On the primacy and irreducible nature of first-person versus third-person information

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Electrophysiological Correlates of Reversal Processes in Ambiguous Figure Perception

Ambiguous figures provide an opportunity to study fluctuations in perceptual experience without corresponding changes in sensory input. Researchers have taken great interest in the mechanisms that generate them using electrophysiology because of the potential to track these processes across time as perceptual reversals of these figures unfold. This work has highlighted brain activity both before and after perceptual reversals involving a wide range of mechanisms. Some of the known electrophysiological correlates of perceptual reversals, like the Reversal Negativity (RN) and Reversal Positivity (RP), have the potential to be explained by demands of the tasks used to elicit them. In addition, many findings on perceptual reversals and ambiguous figure interpretation originate from studies using univariate analyses that do not take full advantage of the multivariate nature of EEG data. In four experiments, I used psychophysics, ERP, and multivariate pattern analysis (MVPA) of EEG data to address the interpretation of two reversal-related ERP components and to identify new multivariate correlates of perceptual reversals. First, I found that the reversal-related RP only appears when reversals are response targets. This result suggests that the RP is not a pure correlate of perceptual processing of endogenous perceptual reversals but rather may reflect response-related monitoring processes. Second, using MVPA, I found that activity linked to perceptual reversals in the post-stimulus period is an ongoing process that involves a wide range of frequency bands (1-30Hz) and spans over a substantial amount of time (~550 ms). Finally, I found that I was able to isolate in time and frequency pre- stimulus activity that is predictive of the upcoming subjective interpretation of the ambiguous stimulus and of perceptual reversals. Overall, these results provide a new interpretation for some extant reversal-related electrophysiological markers as well as identify a set of new phenomena to guide neural theories of perceptual reversals going forward

Integration and disintegration of human visual awareness.

The neuronal underpinnings of visual awareness has recently become the primary question of interest for many researchers, with many theories suggesting distinct mechanisms. The aim of this thesis was to test predictions of the low-level modular theory of visual awareness. This modular view is encapsulated in the 'microconsciousness' framework (Zeki & Bartels, 1999) in which each visual processing system, such as that for colour, is capable of generating a conscious correlate autonomously in parallel across space, within each of the different functionally specialised areas of the visual brain, and across time, with different attributes perceived at different times. Given the scope of this topic, we approached it from three diverse angles: (1) Two psychophysical experiments investigated temporal aspects of visual perception - in particular these addressed the issue of whether the timing of awareness is an 'online* phenomenon rather than integrated into a temporal buffer zone prior to awareness. We measured the relative perceptual times of different magnitudes of direction changes and investigated the 'flash-lag' effect (Nijhawan, 1994) and related illusions of positional localisation. (2) The first two fMRI experiments examined the necessity of frontal and parietal areas for visual awareness in the context of bistable figures, combined with dynamic causal modelling (Friston et al., 2003), and perception outside the focus of attention. (3) We looked to extend the concept of modularity of awareness to that of 'access consciousness', that is the ability to give a report of a conscious experience (Block, 1996), in addition to the previous studies on phenomenal consciousness. To this end, we combined psychophysics with fMRI to investigate the interaction between report modality and visual perception. We conclude that the low-level modular theory of stands up to direct tests of its predictions and remains a viable theory of visual awareness

Evaluation eines Fragebogens zum Populationsscreening auf hereditäre Prosopagnosie

Um ein möglichst großes Kollektiv für weitere Untersuchungen zur Genetik und Pathophysiologie der angeborenen Prosopagnosie zu erhalten, wurde ein Fragebogen mit 21 Aussagen entwickelt, um Personen mit Prosopagnosie zu finden. Dieser wurde an 929 Medizinstudenten, Familienangehörigen von Betroffen, Personen, die sich im Institut für Humangenetik gemeldet haben, verteilt. In dieser Dissertation geht es darum, den Fragebogen statistisch zu testen und herauszufinden, welche der 21 Aussagen einen hohen Beitrag dazu leisten, Prosopagnostiker zu finden. Der getestete Fragebogen ist sehr diskriminativ in seiner Fähigkeit Prosopagnostiker (n=149) und Personen ohne Prosopagnosie (n=780) voneinander zu unterscheiden. Von den 21 Aussagen sind 8 Aussagen besonders geeignet. Sollte der Fragebogen sich an einem unabhängigen Kollektiv reevaluiert als genauso diskriminativ erweisen, hätte man in Zukunft ein gutes Screeningwerkzeug, um Prosopagnostiker in einer großen Population zu finden

