As far as I am aware, the saccades of the eye are central to visual perception. If the eye is held still, the human stops seeing, even if light is reaching the retina and the visual pathway is intact.

But we recently had a question on UX.stackexchange about possible improvements to a visual+EEG interface used for communication with locked-in patients. The designer asking it mentioned that eye tracking devices are unlikely to work, because the patients' eyes can't move. This surprised me, as I would have thought that the patients must be able to move their eyes if they are able to read the device's screen. But I have never had contact with a locked-in person, and assume that the asker knew more about them than I do.

So, are there situations in which a human's eye muscles are paralyzed, so the eyeballs cannot be moved in their orbits, but this human still has visual perception and can mentally process information in his/her field of view?

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    Why will the human stop seeing if eyes are held still? – One Face Jan 25 '15 at 9:21
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    @CRags - through adaptation (image fading). See my answer below. Good question, though. – AliceD Jan 25 '15 at 9:53
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    @ChrisStronks Sorry Chris, now got it. I didn't read the full answer. It is nicely explained. – One Face Jan 25 '15 at 9:58

Short answer
Visual perception persists in the absence of functional eye movement.

Background
An example case would be retinal implants wearers. Retinal implants like the Argus II consist of a set of electrodes placed onto the degenerate retina of blind folks. Electrical stimulation of the electrode array elicits flashes of light (phosphenes). Visual input is provided via a head-mounted camera, and the device translates the camera images into patterned electrode stimuli. Hence, because the camera is head worn, Argus II wearers cannot use eye movements to scan the world. They have to use head-scanning techniques. Still they are able to perceive flashes of light (phosphenes) and can recognize simple shapes and perceive motion.

In fact, eye movements such as saccades are detrimental in Argus II wearers, because the oculomotor system feedbacks muscle movements to the brain when a saccade is made to stabilize the perceptual field of view. In normally sighted people the oculomotor feedback system is important, as it tells the brain to compensate the retinal image position shift when a saccade is made. Compensation occurs through a shift in the brain representation of the retinal image to the opposite direction, thereby stabilizing the image representation. However, this is detrimental in Argus II wearers, because the position of the electrode array and hence retinal position of the image is unaffected by eye position. Hence, when subjects use their implant, any saccades will disrupt visual processing, because the oculomotor system will abberantly compensate the perceived location of the object being viewed.

More importantly, perhaps, are micro-saccades. Micro- saccades are small, involuntary eye movements. When images are present stably to the retina the images fade very quickly due to adaptive processes in retinal neurons. Micro-saccades are thought to prevent fading, as they constantly produce small, involuntary fluctuations in gaze, thereby shifting the image quickly over the retina, and thereby refreshing the retinal image and preventing adaptation (Stingl, 2013). When Argus II wearers stop scanning the image, images indeed fade within seconds (Stronks, 2013).

To get back to your question and summarize:

Title question: No, eye muscles are not needed for sight, given that Argus II wearers have sight (though artificial, crude vision). However, a functional saccade system in normally sighted folks improves scanning, and anecdotally micro-saccades prevent image fading by constantly refreshing the retinal image.

Body question: Argus II implant wearers are an example of vision without oculomotor control. Although they cannot use their eye muscles for scanning, they still have functional vision. However image fading is a problem in Argus II users and folks without functional eye muscles alike, so head scanning techniques will be needed.

PS: Other implant systems, such as the Alpha-IMS use light-capturing methods inside the eye. Hence these subjects can use normal eye scanning and saccades for scanning. Also, albeit anecdotally, fading is reduced in Alpha-IMS wearers, because of the micro-saccades preventing adaptation (Stingl, 2013).

References
- Stingl et al., Proc Royal Soc B (2013); 280(1757)
- Stronks et al., IOVS (2013); 54(6): 3891–901.

  • Can add refs when needed later, I am mobile now unfortunately. – AliceD Jan 24 '15 at 23:20
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    Good answer, why are saccades essential to see in a normal person? – One Face Jan 25 '15 at 9:18
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    @CRags - Thanks! Saccades are not essential, just beneficial for scanning and preventing adaptation. Answer adapted :) – AliceD Jan 25 '15 at 9:32
  • Could you add some references on micro-saccades and image fading? You say "anecdotally" in your answer, does that mean no studies have been performed on it? – MattDMo Jan 25 '15 at 20:48
  • @MattDMo. I am still mobile. Will work on refs when possible. Anecdotal evidence is published, but as far as I can see not thoroughly investigated. – AliceD Jan 25 '15 at 21:15

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