Dissertations / Theses on the topic 'Visual suppression'

Signal transduction in sensory systems is affected by two major neural mechanisms: neural noise and suppression. Both of these factors present limits on the perceptual abilities of the observer. For example, in contrast discrimination both elevate thresholds. Suppression and neural noise have been implicated in normal sensory development, ageing and several neurological disorders. Of particular interest are autism spectrum conditions (ASCs), in which both neural noise and suppressive mechanisms seem to be atypical. This thesis addresses several issues surrounding the measurement and neural implications of neural noise and suppression. Firstly, it investigates where in the brain neural noise affects sensory processing. Using machine learning algorithms to analyse electro- and magneto-encephalography data, it was found that the main source of neural noise is early sensory cortex. Secondly, it compares psychophysical paradigms used to dissociate the effects of noise and suppression, and suggests refined methods, in particular, using double-pass consistency. Thirdly, it investigates the neural effects of modulating neural noise and suppression selectively using transcranial magnetic stimulation (TMS). It reveals that two existing TMS protocols are suitable for this: single pulses suppress neural signals, whereas triple-pulse TMS increases neural noise. Lastly, the thesis investigates neural noise and gain control (a suppressive mechanism) in ASC. The findings show a relationship between sensory noise and autistic traits in the neurotypical population. Furthermore, electrophysiology data from ASC children and adults as well as a genetic Drosophila model of autism revealed a deficit in the transient dynamics of ASC visual systems, which changes over the course of development. Striking similarities between the fruit fly (Nhe3) model and humans suggests that the genetic model is suitable for further research on ASC sensory symptoms. Taken together, this thesis expands the understanding of neural noise and suppression as well as the situations in which these mechanisms are implicated.

Wir untersuchten die Verarbeitung sozial relevanter visueller Reize während “continuous flash suppression” (CFS), einer besonders wirkungsvollen Technik der interokularen Unterdrückung, die benutzt wird um Reize unsichtbar zu machen. In den Studien 1–6 maßen wir die Dauer der perzeptuellen Unterdrückung während CFS um zu testen, ob sozial relevante Reize bevorzugten Zugang zum visuellen Bewusstsein haben. Studie 1 zeigte, dass die Detektion von Gesichtern bei Erwachsenen durch Gesichtsmerkmale beeinflusst wird, welche auch Blickpräferenzen von Neugeborenen beeinflussen. Studie 2 zeigte, dass Gesichtsdetektion durch die Ethnie und Altersgruppe des zu detektierenden Gesichtes beeinflusst wird. In Studie 3 fanden wir größere Effekte der Inversion auf die Detektion von menschlichen Gesichtern und Körpern als auf andere vertraute Objekte. In Studie 4 fanden wir, dass Gesichter mit direktem Blick schneller detektiert werden als Gesichter mit abgewandtem Blick. Studie 5 deckte einen konfundieren Faktor in schematischen emotionalen Gesichtern auf. In Studie 6 fanden wir, dass die schnellere Detektion von furchtsamen im Vergleich zu neutralen Gesichtern auf hohen Raumfrequenzen beruht. Studie 7 zeigte, dass die Messung der visuellen Detektion während CFS keinen eindeutigen Nachweis für unbewusste Verarbeitung unter CFS erbringen kann. In den Studien 8 und 9 maßen wir deshalb Adaptations-Nacheffekte von Reizen, die durch CFS dauerhaft unsichtbar gemacht wurden. Studie 8 zeigte, dass lediglich monokulare Komponenten der Gesichtsform-Adaptation unbewusst ablaufen können, während komplexere Komponenten auf visuelles Bewusstsein angewiesen sind. Studie 9 zeigte, dass nur größenabhängige Komponenten von Blickrichtungen unbewusst repräsentiert werden können, während objektzentrierte Repräsentationen von Blickrichtungen visuelles Bewusstsein benötigen.
We studied the processing of socially relevant visual stimuli during continuous flash suppression (CFS), a potent interocular suppression technique that we used to render stimuli invisible. In Studies 1–6, we measured the duration of perceptual suppression during CFS to test whether socially relevant stimuli have privileged access to visual awareness. Study 1 demonstrated that face detection in adult observers is modulated by facial properties previously shown to modulate looking preferences in newborns. Study 2 revealed own-race and own-age biases in face detection, indicating that visual awareness of faces is shaped by visual experience with one’s own social group. In Study 3, we found larger effects of stimulus inversion on the detection of human faces and bodies than for other familiar objects, suggesting that detection mechanisms are preferentially tuned to conspecifics. Study 4 showed that faces with direct gaze are detected more quickly than faces with averted gaze. Study 5 revealed a confounding factor in schematic emotional faces that are considered to be well-controlled visual stimuli. In Study 6, we found that faster detection of fearful compared to neutral faces relies on high spatial frequencies, arguing against a functional role of a subcortical pathway to the amygdala. Study 7 showed that measures of visual detection during CFS cannot provide unequivocal evidence for unconscious processing under CFS. In Studies 8 and 9 we therefore measured adaptation aftereffects from stimuli rendered permanently invisible by CFS. In Study 8, we measured face shape aftereffects and found that only low-level monocular components of face shape adaptation can proceed unconsciously, whereas higher-level components depend on visual awareness. Study 9 revealed that only size-dependent low-level components of eye gaze can be represented unconsciously, while object-centered higher-level representations of eye gaze directions require visual awareness.

