Neural dynamics of change detection in crowded acoustic scenes

Two key questions concerning change detection in crowded acoustic environments are the extent to which cortical processing is specialized for different forms of acoustic change and when in the time-course of cortical processing neural activity becomes predictive of behavioral outcomes. Here, we address these issues by using magnetoencephalography (MEG) to probe the cortical dynamics of change detection in ongoing acoustic scenes containing as many as ten concurrent sources. Each source was formed of a sequence of tone pips with a unique carrier frequency and temporal modulation pattern, designed to mimic the spectrotemporal structure of natural sounds. Our results show that listeners are more accurate and quicker to detect the appearance (than disappearance) of an auditory source in the ongoing scene. Underpinning this behavioral asymmetry are change-evoked responses differing not only in magnitude and latency, but also in their spatial patterns. We find that even the earliest (~ 50 ms) cortical response to change is predictive of behavioral outcomes (detection times), consistent with the hypothesized role of local neural transients in supporting change detection

Attention, Awareness, and the Perception of Auditory Scenes

Auditory perception and cognition entails both low-level and high-level processes, which are likely to interact with each other to create our rich conscious experience of soundscapes. Recent research that we review has revealed numerous influences of high-level factors, such as attention, intention, and prior experience, on conscious auditory perception. And recently, studies have shown that auditory scene analysis tasks can exhibit multistability in a manner very similar to ambiguous visual stimuli, presenting a unique opportunity to study neural correlates of auditory awareness and the extent to which mechanisms of perception are shared across sensory modalities. Research has also led to a growing number of techniques through which auditory perception can be manipulated and even completely suppressed. Such findings have important consequences for our understanding of the mechanisms of perception and also should allow scientists to precisely distinguish the influences of different higher-level influences

The Effect of Visual Perceptual Load on Auditory Processing

Many fundamental aspects of auditory processing occur even when we are not attending to the auditory environment. This has led to a popular belief that auditory signals are analysed in a largely pre-attentive manner, allowing hearing to serve as an early warning system. However, models of attention highlight that even processes that occur by default may rely on access to perceptual resources, and so can fail in situations when demand on sensory systems is particularly high. If this is the case for auditory processing, the classic paradigms employed in auditory attention research are not sufficient to distinguish between a process that is truly automatic (i.e., will occur regardless of any competing demands on sensory processing) and one that occurs passively (i.e., without explicit intent) but is dependent on resource-availability. An approach that addresses explicitly whether an aspect of auditory analysis is contingent on access to capacity-limited resources is to control the resources available to the process; this can be achieved by actively engaging attention in a different task that depletes perceptual capacity to a greater or lesser extent. If the critical auditory process is affected by manipulating the perceptual demands of the attended task this suggests that it is subject to the availability of processing resources; in contrast a process that is automatic should not be affected by the level of load in the attended task. This approach has been firmly established within vision, but has been used relatively little to explore auditory processing. In the experiments presented in this thesis, I use MEG, pupillometry and behavioural dual-task designs to explore how auditory processing is impacted by visual perceptual load. The MEG data presented illustrate that both the overall amplitude of auditory responses, and the computational capacity of the auditory system are affected by the degree of perceptual load in a concurrent visual task. These effects are mirrored by the pupillometry data in which pupil dilation is found to reflect both the degree of load in the attended visual task (with larger pupil dilation to the high compared to the low load visual load task), and the sensory processing of irrelevant auditory signals (with reduced dilation to sounds under high versus low visual load). The data highlight that previous assumptions that auditory processing can occur automatically may be too simplistic; in fact, though many aspects of auditory processing occur passively and benefit from the allocation of spare capacity, they are not strictly automatic. Moreover, the data indicate that the impact of visual load can be seen even on the early sensory cortical responses to sound, suggesting not only that cortical processing of auditory signals is dependent on the availability of resources, but also that these resources are part of a global pool shared between vision and audition

Pattern of BOLD signal in auditory cortex relates acoustic response to perceptual streaming

