We’ve all had the experience of looking in a mirror and noticing that our eyes don’t seem to move smoothly when we look from side to side. Instead, they seem to “jump” or “flicker” from one position to the next. So why don’t we see our eyes move continuously in the mirror like we would expect? There are a few key reasons.
Saccadic suppression
One major factor is something called “saccadic suppression”. Saccades are the rapid, jerk-like movements our eyes make when shifting from one fixation point to another. It turns out that every time we make a saccade, our visual system actively suppresses the blurry image our eyes see during the movement. This suppression also applies to the image we see of our own eyes in the mirror.
So in reality, our eyes are moving smoothly and continuously as we look from side to side. But because of saccadic suppression, we don’t consciously perceive the blur – we only see the clear image at the end of each saccade. This makes it seem like the eyes are jumping discontinuously in the mirror, when they are actually moving fluidly.
Low spatial resolution
Another reason we can’t see our eyes move smoothly in the mirror is that the eyes themselves have a low spatial resolution compared to the rest of the visual field. The highest concentration of photoreceptors is in the fovea at the very center of the retina. Resolution drops off rapidly as you move towards the periphery.
So when you look at your own eyes in the mirror, the image of the eyes is falling on the poor resolution peripheral areas of your retina. This limits your ability to perceive fine spatial details and smooth motion of the eyes themselves.
Slow temporal resolution
The eyes also have relatively slow temporal resolution, meaning they have a limited ability to perceive continuous changes over time. The fastest movements our eyes can detect are around 20 to 60 changes per second. Any motions faster than this simply get blurred together.
The smooth movements of the eyes during a saccade are extremely fast – up to 700 degrees per second! So the constant micro-changes of the eyes as they move side to side happen way too rapidly for our visual system to distinguish individually.
Low refresh rate of mirrors
An additional factor is the low “refresh rate” of a typical mirror. Refresh rate refers to how many times per second a display updates to show a new image. Mirrors technically don’t have a refresh rate, but we can think of them as being limited by the speed of light – it takes time for light to bounce off the mirror and reach our eyes.
This effectively reduces the temporal resolution that a mirror can convey, similarly to how a low refresh rate monitor would. So some of the smooth motion of the eyes may get lost simply because the mirror can’t keep up.
Fixational eye movements
One final point is that even when you try to hold your eyes still by staring at yourself in the mirror, your eyes are never completely still. They undergo fixational eye movements – tiny involuntary jerks and drifts that happen while you fixate on an object. These movements may also obscure your ability to see smooth motion of your eyes.
Conclusion
In summary, the main reasons we can’t see our eyes move smoothly in the mirror include:
- Saccadic suppression actively hides motion blur during eye movements
- Low spatial resolution of the peripheral retina where eye image lands
- Slow temporal resolution diminishes ability to see rapid motion
- Mirrors have an effective low “refresh rate”
- Fixational eye movements create slight instability
So even though your eyes are moving fluidly, limitations in vision, mirrors, and small fixational movements all conspire to create the illusion that our eyes simply “jump” from point to point when looking in a mirror.
How do our eyes move?
To better understand why we can’t see our eyes move smoothly in the mirror, it helps to review some basics of how our eyes actually move.
Saccades
As mentioned above, saccades are the rapid movements our eyes make several times per second to shift gaze from one object of interest to another. Saccades can move our eyes at extremely fast speeds – up to 700 degrees per second!
Saccades are controlled by burst neurons in the brainstem. These produce a powerful pulse of neural activity lasting about 20-40 milliseconds that drives the extraocular muscles to quickly rotate the eyes.
Smooth pursuit
Smooth pursuit refers to the continuous tracking motion our eyes make when following a moving object. This allows the eyes to closely match the velocity of the target to keep it on the fovea.
Pursuit movements are controlled by a network of brain areas including the frontal eye fields, occipital cortex, cerebellum and brainstem. These areas work together to detect target motion and generate smooth eye movements.
Vergence
Vergence movements allow our eyes to converge or diverge in order to maintain binocular vision and stereopsis. When we look at near objects, our eyes rotate inward (convergence). When we look at far objects, our eyes rotate outward (divergence).
Vergence movements are driven by disparity-sensitive neurons in the visual cortex and other areas that detect misalignment between the two eyes. The vergence system works to minimize this disparity and keep both eyes directed at the object of interest.
Vestibulo-ocular reflex
The vestibulo-ocular reflex (VOR) stabilizes gaze during head movements by producing compensatory rotations of the eyes. When the head turns one direction, the eyes automatically rotate in the opposite direction to keep the visual scene steady.
The VOR originates in the vestibular system of the inner ear, which detects head motion. This sensory input is integrated by brainstem nuclei that control the eye movement muscles.
Optokinetic reflex
The optokinetic reflex (OKR) also serves to stabilize vision, but does so when viewing full-field motion such as a moving visual scene. As the scene drifts across the retina, the OKR generates smooth eye movements at the same speed but in the opposite direction.
This reflex primarily involves integration of visual motion signals from the retina and visual cortex by cerebellar and brainstem circuits controlling eye movements.
Key characteristics of eye movements
Some key characteristics of different types of eye movements that are relevant to why we don’t see our eyes move smoothly in mirrors include:
| Eye Movement | Speed | Function |
|---|---|---|
| Saccades | Up to 700°/s | Quickly shift gaze |
| Smooth pursuit | Up to 100°/s | Track moving objects |
| Vergence | Up to 40°/s | Binocular coordination |
| VOR | Up to 400°/s | Gaze stabilization |
| OKR | Up to 300°/s | Full-field motion stabilization |
As you can see, saccades are by far the fastest eye movements, reaching speeds of up to 700 degrees per second. Smooth pursuit movements can be quite fast as well to accurately track moving objects. And compensatory reflexes like VOR and OKR can also produce high speeds to maintain visual stability.
