Clinicians use early auditory evoked potentials as diagnosis tool for audiological diseases. However, analysis is mostly done visually. This can be challenging. We introduce an approach to get information about the individual wave-region correlation. This has translational potential to clinical applications (e.g. optimization of early auditory evoked activity as a diagnosis tool by facilitating wave detection) and neuroscientific research (e.g. investigation of attentional modulation of early auditory evoked activity). We measured early auditory evoked activity of 18 healthy participants as response to a 30 hz click stimulation at 60 db SPL with magnetic ABR, electrical ABR, ECochG and MEG. We then used

Backward Decoding Model to extract a spatial filter from MEG with reference to ABR. This filter could potentially be used to predict ABR signal from any acoustic stimulated MEG Data. We then decoded MEG data to prove reconstruction of ABR is possible. Additionally, we “took a picture” of activation at the brainstem on its way “racing” through the auditory pathway using

Forward Encoding Model to validate wave-region correlation. In wave V we found no activation at brainstem areas. Repeating source reconstruction for wave VII showed the expected cortical activation. One possible explanation is that reconstruction of very deep subcortical sources is difficult. Another explanation suggests activation to be earlier than expected.

Keywords: Auditory Brainstem Response, ABR, Electrocochleography, ECochG, Early

Auditory Evoked Potentials, Magnetoencephalographie, MEG, Brainstem, Forward Encoding

Model, Backward Decoding Model, Decoding, Encoding, Localization

A Speed Trap on the Auditory Pathway – Investigation of early auditory evoked brainstem activity

Zusammenfassung

In den ersten 10 ms nach akustischer Stimulation wandern afferente Informationen den auditiven Pfad entlang. Ausgehend vom Nervus cochlearis „fährt“ diese frühe akustisch evozierte Aktivität über den Hirnstamm durch Teile des Thalamus in den auditiven Cortex. Sie wird in 5 bis 7 positive Vertex-Wellen unterteilt und kann durch Electrocochleographie

(ECochG) oder Hirnstamm – Messung (ABR) gemessen werden; jeder dieser Vertex-Wellen wird eine Region auf dem Auditiven Pfad zugeordnet. Beispielsweise wird der fünften Welle

Aktivierung im Hirnstamm und der siebten Welle bereits kortikale Aktivierung im auditiven

Cortex zugesprochen. Besonders in der ABR wird bei der fünften Welle ein starker positiver

Ausschlag gemessen. Frühe akustisch evozierte Potenziale werden zur Diagnostik audiologischer Erkrankungen herangezogen. Dabei werden die Wellen zumeist visuell analysiert, was potentielle Schwierigkeiten mit sich bringt. Wir möchten eine Möglichkeit aufzeigen, Informationen über individuelle Wellenlokalisation im Gehirn zu erhalten. Wir testeten an 18 gesunden Probanden frühe akustisch evozierte Aktivität als Antwort auf 30 Hz

Click Stimulation mit 60 db SPL und maßen die elektrische und magnetische ABR, ECochG und Magnetoencephalographie (MEG). Mittels Backward Decoding Model extrahierten wir durch die ABR einen räumlichen Filter für MEG Signal. Zur Überprüfung des Filters dekodierten wir unser eigenen Datensatz und rekonstruierten die ABR. In einem zusätzlichen

Schritt verwendeten wir ein Forward Encoding Model, um den Ursprung der fünften Welle in den Inferior Colliculi des Hirnstamms nachweisen. Es zeigt sich jedoch keine

Hirnstammaktivität bei der fünften Welle. Bei der Lokalisation des Ursprungs der siebten Welle zeigt sich wie erwartet kortikale Aktivität. Eine Erklärung hierfür können Schwierigkeiten beim

Lokalisieren von sehr tiefen subkortikalen Quellen sein. Eine weitere Erklärung wäre eine schnellere Weiterleitung als in der vorherrschenden Literatur angenommen. Unsere

Herangehensweise hat translationales Potential für klinische Anwendungen (z.B.

Erleichterung der Wellendetektion zur Optimierung als Diagnose-Werkzeug) und neurowissenschaftliche Forschung (z.B. Untersuchung von Aufmerksamkeitsmodulation bei früher akustisch evozierter Aktivität). 2

A Speed Trap on the Auditory Pathway – Investigation of early auditory evoked brainstem activity

Schlagwörter: Frühe akustisch evozierte Potentiale, ABR, Magnetoencephalographie, MEG,

Elektrocochleographie, ECochG, Hirnstamm, Auditiver Pfad, Dekodieren, Enkodieren,

Lokalisation.

