The Human Brain : From Neurone to Nervous System

EYE MOVEMENTS


  1. Several types of eye movements have been identified. Movements of the eyes are essential:
    • to stabilise images on the retina during movements of the head using vestibulo-ocuar reflexes
    • to pay attention to the detail of an image or read some text using saccadic movements
    • to assess the distance of an object within the visual field using convergent movements

  2. Stabilisation movements are reflex movements that move both eyes so that the retinal image is stable, i.e. unchanged, despite movements of the head. For example, these reflexes cause the image of the horizon to be maintained in a horizontal position on the fovea during side-to-side movements of the head: the eyes rotate so as to compensate for the side-to-side movement.

  3. These reflex movements depend on the vestibular apparatus and are coordinated by the pontine paramedian reticular formation. Disorders of this system give rise to nystagmus, a continuous uncontrolled movement of the eyes.

  4. Saccadic movements are jerky movements that scan text or an image: attention causes the eyes to examine the detail of the visual image, while skipping areas that contain fewer features. Stimulation of the frontal eye fields in the frontal lobe causes conjugate eye movements - both eyes move simultaneously in the same direction. The frontal eye fields and the superior colliculi are closely involved in the generation of saccades.

  5. Convergence or divergence of the eyes are used to observe near or distant objects. The ability to converge the visual axes allows the depth of the visual field to be perceived and is essential for spatial awareness in three dimensions.

  6. The visual cortex detects disparities in the retinal images of the two eyes, which are used to perceive three dimensional visual space. Failure to achieve accurate convergence on an object gives rise to double vision (diplopia).

  7. Key Words: vestibulo-ocular reflexes, saccadic movements, convergent eye movements, frontal eye fields, superior colliculus, pontine paramedian reticular formation.






Attention to detail

When attention is directed to a face or other object, the eyes move so that the fovea can examine the detail of the image.

Each movement is a saccade: the eyes flick from one position to another, altering the direction of gaze, so that most of the time available is concerned with the details of the image.

Note that the direction of gaze changes allowing the eyes focus on the details of the image of Nefertiti.








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