Eye Movements 3 – Saccades and Fixations

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A sample of some of the great reference on eye movements available at the BBC Motion Gallery

The retina of the human eye is divided into a large outer ring of highly light-sensitive but color-insensitive rods, and a comparatively small central region of color-sensitive cones. This is to allow for diurnal (day and night) vision. That tiny, cone-rich center region is the fovea, and provides our high acuity vision. The outer ring only provides peripheral vision. All detailed observations are made with the fovea, which must constantly be oriented to different parts of the viewed scene by successive fixations.

Frequent, precise eye movements are required because the high acuity vision is restricted to a very small (2º) region surrounding the fixation point. Visual acuity drops off precipitously from that point. The human visual system takes advantage of this high-acuity region by rapidly reorienting the eyes via very fast eye movements called saccades.

For reference, a visual angle of 2° is slightly less than the width of one’s thumb held out at arm’s length, or approximately the width of an average word held at normal reading distance. That’s ALL we normally see with clarity! Detailed observations of a reasonable part of the surrounding world therefore requires moving the eye (and head and body) to successively focus different parts of the ambient light array on the fovea, thus foveating various regions of the observed scene. This is what we must portray through our animation if our characters are to look alive and appear to be observing and interacting with their world.

The human visual system takes advantage of the high resolving power of the fovea by reorienting the fixation point around the viewed scene an average of three times each second via saccadic eye movements, but saccades aren’t the only type of eye movements. Eye movements can be classified into seven different types:

Convergence
is a motion of both eyes relative to each other that ensures that an object is still foveated by both eyes when its distance from the observer is changed. The closer the object is, the more the eyes point towards each other. This movement can be voluntarily controlled, but is normally the result of a moving stimulus.
Saccades
are the principal method for moving the eyes to a different part of the visual scene, and are sudden, rapid movements of the eyes. Saccades can be initiated voluntarily, but are ballistic: that is, once they are initiated, their path of motion and destination cannot be changed. Visual input is suppressed during a saccade.
Pursuit motion
is a much smoother, slower movement than a saccade; it acts to keep a moving object foveated. It cannot be induced voluntarily, but requires a moving object in the visual field. One frequent failing of thoughtless animation is having the eyes demonstrate pursuit motion when there is no object being followed by the character’s eyes.
Nystagmus
is a saw-toothed pattern of eye movements that occurs as a response to the turning of the head (acceleration detected by the inner ear) or the viewing of a moving, repetitive pattern (the train window phenomenon). It consists of smooth `pursuit’ motion in one direction to follow a position in the scene, followed by a fast motion in the opposite direction to select a new position. This is an eye movement that has probably never been animated, and if it was, it would probably get rejected by the supervisor or director because it would look so odd.
Drift and microsaccades
occur during fixations, and consist of slow drifts followed by very small saccades (microsaccades) that apparently have a drift-correcting function. These movements are involuntary, and their function is in question.
Physiological nystagmus
is a high-frequency oscillation of the eye (tremor) that serves to continuously shift the image on the retina, thus calling fresh retinal receptors into operation. Physiological nystagmus actually occurs during a fixation period, is involuntary, and generally moves the eye less than 1°. As with microsaccades, you’d need to be in an extreme close-up for this to register, but it’s another reason the human eye looks ‘alive’ in live-action extreme close-ups.
Rolling
of the eyes is a rotational motion around an axis passing through the fovea and pupil. It is involuntary, and is influenced by among other things the angle of the neck. Although this is also something we can safely ignore as animators, some rigs will automatically provide this when using the ‘eye-target’ controller.

Of the above, it’s really the first three we’re concerned with. Because of the precision that CG animation allows, we can (and should) pay much more attention to these types of movements than is typically done in hand-drawn animation.

Convergence is fairly straightforward. In a properly rigged character, convergence should happen naturally as the eye target is moved close to the character’s face. If a rig lacks this function, or the eye-target control isn’t being used, it should be a simple matter to manually add in some convergence when a character is looking at something very close. You don’t need to go too far with this to portray convergence.

Pursuit is also straightforward. I like to use the eye target to match the motion of the object being tracked, and I like to make sure the head motion (if the head is rotating in the direction of the object being tracked) is somewhat out of phase with the eye movement, otherwise the sense of the eyes actually moving in pursuit motion is lost.

There really aren’t any timing considerations to convergence and pursuit, but there are for saccades. I’ve heard various rules of thumb regarding how many frames* a saccade should take, and how long the eyes should fixate between saccades. Rather than regurgitate those, I’ll lay out some data from physiology studies. I encourage readers to do their own research — look closely at reference like I’ve posted above, and see what the eyes really do.

It takes about 100-300 milliseconds (ms) to initiate a saccade. That is, from the time a stimulus is presented until the eye starts moving takes 0.1 to 0.3 seconds, or between 2 to 7 frames (a common mistake is to have the animated character react immediately — on the same frame — to a visual input).

