Study of Change Blindness EEG

Synchronization using Wavelet Coherence

Analysis

Professor: Liu

Student: Ruby

Motivation & Purpose

• Motivation – The authors wanted to know the brain activity

during the visual detect the change.

• Purpose– Using a wavelet coherence analysis for test the

multi-channel EEG signals when people do the change detection and change blindness.

References

• Two explanations for change blindness:– Visual memory can encoding the information, b

ut can not record all changes.– Visual memory has limitation during the pictur

es was changed. (Markazi et al., 2005)

• Comparing brain activity during change detection and change blindness. (Beck et al., 2001)

The change blindness experiment

• Participants – 9 people whose age from

19-55 years old, have normal vision.

• Material – The topic of the pictures

were faces and places.– EEG signals were taken

1000 Hz from 32 electrodes, 10-20 system.

The change blindness experiment

• Procedure (Main task was to detect a change in the pictures between the two stimulus.)– Step 1: first stimulus included two pictures, display for

500 ms.– Step 2 : 500ms blank display.– Step 3 : second stimulus included two pictures, display

for 500ms.– Step 4 : 500ms blank display.

• Second task was search the letter which was showed at the top and bottom of the screen.

Wavelet coherence method

• The wavelet transform used for investigating the basic characteristics of the non-stationary EEG signals.

• Using the continuous Wavelet Transform (CWT).

• The CWT of a signal x(u) is a function of time (τ) and frequency (f).

Wavelet coherence method

• The continuous wavelet transform of a signal x(u) is a function of time (τ) and frequency (f).

• Choosing the Morlet wavelet.

• The wavelet cross-spectrum between x and y.

• wavelet coherence WCo (t,f) is defined as:

Wavelet coherence analysis

• The length of each epoch was 2000 ms.

• Epochs were classified into four types:

(i) Hit (H)

(ii) Miss (M)

(iii) No- change (correct detection)

(iv) False alarm.

• Only focused on the miss and hit.

Wavelet coherence analysis

Fig. 2: (a) Wavelet coherence map of one epoch for

miss trial between channel FP1 and F3. (b) Wavelet coherence map of one epoch fo

r hit trial between channel FP1 and F3.

The higher coherence

value, the more the

two channels are

synchronized which is

presented as dark areas.

Wavelet coherence analysis

Less degree of coherency comparing to Fig.2.

Fig. 3:(a) Wavelet coherence map of one epoch for

miss trial between channel P8 and C3. (b) Wavelet coherence map of one epoch for

hit trial between channel P8 and C3.

Wavelet coherence analysis• WCO of hit and miss trial were significant differences.

• Fig. 4: P-value maps for WCO of hit and miss trial with p-

value<0.01

(a) The p-value calculated for the pair channels FP1 and F3.

(b) The p-value calculated for the pair channels P8 and C3.

Conclusion

• The frontal electrodes showed higher differences between WCO of hit and miss trials.

• There are different degrees of cross synchronicity and information exchange patterns during visual cognitive activity.