D’après les théories inférentielles de la perception, notre cerveau tire parti des régularités statistiques présentes dans l’environnement pour générer des prédictions qui façonnent nos contenus perceptifs. Le travail réalisé pendant cette thèse inclut 3 études principales, dans le but de caractériser les déterminants neuronaux des fausses perceptions et la nature des prédictions neuronales.Les erreurs perceptives pourraient résulter d’une tentative de notre système visuel d’expliquer des entrées sensorielles imprécises par une hypothèse erronée. Dans une 1ère étude en Imagerie par Résonance Magnétique fonctionnelle (IRMf), nous montrons que les fausses détections sont associées à un état imprécis et biaisé des circuits sensoriels avant la perception.La répétition d’une image génère une activité neurale diminuée (‘Repetition Suppression’) et des temps de réponse plus courts (‘amorçage’). Ces phénomènes pourraient résulter de mécanismes prédictifs, sous une prédiction implicite de répétition. Dans une 2nde étude IRMf, nous montrons que cette prédiction ne peut pas être modulée par l’expérience, ce qui suggère une implémentation locale. Dans une série d’études comportementales, nous montrons que l’amorçage est modulé par les prédictions, suggérant un mécanisme prédictif. Notre 2nde étude IRMf montre aussi qu’une région de moyen niveau sensible aux visages code des prédictions liées à l’identité, ce qui nous informe sur la nature des prédictions visuelles.Nos résultats montrent que notre perception est façonnée par l’interaction de nos entrées sensorielles avec l’état des circuits neuronaux avant stimulation, qu’il s’agisse de l’activité spontanée ou des stimuli précédents
According to theoretical frameworks casting perception as inference, our brain can learn the statistical regularities present in the sensory world, and use this prior information to generate predictions, which in turn shape our perceptual contents. The work conducted in this PhD includes three main studies aimed at characterizing the neural determinants of misperceptions, as well as the nature of neural predictions. Perceptual errors may arise from an attempt of our visual system to 'explain' impreciseinputs with an erroneous hypothesis. In a first functional Magnetic Resonance Imaging (fMRI) study, we show that during a detection task, hallucinations are associated with animprecise and biased state of sensory circuits preceding sensation. Stimulus repetition is associated with decreased neural responses, known as Repetition Suppression, and shorter response times, known as priming. These phenomena may reflectpredictive mechanisms under an implicit prior over repetition. In a second fMRI study, we show that this putative prior cannot be changed by experience, suggesting a local, possibly hard-wired neural implementation. In a series of behavioral experiments, we show thatpriming is modulated by predictions, supporting a predictive account of this phenomenon. Our second fMRI study also shows that a mid-level face-sensitive region codes for exemplarspecific predictions, which sheds light on the nature of the predictions encoded along thevisual hierarchy. Altogether, our results speak to the dependence of perception on prior brain states. Both spontaneous activity in sensory circuits and previous stimulation interact with sensory inputsto shape our perceptual contents

Anatomically, the visual system of non-human primates shows a complicated pattern of cortico-cortical connectivity. The behavioural relevance of many of these connections is unclear, as is the similarity of connectivity with that in the human brain. We used transcranial magnetic stimulation (TMS) and psychophysics to investigate connectivity among visual areas involved in (1) modulating visual perception during saccadic eye movements and (2) perceiving visual motion. Our first study demonstrated that phosphenes induced by TMS of visual cortex are perceived as more intense shortly after the onset of a saccade. This indicates that the cortical areas responsible for saccade generation are connected to those areas responsible for visual perception. Our second study suggested that, when applied with a very short inter-stimulus interval, TMS over an oculomotor region (FEF) can modulate the effect of TMS applied over a region sensitive to visual motion (V5). This suggests a monosynaptic feedback connection from FEF to V5.

Considerable evidence indicates that visual sensitivity is reduced during saccadic eye movement. A central question has been whether saccadic suppression results from a non-visual central signal, or whether the obligate image motion that accompanies saccades is itself sufficient to mask vision. In the first of a series of experiments described here, the visual and non-visual effects of saccades were distinguished by measuring contrast sensitivity to luminance modulated low spatial frequency gratings, at 17 cd·m¯² and 0.17 cd·m¯², in saccade conditions and in conditions in which saccade-like image motion was produced by the rotation of a mirror but when observers’ eyes were kept still. The time course of suppression was examined by making measurements from well before image motion began until well after it had ended. A tenfold decrease in contrast sensitivity was found for luminance-modulated gratings with saccades, but little suppression was found with simulated saccades. Adding high contrast noise to the visual display increased the magnitude and the duration of the suppression during simulated saccades but had little effect on suppression produced by real saccades. At lower luminance, suppression was found to be reduced, and its course shallower than at higher luminance. Simulated saccades produced shallower suppression over a longer time course at both higher and lower luminance. In a second experiment the time course of contrast sensitivity to chromatically modulated gratings, at 17 cd·m¯², was examined. No suppression was found; rather there was some evidence of an enhancement of sensitivity, both before and after saccades, relative to fixation conditions. Differences in the effects of real and simulated saccades in the magnitude and time course of sensitivity loss with luminance modulated gratings suggest that saccadic suppression has an extraretinal component that acts on the magnocellular system; the pattern of enhancement found in the later experiment suggests a selective favouring of the parvocellular system both immediately prior to and immediately after saccades. The possibility that the degree of enhancement in sensitivity varies across the visual field was examined using spatially localized stimuli (either high spatial frequency chromatically modulated gratings or letter combinations). Sensitivity was found to decrease at the initial fixation point during the 75 ms prior to saccadic onset and simultaneously to improve at the saccadic target. In the immediate post-saccadic period, sensitivity at the saccadic target was found to exceed that which had been manifest at the initial fixation point prior to saccades, suggesting that post-saccadic enhancement may improve the temporal contrast between one fixation and the next. The final experiments investigated the possibility that our sense of continuity across saccades (as opposed to stability) is influenced by saccade-induced errors in locating events in time. The results of these experiments suggest that saccades can result in errors in judging (a) the time at which external events occur relative to saccadic onset, (b) the temporal order of visual events, and (c) the magnitude of temporal intervals. It is concluded that apparent time is generally foreshortened prior to saccades. This might be due to selective suppression of magnocellular activity and might function to hide saccades and their effects from our awareness. A speculative synthesis is presented based on the idea that recurrent feedback between the neocortical and cortical structures on the one hand, and the thalamic nuclei on the other, has special importance for perception around the time of saccades

