La reconnaissance visuelle à travers le temps : attentes, échantillonnage et traitement

La reconnaissance visuelle est un processus temporel : d’abord, l’information visuelle est reçue sur notre rétine de manière continue à travers le temps; ensuite, le traitement de l’information visuelle par notre cerveau prend un certain temps à s’effectuer; finalement, notre perception est toujours fonction autant des expériences acquises dans le passé que de l’input sensoriel présent. Les interactions entre ces aspects temporels de la reconnaissance sont rarement abordées dans la littérature. Dans cette thèse, nous évaluons l’échantillonnage de l’information visuelle à travers le temps pendant une tâche de reconnaissance, comment il se traduit dans le cerveau et comment il est modulé par des attentes spécifiques. Plusieurs études indiquent que nos attentes modulent notre perception. Comment l’attente d’un objet spécifique influence nos représentations internes demeure cependant largement inconnu. Dans le premier article de cette thèse, nous utilisons une variante de la technique Bubbles pour retrouver avec précision le décours temporel de l’utilisation d’information visuelle pendant la reconnaissance d’objets, lorsque les observateurs s’attendent à voir un objet spécifique ou non. Nous observons que les attentes affectent la représentation de différents attributs différemment et qu’elles ont un effet distinct à différents moments pendant la réception d’information visuelle. Dans le deuxième article, nous utilisons une technique similaire en conjonction avec l’électroencéphalographie (EEG) afin de révéler pour la première fois le traitement, à travers le temps, de l’information reçue à un moment spécifique pendant une fixation oculaire. Nous démontrons que l’information visuelle n’est pas traitée de la même manière selon le moment auquel elle est reçue sur la rétine, que ces différences ne sont pas explicables par l’adaptation ou l’amorçage, qu’elles sont d’origine au moins partiellement descendante et qu’elles corrèlent avec le comportement. Finalement, dans le troisième article, nous approfondissons cette investigation en utilisant la magnétoencéphalographie (MEG) et en examinant l’activité dans différentes régions cérébrales. Nous démontrons que l’échantillonnage de l’information visuelle est hautement variable selon le moment d’arrivée de l’information sur la rétine dans de larges parties des lobes occipitaux et pariétaux. De plus, nous démontrons que cet échantillonnage est rythmique, oscillant à diverses fréquences entre 7 et 30 Hz, et que ces oscillations varient en fréquences selon l’attribut échantillonné.Visual recognition is a temporal process: first, visual information is continuously received through time on our retina; second, the processing of visual information by our brain takes time; third, our perception is function of both the present sensory input and our past experiences. Interactions between these temporal aspects have rarely been discussed in the literature. In this thesis, we assess the sampling of visual information through time during recognition tasks, how it is translated in the brain, and how it is modulated by expectations of specific objects. Several studies report that expectations modulate perception. However, how the expectation of a specific object modulates our internal representations remains largely unknown. In the first article of this thesis, we use a variant of the Bubbles technique to uncover the precise time course of visual information use during object recognition when specific objects are expected or not. We show that expectations modulate the representations of different features differently, and that they have distinct effects at distinct moments throughout the reception of visual information. In the second article, we use a similar method in conjunction with electroencephalography (EEG) to reveal for the first time the processing, through time, of information received at a specific moment during an eye fixation. We show that visual information is not processed in the same way depending on the moment at which it is received on the retina, that these differences cannot be explained by simple adaptation or repetition priming, that they are of at least partly top- down origin, and that they correlate with behavior. Finally, in a third article, we push this investigation further by using magnetoencephalography (MEG) and examining brain activity in different brain regions. We show that the sampling of visual information is highly variable depending on the moment at which information arrives on the retina in large parts of the occipital and parietal lobes. Furthermore, we show that this sampling is rhythmic, oscillating at multiple frequencies between 7 and 30 Hz, and that these oscillations vary according to the sampled feature

Le décours temporel de l'utilisation des fréquences spatiales dans les troubles du spectre autistique

Notre système visuel extrait d'ordinaire l'information en basses fréquences spatiales (FS) avant celles en hautes FS. L'information globale extraite tôt peut ainsi activer des hypothèses sur l'identité de l'objet et guider l'extraction d'information plus fine spécifique par la suite. Dans les troubles du spectre autistique (TSA), toutefois, la perception des FS est atypique. De plus, la perception des individus atteints de TSA semble être moins influencée par leurs a priori et connaissances antérieures. Dans l'étude décrite dans le corps de ce mémoire, nous avions pour but de vérifier si l'a priori de traiter l'information des basses aux hautes FS était présent chez les individus atteints de TSA. Nous avons comparé le décours temporel de l'utilisation des FS chez des sujets neurotypiques et atteints de TSA en échantillonnant aléatoirement et exhaustivement l'espace temps x FS. Les sujets neurotypiques extrayaient les basses FS avant les plus hautes: nous avons ainsi pu répliquer le résultat de plusieurs études antérieures, tout en le caractérisant avec plus de précision que jamais auparavant. Les sujets atteints de TSA, quant à eux, extrayaient toutes les FS utiles, basses et hautes, dès le début, indiquant qu'ils ne possédaient pas l'a priori présent chez les neurotypiques. Il semblerait ainsi que les individus atteints de TSA extraient les FS de manière purement ascendante, l'extraction n'étant pas guidée par l'activation d'hypothèses.Our visual system usually samples low spatial frequency (SF) information before higher SF information. The coarse information thereby extracted can activate hypotheses in regard to the object's identity and guide further extraction of specific finer information. In autism spectrum disorder (ASD) however, SF perception is atypical. Moreover, individuals with ASD seem to rely less on their prior knowledge when perceiving objects. In the present study, we aimed to verify if the prior according to which we sample visual information in a coarse-to-fine fashion is existent in ASD. We compared the time course of SF sampling in neurotypical and ASD subjects by randomly and exhaustively sampling the SF x time space. Neurotypicals were found to sample low SFs before higher ones, thereby replicating the finding from many other studies, but characterizing it with much greater precision. ASD subjects were found, for their part, to extract SFs in a more fine-to-coarse fashion, extracting all relevant SFs upon beginning. This indicated that they did not possess a coarse-to-fine prior. Thus, individuals with ASD seem to sample information in a purely bottom-up fashion, without the guidance from hypotheses activated by coarse information

