Hello, everybody, and welcome to episode 3 of You Should See This–the very visual blog on how vision works. Today, we’re talking about how the brain interprets visual input, and how it can change the way we witness the world. We’ll take a briefer look at one specific neural mechanism for vision that you may not have ever heard of: visual saccades. Sources are in the description for you to follow along with, and thanks again for tuning in! Let’s get started.
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Last episode, we developed a foundation of how various eye types use different structures and cell types to compile visual data that is sent back into the brain for assessment and compilation. Today, we’re going to talk about how the brain accomplishes just that–how it synthesizes visual data into a complete and cohesive picture that you can actually understand. We’re also going to find out why you can’t ever see your eyes moving in a mirror, and how you see the future every time you look around. Kind of. Read on to find out more!
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So much of what our eyes actually send back to our brain is blurry, incomprehensible nonsense. For example, most of our color-sensitive cones are clustered around our fovea. Technically, we kind of can’t see in focus or color outside a very very small range. To compensate for that, we dart our eyes back and forth in an automatic twitch known as a saccade. I mentioned this last episode but now I’ll actually get into it. Saccades are most noticeable when reading. We move our eyes in very quick back and forth motions that are fully automatic. You might notice that you’re doing it right now!
Saccades are important–without specific quick eye movements punctuated by long pauses in vision, we wouldn’t be able to focus our gaze on anything. Any tiny head twitch or movement would cause our eyes to blur and careen off-center. These saccades are in part made possible by the work of omnipause neurons. Located in the medial pons, right where the brainstem connects to the midbrain, omnipause cells are usually firing, and turn off when saccades are active. According to Kaneko in the Encyclopedia of Neuroscience, “their pause begins just before (about 15 milliseconds) the onset of their movement and a few milliseconds before the burst in medium lead burst neurons.” Burst neurons, by the way, are just what they sound like: they fire quickly and often, and are contrasted against inactivity. Therefore, omnipause cells are the exact opposite of the quick-time bursts of saccades. Instead of bursting to life, they remain on unless otherwise told to pause.
Certain mental illnesses may actually affect the efficiency of sacadic eye movements. According to Bittencourt 2013, depression symptoms may be correlated with “a deficiency in the ability to select or inhibit the responses to competing targets.” This essentially says that depressed patients may be less able to control involuntary reactions like saccadic eye movements. This may not seem like a huge breakthrough, but it could be used as part of a larger diagnostic criteria when looking at patients with potential depression, schizophrenia, or bipolar disorder (the disorders studied in this article).
All of this is happening behind the scenes of our own brain–barring an illness or injury like the aforementioned, you are powerless to actually stop the saccadic movement of your eyes once it’s in action. But that doesn’t mean out can’t see their effects in action. As mentioned, the purpose of saccades is to quickly focus from one point to the next to minimize visual blurriness. But shouldn’t that much quick dashing back and forth eye flickering come with blurriness on its own? The trick is that it does–your brain doesn’t actually get rid of the visual blur that happens during a saccade. It simply edits around it.
In the previous episode, I mentioned that the visual picture you see is actually a pretty heavily chopped and screwed version of reality, processed through a series of filters that contextualize what you see and makes it easy to understand. One of these filters serves to edit around saccadic blur to improve clarity of sight. How does it work? As always, it’s easiest to think of in terms of photograph and film metaphors. If the editor of a movie finds the visuals of one shot to be too blurry or unfocused, they might instead cut to the next shot early, and lay the audio track over the new footage. In a similar way, your brain replaces the blurry “footage” of your saccade with the “shot” that the saccade lands on. Instead of literally seeing the future, you use peripheral vision, complex predictions, as well as a bit of memory manipulation to completly cut the saccadic blur out of your sight. Since saccades are to quick for you to consciously perceive, you don’t even notice that this process is happening every time you look around.
To see this phenomenon for yourself, there’s a few tricks you can try. My favorite involves a clock’s second hand–if you look away from the clock and then quickly glance at the quick moving hand, you’ll notice that it lingers ever so slightly longer before ticking to the next second. It’s not a magic trick–your brain overlayed the image of the second hand over your eye’s saccade, so it seemed like it spent more time in that position. An even simpler trick is to look at your on eyes up close in a mirror. You may have noticed that you can never actually catch your eyes moving while looking from one eye to the other. You brain is so effective at editing out saccades that you can’t see your own eye movement while it’s happening!
Saccades are just one interesting look inside the brain’s involvement in vision and the importance of interpretation and perception. In our final episode, we’ll drive deeper and discover more of what makes human vision so unique. Thanks for reading!
Sources: https://drive.google.com/drive/folders/1eDLUMuFSUlWkMnN162_7eTB87RbqwguM?usp=sharing