L'entrée visuelle rétinienne est discontinue. D'une part les saccades causent un énorme mouvement de l'image sur la rétine 3 à 4 fois par seconde, qui devrait résulter en un floutage des hautes fréquences spatiales et une forte impression de mouvement. D'autre part, les clignements des yeux induisent une diminution temporaire drastique de la luminance toutes les 3 à 4 secondes. Dans des conditions de vision écologiques, ces conséquences visuelles des saccades et des clignements des yeux ne sont pas consciemment perçues et le monde extérieur semble continu et net : deux phénomènes que l'on peut désigner sous le terme d'omission saccadique et d'omission des clignements des yeux. Dans cette thèse, nous avons voulu mieux comprendre comment le système visuel s'accommode de ces interruptions et quels sont les mécanismes qui contribuent à la continuité perceptive autour des saccades et des clignements des yeux. Deux principaux éléments pourraient contribuer à ces omissions : l'entrée visuelle elle-même et un mécanisme extra-rétinien qui informerait le cerveau de l'interruption à venir qui agirait en modifiant le traitement de l'information autour des saccades et des clignements des yeux. Dans une première série d'expériences, nous avons étudié les caractéristiques du masquage du smear saccadique, c'est à dire dans quelle mesure la présence d'images pré et post saccadiques nettes permet de rendre compte de l'omission du smear saccadique. Plus précisément, nous avons élaboré une méthode de mesure objective du masquage du smear et examiné son étendue spatiale et son origine périphérique ou centrale. A l'aide de cette nouvelle méthode, nous avons répliqué les résultats de masquage du smear et mis en évidence que ce masquage a lieu après le site d'interaction binoculaire et résiste à des séparations spatiales entre smear et masque jusqu'à 6 deg. Dans une deuxième étude nous avons comparé la sensibilité à des réseaux sinusoïdaux de basse fréquence spatiale autour des saccades et en fixation lorsque l'entrée visuelle simule les conséquences visuelles des saccades. De plus, nous avons cherché à établir si la plus importante diminution de la sensibilité observée pour de vraies saccades en comparaison des saccades simulées peut être expliquée par les propriétés cinématiques des mouvements oculaires. L'objectif de la troisième étude était de déterminer si le masquage est suffisant pour rendre compte de l'absence de percept de mouvement autour des saccades. Pour ce faire, nous avons présenté en fixation, un stimulus dont le contenu fréquentiel est similaire à celui des scènes naturelles. Ce stimulus était présenté en mouvement avec un profil similaire à celui d'une saccade. Il pouvait être précédé et suivi de l'image statique avant et après le mouvement. Les résultats indiquent que l'amplitude du mouvement perçu diminue considérablement en présence des masques, sans toutefois annuler totalement tout percept de mouvement pour des longues durées de masques. Dans une dernière série d'études nous nous sommes intéressés à la perception de la durée autour des clignements des yeux. Dans la première expérience nous avons quantifié la contribution de la durée d'un clignement des yeux à la durée d'une période d'obscurité plus longue et dans la seconde expérience, nous avons étudié la perception de la durée d'un objet interrompu ou non par un clignement des yeux. Les résultats de ces deux expériences suggèrent l'implication d'un mécanisme extra-rétinien qui supprime la durée perçue de la période d'obscurité causée par les clignements des yeux mais pas la durée des objets visuels chevauchés par le clignement. Pris dans leur ensemble ces résultats précisent notre compréhension des contributions relatives des mécanismes rétiniens et extra-rétiniens à l'omission saccadique et l'omission des clignements des yeux
The retinal input is discontinuous. On the one hand saccades, that occur 3-4 times a minute, cause a huge motion of the image on the retina that should result in smearing of the high frequencies of the image and perceived motion. On the other hand eye blinks induce drastic transient decreases in luminance every 3-4 seconds. Under real-world conditions, those visual consequences of saccades and blinks are barely noticed and the world appears continuous and sharp: two phenomena that can be referred to as saccadic and blink omission. In this thesis we were interested in understanding how the visual system deals with these interruptions and which mechanisms contribute to perceived continuity around saccades and blinks. Two main elements could contribute to those omissions: the visual input itself and an extra-retinal mechanism informing the brain of the impending interruption that would affect information processing around saccades and blinks. In a first series of experiments we studied the characteristics of masking of the saccadic smear, the extent to which clear and still pre- and post-saccadic images are responsible for the perceptual omission of saccadic smear. In particular, we designed an objective method to measure smear masking and studied its spatial extent and whether it is of peripheral or central origin. We replicated previous results of saccadic masking with this new method and found that smear masking seems to take place after the site of binocular interaction and survives separations between smear and mask as much as 6 deg. In a second study we compared sensitivity to low-frequency gratings around saccades and in fixation when the visual input simulates the visual consequences of saccades. Moreover we tried to determine whether the greater decrease in sensitivity around real, as compared to simulated, saccades that we found could be accounted for by the cinematic properties of the eye movement. The goal of the third study was to determine if masking was sufficient to explain the lack of perceived motion during saccades. To do that we presented, during fixation, a natural scene-like stimulus moving at saccadic speeds that could be preceded and followed by the initial or final static image. Results indicate that the amplitude of perceived motion considerably decreased in the presence of pre- and post-masks, even though motion was still perceived for long mask durations. In a final series of studies, we probed duration perception around blinks. In a first experiment we quantified the contribution of the duration of a blink to a longer period of darkness and in a second experiment we tested the perceived duration of an object interrupted or not by a blink. Results suggest the involvement of an extra-retinal mechanism that suppresses the perceived duration of the darkness caused by the blink, but not the duration of visual objects that straddle the blink. Taken together these results refine our understanding of the relative contributions of retinal and extra-retinal mechanisms to saccadic and blink omission