Clarifying the neurophysiological basis of the other-race effect

The other race effect (ORE) is a well-known phenomenon whereby individuals tend to identify more accurately faces from their same-race (SR) as opposed to faces from the other-race (OR). First reported by Feingold (1914), almost a hundred years ago, since then the ORE has found consistent support at the behavioural level. In spite of a general consensus regarding the robustness of this effect, theoretical accounts have thus far failed to reach an agreement concerning the causes underlying this phenomenon. Two main strands exist within the academic literature, differing on the alleged roots of the ORE. One regards this phenomenon as stemming from different levels of expertise individuals hold with SR and OR face (i.e. the expertise based accounts); the other advocates the importance of social cognitive factor (i.e. the social cognitive accounts). Neuroimaging data can provide important insights in understanding the basis of the ORE. These studies though have thus far failed to reach a degree of consistency. EEG data for example are highly contradictory. A number of studies report no race sensitivity on the N170 face preferential component, while others show that this component is in fact modulated by race. However, discrepancy is found even amongst the study reporting N170 modulation to race, with some showing larger N170 to SR faces, while others revealing the opposite pattern. Similarly, fMRI data show the same degree of inconsistency, especially with regards to the role played by the fusiform face area (FFA). The aim of this thesis is to clarify the neurphysiolological basis of the ORE in order to gain further insights into its origins. To this end three studies (two employing EEG and one fMRI) were designed to answer three main questions related to the ORE: when, how and where in the brain does this phenomenon occur. The first study investigates the conjoint effects of race and the face inversion effect (FIE - regarded as a marker of configural face processing) on the N170. Interestingly, no race modulations on this ERP component were observed for upright faces. Race however impacted upon the magnitude of the electrophysiological FIE, with SR faces leading to greater recognition impairment and eliciting larger N170 amplitudes compared to inverted OR faces. These results indicate that race impacts upon early perceptual stages of face processing and that SR and OR faces are processed in a qualitatively different manner. The second study exploites the advantages conferred by adaptation paradigm to test neural coding efficiency for faces of different races. An unbiased spatiotemporal data-driven analysis on the newly developed single-trial repetition suppression (srRS) index, which fully accounts for the paired nature of the design, revealed differential amounts of repetition suppression across races on the N170 time window. These data suggest the SR faces are coded more efficiently than OR faces and, in line with the previous results, that race is processed at early perceptual stages. The final study investigates whether and where in the brain faces are coded according to the laws predicted by valentine’s norm based multidimensional face space model. Representational Dissimilarity Matrices (RDM) showed that faces are coded as a function of experience within the dominant FFA according to the laws of valentine’s theoretical framework Importantly in all experiments I tested both Western Caucasian (WC) and East Asian (EA) observers viewing WC and EA faces. A crossover interaction between the race of the observers and that of the face stimuli is in fact crucial to genuinely relate any observed effect to race, and exclude potential low level confounds that may be intrinsic in the stimulus set. These data, taken together indicate that the ORE is an expertise based phenomenon and that it takes place at early perceptual level of face processing

Altersabhängigkeit visueller Wahrnehmungsleistungen: Bistabile Perzeption und Arbeitsgedächtnis

