Segmentación de mapas de amplitud y sincronía

para el estudio de tareas cognitivas

Alfonso Alba1, José Luis Marroquín2, Edgar Arce1

1 Facultad de Ciencias, UASLP2 Centro de Investigación en Matemáticas

IntroductionElectroencephalography (EEG) consists of voltage measurements recorded by electrodes placed on the scalp surface or within the cortex.

Electrode cap

Varela et al., 2001

• During cognitive tasks, several areas of the brain are activated simultaneously and may even interact together.

EEG synchrony dataSynchrony is measured at specific frequency bands for a given pair of electrode signals.

Typical procedure: Band-pass filter electrode signals Ve1(t) and

Ve2(t) around frequency f. Compute a correlation/synchrony measure

f,t,e1,e2 between the filtered signals Test the synchrony measure for statistical

significance

In particular, we obtain a class field cf,t,e1,e2 which indicates if synchrony was significantly higher (c=1), lower (c=-1) or equal (c=0) than the average during a neutral condition.

Visualization (Figure categorization experiment)

The field cf,t,e1,e2 can be partially visualized in various ways:

Multitoposcopic display of the synchronization pattern (SP) at a

given time and frequency

Time-frequency (TF) map for a given electrode pair (T4-O2)

Time-frequency-topography (TFT) histogram of synchrony increases at

each electrode

• The TFT histogram shows regions with homogeneous synchronization patterns. These may be related to specific neural processes.

Seeded region growingTF regions with homogeneous SP’s can be segmented using a simple region growing algorithm, which basically:

1. Computes a representative synchrony pattern (RSP) for each region (initially the SP corresponding to the seed).

2. Takes a pixel from some region’s border and compares its neighbors against the region’s RSP. If they are similar enough, the neighbors are included in the region and the RSP is recomputed.

3. Repeats the process until neither region can be expanded any further.

Region growing (Figures experiment)

Automatic seed selection

An unlabeled pixel is a good candidate for a seed if it is similar to its neighbors, and all of its neighbors are also unlabeled.

To obtain an automatic segmentation, choose the seed which best fits the criteria above, grow the corresponding region, and repeat the procedure.

Bayesian regularizationThe regions obtained by region-growing show very rough edges and require regularization.

We apply Bayesian regularization by minimizing the following energy function:

lt,f is the label fieldLt,f is a pseudo-likelihood functionNs is the number of electrode pairsV is the Ising potential functiont and f are regularization parameters

Results (Figure categorization experiment)

Automatic segmentation Regularized segmentation

Results (Figure categorization experiment)

Results with induced amplitude

Region optimization

Merge regions with similar RSP’s Two regions i and j are merged if

Delete small regions After merging, regions whose area is

smaller than some d are deleted.

mji

ji

HCHC

RSPRSPd

),(

Region optimization example

Region optimization example

Conclusions We have developed a visualization system for EEG dynamics which

Produces detailed representations of synchrony and amplitude patterns that may be relevant to the task.

Helps neurophysiologists determine TF regions of possible interest.

Can be fully automated and allows for human interaction.

Future work

Validation

Use of segmented maps for the study of a psychophysiological experiment.

Segmentation using combined amplitude+synchrony data?

Homer says thank you!