A Speed Trap on the Auditory Pathway – Investigation of early auditory evoked brainstem activity

Early auditory evoked activity and their applications

Early auditory evoked activity occurs within the first 10 ms of acoustic stimulation and carries the information about the process of acoustic stimulation in humans and animals. The path it takes, called the auditory pathway, contains the cochlear nerve, leading into the brainstem, into parts of the thalamus and ending in the auditory cortex. This is used in audiology to diagnose hearing loss and detect vestibular schwannoma (Eggermont 2017, Ferraro 2000).

Early auditory evoked activity usually consists of five to seven positive vertex potentials. In the literature those waves are corresponded to the underlying subcortical (and cortical) region at which the activity putatively originates.

Wave I emerges about 1.5 to 1.9 ms after stimulus onset in the distal auditory nerve; Wave II following in the proximal auditory nerve. Wave III is presumably located in the cochlear nucleus

(CN, Moore 1987; Møller & Jannetta 1985). Following the auditory pathway through brainstem the next region, the ipsi- and contralateral superior olivary complex, elicits Wave III (Hall 2007) or Wave IV (Moore 1987; Møller & Jannetta 1985). Wave V often shares a peak with Wave IV, appears at 5,0 to 6,0 ms and its source is the lateral lemniscus and inferior colliculi (IC) of the brainstem (Moore 1987; Møller & Jannetta 1985). In a lesion study Durrant, Martin, Hirsch &

Schwegler found 1994 that lesions to the right IC leads to a loss of perception for the left ear, suggesting a contralateral structural organization. The auditory pathway passes the structures of wave I-III ipsilateral, at wave IV bilateral and from wave V on contralateral (Hall 2007). Later

Potentials Wave VI and VII are proposed to be generated by medial geniculate body (Moore

1987; Møller & Jannetta 1985), although Brugge et al. (2008, 2009) even found the first cortical responses at 9 to 12 ms, a time window which could match wave VII. This is supported by Hall

(2007). However, correlating one wave to one specific source is not always that easy because other sources higher or lower in the auditory pathway also attribute to one specific wave. (Hall

2007). Fig. 1 displays each Wave with its underlying region in the auditory pathway.

A Speed Trap on the Auditory Pathway – Investigation of early auditory evoked brainstem activity

Fig. 1.: A display of the Auditory Pathway with the corresponded subcortical regions for Wave I-VII as described by Moore 1987, Møller & Jannetta 1985 and Hall 2007.

There are different measuring methods with slightly different fields of application: Auditory

Brainstem Response (ABR) and Electrocochleography (ECochG).

ABR is measured as a subcortical electrical activation by applying a single EEG electrode on the forehead at FpZ or Cz electrode position (Jasper 1958) and is due to its distance to the measured sources often referred as far-field measurement. The peaks displayed above are usually seen in ABR measurement. But not every wave can be seen in every subject.

Especially wave III and V are found in most subjects. ABR is displaying a strong double peak

A Speed Trap on the Auditory Pathway – Investigation of early auditory evoked brainstem activity for wave IV and wave V (Jewett & Williston 1971). This makes ABR sensitive to activity of brainstem areas, i.e. the lateral lemnisci and the inferior colliculi. In practice, it is used as hearing screening and as diagnosis tool to hearing loss. Parkkonen, Fujiki & Mäkelä (2009) even measured the magnetic auditory brainstem response (mABR) in addition to electrical

ABR by using Magnetoencephalography (MEG). They showed that though there are differences in amplitude latencies of each wave are identical for MEG and EEG measurement.

Another measurement for early auditory activity is ECochG. Here the electrical signal of the nerves is measured in the ear canal by placing electrodes in vicinity of the cochlear nerve as possible, either in an invasive, but non-surgical way with trans-tympanic electrodes pierced through the tympanic membrane, or non-invasive with extra-tympanic electrodes placed in the ear canal. ECochG is used by audiologists for over 75 years, mainly to diagnose tumors or

Meniere`s Disease. The signal contains different aspects which can be considered: The cochlear microphonic (CM) is an alternating current voltage which is generated by outer hair cells (OHC) and reflects cochlear activity. The summating potential (SP) is a direct current which is generated by inner hair cells (IHC) and OHC. The duration of SP depends on the duration of acoustic stimulation. The action potential (AP) is an alternating current which reflects the synchronous activation of hearing nerves and is one way to measure early auditory evoked activity. The AP is independent of stimulus’s duration or phase (Ferraro 2000; Durrant,

Wang, Ding & Salvi 1998). For clinical diagnosis, most often the SP/AP amplitude ratio is used.

