Transcranial Magnetic Stimulation of Early Visual Cortex During Transsaccadic Integration of Object Features

Visual information is integrated across saccades to maintain a continuous spatiotemporal representation of the world. This study investigated the role of early visual cortex (EVC) in trans-saccadic integration using functional magnetic resonance imaging guided repetitive transcranial magnetic stimulation (rTMS) protocol. Triple-pulse rTMS was applied over left and right EVC during the fixation task (participants maintained gaze), and saccade task (participants made an eye movement that either maintained or reversed the visual quadrant of the test stimulus). rTMS had no effect when 1) fixation was maintained, 2) saccades kept the stimulus in the same visual quadrant, or 3) quadrant corresponding to the first Gabor patch was stimulated. However, rTMS affected performance (relative to opposite EVC rTMS) when saccades brought the remembered visual stimulus into the magnetically stimulated quadrant. This effect increased with saccade amplitude. These results show that EVC is involved in the memory and ‘remapping’ of visual features across saccades

The effects of TMS over dorsolateral prefrontal cortex on multiple visual object memory across fixation and saccades

Trans-saccadic memory, the process by which the visual system maintains the spatial position and features of objects across eye movements, is thought to be a form of visual working memory (Irwin, 1991). It has been shown that TMS over the frontal and parietal eye fields degrades trans-saccadic memory of multiple object features (Prime et al., 2008, 2010). We used a similar TMS protocol to investigate whether dorsolateral prefrontal cortex (DLPFC) is also involved in trans-saccadic memory. We predicted that performance would be disrupted similarly during either fixation or saccades. Instead, we found both task and hemisphere-dependent effects. During fixation, TMS over left DLPFC produced inconsistent effects, whereas TMS over right DLPFC reduced performance, consistent with its known role in working memory (Goldman-Rakic, 1987). In contrast, TMS over both sides of DLPFC enhanced trans-saccadic memory, suggesting a dis-inhibition of trans-saccadic processing. These results suggest that visual working memory during fixation and trans-saccadic memory may be supported by different, but interacting, neural circuits

Perceptual stability during saccadic eye movements

Humans and other primates perform multiple fast eye movements per second in order to redirect gaze within the visual field. These so called saccades challenge visual perception: During the movement phases the projection of the outside world sweeps rapidly across the photoreceptors altering the retinal positions of objects that are otherwise stable in the environment. Despite this ever-changing sensory input, the brain creates the percept of a continuous, stable visual world. Currently, it is assumed that this perceptual stability is achieved by the synergistic interplay of multiple mechanisms, for example, a reduction of the sensitivity of the visual system around the time of the eye movement ('saccadic suppression') as well as transient reorganizations in the neuronal representations of space ('remapping'). This thesis comprises six studies on trans-saccadic perceptual stability

When the brain is split, is space still unified?

How does the brain keep track of relevant spatial locations when the eyes move? In extrastriate, parietal and frontal cortex, and in the superior colliculus, neurons update stimulus representations in conjunction with eye movements. This updating reflects a transfer of visual information, from neurons that encode a salient location before the saccade, to neurons that encode the location after the saccade. Copies of the oculomotor command - corollary discharge signals - likely initiate this transfer. Spatial updating, or remapping, is thought to contribute to the maintenance of stable spatial representations as the eyes move. We investigated the circuitry that supports spatial updating in the primate brain. Our central hypothesis was that the forebrain commissures provide the primary route for remapping spatial locations across visual hemifields, which entails the interhemispheric transfer of visual information. Further, we hypothesized that these commissures provide the primary route for corollary discharge signals, generated in one hemisphere, to initiate spatial updating in the opposite hemisphere. We tested these hypotheses by measuring spatial behavior and neural activity in two split-brain macaques. In behavioral experiments, we observed striking initial impairments in the monkeys' ability to update stimuli across visual hemifields. Surprisingly, however, we found that both animals were ultimately capable of performing these across-hemifield sequences. Both monkeys readily performed the same spatial task when updating required an interhemispheric transfer of corollary discharge signals, suggesting that these signals are transferred via subcortical pathways in the normal monkey. In physiological experiments, we found that neurons in lateral intraparietal cortex of the split-brain monkey can remap stimuli across visual hemifields, albeit with a reduction in the strength of remapping activity. These neurons were robustly active when within-hemifield updating was initiated by a saccade into the opposite hemifield. Our findings suggest that both visual and corollary discharge signals from opposite hemispheres can converge to update spatial representations in the absence of the forebrain commissures. These investigations provide new evidence that a unified and stable representation of visual space is supported by a redundant circuit, comprised of cortical as well as subcortical pathways, with a remarkable capacity for reorganization

