It is known that primates are sensitive to the velocity of moving objects. We can also remember velocity information after moving objects disappear. This cognitive faculty has been investigated before, however, the literature on velocity memory to date has been fragmented. For example, velocity memory has been disparately described as a system that controls eye movements and delayed discrimination. Furthermore, velocity memory may have a role in motion extrapolation, i.e. the ability to judge the position of a moving target after it becomes occluded. This thesis provides a unifying account of velocity memory, and uses electroencephalography (EEG) to explore its neural basis. In Chapter 2, the relationship between oculomotor control and motion extrapolation was investigated. Two forms of motion extrapolation task were presented. In the first, participants observed a moving target disappear then reappear further along its path. Reappearance could be at the correct time, too early or too late. Participants discriminated reappearance error with a two-alternative forced choice button press. In the second task, participants saw identical targets travel behind a visible occluder, and they attempted to press a button at the exact time that it reached the other side. Tasks were completed under fixation and free viewing conditions. The accuracy of participant's judgments was reduced by fixation in both tasks. In addition, eye movements were systematically related to behavioural responses, and small eye movements during fixation were affected by occluded motion. These three results imply that common velocity memory and pre-motor systems mediate eye movements and motion extrapolation. In Chapter 3, different types of velocity representation were explored. Another motion extrapolation task was presented, and targets of a particular colour were associated with fast or slow motion. On identical-velocity probe trials, colour still influenced response times. This indicates that long-term colour-velocity associations influence motion extrapolation. In Chapter 4, interference between subsequently encoded velocities was explored. There was robust interference between motion extrapolation and delayed discrimination tasks, suggesting that common processes are involved in both. In Chapter 5, EEG was used to investigate when memory-guided tracking begins during motion extrapolation. This study compared conditions where participants covertly tracked visible and occluded targets. It was found that a specific event related potential (ERP) appeared around 200 ms post occlusion, irrespective of target location or velocity. This component could delineate the onset of memory guided tracking during occlusion. Finally, Chapter 6 presents evidence that a change in alpha band activity is associated with information processing during motion extrapolation tasks. In light of these results, it is concluded that a common velocity memory system is involved a variety of tasks. In the general discussion (Chapter 7), a new account of velocity memory is proposed. It is suggested that a velocity memory reflects persistent synchronization across several velocity sensitive neural populations after stimulus offset. This distributed network is involved in sensory-motor integration, and can remain active without visual input. Theoretical work on eye movements, delayed discrimination and motion extrapolation could benefit from this account of velocity memory.

Nous avons expliqué comment le système visuel intègre les informations de mouvement en manipulant la distribution de vitesse locale à l'aide d'une classe bien contrôlée de stimuli de texture aléatoires à large bande appelée Motion Clouds (TM), avec des spectres de fréquence spatio-temporels naturalistes continus. Nos résultats montrent que le gain et la précision des poursuites se détériorent à mesure que la variabilité de la fréquence de stimulation augmente. Dans l'expérience de discrimination de vitesse perceptuelle, nous avons constaté que les MC ayant une largeur de bande légèrement supérieure à la vitesse étaient perçus comme se déplaçant plus rapidement. Cependant, au-delà d'une bande passante critique, la perception d'une vitesse constante a été perdue. Dans une troisième expérience de discrimination, nous avons constaté que pour les contrôleurs multimédias à large bande passante, les participants ne pouvaient plus discriminer la direction du mouvement. Ces résultats suggèrent que lorsqu’on augmente la bande passante de petites à grandes vitesses, l’observateur expérimente différents régimes de perception. Nous avons finalement réalisé une expérience d’échelle de différence de vraisemblance maximale avec nos stimuli MC afin d’étudier ces différents régimes de perception possibles. Nous avons identifié trois régimes dans la plage des valeurs de différence de vitesse testées qui correspondraient à la cohérence de mouvement, à la transparence de mouvement et à l'incohérence complète. / It is still not fully understood how the visual system integrates motion information across different spatial and temporal frequencies, in order to build a coherent percept of the global motion under complex, noisy naturalistic conditions. We addressed this question by manipulating local speed distribution (i.e. speed bandwidth Bv) using a well-controlled class of broadband random-texture stimuli called Motion Clouds (MCs), with continuous naturalistic spatiotemporal frequency spectra (Sanz-Leon et al., 2012,; Simoncini et al., 2012).Our results show that pursuit gain and precision deteriorate as stimulus speed variability increases. In the perceptual speed discrimination experiment, we found that MCs with moderately larger speed bandwidth were perceived as moving faster. However, beyond a critical bandwidth (Bv > 0.5 °/s), the perception of a coherent speed was lost. In a third direction discrimination experiment, we found that for large bandwidth MCs participants could no longer discriminate motion direction. These results suggest that when increasing speed bandwidth from a small to a large range, the observer experiences different perceptual regimes. We finally ran a Maximum Likelihood Difference Scaling (Knoblauch & Maloney, 2008) experiment with our MC stimuli to investigate these different possible perceptual regimes. We identified three regimes across the range of tested values of velocity difference, that would correspond to motion coherency (and speed integration), motion transparency and complete incoherency.

