The Peri-Saccadic Perception of Objects and Space

Eye movements affect object localization and object recognition. Around saccade onset, briefly flashed stimuli appear compressed towards the saccade target, receptive fields dynamically change position, and the recognition of objects near the saccade target is improved. These effects have been attributed to different mechanisms. We provide a unifying account of peri-saccadic perception explaining all three phenomena by a quantitative computational approach simulating cortical cell responses on the population level. Contrary to the common view of spatial attention as a spotlight, our model suggests that oculomotor feedback alters the receptive field structure in multiple visual areas at an intermediate level of the cortical hierarchy to dynamically recruit cells for processing a relevant part of the visual field. The compression of visual space occurs at the expense of this locally enhanced processing capacity

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

The computational neurology of active vision

In this thesis, we appeal to recent developments in theoretical neurobiology – namely, active inference – to understand the active visual system and its disorders. Chapter 1 reviews the neurobiology of active vision. This introduces some of the key conceptual themes around attention and inference that recur through subsequent chapters. Chapter 2 provides a technical overview of active inference, and its interpretation in terms of message passing between populations of neurons. Chapter 3 applies the material in Chapter 2 to provide a computational characterisation of the oculomotor system. This deals with two key challenges in active vision: deciding where to look, and working out how to look there. The homology between this message passing and the brain networks solving these inference problems provide a basis for in silico lesion experiments, and an account of the aberrant neural computations that give rise to clinical oculomotor signs (including internuclear ophthalmoplegia). Chapter 4 picks up on the role of uncertainty resolution in deciding where to look, and examines the role of beliefs about the quality (or precision) of data in perceptual inference. We illustrate how abnormal prior beliefs influence inferences about uncertainty and give rise to neuromodulatory changes and visual hallucinatory phenomena (of the sort associated with synucleinopathies). We then demonstrate how synthetic pharmacological perturbations that alter these neuromodulatory systems give rise to the oculomotor changes associated with drugs acting upon these systems. Chapter 5 develops a model of visual neglect, using an oculomotor version of a line cancellation task. We then test a prediction of this model using magnetoencephalography and dynamic causal modelling. Chapter 6 concludes by situating the work in this thesis in the context of computational neurology. This illustrates how the variational principles used here to characterise the active visual system may be generalised to other sensorimotor systems and their disorders

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

Change blindness: eradication of gestalt strategies

Arrays of eight, texture-defined rectangles were used as stimuli in a one-shot change blindness (CB) task where there was a 50% chance that one rectangle would change orientation between two successive presentations separated by an interval. CB was eliminated by cueing the target rectangle in the first stimulus, reduced by cueing in the interval and unaffected by cueing in the second presentation. This supports the idea that a representation was formed that persisted through the interval before being 'overwritten' by the second presentation (Landman et al, 2003 Vision Research 43149–164]. Another possibility is that participants used some kind of grouping or Gestalt strategy. To test this we changed the spatial position of the rectangles in the second presentation by shifting them along imaginary spokes (by ±1 degree) emanating from the central fixation point. There was no significant difference seen in performance between this and the standard task [F(1,4)=2.565, p=0.185]. This may suggest two things: (i) Gestalt grouping is not used as a strategy in these tasks, and (ii) it gives further weight to the argument that objects may be stored and retrieved from a pre-attentional store during this task

The role of noise in sensorimotor control

Goal-directed arm movements show stereotypical trajectories, despite the infinite possible ways to reach a given end point. This thesis examines the hypothesis that this stereotypy arises because movements are optimised to reduce the consequences of signal-dependent noise on the motor command. Both experimental and modelling studies demonstrate that signal-dependent noise arises from the normal behaviour of the muscle and motor neuron pool, and has a particular distribution across muscles of different sizes. Specifically, noise decreases in a systematic fashion with increasing muscle strength and motor unit number. Simulations of obstacle avoidance performance in the presence of signal-dependent noise demonstrate that the optimal trajectory for reaching the target accurately and without collision matches the observed trajectories. Isometric force generation is also shown to have systematic changes in variability with posture, which can be explained by the presence of signal-dependent noise in the muscles of the arm. These results confirm the tested hypothesis and imply that consideration of the statistics of action is crucial to human movement planning. To investigate the importance of feedback in the motor system, the impact of static position on motor excitability was examined using transcranial magnetic stimulation and systematic changes in motor evoked potentials were observed. Force generated at the wrist following stimulation was analysed in terms of different possible movement representations, and the differences between force fields arising from stimulation over the cervical spinal cord and from stimulation over primary motor cortex are determined. These results demonstrate the structured influence of proprioceptive feedback on the human motor system. All the experiments are discussed in relation to current theories describing the control of human movements and the impact of noise in the motor system

Visual processing of words in a patient with visual form agnosia: A behavioural and fMRI study

Patient D.F. has a profound and enduring visual form agnosia due to a carbon monoxide poisoning episode suffered in 1988. Her inability to distinguish simple geometric shapes or single alphanumeric characters can be attributed to a bilateral loss of cortical area LO, a loss that has been well established through structural and functional fMRI. Yet despite this severe perceptual deficit, D.F. is able to “guess” remarkably well the identity of whole words. This paradoxical finding, which we were able to replicate more than 20 years following her initial testing, raises the question as to whether D.F. has retained specialized brain circuitry for word recognition that is able to function to some degree without the benefit of inputs from area LO. We used fMRI to investigate this, and found regions in the left fusiform gyrus, left inferior frontal gyrus, and left middle temporal cortex that responded selectively to words. A group of healthy control subjects showed similar activations. The left fusiform activations appear to coincide with the area commonly named the visual word form area (VWFA) in studies of healthy individuals, and appear to be quite separate from the fusiform face area. We hypothesize that there is a route to this area that lies outside area LO, and which remains relatively unscathed in D.F

A computational model of space-variant vision based on a self-organised artificial retina tessellation

Abstract available: p.i-iii