This thesis investigates the involvement of primary auditory cortex (A1) in sound localisation. Experiments were carried out both to assess the effect of A1 inactivation on sound localisation, and to measure the sensitivity of single A1 neurons to sound source location. Ferrets were trained to localise bursts of broadband noise, of varying intensity and duration, from an array of loudspeakers that spanned 360 degrees in azimuth. Bilateral A1 inactivation caused an impairment on this task, but only for short-duration stimuli. Unilateral A1 inactivation also resulted in an impairment for short-duration stimuli, but this was limited to the side of space contralateral to the inactivation, and was only seen in animals which had been highly trained prior to surgery. A feature of the impairment in all animals was the increased number of "front-back confusions", where the animal's response was on the correct side of the midline but the wrong side of the interaural axis. Recordings from ferret A1 showed that the firing rate of individual neurons varied little as sound source location was changed. Further, the neurons' location sensitivity was affected by changes in stimulus intensity and duration. However, mathematical techniques were used to measure the information these neurons provided about sound source location, and it was found that this information was not sensitive to intensity or duration changes. The analysis also showed that the amount of information provided by response latency was greater than that carried by firing rate. Similar mathematical treatment tentatively suggested that the information from different neurons was only slightly redundant, so it may be possible to account for whole-animal localisation performance by assuming that the output of large numbers of neurons is considered. It is concluded that A1 is involved in processing the location of sound sources, but it seems unlikely that sound localisation is A1's primary or only role within the auditory system.

Audio in today’s first person computer games plays a vital role in informing players about their surroundings as well as general gameplay elements. Awareness of the direction, distance, and spatial placement of audio sources can be crucial for players in various contexts. Spatialising audio through stereo panning can pose challenges to players when it comes to accurately localising sound sources in front, behind, below, or above the player. Binaural audio is another technique used for spatialising audio by simulating how sound interacts with the head and ears before reaching the eardrums from a specific direction before the signal is rendered through the headphones. While binaural audio attempts to alleviate the front-back confusion, the cues from binaural audio are lost when the listener moves their head if some form of head tracking solution is not incorporated. Hence, this study’s research question is: ”How is localisation of audio sources in first-person computer games — while wearing headphones— helped by spatialising the soundscape in relation to head movement utilising head tracking technology?”. To be able to answer the research question a prototype of a game was developed following design science guidelines. The objective was to timely and accurately localise 14 invisible targets emitting sound in a virtual three-storey house. During one half of the test the spatilisation with head movements was inactive and the other half had it activated in order to compare the testers ability for localisation with and without the tool. The order for which half would have spatilisation activated was randomised for each test. The research strategy consisted of two types of experiments, blind — with 20 participants, and open — with five testers, that were conducted to measure and evaluate the participants’ head movement and performance in terms of accuracy and time of localising the targets by shooting them. For data collection this study used a mixed methods approach that included questionnaires with closed questions and semistructured interviews. Data about the testers’ performance was automatically logged during the test. The results from the first, blind experiment showed little head movement and no significant impact on localisation performance from the spatialisation. Consequently an open follow-up experiment was performed to discover if the blind experiment design affected the results. The results demonstrated a higher degree of head movement but corroborated the first test in no substantial effect on the testers’ accuracy or time when localising the targets. In summary, there could be found no positive or negative impact on one’s localisation of audio sources in first-person computer games —while wearing headphones— when spatialising the soundscape in relation to head movements by utilising head tracking technology. Additionally, some participants found the tool to be unfit for the genre that the prototype resembled and suggested that spatialisation of audio with head tracking could perhaps be better suited for other genres. This could serve as material for future research on the use of head tracking for spatialisation of audio in computer games.