La schizophrénie, pathologie sévère et fréquente, regroupe des entités différentes au sein desquelles la schizophrénie déficitaire a été identifiée. C’est une entité clinique homogène caractérisée par des symptômes négatifs primaires et durables. Une des principales difficultés de la recherche dans la schizophrénie est la mauvaise corrélation entre le phénotype clinique et le génotype. Les nouvelles approches cherchent à identifier des endophénotypes pertinents permettant de déterminer un sous groupe homogène de la schizophrénie. L’objectif de cette thèse était de déterminer les marqueurs de vulnérabilité les plus pertinents pour la schizophrénie : (1) caractériser les anomalies électrophysiologiques dans la schizophrénie, (2) rechercher les perturbations spécifiques des mouvements oculaires dans la schizophrénie déficitaire, (3) mesurer les performances attentionnelles et exécutives, puis identifier des associations entre ces dernières et 6 polymorphismes des gènes COMT et DRD2. Les patients répondant aux critères DSM-IV de la schizophrénie ont été sélectionnés. Ils sont cliniquement stables depuis au moins 28 jours et ne prennent aucun traitement pouvant altérer les mouvements oculaires. Les patients ont été évalués à l’aide de la traduction française de l’échelle de déficit de Kirkpatrick, la PANSS, l’ESRS et le WCST. Les apparentés sains ont passé la version française de la SADS-LA. Les volontaires sains, recrutés dans la population générale, ont passé la Diagnostic Interview Schedule et le WCST. Les mouvements oculaires de l’ensemble des sujets ont été détectés par réflectométrie infrarouge et analysés sur un ordinateur par le logiciel SAMO. Les paradigmes utilisés sont : la poursuite oculaire sinusoïdale, la poursuite oculaire non prédictive, les saccades et les antisaccades. L’onde P50 des potentiels évoqués auditifs a également été étudiée. Nous avons trouvé chez les schizophrènes et dans une moindre mesure chez leurs apparentés une diminution du gain lors de la poursuite oculaire : la dégradation de la prédiction est à l’origine de l’altération des performances de la poursuite oculaire dans la schizophrénie. Chez les patients comparativement aux volontaires sains, une diminution du nombre d’antisaccades réussies et une augmentation de leur latence ont été mises en évidence. Cette dernière est plus importante chez les apparentés sains par rapport aux volontaires sains. L’augmentation de la latence des antisaccades réussies est plus importante chez les schizophrènes déficitaires par rapport aux non déficitaires et elle y est corrélée au nombre d’erreurs persévératives du WCST. Le ratio T/C de l’onde P50 est significativement plus élevé chez les patients comparativement aux volontaires sains. Pour tous les sujets, l’attention a été évaluée à l’aide du test de Stroop Color-Word et du test des réseaux attentionnels ANT. Le fonctionnement exécutif a été étudié avec le test du Wisconsin. Les patients porteurs du génotype de vulnérabilité TT de rs6275 en DRD2 et ceux porteurs du génotype de vulnérabilité CC de rs2242592 en DRD2, ont des performances significativement plus faibles au Stroop-WC par rapport aux non porteurs. Les patients de génotype Val/Val (COMT) font plus d’erreurs persévératives que les patients porteurs de l’allèle Met. Pour rs165599 (COMT), les patients porteurs de l’allèle de vulnérabilité G font plus d’erreurs persévératives que les patients porteurs du génotype AA. La pathologie schizophrénique et des facteurs génétiques interagissent sur le contrôle exécutif de l’attention, principalement sur le Stroop Color-Word et légèrement sur l’ANT. Les polymorphismes du DRD2, rs6275 et rs2242592, augmentent le conflit, tandis que ceux de la COMT n’auraient pas d’action. Concernant le fonctionnement exécutif, l’allèle G de la COMT et la pathologie schizophrénique interagissent ensemble. (...) / Schizophrenia