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Shocking insights: What electrical stimulation tells us about how we visualize

Why might your mind's eye be blind while your friend can picture crystal-clear images?

Stingrays and Caesars

Your internal experience is uniquely yours. Arguably, it is the thing that is most yours. Only you know precisely what you see in your mind’s eye, and only you are privy to your inner thoughts. So how would you react if you found out your mind’s eye can be manipulated? And stranger yet, what if I told you our story starts with a peculiar headache remedy from the reign of the Roman Empire?

In the first century AD, Scribonius Largus (Roman emperor Claudius’s royal physician) found a treatment for headaches in a most unexpected source: an electric ray. By placing this strange, shocking fish on his patients’ scalps, Scribonius became the first known person to apply electricity to the brain. Although Scribonius was only looking to make a headache go away, he unwittingly became an early pioneer of a technology that would lead to many insights – including a recent insight into how we visualize and why vividness of visual imagery is different for different people.

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Over the centuries, electrical stimulation to the brain has moved from Scribonius’s electric fish to 17th-century hand-crank generators and finally to modern and scientifically-validated techniques. Much of this exploration was for medical treatments – after all, how do you convince someone to hold a strange, electric fish to their head if not as a way to help ease their pain.

In recent years, however, electrical stimulation of the brain has become an important part of the neuroscientist’s toolbox, allowing researchers to manipulate parts of the brain without having to open the skull. Electrical stimulation has been used to treat depression, epilepsy, Alzheimer’s disease, and schizophrenia and has been used in many studies of memory and attention.

Most interestingly for us, though, is a study from 2020 in which researchers used electrical stimulation to adjust the vividness of people’s visual imagery. Their results support a theory of how we visualize and why we visualize differently.

Before we look at how electrical stimulation works and why it proves to be a wonderful tool for understanding visual imagery and aphantasia, let’s take a step back to better understand what it means to have different levels of vividness in your visual imagery.

Chasing waterfalls

Although not often talked about, visual imagery varies wildly from one person to another. When picturing a waterfall, perhaps you see a pleasant but fuzzy image in your mind’s eye while your friend sees a picture-perfect waterfall. Or, like me, you might be one of the 3% of the population who sees nothing at all. If you aren’t familiar with the differences in visual imagery (and the total lack of visual imagery: aphantasia), check out our primer.

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A representation of how visualizing a waterfall might look to the inner eye of someone with very vivid imagery, somewhat vivid imagery, and aphantasia. Photo by the author

In just the past few years, researchers have started realizing just how much visual imagery varies from person to person. The term aphantasia was coined in 2015, which goes to show how new this field is. Many questions remain to be answered, such as what is visualization, why do people experience it differently, and where do these differences live in the brain?

While previous research has found a relationship between imagery vividness and brain activity in the visual cortex (the area of the brain that your eyes connect to), we don’t understand why this difference exists. Is there something about the visual cortex of someone with hyperphantasia that allows them to picture a crystal clear waterfall? 

To get a better idea of what is going on in our brains when we visualize and why some brains might create clearer images than others, researchers took a page from Scribonius’s book and electrically stimulated different parts of people’s brains to see how this might affect their visualizations.

Although you and I understand electricity much better than Scribonius did, we still don’t truly have a clear picture of what happens when we hold an electric current against our scalp (or a stingray, for that matter). One big breakthrough is the knowledge that neurons, the cells of the brain, communicate via electricity and that we can influence this communication by pushing electricity into the brain via the scalp. While it may seem like a magical brain-controlling technology, it’s important to understand the precision (or lack thereof) with which electrical stimulation targets neurons.

In the same way that Scribonius merely held his electric fish against his patient’s forehead, researchers using electrical stimulation place electrically-charged pads or magnets on the head to push electricity into the brain. (Note that researchers sometimes use a battery to create the electricity – called transcranial direct-current stimulation or tDCS – and other times use a strong electromagnet to do so – called transcranial magnetic stimulation or TMS.) 

Although neuroscientists can open a brain and stimulate individual neurons directly, this is complex and dangerous and is only done when a surgeon already needs to open the skull for a medical procedure. The non-invasive method of electrical stimulation, in which electricity is placed on the scalp rather than directly on an open brain, doesn’t cause individual neurons to fire but instead increases the excitability of the targeted neurons. What do we mean by excitability?

