Modulation of EEG-Alpha oscillations during visual spatial attention

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Thesis

Reference

Modulation of EEG-Alpha oscillations during visual spatial attention

RIHS, Tonia

Abstract

Voluntarily directing visual attention to a cued position in space leads to improved processing of forthcoming visual stimuli at this position, due to anticipatory tuning of visual cortex activity.

Recent evidence points to a determining role of modulations of posterior alpha-band activity (8-14Hz) during attention orienting.This thesis investigates the modulation of EEG alpha-oscillations during anticipatory preparation for a visual target. The results show that the topography of alpha band power maps is found to correspond to a retinotopic organisation according to the direction of attention in the time prior to the expected target arrival. Moreover, the thesis presents evidence for a dynamic modulation of alpha amplitudes according to task demands by showing that both alpha power decreases and increases can occur at different times in the course of anticipation of a visual target stimulus. The findings presented here provide further support for an active facilitative versus inhibitory role of alpha-power decreases and increases during attention orienting, by showing retionotopic specificity and dynamic deployment of [...]

RIHS, Tonia. Modulation of EEG-Alpha oscillations during visual spatial attention. Thèse de doctorat : Univ. Genève et Lausanne, 2008, no. Neur. 28

URN : urn:nbn:ch:unige-401093

DOI : 10.13097/archive-ouverte/unige:40109

Available at:

http://archive-ouverte.unige.ch/unige:40109

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DOCTORAT EN NEUROSCIENCES des Universités de Genève et de Lausanne

UNIVERSITÉ DE GENÈVE FACULTÉ FAPSE

Professeur Christoph Michel, directeur de thèse Dr. Gregor Thut, directeur de thèse

TITRE DE LA THÈSE

Modulation of EEG-Alpha Oscillations during visual spatial attention

THÈSE Présentée à la

Faculté de Psychologie et des Sciences de l'Education de l’Université de Genève

pour obtenir le grade de

Docteure en Neurosciences par

Tonia Anahi RIHS de Safnern, BE

Thèse N° 28

Imprimé à Genève 2008 FACULTÉ DE PSYCHOLOGIE

ET DES SCIENCES DE L’EDUCATION

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Publications

Rihs, T.A., Michel, C.M., Thut, G. (2007). Mechanisms of selective inhibition in visual spatial attention are indexed by alpha-band EEG synchronization.

European Journal of Neuroscience, 25, 603-610.

Rihs, T.A., Michel, C.M., Thut. G. (2009). A bias for posterior alpha-band power suppression versus enhancement during shifting versus maintenance of spatial attention. NeuroImage 44, 190-199.

Romei V, Rihs T, Brodbeck V, Thut G. (2008). Resting electroencephalogram alpha- power over posterior sites indexes baseline visual cortex excitability.

Neuroreport. Jan 22;19(2):203-8.

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REMERCIEMENTS

Un grand merci à mes directeurs de thèse Gregor Thut et Christoph Michel pour leur soutien exceptionnel durant ma thèse. Je remercie Theodor Landis pour le soutien qui m'a permis de terminer ma thèse à Genève. Merci à Gilles Pourtois pour son expertise durant mon examen de thèse. Tous mes remerciements vont également aux membres du jury de thèse: John Foxe, Micah Murray and Patrik Vuilleumier ainsi qu' à Gregor Thut and Christoph Michel

Merci aux membres du laboratoire de Christoph Michel que j'ai eus la chance de connaître durant le temps de cette thèse: Colette Boex, Juliane Britz, Verena Brodbeck, Denis Brunet, Nouara Chekhar, Mélanie Genetti, Narly Golestany, Mary Kurian, Göran Lantz, Agustina Lascano, Adrien Martin, Pierre Mégevand, Anne Normandin, Stéphanie Ortigue, Fabienne Picard, Nadia Rosenberg, Vincenzo Romei, Laurent Spinelli, François Tadel, Remi Tyrand et Lilliann Zamora. Beaucoup d'autres personnes ont contribué à rendre mon temps de PhD lémanique aussi riche: Shahar Arzy, Marzia de Lucia, Karim N'Diaye, Stéphanie Duhoux, Sandra Lehmann, Nadia Lucas, Manuel Mercier, Stéphanie Morand, Karsten Rauss, Marie Schaer, Sophie Schwartz and Ulrike Toepel.

Je remercie mes amis, ma famille et Shane Hofmann qui m'ont soutenus d'une manière incroyable. Et je remercie tous ceux cités ci-dessus de lire aussi les remerciements en anglais (voir acknowledgements).

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ACKNOWLEGDEMENTS

First, I would like to express my highest gratitude to my thesis directors Gregor Thut and Christoph Michel for their guidance and support. Gregor, I am grateful to you for welcoming me in your team as a PhD student, for being a true mentor, always open to new ideas and suggestions, with a contagious enthusiasm for research. Your commitment, the highly focused way of thinking, your sense of humour and your understanding have allowed me to learn many things that will reach far beyond the topic of my thesis. Christoph, I would like to express my deepest gratitude for your support and encouragement, for allowing me to work in such an enriching environment, for being accessible, listening to even the smallest questions and for encouraging us to think outside the box on many occasions. I would also like to express my sincere gratitude to Theodor Landis, whose support has enabled me to complete my thesis here in Geneva. Many thanks also go to Gilles Pourtois for his expertise during my thesis exam. To John Foxe, Micah Murray and Patrik Vuilleumier as well as Gregor Thut and Christoph Michel I would like to express my highest gratitude for taking part in my thesis jury.

For the times of collaborative work we shared, I thank Verena Brodbeck, Vincenzo Romei, and Gregor Thut. The hours, during which we were locked in the lab room, trying out new experiments, are some of the best memories of my time here. I also immensely appreciated and cherished the time with the lab members of the past and present. Sometimes, just the subtle joke at lunch would be enough to return to the data analysis. For this I would like to thank: Colette Boex, Juliane Britz, Verena Brodbeck, Denis Brunet, Nouara Chekhar, Mélanie Genetti, Narly Golestany, Mary Kurian, Göran Lantz, Agustina Lascano, Adrien Martin, Pierre Mégevand, Anne Normandin, Stéphanie Ortigue, Fabienne Picard, Nadia Rosenberg, Vincenzo Romei, Laurent Spinelli, François Tadel, Remi Tyrand and Lilliann Zamora. There are many more people from other labs that contributed to making my lemanic PhD time so enriching, inside and outside of research concerns, among which I would like to thank Shahar Arzy, Marzia de Lucia, Karim N'Diaye, Stéphanie Duhoux, Sandra Lehmann, Nadia Lucas, Manuel Mercier, Stéphanie Morand, Karsten Rauss, Marie Schaer, Sophie

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letting us marvel at the beauty of the world that surrounds us and for stimulating our curiosity to explore it in more detail. I thank you for the energy of our dinner discussions, ranging from aphlatoxins and metaphysics to literature and politics. But first and foremost, I thank you for your love and unfaltering support. To my sister Sandra Rihs, to Léonard Chabloz, my brother Simon Rihs and Monica Bachmann: I thank you for your love and unconditional acceptance, for always being there for me and for your wicked sense of humour. Laszlo, I thank you for letting me know about your world, for racing around in fastest record TGV's and supersonic planes and for finding different angles to the simplest things. To my aunt, Rosemarie Waltermann, I thank you for your understanding, love and support through all stages of my life, which are tremendously important to me. I thank my grandparents and Kurt Rihs: your guiding words, warmth and humanity will remain.

