Types of eye movement

Visual system Human eye Eye movement

The aim of this page is top give a brief description of the different main types of eye movements and there function. 

The spatial and temporal sampling ability of the human eye limits the manner in which we extract visual information from events in the world. Because visual acuity decreases rapidly when we move away from the center of our visual field, we possess a repertoire of eye movements that allow us to point our eyes at target locations of interest.

Saccades are the type of eye movement used to move the fovea rapidly from one point of interest to another, while a fixation is the period of time where the eye is kept aligned with the target for a certain duration, allowing for the image details to be processed. Our perception is guided by alternating these sequences of fixations and saccades (see figure on the left). Due to the fast movement during a saccade, the image on the retina is of poor quality and information intake thus happens mostly during the fixation period.

Saccade facts:

  • can be triggered voluntarily or involuntarily
  • both eyes move in the same direction
  • 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. larger jumps produce longer durations
  • the end point of a saccade cannot be changed when the eye is moving
  • Saccades do not always have simple linear trajectories

Fixation facts:

  • a fixation is composed of slower and minute movements (microsaccades, tremor and drift) that help the eye align with the target and avoid perceptual fading (fixational eye movements)
  • the duration varies between 50-600 ms (however longer fixations have been reported)
  • the minimum duration required for information intake depends on the task and stimulus

When we look at a static object with our heads relatively still, we mainly perform saccades and fixational eye movements. However in more dynamic situations where either we are moving, or the object itself is moving, other eye movements are triggered to keep the fovea aligned with the point of interest. Vergence movements are recruited to help us focus on objects placed at different distances, smooth pursuit is used to keep the fovea aligned with moving objects and the vestibular ocular reflex is used to maintain our fovea pointed at a point of interest when our head and body are moving.

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
  • are generally slower than saccades

Smooth pursuit facts:

  • cannot be triggered voluntarily, in absence of a moving target
  • 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 we start to employ catch up saccades to keep up with the target

Vestibular ocular reflex

  • the eyes move in the opposite direction of the head
  • normally the speed of the eye equals the speed of the head

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