Metaphorical visual cortex neurons
Photo by Teresa Wilde

Imagine (without visualizing, if you wish) a pot of popcorn, full of kernels flying in every direction and causing other kernels to pop when they land. If each kernel is a neuron in your visual cortex, then open brain surgery is like taking the lid off the pot and poking specific kernels, causing them to pop. In non-invasive electrical stimulation, we leave the lid on the pot and turn up the stove burner, not causing any specific kernels to pop but increasing the temperature of each kernel, bringing it closer to popping. This is what we mean by increased excitability: we don’t cause specific kernels to pop but make it easier for any given kernel to pop. In the brain, we don’t cause any specific neuron to fire, but instead bring each one closer to firing. Without this increased excitability, a neuron might only fire if several other neurons trigger it at once. With the electrical stimulation, it might trigger with just a single neuron’s input.

In this way, electrical stimulation (without opening the skull) is not a precise practice in which individual neurons are targeted. But, by changing the excitability of the entire brain region, the neurons start chattering with one another a lot more, shifting the activity in that area. And, perhaps unsurprisingly, when the activity of a brain area shifts, you start experiencing some strange things.

Seeing stars

Let’s say I place my electromagnet at the back of your head, nearest the part of your brain that your eyes connect to and where vision is processed (this is the visual cortex). By pumping electricity into this area, I can make you see stars, just as if you bumped your head, stood up too quickly, or rubbed your eyes too hard. These stars you see – known to scientists as phosphenes – are a result of your neurons firing despite a lack of appropriate visual input. Why do we see stars when we add electricity to our visual cortex?

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Photo by Yash Raut

Think back to our popcorn: if you crank up the burner for a brief moment, any kernel that was close to popping will fire off and you’ll see a sudden bursting of kernels. Pushing a large amount of electricity into your visual cortex is the same idea: it causes some number of neurons to suddenly fire off and thus you see flashing phosphenes.

As researchers induced these phosphenes in more people, they came to realize that some people require more or less electricity to see a flash of light than others. It was almost as if some people were more “ready” to see phosphenes. Returning to our pot of popcorn, this is as if my pot is quite hot with many kernels ready to pop, while yours is on low heat and isn’t close to done. If someone wanted to cause a burst of pops in each pot by turning the burner up for a moment, they’d have to crank up the heat under your pot but might be able to get the same effect with only a small amount of heat for my pot. The same is true in our visual cortices: my neurons might already be more excitable than yours, meaning that a scientist can push just a bit of electricity into my brain to make me see stars while they’d have to push quite a lot into yours.

With this in mind, researchers can measure how “excitable” someone’s visual cortex is by seeing just how much electricity they need to pump in before causing the light show. So, in an attempt to find out what differences exist in the brains of people with very different levels of visual imagery, scientists used this method to measure the excitability of the visual neurons in people with different levels of imagery vividness. Their question was – does the ability to create vivid visual imagery in one’s mind’s eye have something to do with how ready and able that person’s visual neurons are to fire? Does excitability in visual areas relate to vividness of visual imagery?

Vividness of visual imagery and excitability of visual cortex
The more excitable someone’s visual cortex, the less vivid their visual imagery. Cortical excitability is measured by seeing how much electricity is needed to induce phosphenes while imagery vividness is measured through the binocular rivalry task, explained later in this article.

And their finding: it absolutely does. The more vivid someone’s mind’s eye, the less excitable their visual cortex. If I can push just a bit of electricity into the back of your head and cause you to see bright flashes of light, chances are you have a weak ability to visualize. On the other hand, hyperphantasics, who see picture-perfect imagery, require a high level of electricity to see phosphenes. 

“When we found that cortical excitability was negatively correlated with imagery strength, we were at first surprised,” lead researcher Rebecca Keogh told me.
“But as all of the other experiments started to line up showing the same trend, we became excited that we had found a potential underlying mechanism that explains individual difference in imagery ability.”

As exciting and satisfying these results were, the researchers couldn’t stop there. They had shown that phosphene thresholds can predict the vividness of someone’s imagery, but they weren’t sure if an excitable visual cortex causes low vividness. Dr. Keogh explained that “it might be that people with stronger imagery tend to have less excitable cortexes but this plays no causative role in imagery ability. So to be sure that cortical excitability really is playing a part in individual differences in imagery ability we needed to show that changing cortical excitability likewise changed imagery strength.”

And that’s where the sci-fi-like research of modifying your internal experience comes in.

Turning up the heat

We’ve seen how researchers can measure your visual cortex’s excitability using electricity: pump in a jolt of electricity and see how much is required to cause you to see flashes of light. The smaller the jolt that worked, the more excitable your cortex is. This strong but short-lived jolt of electricity is caused by an electromagnet (TMS) and is akin to turning the burner up high for just a second. But neuroscientists can also use a slightly different flavor of electrical stimulation that relies on a battery instead of a magnet and applies a weaker but consistent electric current (tDCS). This is more like raising the temperature of the burner slightly and leaving it there.

