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Covert Visual Attention COVERT VISUAL ATTENTION An event-related potential study of the N2pc and PD components Master Degree Project in Cognitive Neuroscience One year Advanced level 30 ECTS Spring term 2020 Andreas Karske Supervisor: Oskar MacGregor Examiner: Andreas Kalckert Abstract In the study of covert visual attention, two event-related potential (ERP) components have been identified by earlier research. The N2 posterior contralateral (N2pc) component has been suggested to index the enhancement of attention to a specific lateralized target item. The distractor positivity (PD) component has been suggested to index the suppression of distractors appearing in the same search array. Earlier studies have reported different latencies for the PD component depending on the task and experiment. Furthermore, the N2pc and the PD component are not always elicited in the same experiment. Relative target-to-difficult-distractor placement have been shown to affect the mean amplitude of the N2pc. Less is known about how different relative placements affect the PD component. The aim of the present study was to try and elicit both an N2pc and a PD component in the same visual search paradigm. The PD was recorded later time-window which previous studies have suggested to indicate the ending of attention to a previously attended target. Three relative placements were analysed, horizontally opposite, vertically opposite and diagonally opposite. When combining all three relative placements an N2pc component was elicited contralateral to the target. No PD component was found when combining all relative placements. A larger mean amplitude N2pc was measured for the vertically opposite condition. The results are not in line with previous research, that have found the N2pc to be smaller in conditions where both target and distractor are on the same side of the visual field. However, when comparing upper and lower visual field targets the N2pc was found to be larger for lower visual field stimuli, which is in line with previous research. A larger mean amplitude for the PD was found in the diagonally opposite condition. Earlier research has suggested that when difficult distractor and target are located on separate sides of the visual field, this leads to successful inhibition, indexed by the PD component. In contrast to earlier research a larger PD component was not found for upper visual field stimuli. The present study differs from previous studies in the way the target and difficult distractor were placed and analysed. By separating what has previously been called “opposite side” condition into two separate conditions diagonally opposite and horizontally opposite the results from the present study seem to suggest that these two conditions are not synonymous. However, the results should be regarded with caution due to the small sample size. Furthermore, the horizontally opposite side condition also differs from previous studies with regards to relative target and distractor distances, which could have had an effect on the results. Keywords: N2pc, PD, visual search, attentional capture, attention, event-related potential, cognitive neuroscience 3 The following story is said to have taken place between Ikkyū Sōjun, a 15th century zen master and his student. One day a man approached Ikkyū and asked: “Master, will you please write for me some maxims of the highest wisdom?” Ikkyū took his brush and wrote: “Attention.” “Is that all?” asked the man. Ikkyū then wrote: “Attention, Attention.” “Well,” said the man, “I really don’t see much depth in what you have written.” Then Ikkyū wrote the same word three times: “Attention, Attention, Attention.” Half-angered, the man demanded, “What does that word ‘Attention’ mean, anyway?” Ikkyū gently responded, “Attention means attention.” (Schiller, 1994, p.17). 4 Acknowledgements The visual search paradigm used in the present thesis was a collaborate effort between myself and two other fellow students, Noah Kaufman and Oscar Magnusson. As a group we faced the challenge of both designing and programming a visual search paradigm that could be used for three separate aims that in turn would lead to three separate theses. It goes without saying that this would never have been possible without the joint effort from all of us. I would like to thank Noah Kaufman for having the patience and pedagogical skills to explain to me some of the (according to me) more difficult aspects of programming while we were working together. I would also like to thank Oscar Magnusson for helping me to design an experiment that could be used to study the N2pc. After working together with both Noah and Oscar I think it is safe to say that I hold both in high regard and couldn’t have asked for two more well- humoured and compassionate collaborators. I would also like to thank Oskar MacGregor, my supervisor, who through his more hands-off approach let me make many mistakes during the process of both designing and writing the present thesis. But also, for knowing when to step in and give more advice on how to fine-tune aspects of both the design of the experiment as well as the terminology used to communicate the finer details of the experiment to the reader. Finally, I would like to thank all the participants who volunteered for the project. 5 Table of Contents Introduction ............................................................................................ 7 Background ............................................................................................ 9 Attention, attention, attention .............................................................. 9 The N2pc component ........................................................................... 11 N2pc and PD components .................................................................... 12 Neuronal basis of visual attention ....................................................... 15 Focus of the present study .................................................................. 18 Method .................................................................................................. 20 Participants ........................................................................................ 20 Design ................................................................................................ 20 Stimuli ................................................................................................ 21 Procedure ........................................................................................... 22 Electrophysiological recording ........................................................... 24 Data analysis ...................................................................................... 25 Results .................................................................................................. 26 Behaviour ........................................................................................... 26 Analysis N2pc ..................................................................................... 26 Analysis PD ........................................................................................ 29 Discussion ............................................................................................. 30 The N2pc component .......................................................................... 32 The PD component ............................................................................. 35 Conclusion ............................................................................................ 40 References ............................................................................................. 42 6 Introduction Attention is a broad umbrella term reflecting many different processes. In everyday life we use attention to select out information relevant to our personal goals. How this selection is carried out is a question that has been the subject of considerable research (Luck & Gold, 2008). The visual search paradigm has been used to inform many influential theories regarding visual selective attention. During a visual search experiment subjects search for a specific target among a number of distractors (Hickey, Di Lollo, & McDonald, 2009). Another question that has occupied the field of attention, is whether focused attention is the result of target enhancement or distractor suppression. Earlier research suggested that focused attention was mainly the result of enhancement of features related to target items (Luck & Hillyard, 1994). Later studies, however, found that suppression of features belonging to distracting objects also played a part in focused attention (Hickey, et al., 2009). Since the behavioural results are the same irrespective of the actual underlying mechanism, (i.e., attention is focused on a single target at the expense of irrelevant distractors), researchers have looked for answers on a neural basis to resolve the question (e.g. Sawaki, Geng, & Luck, 2012). The use of event-related potentials (ERPs) is a common way to investigate cognitive processes that happen on a short time scale. Research into visual perception has revealed different potential ERP components that relate to different stages of visual perception. Earlier ERP studies found that when subjects focused on a lateralized target, this elicited a more negative amplitude waveform in the contralateral hemifield compared to the waveform recorded at the ipsilateral hemifield relative to the target. The difference between the contralateral and ipsilateral waveforms was dubbed the N2 posterior contralateral
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