Sound localisation is one of the most representative function of the auditory system and, as such, it has been extensively investigated across species. Spatial hearing can be dramatically altered across the life span, yet research in humans have highlighted the remarkable capacity of the brain to adapt to changes of listening conditions, such as temporary ear plugging or long lasting hearing impairments. Although several investigations have examined accommodation to altered auditory cues (Chapter 1), a common theoretical framework seems to lack and a number of questions remain open. This limits the possibility to translate our current knowledge into concrete clinical applications for individuals who experience spatial hearing difficulties after hearing loss. The current dissertation reflects the attempt to answer specific questions regarding the process of sound localisation. The first study (Chapter 2) aimed to investigate the relation between different reference frames in spatial hearing, namely egocentric and allocentric sound representation. We studies this topic in the context of a learning paradigm, assessing to what extent localisation of single sounds in simulated monaural hearing (unilateral ear plugging) can improve following an audio-visual spatial hearing training focused on egocentric sound processing vs allocentric sound processing. An untrained group was also included in the study. We found that localisation performance in the horizontal plane improved specifically in the side ipsilateral to the ear-plug for all groups. Yet, the trained groups showed a qualitatively different change of performance after four days of multisensory ego/allocentric training compared to the untrained group, providing initial evidence of the possible role of allocentric coding in acoustic space re-learning. These results further highlight the importance of including a test-retest group in paradigms of sound localisation training. The second study (Chapter 3) focused on a specific aspect of the phenomenological experience of spatial hearing, namely the subjective confidence about the perceived sound position. We examined the relation between objective localisation accuracy and subjective certainty while participants localised sounds in two different listening conditions – binaural or simulated monaural hearing. Results showed that overall subjective certainty on sound position decreased in the altered listening condition (unilateral ear-plugging). In simulated monaural hearing, localisation accuracy and spatial confidence dissociated. For instance, there were trials in which participants were accurate, but felt uncertain, and trials in which they were less accurate but expressed higher ratings of spatial confidence on sound position. Furthermore, subjective confidence increased as a function of time within the testing block, and it was related to the spatial distribution of the perceived sound-source position. The third study (Chapter 4) exploited magnetoencephalography (MEG) to study the dynamics of the cortical network implied in active sound localisation. We implemented a novel apparatus to study sound localisation in MEG with real sounds in external space, and collected behavioural and subjective responses (i.e., accuracy and confidence, as in Study 2) during this altered listening condition. Results showed that participants were able to perceive the spatial difference between the positions of stimulation, thus proving the reliability of our novel setting for the study of spatial hearing in MEG. MEG data highlight a distributed bilateral cortical network involved in active sound localisation, which emerged shortly after stimulus presentation (100—125 ms). The network comprise the classical dorsal auditory pathway plus other cortical regions usually underestimated in previous literature – most notably, regions in the central sulcus/precentral gyrus possibly involved in head movements. Connectivity analysis revealed different patterns of neural coupling, as a function of frequency band. In particular, coherence in high gamma revealed significant connections involving the parietal cortex and the posterior superior temporal cortex. In the final chapter (Chapter 5), I summarise the main findings of the three studies, discuss their implications and outline potential future directions.

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal.

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal

Le vieillissement dit "naturel", affecte physiologiquement les structures auditives périphériques; il en va de même du collicule supérieur (CS) lors du traitement des signaux auditifs et visuels. Chez le rat âgé, la sensibilité auditive périphérique diminue et l’extraction des attributs des signaux auditifs est modifiée, et ce, dès les noyaux cochléaires (premiers relais centraux de la voie auditive ascendante). De plus, les propriétés spectrales, temporelles et directionnelles des neurones auditifs du CS sont altérées lors du vieillissement. Ceci se manifeste aussi au niveau de l’organisation topographique de la sensibilité à la direction qui est abolie au sein des couches profondes du CS. Dans la même optique, l’extraction des attributs des fréquences spatiales concentriques mobiles (en présence ou en absence d’objet sonore modulé en amplitude) est altérée aussi au niveau des neurones audiovisuels du CS lors du vieillissement. En effet, au niveau spatial, chez l’animal âgé, la présence de déficits visuels est particulièrement marquée par une diminution de la sensibilité aux stimuli visuels et audiovisuels mobiles et rapides lors du déplacement de l’organisme dans son environnement. Compte tenu de l’ampleur des changements qui s’installent avec le vieillissement au niveau des structures périphériques et centrales, inévitablement, les mécanismes nerveux de la plasticité audiovisuelle de bas niveau, i.e. au niveau des neurones du CS, sont perturbés. En outre, chez l’animal âgé, le gain audiovisuel induit par l’activité des neurones du CS ne produit pas d’amélioration notable par rapport à la modalité unisensorielle. Dans l’ensemble, ces résultats montrent que le vieillissement perturbe l’activité neuronale du CS permettant de détecter les informations sensorielles pertinentes dans un environnement audiovisuel complexe. / Age-related physiological changes affect the peripheral auditory structures; this also applies to the superior colliculus (SC) auditory and visual processes. In aged rats, the peripheral hearing sensitivity decreases and at more central regions, particularly the first central node, in the ascending auditory projections, auditory processing of sounds is altered. Furthermore, at the level of the SC, spectral, temporal and directional properties of auditory neurons are also altered during aging. In addition, no systematic directional spatial arrangement is encountered among the neurons of aged rats, implying that the topographical organization of the auditory directional map is abolished in the deep layers of the SC. Moreover, in a condition where SC visual neurons are stimulated with looming concentric sinusoidal gratings (in the presence or in the absence of modulated audio signals), visual deficits in aged animals are particularly marked by a decrease in sensitivity to fast moving visual and audiovisual stimuli during self-motion. Given the links of age-related changes in the peripheral and in the central structures, inevitably, the mechanisms underlying the neuronal audiovisual plasticity, in the low-level (SC), are somehow disrupted. Specifically, in aged animals, the presence of the auditory stimulus when coupled with the visual signal did not enhance the response activity of the visual neurons. This seems to suggest that the mechanism that may serve to amplify the visual signal under weak or ambiguous and noisy conditions thus improving greater behavioral relevance of detecting rapidly approaching visual and audiovisual moving objects during self-motion is altered with aging. Overall, these results show that aging disrupts the SC neuronal activity that enables detection of relevant sensory information in a complex audiovisual environment.