A saccade takes another 30-120 ms to complete, depending on, among other things, the visual angle traversed. That means the typical saccade takes from 1 to 3 frames. This is where I see a lot of mistakes, usually with saccades that last 5, 6, even 8 frames, or sometimes very large saccades that occur in a single frame. Basically, you can reserve 1-frame saccades for very small movements, 2-3 frame saccades the rest of the time, and very rarely use a 4-frame saccade for a large, deliberate eye movement.

Again, saccades are ballistic, and they do not have any anticipatory movement (unless you’re going for an overtly comic or bizarre effect), nor do they have any overshoot. The fact that the saccades cannot be adjusted on the fly means that our brain makes a best guess for the fixation target of the saccade. The result is that often a relatively large eye dart is followed almost immediately by a smaller ‘adjustment’ saccade. This can be seen clearly in some of the BBC Motion reference footage.

Interestingly, not only is visual processing suppressed during saccades, but this suppression actually begins about 50ms (about 1 frame) before the saccade is initiated. I’ve heard animators speculate that one reason we blink during eye movements is that the closing of the eyes keeps us from getting confused by a ‘blurry’ visual image during an eye dart, or that the blinking ‘resets’ the eye’s focus. Since visual input is suppressed just before, and during, the saccade, we can see this logic is reversed. Our visual system takes advantage of the brief moment of visual ‘down time’ during the saccade to get some lubrication to the surface of the eye with a quick blink. The blink during the head movement also provides some physical protection to the surface of the eye from potential external trauma.

So processing of the retinal image takes place mainly between the saccades, during the fixations. These fixations last for about 200-600 ms (5-15 frames), with 300 ms (7-8 frames) being average, during scanning behavior. Obviously we can fixate on a target for much longer periods, depending on the situation.

Because of the above physiology, I like to use the eye-target control as often as possible. Most of the time this is how our eyes actually work: fixating on a point even as our head and body move. When it comes to the eyes, by far the most common mistake I see in beginning animator’s work is letting the eyes drift around with the head and body movement. In these kinds of scenes, the eyes tend to fixate only when the head and body settle. This will undermine even the most polished and nuanced acting scene.

Of course, if you’re using the eye-target, you need to build in appropriate fixations and saccades. This can take a lot of time, and once done, the lids need to be adjusted properly (more on this in a later post). One problem is that the eyes can look overly busy when they’re animated realistically in this manner. Maybe this is because animated characters tend to have abnormally large eyes (so we notice the eye darts more), or perhaps because we tend to accept ‘under-animated’ eyes in the animation we’ve grown up with. If you really pay attention to what people do with their eyes in everyday situations, you quickly see that we edit out a lot when we’re animating. Frankly, I think we edit out too much, and that there’s a tremendous amount of good acting that could be done with very subtle movements of the eyes and lids.

One trick I’ve found useful to avoid having too many saccades and fixations is to cheat the eye-target during a fixation. Imagine the character’s head is rotating from right to left. The eyes (and the eye-target controller) make a saccadic motion to the left (with the head motion), then fixate. If you ever-so-slightly drift the eye-target further to the left during the fixation, the eyes will appear to remain fixed on the same spot, and can be held for a longer fixation without the appearance of nystagmus. This way during a long close-up scene the number of saccades can be kept down.

What I haven’t discussed here is the relationship to these kinds of eye movements to acting, how to use these eye movements to show both intent and to show a character’s internal process, or how the eye lids and brows respond during various eye movements. Hopefully that’s coming up.

*As always, I’m using the feature film standard of 24 frames per second.

12 Responses to “Eye Movements 3 – Saccades and Fixations”

  1. Cassidy Says:

    “Why… are you always… shaking your eyes? Don’t shake your eyes at me lady, that’s what they say! Shake ‘em here, shake ‘em there… why don’tcha go join some shakin’ rock and roll band!”

    Thanks for another excellent post, Kevin!

  2. Randall Says:

    Thank you for creating another brilliant post. I really appreciate you taking the time to provide this incredibly detailed information. I hope that others (myself is included in others) will be able to use this to further the art form.

    Keep up the fantabulous work!

  3. Lluis Says:

    Awesome awesome post !!! Thanks for taking the time to write this down Kevin !!!

    HONK –

    Lluís

  4. Miles Says:

    Great post! Very interesting info, especially that bit about blinks.

  5. alonso Says:

    Thanks for the interesting information. Is the common eye movement we see in close ups saccades jumping from 1 eye to the other of the listener, or is it drift or tremors? If we have such a small area of strong focus, I wonder how much information we are getting in our periphery and what it is that makes things in our periphery demand a saccade to see it clearly. I know that our periphery is better able to pick up movement, which is why those flashing popups are so annoying. Convergence appears relative right, so a crosseyed character would become even more strongly cross eyed looking at something close? (don’t they sometimes make “cute” characters cuter by slightly crossing their eyes? ) Thanks for the interesting thoughts.

  6. Renaud Says:

    Awesome post!
    Like you said, no book speak about eyes’s movements and I don’t really know how to move my eyes. But your article learn to me lot of things and I can’t wait for the next!
    You blog is like a goldmine, and I thank you a lot to put all this great advice on the web!
    Thank!

