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Before embarking on an eye tracking journey, it is essential to understand the different types of eye movements and their purpose. This knowledge will help you to effectively design your eye tracking study, interpret the data, and draw sound conclusions about your study results. Furthermore, different types of eye movements can provide insights into different aspects of cognitive processes. For instance, saccades can reveal information about the decision-making process, while fixation duration may indicate mental effort (Ryan and Shen, 2020; Spering, 2022). This article will walk you through the primary eye movements and their function, bringing you one step closer to understanding the basis of the visual system.
Eye movements can be captured with an eye tracker. Eye tracking technology is used in various application fields, such as scientific research, consumer research and user experience, skills assessment, healthcare, sports, and gaming. Learn more about eye tracking application areas.
Fixation is the period when eyes essentially stop scanning a visual scene and remain relatively still. Fixations allow holding a stationary object of interest on the fovea for a detailed visual information intake (Hessels et al., 2018.)
Saccades are rapid, ballistic eye movements between fixations that bring an area of the visual scene onto the fovea (Hessels et al., 2018). The vision is highly suppressed during saccades, which allows for continuous and stable perception during saccadic reorientation. Human perception is guided by alternating sequences of fixations and saccades (Figure 2).
Although the eyes appear still during fixations, small fixational eye movements are always present when attending to a fixed point. There are three fixational eye movements: tremors, microsaccades, and drifts. Fixational eye movements are linked to various cognitive processes and thus have increasingly received more interest from experimental psychologists and neuroscientists (Martinez-Conde et al., 2013).
A microsaccade is a small, fast, jerk-like eye movement that occurs during a voluntary fixation (Martinez-Conde et al., 2004). Microsaccades carry the retinal image across up to several hundred photoreceptors’ width, preventing perceptual fading (Martinez-Conde et al., 2000). Contrary to the common concept that microsaccades are exclusively involuntary eye movements, research shows that microsaccades can also be generated on demand (Willeke et al., 2019).
A tremor, also known as physiological nystagmus, is an aperiodic, wavelike eye movement. Tremors help to retain visual acuity during prolonged fixations (Mahanama et al., 2022).
Drifts are slow, irregular, smooth-motion eye movements that occur during attempted fixation. Drift’s role is to maintain a stable vision during a fixation in the absence or poor compensation by microsaccades (Martinez-Conde et al., 2004).
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Humans mainly make saccades and fixational eye movements when looking at static objects with relatively still heads. However, in more dynamic situations where either the viewer or the object itself is moving, the additional repertoire of eye movements is there to help keep the fovea aligned with the point of interest.
Vergence is the eye movement that occurs when tracking an object moving in depth – either forward or away from us. Vergence can be triggered by binocular disparity, blur, and the perceived nearness of surrounding objects (Giesel et al., 2019).
Smooth pursuit is a tracking eye movement used to maintain a moving object of interest on the fovea.
The vestibular ocular reflex is a reflex to stabilize gaze and, thus, stable vision during head movement.
Eyelid movements, known as blinks, accompany eye movements. Learn more about different types of eyelid movements and how to measure them with an eye tracker.
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SCIENCE WRITER, TOBII
As a science writer, I get to read peer-reviewed publications and write about the use of eye tracking in scientific research. I love discovering the new ways in which eye tracking advances our understanding of human cognition.
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