AN SSVEP-ACTUATED BRAIN COMPUTER
INTERFACE USING PHASE-TAGGED
FLICKERING SEQUENCES: A CURSOR SYSTEM
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
Chairman ：Dr. Hung-Chi Yang
Presenter：HSUAN-CHIA KUO
Adviser ：Dr. Shih-Chung Chen
Date
：2013/12/25
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 uses
only 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
Other
1-h experience
Naïve subjects
Visual stimulation
MATERIALS AND METHODS
Six months later…
MATERIALS AND METHODS
Subject I and II
Other
1.5-h experience
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
OFF
BL
BR
Pic 2. Flickering LEDs
APPLICATION TASK
II
ON
OFF
BL
BR
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
OFF
BL
BR
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
SSVEPgaze
of
maximum
Time lag (td)：
Td = tpeak − tref
 Θdetected：


Θd：
amplitude
peak
in
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 gazedtargets.
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




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Basar-Eroglu. Berlin: Springer-Verlag, 1999, pp. 27–
59.
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Carney.The topography of visual evoked response
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Kubler, andJ. Perelmouter. A spelling device for the
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Ophthal.Physl. Opt. 17:25–31, 1997.