**2.1 Retina–cornea voltage source**

The EOG measurement is based on the voltage measurement that depends on the eye orientation (Fig. 1). The voltage between retina–cornea is about ±1 mV [Northrop (2002); Schlgöl et al. (2007)], which is a very high value in comparison to other biosignals, but it also depends on numerous factors, for instance, the light conditions [Denney & Denney (1984)], the contact between electrodes and skin, which is the source of amplitude instability [Augustyniak (2001); Krogh (1975)].

One of the advantages of the EOG measurements over other techniques is that the field–of–view is not reduced by the glasses that are used as mounting platform for video or other sensors and illuminators. The electrodes are placed around eyes and are distant for the eye safety reasons.

Fig. 1. Model of retina–cornea measurements for two different orientations

### **2.2 Measurement system configurations**

There is no single unique electrodes placement, so many configurations are possible and used in practice [Brown et al. (2006)]. Such situation complicates the comparison of different algorithms used by researchers. The properties of the EOG signal change depending on the placement of electrodes, so the estimation technique should be robust. Additional calibration is necessary if the precise estimation is expected.

There are a few electrode configurations (Fig. 2) and in this chapter the 3/4 configuration is assumed: three main electrodes, for two differential measurements: LEFT–UP and RIGHT–UP, and an additional fourth reference electrode (REF).

The 3/4 configuration is minimal one for the full estimation of eye orientation in both directions and blinking too. Further reduction is possible if the single orientation is sufficient for a specific application. Another configuration 7/8 is the maximal variant [Thakor (1999)] that allows the measurement of very precise movement including every eye separately. It 2 Will-be-set-by-IN-TECH

possibilities of application of the HCI system based on EOG depend on the biosignal measurement system and digital signal processing techniques applied to the obtained signals.

The EOG measurement is based on the voltage measurement that depends on the eye orientation (Fig. 1). The voltage between retina–cornea is about ±1 mV [Northrop (2002); Schlgöl et al. (2007)], which is a very high value in comparison to other biosignals, but it also depends on numerous factors, for instance, the light conditions [Denney & Denney (1984)], the contact between electrodes and skin, which is the source of amplitude instability

One of the advantages of the EOG measurements over other techniques is that the field–of–view is not reduced by the glasses that are used as mounting platform for video or other sensors and illuminators. The electrodes are placed around eyes and are distant for the

**retina**

**cornea**

**V**

E**RC**

>0V

**2. Electrooculography signal**

**2.1 Retina–cornea voltage source**

[Augustyniak (2001); Krogh (1975)].

E**RC**

**retina**

**cornea**

**V**

~0V

is necessary if the precise estimation is expected.

and an additional fourth reference electrode (REF).

**2.2 Measurement system configurations**

Fig. 1. Model of retina–cornea measurements for two different orientations

There is no single unique electrodes placement, so many configurations are possible and used in practice [Brown et al. (2006)]. Such situation complicates the comparison of different algorithms used by researchers. The properties of the EOG signal change depending on the placement of electrodes, so the estimation technique should be robust. Additional calibration

There are a few electrode configurations (Fig. 2) and in this chapter the 3/4 configuration is assumed: three main electrodes, for two differential measurements: LEFT–UP and RIGHT–UP,

The 3/4 configuration is minimal one for the full estimation of eye orientation in both directions and blinking too. Further reduction is possible if the single orientation is sufficient for a specific application. Another configuration 7/8 is the maximal variant [Thakor (1999)] that allows the measurement of very precise movement including every eye separately. It

eye safety reasons.

Fig. 2. Different measurement systems: 3/4, 4/5, and 7/8

is important especially for the medical purposes. The configuration 4/5 is the compromise between both mentioned configurations. The large number of electrodes reduces the long time reliability due to the degradation of skin contact. The reduced number of electrodes is preferred for the applications where the touching of the human face is possible. The wires located especially below eyes in the 4/5 and 7/8 configuration is one of disadvantage. The 3/4 configuration allows the placement of electrodes in less visible parts of a face.