Le rétablissement des positions d un objet dans l espace à travers des mouvements des yeux et de la tête

Le système visuel a évolué de manière à prendre en compte les conséquences de nos mouvements sur notre perception. L évolution nous a particulièrement doté de la capacité à percevoir notre environnement visuel comme stable et continu malgré les importants déplacements de ses projections sur nos rétines à chaque fois que nous déplaçons nos yeux, notre tête ou notre corps. Des études chez l animal ont récemment montré que dans certaines aires corticales et sous-corticales, impliquées dans le contrôle attentionnel et dans l élaboration des mouvements oculaires, des neurones sont capables d anticiper les conséquences des futurs mouvements volontaires des yeux sur leurs entrées visuelles. Ces neurones prédisent ce à quoi ressemblera notre environnement visuel en re-cartographiant la position des objets d importance à l endroit qu ils occuperont après l exécution d une saccade. Dans une série d études, nous avons tout d abord démontré que cette re- cartographie pouvait être évaluée de manière non invasive chez l Homme avec de simples cibles en mouvement apparent. En utilisant l enregistrement des mouvements des yeux combinés à des méthodes psychophysiques, nous avons déterminé la distribution des erreurs de re-cartographie à travers le champ visuel et ainsi découvert que la compensation des saccades oculomotrices se faisait de manière relativement précise. D autre part, les patterns d erreurs observés soutiennent un modèle de la constance spatiale basé sur la re-cartographie de pointeurs attentionnels et excluent d autres modèles issus de la littérature. Par la suite, en utilisant des objets en mouvement continu et l exécution de saccades au travers de leurs trajectoires, nous avons mis à jour une visualisation directe des processus de re-cartographie. Avec ce nouveau procédé nous avons à nouveau démontré l existence d erreurs systématiques de correction pour les saccades, qui s expliquent par une re-cartographie imprécise de la position attendue des objets en mouvement. Nous avons par la suite étendu notre modèle à d autres types de mouvements du corps et notamment étudié les contributions de récepteurs sous-corticaux (otoliths et canaux semi-circulaires) dans le maintien de la constance spatiale à travers des mouvements de la tête. Contrairement à des études décrivant une compensation presque parfaite des mouvements de la tête, nous avons observé une rupture de la constance spatiale pour des mouvements de roulis et de translation de la tête. Enfin, nous avons testé cette re-cartographie de la position des objets compensant un déplacement oculaire avec des cibles présentées à la limite du champ visuel, une re-cartographie censée placer la position attendue de l objet à l extérieur du champ visuel. Nos résultats suggèrent que les aires visuelles cérébrales impliquées dans ce processus de re-cartographie construisent une représentation globale de l espace allant au-delà du traditionnel champ visuel. Pour finir, nous avons conduit deux expériences pour déterminer le déploiement de l attention à travers l exécution de saccades. Nous avons alors démontré que l attention capturée par la présentation brève d un stimuli est re-cartographiée à sa position spatiale correcte après l exécution d une saccade, et que cet effet peut être observé avant même l initiation d une saccade. L ensemble de ces résultats démontre le rôle des pointeurs attentionnels dans la gestion du rétablissement des positions d un objet dans l espace ainsi que l apport des mesures comportementales à un champ de recherche initialement restreint à l électrophysiologieThe visual system has evolved to deal with the consequences of our own movements onour perception. In particular, evolution has given us the ability to perceive our visual world as stableand continuous despite large shift of the image on our retinas when we move our eyes, head orbody. Animal studies have recently shown that in some cortical and sub-cortical areas involved inattention and saccade control, neurons are able to anticipate the consequences of voluntary eyemovements on their visual input. These neurons predict how the world will look like after a saccadeby remapping the location of each attended object to the place it will occupy following a saccade.In a series of studies, we first showed that remapping could be evaluated in a non-invasive fashion in human with simple apparent motion targets. Using eye movement recordingsand psychophysical methods, we evaluated the distribution of remapping errors across the visualfield and found that saccade compensation was fairly accurate. The pattern of errors observedsupport a model of space constancy based on a remapping of attention pointers and excluded otherknown models. Then using targets that moved continuously while a saccade was made across themotion path, we were able to directly visualize the remapping processes. With this novel method wedemonstrated again the existence of systematic errors of correction for the saccade, best explainedby an inaccurate remapping of expected moving target locations. We then extended our model toother body movements, and studied the contribution of sub-cortical receptors (otoliths and semi-circular canals) in the maintenance of space constancy across head movements. Contrary tostudies reporting almost perfect compensations for head movements, we observed breakdowns ofspace constancy for head tilt as well as for head translation. Then, we tested remapping of targetlocations to correct for saccades at the very edge of the visual field, remapping that would place theexpected target location outside the visual field. Our results suggest that visual areas involved inremapping construct a global representation of space extending out beyond the traditional visualfield. Finally, we conducted experiments to determine the allocation of attention across saccades.We demonstrated that the attention captured by a brief transient was remapped to the correctspatial location after the eye movement and that this shift can be observed even before thesaccade.Taken together these results demonstrate the management of attention pointers to therecovery of target locations in space as well as the ability of behavioral measurements to address atopic pioneered by eletrophysiologists.PARIS5-Bibliotheque electronique (751069902) / SudocSudocFranceF