Previous studies have demonstrated that observers may ignore highly salient feature singletons during a conjunction search task through focusing the attentional window (Belopolsky, Zwaan, Theeuwes, & Kramer, 2007), or by the suppression of bottom-up information (Treisman & Sato, 1990). In the current study, observers’ eye movements were monitored while performing a search task in which a feature singleton was present and corresponded with the target at a chance level. With practice, observers were less likely to make an initial saccade toward the singleton item, but initial saccades directed at the target were likely throughout. Results demonstrate that, in an effort to ignore the singleton, observers were more likely to suppress bottom-up information than adjust the size of the attentional window.

The understanding of the subjective experience of a visually stable world despite the occurrence of an observer's eye movements has been the focus of extensive research for over 20 years. These studies have revealed fundamental mechanisms such as anticipatory receptive field (RF) shifts and the saccadic suppression of stimulus displacements, yet there currently exists no single explanatory framework for these observations. We show that a previously presented neuro-computational model of peri-saccadic mislocalization accounts for the phenomenon of predictive remapping and for the observation of saccadic suppression of displacement (SSD). This converging evidence allows us to identify the potential ingredients of perceptual stability that generalize beyond different data sets in a formal physiology-based model. In particular we propose that predictive remapping stabilizes the visual world across saccades by introducing a feedback loop and, as an emergent result, small displacements of stimuli are not noticed by the visual system. The model provides a link from neural dynamics, to neural mechanism and finally to behavior, and thus offers a testable comprehensive framework of visual stability.

L’objectif de cette thèse était d’étudier les variations possibles des processus impliqués dans la reconnaissance visuelle des mots selon les différences d’habiletés lexicales de lecteurs adultes (niveaux de lecture, orthographe et vocabulaire). Ainsi, nous avons testé l’effet de fréquence du voisinage orthographique par suppression et par substitution d’une lettre dans des tâches de décision lexicale (Exp. 1, 4), démasquage progressif (Exp. 2, 5), dénomination (Exp. 3, 6), et catégorisation de couleur (Exp. 7). Un effet inhibiteur de fréquence du voisinage orthographique par suppression (Exp. 1-3) et par substitution (Exp. 6) d’une lettre a été obtenu. Les temps de réponse étaient plus longs et les taux d’erreurs plus élevés pour les mots avec au moins un voisin orthographique plus fréquent que pour ceux sans un tel voisin, ce qui peut être expliqué en termes de compétition lexicale. De plus, la compétition lexicale du voisin par suppression d’une lettre était plus importante pour les individus ayant des habiletés lexicales hautes plutôt que basses (Exp. 1, 3), ces derniers individus témoignant de difficultés d’inhibition du compétiteur. Par ailleurs, les individus ayant des habiletés lexicales basses étaient moins rapides et moins précis que ceux ayant des habiletés lexicales hautes (Exp. 1-7). Les données de la tâche de catégorisation (Exp. 7) suggèrent des difficultés dans la mise en place de l’inhibition pour ces individus. Enfin, nous avons montré que l’effet de fréquence du voisinage orthographique était influencé par la confusabilité de la lettre substituée, ce qui différait selon les habiletés lexicales des individus (Exp. 4-6). Dans le cadre théorique de l’activation interactive et de codage spatial des lettres, les données soulignent l’importance des différences d’habiletés lexicales des lecteurs pour rendre compte des différences dans la diffusion de l’activation et de l’inhibition lexicales dans la reconnaissance visuelle des mots
Word recognition, according to the adult readers’ lexical skill differences (reading, spelling and vocabulary levels). To do so, we tested the orthographic neighborhood frequency effect by deletion and substitution of a letter in lexical decision (Exp.1, 4), progressive demasking (Exp 2, 5), denomination (Exp. 3, 6), and color categorization tasks (Exp.7). Response times were longer and the error rates were higher for words with at least one higher frequency neighbor than for words without such a neighbor, which can be explained in terms of lexical competition. In addition, the lexical competition of the higher-frequency deletion neighbor seems more important for individuals with high lexical skills than for those with low lexical skills (Exp 1, 3), the latter showing difficulties in inhibiting the competitor. Furthermore, individuals with low lexical skills were slower and less accurate than those with high lexical skills (Exp 1-7). Data from the categorization task (Exp. 7) suggest difficulties in setting up inhibition for these individuals. Finally, we have shown that the orthographic neighborhood frequency effect was influenced by the confusability of the substituted letter, differing according to the lexical skills of the individuals (Exp 4-6). In the theoretical framework of activation-interactive and spatial coding, the data highlight the importance of readers’ differences in lexical skills for the diffusion of lexical activation and inhibition in visual word recognition