Bilateral cochlear implantation is accepted medical practice since 2008 in clinically suitable adults and children to enhance bilateral processing benefits. Bilateral implantation may lead to the restoration of some bilateral hearing advantages, such as improved speech recognition in noise, localisation, head shadow effect, summation, and squelch. The majority of the advantages stated in literature, though, are characteristic of the simultaneously implanted cochlear implant population. Simultaneous implantation is not yet a reality in South Africa due to funding constraints, therefore determining the bilateral processing abilities in sequentially implanted adults is essential. Determining bilateral processing benefits achievable with sequential implantation could result in evidence-based recommendations in terms of candidacy considerations, surgery protocols, motivations for medical aid funding for simultaneous cochlear implantation, and relevant measures to determine the bilateral processing benefit attainable. Furthermore, it might enhance audiologists‟ insight regarding post-implantation performance of sequentially implanted patients and enable them to counsel prospective candidates realistically. The aaim of this study was to determine the bilateral benefit attained by sequentially implanted adults. A quantitative, cross-sectional research approach was followed in a one group post-test-only exploratory research design. A purposive convenient sampling method with specified selection criteria was used to select 11 adult clients of an established cochlear implant programme in Pretoria. Tests of sound localisation in the horizontal plane and speech perception in noise were performed. During the test of sound localisation, performance with only the first or only the second implant was found to be very similar. For the majority of participants the second cochlear implant (CI 2) was the superior performing implant during xviii speech perception in noise testing, in spatially separated speech and noise conditions where noise was directed to the first implant, as well as in spatially coincident speech and noise. A statistical significant bilateral benefit (p < 0.05) was attained by sequentially implanted adults for sound localisation. A bilateral benefit for speech perception in noise was observed when noise was directed to the first implant and in the diotic listening condition with average benefits of 1.69 dB and 0.78 dB, respectively. It was not statistically significant (p > 0.05), however, and was smaller than bilateral benefit values achieved by simultaneously implanted adults in previous studies. The head shadow effect at 180° was found to be the strongest and most robust bilateral spatial benefit. Squelch and summation benefit values ranged from negative values to 2 dB and 6 dB, respectively. This corresponded with values found in previous studies. The improvement in speech perception in spatially distinct speech and noise from adding the ear with a better SNR (signal to noise ratio) indicated that the contribution of CI 2 seems to be greater than that of CI 1 for bilateral spatial benefit. It can be concluded that adults with sequential implants may achieve some extent of bilateral benefit even with many years of unilateral implant use, when speech processors differ, when the second implant is done ≥ 10 years after the first implant, and in cases of prelingual deafness. A key benefit of sequential implantation appears to be related to the advantage of having hearing on both sides so that the ear with the more favourable environmental signalto-noise ratio is always available. AFRIKAANS : Bilaterale kogleêre inplanting is sedert 2008 aanvaarde mediese praktyk vir klinies geskikte volwassenes en kinders, ten einde bilaterale prosesseringsvoordeel te verhoog. Bilaterale inplanting kan lei tot die herstel van sommige van die voordele van bilaterale gehoor, soos verbeterde spraakherkenning