The Heterogeneous Nature of Number–Space Interactions

It is generally accepted that the mental representation of numerical magnitude consists of a spatial “mental number line” (MNL) with smaller quantities on the left and larger quantities on the right. However, the amount of dissociations between tasks that were believed to tap onto this representational medium is accumulating, questioning the universality of this model. The aim of the present study was to unravel the functional relationship between the different tasks and effects that are typically used as evidence for the MNL. For this purpose, a group of right brain damaged patients (with and without neglect) and healthy controls were subjected to physical line bisection, number interval bisection, parity judgment, and magnitude comparison. Using principal component analysis, different orthogonal components were extracted. We discuss how this component structure captures the dissociations reported in the literature and how it can be considered as a first step toward a new unitary framework for understanding the relation between numbers and space

Mapping and modulating spatial attention asymmetries in young and older adults

Healthy young adults demonstrate a group-level, systematic preference for stimuli presented in the left side of space relative to the right (‘pseudoneglect’) (Bowers & Heilman, 1980). This results in an overestimation of features such as size, brightness, numerosity and spatial frequency in the left hemispace, probably as a result of right cerebral hemisphere dominance for visuospatial attention. This spatial attention asymmetry is reduced in the healthy older population, and can be shifted entirely into right hemispace under certain conditions. Although this rightward shift has been consistently documented in behavioural experiments, there is very little neuroimaging evidence to explain this effect at a neuroanatomical level. In this thesis, I used behavioural methodology and electroencephalography (EEG) to map spatial attention asymmetries in young and older adults. I then use transcranial direct current stimulation (tDCS) to modulate these spatial biases, with the aim of assessing age-related differences in response to tDCS. In the first of three experiments presented in this thesis, I report in Chapter Two that five different spatial attention tasks provide consistent intra-task measures of spatial bias in young adults across two testing days. There were, however, no inter-task correlations between the five tasks, indicating that pseudoneglect is at least partially driven by task-dependent patterns of neural activity. In Chapter Three, anodal tDCS was applied separately to the left (P5) and right (P6) posterior parietal cortex (PPC) in young and older adults, with an aim to improve the detection of stimuli appearing in the contralateral visual field. There were no age differences in response to tDCS, but there were significant differences depending on baseline performance. Relative to a sham tDCS protocol, tDCS applied to the right PPC resulted in maintained visual detection across both visual fields in adults who were good at the task at baseline. In contrast, left PPC tDCS resulted in reduced detection sensitivity across both visual fields in poor performers. Finally, in Chapter Four, I report a right-hemisphere lateralisation of EEG activity in young adults that was present for long (but not short) landmark task lines. In contrast, older adults demonstrated no lateralised activity for either line length, thus providing novel evidence of an age-related reduction of hemispheric asymmetry in older adults. The results of this thesis provide evidence of a highly complex set of factors that underlie spatial attention asymmetries in healthy young and older adults

Retinotopy of emotion: Perception of negatively valenced stimuli presented at different spatial locations as revealed by event-related potentials

Scarce previous data on how the location where an emotional stimulus appears in the visual scene modulates its perception suggest that, for functional reasons, a perceptual advantage may exist, vertically, for stimuli presented at the lower visual field (LoVF) and, horizontally, for stimuli presented at the left visual field (LeVF). However, this issue has been explored through a limited number of spatial locations, usually in a single spatial dimension (e.g., horizontal) and invariant eccentricities. Event-related potentials (ERPs) were recorded from 39 participants perceiving brief neutral (wheels) and emotional stimuli (spiders) presented at 17 different locations, one foveal and 16 at different peripheral coordinates. As a secondary scope, we explored the role of the magnocellular (M) and the parvocellular (P) visual pathways by presenting an isoluminant/heterochromatic (P-biased) and a heteroluminant/isochromatic version (M-biased) of each stimulus. Emo > Neu effects were observed in PN1 (120 ms) for stimuli located at fovea, and in PN2 (215 ms) for stimuli located both at fovea and diverse peripheral regions. A factorial approach to these effects further revealed that: (a) emotional stimuli presented in the periphery are efficiently perceived, without evident decrease from para- to perifovea; (b) peripheral Emo > Neu effects are reflected 95 ms later than foveal Emo > Neu effects in ERPs; (c) LoVF is more involved than UVF in these effects; (d) our data fail to support the LeVF advantage previously reported, and (e) Emo > Neu effects were significant for both M and P stimuliComunidad de Madrid, Grant/Award Number: HUM19-HUM5705; Ministerio de Ciencia, Innovación y Universidades, Grant/Award Number: PGC2018-093570-B-IO

Slipping into Sleep: neurodynamics of alertness transitions in humans and fruit flies

The ability to react to events in the external world determines the fate of every living organism, this general state of readiness is called as ’alertness’. What happens to neurodynamics in the brain when alertness fades away as we fall asleep? How is behaviour affected? These questions will help us understand the organizing principles in the brain and functions of sleep itself. Here I use two distant animal models, the richness in behaviour and complexity of the human brain to understand how alertness transitions affects attention; and the experimental flexibility of the fruit fly, to understand its effect over longer time intervals. I first develop an objective method to track alertness using Electroencephalography (EEG). Then, I investigate the behavioural dynamics using an auditory spatial attention task while participants fall asleep. By using multilevel modelling and psychophysics, I show that participants systematically misclassify tones from the left side when drowsy, and further with a hierarchical drift diffusion model (HDDM) show how drift-rate (evidence accumulation) explains errors. Then, I show convergent evidence in the neural dynamics using multivariate pattern analysis (MVPA). Next, I probe the effect of handedness on the same task. Handedness affects behaviour only under drowsy condition and I show how neural dynamics are affected by a combination of handedness and alertness. To approach alertness transitions in a system with reduced neural complexity, I explore those dynamics in the fruit fly (Drosophila melanogaster), using both single and multichannel local field potential (LFP) data to show how alertness transitions and sleep modulate different regions of the fly brain. Further, I validate the results by converging evidence from causal manipulations. Finally, I discuss how the mapping of alertness transitions -under natural conditions- can help us understand fundamental questions in neuroscience such as the functions of sleep or the mechanisms of general anaesthesia.Gates Cambridg