Deepest thanks go to Olivier Aebischer, Kim Begley, Alain and Myriam Birchmeier, Isabelle Feijo, Marie-Madeleine Friberg, Milan Gygax, Ruth Hennessy, Andrea Horn, Louise Houtzager, Nicola Jacobshagen, Adrian Kramp, Isabelle Krieg, Alexandra Kunz, Gillian Macnamara, Jega Sarangapany and Nicolas and Michèle Senn who remained understanding and supportive in this PhD time and to whom I owe a lot. I would also like to thank Richard Hahnloser, Nicola Jacobshagen and Ignaz Strebel for encouraging me in their own ways to find the path to this thesis. I am grateful for the ongoing support and friendship of Ariane Gonthier and Muriel Stilli who shared my experiences and concerns in happy but also more difficult times. To René Perler; I am deeply grateful for your incredible generosity and all those little signs of your friendship that cannot be measured. And I would like to thank you, Shane Hofmann, for your encouragement to undertake this project from the very beginning, to being there for me in the last phases of it. I thank you for everything we share and for believing in me.

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FOREWORD

Jorge Luis Borges, The Aleph, House of Sand, 1945

The Aleph is found as the first letter in the ancient Phoenician alphabet and is thought to be the precursor of the Syriac alaph, the Arabic alif and finally the Greek alpha. It is still used as the first letter of the Hebrew alphabet, in which it is considered a symbol of the infinite.

Through time, the Aleph remains as a symbol of the infinite and re-occurs under many different forms. Cantor named his entities of infinity after the Aleph, it appears as a term in informatics and in the influential writings of Borges, to name just a few.

Without being able to delve much deeper into the many connotations and meanings attributed to the Aleph over time, I found it worthy of note that Hans Berger named the first detected oscillation during electroencephalography, with a frequency of about 10 Hz, the Alpha-rhythm. Yet, while it is unlikely that this was more than a coincidence, the designation opens a link to the age old fascination exerted by infinite phenomena and our recurrent attempts to understand them.

To me, the fact that an oscillation is a more intuitively accessible form of a continuous and recursive phenomenon and the fact that it is found within neuronal processing has been an underlying fascination and motivation to

"He explained that an Aleph is one of the points in space that contains all other points."

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In a little more prosaic terms this thesis will investigate the influence of alpha oscillations occurring over visual cortical areas at baseline and its modulation during the process of endogenous visual spatial attention orienting.

The thesis is structured as follows: as the major part of this work is focused on modulations of alpha activity during attention orienting, the introduction will start by presenting models of attention. In a next step, the role of alpha band oscillations in attention research is discussed in more detail. This will lead over to the research questions and hypotheses that were investigated in this thesis. The following method and result sections give a summary of the findings, which are also presented in the attached published papers. A discussion follows in part four.

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ABSTRACT

Voluntarily directing visual attention to a cued position in space leads to improved processing of forthcoming visual stimuli at this position, due to anticipatory tuning of visual cortex activity. Recent evidence points to a determining role of modulations of posterior alpha-band activity (8-14Hz) for anticipatory facilitation (alpha- power decreases) vs. inhibition (alpha-power increases) during attention orienting.

This thesis investigates the modulation of alpha-oscillations during anticipatory preparation for a visual target. The results show that the topography of alpha band power maps is found to correspond to a retinotopic organisation according to the direction of attention in the time prior to the expected target arrival. Moreover, the thesis presents evidence for a dynamic modulation of alpha amplitudes according to task demands by showing that both alpha power decreases and increases can occur at different times in the course of anticipation of a visual target stimulus. Alpha-decreases are found to occur relatively early in time with their maximal decrease over visual cortex due to process the expected target. This points to a task specific activation of relevant areas to facilitate visual target processing during shifting of spatial attention.

During sustained attention over longer intervals, alpha increases were found over cortex located contralaterally to unattended space, indicating an active inhibitory role of increased alpha-activity to impair processing of spatial locations that need to be ignored.

Furthermore, the oscillatory alpha-band changes depended on the participants’

resting alpha-band power, whereas anticipatory event related potentials did not.

Subjects with high vs. low alpha power at baseline also showed different levels of cortical excitability. This indicates that subjects differ on their use of alpha-power modulations during shifting and maintenance of attention towards a point in space.

The findings presented here provide further support for an active facilitative versus inhibitory role of alpha-power decreases and increases during attention orienting, by showing retionotopic specificity and dynamic deployment of attentional resources to prepare versus maintain the cortex for optimal target processing.

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SUMMARY IN FRENCH

Depuis les investigations de Helmholtz en 1867, il est connu que nous sommes capables de diriger notre attention visuelle vers un point spécifique de l'espace sans avoir recours à des mouvements oculaires vers celui-ci.

Helmholtz a également constaté que lorsque notre attention est dirigée vers un point particulier, la détection de stimuli visuels dans la région cible de l'attention est facilitée par rapport à la détection de stimuli présentés ailleurs. Ces observations ont été reprises de manière systématique plus d'un siècle plus tard, en 1985, par Michael Posner qui a investigué les processus d'attention volontaire, ou endogène, ainsi que l'attention exogène, ou réflexive, en utilisant des tâches expérimentales spécifiques. Pour cela, il a créé une tâche d'amorçage spatial qui permet ainsi de guider l'attention vers un point spécifique de l'espace, en préparation à l’arrivée d’un stimulus visuel. Ce paradigme a pu établir que l'attention visuo-spatiale volontaire influence avantageusement le comportement : cela se traduit par des temps de réactions plus courts ainsi que par une meilleure détection des stimuli attendus comparés aux stimuli qui ne sont pas au centre de l'attention. Il a été démontré que ces observations comportementales s'expriment au niveau neuronal par un processus selon lequel les réponses neuronales aux stimuli cibles de l’attention sont amplifiées de manière sélective, alors que les réponses neuronales aux stimuli hors du champ attentionnel sont diminuées. Il s'agit donc de favoriser spécifiquement le traitement de l'information qui est l'objet de l'attention.

