Perceptual Grouping and Visual Enumeration

We used lateralized Event-Related Potential (ERP) measures – the N2pc and CDA/SPCN components – to assess the role of grouping by target similarity during enumeration. Participants saw a variable number (0, 1, 2 or 3) of same- or differently-colored targets presented among homogeneous distracters, and performed an enumeration task. Results showed that the N2pc, but not the CDA, was larger for multiple targets of identical color relative to targets of different colors. The findings are interpreted in terms of the effects of grouping on early versus late stages of multiple object processing. Within this framework, they reveal that grouping has an effect on early individuation mechanisms, while later processing mechanisms are less prone to such an influence.Psycholog

Early multisensory attention as a foundation for learning in multicultural Switzerland

Traditional laboratory research on visual attentional control has largely focused on adults, treated one sensory modality at a time, and neglected factors that are a constituent part of information processing in real-world contexts. Links between visual-only attentional control and children’s educational skills have emerged, but they still do not provide enough information about school learning. The present thesis addressed these gaps in knowledge through the following aims: 1) to shed light on the development of the neuro-cognitive mechanisms of attention engaged by multisensory objects in a bottom-up fashion, together with attentional control over visual objects in a top-down fashion, 2) to investigate the links between developing visual and multisensory attentional control and children’s basic literacy and numeracy attainment, and 3) to explore how contextual factors, such as the temporal predictability of a stimulus or the semantic relationships between stimulus features, further influence attentional control mechanisms. To investigate these aims, 115 primary school children and 39 adults from the French-speaking part of Switzerland were tested on their behavioural performance on a child-friendly, multisensory version of the Folk et al. (1992) spatial cueing paradigm, while 129-channel EEG was recorded. EEG data were analysed in a traditional framework (the N2pc ERP component) and a multivariate Electrical Neuroimaging (EN) framework. Taken together, our results demonstrated that children’s visual attentional control reaches adult-like levels at around 7 years of age, or 3rd grade, although children as young as 5 (at school entry) may already be sensitive to the goal- relevance of visual objects. Multisensory attentional control may develop only later. Namely, while 7-year-old children (3rd grade) can be sensitive to the multisensory nature of objects, such sensitivity may only reach an adult-like state at 9 years of age (5th grade). As revealed by EN, both bottom-up multisensory control of attention and top-down visual control of attention are supported by the recruitment of distinct networks of brain generators at each level of schooling experience. Further, at each level of schooling, the involvement of specific sets of brain generators was correlated with literacy and numeracy attainment. In adults, visual and multisensory attentional control were further jointly influenced by contextual factors. The semantic relationship between stimulus features directly influenced visual and multisensory attentional control. In the absence of such semantic links, however, it was the predictability of stimulus onset that influenced visual and multisensory attentional control. Throughout this work, the N2pc component was not sensitive to multisensory or contextual effects in adults, or even traditional visual attention effects in children, and it was owing to EN that the mechanisms of visual and multisensory attentional control were clarified. The present thesis demonstrates the strength of combining behavioural and EEG/ERP markers of attentional control with advanced EEG analytical techniques for investigating the development of attentional control in settings that closely approximate those that we encounter in everyday life

Distractor intrusions are the result of delayed attentional engagement: a new temporal variability account of attentional selectivity in dynamic visual tasks

When observers have to identify targets among distractors in a rapid serial visual presentation (RSVP) stream, distractor intrusion errors are frequent, demonstrating the difficulty of allocating attention to the right object at the right moment in time. However, the mechanisms responsible for such intrusion errors remain disputed. We propose a new attentional engagement account of selective visual processing in RSVP tasks. Engagement is triggered by the pre-attentive detection of target-defining features. Critically, the success versus failure of target identification is determined by the speed of such engagement processes on individual trials. To test this account, we measured electrophysiological markers of attentional engagement (N2pc components) in three experiments where observers had to report the identity of a target digit in one of two lateral RSVP streams. On most trials, the target was immediately followed by a digit distractor, resulting in many post-target distractor intrusions. Critically, N2pcs components measured on distractor intrusion trials were significantly delayed relative to trials with correct target reports. This was the case regardless of whether the target was defined by a shape cue or by its colour, and even when the location of shape-defined targets was known in advance. These findings show that distractor intrusions are the result of delayed attentional engagement. They demonstrate that temporal variability in attentional selectivity across trials can strongly affect visual awareness and perceptual reports. Our temporal variability account of attentional engagement offers a new framework for assessing the temporal dynamics of attention in visual object recognition