It also has been suggested to detect hidden hearing loss in humans (Liberman, Epstein,

Cleveland, Wang & Maison 2016). The Action Potential itself is congruent with the Wave I and thus measures the early auditory activity in the cochlear nerve (Fig.2). Compared to the ABR measurement Wave I elicits a huge peak in ECochG due to the reduced distance to the cochlear nerve (Minaya & Atcherson 2015)

A Speed Trap on the Auditory Pathway – Investigation of early auditory evoked brainstem activity

Fig. 2.: The time course of exemplary ABR and ECochG are contrasted. ABR signal (top) displays Wave

I, III and V, ECochG signal displays baseline, SP and AP. The accordance of Wave I and AP is accentuated. The polarity of ECochG is arbitrary. This figure is adapted from Minaya & Atcherson 2015.

It must be mentioned that ECochG and ABR signal highly differs between subjects in aspects of amplitude and latency. Mitchell, Philips and Trune (1989) found that ABR measurement is dependent from factors like gender, head size, age and hearing threshold. The Amplitude and

Latency of ABR depends on volume of acoustic stimulation. Lewis, Kopun, Neely, Schmid &

Gorga (2015) found that amplitude and latency can differ for all waves depending on volume level, i.e. Wave V latency differs between 6ms at 90 dB SPL to 11 ms at 20 dB SPL. For our volume level of 60 dB SPL Lewis et al. found latencies of 6,9 ms to 8,5 ms at 4kHz tone burst stimulation in normal hearing subjects. ECochG signal is also affected from different aspects of the presented auditive stimuli. Tones are usually presented as click bursts. Kaf et al. (2017) showed that lower click rates and lower volume can lead to deterioration of the measured signal.

Although early auditory evoked activity is measured with ECochG and ABR since many years and wave-region correlation is widely accepted and published in textbooks, diagnosis is usually

A Speed Trap on the Auditory Pathway – Investigation of early auditory evoked brainstem activity based on the clinicians’ decision about labelling waves with help of latency and amplitude. As mentioned before, this can be quite challenging because wave-region correlation cannot be done in a one-to-one like manner and is interfered with huge interindividual differences. A tool is needed that facilitates the detection of waves by providing additional information about the waves’ sources on the individual level and by unraveling the wave-region correlation more clearly.

Magnetoencephalography (MEG) can be used to reconstruct the source of oscillating activity for deeper brain regions. Parkkonen et al. (2009) already tracked the originating locations of

Wave I, Wave II and Wave V by using simultaneous MEG and EEG. They used source reconstruction on the magnetic ABR signal of seven healthy patients to locate them. This however is not very practicable for clinical applications because it includes again manual wave detection, time-consuming source reconstruction and requires an individual structural MRI for each patient.

In this master thesis we introduce a new perspective to capture early auditory evoked activity and relate it to their spatial domain.

One upcoming mathematical way to correlate neural activation to stimulus features are

Backward Decoding Models (BDM) and Forward Encoding Models (FEM). BDMs can predict stimulus features based on the neural activation pattern. One of BDMs’ output are their correlation coefficients, so called weights. These weights could potentially be used on another set of neural activation pattern (Crosse, Di Liberto, Bednar & Lalor 2016). FEMs use an opposite approach. They express the observed neural data as functions for the stimulus features and provide a model for the generation of the neural data. FEM weights are meaningful in the sense that they display neural activation (Haufe et al. 2014). Therefore, they can be plotted and interpreted topographical or on source space.

We measure the early auditory evoked activity of 18 participants simultaneously in MEG,

ECochG and electric / magnetic ABR. We then use BDM to correlate the neural activation in the MEG to the signal of the measuring methods (ECochG, ABR, mABR). We take the weights

A Speed Trap on the Auditory Pathway – Investigation of early auditory evoked brainstem activity of the BDM as output to use them as a spatial filter for MEG dataset. This filter can be adapted for more sensitivity by using different measuring methods as reference. While ABR is known to elicit a huge peak for wave V, one may take ABR as reference for wave V detection. In ear measured ECochG however should enable a better estimation for activity of auditory nerve fibers., therefore it is possible to use ECochG as a reference for wave I.

To prove the potential of the weights as spatial filter we use BDM to reconstruct the original signal and FEM to locate the underlying regions via source reconstruction.

Our aim is to lay steps for the development of a gauge which has translational potential to clinical settings and neuroscientific research. As a clinical application, early auditory evoked activity as a diagnosis tool could be optimized to facilitate the wave detection on the individual level. In neuroscientific research such gauge could provide new perspectives to investigate various aspects of the physiological basis and behavioral modulation of early auditory evoked activity. One interesting concept is the investigation of attentional modulation of this early activation.