Plus récemment, des études d'électrophysiologie, d'électro- encéphalographie et d'imagerie par résonance magnétique fonctionnelle ont pu établir que certains processus sélectifs d'amplification neuronale semblent déjà être actifs en préparation à un événement anticipé avant même que le stimulus attendu soit présenté.

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particulièrement investiguer dans les travaux de cette thèse en étudiant les modulations par certains mécanismes attentionnels de la fréquence dite alpha (de 8-14 Hz), mesurée par l'électroencéphalographie (EEG) de haute résolution.

La fréquence alpha est une composante de l'EEG connue depuis les premiers enregistrements de Helmut Berger en 1929. Néanmoins, à ce jour, la fonction exacte des modulations de la bande alpha reste à élucider. A cette époque, Berger avait déjà rapporté une suppression du rythme alpha liée à une activité mentale. Depuis ce temps, une réduction de la puissance en alpha a été mise en lien avec une activation corticale ainsi qu'avec des processus de type attentionnel, alors qu'une augmentation du rythme alpha est restée pendant longtemps associée à une oscillation d'inactivité généralisée.

Afin de mieux cerner ces mécanismes, j'ai étudié les modulations de la puissance en fréquence alpha lors d'une tâche d'amorçage spatial durant la période d'orientation attentionnelle, et donc pendant l'anticipation d'une cible à une certaine position. Dans la première étude de ce travail, j'ai pu démontrer que les changements relatifs d'alpha montrent une topographie rétinotopique et donc spécifique à la position attentionnelle. De plus, les changements étaient dominants pour une augmentation relative en alpha dans les 200 ms avant l'apparition de la cible, et localisés sur le cortex occipito-pariétal controlatéral à l'hémichamp qui devait être ignoré. Ceci a permis les conclusions suivantes: en vue de la distribution rétinotopique située sur le cortex qui couvre l'hémichamp non couvert par l’attention, l'augmentation en alpha est vraisemblablement liée a un processus actif d'inhibition spécifique et anticipatoire, et non pas à une oscillation d'inactivité. De manière surprenante, nous n'avons pas trouvé dans cette étude de réductions significatives en puissance alpha situées sur le cortex occipito-pariétal qui se prépare à traiter l'information de la cible (cortex controlatéral à la cible). L'hypothèse émise lors de cette première étude a été que la dominance de la puissance alpha était due à un processus de maintien

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étude présentée ici, nous avons investigué plus précisément le décours temporel lié à l'orientation de l'attention en relation avec les modulations dans la bande alpha. Dans cette étude, nous avons pu démontrer que, lors d'intervalles relativement brefs, nous observons une réduction relative en puissance alpha sur les parties occipitales controlatérales à la cible, alors que le maintien de l'attention engendre une augmentation relative en puissance alpha controlatérale aux positions non-attendues, confirmant ainsi les résultats obtenus lors de la première étude. De plus, nous avons montré que les modulations attentionnelles en alpha ne sont observées que pour les sujets qui présentent une relativement haute puissance en alpha lors de l’état de repos.

Une étude TMS a également mis en évidence un lien fonctionnel entre la puissance alpha au repos et l'excitabilité corticale mesurée par le seuil d'induction de phosphènes sur le cortex occipital.

Ensemble, les éléments présentés dans cette thèse ont pu démontrer que des changements spécifiques en puissance alpha sont liés à l'attention spatiale dans le sens d'une réduction en alpha sur le cortex qui s'apprête à traiter l'information attendue, ainsi que dans le sens d'une augmentation relative d'alpha sur le cortex opposé qui traiterait l'information non-attendue. Ceci pourrait ainsi indiquer des processus plus macroscopiques d'activation et de suppression lors de l'orientation attentionnelle durant l'anticipation d'une cible visuelle. De plus, ces travaux ont mis en évidence des différences individuelles de la fréquence alpha au repos qui se traduisent par une modulation attentionnelle ainsi qu'une excitabilité corticale différente. Ceci pourrait indiquer que certains sujets ont recours à des mécanismes de modulation neuronale différente pour orienter leur attention spatiale vers un point de l'espace. Tout en soulignant l'importance de futures études différentielles pour caractériser les modulations individuelles des mécanismes oscillatoires, ces éléments renforcent le role des variations ciblées des oscillations alpha pour mieux préparer le cortex occipital à un événement visuel attendu.

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1. INTRODUCTION

"Every one knows what attention is. It is the taking possession by the mind, in clear and vivid form, of one out of what seem several simultaneously possible objects or trains of thought. Focalization, concentration, of consciousness are of its essence. It implies withdrawal from some things in order to deal effectively with others."

William James, The Principles of Psychology, 1890 (James, 1890).

With this definition, William James points to the capacity of attention to selectively enhance processing of attended content as well as its faculty to withdraw processing resources from irrelevant stimuli. William James thus captures an intuitive concept of attention while outlining some of the major themes that are still the core of attention research to this day.

The aim of this introduction is to relate the papers of the thesis to frameworks of attention research as well as to link them to the literature on oscillatory mechanisms. I will begin with an overview on attention research, first by presenting a conceptual framework for mechanisms of attention, then by introducing networks of attention for endogenous and exogenous attention. In a next step, anticipatory control processes will be differentiated from attentional modulation of stimulus processing. I will then present event related electroencephalographic studies of visual spatial attention. From there, I will present oscillations in different frequency bands and their contribution to attention research. I then discuss the role of alpha-modulation in attention research.

Finally, I will present the questions and hypotheses that led to the studies of this thesis.

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While William James is often quoted, Helmholtz was one of the first to investigate visual attention systematically with a device similar to a tachistoscope, in which a large array of letters was briefly illuminated. He noted that the focus of attention could be shifted independently of the eye's fixation point, and that target detection was greatly enhanced at the location of attentional focus compared to unattended locations (Helmholtz and Southall, 1867/1962; Wright and Ward, 2008). Helmholtz was thus able to show that covert shifts of attention can be performed independently of ocular fixation.

