Development and evaluation of an online Transcranial Doppler Ultrasonographic brain- computer interface for communication by Jie Lu A thesis submitted in conformity with the requirements for the degree of Master of Applied Science Institute of Biomaterials and Biomedical Engineering University of Toronto © Copyright by Jie Lu 2013 Abstract Development and evaluation of an online transcranial Doppler ultrasonographic brain-computer interface for communication Jie Lu Master of Applied Science Graduate Department of Institute of Biomaterials and Biomedical Engineering University of Toronto 2013 We investigated an emerging brain-computer interface (BCI) modality, namely, transcranial Doppler ultrasonography (TCD), which measures cerebral blood flow velocity. We hypothesized that a bilateral TCD-driven online BCI would be able to dichotomously classify a user’s intentions with at least 70% accuracy. To test this hypothesis, we had three objectives: (1) to develop a signal classifier that yielded high (>80%) offline accuracies; (2) to develop an online TCD-BCI system with an onscreen keyboard; and, (3) to determine the achievable online accuracy with able-bodied participants. With a weighted, forward feature selection and a Naïve Bayes classifier, sensitivity and specificity of 81.44 ± 8.35% and 82.30 ± 7.39%, respectively, were achieved in the ii online differentiation of two mental tasks. The average information transfer rate and throughput of the system were 0.87 bits/min and 0.35 ± 0.18 characters/min, respectively. These promising online results encourage future testing of TCD-BCI systems with the target population. iii Acknowledgments First off, I would like to thank my wonderful and supportive parents, Yanping Lu and Aixue Jia, without whom I would not be where I am today. Also, great love goes to my younger sister, Luna Lu, who brings joy into my world. I would like to thank Dr. Tom Chau for his brilliant guidance and supervision along the way. His passion and energy has truly been inspirational. Many thanks to my committee members, Dr. Michelle Keightley and Dr. Elaine Biddiss, and external examiner, Dr. Anne-Marie Guerguerian for their suggestions and comments. Special thanks to Colleen Smith, Andrew Myrden, Larissa Schudlo, Dr. Saba Moghimi, Dr. Young Don Ko, and Dr. Khondaker Mamun for all of their kind help and wonderful company. I would also like to thank all members of the PRISM lab at Bloorview Research Institute for their support. Finally, I would like to acknowledge the financial support of donors of the Kimel Family Graduate Student Scholarship in Paediatric Rehabilitation, Holland Bloorview Kids Rehabilitation Hospital, donors of the James F. Crothers Family Fellowships in Peripheral Nerve Damage, and the Institute of Biomaterials and Biomedical Engineering at the University of Toronto. iv Table of Contents Abstract ................................................................................................................................ i Acknowledgments.............................................................................................................. iii Table of Contents ............................................................................................................... iv List of Figures .................................................................................................................... ix List of Tables .................................................................................................................... xii List of Appendices ........................................................................................................... xiii Chapter 1. Introduction ....................................................................................................... 1 1.1 Motivation ................................................................................................................. 1 1.2 Research Question and Objectives............................................................................ 2 1.3 Overview ................................................................................................................... 2 Chapter 2. Background ....................................................................................................... 4 2.1 Brain-Computer Interfaces ................................................................................... 4 2.2 Existing BCI Modalities ....................................................................................... 5 2.2.1 Intra-cortical BCIs ........................................................................................ 5 2.2.2 Cortical BCIs ............................................................................................... 5 2.2.3 Non-invasive BCIs ........................................................................................ 5 2.3 Transcranial Doppler Sonography ....................................................................... 6 v 2.3.1 Measuring Blood Flow Velocities ................................................................ 8 2.3.2 Hemodynamic Lateralization ........................................................................ 8 2.3.3 TCD-BCI Applications ................................................................................. 9 2.3.4 Ultrasound Probe Configuration ................................................................... 9 2.4 Communication Systems .................................................................................... 10 2.4.1 Existing Virtual Keyboards ......................................................................... 10 2.4.2 Dynamic Keyboard ..................................................................................... 10 2.5 Mental Tasks ...................................................................................................... 12 Chapter 3. Signal Processing and Classification Techniques to Optimize Transcranial Doppler Ultrasonography Performance ............................................................................ 14 3.1 Abstract .............................................................................................................. 14 3.2 Introduction ........................................................................................................ 15 3.3 Materials and Methods ....................................................................................... 17 3.3.1 Participants .................................................................................................. 17 3.3.2 Instrumentation ........................................................................................... 17 3.3.3 Mental Tasks ............................................................................................... 18 3.3.4 Experimental Protocol ................................................................................ 19 3.3.5 Pre-processing & Feature Extraction .......................................................... 20 3.3.6 Feature Selection ......................................................................................... 21 3.3.7 Classification .................................................................................................. 23 vi 3.4 Results ................................................................................................................ 24 3.4.1 Linear Discriminant Analysis ..................................................................... 24 3.4.2 Naïve Bayes ................................................................................................ 25 3.4.3 Features ....................................................................................................... 27 3.5 Discussion .......................................................................................................... 28 3.5.1 Classification of Word Repetition .............................................................. 28 3.5.2 Feature Selection ......................................................................................... 29 3.5.3 Classifier ..................................................................................................... 30 3.6 Conclusion .......................................................................................................... 30 Chapter 4. Online Transcranial Doppler Ultrasonographic Control of an Onscreen Keyboard ........................................................................................................................... 31 4.1. Introduction ........................................................................................................ 32 4.2. Methods .............................................................................................................. 34 4.2.1. Participants .................................................................................................. 34 4.2.2. Instrumentation ........................................................................................... 34 4.2.3. Mental tasks ................................................................................................ 35 4.2.4. Dynamic Keyboard ..................................................................................... 36 4.2.5. Experimental Protocol ................................................................................ 38 4.2.6. Data Processing and Classification ............................................................. 40 4.2.7. Performance evaluation .............................................................................