A Speed Trap on the Auditory Pathway – Investigation of early auditory evoked brainstem activity

Hypothesis

This research is divided into two different master`s theses. One potential advantage of our approach is definitively the possibility to detect very early activity in the hearing nerve. A spatial filter for auditory nerve activity may have important applications in auditory neuroscience. This is researched in another work. On the other side, there is importance to detect wave V because it is known to be the most reliable wave in humans and it may contain potential for the diagnosis of hidden hearing loss (Mehraei et al. 2016). This master thesis mainly focuses on brainstem activation and therefore uses ABR to “trap” activity there.

Comparing all measuring methods, we expect ABR and mABR to show the best sensitivity

(through the highest deviation in amplitude) at Wave V time points. The Latency of Wave V in

ABR should be shifted due to volume level at 60 dB SPL which is lower compared to diagnostic measurements. While electrical and magnetic ABR differ in amplitude, the latency remains the same. Therefore, the most prominent peak in electrical and magnetic ABR should occur between 7.0 to 8.5 ms after stimulus.

ABR is now taken as reference for BDM to calculate the spatial filter. As a proof of principle, we decode the ABR from MEG data and compare it to the original ABR.

Using source reconstruction on FEM weights we localize the source of activity at the peak found for Wave V and Wave VII in ABR. The corresponding brain areas for wave V should be the brainstem, in particular the lateral lemnisci or inferior colliculi, and for wave VII the auditory cortex.

A Speed Trap on the Auditory Pathway – Investigation of early auditory evoked brainstem activity

Methods & Materials

Participants

Eighteen healthy participants, 13 of them male, with age ranging from 21 to 38 years (mean =

26,17 years, SD = 4,09 years) were tested. They had normal or corrected-to-normal vision and reported no history of psychiatric or neurological disorders and gave written informed consent.

The measurement was part of an experiment of two hours which was approved by University

Salzburg’s ethics committee. Participants were rewarded with either 20 Euro or experiment credits for psychology undergraduate students.

MEG Acquisition

The experiment took place in a shielded room (AK38, Vakuumschmelze, Hanau, Germany) in the Christian-Doppler Klinik Salzburg, isolated from outer magnetic fields and environmental sounds. The neurophysiological data were acquired with Elekta Neuromag Triux MEG with

306 MEG channels (102 magnetometers, 204 planar gradiometers) by ElektaOy (Helsinki,

Finland). Head Position Indication (HPI) was done by constructing a head shape model via 5

HPI Coils and around 300 additional points, including 2 preauricular points and the nasion, with Polhemus Fastrak Digitizer (Polhemus, Vermont, USA). Acquisition was done for MEG,

ECochG and ABR with a 10000 Hz sampling rate.

ECochG and ABR Acquisition

For ECochG measurement extra-tympanic tiptrode electrodes (ER3-26, 13 mm by MedCaT,

Netherlands) were used for both presenting auditory stimuli through a pneumatic tube and measuring electrical activity from the ear canal via the tiptrodes’ gold foil. For better impedances Nuprep® Electrode gel (Weaver and Company, Colorado, USA) was applied carefully on the gold foil. ECochG of both ears were measured in a unipolar setup. ABR was measured via a single EEG electrode placed at FpZ of commonly used 10-20 EEG placement system (Jasper 1958). A ground electrode was attached on the forehead and a reference electrode on the neck. Impedances of ECochG and ABR were checked and kept below 5 kΩ.

A Speed Trap on the Auditory Pathway – Investigation of early auditory evoked brainstem activity

Additional electrodes for horizontal and vertical electrooculography (EOG) and electrocardiography (ECG) were applied to detect heartbeat and eye movement artefacts.

Stimulus Material and Experiment Design

Participants were presented 10 000 rarefaction and condensation clicks binaurally at 60db

SPL. There was no time shift between ears. Clicks were presented at 30 Hz frequency for 5:45 minutes. One single click had a duration of 80μs. Clicks were produced with SOUNDPixx system (Vpixx technologies, Canada) and it took 16,5ms to reach participant´s ear. This was compensated in data analysis. Participants had no specific task other than to listen to the stimulation with eyes opened. The stimulation was programmed in Matlab 2016b (The

MathWorks, Inc., USA) with the Psychophysics Toolbox 3 (Brainard, 1997).

Filter and Data Processing

Raw MEG, ABR and ECochG Data was filtered with Signal Space Separation (SSS) method via MaxFilterTM (Elekta Oy, Finland) and preprocessed using the Fieldtrip Toolbox by

Oostenveld, Fries, Maris, Schoffelen (2011) of Matlab 2016b. We used 8th order butterworth filter at 150 hz highpass and 2500 hz lowpass to clean data from environmental noise.

Remaining artefacts were rejected visually based on their kurtosis, maxabs and z-score values.

Magnetic ABR was derived from the (original) MEG signal calculated as global mean field power (Esser, Huber, Massimini, Peterson, Ferrarelli & Tononi 2006). This is one approach to calculate the total mean of all MEG Channels.