When researchers use tDCS to drive a small amount of electricity in a sustained manner, they make an area of your brain more excitable – or, if they swap around the negative and positive terminals, less excitable (like turning down the burner). The difference here is that, instead of a short-lived effect that lets researchers measure excitability, this electrical stimulation holds its effect long enough to allow researchers to temporarily modify the excitability of part of your brain.

With this tool in hand, all the researchers must do is adjust the excitability of someone’s visual cortex and see if this causes them to visualize more or less vividly. Based on the previous data with phosphene measurements, we’d expect that if we make your visual cortex more excitable, you’ll see less vivid imagery, and if we make it less excitable, your imagery will be more vivid.

While this seems straightforward, there’s another issue to tackle: how can we know for sure if your vividness is changing? You may feel like the images you are seeing are more or less vivid than before, but it can be difficult to know for sure how this is changing (particularly if the change is subtle).

Luckily, there are ways to measure how vivid your visual imagery is without relying on your subjective feedback. The test these researchers used for this purpose is the binocular rivalry task, which, in brief, works as such:

I show your left eye a blue circle and your right eye a red circle. Because these images perfectly overlap, you can’t see both at once and your brain has to pick which to see, often flipping back and forth between seeing the blue and the red circle. By showing you a blue circle before showing you the overlapping circles, I prime your brain to land on the blue circle and thus you’re more likely to see that instead of the red circle. If instead of showing you a blue circle before showing you the overlapping circles, I merely ask you to picture the blue circle, you’ll still be primed to land on the blue circle instead of the red circle. Importantly, the more vivid your visual imagery, the stronger this priming effect. Thus, by asking you many times to visualize a blue circle, showing you the overlapping circles, and then asking which circle you saw, we can objectively measure how vivid your imagery is.

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Some binocular rivalry setups use 3D glasses such as these to present different images to each eye. Other setups use a series of mirrors or virtual reality headsets. Photo by Sigmund.

You can read more about the binocular rivalry task in our summary, but for the sake of this article, all you need to know is that there is an objective way to measure the vividness of your imagery. 

So now our researchers yield two tools: one to adjust the excitability of the visual cortex and another to objectively measure imagery vividness. With these in hand, they can address the question of causality: of whether or not the excitability in the visual cortex is responsible for how vivid your imagery is. Researchers hooked up 60 people to electrical stimulation that increased or decreased the excitability of their visual cortex and then tested their level of vividness with the binocular rivalry task. Here is what the researchers found:

Changes in Imagery Vividness from Electrical Stimulation
When electrical stimulation is used to decrease the excitability of the visual cortex, visual vividness increases – and vice versa. Electrical stimulation is via tDCS. Vividness is measured by the binocular rivalry task.

Amazingly, they were able to change participants’ internal experience of visualization by changing how excitable their visual cortex is. Though you may believe your visualizations to be something you alone control, these researchers can flip a switch and change how vivid your imagery is.

One thing to note is that this change in vividness is not day and night. The participants didn’t report an obvious change in their imagery when experiencing the electrical stimulation. It’s even possible that their visual vividness didn’t change and that, instead, the incoming electricity caused some other shift in their brain which in turn caused a difference in the binocular rivalry task. However, there’s a large amount of research showing changes in the binocular rivalry task do align with changes in vividness of imagery, suggesting that these participants really were experiencing a shift in their mind’s eye.

The researchers have laid out a solid study, showing that, most probably, a more excitable visual cortex leads to less vivid imagery and vice versa. Why would this be and what does it mean for how we visualize?

The popcorn artist

Let’s return to our pot of popcorn, representing the neurons in our visual cortex. Remember that a very excitable visual cortex is like a hot pot of kernels, ready to pop at a moment’s notice. Now pretend that you’re an experimental artist who only draws pictures in pots of popcorn, poking this kernel and that kernel with a hot poker until the Mona Lisa appears in popped kernels. 

Stimulating primary visual cortex neurons
Photo by Georgia Vagim.

If this is your goal, would you rather work with a hot pot of kernels ready to pop at the slightest nudge or a cool, tranquil pot with not much going on? Remember that, when one kernel pops, it’s likely to set off other kernels. So which pot would let your masterpiece shine, and which would cause it to be a fuzzy and unclear art piece? Hopefully, you’ve picked the cold, unexcitable pot. It’d be impossible to draw a picture in a hot pot of kernels that pop at the lightest touch, as your Mona Lisa would end up being lost among all the accidentally popped popcorn.

For perhaps the same reason a hot, excitable pot leads to a fuzzier image, a more excitable visual cortex leads to less vivid imagery: there is too much noise to draw a clear picture. The less excitable your visual cortex, however, the clearer a picture you can draw.

