Introduction

Eye tracking

Eye tracking is an established method to do research on a wide range of cognitive processes, like perception, attention or social interaction. Our eye tracking is based on video recordings, and features to be tracked are the pupils and infrared light that is reflected from the cornea.
In reading experiments, for example, participants are to read simple sentences while cameras record their eye movements. Straight lines, skips, curves and regressions show strategies that readers develop to perceive, understand and memorize sentences. The following kinds of eye movements during reading can occur: saccades and fixations.
Six small muscles are responsible for moving each eyeball horizontally and vertically. Contrary to subjective perception, eyes never move smoothly, unless they directly follow the movement of an object. Their natural mode of movement is to rapidly move from one fixated point to another. This movement is called a saccade. The duration of a saccade ranges from ca.15 ms up to 100 ms. The time between the appearance of a visual stimulus and the onset of the saccade is called saccade latency. During a saccade, the sensitivity for visual input is greatly reduced, which can cause phenomenons such as change blindness.

In between saccades fixations occur, which last for approximately 200 ms while reading. During a fixation the eye is almost at rest, to allow for a word being processed, for instance. However, our eyes are never completely static. They do perform miniature movements at all times.
         
The focus area of the retina, called fovea, is actually quite small. The visual accuity decreases with increasing distance to the fovea. This area of blurred vision is called parafovea. In order to counter the effects of this, our eyes compensate by moving at all times, but we still get the impression of a clear and stable field of vision.

What happens while reading?

What controls eye movements? Are there different types of readers? What strategies do they pursue? The main objectives of our eye movement experiments are the theories regarding dynamic eye movement control while reading.
The experimental question asked determines which eye trackers are being used. For reading experiments a sample rate of > 50 Hz is necessary to be used. For example, while reading two sample words like "the eye", a high spatial resolution is essential to decide whether a participant looks at ìtheî or ìeyeî and which letter within the word he fixates.