<p>Abstract</p> <p>Background</p> <p>Segregating auditory scenes into distinct objects or streams is one of our brain's greatest perceptual challenges. Streaming has classically been studied with bistable sound stimuli, perceived alternately as a single group or two separate groups. Throughout the last decade different methodologies have yielded inconsistent evidence about the role of auditory cortex in the maintenance of streams. In particular, studies using functional magnetic resonance imaging (fMRI) have been unable to show persistent activity within auditory cortex (AC) that distinguishes between perceptual states.</p> <p>Results</p> <p>We use bistable stimuli, an explicit perceptual categorization task, and a focused region of interest (ROI) analysis to demonstrate an effect of perceptual state within AC. We find that AC has more activity when listeners perceive the split percept rather than the grouped percept. In addition, within this ROI the pattern of acoustic response across voxels is significantly correlated with the pattern of perceptual modulation. In a whole-brain exploratory test, we corroborate previous work showing an effect of perceptual state in the intraparietal sulcus.</p> <p>Conclusions</p> <p>Our results show that the maintenance of auditory streams is reflected in AC activity, directly relating sound responses to perception, and that perceptual state is further represented in multiple, higher level cortical regions.</p

Insertion and Evolution of an Endogenous Retrovirus into KIT is Responsible for Multiple Phenotypes at the White Locus in the Domestic Cat

The Dominant White locus (W) in the domestic cat demonstrates pleiotropic effects exhibiting complete penetrance for absence of coat pigmentation and incomplete penetrance for deafness and iris hypopigmentation. I preformed linkage analysis using a pedigree segregating White to identify KIT (Chr. B1), as the feline W locus. Segregation and sequence analysis of the KIT gene in two pedigrees (P1 and P2) revealed the remarkable retrotransposition and evolution of a feline endogenous retrovirus (FERV1) as responsible for two distinct phenotypes of the W locus, Dominant White, and White Spotting. The retrotransposition interrupts a DNase I hypersensitive site in KIT intron 1 that was previously demonstrated to regulate temporal and tissue specific expression of KIT in mice. A large population-genetic survey of cats (n=269), supports our findings and demonstrates statistical significance of the FERV1 LTR and full-length element with Dominant White (p < 0.0001) and White Spotting (p< 0.0001), respectively

Brain responses in humans reveal ideal observer-like sensitivity to complex acoustic patterns

This study was funded by a Deafness Research UK fellowship and Wellcome Trust Project Grant 093292/Z/10/Z (to M.C.)

