Fixations and Saccades in Eye Tracking

A recent blog post on eye tracking brings to light a variable that many researchers who use eye tracking technology have to deal with throughout their studies. When using gaze or eye movement for research, you must distinguish between fixations and saccades. And to understand this difference, we have to look at the physical structure of the eye itself.

The fovea centralis, also known simply as the fovea, is a part of the eye located at the center of the macular region of the retina. It is responsible for sharp central vision (foveal vision) – a characteristic in humans that enables us to read, watch television, drive, watch a movie, or play a video game. Foveal vision comes into play in any human activity where visual detail is of primary importance. In fact, it’s the only part of the human eye that permits 100% visual activity. Your “line of sight” is an imaginary line that connects the fovea with a particular point of fixation in the outside world. It’s where you’re looking. Actually, Leonardo Da Vinci was responsible for distinguishing between foveal vision – your precise point of focus – and peripheral vision, the part of vision that occurs outside the center of the human gaze.

A fixation occurs when our foveal attention lingers on a particular object; the movements of our eyes from one fixation to another are known as saccades. Our readers familiar with Tobii’s eye tracking systems might know that an eye tracking-results graph with dots and lines is representative of the subject’s fixations and saccades – the fixations are the dots where vision was focused (even for a millisecond) and the lines connecting them are the saccades.

Movements between fixations happen subconsciously. As the human eye scans over the scene or image, the foveal focus shifts about 25 times per second, darting from place to place and taking in the disparate parts of what lay in its field of vision. The movements and information combine, and the brain forms a cohesive vision of the scene. Where the eyes dart to and from is based on many varying qualities of the scene – what you are interested in. It could be color, a shape, a body, a form, or an object – really anything that catches the eye and, understandably, our attention.

The blog entry cites MIT’s Dr. Earl Miller who, knowing that the eye shifts from spot to spot subconsciously, decided to study the timing of the shifts and what exactly causes them. Miller found that human brains appear to have a built-in timer that keeps their eyes moving, and that the foveal focus seems to be regulated by brain waves. As you can imagine, there are periods of high activity and periods of low activity, and Miller’s study shows “a conclusive link between these wave cycles and the refocusing of visual attention. It appears that our brains have a built in metronome that dictates how we engage with visual stimuli. The faster the cycles, the faster we “think.”

How Our Brain Decides How Long We Look at Something