Neural Mechanisms of Transsaccadic Integration of Visual Features

This thesis explores the neural mechanisms of transsaccadic integration of visual features. In the study, I investigated the cortical correlates of transsaccadic integration of object orientation in multiple reference frames. In a functional MRI adaptation (fMRIa) paradigm, participants viewed sets of two orientation stimuli in each trial and were asked to indicate if the orientations were the same (Repeat condition) or different (Novel condition). Stimuli were presented in one of three spatial conditions: 1) space-fixed, 2) retina-fixed and 3) frame-independent. Results indicate that, in addition to common activation in frontal motor cortical regions in all three spatial conditions, parietal and occipitotemporal regions are active in the space-fixed condition, parietofrontal regions are active in the retina-fixed condition, and parietofrontal and occipitotemporal regions are active in the frame-independent condition. In conclusion, these results indicate that transsaccadic integration involves differential activation of cortical areas, depending on the frame of reference

Predictive feedback to the primary visual cortex during saccades

Perception of our sensory environment is actively constructed from sensory input and prior expectations. These expectations are created from knowledge of the world through semantic memories, spatial and temporal contexts, and learning. Multiple frameworks have been created to conceptualise this active perception, these frameworks will be further referred to as inference models. There are three elements of inference models which have prevailed in these frameworks. Firstly, the presence of internal generative models for the visual environment, secondly feedback connections which project prediction signals of the model to lower cortical processing areas to interact with sensory input, and thirdly prediction errors which are produced when the sensory input is not predicted by feedback signals. The prediction errors are thought to be fed-forward to update the generative models. These elements enable hypothesis driven testing of active perception. In vision, error signals have been found in the primary visual cortex (V1). V1 is organised retinotopically; the structure of sensory stimulus that enters through the retina is retained within V1. A semblance of that structure exists in feedback predictive signals and error signal production. The feedback predictions interact with the retinotopically specific sensory input which can result in error signal production within that region. Due to the nature of vision, we rapidly sample our visual environment using ballistic eye-movements called saccades. Therefore, input to V1 is updated about three times per second. One assumption of active perception frameworks is that predictive signals can update to new retinotopic locations of V1 with sensory input. This thesis investigates the ability of active perception to redirect predictive signals to new retinotopic locations with saccades. The aim of the thesis is to provide evidence of the relevance of generative models in a more naturalistic viewing paradigm (i.e. across saccades). An introduction into active visual perception is provided in Chapter 1. Structural connections and functional feedback to V1 are described at a global level and at the level of cortical layers. The role of feedback connections to V1 is then discussed in the light of current models, which hones in on inference models of perception. The elements of inferential models are introduced including internal generative models, predictive feedback, and error signal production. The assumption of predictive feedback relocation in V1 with saccades is highlighted alongside the effects of saccades within the early visual system, which leads to the motivation and introduction of the research chapters. A psychophysical study is presented in Chapter 2 which provides evidence for the transference of predictive signals across saccades. An internal model of spatiotemporal motion was created using an illusion of motion. The perception of illusory motion signifies the engagement of an internal model as a moving token is internally constructed from the sensory input. The model was tested by presenting in-time (predictable) and out-of-time (unpredictable) targets on the trace of perceived motion. Saccades were initiated across the illusion every three seconds to cause a relocation of predictive feedback. Predictable in-time targets were better detected than the unpredictable out-of-time targets. Importantly, the detection advantage for in-time targets was found 50 – 100 ms after saccade indicating transference of predictive signals across saccade. Evidence for the transfer of spatiotemporally predictive feedback across saccade was supported by the fMRI study presented in Chapter 3. Previous studies have demonstrated an increased activity when processing unpredicted visual stimulation in V1. This activity increase has been related to error signal production as the input was not predicted via feedback signals. In Chapter 3, the motion illusion paradigm used in Chapter 2 was redesigned to be compatible with brain activation analysis. The internal model of motion was created prior to saccade and tested at a post-saccadic retinotopic region of V1. An increased activation was found for spatiotemporally unpredictable stimuli directly after eye-movement, indicating the predictive feedback was projected to the new retinotopic region with saccade. An fMRI experiment was conducted in Chapter 4 to demonstrate that predictive feedback relocation was not limited to motion processing in the dorsal stream. This was achieved by using natural scene images which are known to incorporate ventral stream processing. Multivariate analysis was performed to determine if feedback signals pertaining to natural scenes could relocate to new retinotopic eye-movements with saccade. The predictive characteristic of feedback was also tested by changing the image content across eye-movements to determine if an error signal was produced due to the unexpected post-saccadic sensory input. Predictive feedback was found to interact with the images presented post-saccade, indicating that feedback relocated with saccade. The predictive feedback was thought to contain contextual information related to the image processed prior to saccade. These three chapters provide evidence for inference models contributing to visual perception during more naturalistic viewing conditions (i.e. across saccades). These findings are summarised in Chapter 5 in relation to inference model frameworks, transsacadic perception, and attention. The discussion focuses on the interaction of internal generative models and trans-saccadic perception in the aim of highlighting several consistencies between the two cognitive processes