Distilling the neural correlates of conscious somatosensory perception

The ability to consciously perceive the world profoundly defines our lives as human beings. Somehow, our brains process information in a way that allows us to become aware of the images, sounds, touches, smells, and tastes surrounding us. Yet our understanding of the neurobiological processes that generate perceptual awareness is very limited. One of the most contested questions in the neuroscientific study of conscious perception is whether awareness arises from the activity of early sensory brain regions, or instead requires later processing in widespread supramodal networks. It has been suggested that the conflicting evidence supporting these two perspectives may be the result of methodological confounds in classical experimental tasks. In order to infer participants’ perceptual awareness in these tasks, they need to report the contents of their perception. This means that the neural signals underlying the emergence of perceptual awareness often cannot be dissociated from pre- and postperceptual processes. Consequently, some of the previously observed effects may not be correlates of awareness after all but instead may have resulted from task requirements. In this thesis, I investigate this possibility in the somatosensory modality. To scrutinise the task dependence of the neural correlates of somatosensory awareness, I developed an experimental paradigm that controls for the most common experimental confounds. In a somatosensory-visual matching task, participants were required to detect electrical target stimuli at ten different intensity levels. Instead of reporting their perception directly, they compared their somatosensory percepts to simultaneously presented visual cues that signalled stimulus presence or absence and then reported a match or mismatch accordingly. As a result, target detection was decorrelated from working memory and reports, the behavioural relevance of detected and undetected stimuli was equated, the influence of attentional processes was mitigated, and perceptual uncertainty was varied in a controlled manner. Results from a functional magnetic resonance imaging (fMRI) study and an electroencephalography (EEG) study showed that, when controlled for task demands, the neural correlates of somatosensory awareness were restricted to relatively early activity (~150 ms) in secondary somatosensory regions. In contrast, late activity (>300 ms) indicative of processing in frontoparietal networks occurred irrespective of stimulus awareness, and activity in anterior insular, anterior cingulate, and supplementary motor cortex was associated with processing perceptual uncertainty and reports. These results add novel evidence to the early-local vs. late-global debate and favour the view that perceptual awareness emerges at the level of modality-specific sensory cortices.Die Fähigkeit zur bewussten Wahrnehmung bestimmt maßgeblich unser Selbstbild als Menschen. Unser Gehirn verarbeitet Informationen auf eine Weise, die es uns ermöglicht, uns der Bilder, Töne, Berührungen, Gerüche und Geschmäcker, die uns umgeben, bewusst zu werden. Unser Verständnis davon, wie neurobiologische Prozesse diese bewusste Wahrnehmung erzeugen, ist jedoch noch sehr begrenzt. Eine der umstrittensten Fragen in der neurowissenschaftlichen Erforschung des perzeptuellen Bewusstseins besteht darin, ob die bewusste Wahrnehmung aus der Aktivität früher sensorischer Hirnregionen entsteht, oder aber die spätere Prozessierung in ausgedehnten supramodalen Netzwerken erfordert. Eine mögliche Erklärung für die widersprüchlichen Ergebnisse, die diesen beiden Perspektiven zugrunde liegen, wird in methodologischen Störfaktoren vermutet, die in klassischen experimentellen Paradigmen auftreten können. Um auf die Wahrnehmung der Versuchspersonen schließen zu können, müssen diese den Inhalt ihrer Wahrnehmung berichten. Das führt dazu, dass neuronale Korrelate bewusster Wahrnehmung häufig nicht sauber von prä- und postperzeptuellen Prozessen getrennt werden können. Folglich könnten einige der zuvor beobachteten Effekte, anstatt tatsächlich bewusste Wahrnehmung widerzuspiegeln, aus den Anforderungen experimenteller Paradigmen entstanden sein. In dieser Arbeit untersuche ich diese Möglichkeit in der somatosensorischen Modalität. Um zu überprüfen, inwiefern neuronale Korrelate bewusster somatosensorischer Wahrnehmung von den Anforderungen experimenteller Aufgaben abhängen, habe ich ein Paradigma entwickelt, dass die häufigsten experimentellen Störfaktoren kontrolliert. In einer somatosensorisch-visuellen Vergleichsaufgabe mussten die Versuchspersonen elektrische Zielreize in zehn verschiedenen Intensitätsstufen detektieren. Anstatt diese jedoch direkt zu berichten, sollten sie ihre somatosensorischen Perzepte mit gleichzeitig präsentierten visuellen Symbolen vergleichen, die entweder Reizanwesenheit oder -abwesenheit signalisierten. Entsprechend wurde dann eine Übereinstimmung oder Nichtübereinstimmung berichtet. Dadurch wurde die Reizwahrnehmung von Arbeitsgedächtnis und Berichterstattung dekorreliert, die Verhaltensrelevanz detektierter und nicht detektierter Reize gleichgesetzt, der Einfluss von Aufmerksamkeitsprozessen reduziert und die mit der Detektion verbundene Unsicherheit auf kontrollierte Weise variiert. Die Ergebnisse aus einer funktionellen Magnetresonanztomographie (fMRT)-Studie und einer Elektroenzephalographie (EEG)-Studie zeigen, dass die neuronalen Korrelate bewusster somatosensorischer Wahrnehmung auf relativ frühe Aktivität (~150 ms) in sekundären somatosensorischen Regionen beschränkt sind, wenn experimentelle Störfaktoren kontrolliert werden. Im Gegensatz dazu trat späte Aktivität (>300 ms), die auf die Verarbeitung in frontoparietalen Netzwerken hindeutet, unabhängig von der Reizwahrnehmung auf, und Aktivität im anterioren insulären, anterioren cingulären und supplementär-motorischen Kortex war mit der Verarbeitung von Detektionsunsicherheit und der Berichterstattung verbunden. Diese Ergebnisse liefern neue Erkenntnisse zur Debatte um die Relevanz früher, lokaler vs. später, globaler Hirnaktivität und unterstützen die Ansicht, dass perzeptuelles Bewusstsein in modalitätsspezifischen sensorischen Kortizes entsteht

