Eye Movement: Types and functions explained

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.

Main types of eye movements:

• Saccades
• Fixations
• Microsaccades
• Tremors
• Drifts
• Smooth pursuit
• Vergence
• Vestibular-ocular reflex (Mahanama et al., 2022)

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.

Fixations

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.)

Fixations facts:

• Composed of slower, smaller-scale eye movements (microsaccades, tremors, and drifts) that help the eye align with the target and avoid perceptual fading (fixational eye movements).
• The duration varies between 50-600 ms.
• The minimum duration required for information intake depends on the task and stimulus at hand (Land & Tatler, 2012; Rayner, 2009).

Saccades

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).

Saccades facts:

• Occur voluntarily or involuntarily.
• Binocular and conjugate.
• The time to “plan” a saccade (latency) is task-dependent and varies between 100-1000 ms.
• The average duration of a saccade is 20-40 ms.
• The duration of a saccade and its amplitude are linearly correlated, i.e., more significant jumps produce longer durations.
• The endpoint of a saccade cannot be changed when the eye is moving (Land & Tatler, 2012; Rayner, 2009).

Fixational eye movements: microsaccades, tremors, and drifts

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).

Microsaccade

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).

Microsaccades facts:

• Typically occur at a rate of 1-3 Hz.
• Approximately 25 ms in duration.
• Magnitude of around 0.5°, can go up to 1° (Martinez-Conde et al., 2013).
• Binocular and conjugate.
• Can indicate the orientation of covert attention (Engbert & Kliegl, 2003).
• One of the main eye movements involved in the perception of illusory rotation (BOX 2) (Otero-Millan et al., 2012).

Tremor

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).

Tremor facts:

• Occurs at a frequency of 90 Hz.
• The amplitude of about the diameter of a foveal cone.
• The smallest type of all eye movements.
• Challenging to record accurately due to similar amplitudes and frequencies of a recording system’s noise.
• Conjugate movement (Martinez-Conde et al., 2004).

Drifts

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).

Drift facts:

• Co-occurs with tremor.
• Can be both conjugate and non-conjugate.
• Less than 0.13° in size.
• Average velocity is around  0.5°/sec (Rolfs, 2009).

Eye movements in dynamic settings: vergence, smooth pursuit, and vestibular ocular reflex

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

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).

Vergence facts:

• The left and right eye move in opposite directions.
• Can be classified into two types of movements - far-to-near focus triggers convergent movements, and near-to-far focus triggers divergent movements (Giesel et al., 2019).

Smooth pursuit

Smooth pursuit is a tracking eye movement used to maintain a moving object of interest on the fovea.

Smooth pursuit facts:

• Can be executed only in the presence of a moving target.
• Latency of 100-125 ms.
• Eye velocity is most often less than 30 deg/sec (however, some individuals can smooth pursuit at velocities as high as 100 deg/sec).
• When the target moves at a higher speed than 30 deg/sec, a viewer starts to employ catch-up saccades to keep up with the target (Land & Tatler, 2012).

Vestibular ocular reflex

The vestibular ocular reflex is a reflex to stabilize gaze and, thus, stable vision during head movement.

Vestibular ocular reflex:

• The eyes move in the opposite direction of the head.
• The speed of the eye equals the speed of the head (Land & Tatler, 2012).

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.

Cited publications

Anderson, J., Barlow, H. B., Gregory, R. L., Land, M. F., & Furneaux, S. (1997). The knowledge base of the oculomotor system. Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences, 352(1358), 1231–1239.

Bradley, A., Applegate, R. A., Zeffren, B. S., & van Heuven, W. a. J. (1992). Psychophysical measurement of the size and shape of the human foveal avascular zone. Ophthalmic and Physiological Optics, 12(1), 18–23.

Engbert, R., & Kliegl, R. (2003). Microsaccades uncover the orientation of covert attention. Vision Research, 43(9), 1035–1045.

Giesel, M., Yakovleva, A., Bloj, M., Wade, A. R., Norcia, A. M., & Harris, J. M. (2019). Relative contributions to vergence eye movements of two binocular cues for motion-in-depth. Scientific Reports, 9(1), Article 1.

Hessels, R. S., Niehorster, D. C., Nyström, M., Andersson, R., & Hooge, I. T. C. (n.d.). Is the eye-movement field confused about fixations and saccades? A survey among 124 researchers. Royal Society Open Science, 5(8), 180502.

Land, M., & Tatler, B. (2012). Looking and Acting: Vision and eye movements in natural behaviour. Oxford University Press.

Mahanama, B., Jayawardana, Y., Rengarajan, S., Jayawardena, G., Chukoskie, L., Snider, J., & Jayarathna, S. (2022). Eye Movement and Pupil Measures: A Review. Frontiers in Computer Science, 3.

Martinez-Conde, S., Macknik, S. L., & Hubel, D. H. (2000). Microsaccadic eye movements and firing of single cells in the striate cortex of macaque monkeys. Nature Neuroscience, 3(3), Article 3.

Martinez-Conde, S., Macknik, S. L., & Hubel, D. H. (2004). The role of fixational eye movements in visual perception. Nature Reviews Neuroscience, 5(3), Article 3.

Martinez-Conde, S., Otero-Millan, J., & Macknik, S. L. (2013). The impact of microsaccades on vision: Towards a unified theory of saccadic function. Nature Reviews Neuroscience, 14(2), Article 2.

Otero-Millan, J., Macknik, S. L., & Martinez-Conde, S. (2012). Microsaccades and Blinks Trigger Illusory Rotation in the “Rotating Snakes” Illusion. Journal of Neuroscience, 32(17), 6043–6051.

Rayner, K. (2009). Eye movements and attention in reading, scene perception, and visual search. Quarterly Journal of Experimental Psychology (2006), 62(8), 1457–1506.

Rolfs, M. (2009). Microsaccades: Small steps on a long way. Vision Research, 49(20), 2415–2441.

Ryan, J. D., & Shen, K. (2020). The eyes are a window into memory. Current Opinion in Behavioral Sciences, 32, 1–6.

Spering, M. (2022). Eye Movements as a Window into Decision-Making. Annual Review of Vision Science.

Troncoso, X. G., Macknik, S. L., Otero-Millan, J., & Martinez-Conde, S. (2008). Microsaccades drive illusory motion in the Enigma illusion. Proceedings of the National Academy of Sciences, 105(41), 16033–16038.

Willeke, K. F., Tian, X., Buonocore, A., Bellet, J., Ramirez-Cardenas, A., & Hafed, Z. M. (2019). Memory-guided microsaccades. Nature Communications, 10(1), Article 1.