Seeing 3D surfaces: neural stimulation, learning and masking Vassilis Pelekanos A thesis submitted to the University of Birmingham for the degree of Doctor of Philosophy School of Psychology College of Life and Environmental Sciences University of Birmingham June 2015 University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder. Abstract Our everyday visual perception experience is of a richly detailed world full of bounded objects, slanted and extensive surfaces. Estimating surfaces’ depth is critical for visually guided interactions, yet, challenged by the limited two-dimensional input projected on each eye’s retina. Binocular disparity, the positional differences that objects project on the two retinae, is a powerful depth cue inducing stereopsis. In the present dissertation, I assessed the neural stages of the visual hierarchy that support the visual perception of disparity- defined three-dimensional (3D) surfaces. In the first experimental chapter (chapter 3), I used fMRI-guided rTMS to probe the cortical areas involved in the perception of slanted surfaces. Results hint at a functional contribution of the dorso-parietal visual stream (posterior parietal cortex; PPC) to slant estimation, however, further work is needed to fully understand the nature of its involvement. In chapter 4, I used fMRI-guided rTMS to show that stimulation-induced disruption of the ventral stream (area LO) eliminates the facilitation observed in 3D surface discrimination when disparity and motion cues inform depth in a congruent manner. This finding indicates that LO encodes signals for the integration of depth cues. In chapter 5, I found rTMS evidence that disparity and orientation signal-in- noise discriminations causally relate to the PPC’s function. Interestingly, this relation diminished after training on a visual feature other than the one employed during the rTMS testing (e.g., testing on depth - training on orientation discrimination, or vice versa). This finding indicates that learning generalisation, even across visual features, can influence neuronal organization. In chapter 6, I used metacontrast backward masking to show that brightness masking incorporates the 3D information of disparity-defined slant. This finding suggests that brightness estimation is mediated by mid-level neuronal mechanisms, at a cortical stage where binocular signals have been combined to extract disparity edge structure. Finally, considering the eye-tracking data I acquired during rTMS experiments, I found that neural stimulation did not affect eye movements systematically. These supplementary results provide reassurance that participants’ vergence was stable throughout the experiments and there was no stimulation-induced disruption of stereopsis. 2 Acknowledgements First and foremost I would like to thank my PhD advisor Andrew Welchman. Thank you for all your contributions of time, guidance and ideas. Your enthusiasm for science and passion for your research have been extremely motivational for me. Even during tough times of my PhD, your professionalism and high quality working standards were strongly stimulating. I appreciate all the skills you taught me. Many of those are skills for life. I am very grateful to Zoe Kourtzi for her advice and suggestions. Thank you for the insightful comments on my experiments and discussions about their inferences. Thank you for sharing your expertise in vision science in lab meetings and similar occasions. In general, thank you for your continuous support. Special thanks to Hiroshi Ban. I will forever be thankful to you as a colleague, mentor and friend. Thank you for the endless discussions about my work, your tremendous help throughout my PhD, your astonishing kindness and your support. You have been my best role model as a scientist. Working with you has been a great honour and privilege. I also thank all the past and present members of the lab. I will never forget the great moments we had in Birmingham for 2,5 years. Still, after the lab’s relocation to Cambridge, the same brilliant environment and team-working ethos continued to be strongly influential and stimulating. I want to thank you for all the fruitful scientific discussions and of course for spending several hours to participate in my experiments. Special thanks to Matthew Dexter and Matthew Dyson for managing the lab in Birmingham and Cambridge respectively and providing very helpful technical support. I thank all my friends, old and new ones. Sorry that I do not name you. You know who you 3 are! Thank you for sharing great moments and true feelings which I will forever appreciate. I would especially like to thank my parents and brother for always being there to discuss and offer wise advice about every possible aspect of my PhD life in the UK. Thank you for your love, fun and support. 4 CONTENTS Page Abstract ..................................................................................................................2 Acknowledgements ......................................................................................................... 3 Contents ......................................................................................................................... 5 List of figures .................................................................................................................. 9 List of tables .................................................................................................................... 11 List of abbreviations ........................................................................................................ 12 1. General introduction ................................................................................14 1.1 Depth perception: binocular disparity is a powerful cue................................ 16 1.2 The neurophysiology of binocular disparity .................................................. 18 1.3 The dissociation between the ventral and the dorsal visual stream .............. 21 1.3.1 ‘What’ versus ‘where’ ......................................................................... 21 1.3.2 ‘Fine’ versus ‘coarse’ ......................................................................... 23 1.4 Seeing 3D surfaces: slant estimation ........................................................... 24 1.5 Cue integration ............................................................................................ 26 1.6 Visual learning and performance optimization .............................................. 26 1.7 ‘Filling-in’ and surface perception ................................................................. 28 1.8 Surface brightness estimation ...................................................................... 30 1.9 Neural mechanisms for perceptual completion ............................................ 31 2. General methods ......................................................................................35 2.1 Participant recruitment, screening and ethics ............................................... 35 2.2 Monitors configuration and gamma calibration ............................................. 35 2.3 Stimuli and data manipulation ...................................................................... 36 2.4 Transcranial magnetic stimulation ................................................................ 36 2.5 MRI imaging and analysis ............................................................................ 38 2.6 fMRI-guided TMS protocol ........................................................................... 40 2.7 Eye tracking ................................................................................................. 41 2.8 Method of single stimuli ............................................................................... 43 2.9 Visual masking and the stimulus onset asynchrony ..................................... 45 5 3. Seeing slanted surfaces: probing the contributions of dorsal and ventral visual brain areas ........................................................................46 Abstract .............................................................................................................. 46 3.1 Introduction ................................................................................................. 47 3.2 Methods ....................................................................................................... 49 3.2.1 Participants ........................................................................................ 49 3.2.2 Regions of interest ............................................................................. 49 3.2.3 Stimuli ................................................................................................ 50 3.2.4 Design of the method of single stimuli ................................................ 51 3.2.5 Psychophysics ................................................................................... 53 3.2.6 Procedure .........................................................................................