The dynamics of statistical learning in visual search and its interaction with salience processing: an EEG study

Visual attention can be guided by statistical regularities in the environment, that people implicitly learn from past experiences (statistical learning, SL). Moreover, a perceptually salient element can automatically capture attention, gaining processing priority through a bottom-up attentional control mechanism. The aim of our study was to investigate the dynamics of SL and if it shapes attentional target selection additively with salience processing, or whether these mechanisms interact, e.g. one gates the other. In a visual search task, we therefore manipulated target frequency (high vs. low) across locations while, in some trials, the target was salient in terms of colour. Additionally, halfway through the experiment, the high-frequency location changed to the opposite hemifield. EEG activity was simultaneously recorded, with a specific interest in two markers related to target selection and post-selection processing, respectively: N2pc and SPCN. Our results revealed that both SL and saliency significantly enhanced behavioural performance, but also interacted with each other, with an attenuated saliency effect at the high-frequency target location, and a smaller SL effect for salient targets. Concerning processing dynamics, the benefit of salience processing was more evident during the early stage of target selection and processing, as indexed by a larger N2pc and early-SPCN, whereas SL modulated the underlying neural activity particularly later on, as revealed by larger late-SPCN. Furthermore, we showed that SL was rapidly acquired and adjusted when the spatial imbalance changed. Overall, our findings suggest that SL is flexible to changes and, combined with salience processing, jointly contributes to establishing attentional priority

CES-531: Collaborative Brain-Computer Interfaces for Target Detection and Localisation in Rapid Serial Visual Presentation

The rapid serial visual presentation protocol can be used to show images sequentially on the same spatial location at high presentation rates. We used this technique to present aerial images to participants looking for predefined targets (airplanes) at rates ranging from 5 to 12 Hz. We used linear support vector machines for the single-trial classification of event-related potentials from both individual users and pairs of users (in which case we averaged either their individual classifiers' analogue outputs before thresholding or their electroencephalographic signals associated to the same stimuli) with and without the selection of compatible pairs. We considered two tasks - the detection of targets and the identification of the visual hemifield in which targets appeared. While single users did well in both tasks, we found that pairs of participants with similar individual performance provided significant improvements. In particular, in the target-detection task we obtained median improvements in the area under the receiver operating characteristic curve (AUC) of up to 8.3% w.r.t. single-user BCIs, while in the hemifield classification task we ob- tained AUCs up to 7.7% higher than for single users. Furthermore, we found that this second system allows not just to say if a target is in on the left or the right of an image, but to also recover the target's approximate horizontal position

Attentional gain is modulated by probabilistic feature expectations in a spatial cueing task: ERP evidence

Several theoretical and empirical studies suggest that attention and perceptual expectations influence perception in an interactive manner, whereby attentional gain is enhanced for predicted stimuli. The current study assessed whether attention and perceptual expectations interface when they are fully orthogonal, i.e., each of them relates to different stimulus features. We used a spatial cueing task with block-wise spatial attention cues that directed attention to either left or right visual field, in which Gabor gratings of either predicted (more likely) or unpredicted (less likely) orientation were presented. The lateralised posterior N1pc component was additively influenced by attention and perceptual expectations. Bayesian analysis showed no reliable evidence for the interactive effect of attention and expectations on the N1pc amplitude. However, attention and perceptual expectations interactively influenced the frontally distributed anterior N1 component (N1a). The attention effect (i.e., enhanced N1a amplitude in the attended compared to the unattended condition) was observed only for the gratings of predicted orientation, but not in the unpredicted condition. These findings suggest that attention and perceptual expectations interactively influence visual processing within 200 ms after stimulus onset and such joint influence may lead to enhanced endogenous attentional control in the dorsal fronto-parietal attention network

My mind's playing tricks on me: Understanding event integration in rapid stimulus streams.