Current models define attention as a process by which the influence of neuronal responses that convey attended information is selectively enhanced (Kastner et al., 1999; Luck et al., 1997; Muller et al., 1998; Reynolds and Chelazzi, 2004;

Treue and Maunsell, 1996), as well as a process resulting in the suppression of activity in unattended neural circuits which will lead to attenuated processing of competing stimuli (Luck et al., 1997; Slotnick et al., 2003; Smith et al., 2000;

Treue and Maunsell, 1996; Vanduffel et al., 2000). Attention affects the processing of different classes of stimuli in the external environment such as stimuli defined by spatial location (Posner et al., 1980), movement, (Treue and Maunsell, 1996), objects (Blaser et al., 2000; Roelfsema et al., 1998; Yantis and Serences, 2003), or specific features (Corbetta et al., 1990; Motter, 1994;

Roelfsema et al., 1998; Treue and Martinez Trujillo, 1999); but attention may also be directed towards mental activities such as our expectation of time (Coull et al., 2000; Ghose and Maunsell, 2002; Nobre et al., 2007), memory content (Griffin and Nobre, 2003; Summerfield et al., 2006) or even semantic information (Cristescu et al., 2006). In the case of attention to sensory events, attention amongst other factors may ultimately influence perceptual decision making (Heekeren et al., 2008). The overlying goal of promoting decision making, action or planning (Nobre and Shapiro, 2006) is postulated for attention to mental processes.

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1.1 Orienting of attention

To describe and measure attention in more detail, the work of Michael Posner proved to be highly influential (Posner et al., 1980). Through systematic investigation, he has opened a rich field of attention related research, which has focused to a large extent on the process of orienting of attention, often also defined as attentional selection. Posner separated attention orienting into three components: (i) the disengagement of attention from a current focus, followed by (ii) shifting attention to a new focus and (iii) the maintenance of attention at this point until target appearance (Posner et al., 1984). In the following paragraphs, I will outline the bases of attention orienting in more detail, beginning with the distinction between endogenous and exogenous attention.

1.1.1 Endogenous and exogenous attention orienting

It became evident early in attention research that attentional enhancement can be characterized by two mechanisms (Titchener, 1910/1980; Wertheimer, 1925;

Wundt, 1911), reviewed in (Wright and Ward, 2008). One is the top-down mediated process of endogenous attention. In other words it is voluntary attention guided by knowledge, expectation or goal oriented behaviour to a particular aspect of the environment, or to a portion of space, in the case of spatial attention. This process can be differentiated from the sensory driven mechanisms of reflexive orienting or exogenous attention, whereby a stimulus that is salient, highly different from its surround, or potentially dangerous will automatically attract attentional resources to the portion of space at which it appears. Both mechanisms were shown to have partially segregated neuronal substrates: endogenous attention is most likely modulated by top-down signals from dorsal posterior parietal, and frontal cortex (Corbetta et al., 2008; Corbetta and Shulman, 2002; Coull et al., 2000; Hopfinger et al., 2000; Kastner and Ungerleider, 2000), while exogenous attention is linked to neuronal pathways in the right temporoparietal and bilateral ventral frontal cortex (Corbetta and

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Shulman, 2002; Coull et al., 2000) to enhance detection of sensory salient events.

The two forms of attention can also be distinguished by different behavioural responses. These responses are characterized by the benefits in accuracy or reaction times when responding to an attended stimulus, but also the costs of responding to unattended stimuli. The well-known location-cueing method by Posner (Posner et al., 1980) enables us to test these costs and benefits, by presenting a previously cued target either at the indicated location (valid cueing) or at another location (invalid cueing), and is mostly tested by maintaining eyes at a central fixation point (covert attention orienting). This paradigm also enables us to differentiate between endogenous and exogenous forms of attention. When exogenous attention is investigated, reflexive attention is drawn to a location by a sudden change in stimulus salience (i.e. the brightening or change in colour at the target location). To investigate endogenous attention, the cues are symbolic indexes, and are not spatially contiguous with the target location (centrally presented cues, often symbols or letters). In exogenous attention, an initial behavioural benefit in response to a salient attentional cue can be observed for up to 200ms after the cue. The benefit reverses and turns into a slowing of response times when a stimulus appears at the location of the exogenous attention attractor during time intervals lasting longer than 200 ms.

This phenomenon is called inhibition of return (Klein, 2000; Posner, 1985), is observed in the case of exogenous attention orienting and presumably results from a mechanism by which a shift towards a recently attended location is impeded in order to facilitate reflexive orienting to other locations in the visual field (Klein, 2000; McDonald et al., 1999; Ro et al., 2003). In the case of endogenous or voluntary orienting of attention, a behavioural benefit is observed for much longer time intervals, ranging from around 300ms to about 4 seconds between the cue and the target (Ling and Carrasco, 2006; Muller et al., 1998), showing that once attention is voluntarily shifted, it can be maintained at the attended position for a prolonged duration. Targets appearing at the attended position will be detected faster, with higher accuracy and needing a

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lower perceptual threshold (Cave and Bichot, 1999; Ling and Carrasco, 2006;

Posner et al., 1980).

Based on these findings, a separation between endogenous and exogenous attention is meaningful in order to investigate attentional mechanisms.

However, it is also evident that both processes will interact in real life behaviour.

There are occurrences in which bottom-up attention acts as a "circuit breaker"

and attracts the subjects attention away from its endogenous attentional focus (Corbetta and Shulman, 2002). The strength of interference by exogenous attention attractors, depends on their physical characteristics (i.e. perceptual difference from the environment, saliency, etc.) as well as internal variables such as novelty, but also the strength of the endogenous attentional focus. This is particularly evident when an otherwise easy to detect exogenous attention attractor is ignored if it occurs while endogenous attention is highly focused on another task. One example of such a mechanism is described in the inattentional blindness paradigms (Simons and Chabris, 1999).

While endogenous and exogenous systems mostly interact in natural situations, the following paragraph will now turn to models of endogenous attention orienting to describe the conceptual framework that underlies attention research in this field.

1.1.2 Models of endogenous attention orienting

As most attention research would agree that endogenous attention is achieved by top-down signals to modulate sensory-evoked processing, aspects of this modulation are investigated in different areas of neuroscience ranging from network modelling approaches (Hahnloser et al., 1999) to neurophysiology in animals (Fries et al., 2001; Luck et al., 1997; Moran and Desimone, 1985;

Roelfsema et al., 1998; von Stein et al., 2000; Womelsdorf and Fries, 2007), to fMRI and EEG/MEG studies (Corbetta and Shulman, 2002; Foxe et al., 1998;

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Hopf and Mangun, 2000; Hopfinger et al., 2000; Kastner and Ungerleider, 2000;

Lakatos et al., 2008; Worden et al., 2000) and TMS research in humans

(Grosbras and Paus, 2002; Muri et al., 2002; Pourtois et al., 2001; Silvanto et al., 2008; Thut et al., 2004).

