We live in a visually rich environment yet our brains are only equipped to process a small fraction of all available information at any point in time. For successful and efficient perception, the brain relies on attention to differentiate and select specific stimuli for further analysis. Attention can be directed to features – feature based attention – which enhances the processing of other similar features independent of spatial location. I have recently shown that the benefits of feature-based attention not only apply to lower-level features, but also to processes of object perception. The aim of the thesis was to examine the behavioural and electrophysiological correlates underlying the influence of feature-based attention on object perception. Chapter 1 measured the electric field activity associated with attending to higher-level features (object contours) and comparing it with the neural activity while attending to motion stimuli. We found temporally later effects for contours relative to motion, suggesting that feature-based attention to objects might be mediated by higher-tier visual areas, such as the lateral occipital cortex. In Chapter 2, I describe a study designed to investigate the time course of neural activity while cueing attention within the feature dimension of shape that more directly targets higher-tier visual areas. Consistent with Chapter 1, I iii found temporally late modulation, but behavioural effects that were weaker than expected. To account for these findings, I proposed a “wrong-turn” model which explains the perceptual benefits and costs coupled to expecting the correct or incorrect feature by taking into consideration the hierarchical structure of the visual system. Moreover, the model also makes specific predictions about the pattern of behavioural and electrophysiological activity while attending to features of varying complexity during object perception. The aim of Chapter 3 was to test the predictions of the model; I cued attention to colour, a lower-level feature essential to perceiving the object. I found much stronger behavioural cueing effects, and a biphasic pattern (early and late) electric brain activity that confirmed the predictions of the model. Together the results indicate that feature-based attention plays an important role in object perception that is mediated by a flexible perceptual system.

We live in a visually rich environment yet our brains are only equipped to process a small fraction of all available information at any point in time. For successful and efficient perception, the brain relies on attention to differentiate and select specific stimuli for further analysis. Attention can be directed to features – feature based attention – which enhances the processing of other similar features independent of spatial location. I have recently shown that the benefits of feature-based attention not only apply to lower-level features, but also to processes of object perception. The aim of the thesis was to examine the behavioural and electrophysiological correlates underlying the influence of feature-based attention on object perception. Chapter 1 measured the electric field activity associated with attending to higher-level features (object contours) and comparing it with the neural activity while attending to motion stimuli. We found temporally later effects for contours relative to motion, suggesting that feature-based attention to objects might be mediated by higher-tier visual areas, such as the lateral occipital cortex. In Chapter 2, I describe a study designed to investigate the time course of neural activity while cueing attention within the feature dimension of shape that more directly targets higher-tier visual areas. Consistent with Chapter 1, I iii found temporally late modulation, but behavioural effects that were weaker than expected. To account for these findings, I proposed a “wrong-turn” model which explains the perceptual benefits and costs coupled to expecting the correct or incorrect feature by taking into consideration the hierarchical structure of the visual system. Moreover, the model also makes specific predictions about the pattern of behavioural and electrophysiological activity while attending to features of varying complexity during object perception. The aim of Chapter 3 was to test the predictions of the model; I cued attention to colour, a lower-level feature essential to perceiving the object. I found much stronger behavioural cueing effects, and a biphasic pattern (early and late) electric brain activity that confirmed the predictions of the model. Together the results indicate that feature-based attention plays an important role in object perception that is mediated by a flexible perceptual system.

Feature-based attention refers to the phenomenon that attending to a feature value (e.g., a specific shade of red) enhances the detection of similar feature values (e.g., the same shade of red or other shades of red similar to the attended shade) relative to different feature values (e.g., green) that belong to a different object, and that this facilitation effect can be found across the visual field. In previous studies, the participants’ task was primarily the detection or discrimination of simple features such as orientation, colour or motion. The experiments reported in this thesis investigated whether feature-based attention could also influence the speed and/or accuracy of discriminating alphanumeric stimuli such as letters and numbers. In three experiments, participants saw displays that consisted of a series of stimulus patterns at a central location followed by the appearance of an alphanumeric stimulus at one of two peripheral locations. Experiment 1 tested whether paying attention to a specific orientation in a central stimulus would affect the speed and/or accuracy of identifying a peripheral letter whose principal axis was either the same as or different from the attended orientation of the central stimulus. Experiment 2 changed the peripheral stimulus from a letter to a number. In Experiment 3, a peripheral stimulus occurred randomly on 50% of the trials instead of on 100% of the trials. The results showed that attending to a specific orientation of a central stimulus could affect the processing efficiency of both letters and numbers at a peripheral location when the alphanumeric stimulus occurred on every trial (Experiments 1 and 2), but not when it appeared on 50% of the trials. These results suggest that feature-based attention could influence the identification of alphanumeric stimuli. However, the effect may be quite short-lived.

