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AN SSVEP-ACTUATED BRAIN COMPUTER INTERFACE USING PHASE-TAGGED
FLICKERING SEQUENCES: A CURSOR SYSTEM
Chairman : Dr. Hung-Chi Yang
Presenter: HSUAN-CHIA KUO
Adviser : Dr. Shih-Chung Chen
Date : 2013/12/25
PO-LEI LEE, JYUN-JIE SIE, YU-JU LIU, CHI-HSUN WU, MING-HUAN LEE, CHIH-HUNG SHU,PO-HUNG LI, CHIA-WEI SUN, and KUO-KAI SHYU
Annals of Biomedical Engineering, Vol. 38, No. 7, July 2010 (© 2010) pp. 2383–2397
OUTLINE
INTRODUCTION MATERIALS AND METHODS RESULTS CONCLUSIONS REFERENCES
INTRODUCTION
Patients suffering from severe motor disabilities, such as amyotrophic lateral scleroses (ALS)
Novel techniques allow users to control external devices or express their intentions independent of peripheral neuromuscular functions
INTRODUCTION
Among those proposed solutions, one promising technique, called brain computer interface (BCI)
This paper proposes a new SSVEP usesonly one Oz EEG channel for SSVEP recordings and employs a simple architecture for SSVEP extraction.
MATERIALS AND METHODS
Seven volunteers (Six males and one female), ages from 24 to 32 years.
MATERIALS AND METHODS
Application study I
Control Study Application Study
MATERIALS AND METHODS
Application study I
Came back 6 months later
MATERIALS AND METHODS
Application study II
More complicated application study !
CONTROL STUDY
Phase difference the predicted phase delay the detected phase lags
In the induced SSVEPs using averages of different epoch lengths.
APPLICATION STUDIES
Aimed to demonstrate the feasibility of the proposed system by inputting command sequences.
MATERIALS AND METHODS
Subject I
1-h experience
Visual stimulation
Other
Naïve subjects
MATERIALS AND METHODS
Six months later…
MATERIALS AND METHODS
Subject I and II
1.5-h experience
Other
0.5-h experience
MATERIALS AND METHODS
Pic 1. The schematic diagram of the proposed SSVEP-actuated BCI system
APPLICATION TASK I
Produce a sequence of eight cursor commands
ON
BL BR
OFF
Pic 2. Flickering LEDs
APPLICATION TASK II
ON
BL BR
OFF
3
Pic 3. Flickering LEDs
EEG RECORDINGS
Used only one bipolar EEG channel
One electrode (oz(+)) and (oz(−))
A ground electrode
Bandpass, 0.5–50 hz
Pic 4. Electrode Position
VISUAL STIMULI
Square wave Oscillating at 31.25 Hz
(32 ms duration for each ON–OFF cycle)
ON
BL BR
OFF
Pic 5. Flickering LEDs
VISUAL STIMULI
The ith LED flicker (LEDi) was set as:
θi = (i − 1) * 45°
Full-phase cycle (360°) with a ±22.5° phase margin.
VISUAL STIMULI
The flickering frequency is known as 31.25 Hz
The phase delay can be controlled by setting a time delay on the square wave generation:
VISUAL STIMULI
Pic 6. Visual Stimuli
SIGNAL PROCESSING OF SSVEP
SSVEP-based BCI
The flickering sequences: Set at 31.25 Hz Tagged with distinct phases
The Oz EEG signals: Band-Pass-Filtered between 29.25 and 33.25 Hz
SIGNAL PROCESSING OF SSVEP
LED1: Estimate the subject-specific phase lag
SSVEP ref The induced SSVEP from LED1 Averaging the epochs in the 1-min recording for
each subject
SSVEP gaze Epochs induced from each LED flicker Excluding LED1 Were averaged over 60 epochs No overlaps
SIGNAL PROCESSING OF SSVEP
Tref The latency of the maximum amplitude peak
Accomplished recognition of user’s gazed-target by: The phase lag between SSVEPgaze and the
SSVEPref
GAZED-TARGET IDENTIFICATION
Tpeak The latency of maximum amplitude peak in
SSVEPgaze Time lag (td):
Td = tpeak − tref
Θdetected:
Θd:
GAZED-TARGET IDENTIFICATION
Di:
The ith LED (flicker LEDi) with minimum angle distance Di is recognized as the gazed-target.
Pic 7. Oz EEG RECORDINGS
RESULTS
Pic 8. SSVEP-Based BCI Suing Phase Encoded Flickering Sequences
Pic 9. LEE
Pic 9. LEE et al
CONCLUSIONS
This work proposes a SSVEP-based BCI using phase-tagged flickering sequence to produce cursor commands for communication purposes.
Subjects shift their gazes at different LED flickers and phase information of the induced SSVEP is extracted for recognizing the gazed-targets.
CONCLUSIONS-FEATURES
SSVEP: Stable Reliable Noise can be removed by simply Bandpass Filter
Only one flickering frequency
Avoid interferences from low-frequency noise A more comfortable visualization.
REFERENCES [1] Basar, E. Brain functions and oscillation. In: Cross-
Modality Experiments on the Cat Brain, edited byE. Basar, T. Demiralp, M. Schurmann, and C. Basar-Eroglu. Berlin: Springer-Verlag, 1999, pp. 27–59.
[2] Baseler, H. A., E. E. Sutter, S. A. Klein, and T. Carney.The topography of visual evoked response propertiesacross the visual field. Electroencephalogr. Clin. Neurophysiol.90:65–81, 1994.
[3] Birbaumer, N., H. Flor, N. Ghanayim, T. Hinterberger,I. Iverson, E. Taub, B. Kotchoubey, A. Kubler, andJ. Perelmouter. A spelling device for the paralyzed. Nature398:297–298, 1999.
[4] Brown, B., and M. Z. Yu. Variation of topographic visuallyevoked potentials across the visual field. Ophthal.Physl. Opt. 17:25–31, 1997.