Then the averages of all trials were calculated for each participant. To enable comparison, all measuring methods (ECochG, ABR, mABR) and MEG data were z-transformed.

Average over all subjects was calculated on different points. For method comparison the average was calculated before z-transformation. The weights of FEM are averaged after z- transformation. Source reconstruction was first performed on individual data and then brought into common space.

A Speed Trap on the Auditory Pathway – Investigation of early auditory evoked brainstem activity

Then the averaged signals were inspected for their wave informations. In clinical settings this is done manually by the clinician/researcher. We used the function “findpeaks” of Matlab to mark every local maximum for the different measuring methods with z-scores over 0.5. Those marked peaks were considered as waves and labelled manually (after latency, expectation).

Additional peaks could also manually be considered as waves if they were meaningful additions to the time course of the already marked waves and if their latencies are consistent over almost all methods.

Peaks found with Matlab`s “findpeaks” were compared to the baseline with Bonferroni corrected t-tests.

BDM and FEM

Each measuring method (ECochG left and right, ABR, mABR) was used in a ridge regression model (BDM and FEM) with the MEG data to solve for their linear mapping function. They were processed with the mTRF toolbox (Crosse et al. 2016) in Matlab. FEM was used to provide a model for generation of neural data, the temporal response function. Unlike the weights of

BDM, the weights of FEM can be interpreted directly because they display neural activation.

This method was used to provide data for source reconstruction of wave V and VII. BDM was used to extract weights to decode magnetic neural signal. We used these weights to decode the MEG signal and reconstruct the reference signal, either EcochG left and right, ABR or mABR (Crosse et al. 2016, Haufe et al 2014).

Source Analysis

The source reconstruction of FEM weights required individual structural brain images. Seven of our participants were measured in the MRI (MAGNETOM TIM Trio 3 Tesla by Siemens) of the Centre of Cognitive Neuroscience (CCNS) in Salzburg and provided their structural T1 images. For the other participants, the head shape model of MEG head position indication was used to construct individual single shell head models (Nolte 2003). Co-registration was done with 2 preauricular points and the nasion to adapt the individual brains to the Montreal

Neurological Institute (MNI) standard brain. Linearly constrained minimum variance (LCMV)

A Speed Trap on the Auditory Pathway – Investigation of early auditory evoked brainstem activity beamformer analysis (Veen, Drongelen, Yuchtman, & Suzuki, 1997) was used to project data on source level. The beamformer filters of each subject were then multiplied with the weights of FEM. The time points of source reconstruction were the timepoints of the highest peak in this wave over all subjects. The source reconstruction images were then baseline corrected and averaged over participants. A mask covering the only the brain was used to cut overlapping activity of hearing nerve to highlight brainstem and cortex for wave V and VII.

Activation in source reconstruction images displays the weights of FEM.

A Speed Trap on the Auditory Pathway – Investigation of early auditory evoked brainstem activity

Findings and Validation of the Spatial Filter

Comparison of used Methods

At first, we compared the signals of different methods. In Figure 3 the different signals for mABR, ABR and both left and right ECochG are displayed. The x-axis displays the time in seconds after stimulus onset ranging from 0 to 0.01 seconds, the y-axis shows z-scored amplitude. The lines show the average signal of all 18 subjects with their respective error bars

(standard error). Colored diamonds mark timepoints where the signal is significantly different from the baseline. Waves I-VII were identified and marked via visual inspection.

Fig. 3: Display of differences between mABR, ABR and ECochG Signals. X-axis displays time after stimulus onset in ms, y-axis displays the amplitude of methods in z-scores. The color for ABR is yellow, for mABR red, for the left ECochG green and for the right ECochG blue. Errorbars are calculated with the standard error. Waves are marked in black if there is at least one method which detected a significant peak. Wave I is displayed for every method to provide a better overview of the delayed peaks. Wave II is only presented in mABR. ABR shows the best signal for wave V, mABR shows the best sensitivity at wave I and II. ECochG left and right show wave I with a latency delay to mABR and wave V with no delay.

Magnetic ABR shows significant timepoints from 1.1ms to 4.0 ms, 4.7m to 4.9ms, 5.1ms to

5.2ms, 5.5ms to 5.7ms, 6.4ms to 6.5ms and one single significant timepoint at 7.3 ms. We labelled the peaks at 2.0 ms as Wave I, 2.6ms as Wave II, 3.8ms as Wave III, 6.5ms as Wave

IV, 7.3 ms as Wave V. Visually one could also detect later peaks at 8.5ms (Wave VI) and

9.5ms (Wave VII).

