It can be overwhelming to enter the world of neurological disorders because so many things can go wrong with your brain. The more you read and learn, the more it seems as though there are an infinite number of ways to corrupt your conscious experiences. For example, when it comes to your own body, you can lose control of how your limbs behave and lose the belief that your body belongs to you. These are rare problems, but they shine a fascinating light on what our brains must be doing. The back of our brain — the occipital lobe — is particularly involved in processing visual information. You could argue that vision is our richest sense and the one we most rely on in everyday life, so blindness is a terrifying prospect. But blindness isn’t the only thing that can go wrong with your vision. When your occipital lobe is damaged in a specific location, you may see pictures perfectly well, but videos can become impossible.
Your occipital lobe can be broken down into subsections with different visual functions. Your primary visual cortex (also known as area V1) processes many visual features of the light hitting your retina, but as you move deeper into the occipital lobe, you find more specialized areas for features such as color (V4) and motion (V5).
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Think, for a moment, about how much of your visual experience is dominated by motion. I’m sitting in a cafe as I write this, and the world around me never stops moving. Baristas are shifting back and forth behind the counter as they prepare drinks, customers pace as they wait for their coffee, friends smile and chat at their tables, and people like me sit typing away on their laptops. Even when I’m being lazy at home, it’s likely that I’ll see light patterns moving around on my television or pages turning in my book. So what happens if you damage the parts of your brain that process visual motion?
Patients with damage to their V5 area have major problems with seeing objects move in the world, a disorder that has been referred to as akinetopsia (note that damage to other visual areas can also impair visual motion processing). It is difficult to imagine exactly what it feels like to see no motion, but one famous patient known as LM described her experiences particularly well: “People, dogs, and cars appear restless, are suddenly here and then there, but disappear in between”. Pouring a cup of coffee was a daily pain for her. As the liquid flowed from the spout, it would appear frozen like a glacier.
When you can’t see the continuous motion of fluid as it is poured, or the rising liquid in your cup, you will regularly end up with coffee all over the table, because you cannot time your actions effectively. Life becomes a series of frozen frames rather than a smooth, continuous dynamic experience. That is a dangerous world to live in. Just imagine trying to cross the road as you watch an approaching car…
The damage to LM’s brain came from a brain hemorrhage, but similar reports have come from patients who suffer from epileptic seizures that disrupt the normal functioning of area V5.
There are also ways to temporarily disrupt healthy brain activity using transcranial magnetic stimulation (TMS). This method uses a magnetic field on the scalp to stimulate electrical activity in particular areas of the brain. When you target area V5, and click the TMS button with settings that disrupt brain activity, people struggle to correctly identify the direction in which a bunch of dots are moving on a computer screen. They are also slower in trying to catch moving objects with their hand, but perform perfectly well when reaching to grasp a static object. For a brief moment, you give them symptoms similar to akinetopsia.
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Stimulating the brain can create illusory perceptions. You can test TMS at different intensities over the brain to find the thresholds for different types of outcome. Some settings will disrupt activity in the brain while others will introduce activity that has a closer relationship to normal brain function. When you stimulate area V1 at the right intensity, people perceive static spots of light in their visual field known as phosphenes. When you stimulate area V5 at the right intensity, people see moving phosphenes. Although nothing is happening in the external world, stimulated activity in the brain introduces a specific conscious experience that reflects the function of the targeted area.
Visual illusions allow you to experience brain-fabricated motion while looking at perfectly static images. When an image is designed in just the right way to stimulate neurons in area V5, it can appear as though it is moving. The illusion below is a great example. Each consecutive shape in the image is slightly rotated, and every time you move your eyes, your brain interprets the image as if it is physically moving. We are still learning about exactly how these types of images fool the brain into seeing illusory motion, but we know that V5 has a crucial role in producing the effect alongside other brain areas. In fact, if you disrupt your V5 activity with TMS just before looking at illusions like this, you are delayed in seeing them move.
People who suffer from migraines often describe disturbing visual experiences in their everyday life. They report discomfort when viewing striped patterns and are particularly sensitive to light. Motion sickness is also a common symptom when they view particular patterns of visual movement. So when researchers examined the brains of people with migraines, area V5 was a natural target for testing. Brain scans showed that patients who were vulnerable to migraines had a thicker cortex around the V5 area. This may explain part of the story behind their abnormal visual experiences.
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There are some surprising interactions between our perceptions of visual motion and our other mental processes. If I played a continuous whistle sound that increased in pitch, and another that decreased in pitch, which whistle would you connect more closely to an image of a falling rock? The vast majority of you would probably find a stronger connection with the descending pitch sound. The falling pitch just seems to fit with the falling rock, even though there is no obvious natural connection between the two. Cartoons make great use of this association, as you can see below:
Researchers have tested the strength of the association between sound and motion within our psychology. They showed participants a superimposed image of two moving patterns: one moving upward and one moving downward. They asked participants to decide which way these ambiguous images were moving overall. As participants looked at the images, the researchers also played a pure auditory tone that ascended or descended in pitch. When looking at exactly the same image, participants were more likely to report seeing it move upward when listening to an ascending pitch, and downward when listening to a descending pitch. In other words, we don’t just vaguely associate sound with motion. Our sense of sound directly impacts how we see motion in the world. But I guess Wile E. Coyote and his animators could have told you that a long time ago.
In another similar study, participants saw individual written words and had to decide whether they were real or fake words as fast as possible. Some of the real words were related to upward motion (e.g. “rise”) and others were related to downward motion (e.g. “fall”). Behind each of the words, participants saw a pattern of dots which could move in directions that matched or mismatched with the word they saw. Participants were significantly faster to detect real worlds when their meaning matched the background motion, at least when the motion wasn’t too obvious. When there was a mismatch, they could not stop themselves from processing the conflict, even though they presumably wanted to ignore the distracting dots. So if participants saw the word “fall”, but the dots in the background moved primarily upward, the mismatch automatically interfered with their brain and slowed them down in detecting the word. We cannot help but create a link between the meanings of what we see, read, and hear when it comes to motion.
Visual motion is everywhere. Without the ability to detect it, our life loses a considerable amount of safety and excitement. Movement patterns provide a flag for approaching dangers when crossing a road, and help us to stop at the right time when pouring a cup of coffee. They transform the world from a dull photograph to a dynamic movie. Whenever you explicitly notice the pleasures of visual motion, perhaps when watching a stunning ballet or sports performance, feel free to tap the occipital lobe at the back of your head and thank your V5.