Neuroanatomical and perceptual deficits in auditory agnosia : a study of an auditory agnosia patient with inferior colliculus damage

Auditory agnosia is a rare disorder in which individuals lose the ability to understand sounds. In this thesis, I examine an auditory agnosia patient with brainstem damage, but intact cortex. The patient was severely impaired when instructed to type the names of sounds. The patient, however, was only mildly impaired when instructed to choose the correct sound out of four written alternatives, which implies partial auditory perception. In two fMRI scans, conducted a year apart, passive listening to sounds resulted with a unique activation pattern in her auditory cortices. In particular, her anterior primary and associative auditory fields were much less responsive to sounds than more posterior primary and associative auditory fields. The functional dissociation between these regions suggests connections between the anterior primary and associative regions, and between the posterior primary and associative regions. Hitherto, these connections were only reported in monkeys. An EEG study that examined mismatch negativity for frequency, duration, and intensity of sounds, demonstrated that the patient’s ability of detecting changes to frequency and duration of sounds is bilaterally impaired, whereas the detection of changes to sound’s intensity is impaired in the left hemisphere but intact in the right hemisphere. Behavioral studies also show that the patient’s auditory perceptual deficit is partially due to impaired perception of the duration of sounds. For instance, when the patient heard two subsequent clicks, she was impaired at discriminating these sounds by the duration of their intervening interval. In a spoken word discrimination task, she was also impaired at discriminating words that could only be distinguished by their temporal properties (voice onset-time). Based on these findings, I argue that the patient experiences auditory agnosia be- cause the brain stem injury prevents the transmission of critical auditory information to the auditory cortex. As a result of this absence, the auditory fields responsible for sound recognition, the anterior auditory fields, are not recruited. In a dichotic listening task, the patient extinguished sounds presented to the right ear, and in a sound localization task she perceived sounds as emerging from the left auditory hemi-field. Given cumulative evidence that associates the posterior auditory cortex with sound localization and phonological-acoustic analysis of verbal material from the contra-lateral hemi-field, the patient’s performance in these tasks suggest that her spared auditory abilities is due to processing in her right posterior auditory cortex. This role of the patient’s right posterior auditory cortex is consistent with both the fMRI study, in which the right posterior auditory cortex was consistently responsive to sounds, and the EEG study, in which detection of changes to sound intensity was restricted to the right hemisphere

Exploring the adaptive capabilities of the brain following unilateral conductive hearing loss

Plasticity in adulthood potentially allows the recovery of perceptual abilities following sensory impairments. Training-dependent recovery of sound localisation accuracy following unilateral conductive hearing loss has been widely used to study the adaptability of the auditory system. Previous studies have implicated the inferior colliculus (IC) in this plasticity, but have not examined whether neural correlates of training-induced plasticity are found in this midbrain structure. To explore this question, we performed chronic electrophysiological recordings in the IC of ferrets while they were trained to adapt to unilateral hearing loss, induced by reversibly plugging one ear. We found that IC responses were initially disrupted by monaural hearing loss, which coincided with impaired localisation behaviour. With daily training, however, adaptive changes occurred in the spatial response properties of IC neurons and in the animals’ localisation behaviour. Neural correlates of training-induced adaptation are therefore found in the IC, supporting its role in auditory spatial plasticity. These results also provide insight into how sound azimuth is encoded by IC neurons. Demonstrating that training-dependent adaptation has potential therapeutic value for people with hearing disorders requires an understanding of the persistence and context dependence of this plasticity, and of the extent to which adaptation generalises to untrained sounds. By measuring the behavioural effects in ferrets of repeated periods of monaural occlusion in the same ear and then of the opposite ear, we show that adaptation leaves a “memory trace” that determines how localisation accuracy is affected when unilateral hearing loss is experienced again. To evaluate the generalisation of learning, we measured localisation accuracy after adaptation using different bandwidth stimuli presented in silence or against background noise. Our results show a positive relationship between generalisation and the degree of adaptation achieved. Finally, a virtual reality setup was developed for investigating these questions in human participants