Sakkadische Blickbewegungen sind die häufigsten und schnellsten aller menschlichen Bewegungen und führen zur wiederholtem und rapiden Verschiebung von Objektprojektionen über die Retina. Entgegen der verbreiteten Annahme der Suppression untersucht diese Arbeit Ausmaß und Funktion intrasakkadischer visueller Wahrnehmung. Studie I beschreibt eine individuell gefertigte LED-Installation zur ausschließlich intrasakkadischen Präsentation von Text und Bildern, während Studie II einen Algorithmus zur Detektion von Sakkaden vorstellt, welcher blickkontingente Stimulusmanipulationen mithilfe eines DLP Projektionssystems mit einer Bildwiederholungsrate von 1440 Hz ermöglicht. Studien III und IV untersuchten ob visuelle Bewegungsspuren (sog. motion streaks), welche durch die schnelle Bewegung von Objekten über die Retina erzeugt werden, Korrespondenz zwischen Objekten über Sakkaden hinweg herstellen könnten. Diese Bewegungsspuren erlaubten Versuchsteilnehmern nicht nur einen präsakkadischen Stimulus aus zwei identischen postsakkadischen Stimuli zu identifizieren, während diese Fähigkeit von der Deutlichkeit der Bewegungsspur abhing, sondern auch Korrektursakkaden zu einem ursprünglichen präsakkadischen Stimulus zu erleichtern, falls dieser während der Sakkade versetzt wurde. Studie V untersuchte die subjektive Wahrnehmung und Lokalisierung von intrasakkadischen Bewegungsspuren, indem Teilnehmer gezeichnete Berichte angaben. Die Modellierung letzterer ergab, dass retinale Positionssignale mit einer zeitlich gedämpften mentalen Repräsentation von Augenposition kombiniert wurden, um eine Lokalisation in weltzentrierten Koordinaten zu ermöglichen. Diese Ergebnisse legen nahe, dass intrasakkadische visuelle Signale einen Einfluss auf transsakkadische perzeptuelle und motorische Prozesse haben könnten. Letztlich werden die mögliche Funktionen intrasakkadischer Wahrnehmung, sowie Möglichkeiten für zukünftige wissenschaftliche Untersuchungen, diskutiert.
Rapid eye movements, so-called saccades, are the fastest and most frequent human movements and cause projections of objects in the world to constantly shift across the retina at high velocities, thereby producing large amounts of motion blur. In contrast to accounts of saccadic suppression, this work explores the extent and potential functional role of intra-saccadic perception. As saccades are fast and brief events, technical challenges were addressed. Study I describes a custom LED-based anorthoscopic presentation setup capable of displaying text and images strictly during saccades. In study II, a novel online saccade detection algorithm enabled rapid, gaze-contingent display changes using a DLP projection system running at 1440 fps. Studies III and IV investigated whether intra-saccadic motion streaks, i.e., blurred traces routinely induced by stimuli moving at saccadic speeds, could serve as cues to establishing object correspondence across saccades. Motion streaks not only enabled perceptual matching of pre- and post-saccadic object locations, while performance depended strongly on streak efficiency, but also facilitated gaze correction in response to intra-saccadic target displacements, that was previously found to be mainly driven by objects’ surface features. Finally, study V explored the subjective appearance and localization of intra-saccadic motion streaks, tasking observers to reproduce their trajectories. Computational modeling of resulting response patterns suggested that retinal positions over time were combined with a damped eye position signal to readily localize intra-saccadic input in world-centered coordinates. Taken together, these results invite the intriguing hypothesis that intra-saccadic visual signals are not discarded from processing and might affect trans-saccadic perceptual and motor processes. The potential role of intra-saccadic perception for active vision, as well as directions for future research, are discussed.

La prosopographie est utilisée par les historiens pour désigner des notices biographiques afin d'étudier des caractéristiques communes d'un groupe d'acteurs de l'histoire au moyen d'une analyse collective de leur vie. La visualisation d'informations présente des perspectives intéressantes pour analyser les données prosopographiques. C'est dans ce contexte que se situe le travail présenté dans ce mémoire. Dans un premier temps, nous présentons la plateforme ProsoVis conçue pour analyser et naviguer dans des données prosopographiques. Nous décrivons les différents besoins exprimés et détaillons les choix de conception ainsi que les techniques de visualisation employées. Nous illustrons son utilisation avec la base Siprojuris qui contient les données sur la carrière des enseignants de droit de 1800 à 1950. La visualisation d'autant de données pose des problèmes d'encombrement visuel. Dans ce contexte, nous abordons la problématique des chevauchements des nœuds dans un graphe. Différentes approches existent mais il est difficile de les comparer car leurs évaluations ne sont pas basées sur les mêmes critères de qualité. Nous proposons donc une étude de l'état de l'art et comparons les résultats des algorithmes sur une liste homogène de critères. Enfin, nous abordons une autre problématique d'encombrement visuel au sein d'une carte et proposons une approche de regroupement spatial agglomératif, F-SAC, beaucoup plus rapide que les propositions de l'état de l'art tout en garantissant la même qualité de résultats
Prosopography is used by historians to designate biographical records in order to study common characteristics of a group of historical actors through a collective analysis of their lives. Information visualization presents interesting perspectives for analyzing prosopographic data. It is in this context that the work presented in this thesis is situated. First, we present the ProsoVis platform to analyze and navigate through prosopographic data. We describe the different needs expressed and detail the design choices as well as the different views. We illustrate its use with the Siprojuris database which contains data on the careers of law teachers from 1800 to 1950. Visualizing so much data induces visual cluttering problems. In this context, we address the problem of overlapping nodes in a graph. Even if approaches exist, it is difficult to compare them because their respective evaluations are not based on the same quality criteria. We therefore propose a study of the state-of-the-art algorithms by comparing their results on the same criteria. Finally, we address a similar problem of visual cluttering within a map and propose an agglomeration spatial clustering approach, F-SAC, which is much faster than the state-of-the-art proposals while guaranteeing the same quality of results