Animal studies with monkeys found a characteristic modulation of neuronal response rates when two stimuli were presented in the same receptive field of area V4 (Luck et al., 1997; Moran and Desimone, 1985). When no attention was directed to the stimuli, neuronal responses were attenuated, since the two stimuli competed for resources within the same receptive field. However when one of the two stimuli was attended, the resulting response was strongly influenced by the response to the attended stimulus, thereby indicating that the response to the unattended stimulus was suppressed while neuronal responses to the attended stimuli were enhanced. This lead to the biased competition accounts of spatial attention which have been highly influential in shaping our understanding of voluntary or top-down attention, reviewed by (Kastner and Ungerleider, 2000). The model proposes that the stimuli in the visual input compete for our limited processing capacity when they appear. This competition will be resolved if the system prepares itself to favour stimuli that are behaviourally relevant. Evidence for anticipatory biasing was given by Luck (1997), who showed that baseline firing rates in receptive fields were increased, whenever attention was directed towards the receptive field. These shifts in baseline activity were one of the first measurable consequences in single cell recordings of attentional bias during anticipation of a visual target.

While the biased competition model proved to be important in conceptualising mechanisms of selective attention, one difficulty emerged when the model was restricted to attentional modulation of stimuli that would fall either inside or outside a receptive field (Desimone and Duncan, 1995). Firstly, this contrasted with behavioural (Carrasco et al., 2004) but also single cell (Ito and Gilbert, 1999) and neuroimaging studies in humans, (Kastner et al., 1999; Martinez et

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al., 1999; Pinsk et al., 2004) some of which described attentional modulation in area V1 (Schwartz et al., 2005; Silver et al., 2005; Somers et al., 1999; Tootell et al., 1998; Yamagishi et al., 2005). This provided evidence for attentional

modulation which could not be explained by competition within a receptive field;

as the receptive fields in the studies finding V1 activity would be too small to contain both the attended and ignored stimulus (Schroeder et al., 2001).

Furthermore, early studies showed that other factors such as stimulus contrast (Carrasco et al., 2004; Reynolds et al., 2000), task difficulty or attentional load (Pinsk et al., 2004; Rauss et al., 2008; Schwartz et al., 2005; Spitzer et al., 1988) changed the extent of attentional modulation and could not be explained by simple competition mechanisms, in which responses would only be enhanced if they were within a receptive field. The biased competition model also implies that distractors need to be present in order to create a competition, which is not always the case (Kastner et al., 1999; Luck et al., 1997). This lead to a different, but related conceptualisation of attention by which the allocation of attention is a dynamic process that can be targeted flexibly to the current dimension of the attended target, such as features, objects, locations or time, and will affect processing in respective cortical areas. Attentional models that encompass these findings are similar to the Feature Similarity Gain Model by Treue and Martinez-Trujillo (Treue and Martinez Trujillo, 1999); reviewed by (Schroeder et al., 2001; Treue, 2001). These models can thus account for attentional modulation at the size of receptive fields but also for the increase in magnitude of the modulation from early cortical areas to higher order areas.

Lavie (Lavie, 1995) proposes in a related model that attention operates flexibly and can affect early stages of perceptual processing if they are task relevant.

Treue (2001) and others further postulate that attentional modulation is a function of feedback projections from higher- to lower order visual areas (Lamme and Roelfsema, 2000). In summary, these models suggest that attention to any specific feature will enhance the responsiveness of all neurons that prefer the stimulus dimension, and not only neurons whose receptive fields cover the attended stimulus. They indicate that neuronal components that are

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used in the attentional modulation of processing are flexible and influenced by task demands.

Yet, by focusing on the gains of processing attended stimulus dimensions, those models do not always explicitly account for the biased processing before

target arrival, such as the experimental evidence that covert visuo-spatial attention to an expected stimulus location generates anticipatory activity that is both an enhancement of pre-stimulus activity over areas expecting the arrival of the stimulus (Chelazzi et al., 1993; Hopfinger et al., 2000; Kastner et al., 1999;

Luck et al., 1997); but also a concomitant deactivation of visual cortex that represents unattended locations (Muller and Kleinschmidt, 2004; Silver et al., 2007; Sylvester et al., 2008; Sylvester et al., 2007). In addition, the finding that attentional effects are increased when distractors are present (Awh et al., 2003;

Ruff and Driver, 2006; Serences et al., 2004) point to the importance of suppressive mechanisms during attention orienting that should not be ignored.

More recently, several studies have integrated neuronal oscillations as an alternative explanation to enhance attended stimulus impact (Fries et al., 2001;

Taylor et al., 2005; Womelsdorf and Fries, 2007) as well as top-down influence on pre-target processing (Foxe et al., 1998; Kelly et al., 2006; von Stein et al., 2000; Worden et al., 2000) that can account both for activation and inhibition of attended versus unattended stimuli.

By integrating findings form neuronal oscillation dynamics, current findings might thus contribute to a theory of endogenous attention orienting that can account for push-and pull mechanisms at the macroscopic level of interactions between cortical areas, while integrating the observation that attentional modulation can dynamically adapt itself to the affordances of the task. This thesis aims to contribute to this approach by specifically investigating anticipatory modulations of alpha-band activity during attention orienting over

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occipital cortex. In the following paragraphs the networks of attention orienting will be presented in more detail.

1.1.3 Networks of endogenous attention orienting

The papers presented above indicate that attentional orienting is dependent on the stimulus attributes or factors that guide selection, and might thus involve different neuronal mechanisms and structures related to the attended activity.

Nonetheless, common systems and anatomical structures emerge that characterize the attentional input from higher order areas towards sensory processing areas (Hopfinger et al., 2000; Nobre et al., 2006). Top-down influences have been postulated to stem from a distributed fronto-parietal network involving the posterior parietal cortex, with the dorsal posterior parietal cortex and intraparietal sulcus (Corbetta and Shulman, 2002; Hopfinger et al., 2000; Muri et al., 2002; Nobre et al., 2006; Ruff et al., 2008; Silvanto et al., 2008; Thut et al., 2004) but also the prefrontal cortex, including the frontal eye fields (Grosbras and Paus, 2002; Kelley et al., 2008; Kincade et al., 2005; Ruff et al., 2008; Taylor et al., 2007) and the cingulate cortex (Gitelman et al., 1999;

Nobre et al., 1997) but see also (Mesulam et al., 2001). The existence of an attention network of endogenous attention orienting also integrates with clinical findings in the field of visuospatial neglect, in which spatial attention is impaired when directed to the opposite hemifield of a lesion, particularly for lesions over right posterior parietal cortex (Corbetta and Shulman, 2002; Rafal, 1994; Vallar and Perani, 1986). The right hemispheric dominance in spatial attention orienting (Corbetta et al., 2005; Coull et al., 2001; Foxe et al., 2003) can also be observed in normal participants with the so-called pseudoneglect phenomenon, which is the tendency to judge the midpoint of a horizontal line leftwards of the actual center. This is indicative of a slight hyperattention to the left hemispace, or a dominance of right parietal cortex in the modulation of spatial attention under normal conditions. These observations have led to a model of right hemispheric dominance in spatial attention, where both the right and left hemispheres are able to direct attention to the right hemifield, while the right