A Speed Trap on the Auditory Pathway – Investigation of early auditory evoked brainstem activity

Left ECochG shows significant timepoints at 4.0 to 4.3 ms (4.0ms peak labelled as Wave III) and 6.9 to 7.4 ms (7.3 ms peak labelled as Wave V). Wave III in left ECochG has a 0.2ms latency delay to the Wave III of mABR. Visually we can detect a peak at 2.2ms which we labelled as Wave I. Left ECochG Wave I has a latency delay of 0.2 ms to mABR Wave I. We can also suggest a peak at 8.4 ms to be Wave VI and a peak at 9.5 ms to be Wave VII.

Right ECochG shows only a negative significant peak at 1,2 to 1,4 ms, which can be no wave after definition. Visually, right ECochG peaks additionally at Wave I at 2.4 ms with a latency delay to Wave I of left ECochG (0.2 ms) and to Wave I of mABR (0.4 ms). There are also non- significant peaks at 6.3 ms (Wave IV), 7.3 ms (Wave V) and 9.4 ms (Wave VII).

ABR only shows significance at 7.0 to 7.5 ms. This peak at 7.3 ms is marked as Wave V and doesn’t differ in latency between all methods. Non-significant peaks at 2.3ms, 3.1ms, 4.1 ms,

6.5 ms, 8.3 ms and 9.4 ms are labelled as Wave I, II, III, IV, VI and VII respectively.

The results are displayed in Tab. 1.

Wave I Wave II Wave III Wave IV Wave V Wave VI Wave VII

mABR 2.0 * 2.6 * 3.8 * 6.5 * 7.3 * 8.5 9.5

Left 2.2 4.0 * 7.3 * 8.4 9.5

ECochG

Right 2.4 6.2 7.3 9.4

ECochG

ABR 2.3 3.1 4.1 6.5 7.3 * 8.3 9.4

Tab. 1: The table contents the found latency timepoints (in ms) for wave I – VII (rows) for the different measuring methods mABR, Left ECochG, Right ECochG and ABR (columns). Stars indicate time points which are detected by the Matlab function “findpeaks”. Time points without stars are added after they were found as senseful addition in the time course. Empty arrays indicate that there was no clear wave found for this method.

A Speed Trap on the Auditory Pathway – Investigation of early auditory evoked brainstem activity

Analysis of Backward Modell and Prediction of ABR

As the comparison of measurement methods has shown ABR has the most pronounced result in Wave V. Therefore, we used ABR as reference extract the BDM weights. By applying these weights we can decode MEG data to predict the ABR. This can theoretically be done on every dataset with auditory stimulation and allows us to use the weights as a filter to extract information about early auditory evoked activity from MEG signal. Taking ABR as reference allows us a good estimation about wave V activity. Using the weights on a different MEG dataset allows us to evaluate the similarity to ABR wave V. Correlating the activation of wave

V to a specific region still needs a theoretical foundation.

To prove the capability of the weights to do this, we used the MEG data to reconstruct the ABR signal. Figure 4 shows the results of reconstruction of ABR together with the original ABR signal. The rhythm of reconstructed ABR follows the rhythm of original ABR. The correlation of the original and the reconstructed ABR is r = .998, p < 0.05.

Fig. 4: Comparison between original ABR signal (blue) and the reconstructed ABR signal (orange). The latencies are identical. X-axis displays the time in ms from 2ms before to 10 ms after acoustic stimulation, y-axis shows the amplitude standardized in z-scores. Reconstructed ABR follows the rhythmic of the original ABR, correlation r = .998, p < 0.05.

A Speed Trap on the Auditory Pathway – Investigation of early auditory evoked brainstem activity

Figure 5 is another way to display the outcome of reconstruction. Activity is displayed color coded in a matrix. This is a typical way to display the efficiency of decoding. Usually, y- axis represent training items while x-axis represent testing items. In our case, we compared the used reference to the output of the BDM, that is the predicted signal. X-axis refers to the time of from MEG reconstructed ABR and is displayed in ms, y -axis refers to the time of the original

ABR signal and is also displayed in ms. The amplitude of each signal is color coded with blue for very low amplitude and yellow for very high amplitude on both signals. If the signal of ABR matches the signal of prediction, it is displayed in a linear diagonal. If the signals don’t match, the matrix would blur out. For our data we see a linear correlation between the original and the reconstructed ABR. The yellow bead at at ~7 to ~8 ms for x-axis and y-axis alike indicates the sensitivity of our filter for wave V.

Figure 5: A decoding typical display of the efficiency of prediction. X-axis shows the BDM output, that is the time of reconstructed ABR in milliseconds. Y-axis shows the used reference, that is the time of original ABR in ms. A dark blue indicates low activity, yellow indicates high activity. The diagonal line without blurring suggests a good correlation between those data sets. The yellow bead indicates sensitivity for wave V.