When dealing with significant sensory stimuli, performance can be hampered by distracting events. Attention mechanisms lessen such negative effects, enabling selection of relevant information while blocking potential distraction. Recent work shows that preventing the negative impact of forthcoming distraction is actively achieved by attentional selection processes. Thus, I hypothesize that the engagement of a distraction-filtering mechanism to counteract distraction, although indisputably beneficial when distraction occurs, also taxes cognitive-brain systems when distraction is expected but does not occur, leading to performance costs. In my thesis, I seek the behavioral and brain signature of a mechanism for the filtering of potential distraction within and between sensory modalities. I show that, when potential distraction is foreseen in a stimulus-processing context, a cognitive mechanism is engaged for limiting negative impact of irrelevant stimuli on behavioral performance, yet its engagement is resource-demanding and thus incurs a performance cost when distraction does not occur. This cost consists of slower response times to a simple sensory stimulus when presented alone but in a potentially-distracting context, as compared to the same stimulus presented in a completely distraction-free context. This cost generalizes across different target and distracters sensory modalities, such as touch, vision and audition, and to both space-based and feature-based attention tasks. The activation of the filtering mechanism relies on both strategic and reactive processes, as shown by its dynamic dependence on probabilistic and cross-trial contingencies. Probability of conflict substantially modulates the magnitude of the filtering cost, which results larger in contexts where the probability of experiencing conflict is high. Crucially, across participants, the observed strategic cost is inversely related to the interference exerted by a distracter on distracter-present trials. The strategic filtering mechanism is predominantly adopted as a longer-term, sustained, cognitive set throughout an extended time period. Its activation is associated with sustained brain activity in prefrontal areas and in the frontoparietal attentional network. Sustained brain activity in prefrontal areas correlates across participants with the filtering cost, thus confirming a close relationship between this sustained activation and the observed behavioral cost. I also show that the recruitment of the distraction filtering mechanism in a potentially distracting context guides attention and behavior through different top-down modulations. In fact, when potential distraction is foreseen, the activation of a filtering mechanism promotes both the attenuation of sensory representation of distracting stimuli in extrastriate visual cortex and the prevention of involuntary activations of conflict-driven motor responses in the premotor cortex. These results attest to the existence of a system for the monitoring and filtering of potential distraction in the human brain that likely reflects a general mechanism of cognitive-attentional control.

The brain displays moment-to-moment activity fluctuations that reflect various levels of engagement with the outside world. Processing external stimuli is not only associated with increased brain metabolism but also with prominent deactivation in specific structures, collectively known as the default-mode network (DMN). The role of the DMN remains enigmatic partly because its electrophysiological correlates and temporal dynamics are still poorly understood. Using unprecedented wide-spread depth recordings in epileptic patients, undergoing intracranial EEG during pre-surgical evaluation, we reveal that DMN neural populations display task-related suppressions of gamma (60-140 Hz) power and, critically, we show how millisecond temporal profile and amplitude of gamma deactivation tightly correlate with task demands and subject performance. The results show also that during an attentional task, sustained activations in the gamma band power are presented across large cortical networks, while transient activations are mostly specific to occipital and temporal regions. Our findings reveal a pivotal role for broadband gamma modulations in the interplay between activation and deactivation networks mediating efficient goal-directed behavior

