COGNITIVE PROCESSES OF LETTER AND DIGIT IDENTIFICATION by Teresa Marie Schubert A dissertation submitted to Johns Hopkins University in conformity with the requirements for the degree of Doctor of Philosophy Baltimore, Maryland January, 2015 © Teresa Marie Schubert 2015 All Rights Reserved ABSTRACT Letter identification, the process of determining letter identities and their positions from a visual stimulus, has a long research history. Debate remains concerning the levels of representation involved, as well as the manner in which letter position is coded: letters must be specified at particular positions. Digit identification, a cognitive process with arguably parallel goals to letter identification, has been less studied. A central question concerns overlap between letter and digit identification processes: Given the similarities between digits and letters as visual symbols, do they share a single identification process or does processing differ based on stimulus category? This thesis combines two investigations, concerning the letter position code employed in identification, and the extent of overlap of digit and letter processing, including variables determining letter and digit identification speed, accuracy, and error patterns. To address the first topic, I elicit letter perseveration errors in a character identification task, seeking evidence for the type of position code which can successfully predict both letter position specificity and flexibility in word recognition. Consistent with prior results from other paradigms, conclusions from this study suggest a key role for representing letters based on their distance from both the beginning and end of the word. Addressing the second topic, I explore the ability of character frequency, visual similarity, and visual complexity of letters and digits to predict a categorical distinction in identification performance. In behavioral experiments with unimpaired readers, I elicit identification errors for mixed letter and digit strings and same/different judgment response times to pairs of characters, providing a rich empirical dataset on which to further test the influence of these variables. The results have implications for our knowledge of letter and digit processing, and are ii consistent with a shared identification system in which both character types are identified. Together these investigations inform cognitive processes of letter and digit identification, which is the first stage in the reading process. Reading is a skill which is remarkably automatic, yet not innate, and the study of reading introduces broad questions about the nature of cognitive and neural processes that evolved to solve other problems in visual cognition. Thesis Committee Michael McCloskey (primary advisor), Cognitive Science Brenda Rapp, Cognitive Science Colin Wilson, Cognitive Science Howard Egeth, Psychological and Brain Sciences Marina Bedny, Psychological and Brain Sciences iii ACKNOWLEDGMENTS The completion of this dissertation would not have been possible without the support of numerous people; I mention only the most salient here. I would like to thank Mike and Brenda, who never stopped asking hard questions and pointing out the ugly facts to ruin my beautiful theories. I could not have asked for better mentors. Thanks also to my other committee members, Colin, Marina, and Howard, for insightful questions about my research and productive discussions of big picture issues. Thanks to my parents, Margaret and Peter, and my sister, Christina, for always believing in me (and teaching me how to make chocolate cake along the way). Thanks to Darko, Laura, and Megan for beautiful friendships near and far. Thanks to my officemates, past and present (but especially past, but especially present), for keeping me sane with coffee, chocolate, and snowflake breaks. All remaining stupid questions will be sent via email. Thanks to Bob, Dan, and Chris for always being on-call to play board games, often with very little notice. And lastly, thank you to my husband, Carter, for his unwavering faith in me through everything. I would like to dedicate this dissertation to my brother Joe, whose life inspired me in countless ways. iv TABLE OF CONTENTS ABSTRACT ....................................................................................................................... ii ACKNOWLEDGMENTS .............................................................................................. .iv TABLE OF CONTENTS ................................................................................................. v LIST OF TABLES ............................................................................................................ x LIST OF FIGURES ......................................................................................................... xi CHAPTER 1: INTRODUCTION .................................................................................... 1 Overview of the dissertation ........................................................................................ 3 CHAPTER 2: LETTER IDENTIFICATION ................................................................ 5 Goals of letter identification ........................................................................................ 7 Invariance over visual forms ................................................................................ 8 Representing position ......................................................................................... 10 Invariance over position ..................................................................................... 10 Theories of letter identification ................................................................................. 12 Bigram-Based Theories of Letter Identification ................................................. 13 Letter-Based Theories of Letter Identification ................................................... 17 Evidence for stored letter-form representations ........................................................ 21 Evidence from unimpaired participants .............................................................. 23 Evidence from neuropsychological studies ........................................................ 26 Evidence from LHD ........................................................................................... 27 Computational modeling .................................................................................... 33 Revised Caramazza & Hillis theory of letter identification ....................................... 35 v Position representation in letter identification ........................................................... 36 Letter string similarity ........................................................................................ 37 Empirical evidence for flexible position coding ................................................ 39 Reconciling graded both-edges position coding with previous results ..................... 50 Summary: Letter identification theory under consideration ...................................... 52 CHAPTER 3: DIGIT IDENTIFICATION AND RELATIONSHIP TO LETTER IDENTIFICATION ........................................................................................................ 54 Digit identification ..................................................................................................... 54 Theories of digit identification .................................................................................. 54 Shared letter and digit identification .................................................................. 57 Separate letter and digit identification ................................................................ 59 Shared identification systems .................................................................................... 64 McCloskey and Schubert (2014) ........................................................................ 65 Review of evidence for shared system ............................................................... 68 Reconciling differential digit and letter accuracy with shared identification ............ 77 Digits comprise a smaller set .............................................................................. 78 Differential role of semantics and/or the right hemisphere ................................ 79 Within-set visual similarity ................................................................................ 80 Character frequency differences ......................................................................... 82 Summary of hypotheses ..................................................................................... 84 Overview of Studies 1-3 ..................................................................................... 84 vi CHAPTER 4: EXPLORING HYPOTHESES FOR LETTER AND DIGIT ACCURACY DISCREPANCY (STUDY 1) ................................................................. 86 Methods ..................................................................................................................... 88 Frequency ........................................................................................................... 88 Visual similarity ................................................................................................. 89 Visual complexity ............................................................................................... 92 Discriminability .................................................................................................