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Visual attention improves sensory processing, as well as perceptual readout and behavior. Over the last decades, many proposals have been put forth to explain how attention affects visual neural processing. These include the modulation of neural firing rates and synchrony, neural tuning properties, and rhythmic, subthreshold activity. Despite the wealth of knowledge provided by previous studies, the way attention shapes interactions between cortical layers within and between visual sensory areas is only just emerging. To investigate this, we studied neural signals from macaque V1 and V4 visual areas, while monkeys performed a covert, feature-based spatial attention task. The data were simultaneously recorded from laminar electrodes disposed normal to cortical surface in both areas (16 contacts, 150 μm inter-contact spacing). Stimuli presentation was based on the overlap of the receptive fields (RFs) of V1 and V4. Channel depths alignment was referenced to laminar layer IV, based on spatial current source density and temporal latency analyses. Our analyses mainly focused on the study of Local Field Potential (LFP) signals, for which we applied local (bipolar) re-referencing offline. We investigated the effects of attention on LFP spectral power and laminar interactions between LFP signals at different depths, both at the local level within V1 and V4, and at the inter-areal level across V1 and V4. Inspired by current progress from literature, we were interested in the characterization of frequency-specific laminar interactions, which we investigated both in terms of rhythmic synchronization by computing spectral coherence, and in terms of directed causal influence, by computing Granger causalities (GCs). The spectral power of LFPs in different frequency bands showed relatively small differences along cortical depths both in V1 and in V4. However, we found attentional effects on LFP spectral power consistent with previous literature. For V1 LFPs, attention to stimuli in RF location mainly resulted in a shift of the low-gamma (∼30-50 Hz) spectral power peak towards (∼3-4 Hz) higher frequencies and increases in power for frequency bands above low-gamma peak frequencies, as well as decreases in power below these frequencies. For V4 LFPs, attention towards stimuli in RF locations caused a decrease in power for frequencies < 20 Hz and a broad band increase for frequencies > 20 Hz. Attention affected spectral coherence within V1 and within V4 layers in similar way as the spectral power modulation described above. Spectral coherence between V1 and V4 channel pairs was increased by attention mainly in the beta band (∼ 15-30 Hz) and the low-gamma range (∼ 30-50 Hz). Attention affected GC interactions in a layer and frequency dependent manner in complex ways, not always compliant with predictions made by the canonical models of laminar feed-forward and feed-back interactions. Within V1, attention increased feed-forward efficacy across almost all low-frequency bands (∼ 2-50 Hz). Within V4, attention mostly increased GCs in the low and high gamma frequency in a 'downwards' direction within the column, i.e. from supragranular to granular and to infragranular layers. Increases were also evident in an ‘upwards’ direction from granular to supragranular layers. For inter-areal GCs, the dominant changes were an increase in the gamma frequency range from V1 granular and infragranular layers to V4 supragranular and granular layers, as well as an increase from V4 supragranular layers to all V1 layers.

Chaque saccade oculaire décale brusquement l'image projetée sur la rétine. Pourtant notre perception du monde reste stable et uniforme car le système visuel fait correspondre les informations avant et après chaque saccade. Pour établir cette correspondance, les mécanismes attentionnels seraient fondamentaux. Jusqu'à présent, ce lien transsaccadique a été mis en évidence par des études portant essentiellement sur le traitement des informations spatiales - à savoir, comment la position rétinienne d'un objet est corrigée à chaque saccade pour maintenir une perception stable du monde. Le traitement des traits visuels tels que la couleur ou la forme est encore mal compris et leur rôle dans l'impression de stabilité reste à établir. Est-ce que les traits et l'attention dédiée aux traits (feature-based attention), par définition indépendants de l'espace, participent aussi à la correspondance transsaccadique ? Pour analyser la relation entre le traitement des traits et celui des positions lors des saccades oculaires, cette thèse a suivi deux approches. La première s'est intéressée à la perception des attributs visuels, uniforme malgré l'hétérogénéité du système visuel. Les résultats ont montré que si la perception uniforme des attributs visuels s'appuie sur un apprentissage, les mécanismes sous-jacents ne seraient pas spécifiques aux mouvements oculaires. L'uniformité de la perception s'appuierait plutôt sur un mécanisme d'apprentissage associatif général. La seconde approche a cherché à mieux comprendre la nature de l'attention sélective transsaccadique. Les résultats ont montré que l'attention allouée à la cible d'une saccade ne contribue pas à aux mécanismes sélectifs guidés par les traits et engagés juste après l'exécution d'un mouvement oculaire. L'attention allouée à une cible saccadique et l'attention aux traits seraient alors indépendantes. Enfin, la dernière étude a montré que, lorsque l'attention sélective basée sur les traits est engagée pendant la préparation de la saccade en dehors de la cible saccadique, les traits sont maintenus pendant la saccade et affectent les processus sélectifs engagés juste après la saccade. L'attention transsaccadique ne serait alors pas de nature purement spatiale. L'ensemble de ces résultats suggère que les traits et l'attention aux traits joueraient un rôle dans la correspondance transsaccadique. / With each saccade, the image on the retina shifts abruptly but our perception of the surrounding world remains stable and uniform, because the visual system matches pre- and post-saccadic visual information. Attentional mechanisms could play a fundamental role in this process and numerous studies have examined the role of spatial attention. The processing of feature-based attention across saccades remains unclear and its role in matching pre- to post-saccadic visual information is not known. Do visual features and feature-based attention, assumed to enhance the feature-specific representations throughout the visual field, take part in the transsaccadic correspondence? To examine the relationship between feature and spatial processing, this thesis chose two approaches. The first one considered the uniform perception that we have for features despite the heterogeneity of the retina. Results show that, if the transsaccadic correspondence of visual features relies on learning, the underlying mechanisms would not be specific to eye movements. Visual constancy is more likely to arise from a general associative learning. The second approach examined the nature of transsaccadique attention. Results show that attention drawn to the saccade target did not contribute to selective mechanisms engaged just after an eye movement, suggesting a dissociation between feature-based attention and saccade programming. Finally, the last study show that feature-based selectivity is maintained across saccades to ensure spatiotopic correspondence, pointing out the potential role of feature-based attention in matching pre- to post-saccadic information.