Nell’apparentemente semplice processo di “vedere” sono coinvolti meccanismi neurali complessi, non del tutto compresi. La percezione visiva consiste di due componenti chiave: l’analisi dell’informazione e la consapevolezza soggettiva. Questa tesi riporta degli esperimenti comportamentali aventi lo scopo di far luce su due condizioni in cui l’esperienza e la sensazione percettiva sono dissociate: l’immaginazione e la soppressione visiva. IL CASO DELL'IMMAGINAZIONE VISIVA Una questione da lungo dibattuta è se la percezione ed immaginazione visiva condividono analoghi meccanismi cognitivi e neurali. Per chiarire questo problema abbiamo confrontato gli effetti di stimoli visivi reali e generati mentalmente sul tempo di reazione semplice (TR). In cinque esperimenti sono stati studiati gli effetti di differenze in luminanza, contrasto, frequenza spaziale, velocità di movimento ed orientamento. Con l’interessante eccezione della frequenza spaziale, in tutti gli altri compiti, percezione ed immaginazione hanno mostrato effetti qualitativamente simili. E’stata trovata una corrispondenza fra gli effetti al compito percettivo ed immaginativo per luminanza, contrasto, velocità di movimento ed orientamento di linee. Diversamente, l’interazione fra frequenza spaziale e compito era significativa: le risposte erano più veloci per reticoli di bassa rispetto ad alta frequenza spaziale solo per gli stimoli presentati visivamente. Il presente studio, dunque, mostra come variabili dipendenti di base esercitino simili effetti sul TR sia quando gli stimoli sono presentati visivamente che immaginati. Tali risultati sono indubbiamente a favore di una sovrapposizione fra le rappresentazioni strutturali della percezione e dell’immaginazione visiva. IL CASO DELLA SOPPRESSIONE VISIVA Quando gli input provenienti dai due occhi sono incompatibili e non possono essere fusi in un singolo percetto coerente, si verifica un fenomeno chiamato rivalità binoculare. Gli input visivi raggiungono alternativamente la consapevolezza per pochi secondi cosicché mentre un percetto è visibile, cioè dominante, l’altro è invisibile, cioè soppresso. I meccanismi neurali sottostanti la rivalità binoculare sono ampiamente dibattuti. Dati recenti mostrano una correlazione fra l’attività nella corteccia striata e lo stato percettivo riportato soggettivamente. Ad ogni modo, si sa ancora poco sul ruolo degli stadi di elaborazione precoce a livello sottocorticale. La presente Tesi fornisce una prova del coinvolgimento della via sottocorticale durante la rivalità binoculare. Il paradigma sperimentale impiegato è l’effetto degli stimoli ridondanti (noto come redundant target effect, RTE) con stimoli presentati in modo da generare una rivalità binoculare. Il RTE consiste nella maggiore velocità a rispondere a due (o più) target, rispetto ad uno solo. Tale effetto è mediato da un processo di sommazione spaziale in cui le strutture sottocorticali, compreso probabilmente il collicolo superiore, hanno un ruolo importante. In questo studio, abbiamo presentato dei Gabor in modo tale da generare una rivalità binoculare in un lato del punto di fissazione e una fusione nell’altro. Il/I target era/erano un incremento di contrasto del/i Gabor, che poteva essere visto oppure soppresso. L’effetto dello stesso target ridondante è stato confrontato quando questo era nella fase di dominanza e di soppressione. Il RTE trovato quando il target ridondante era dominante scompariva quando lo stesso target era percettivamente soppresso. Poiché il RTE era attribuibile ad una coattivazione neurale, tale dato suggerisce che il processo di soppressione coinvolge la via neurale del RTE. Questi risultati sono in linea con l’idea che la soppressione binoculare coinvolga processi visivi a stadi precoci, probabilmente antecedenti alla corteccia visiva primaria.
Behind the apparently simple process of “seeing”, there are complex neural mechanisms involved that are not completely understood. Visual perception consists of two key components: information analysis and subjective awareness. This thesis reports some behavioural experiments to cast further light on two conditions in which perceptual experience and sensation are dissociated: visual imagery and visual suppression. THE CASE OF VISUAL IMAGERY: A long standing issue is whether perception and mental imagery share similar cognitive and neural mechanisms. To cast further light on this problem we compared the effects of real and mentally generated visual stimuli on simple reaction time (RT). In five experiments we tested the effects of difference in luminance, contrast, spatial frequency, motion and orientation. With the intriguing exception of spatial frequency in all other tasks perception and imagery showed qualitatively similar effects. We found a correspondence between perception and imagery effects for luminance, contrast, speed of motion, and line orientation. In contrast, we found an interaction between perception and imagery for spatial frequency: gratings of low spatial frequency were responded to more quickly than those of higher spatial frequency only for visually presented stimuli. Thus, the present study shows that basic dependent variables exert similar effects on visual RT either when retinally presented or imagined. The present results undoubtedly provide support for some overlap between the structural representation of perception and imagery. THE CASE OF VISUAL SUPPRESSION: Binocular rivalry occurs when the inputs from the two eyes are incompatible and cannot be fused into a single, coherent percept. The visual inputs reach alternatively consciousness for a few seconds and while one percept is seen (dominant) the other is invisible (suppression). The neural mechanisms underlying binocular rivalry have been much debated. Recent evidence shows a correlation between the activity in the striate cortex and the subjectively reported state of rivalry. However, little is known about the role of subcortical processing stages. Here we provide evidence for an involvement of subcortical pathways during binocular rivalry. To this purpose, we employed the redundant target effect (RTE) with stimuli undergoing binocular rivalry. RTE simply means that response to two (or more) targets is faster than to one. It is mediated by a spatial summation process where subcortical structures, likely involving the superior colliculus, play an important role. In this study, Gabors were presented in a way to generate binocular rivalry on one side and fusion on the other side of the fixation cross. Target/s was/were a contrast increment of the Gabor that could be visible or perceptually suppressed. The effects of the same redundant target were compared when it was dominant and it was suppressed. The RTE found when the redundant target was dominant disappeared when the same target was perceptually suppressed. Since the RTE was ascribable to a neural coactivation, the present finding suggests that visual suppression involves the RTE pathway. These results support the idea that binocular suppression involves visual processes at early stages, prior to the primary visual cortex.

The brain selectively processes incoming sensory information and plans adequate motor output aimed at behaviourally relevant objects in the environment, based on different attentional control (AC) mechanisms. The contribution of single AC mechanisms to visual attention has been extensively investigated; still, it remains unclear how those different biasing signals interact with one another in order to reach the final choice of which spatial location (or stimulus) is worth of attention. The main goal of my PhD project was to investigate whether different AC mechanisms jointly act to shape priority of given stimuli and locations or whether, at any given moment, one mechanism prevails over the others, gaining precedence onto the neural representation of the visual space, known as spatial priority map. By using variants of the same visual search task, we implemented a series of behavioural and EEG experiments to test the unique and combined effect of two AC mechanisms: top-down control, modulated via endogenous cueing (valid vs. neutral cues), and experience-dependent control, implemented through a statistical learning (SL) protocol (high vs. low target frequency locations). Our results revealed that both cue validity and SL enhanced performance respectively for targets predicted by valid (vs. neutral) cue and for targets at high (vs. low) frequency locations. The benefit of top-down control was also confirmed by larger CNV and P1, i.e. EEG markers of general preparation and early categorization for target selection, respectively. Most importantly, when activated together, top-down control and SL display an interesting interaction, with the behavioural effect of the latter being overridden by the presence of the former. However, in terms of N2pc, an EEG index of selection, the cueing effect selectively emerged for targets in the low- (vs. high) frequency location, suggesting that, even if not behaviourally evident, the SL effect was not totally blocked by top-down guidance; rather, it could affect attentional deployment, at least at some point of the target selection process. Finally, in our tasks, we could also indirectly assess the impact of a salient distractor on individuals’ performance; this irrelevant bottom-up AC signal indeed diverted attention from the target and interfered with the task, regardless of the presence or absence of the other AC mechanisms.