is a complex disorder where the deficit syndrome has been identified as the presence of primary, enduring negative symptoms such as restricted affect, diminished emotional range, poverty of speech, curbing of interest, diminished sense of purpose, diminished social drive. The deficit syndrome is associated with specific neurological and neuropsychological impairments, biochemical characteristics, attentional impairments, structural and functional brain abnormalities. The identification of intermediate phenotypes associated with schizophrenia may improve understanding of the neurobiology of the disease and contribute to the genetic dissection of this disease. Endophenotypes would also be useful for establishing a biological underpinning for diagnosis and classification of schizophrenia. The aim of this work was to determine the most relevant vulnerability markers of schizophrenia: (1) describe electrophysiological abnormalities in schizophrenia, (2) specify eye movements impairments in deficit schizophrenia, (3) assess attentional, executive function and analyze their association with four SNPs in the DRD2/ANKK1 locus (rs6275, rs6277, rs2242592 and rs1800497) and two SNPs in the COMT gene (rs4680 and rs165599). Patients meeting DSM IV criteria for schizophrenia were recruited. They were clinically stabilized for a minimum of 28 days with no change in neuroleptic dose at the time of their participation in the study. None of them was treated with drugs known to affect eye movements. The patients were subtyped into deficit and nondeficit subgroups, using the french translation of the Schedule for the Deficit Syndrome. Patients were also assessed using PANSS, ESRS and WCST. Healthy parents of patients were interviewed using the french translation of the SADS-LA. The unrelated healthy controls (no personal or family history of neurological or psychiatric disease, free of any psychotropic treatment) were recruited from the hospital staff and screened by the Diagnostic Interview Schedule and the WCST. Horizontal eye movements were recorded by an infrared photoelectric limbus eye tracking device. Subjects were tested in five tasks: smooth pursuit, unpredictable pursuit, P50 eventrelated potential, reflexive saccades and antisaccades. Schizophrenia patients and parents had a high prevalence of eye tracking dysfunction: the impaired predictive mechanisms were involved. Both groups showed a high rate of inhibitory deficits measured by the P50 event-related potential and antisaccade paradigms. The latency of successful antisaccades was significantly increased in deficit patients as compared with non-deficit patients and healthy controls. In deficit patients only, we found a significant correlation between the latency of successful antisaccades and the WCST perseverative errors. Patients and healthy controls have performed the Stroop Color-Word Test, the Attention Network Test, and the Wisconsin Card Sorting Test to assess attention and executive functions. Patients with schizophrenia performed significantly worse than controls in all cognitive performance. The TT genotype of rs6275 and CC genotype of rs22422592 were associated with a marked deterioration of selective attention (Stroop Color-Word) and this effect is more important in participants with schizophrenia. We showed an interaction between schizophrenia and the genetic effect of rs6275 and rs22422592 on Stroop-PI, but the effect of the disease appears to be more prominent. The COMT Val/Val genotype and schizophrenia were associated with increased number of perseverative errors. Schizophrenia and genetics interact on the executive control of attention, mostly on Stroop-PI and slightly on ANT. Rs6275 and rs2242592 of the DRD2 increased the conflict while COMT SNPs do not. Regarding executive cognition, the COMT and schizophrenia also interact. (...)