Behavioural and electrophysiological correlates of lightness contrast and assimilation ACASTER, Steph Available from Sheffield Hallam University Research Archive (SHURA) at: http://shura.shu.ac.uk/24340/ This document is the author deposited version. You are advised to consult the publisher's version if you wish to cite from it. Published version ACASTER, Steph (2018). Behavioural and electrophysiological correlates of lightness contrast and assimilation. Doctoral, Sheffield Hallam University. Copyright and re-use policy See http://shura.shu.ac.uk/information.html Sheffield Hallam University Research Archive http://shura.shu.ac.uk Behavioural and Electrophysiological Correlates of Lightness Contrast and Assimilation Stephanie Louise Acaster A thesis submitted in partial fulfilment of the requirements of Sheffield Hallam University for the degree of Doctor of Philosophy October 2018 Abstract Lightness contrast and lightness assimilation are examples of the perception of a surface being influenced by surrounding areas. In contrast, a grey target is perceived lighter when neighbouring a dark surface, and darker when neighbouring a light surface. The reverse is true for assimilation. The general aims of this thesis were to investigate contrast and assimilation in parallel, using behavioural and electrophysiological methods to examine the responses. The first part of the project used a matching-chart method to assess the effect of depth separation on the perception of stimuli shown to elicit either contrast or assimilation, making a direct comparison between the effect on contrast and on assimilation. The second part of the project developed a forced-choice (lighter/darker) task to investigate the electrophysiological (ERP) responses associated with contrast and assimilation, thus investigating the time course of the associated neural processing, and whether contrast and assimilation result from different underlying processing. Throughout the studies, contrast effects were stronger with white inducers, whereas assimilation effects were stronger with black inducers. Both contrast and assimilation effects were disrupted when the target and inducer were separated by depth. When comparing contrast and assimilation responses, there was a difference in N1 amplitude in the left occipital area, but this was only apparent in one condition. Within other conditions, the P1 amplitude decreased as the strength of contrast effects increased and increased as the strength of assimilation increased. When comparing between conditions, a stronger contrast effect (white inducers) resulted in smaller amplitude than a weaker contrast effect (black inducers). The observation that both effects change after depth separation despite the 2D retinal image remaining equivalent, and the ERP activity throughout occipital and parietal areas suggest that contrast and assimilation require processing at the cortical level, rather than retinal processing alone. 2 Dedication To those who helped this come to be; To old friends, lost along the way; And to new friends, who found me. 3 Acknowledgements I would like to thank my supervisors, Naira, Alessandro, and John, for their time, feedback, and for each bringing their own expertise to this project. I have learnt from each of them in different ways, both specifically relating to this work, and things to apply more generally. I would also like to thank my colleagues in the PhD office for various elements of practical and moral support over the last 4+ years. Particular thanks go to Gary, for much-needed comradeship and guidance in getting to grips with the EEG lab. There would be no data without the contribution made by my various research participants, so I’d like to express gratitude to them all (especially those who sat through long EEG sessions). I’d also like to thank Lee and Dan for providing technical support – and sometimes moral support too! My family deserve special thanks for continued support throughout the time I have spent on this project. I’d particularly like to thank my dad, Ian, for being an essential voice of reason, for proofreading tricky paragraphs; and for making sure I always had space for time away. I would also like to thank James, for teaching me to tame a dragon and reminding me of the rewards of perseverance. Finally, I would like to give thanks to Linda, who helped me to get started in psychology and perception, and who would have been so proud to see this come together. 4 Candidate’s Statement The work presented in this theses is solely my own work, and has not been submitted for any other award. A complete list of references to material cited, and acknowledgements of those contributing to the progress of the project have been included. 5 Table of Contents Abstract ............................................................................................................................. 2 Dedication ......................................................................................................................... 3 Acknowledgements ........................................................................................................... 4 Candidate’s Statement ....................................................................................................... 5 Table of Contents .............................................................................................................. 6 Table of Figures .............................................................................................................. 11 Chapter 1: Introduction ................................................................................................... 14 1.1. Defining Contrast and Assimilation ......................................................................... 15 1.2. Theoretical Accounts of Contrast and Assimilation ................................................ 18 1.2.1. Low-level physiological explanations....................................................... 19 1.2.2. Attributing contrast and assimilation to different levels of processing..... 21 1.2.3. Higher-level explanations. ........................................................................ 24 1.3. Behavioural and Psychophysical Investigations of Contrast and Assimilation ....... 32 1.3.1. The effects of the luminance of surfaces................................................... 32 1.3.2. The effects of surface size. ........................................................................ 38 1.3.3. The effects of depth and distance. ............................................................. 41 1.4. Neurophysiology of Lightness Perception ............................................................... 43 1.4.1. Findings from neuroimaging studies. ........................................................ 48 1.4.2. Findings from electrophysiological studies. ............................................. 50 1.4.3. ERP investigations of lightness effects ..................................................... 52 1.5. Aims and Scope of the Project ................................................................................. 55 6 Chapter 2: The Effects of Depth Separation on Lightness Contrast and Lightness Assimilation .................................................................................................................... 58 2.1. Method……………………………………………………………………………..60 2.1.1. Participants. .................................................................................................... 60 2.1.2. Design and stimuli. ........................................................................................ 60 2.1.3. Procedure. ...................................................................................................... 63 2.2. Results…………………………………………………………………………….. 64 2.3. Summary and Discussion ......................................................................................... 66 2.3.1. The effects of depth separation. ..................................................................... 68 2.3.2. Asymmetries associated with colour of inducers. .......................................... 69 2.3.3. Perceptual grouping and figure-ground segmentation ................................... 70 2.4. Conclusion………………………………………………………………………… 72 Chapter 3: Considerations for the ERP Task and Methodological Studies .................... 73 3.1. Brief Overview of ERPs........................................................................................... 73 3.2.Participants………………………………………………………………………... 74 3.3.Task Design………………………………………………………………………...75 3.4. Stimulus Design – Methodological Studies ............................................................. 76 3.4.1. ‘Contrast’ stimuli – large inducer. ................................................................. 78 3.4.2. ‘Assimilation’ stimuli – small inducers. ........................................................ 79 3.4.3. Fixation. ......................................................................................................... 80 7 3.4.4. Control stimuli and practice trials. ................................................................. 81 3.5. Task Procedure and Inter-Stimulus Interval ............................................................ 82 3.6. Stimulus Presentation and EEG Recording Equipment ........................................... 83 3.7. ERP Analysis Methodology ..................................................................................... 84 3.8. Quantification of ERP Measurements .....................................................................