碩士
國立成功大學
心理學系認知科學碩士班
101
To what extent unconscious information can be process and integrated is still of great debate. Backward masking (BM) and interocular suppression had been widely used to probe this problem for a long time. But the cross form effect of word and picture remain unclear. In this dissertation we applied CFS paradigm to study this problem. The difference of response time (RT) to breakthrough the suppression under congruent and incongruent condition was used as an indicator of the effect of semantic integration. We supposed that perception may occur by multilevel neuro circuit connection of bottom-up and top-down information to achieve semantic integration of word and picture . However our results did not showed significant congruency effect of picture to word and no evidence of integrated effect or accumulation effect in cross-form (word-picture-word) condition. The absence of congruent effect of the stimuli from different form may implicit either failure in neural information transformation or failure in integrated with top down information. At last we would like to make some modifications of our paradigm for future research in this topic.

Salient distractors appearing in the visual field trigger an involuntary oculomotor capture, so being able to ignore them is paramount for an efficient attentional selection. Recent findings have revealed that past experience of distractor filtering greatly affects the deployment of attention such that it can reduce the priority of locations frequently associated with irrelevant information and, accordingly, weaken the interference of distractor appearing therein. Such benefit associated with suppression history suggests that selective attention has adaptive experience-dependent features. There are still gaps however in the knowledge of the mechanisms underlying these phenomena, that need to be more clearly identified and detailed. In a series of experiments, we addressed this topic by exploring the effect of suppression history on the immediate behavioral measures of attentional deployment - i.e. eye-movements - and on their neural correlates. Using variants of a visual search task, we manipulated the probability of occurrence of a salient distractor such that it occurred more frequently at two locations on the visual display, unbeknown to the participants (High Frequency locations - HF). The results showed that the amount of oculomotor capture triggered by the distractors appearing at HF locations was dramatically reduced relative to distractors appearing at other locations, consistently with the improvement also shown on task performance. Testing the permanence over time of these benefits, we found that some residual effects of suppression history were still detectable after the frequency unbalances were no longer in place, but their traces lingered for a very short time, vanishing definitively 24-hours later. Importantly, the bias induced by suppression history was accomplished by changes in neural activity at a relatively early stage of cortical visual processing. Indeed, the distractor-related cortical activities explored at posterior-occipital areas showed a reduced neural activation for distractors appearing at HF locations as indexed by a smaller N2pc, hence providing evidence of a decreased deployment of selective attention towards these stimuli, prior to saccadic planning. In summary, this work provides compelling evidence that suppression history affects attentional spatial priority by dynamically down-weighting the representation of spatial locations that have been more frequently associated with distraction, and whose selection has been therefore inhibited. Our data suggest that such plasticity, within topographic maps of the visual space, is transient and functional, and supported by neural changes in cortical visual processing that sustains ongoing oculomotor control.

Le traitement visuel répété d’un visage inconnu entraîne une suppression de l’activité neuronale dans les régions préférentielles aux visages du cortex occipito-temporal. Cette «suppression neuronale» (SN) est un mécanisme primitif hautement impliqué dans l’apprentissage de visages, pouvant être détecté par une réduction de l’amplitude de la composante N170, un potentiel relié à l’événement (PRE), au-dessus du cortex occipito-temporal. Le cortex préfrontal dorsolatéral (CPDL) influence le traitement et l’encodage visuel, mais sa contribution à la SN de la N170 demeure inconnue. Nous avons utilisé la stimulation électrique transcrânienne à courant direct (SETCD) pour moduler l’excitabilité corticale du CPDL de 14 adultes sains lors de l’apprentissage de visages inconnus. Trois conditions de stimulation étaient utilisées: inhibition à droite, excitation à droite et placebo. Pendant l’apprentissage, l’EEG était enregistré afin d’évaluer la SN de la P100, la N170 et la P300. Trois jours suivant l’apprentissage, une tâche de reconnaissance était administrée où les performances en pourcentage de bonnes réponses et temps de réaction (TR) étaient enregistrées. Les résultats indiquent que la condition d’excitation à droite a facilité la SN de la N170 et a augmentée l’amplitude de la P300, entraînant une reconnaissance des visages plus rapide à long-terme. À l’inverse, la condition d’inhibition à droite a causé une augmentation de l’amplitude de la N170 et des TR plus lents, sans affecter la P300. Ces résultats sont les premiers à démontrer que la modulation d’excitabilité du CPDL puisse influencer l’encodage visuel de visages inconnus, soulignant l’importance du CPDL dans les mécanismes d’apprentissage de base.
Repeated visual processing of an unfamiliar face suppresses neural activity in face-specific areas of the occipito-temporal cortex. This "repetition suppression" (RS) is a primitive mechanism involved in learning of unfamiliar faces, which can be detected through amplitude reduction of the N170 event-related potential (ERP). The dorsolateral prefrontal cortex (DLPFC) exerts top-down influence on early visual processing. However, its contribution to N170 RS and learning of unfamiliar faces remains unclear. Transcranial direct current stimulation (tDCS) transiently increases or decreases cortical excitability, as a function of polarity. We hypothesized that DLPFC excitability modulation by tDCS would cause polarity-dependent modulations of N170 RS during encoding of unfamiliar faces. tDCS-induced N170 RS enhancement would improve long-term recognition reaction time (RT) and/or accuracy rates, whereas N170 RS impairment would compromise recognition ability. Participants underwent three tDCS conditions in random order at ~72 hour intervals: right anodal/left cathodal, right cathodal/left anodal and sham. Immediately following tDCS conditions, an EEG was recorded during encoding of unfamiliar faces for assessment of P100 and N170 visual ERPs. P300 was analyzed to detect prefrontal function modulation. Recognition tasks were administered ~72 hours following encoding. Results indicate the right anodal/left cathodal condition facilitated N170 RS and induced larger P300 amplitudes, leading to faster recognition RT. Conversely, the right cathodal/left anodal condition caused increases in N170 amplitudes and RT, but did not affect P300. These data are the first to demonstrate that DLPFC excitability modulation can influence early visual encoding of unfamiliar faces, highlighting the importance of DLPFC in basic learning mechanisms.