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Spectral Components of the P300 Speller Response in Electrocorticography

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Spectral Components of the P300 Speller Response in Electrocorticography Proceedings of the 5th International FrA1.4 IEEE EMBS Conference on Neural Engineering Cancun, Mexico, April 27 - May 1, 2011 Spectral Components of the P300 Speller Response in Electrocorticography Dean J. Krusienski, SeniorMember, IEEE , and Jerry. J. Shih Abstract —Recent studies have demonstrated that the P300 and showed predictable time courses in the delta, theta, and Speller can be used to reliably control a brain-computer alpha bands. With the increased spatial resolution and interface via scalp-recorded electroencephalography (EEG) and spectral bandwidth afforded by ECoG, and the recent intracranial electrocorticography (ECoG). However, the exact association of gamma band features with a wide variety of reasons why some disabled individuals are unable to cortical processes [7][23][32], an examination of the spectral successfully use the P300 Speller remain unknown. The characteristics of the P300 Speller response using ECoG is superior bandwidth and spatial resolution of ECoG compared justified. This paper provides a preliminary spatio-temporal to EEG may provide more insight into the nature of the responses evoked by the P300 Speller. This study examines the analysis of the P300 Speller responses in the conventional spatio-temporal progression of the P300 Speller response in delta, theta, alpha, beta, low-gamma, and high-gamma bands. ECoG for six conventional frequency bands. II. METHODOLOGY I. INTRODUCTION A. Subjects BRAIN -COMPUTER INTERFACE (BCI) is a device that Six subjects with medically intractable epilepsy who Auses brain signals to provide a non-muscular underwent Phase 2 evaluation for epilepsy surgery with communication channel [34], particularly for individuals with temporary placement of intracranial grid or strip electrode severe neuromuscular disabilities. One of the most arrays to localize seizure foci prior to surgical resection were investigated paradigms for controlling a BCI is the P300 tested for the ability to control a visual keyboard using ECoG Speller [10]. Based on multiple scalp-recorded signals. All six subjects were presented at Mayo Clinic electroencephalogram (EEG) studies in healthy volunteers Florida’s multidisciplinary Surgical Epilepsy Conference [13][15][18][25], and initial results in persons with physical where the consensus clinical recommendation was for the disabilities [24][28][33], the P300 Speller has the potential to subject to undergo invasive monitoring primarily to localize serve as an effective communication device for persons who the epileptogenic zone, and also to map out language and have lost or are losing the ability to write and speak. sensorimotor cortex if appropriate. The study was approved More recently, studies have shown that both intracranial by the Institutional Review Boards of Mayo Clinic, electrocorticography (ECoG) and depth electrodes can be University of North Florida, and Old Dominion University. used to reliably control the P300 Speller, with performance All subjects gave their informed consent. comparable to EEG [6][16][17]. ECoG has been shown to B. Electrode Locations and Clinical Recordings have superior signal-to-noise ratio, immunity to artifacts such as EMG, and spatial and spectral characteristics compared to Electrode (AD-Tech Medical Instrument Corporation, WI, EEG [5][11][26]. It is believed that a thorough USA) placements and duration of ECoG monitoring were characterization of the P300 Speller response properties in based solely on the requirements of the clinical evaluation, without any consideration of this study. All electrode ECoG will also lead to improved scalp-EEG P300 placements were guided intra-operatively by Stealth MRI performance. Several ECoG studies have attempted to characterize neuronavigational system (Medtronics, Inc., MN, USA). various event-related potentials [14][21][22][30], and a few Each subject had post-operative anterior–posterior and lateral recent studies have explored the responses to the P300 radiographs to verify electrode locations. After electrode Speller [6][16][17]. Because the traditional control signal for implantation, all subjects were admitted to an ICU room with epilepsy monitoring capability. Clinical ECoG data were the P300 Speller is an event-related potential (ERP), the gathered with a 64-channel clinical video-EEG acquisition majority of prior analyses have focused on low frequency features in the time domain. Bernat et al. (2007) [3] system (Natus Medical, Inc.; CA, USA). Electrode locations performed a time-frequency decomposition of P300 activity are detailed in Table 1, with the superposition of approximate electrode locations of all subjects illustrated in Figure 1. C. BCI Data Acquisition This work was supported by the National Institutes of Health (NIBIB/NINDS EB00856) and the National Science Foundation (0905468/ All subjects performed BCI testing between 24-48 hours 1064912). after electrode implantation. Testing was performed only D.J. Krusienski is with the Dept. of Electrical and Computer Engineering when the subject was clinically judged to be at cognitive at Old Dominion University, Norfolk, VA 23529 (corresponding author e- baseline and free of physical discomfort that would affect mail: [email protected]). J.J. Shih is with the Department of Neurology at the Mayo Clinic, attention and concentration. Testing was performed at least Jacksonville, FL 32224. 