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.

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[4] Brown, B., and M. Z. Yu. Variation of topographic visuallyevoked potentials across the visual field. Ophthal.Physl. Opt. 17:25–31, 1997.