The speeds of these various eye movements far exceed the temporal resolution of our visual system. Our eyes simply cannot see changes happening faster than about 60 times per second. So when our eyes are moving rapidly in the mirror, the continuous motion gets blurred together into discontinuous jumps due to these visual limits.
Saccadic suppression
Saccadic suppression is one of the main reasons we can’t see our eyes move smoothly in the mirror. This is an active neural process that actively inhibits visual processing during saccades.
Because the eyes move so rapidly during saccades, the image that falls on the retina is a blurry smear during the movement. But we never consciously perceive these smears thanks to saccadic suppression.
Research shows that suppression starts about 50 milliseconds before a saccade begins. Neural inhibition then reduces sensitivity to motion and blur during the saccade itself. The suppression also lingers for around 20 milliseconds after the eye stops moving.
Without this active suppression, our vision would be full of annoying motion streaks every time our eyes moved. Saccadic suppression prevents this by blocking visual processing during saccades.
The effect is strong enough that we are essentially blind to external visual inputs during saccades. Experiments utilizing brief flashes during saccades show that people cannot even detect a flash presented during a saccade.
Mechanisms of saccadic suppression
Exactly how saccadic suppression is achieved is still not completely understood, but several mechanisms have been identified:
- Burst cell inhibition – Burst cells driving saccades inhibit neurons involved in visual processing.
- Visual masking – The smear of a saccade masks and overwrites previous and subsequent visual input.
- Fading – Visual neurons show a reduced response to stimuli presented around the time of a saccade.
- Spatial remapping – Receptive fields of visual neurons shift before each saccade, disrupting visual continuity.
Through a combination of these mechanisms, the visual system essentially becomes blind each time we make a fast saccade. This prevents us from seeing the blurring that would otherwise occur with such rapid eye movements.
Mirrors and eyes
So how exactly do the challenges of perceiving our own eye movements relate to looking in a mirror?
For one thing, mirrors can only reflect the amount of light coming into them and project it back at the same intensity. This limits the effective contrast and refresh rate a mirror can convey, similar to a low quality digital display.
But more importantly, when looking in a mirror our eyes rotate to move the image of our eyes onto the peripheral areas of the retina. Unfortunately, visual acuity and motion perception are poor in the periphery.
Distribution of photoreceptors
The distribution of photoreceptors is very uneven across the retina. The highest density of cones providing high acuity vision is found in the fovea at the very center of gaze. Cone density drops off rapidly as you move towards the peripheral retina.
For example, cone density is maximal at the fovea with nearly 200,000 cones per mm2. But at 20 degrees eccentricity, there are only 2,500 cones per mm2. That represents nearly a 100x reduction in cone density and subsequent loss of visual acuity.
Cortical magnification
Related to the change in photoreceptor density, cortical magnification describes how much more area in the visual cortex is devoted to processing central versus peripheral visual signals. Cortical magnification follows the same pattern, with much more cortical circuitry dedicated to foveal signals.
Again using 20 degree eccentricity as an example, cortical magnification may be reduced by over 10x compared to the foveal representation. This results in far less processing bandwidth and resolution for peripheral objects.
Saccadic suppression effects
On top of the limitations in peripheral vision itself, saccadic suppression effects are also most pronounced toward the periphery. Even though we are not executing saccades when looking in the mirror, the perception of blurring is stronger away from central vision.
This matches the distribution of burst neurons driving saccadic suppression. Their inhibitory effects on visual processing are strongest for circuitry representing the peripheral visual field.
Together, the poor resolution and motion sensitivity of peripheral vision combines with enhanced saccadic suppression effects in the periphery to maximum the loss of detail and blurring when viewing our own eye movements reflected off-center in a mirror.
Implications and applications
Understanding why we can’t easily see our own eye movements in a mirror provides insight into the limitations and quirks of the human visual system. This also has implications for technology applications.
Virtual reality
In virtual reality environments, it is critical to update the display precisely in sync with a user’s eye movements. Otherwise, motion blur or discontinuities will be readily apparent.
The limitations around perceiving our own eye motions in the mirror demonstrate that we have a high tolerance for artifacts related to eye movements. This knowledge can help guide development of VR technology.
Vision science
The fact that we can’t observe our own eye movements directly also has importance for vision science. It means much of what we understand about eye movements comes from objective recordings, rather than subjective experience and perception.
Understanding these limitations is helpful context when interpreting results from eye tracking studies in both science and usability research.
Vision rehabilitation
For people with eye movement disorders resulting from brain injury or neurodegenerative disease, the inability to see their own eye movements could potentially have therapeutic benefits. Just as we are unaware of our own eye movement deficits when looking in a mirror, this could mask or reduce symptoms from their perspective.
This potential positive illusion could improve subjective well-being for those with eye movement disorders impacting activities like reading.
Conclusion
In summary, the main factors that prevent us from seeing our eyes move smoothly in the mirror include:
- Saccadic suppression actively inhibits vision during rapid eye movements
- Low visual resolution and acuity in peripheral retina where eye image falls
- Limited temporal resolution fails to capture high speed eye motions
- Mirrors have an effective low refresh rate
- Small fixational drifts add instability
The visual system essentially becomes “blind” during saccades and lacks the spatial and temporal resolution to distinguish rapid eye movements in our peripheral vision. So while the eyes move smoothly, we only perceive discrete jumps when looking at our eyes in the mirror.
Understanding these visual system limits provides insight into perception while also informing applications from virtual reality to vision rehabilitation.