A Speed Trap on the Auditory Pathway – Investigation of early auditory evoked brainstem activity

Source Reconstruction of Brainstem Activity (Wave V)

To further validate our filter, we performed a Forward Encoding Model of the ABR to obtain a wave V sensitive function. We then used source reconstruction to detect the sources of wave

V and try to locate it in the lateral lemnisci and the inferior colliculi of the brainstem.

However, at Wave V timepoint (7.3ms) there is nearly no activation in brainstem areas and the inferior colliculi compared to background noise. Instead there is activation in the white matter under postcentral gyrus of the right hemisphere and in the white matter rom thalamus to

Heschl’s gyrus on the left hemisphere.

Fig. 6: Displayed is the averaged ABR wave form with marked wave V (cf. Fig 3) and the source reconstruction of FEM weights for the time point with highest peak in ABR (7.3ms, wave V), averaged over all subjects. Also displayed are the transverse and saggital source reconstruction images of a standard brain. FEM weights indicate activation in the white matter under postcentral gyrus of the right hemisphere and in the white matter rom thalamus to Heschl’s gyrus on the left hemisphere, but no activation of lateral lemniscus or inferior colliculus.

A Speed Trap on the Auditory Pathway – Investigation of early auditory evoked brainstem activity

Source Reconstruction of Cortical Activity (Wave VII)

Because of the difficulties we had with reconstructing the source of wave V it was necessary to validate the source reconstruction approach for our ABR trained MEG data by reconstruct the source of Wave VII. Since Wave VII should show already cortical activity and its source is therefore nearer to the surface than the brainstem, thus easier to track.

At 9,4 ms source reconstruction reveals a widespread activation in both hemispheres ranging from the thalamus in the middle of the brain to cortical areas on the side of the brain, like

Heschl’s gyrus. On the left hemisphere the activation pattern is broader and the cortex is more activated than on the right hemisphere.

Fig. 7: Displayed is the averaged ABR wave form with marked wave VII (cf. Fig 3) and the source reconstruction of FEM weights for the estimated wave VII time point (9.4ms), averaged over all subjects. Also displayed are the transverse, sagittal and horizontal source reconstruction images of a standard brain. FEM weights indicate a widespread activation in both hemispheres ranging from the thalamus in the middle of the brain to cortical areas on the side of the brain, like Heschl´s gyrus. On the left hemisphere the activation pattern is broader and the cortex is more activated than on the right hemisphere.

A Speed Trap on the Auditory Pathway – Investigation of early auditory evoked brainstem activity

Limitations, improvements and perspectives for research in attentional modulation

As we suggested ABR was the method which showed the most pronounced early auditory evoked activity at Wave V. We therefore use it as reference signal to extract wave V.

Surprisingly, mABR acted different from its electrical counterpart. While mABR showed a peak at Wave V at the same latency as ABR, its amplitude was much lower. Though, mABR has apparently a higher sensitivity for Wave I and II as both left and right ECochG and ABR. This was also found from Parkkonen et al. (2009) as they compared mABR to ABR. They suggested mABR to contain more hidden information than ABR. With this result, mABR should be used instead of ECochG to measure activation in the hearing nerve and the cochlear nucleus, but should not be used as a reference to extract wave V.

As next step, we used our filter approach to predict ABR out of MEG data. The same MEG data which were used to calculate the filter weights, were decoded to reconstruct the ABR with no latency differences, small amplitude differences and a near-to-perfect correlation. This of course only works as a proof of principle. To further validate our approach, it is necessary to apply the filter weights on a different MEG dataset. First steps in this direction are already done. Our participants took part in a longer experiment setup in MEG and those datasets are going to be analyzed with the filter weights.

As we wanted to locate the source of Wave V in the inferior colliculi of the brainstem, we found no activation in expected areas. Instead we found a broad activation spreading from the middle of the brain (e.g. thalamus) to cortical areas, presumably auditory cortex. This could have multiple reasons. Reconstructing subcortical sources which are as deep in the brain as the brainstem is known to be difficult, because the overall brain noise disguises activation there.

Parkkonen et al. (2009) mentioned difficulties at brainstem source reconstruction as well even though they managed to locate the source of Wave V near IC.

A Speed Trap on the Auditory Pathway – Investigation of early auditory evoked brainstem activity

There are some points in our analysis pipeline which could be addressed to better ECochG and ABR signal for a higher signal-to-noise ratio. In our work, wave detection was performed on the grand average over all subjects. Due to the high interindividual variability in amplitude and importantly latency, this could lead to an underestimation of the signal`s strength. A vice versa pipeline with wave detection on individual subject´s signal and averaging of individual waves instead of a specific timepoint would probably enhance ABR and ECochG signal.