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hemisphere is dominant for directing attention to the left hemifield. This would explain why visuospatial neglect occurs less frequently for lesions to left hemispheric parietal cortex. An additional explanation comes from Kinsbourne's model of spatial attention (Kinsbourne, 1977), which proposes that attention is mediated by equivalent processors in each hemisphere that are responsible for directing attention to the contralaterateral hemifield, and which are mediated by reciprocal transcallosal inhibition. In the case of damage to the right parietal cortex, attention directing to the left hemifield would be impaired in addition to a lack of callosal inhibition from the right parietal cortex to the left, resulting in a phenomenon of hyperattention to the right hemispace and thus exacerbating the neglect symptoms. So, the right hemispheric dominance in attention orienting observed in visual spatial neglect, and normal pseudoneglect could be caused by a stronger attentional modulation by right parietal cortex, but also a stronger transcallosal inhibition from the right to the left hemisphere.

In addition to describing the models and structures underlying the top-down modulation of endogenous attention, the process of attention can be separated into the attentional biasing of the system versus a post-stimulus attentional modulation of processing which will be outlined in the next paragraph.

1.2 Anticipatory control and attentional modulation of stimulus processing

The process, which enables us to select behaviourally relevant stimuli for further processing as well as the resulting change of neuronal responses has also been called the attentional set (Corbetta and Shulman, 2002). Within this attentional set, a useful distinction can be made between the anticipatory bias of the system; that is the selection, orientation and maintenance of attention towards expected stimulus entities, as opposed to the facilitation of processing of attended stimuli vs. the inhibition of processing of unattended stimuli.

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In other words, selective attention can be separated into anticipatory attention and the modulation of processing after a stimulus appeared in the environment.

We are thus using a behaviourally relevant stimulus (or the target) as the point on a time axis to which there is the "before" or anticipatory attention towards a selected stimulus or stimulus category as opposed to the responses to the occurrence of a target stimulus. In the same way, the top-down interaction to inhibit processing of stimuli that would interfere with the attentional goal can also be separated into a biasing or anticipatory component versus a process that describes the attenuated processing of unattended stimuli. The focus in this work lies on investigating the much less studied anticipatory phase of spatial attention orienting. One particular aspect of interest during this phase of attention orienting is the question of the time course of anticipatory spatial attention, which will follow in the next paragraph.

1.2.1 The time course of anticipatory spatial attention

The literature on the time course of anticipatory spatial attention has identified certain constants in the timing of spatial attention processing. Several studies report that orienting of endogenous attention starts showing behavioural effects for intervals between 200 and 300 ms after presentation of an indicative cue;

see (Griffin et al., 2001). Other studies also show that within endogenous attention, shorter intervals (<600 ms) between the cue and the target lead to slightly better behavioural benefits (Nobre, 2001) compared to longer cue target intervals; indicating that maintaining attention becomes more difficult with increasing intervals between the cue and the target. Despite the slight disadvantages of increasing intervals, behavioural benefits of maintaining voluntary attention appear to last for up to 4 seconds (Muller et al., 1998).

However, it is important to note that spatial and temporal attention are often linked and most likely operate by interdependent mechanisms (Nobre et al., 2007). As a result, it is often difficult to describe the time course of anticipatory spatial attention independently of the expected timing of a task. This was shown

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in a study by Ghose and Maunsell (2002) who found that the timing of responses in single cell recordings of area V4 of macaque monkeys in a task, where a change of orientation needed to be detected in a Gabor patch, was dependent on the temporal likelihood of the change to occur. By using a blocked design and gradually increasing the temporal probability of a target occurring at a spatially cued location, they demonstrated that the monkeys formed an expectation of the timing of the spatial target and modulated their responses in the corresponding receptive fields of V4 around the expected time of target presentation (Ghose and Maunsell, 2002). This was shown for two different timing delays, presented in a blocked design and jittered either around a 500 ms delay or a 2300 ms delay respectively (note, that no target was presented in the first 500 ms of the long delay blocks). This indicates that the modulation of spatial attention over time can itself be tuned to the timing prediction for the expected target.

After introducing the frameworks that are guiding current attention research, I will review the related research in the fields of event-related and oscillatory EEG and MEG studies to investigate the selective bias of processing during anticipatory and voluntary visual spatial attention.

1.3 Event-related EEG and MEG studies of visual spatial attention

Electroencephalographic studies showed early on that the evoked response to attended stimuli differed significantly when compared to unattended stimuli, in particular for the components of the visual evoked potential such as the P100 and N100 component, showing mostly an amplitude enhancement for validly cued versus invalidly cued stimuli (Di Russo et al., 2003; Hillyard and Anllo- Vento, 1998; Luck et al., 2000). This difference was interpreted as evidence for top-down feedback from higher order areas to influence processing of attended stimuli, mostly at extrastriate areas (Di Russo et al., 2003; Hillyard and Anllo-

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Vento, 1998; Martinez et al., 1999). Recently, evidence for changes in evoked activation in the calcarine was also found in EEG and MEG studies (Di Russo et al., 2003, Yamagishi et al., 2003) and even the early C1 component was shown to be modulated by spatial attention (Kelly et al., 2008) or attentional load (Rauss et al., 2008). These results fuelled ERP research to study the biasing of attention before a visual target is presented (Grent-'t-Jong and Woldorff, 2007;

Harter et al., 1989; Hopf and Mangun, 2000; Muller et al., 1998; Nobre et al., 2000; Slagter et al., 2005). Some of these studies identified a waveform component named EDAN (posterior early directing attention negativity) at around 200 to 400 ms post-cue, which was hypothesized to reflect encoding of the cue-meaning and initiation of the attentional shift (Nobre et al., 2000; Hopf and Mangun, 2000), while a component defined as ADAN (anterior directing attention negativity) occurring at around 380- 520 ms post-cue was related to attentional control and the redirection of attention in space (Nobre et al., 2000;

Hopf and Mangun, 2000). Finally, a third component, starting around 500 ms after the cue, was named LDAP (late directing attention positivity and was thought to reflect supramodal attentional control processes in posterior parietal areas, as it was observed in cross-modal attention studies (Eimer and Driver, 2001). However it was also linked to encoding of attended positions and shifting of attention to attended positions (Green and McDonald, 2006) while in some studies no LDAP was found (Nobre et al., 2000; Yamaguchi et al., 1994).