in lawaai, klanklokalisering, die kopskadueffek, sommering en selektiewe onderdrukking (“squelch”). Die meeste van die voordele wat in die literatuur bespreek word, is egter kenmerkend van dié persone by wie twee kogleêre inplantings gelyktydig gedoen is. Gelyktydige inplanting is as gevolg van beperkte befondsing nog nie in Suid-Afrika 'n werklikheid nie, daarom is dit noodsaaklik om te bepaal watter bilaterale prosesseringsvoordele by opeenvolgend-geïnplanteerde volwassenes voorkom. Die bepaling van watter bilaterale prosesseringsvoordele met opeenvolgende inplanting bereik kan word, sou kon lei tot getuienis-gebaseerde aanbevelings met betrekking tot besluite oor die geskiktheid van kandidate, protokol vir sjirurgie, motiverings vir die befondsing van gelyktydige kogleêre inplantings deur mediese voorsorgfondse, en toepaslike maatstawwe om te bepaal watter mate van bilaterale prosesseringsvoordeel haalbaar sou wees. Dit sou verder oudioloë se insig kon verbreed met betrekking tot die na-operatiewe prestasie van opeenvolgend-geïnplanteerde persone en hulle sodoende in staat stel om voornemende kandidate van realistiese raad te bedien. Die doel van hierdie studie was om te bepaal wat die bilaterale prosesseringsvoordele is wat deur opeenvolgend-geïnplanteerde volwassenes verkry kan word. 'n Kwantitatiewe navorsingsbenadering met 'n dwarsprofiel van „n enkelgroep is gevolg, met 'n post-toets verkennende navorsingsontwerp. 'n Doelgerigte gerieflikheidssteekproef met 'n gespesifiseerde seleksiekriteria is gebruik om 11 volwasse kliënte van 'n gevestigde kogleêre inplantprogram in Pretoria te selekteer. Klanklokalisering in die horisontale vlak en die waarneming van spraak in lawaai is getoets. Tydens die toets vir klanklokalisering is gevind dat prestasie met slegs die eerste of slegs die tweede inplanting soortgelyk was. Vir die meeste deelnemers aan die studie het die tweede kogleêre inplanting (KI 2) die beste prestasie gelewer tydens spraakwaarneming in lawaai, in omstandighede waar spraak en lawaai ruimtelik geskei is en die lawaai op die eerste inplanting gerig is, asook in omstandighede waar spraak en lawaai ruimtelik saamvoorkomend aangebied is. 'n Statisties beduidende bilaterale voordeel (p < 0.05) is deur opeenvolgend-geïnplanteerde volwassenes vir klanklokalisering behaal. 'n Bilaterale voordeel vir spraakwaarneming in lawaai is waargeneem waar lawaai op die eerste inplanting gerig is en ook in diotiese luistertoestande, met 'n gemiddelde voordeel van 1.69 dB en 0.78 dB, onderskeidelik. Dit was egter nie statisties beduidend nie en was ook kleiner as die bilaterale voordeelwaardes wat in vorige studies deur gelyktydig-geïnplanteerde volwassenes behaal is. Die kopskadu-effek by 180° was die sterkste en mees robuuste bilaterale ruimtelike voordeel. Voordeelwaardes vir selektiewe onderdrukking en sommering het gewissel van negatiewe waardes tot 2 dB en 6 dB onderskeidelik. Dit stem ooreen met waardes wat in vorige studies gevind is. Die verbetering in spraakwaarneming in ruimtelik geskeide spraak en lawaai wat verkry is deur die oor met 'n beter STR (sein-tot-ruis ratio) by te voeg, het daarop gedui dat die bydrae van KI 2 tot bilaterale ruimtelike voordeel waarskynlik groter as die bydrae van KI 1 is. Die gevolgtrekking kan gemaak word dat volwassenes met opeenvolgende inplantings 'n mate van bilaterale voordeel verkry selfs na vele jare van unilaterale inplantingsgebruik, wanneer die spraakprosesseerders in die twee inplantings van mekaar verskil, wanneer die tweede inplanting ≥ 10 jaar na die eerste plaasvind, en in gevalle van prelinguale doofheid. 'n Sleutelvoordeel van opeenvolgende inplanting hou klaarblyklik verband met die voordeel van gehoor aan albei kante te hê sodat die oor met die gunstigste sein-tot-lawaai ratio altyd beskikbaar is. / Dissertation (MCommunication Pathology)--University of Pretoria, 2011. / Speech-Language Pathology and Audiology / Unrestricted