In our experiment participants installed the ECochG electrodes themselves in their ear canals which makes an uneven acoustic stimulation likely. This could be avoided with a fixed application process including a specialist installing ECochG electrodes in participants’ ear.

Switching to another kind of electrodes would then be possible and could additionally enhance acoustic stimulation and ECochG signal.

A huge impact on the ABR and ECochG signal comes from the choice of signal filter. The use of high pass filter can distort signal of early auditory evoked potentials (Tabachnik & Toscano

2016). For our analysis, we tested 9 different lowpass filters at 0.1 Hz, 10 Hz, 50 Hz, 100 Hz,

150 Hz, 200 Hz, 300 Hz, 400 Hz and 500 Hz. Results can be seen in Supplementary 1. We also checked if the use of finite impulse response (FIR) filters could have an advantage for the signal of early auditory evoked potentials (Widmann, Schröger & Maess 2015), but they did not differ compared to signals filtered by infinite impulse response (IIR, butterworth) filters (cf

Sup. 2). In the end, we used 8th order butterworth filter at 150 hz highpass and 2500 hz lowpass.

Another reason for our results to wave V would be that the widely accepted correlation between subcortical region and wave doesn’t hold for our experimental setup. Source reconstruction of wave V reveals presumably even further processed activation. The expected results of the source reconstruction of wave VII speaks in favor for the correctness of wave V source reconstruction. This is supported by reconstruction of other timepoints earlier than wave V. (cf

Sup. 3). This would also have an impact of the application of our spatial filter. Detection of early auditory activity depends on the signal peak in the used measuring method. Connecting it to

A Speed Trap on the Auditory Pathway – Investigation of early auditory evoked brainstem activity an underlying region is a theoretical problem. If the theoretical assumed region is not the origin of activation for the auditory pathway anatomy due to different stimulation or different anatomy of the subject`s brain, then there is no spatial assignment possible. Still, in MEG source reconstruction is usable to verify this theoretical assumed region. Further research about the generating regions of early auditory evoked activity, especially for wave V, are advised.

Source reconstruction of wave VII showed unlike wave V the expected regions activated. There is bilateral activation in cortical areas near Heschl´s gyrus, known as auditory cortex and in subcortical areas underneath. Activation on the left hemisphere appears broader than on right hemisphere. This could be due to uneven stimulation on both ears, i.e. if participants installed the tiptrodes uneven in their ears as mentioned earlier. These findings are going along with

Brugge et al. (2008, 2009) and Parkkonen et al. (2009) and claim the source of wave VII to be in auditory cortex.

Forte, Etard and Reichenbach investigated 2017 the modulation of attention on early auditory evoked activity. To achieve this, they developed a method to measure early auditory evoked activity in response to running speech instead of repetitive click stimulation. They claimed attention to higher the signal of the overall ABR. On basis of the findings of Hashimoto,

Ishiyama, Yoshimoto & Nemoto (1981) they denied the participation of auditory cortex in the first ten ms of ABR. Our results, in line with literature in humans (Brugge et al. 2008, 2009) and monkeys (Steinschneider et al. 1992), however reveal cortical activation already at 9.4 ms after stimulus onset. It may be that their results are biased by the participation of auditory cortex.

This puts a question mark behind the attentional modulation of ABR on the level of brainstem or auditory nerve.

To examine the findings of Forte et al. we included in our experimental setup an additional attention task. As further step we are going to use our spatial filter to calculate early auditory evoked activity for either attended or unattended stimuli on hearing nerve and brainstem in the auditory pathway.

A Speed Trap on the Auditory Pathway – Investigation of early auditory evoked brainstem activity

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A Speed Trap on the Auditory Pathway – Investigation of early auditory evoked brainstem activity

Supplementary

Supp. 1: This graphic shows the effect of different filters on the ABR signal (without z-score standardization). Compared are different highpass filters for an eight order butterworth filter with 2500 hz highpass. The chosen signal is marked as the red line.

Supp. 2: This graphic shows the effect of different filters on the ABR signal (without z-score standardization). Compared are IIR butterworth filter against FIR filter. There is no significant difference between those filtering methods.

A Speed Trap on the Auditory Pathway – Investigation of early auditory evoked brainstem activity

Supp. 3: This graphic display different source reconstruction images of FEM weights of the ABR on a standard brain at the time points 5.3 ms and 6.1 ms after acoustic stimulation. At 5.3 ms, we possibly see activation in the lateral lemnisci and the inferior colliculi as well as activation spreading to the medial geniculate body on the left hemisphere. At 6.1 ms, there is high activation in areas around the medial geniculate body of the right hemisphere. The activation pattern suggests an earlier than expected processing of early auditory evoked potentials.