The McGill Physiology Virtual Lab

Biological Signals Acquisition

EOG tests > saccades

Saccadic eye movements (saccades) are generated to move the eye rapidly to a specific target of interest in visual space.

The word saccade originated from the French term for "jerk", named so to reflect the great speed at which these eye movements occur. Eye movements are generally described in terms of the angle (in degrees) that the eye has rotated. Saccades last for only a fraction of a second and can reach speeds up to 900 deg/s.
Saccades are under voluntary control and can be made in the dark, or with the eye closed. During our daily activities, we constantly generate saccadic eye movements to scan our visual surroundings. When viewing an object of interest, we make saccadic eye movements to specific features of the image in order to analyze it; in the case of the human face, saccades are made primarily to areas containing the eyes (see figure to the right).
(from Yarbus, "Eye Movements and Vision", Plenum Press, NY, 1967)

Likewise while reading we generate small saccadic eye movements to move from word to word, phrase to phrase and line to line.

Saccades/Calibration Curve: Procedure
Eye movements are measured in degrees. However the signal that you will record from the EOG electrodes will be in volts. In order to determine the relationship between the amplitude of the measured voltage and the actual eye movement generated by the subject, it is necessary to construct a calibration curve.
  • The subject sits 7 inches from the monitor and the "target" option is chosen

  • First the leftward targets: The subject fixates on the center ball (blue), and then focuses on the red balls as they appear at -5, -10, -20, -30 and -40 degrees. After this trial is completed, the data is saved.

  • Then the rightward targets: The subject focuses on the central blue ball, and then the red balls as they appear at 5, 10, 20, 30 and 40 degrees . After this trial is completed, the data is saved.

Sample Targets

Saccades/Calibration Curve: Results

Saved Data for Leftward Moving Targets

Saved Data for Rightward Moving Targets

We now have a set of data that relates voltage to the actual eye position. From this data we will construct a calibration curve to obtain the calibration factor or coefficient, which we can use to convert voltages obtained during various eye tests to the actual displacement in degrees the eye made.
Degrees -40 -30 -20 -10 -5 0 +5 +10 +20 +30 +40
Voltage 0.752 0.767 0.78 0.79 0.793 0.8 0.808 0.811 0.82 0.832 0.844
The graph should be linear over most of the range -40 to +40 degrees. After fitting the best fit line, the slope is determined. For this example,

the slope or calibration factor = 0.0011 volts/degree.

To continue with the next topic, Smooth pursuit, click here