Humans generally perceive the external world in a coherent manner. This perceptual coherence exists in most individuals despite the supposedly overwhelming number of individual bits of perceptual information available at any given moment. A substantial body of scientific work has been dedicated to understanding the cognitive reasons underlying the coherence of human perception. A key notion in this context was put forth almost a century ago by Gestalt psychologists (Von Ehrenfels, 1937; Wagemans et al., 2012), stating that humans group and categorise sensory input into objects because the human mind is dispositioned to perceive patterns. Grouping sensory input into distinct objects is achieved via perceptual integration. Integration can occur either temporally, spatially, as well as multimodally, allowing us to perceive the world as trees, humans, and melodies, thereby preventing sensory overload. The study of human perception often analyses difficult perceptual tasks in which processes, such as integration, regularly fail. The idea underlying this approach is that insight about the circumstances leading a system, such as that of human perception, to reach its limits and fail simultaneously informs about how the system functions when it functions well. This thesis will study the limits of perceptual integration in the time domain. We will specifically analyse two cases in which visual stimuli presented in rapid sequence are integrated erroneously, leading to perceptions that were not presented as such in the physical world. The distractor intrusion phenomenon, in which multi-dimensional stimulus features are bound together wrongly, will be investigated first. The 2-feature Simultaneous Type/ Serial Token neural model (2f-ST2 model) will be presented to account for distractor intrusions across a range of different empirical paradigms studied in humans. We will provide behavioural as well as virtual electrophysiological (EEG) results generated by the 2f-ST2 model that qualitatively match those found with humans, providing evidence in favour of the 2f-ST2 model’s validity. Besides, we will provide a series of empirical analyses that further elucidate the cognitive mechanisms underlying distractor intrusions. In essence, these results suggest that whether integration occurs correctly or erroneously depends largely on the timing with which transient attentional enhancement (TAE) impacts relevant cognitive processes. Temporal event integration describes the cognitive process that binds two rapidly and successively presented visual stimuli into a single percept, given that there is a perceptually meaningful way of doing so. We analysed temporal event integration applying a machine learning approach to EEG data. Our results replicate previous findings on a whole-brain basis and further suggest that temporal event integration occurs about 300 – 450 ms after stimuli were presented specifically, and, more generally, with characteristics that are in line with those of distractor intrusions and the dynamics proposed by the 2f-ST2 model. We finally provide a series of simulation analyses that demonstrate the easily observable and dire methodological risk of overhyping when applying machine learning algorithms to neuroimaging datasets, such as those adopted by ourselves when investigating temporal event integration. Overhyping describes that choosing classification hyperparameters based on which generate the most desirable results (e.g., maximal classification accuracy) can threaten the external validity (i.e., generalisability) of an analysis if the necessary precautions are not taken. In this context we demonstrate that overhyping can lead to classifiers generating spurious above chance-level accuracies even though no signal was present in the data before providing effective safeguards that limit the risk of overhyping

Neural Processes Underlying the Flexible Control and Learning of Attentional Selection

In every-day life we are usually surrounded by a plethora of stimuli, of which only some may be relevant to us at a given moment in time. The dynamic interaction between internal factors, such as our previous experience and current goals, and external factors, such as salient sensory stimulation, determine where, how and what we attend to in our environment. This dissertation investigated some of the neural mechanisms that underlie successful goal-directed behavior in two conditions 1. when attention was actively cued to a target stimulus, and 2. when the attentional target had to be actively and repeatedly learned, in macaque monkeys and in humans. In Chapter 2, I investigated inter-areal spiketrain correlations in neuron pairs across the fronto-cingulate cortex when macaque monkeys are cued to shift their attention to one of two target stimuli. I found that neuron pairs in anterior cingulate cortex (ACC) and dorsal prefrontal cortex (PFC) with similar spatial preferences correlate their spiketrains at the time when attention needs to be actively shifted, suggesting that the flexible interaction between these two areas may support successful covert attention shifts. In Chapter 3, I show that when the attentional target stimulus needs to be repeatedly learned and is defined by only one of several stimulus features, neurons in macaque frontal and striatal regions encode prediction error signals that carry specific information about the stimulus feature that was selected in the preceding choice. These signals may be involved in identifying those synapses that require updating to allow flexible adjustments in goal-directed behavior. In Chapter 4, I found that when humans must repeatedly learn the identity of an attentional target, a human event-related potential over visual cortex that is thought to index attentional target selection, selectively decreases after successful learning, in particular for the distracting stimulus, and selectively increases for the target stimulus following negative feedback during learning. Overall, this dissertation provides novel insights into some of the complex neural mechanisms that support flexible control and learning of attention across brain regions of the human and non-human primate brain