978-1-4244-4141-9/11/$25.00 ©2011 IEEE 282 six hours after a clinical seizure. Stimuli were presented and times the target character flashed, until a new character was the ECoG data were recorded using the general-purpose BCI specified for selection. All data was collected in the copy system BCI2000 [27]. All electrodes were referenced to a speller mode: words were presented on the top left of the scalp vertex electrode, amplified, band pass filtered (0.5–500 video monitor and the character currently specified for Hz), digitized at 1200 Hz using 16-channel g.USB amplifiers selection was listed in parentheses at the end of the letter (Guger Technologies, Graz, Austria), and stored. A laptop string as shown in Figure 2. Each session consisted of 8-11 with a 2.66 GHz Intel Core 2 Duo CPU, 3.5 GB of RAM, and experimental runs of the P300 Speller paradigm; each run Windows XP was used to execute BCI2000. The signals for was composed of a word or series of characters chosen by the the BCI experiments were acquired concurrent with the investigator. This set of characters spanned the set of clinical monitoring via a 32-channel electrode splitter box characters contained in the matrix and was consistent for (AD-Tech Medical Instrument Corporation, WI, USA). each subject and session. Each session consisted of between 32-39 character epochs. A single session lasted approximately one hour. One to three sessions were Age Electrode # BCI Sub /Sex Locations Elect collected for each subject, depending on the subject’s 36-contact grid left physical state and willingness to continue. A 41/M temporal-occipital 16 1 x 4 left temporal strip Four 1x6 strips covering left B 29/M frontal and lateral temporal 26 2 left hippocampal single depths C 20/F 24-contact grid left parietal 24 36-contact grid left frontal-parietal D 27/M 32 2 left hippocampal single depths 16-contact grid left temporal-parietal 1 x 4 superior temporal gyrus strip E 60/M 1 x 6 inferior temporal gyrus strip 32 1x6 inferior frontal strip 2 left hippocampal single depths 20-contact grid left temporal neocortex F 38/F 30 1 x 4 left inferior frontal 1 x 6 left medial and basal temporal Table I. The demographics, electrode locations, and number of electrodes used for the BCI experiments for each subject. Fig. 2: The 6x6 matrix used in the current study. A row or column intensifies for 100 ms every 175 ms. The letter in parentheses at the top of the window is the current target letter “D.” A P300 should be elicited when the fourth column or first row is intensified. After the intensification sequence for a character epoch, the result is classified and online feedback is provided directly below the character to be copied. E. Data Analysis The first uncorrupted session from each subject was used for the analysis. The band-power was determined for six frequency bands: delta (0-3 Hz), theta (4-7 Hz), alpha (8-12 Hz), beta (13-30 Hz), low-gamma (31-70 Hz), and high- gamma (71-200 Hz). The band-power was computed by forward and reverse filtering each channel with a precise FIR filter to produce zero-phase distortion. The resulting channel values were then squared and smoothed using the same zero- phase approach to extract the band-power envelope. For each channel, 800 ms segments of data following each Fig. 1. The superposition of approximate ECoG electrode locations for all flash were extracted for the analysis. Each segment was six subjects. divided into non-overlapping 100 ms partitions and the average band-power for each of the six frequency bands was D. Task, Procedure, & Design computed for each partition. The average band-power was The experimental protocol was based on the protocol used then correlated with the binary target labels (i.e., target or in an EEG-based P300 Speller study [15]. Each subject sat in non-target flash) and the resultant p-values were computed, a hospital bed about 75 cm from a video monitor and viewed where the p-value corresponds to the hypothesis that the the matrix display. The task was to focus attention on a correlation between the band-power and the task is zero. The specified letter of the matrix and silently count the number of p-values were transformed using –log(p-value) to make the 283 Fig. 3. The transformed p-value topographies for each of the six frequency bands over the 100 ms increments (centered on the values provided on the x- axis). The corresponding color-scale begins fading at a significance level of p = 0.05, which corresponds to approximately a value of 3 on the color- scale.
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