Who is doing this drawing? Is there an experimental artist in your brain, poking neurons to draw on your visual cortex? In a sense, there is: your prefrontal cortex. You can think of this part of your brain, which sits just behind your eyes, as the “final stop” for normal vision. 

When you look at the Mona Lisa, the picture first enters your eyes, where it is then sent to your visual cortex in the back of your brain, and then (via a few other stops on a very complex track) reaches your prefrontal cortex, which seems to help you be conscious of this perception. But when you visualize the Mona Lisa in your mind’s eye, the opposite happens: your prefrontal cortex sends the signal straight to your visual cortex.

Visualization, then, is when our popcorn artist feels an urge to draw an image on the kernels. Brain imaging studies support this analogy, showing that when you visualize an image, your prefrontal cortex activates and subsequently causes neurons in your visual cortex to fire. 

Visually seeing vs mental imagery in the mind's eye
When the man on the left sees an image, the signal travels from his eyes to his visual cortex and up to his prefrontal cortex. When the man on the right pictures an image, the signal originates in his prefrontal cortex and is sent back to his visual cortex.

We’ve seen why electrical stimulation on the visual cortex might lead to more or less vivid imagery, but what can it say about the prefrontal cortex’s involvement? Can we use electricity to improve the skills of our experimental popcorn artist?

Using a similar setup as before, the researchers can show how electrical stimulation to the prefrontal cortex (rather than to the visual cortex) changes visual imagery.

Changes in Imagery Vividness from Electrical Stimulation
We’ve seen the left side of this plot above: it shows that a less excitable visual cortex leads to more vivid imagery and vice versa. The right side shows that the opposite is true in the prefrontal cortex: increased excitability leads to more vivid imagery.

It turns out that, contrary to our visual cortex, increasing the excitability of our prefrontal cortex leads to more vivid imagery while decreasing this excitability leads to fuzzier imagery. Whereas excitability of our corn kernels (or visual cortex) makes for a worse art piece, excitability of our artist (our prefrontal cortex) leads to a stunning work of art. You can think about this as if, by pumping electricity into the front of our brain and exciting our prefrontal cortex, we’re giving the artist a very hot poker instead of just a warm poker, allowing her to more precisely draw her picture on the corn kernels. Alter the electrical stimulation to instead decrease the excitability of the prefrontal cortex, and the artist’s tools become too cold to draw a clear picture.

To summarize this theory, we can say that someone who has very vivid imagery has either a very excitable prefrontal cortex, a not-excitable visual cortex, or both, and, inversely, an aphantasic has either a less excited prefrontal cortex, a more excited visual cortex, or both.

Visual cortex excitability vs prefrontal cortex excitability
If this theory of visualization is correct, then this grid shows how vividly you might visualize a waterfall based on the excitability of your visual cortex and prefrontal cortex.

Many readers, especially those with aphantasia, will be quick to ask: can electrical stimulation give you imagery? If you excite my prefrontal cortex and inhibit my visual cortex, will my mind’s eye see for the first time?

The answer is an unfortunate “we don’t know”. This study involves people with a wide range of imagery vividness and did not specifically recruit people with aphantasia, and thus cannot speak to whether someone with no visual imagery would suddenly experience imagery. If electrical stimulation increases the vividness of someone with middle-of-the-road visual imagery, there’s certainly a chance that it’d cause an aphantasic to see imagery. However, it’s also entirely possible that the cause of aphantasia is a fundamental difference in the brain that goes beyond the excitability of these brain regions, suggesting that no amount of electric batteries, magnets, or stingrays would open someone’s blind mind’s eye. 

Although there’s much to be learned around visualization and aphantasia, and while vividness of imagery is surely more complex than just an interaction between the prefrontal and visual cortices, this research has given us great insight into imagery. As we’ve seen, when researchers use electrical stimulation to change the excitability of your visual or prefrontal cortex, they appear to adjust the vividness of imagery you experience. This certainly bodes well for the theory that visualization is the reverse of sight – that, while sight involves images entering your eyes and passing through your visual cortex to your prefrontal cortex, your mind’s eye “sees” by passing internal images from the prefrontal cortex to the visual cortex.

So while Scribonius was only looking for a headache remedy, he may have been the first human to manipulate a mind’s eye other than his own. And although they couldn’t have known it, his patients were paving the way to a better understanding of how and why we (well, not all of us) visualize images.

There’s much more to be learned about visualization and the mechanisms in the brain that lead to aphantasia, but a large part of it seems to have to do with how excitable your visual cortex and prefrontal cortex are. Why these differences exist and exactly how they lead to vivid or non-vivid imagery remains to be seen. Best to keep your stingray nearby.

Shocking insights: What electrical stimulation tells us about how we visualize

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