The number of electrodes depends on the acquisition systems. The first number represents the number of active electrodes used for measurements and the second number is the total number of electrodes. The additional electrode (the number 4 in the 3/4 configuration) is the reference electrode (REF). The acquisition systems typically use differential inputs. Such input type is preferred due to better SNR. Two channels are used and the first one is the LEFT–UP and the second one is the RIGHT–UP. The example signals are shown in Fig. 3. High impedance inputs are necessary due to the high resistance of the voltage source. The additional suppression of the 50/60 Hz interference is necessary [Prutchi & Norris (2005)], because the power lines are the source of the biosignal disturbances for the EOG signal, which has bandwidth about 200 Hz. Filtering techniques and appropriate wiring are used for the reduction of power lines interference. High frequency power line interference is omitted if a measurement system has the low–pass properties. The main source of high frequency interference is an incandescent light source. The power line interference is additive to a biosignal, especially, if the measurement systems wires are not shielded. Some portion of the power line interference occurs between electrodes on the human body.

The long time stability of the skin contact is obtained using the adhesive electrodes or an electrogel. The Ag/Ag–Cl electrodes are used typically. Such electrode types are conductive, but the other types (e.g. capacitance–based) are also used. The conductive electrodes support a DC signal.

#### **2.3 Measurements settings and signal processing**

The EOG biosignal needs a much higher sampling rate in comparison to other biosignal measurement systems. The sampling rate should be about a few hundreds samples per second. The low–pass filtering property of the measurement system is necessary. The AC coupling application used for the suppression of the DC signal is not correct. The DC level and low–frequency components corresponds to the eye orientation. The band–pass filtering in some measurement systems makes the differentiation of signals and the correction of measurements very hard or not possible.

**3. Signal properties**

Fig. 4. Example of blinking pulses

typical blinking is assumed.

**3.2 Saccades**

**3.3 Smooth pursuits**

movements are named as smooth pursuits.

movements of eyes but in the smaller scale.

The EOG signal has one important artifact and it is the blinking signal. The blinking occurs if the eyelid makes movements (vertical one). The disturbance depends on the electrode

Real–Time Low–Latency Estimation of the Blinking and EOG Signals 317

4 5 6 7 8 9 **time [s]**

There are also atypical blinking cases when the eyelid moves very slowly or if the eyelid closing time is different than the eyelid opening time. There are much more cases when blinking is non–Gaussian, but they are not considered in typical systems. In this chapter

The saccade is the rapid change (Fig. 5) of eye orientation [Becker (1989); Gu et al. (2008); Mosimann et al. (2005)]. The rapid changes require the acquisition of high frequency components of a signal. This is one of the reasons why the necessary sampling rate is higher in comparison to the other systems. The low–pass filtering for the removal of the 50/60 Hz

There are also interesting cases, for instance, when the blinking is near to the saccades. This situation is not rare in real measurements, but it is very often not considered by researchers.

During the tracking of a slowly moving object, the eyes move smoothly (Fig. 6). Such

There are also other features of the EOG measurements, like the microsaccades that are rapid

The blinking pulse is similar to the Gaussian pulse for typical blinking (Fig. 4).

component using the cut of frequency about 30–40 Hz disturbs a saccade signal.

configuration and it is additive for the 3/4 configuration.

**3.1 Blinking**

Fig. 3. Example of the EOG signal – two channels for the 3/4 configuration

The reliable measurements should be obtained using the flat band–pass filters (e.g. the Buttherworth filter), but the phase distortions are introduced by the analog filters. The combined filtering of a signal using the analog and digital filters and higher sampling rates is necessary.

The number of quantization levels (the number of bits per sample) for EOG should be carefully set depending on the DC level processing. Even a 8–bit per sample is enough for not demanding applications with correct DC level maintenance and 50/60 Hz interference suppression before sampling. The higher resolution of quantization is used (e.g. 12–16 bits per sample) if both mentioned components are hard to control by electronics. The higher resolution of quantization allows the signal processing using the digital signal processing algorithms.

The higher sampling rate and better quantization create the possibilities of precise observation of the EOG signal, which is important especially for the medical purposes.

The calibration of the system is necessary. A few extreme orientations of eyes are used and the intermediate orientations are interpolated. The HCI system requires the calibration before the beginning of measurements. The non real–time applications support the additional calibration at the end and intermediate calibration if they are necessary. More than single calibration allow the correction of the measurements and improve the acquisition results.

High quality measurements are recommended, but the signal processing of the obtained biosignal is necessary for the separation of the EOG and blinking signals. The estimation of parameters for both signals is necessary.