Disentangling presentation and processing times in the brain

Visual object recognition seems to occur almost instantaneously. However, not only does it require hundreds of milliseconds of processing, but our eyes also typically fixate the object for hundreds of milliseconds. Consequently, information reaching our eyes at different moments is processed in the brain together. Moreover, information received at different moments during fixation is likely to be processed differently, notably because different features might be selectively attended at different moments. Here, we introduce a novel reverse correlation paradigm that allows us to uncover with millisecond precision the processing time course of specific information received on the retina at specific moments. Using faces as stimuli, we observed that processing at several electrodes and latencies was different depending on the moment at which information was received. Some of these variations were caused by a disruption occurring 160–200 ​ms after the face onset, suggesting a role of the N170 ERP component in gating information processing; others hinted at temporal compression and integration mechanisms. Importantly, the observed differences were not explained by simple adaptation or repetition priming, they were modulated by the task, and they were correlated with differences in behavior. These results suggest that top-down routines of information sampling are applied to the continuous visual input, even within a single eye fixation

Atypical Time Course of Object Recognition in Autism Spectrum Disorder

International audienceIn neurotypical observers, it is widely believed that the visual system samples the world in a coarse-to-fine fashion. Past studies on Autism Spectrum Disorder (ASD) have identified atypical responses to fine visual information but did not investigate the time course of the sampling of information at different levels of granularity (i.e. Spatial Frequencies, SF). Here, we examined this question during an object recognition task in ASD and neurotypical observers using a novel experimental paradigm. Our results confirm and characterize with unprecedented precision a coarse-to-fine sampling of SF information in neurotypical observers. In ASD observers, we discovered a different pattern of SF sampling across time: in the first 80 ms, high SFs lead ASD observers to a higher accuracy than neurotypical observers, and these SFs are sampled differently across time in the two subject groups. Our results might be related to the absence of a mandatory precedence of global information, and to top-down processing abnormalities in ASD

Hand Position Alters Vision by Modulating the Time Course of Spatial Frequency Use

International audienceThe nervous system gives preferential treatment to objects near the hands that are candidates for action.It is not yet understood how this process is achieved. Here we show evidence for the mechanism thatunderlies this process having used an experimental technique that maps the use of spatial frequencies(SFs) during object recognition across time. We used this technique to replicate and characterize withgreater precision the coarse-to-fine SF sampling observed in previous studies. Then we show that thevisual processing of real-world objects near an observer’s hands is biased toward the use of low-SFinformation, around 288 ms. Conversely, high-SF information presented around 113 ms impaired objectrecognition when objects were presented near the hands. Notably, both of these effects happenedrelatively late during object recognition and suggest that the modulation of SF use by hand position is atleast partly attentional in nature

Real-world expectations and their affective value modulate object processing

International audienceIt is well known that expectations influence how we perceive the world. Yet the neural mechanisms underlying this process remain unclear. Studies about the effects of prior expectations have focused so far on artificial contingencies between simple neutral cues and events. Real-world expectations are however often generated from complex associations between contexts and objects learned over a lifetime. Additionally, these expectations may contain some affective value and recent proposals present conflicting hypotheses about the mechanisms underlying affect in predictions. In this study, we used fMRI to investigate how object processing is influenced by realistic context-based expectations, and how affect impacts these expectations. First, we show that the precuneus, the inferotemporal cortex and the frontal cortex are more active during object recognition when expectations have been elicited a priori, irrespectively of their validity or their affective intensity. This result supports previous hypotheses according to which these brain areas integrate contextual expectations with object sensory information. Notably, these brain areas are different from those responsible for simultaneous context-object interactions , dissociating the two processes. Then, we show that early visual areas, on the contrary, are more active during object recognition when no prior expectation has been elicited by a context. Lastly, BOLD activity was shown to be enhanced in early visual areas when objects are less expected, but only when contexts are neutral; the reverse effect is observed when contexts are affective. This result supports the proposal that affect modulates the weighting of sensory information during predictions. Together, our results help elucidate the neural mechanisms of real-world expectations

Flexible time course of spatial frequency use during scene categorization

Human observers can quickly and accurately categorize scenes. This remarkable ability is related to the usage of information at different spatial frequencies (SFs) following a coarse-to-fine pattern: Low SFs, conveying coarse layout information, are thought to be used earlier than high SFs, representing more fine-grained information. Alternatives to this pattern have rarely been considered. Here, we probed all possible SF usage strategies randomly with high resolution in both the SF and time dimensions at two categorization levels. We show that correct basic-level categorizations of indoor scenes are linked to the sampling of relatively high SFs, whereas correct outdoor scene categorizations are predicted by an early use of high SFs and a later use of low SFs (fine-to-coarse pattern of SF usage). Superordinate-level categorizations (indoor vs. outdoor scenes) rely on lower SFs early on, followed by a shift to higher SFs and a subsequent shift back to lower SFs in late stages. In summary, our results show no consistent pattern of SF usage across tasks and only partially replicate the diagnostic SFs found in previous studies. We therefore propose that SF sampling strategies of observers differ with varying stimulus and task characteristics, thus favouring the notion of flexible SF usage