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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 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 1 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. 3 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. 4 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 5 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
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