    Renaud

  7. Kevin Says:

    Thanks, everyone. Alonso, those distracting little eye darts you’re describing are definitely saccades.

    Regarding peripheral vision, that part of our nervous system is particularly attuned to CHANGE. So something new popping into our peripheral vision will automatically tend to trigger a saccade. It’s a basically a very sensitive warning system.

    That said, if you concentrate and force yourself to attend to your peripheral visual field, you CAN get a tremendous amount of information without moving your eyes. And since your peripheral vision is better in low light, there’s an old trick for seeing detail in dim light: focus your fovea just beside whatever it is you want to see, hold that fixation point, but attend to the object just beside the center of your visual field. Suddenly you’ll be able to see much more detail, because you’re using mostly rods instead of mostly cones.

    As for crossed eyes, or esotropia, that’s much more complex, since there are a variety of types of esotropia. Babies often appear to have esotropia, because the bridge of the nose isn’t fully formed (plus a lot of newborns have a form of esotropia that resolves), so I think that’s why a touch of esotropia might appear cute. Plus it’s what people do when they’re very close to you, so it implies a certain physical intimacy I guess.

  8. Joel Brinkerhoff Says:

    I’ve heard people look up when they try and remember something that happened recently and down if they think about something from the past. Maybe I have that reversed?

  9. Kevin Says:

    Joel, I don’t think it’s nearly that simple. At some point in the future I’ll might do a post pulling together some data on what the eyes tend to do when we’re remembering, but be careful of overly simple rules.

  10. Joel Brinkerhoff Says:

    I don’t know of any rules about eye movements and go on observation. The reflective look up or down has something to do with where recent and past memories are stored in the brain. I look forward to your future post about it.

    Thanks

  11. Rodrigo Says:

    You make my head explode, but I like it.

  12. Dana Says:

    My verbosity kind of just happens when I get excited. I’m not mad or haughty or anything. I’m just OCD. Try and enjoy this:

    Memories aren’t “stored.” They are most probably an illusion caused by the heightened efficiency of neurotransmitters through the voltage gates of those neurons most frequently or strongly “visited,” (i.e., increased long-term conductance by way of either frequent mild voltage fluctuations or occasional hyperstimulation, (neither of which literally *causes* an increase in the number of voltage gates on the stomatic membrane, but instead think of these more conductive cells being “selected for” by the demands of said energy propagation, in, sure, a sort of biochemical Process of Natural Selection)). Every brain is uniquely wired and so follows the “localization map” one learns about in introductory psychology/physiology only *probabilistically* for any given region. For any given brain, in other words, there will not be a guaranteed locus of Broca’s area, nor will there be just one for it.

    The reflective look up or down mentioned above is not a universal, cross-cultural truth, thus there are unfathomably many context variables to consider–e.g., whom are we usually speaking to when summoning distant memories, and what impact does that have on social-role norms w/r/t gaze orientation; could years and years of practicing these interaction expectations heighten the likelihood of role-based behavior even outside of the specific interactions which would have governed such a choice of movement (i.e., could it develop into reflex with practice, such that one might do look down even when the situation doesn’t call for it?); with the consideration of years and years in tow, does this not also imply an increase in both the number of distant memories and the *distance* of distant memories, which then only feeds back into the practice loop; and so on. The number of confounds shoots through the roof when you consider that no one *always* looks up when thinking of some recent event. So we adhere to Occam’s razor. And believe it or not, all that neurological mumbo-jumbo at the the top of this post is a simplification. It seems inelegant, at first, but let me insist that it’s drop dead gorgeous; consider the fact that the amazing screen you’re reading from is just ones and zeros, binary code, and then compare that to the fact that your body, nervous system and all, is coded in *quaternary* (DNA = {T, C, G, A}). Which means it’s massively powerful. My favorite neuroscientist gave me the best turn of phrase to describe the unfathomably intricate interaction of DNA with its organic environment, by calling it “a network of feedback loops.” Trust me: this super-complex notion is enough to compensate for the experience of, well, experience. The task of translating this language in which genetic matter speaks now lies in the hands of genomists, neuroscientists, and whomever else has the masochistic patience demanded of such an intense endeavor.

    As for saccadic eye movement: you neglect to mention the blindness of the fovia. It sees nothing! Saccades are rather more about the essentialness of relative information w/r/t color pitch, i.e. why the exact same color can look significantly different in different contexts, (e.g., bright gray will look bluer amid yellow, but yellower amid blue) and this means that the eye must constantly move around if it wishes to increase its opportunities for meaningful input, whether it be novel or informative w/r/t prior input. Also, to say that rods are color insensitive is to neglect their role in appreciating several properties of visible light other than hue, e.g., luminosity, saturation, valence, etc.; as an animator, you probably know better than anyone that color space is 3-D. I’m not just being nit-picky. These little capabilities are what make letting go of the notion of “undefinable” experience into a poignant release instead of a painful relinquishment.

    Btw, I love your site. I wouldn’t have contributed if I didn’t think it’d help.

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The animation and animation-related musings of Kevin Koch