Unfortunately to this date, few ERP studies have investigated the effects of attention orienting with reference independent methods of topographic spatio- temporal EEG analysis (Foxe and Simpson, 2005) that would allow for an easier comparison of the obtained results.

In addition to these earlier components in attention orienting, recent research has turned again to classical slow potential shifts similar to the contingent negative variation (CNV) (Rohrbaugh et al., 1976), when attention has to be maintained over long time intervals from 700 ms to 2000ms (Foxe and Simpson, 2005; Grent-'t-Jong and Woldorff, 2007). A late CNV was proposed to represent a generalized state of endogenous sensory and motor anticipation

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with a temporal prediction component during the expectation of a target (Bender et al., 2005). The CNV topography has also been linked in previous studies to generators over prefrontal, cingulate and posterior parietal areas in addition to premotor and motor areas (Bender et al., 2005; Gomez et al., 2007) and a CNV like fronto-central negativity was shown in both auditory and visual anticipatory attention (Foxe and Simpson, 2005).

1.4 Oscillations and anticipatory spatial attention

The studies presented thus far show that selective and endogenous attention is active at the local level by changing responses in areas that are tuned to the attended stimulus dimension. However if we assume that these local responses are modulated by attentional input, this also implies a targeted top-down communication over a wider spatial scale (Womelsdorf and Fries, 2007).

Recent evidence suggests that this kind of information both at local and long range spatial scales might be conveyed by means of synchronous neuronal oscillations as they are able to reflect rhythmic changes of neuronal excitability, and might thus act as a temporal window for coordinated processing (Buzsáki, 2006; Buzsaki and Draguhn, 2004; Fries, 2005; Lakatos et al., 2008).

On a local scale, gamma band synchronization (40-80 Hz) has often been investigated during feature based attention tasks in single cell recordings (Bichot et al., 2005) or human subjects (Tallon-Baudry et al., 2005). Specific oscillatory responses in the gamma frequency band are also found when spatially cued stimuli are attended by monkeys in area V4, showing coherence of spikes and local field potentials (Fries et al., 2001; Womelsdorf et al., 2006) as well as a correlation between selective synchronization in the gamma frequency band and behavioural performance in a spatial attention task (Womelsdorf et al., 2006). In addition, the lowest gamma synchronization in receptive fields treating unattended stimuli was found when the response to attended stimuli was fastest (Womelsdorf et al., 2006). Likewise, attentional

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enhancement of sensory processing was linked to increased gamma-band synchronization in human subjects in spatial attention tasks (Wyart and Tallon- Baudry, 2008). While a few studies also showed that gamma-band synchronization immediately before target arrival was able to predict behavioural performance in spatial attention tasks (Gonzalez Andino et al., 2005; Womelsdorf et al., 2006), thus far most studies on attentional modulation found gamma band synchronization over visual cortex after the target stimulus was presented (Bichot et al., 2005; Fries, 2005; Tallon-Baudry et al., 2005;

Wyart and Tallon-Baudry, 2008). In summary, this indicates that the degree of gamma band synchronization might index whether a neuronal group is processing an attended stimulus effectively (Womelsdorf and Fries, 2007).

While gamma-band oscillations seem to be active mainly at the local level (Fries, 2005; von Stein et al., 2000), lower frequencies have been found in inter- areal communication within the attentional network, as their longer cycle length might also be more suited to convey information over longer distances (Buzsáki, 2006). Frequencies such as beta (14-30 Hz) (Roelfsema et al., 1997) theta (4-8 Hz), alpha (8-14 Hz) (Sauseng et al., 2005b; Schack et al., 2005;

von Stein et al., 2000) and delta (0.5-4 Hz) (Lakatos et al., 2008) have all been proposed to reflect inter-area communication within neuronal networks related to attention or working memory. However the proposed role of gamma coherence in integrating visual feature binding over distant areas would also argue against a purely local function of gamma synchrony (Engel et al., 2001).

Interestingly, recent studies indicate that gamma-band synchronization might be modulated by the theta rhythm (Canolty et al., 2006; Csicsvari et al., 2003;

Jensen et al., 2007), or even phase locked to entrained delta oscillations that showed attentional modulation (Lakatos et al., 2008), while others show that increased gamma-band activity coincides with decreased alpha-band synchrony in a spatial attention task (Fries et al., 2001) and feature based attention task (Fries et al., 2008).

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As theta band activity has previously been linked to successful working memory performance in monkeys (Lee et al., 2005) and man (Jensen and Tesche, 2002;

Klimesch et al., 2005) the findings of theta band activity within anticipatory spatial attention tasks might also reflect the working memory component often inherent to spatial attention tasks (Jensen et al., 2002). The above mentioned studies and other studies using rhythmic stimulation conditions (Kelly, 2006), in which oscillatory responses are found to be specific to the attended stimulation frequency, indicate that phase alignment can be controlled by attention within different frequency bands. Moreover, the study by Lakatos (2008) shows that the amplitude of higher frequency oscillations might be modulated by the phase of lower frequencies. While more information is still to be gathered to describe the functionality and complex interactions between different frequency bands (Varela et al., 2001) and to disentangle the "blend of rhythms" found in neuronal brain activity (Buzsaki, 2006) for long range communications between cortical areas, one particular frequency was consistently found to be specifically related to visual-spatial attention orienting at local levels: that is the alpha-frequency band.

1.5 The alpha frequency band

Despite of the fact that the alpha rhythm was the first oscillation to occur over occipital cortex that was described early in EEG-history by Hans Berger (Berger, 1929), its neuronal generators are still largely unknown to this date (Buzsaki, 2006; Klimesch, 2007). In addition to thalamic generators (Lopes da Silva et al., 1980), alpha is also generated in different cortical populations and shows slightly different frequency characteristics in these areas (de Munck et al., 2007; Laufs et al., 2003; Lopes da Silva, 1991; Yamagishi et al., 2005).

Cortical alpha rhythms may also result from an interaction between thalamo- cortical and cortico-cortical mechanisms (Lopes da Silva et al., 1980;

Suffczynski et al., 2001). Alpha-band power at rest or baseline was found in different EEG studies to be stable within individuals over time (Hanslmayr et al.,

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2007; Klimesch et al., 2007; Napflin et al., 2007), indicating that alpha-power might reflect a trait-like variable in each individual.

The early observation that alpha is a prominent oscillatory frequency over occipital cortex during rest and with closed eyes, led to the conclusion that alpha was an idling rhythm. This was reinforced by the findings of alpha-power suppression after eye opening, visual stimulus presentation and mental activity.