Attentional competition between visual stimuli in healthy individuals and neurological patients

In the rich and complex visual environment that surrounds us, visual stimuli compete for attention in a limited capacity perceptual system (Broadbent, 1958; Duncan, 1980; Treisman, 1969). In this competition, the winners reach perceptual awareness and the losers are disregarded and fail to reach awareness (Ward, Goodrich & Driver, 1994; Mattingley, Davis & Driver, 1997). Theories of visual attention can be guided and informed by the study of brain damaged patients who show specific impairments in attending to visual stimuli, in particular visual extinction, commonly following right hemisphere damage and resulting in an inability to perceive a contralesional stimulus when it appears with a simultaneous ipsilesional item, but no such impairment when it appears alone. The studies reported in this thesis created an extinction-like pattern of errors in healthy volunteers using a bottom-up (stimulus- driven) paradigm when a simple task of detection was employed. When a more demanding task of stimulus identification employed, both in bottom-up and top-down (cueing) paradigms, a rarely previously described pattern of anti-extinction was observed, in which perception of a weaker item was facilitated (rather than impaired) by a simultaneous ‘stronger’ item in the display. Extinction and anti-extinction were then explored in brain damaged patients. A novel ‘attentional waiting’ hypothesis was discussed, which proposes that extinction and anti-extinction may be part of the same attentional mechanism where the latter manifestation may be observed in larger proportion of patients showing extinction if duration of stimuli is increased

Lateral biases in attention and memory in neurologically healthy adults

Attention enables us to experience the world around us and to prioritise relevant sensory information. Attentional capacity is, however, limited and the mechanisms underlying the ability to focus attention are not symmetrically presented in the brain. Healthy populations do not attend to their left and right sides equally when viewing the visual world. Some information may, therefore, be over attended while other stimuli are ignored. Pseudoneglect is the tendency to demonstrate a leftward bias in spatial attention. The strong link between attention and memory suggests that this leftward attentional bias may impact what is encoded to memory. This research study explored the impact of pseudoneglect on visual long-term memory and attention by using an eye tracker to record eye-movements. Pseudoneglect was measured using a computerised version of the line bisection task (LBT), consisting of different line lengths presented in different positions. Male and female LBT performance was also explored. Participants demonstrated a tendency to bisect lines, of different lengths more towards the left of the true midpoint. No significant gender differences with regard to LBT performance were found. The eye-tracking data produced significant differences between the number of left and right fixations according to the items viewed, F(14, 28) = 2.74 p =.01, η2 = .58, indicating a large effect size. The findings also demonstrated that more items on the left were correctly recalled when compared to the right. On average, participants recalled more items on the left (M = 66.49, SE = 1.8) than on the right (M = 61.60, SE = 2.1), t(34) = 2.86, p = .004 (one-tailed). The eta squared (.483) indicated a small to medium effect size. Although a higher number of leftward fixations were observed and more items on the left were correctly recalled, the data revealed no significant correlations between leftward biases in attention and memory. There were no significant associations between the number of fixations and the number of items recalled. The study concludes that pseudoneglect impacts attention with a higher number of fixations recorded for the left-hemifield, but no significant differences were observed concerning memory encoding.Thesis (PhD (Psychology))--University of Pretoria, 2021.PsychologyPhD (Psychology)Unrestricte

Functional cerebral asymmetries of emotional processes in the healthy and bipolar brain

The perception and processing of emotions are of primary importance for social interaction, which confers faculties such as inferring what another person’s feels. Brain organisation of emotion perception has shown to primarily involve right hemisphere functioning. However, the brain may be functionally organised according to fundamental aspects of emotion such as valence, rather than involving processing of emotions in general. It should be noted, however, that emotion perception is not merely a perceptual process consisting in the input of emotional information, but also involves one’s emotional response. Therefore, the functional brain organisation of emotional processing may also be influenced by emotional experience. An experimental model for testing functional cerebral asymmetries (FCAs) of valenced emotional experience is uniquely found in bipolar disorder (BD) involving impaired ability to regulate emotions and eventually leading to depressive or manic episodes. Previous models have only explained hemispheric asymmetries for manic and depressive mood episodes, but not for BD euthymia. The present thesis sought to investigate FCAs in emotional processing in two major ways. First, FCAs underlying facial emotion perception under normal functioning was examined in healthy controls. Secondly, functional brain organisation in emotional processing was further investigated by assessing FCAs in the bipolarity continuum, used as an experimental model for studying the processing of emotions. In contrast with previous asymmetry models, results suggested a right hemisphere involvement in emotional experience regardless of valence. Atypical FCAs were found in euthymic BD patients reflecting inherent aspects of BD functional brain organisation that are free of symptomatic influence. Also, BD patients exhibited atypical connectivity in a default amygdala network particularly affecting the right hemisphere, suggesting intrinsic mechanisms associated with internal emotional states. Last, BD patients were associated with a reduced right hemisphere specialisation in visuospatial attention, therefore suggesting that right hemisphere dysfunction can also affect non-emotional processes. Taken together, the findings emphasize a BD continuum model relying on euthymia as a bridging state between usual mood and acute mood phases

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