Es besteht in der heutigen neurologischen Forschung ein fundamentales Interesse daran, die Prozesse des normalen Alterns und solche der krankhaften Degeneration besser zu charakterisieren und voneinander zu unterscheiden. Vor dem Hintergrund, dass das visuelle System des Gehirns in vieler Hinsicht als Modellsystem gelten darf, liegt es nahe, Alterungsprozesse beispielhaft für die visuelle Wahrnehmung zu studieren. Visuelle Wahrnehmung wiederum beruht darauf, dass Reize um kategoriale Repräsentationen im Gehirn miteinander konkurrieren, welche ausreichend lange gespeichert bleiben, um dem Betrachter bewusst zu werden. Die bistabile Wahrnehmung mehrdeutiger Reize und das visuelle Arbeitsgedächtnis sind deshalb zentral für das Verständnis von Wahrnehmungsprozessen. Ziel der vorliegenden Untersuchung war es, diese beiden Funktionen in Abhängigkeit des normalen Alterns zu charakterisieren. Hierzu wurden 50 normalsichtige und neurologisch gesunde Probanden unterschiedlichen Alters einer Reihe visueller Wahrnehmungstests unterzogen. Die Probanden wurden bezüglich ihres Alters einer von fünf Dekaden zugeordnet (jeweils zehn Probanden pro Dekade, 20-29 Jahre, 30-39 Jahre, 40-49 Jahre, 50-59 Jahre, 60-69 Jahre). Zur Untersuchung der bistabilen Wahrnehmung mehrdeutiger Figuren wurden Experimente unter Verwendung des Necker Würfels, des Prinzips der transparenten Bewegung (vgl. Stoner et al. 1990) und des Speichenradeffektes (vgl. VanRullen 2006, VanRullen et al. 2006) durchgeführt. In diesen Experimenten wurden die Häufigkeit von Perzeptwechseln und ggf. auch der jeweilige perzeptuelle Inhalt erfasst. Auswirkungen des Alters auf die visuelle Arbeitsgedächtnisleistung wurden durch Reize getestet, in welchen die Probanden Bewegungsrichtungsinformationen sequenziell präsentierter Reize miteinander vergleichen mussten. Im Ergebnis zeigte sich, dass ältere Probanden in allen Tests zur bistabilen Wahrnehmung eine Tendenz zu weniger Perzeptwechseln aufwiesen. Diese Abhängigkeit war für bistabile transparente Bewegung und den Speichenradeffekt statistisch hoch signifikant, nicht jedoch für den Necker Würfel. Ältere Probanden bevorzugten beim Test zum Speichenradeffekt das Perzept der physikalisch präsentierten Bewegungsrichtung signifikant häufiger als jüngere Probanden. Mit anderen Worten, sie zeigten die Speichenrad-Illusion, d.h. die illusionäre Wahrnehmung einer der physikalischen Raddrehung entgegengesetzten Bewegung, nur selten. Im Test zur bistabilen transparenten Bewegung ergab sich keine weitere altersabhängige Präferenz perzeptueller Inhalte. Im Test zum Arbeitsgedächtnis zeigten ältere Probanden tendenziell schlechtere Leistungen, die jedoch nicht von der Anzahl der präsentierten Bewegungsinformationen abhingen. Mit anderen Worten, es zeigte sich hier kein für das visuelle Arbeitsgedächtnis spezifisches Defizit. In umfangreichen Korrelationsanalysen konnte gezeigt werden, dass unabhängig vom Alter der Probanden die Wahrnehmungsleistungen beim Necker Würfel und zur transparenten Bewegung miteinander korrelierten, alle anderen Tests jedoch keine signifikante Korrelation aufwiesen. Zusammenfassend konnte somit gezeigt werden, dass im Alter die Bereitschaft für perzeptuelle Wechsel unabhängig des (erhaltenen) visuellen Arbeitsgedächtnisses abnimmt und dass die Mechanismen bistabiler Wahrnehmung reizspezifisch sind. Die hier erarbeiteten Ergebnisse können im Sinne eines Normalkollektivs für zukünftige klinische Untersuchungen verwendet werden

The interplay between spontaneous and evoked brain activity during visual perception

The vast majority of studies in cognitive neuroscience have focused on the brain’s response to a task or stimulus. However, the brain is very active even in the absence of explicit input or output, as its enormous energy consumption at rest suggests. This ongoing brain activity is present at all timescales; as spontaneous neuronal firing measured by electrophysiology, and as slow fluctuations in the BOLD signal measured by functional magnetic resonance imaging (fMRI). Its significance for behaviour is still unclear. This thesis explores the nature of the brain’s spontaneous activity, with an emphasis on its interaction with brain activity devoted to visual perception. Using a theoretical approach, I first show that the amount of energy expended on evoked brain activity related to a perceptual decision is minute compared to the energy expenditure associated with spontaneous activity. I then focus on spontaneous brain activity measured in the fMRI signal, the so-called resting-state fluctuations. Using simultaneous fMRI-electrophysiology in awake monkeys, I demonstrate that these fMRI resting-state fluctuations are strongly correlated to underlying fluctuations in neural activity, and are therefore likely to be neural in origin. A further fMRI study in humans shows that resting-state fluctuations in visual cortex can account for a significant degree to the variability in cortical, and to a lesser degree to the variability in behavioural responses to a visual stimulus at perceptual threshold. Lastly, I use a visual illusion called motion-induced blindness as a model system for studying the effect of spontaneous fluctuations in internal brain state on bistable perception. Using fMRI in humans, I show that while the retinal input remains constant, activity in early visual cortex reflects awareness of the stimulus. In the final, behavioural experiments, I manipulate the brain’s internal state by examining the influence of endogenous attention on the temporal dynamics of motion-induced blindness. Taken together, these studies show that spontaneous brain activity plays an important role in visual perception, and argue that understanding the brain’s internal dynamics is essential to understanding the brain as a whole