Later, alpha oscillations were found to occur over frontal cortex, i.e. the frontal eye fields, as well as over sensory motor cortex and the supplementary motor area (named the mu rhythm), (Pfurtscheller and Andrew, 1999) and over primary auditory cortex (Hari and Salmelin, 1997). In these early studies, the most prominent feature was the blocking of the spontaneous rhythm by activity.

However, the studies also showed suppression of the mu rhythm that began before a movement was planned over related cortical areas (Pfurtscheller et al., 1997). The reduction of alpha-power was subsequently related to be a functional correlate of brain activation or a state of high cortical excitability (Pfurtscheller 1992, reviewed by Klimesch, 2006). The so-called alpha suppression effect was considered as the prevailing functional indicator of cortical activation, while increased alpha synchronization was described for a long time as a cortical idling phenomenon (Pfurtscheller, 1992) in spite of early findings showing task induced increases of alpha-band power that argued against a pure idling hypothesis (Basar 1989, Klimesch 1999).

Changes in oscillatory alpha-band activity were also found in working memory tasks, showing relative increases in alpha power during retention and with increasing working memory load (Jensen et al., 2002; Jokisch and Jensen, 2007; Michels et al., 2008). While working memory tasks and attention may share partially overlapping mechanisms, the observed increase in alpha power in the above mentioned studies was related to an inhibitory function of distracting inputs as it was observed over areas that needed inhibition in a delayed match to sample task (Jokisch and Jensen, 2007).

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1.6 Attentional modulation in the alpha-band

1.6.1 The role of alpha power suppression

In the field of endogenous spatial attention studies, evidence for functional links between alpha suppression (as measured by relative decreases in alpha-band power) and targeted activation was found. This was shown in cued attention tasks by the topography of anticipatory alpha power decreases just before a target was expected, occurring over visual cortex contralateral to the attended hemifield (Sauseng et al., 2005a; Thut et al., 2006; Yamagishi et al., 2005), thus indicating anticipatory activity over areas preparing to process the target. Other studies provided evidence for enhanced behavioural performance with reduced alpha band power over relevant cortical areas occurring either spontaneously (Ergenoglu et al., 2004; van Dijk et al., 2008) or as modulated by attention (Hanslmayr et al., 2007; Thut et al., 2006). In addition, the results of combined TMS and EEG studies show that visual cortical excitability as measured by TMS induced phosphene perception is dependent on the momentary alpha desynchronization before the TMS pulse is delivered (Romei et al., 2007).

1.6.2 The role of alpha power enhancement

Recently, the role of alpha synchronization (as measured by relative increases in alpha-band power) was studied in greater detail during anticipatory spatial attention orienting, finding increasing evidence for a relationship between alpha synchronization and functional inhibition. Recent studies found alpha power increases over posterior sites contralateral to the unattended location (i.e., ipsilateral to the attended position), which might serve to actively suppress visual input from task-irrelevant or unattended positions (Kelly et al., 2006;

Worden et al., 2000). Increases in alpha amplitude over occipital cortex were also found using cross-modal attention paradigms when attention was directed to auditory inputs (Foxe et al., 1998; Fu et al., 2001). Likewise, a TMS study investigating spontaneous fluctuations of alpha-band power prior to TMS

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induced phosphene perception failed to induce a phosphene percept (TMS intensities at detectability threshold) when alpha power was high (Romei, 2007).

These studies provided evidence for an active role of alpha desynchronization and synchronization during spatial attention orienting.

The studies presented above provided evidence for alpha decreases over cortex expecting a target to occur (Sauseng et al., 2005a; Thut et al., 2006;

Yamagishi et al., 2005) as well as alpha increases over cortex ipsilateral to the target site. However, those results showing either decreases or increases were obtained by presenting two possible target positions at opposite locations (left vs. right and valid vs. invalid) mostly for the lower visual field (Sauseng et al, 2005a; Thut et al., 2006; Yamagishi et al. 2005; (Kelly et al., 2006; Worden et al., 2000). In the study of Worden (Worden et al., 2000), two mirror locations were investigated in a blocked design either in the lower- or upper visual field.

The "to be ignored" position was therefore pre-determined in all of those studies, whether a distractor was present or not. Thus, no study had so far investigated whether specificity of topographic maps in the alpha band could be shown for more than two positions in the visual field in which the location of the invalid cue was equally distributed. In addition, the previous studies finding relative increases in alpha-band power involved either the presence of a simultaneous distractor stimulus at the opposite field location (Kelly et al., 2006) or the instruction to ignore the target when presented at the uncued location (Worden et al., 2000). It could therefore not be excluded that relative alpha- increases were only observed in specific tasks in which the 'to be ignored' location was entirely predictable.

1.7 Research questions

These observations led to the research questions of the first study, in which we wanted to investigate whether we could find topographic maps in the alpha frequency band during anticipation of a visual target that were retinotopically

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specific to one of eight attended positions in the visual field. One of the questions was therefore whether earlier findings of specific maps of alpha- synchronization (Kelly et al, 2006; Worden et al., 2000) would generalise to a task with more cued locations. To investigate this, we used a Posner cueing paradigm in which the cue pointed to one out of eight locations in a circular array. The cue was predictive of the target location with an 88% probability, while each of the other 7 locations was a possible "uncued" location. This allowed us to test whether alpha-band increases only occur in cases, in which the distractor location is easily predictable or whether they reflect an underlying mechanism that is independent of distractor processing. Based on the previous findings of alpha-decreases to occur in situations where no distractors were present, we also hypothesized that we would see evidence for alpha-power decreases over cortical areas that were expecting the target in the first study.

However, while we found evidence for a topographic distribution of alpha-power changes, we found a dominance of alpha-increases, whereas alpha-decreases were absent or small. This corresponded to the findings of the studies presented earlier (Kelly et al., 2006; Worden et al., 2000) as well to as newer studies that found either alpha-decreases (Trenner et al., 2008; Wyart and Tallon-Baudry, 2008; Yamagishi et al., 2008) or alpha increases (Doesburg et al., 2008). The second study was thus aimed at investigating whether both alpha-decreases and alpha-increases can co-occur and which factors influence the alpha-modulation during attention orienting. One possible explanation for this apparent discrepancy is that alpha-modulation is resulting from a flexible adaptation to task demands.

We hypothesised that one of those task demands was the timing of the expected target, since alpha-decreases were mainly observed in earlier windows corresponding to a shifting phase, while alpha-increases were more likely related to maintaining attention at one point for a relatively long time.

Another question of the second study was to investigate whether the observed differences could be related to different probabilities of cue-target predictability,

Modulation of EEG-Alpha oscillations during visual spatial attention

Thesis

Reference