Attention induced distortions of neural population responses, receptive fields, and tuning curves

Selektive visuelle Aufmerksamkeit bezeichnet im Allgemeinen die zielgerichtete Steuerung des Informationsflusses. Zahlreiche Studien im Bereich der raum- und merkmalsbasierten Aufmerksamkeit haben gezeigt, dass das visuelle System diese Kontrolle durch aktivitätsmodulierende Mechanismen ausübt. Es wird angenommen, dass diese Mechanismen zu einer verstärkten neuronalen Repräsentation von relevanten Stimuli oder Merkmalen führen, während irrelevante Aspekte unterdrückt werden. Dies bedeutet, dass Aufmerksamkeit lediglich die Stärke der neuronalen Repräsentationen, nicht aber die repräsentierten Inhalte selbst ändert. In dieser Arbeit wird argumentiert, dass Aufmerksamkeit die neuronalen Repräsentationen grundlegend sowohl auf Populationsebene als auch auf der Ebene einzelner Neurone verändern kann. Dies wird anhand offener Aufmerksamkeitsverlagerungen und der Ausrichtung von merkmalsbasierter Aufmerksamkeit gezeigt werden. Selective visual attention is generally conceptualized to control the flow of information with respect to the task at hand. Various studies in the space-based and feature-based domain of attention have demonstrated that the visual system achieves this via gain-control mechanisms. These mechanisms are supposed to result in an enhanced neural representation of relevant stimuli or features while irrelevant ones are suppressed. Thus, attention is suggested to modulate the strength of neural representations without altering their content. In this thesis, however, it will be argued that attention is able to change the very nature of these neural representations both at the level of population responses and of single neurons. This will be demonstrated for overt shifts of space-based attention as well as for the directing of feature-based attention

The interaction between human vision and eye movements in health and disease

Human motor behaviour depends on the successful integration of vision and eye movements. Many studies have investigated neural correlates of visual processing in humans, but typically with the eyes stationary and fixated centrally. Similarly, many studies have sought to characterise which brain areas are responsible for oculomotor control, but generally in the absence of visual stimulation. The few studies to explicitly study the interaction between visual perception and eye movements suggest strong influences of both static and dynamic eye position on visual processing and modulation of oculomotor structures by properties of visual stimuli. However, the neural mechanisms underlying these interactions are poorly understood. This thesis uses a range of fMRI methodologies such as retinotopic mapping, multivariate analsyis techniques, dynamic causal modelling and ultra high resolution imaging to examine the interactions between the oculomotor and visual systems in the normal human brain. The results of the experiments presented in this thesis demonstrate that oculomotor behaviour has complex effects on activity in visual areas, while spatial properites of visual stimuli modify activity in oculomotor areas. Specifically, responses in the lateral geniculate nucleus and early cortical visual areas are modulated by saccadic eye movements (a process potentially mediated by the frontal eye fields) and by changes in static eye position. Additionally, responses in oculomotor structures such as the superior colliculus are biased for visual stimuli presented in the temporal rather than nasal hemifield. These findings reveal that although the visual and oculomotor systems are spatially segregated in the brain, they show a high degree of integration at the neural level. This is consistent with our everyday experience of the visual world where frequent eye movements do not lead to disruption of visual continuity and visual information is seamlessly transformed into motor behaviour