The colors you see behind your eyes could hold clues for scientists about treating depression

The fireworks your brain creates.
The fireworks your brain creates.
Image: Reuters/Bobby Yip
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As sophisticated as our brains are, they can be easily fooled.

Take vision for example: Our eyes gather in information about the outside world and signal to the optic nerve, which turns this info into electrical signals in a part of the brain called the visual cortex. Our visual cortex tells us what we’re looking at: a loved one, a landscape, or a computer screen. (They also flip the image right-side-up for us; our retinas see everything upside down.)

But we can fairly easily trick our brains into thinking we’re seeing light when our eyes are shut and are sitting in the dark.

Any time you apply (gentle) pressure to your eyes—say, with the palms of your hand onto closed eyelids—you directly tickle your optic nerve, and your visual cortex interprets the pressure into flashes of color against the dark of your lids. These colors are called phosphenes, and they’re a harmless consequence of our physiology.

But they might actually be useful for medical science. Researchers at Stanford University and New York University have found a way to use phosphenes as a metric for the correct dosage of electrical brain stimulation for targeted future therapies.

This kind of stimulation uses tiny amounts of electricity applied directly to brain. Currently, the technique is only used to treat the uncontrollable movement symptoms in extremely sick patients with Huntington’s or Parkinson’s—but it’s not refined enough to target less severe conditions. Researchers believe that it could be used to treat mental illnesses, like depression or obsessive compulsive disorder, if they can understand exactly how neurons responses of low doses of electricity on a smaller scale.

In the new study, published Dec. 8 in the journal Neuron, Josef Parvizi, a neurologist at Stanford and his colleague Jonathan Winawer, a psychologist at NYU, looked at the brains of four patients who were undergoing seizure monitoring as part of their treatment for epilepsy. These patients already had tiny arrays of electrodes implanted in their brains to track the origin of their seizures (asking volunteers to undergo brain surgery for a basic clinical trial would otherwise be too risky).

Parvizi and Winawer applied low levels of electricity directly to patients’ visual cortices. They tested frequencies (at most 10 times the speed of a typical resting heart rate) and amplitudes (up to 5/1000ths of an amp), separated by microseconds, for up to a second at a time. This electricity caused the patients to see floating phosphenes in their field of vision. They asked patients to draw the shapes they saw, and used the drawings to map out exactly how much of the patient’s visual cortex had been stimulated through these electrical pulses.

“You can’t do this in other areas [of the brain] where you don’t have a pre-established map,” Parvizi says. Scientists have a pretty good map of which neurons were connected to each part of our field of vision. This means that they could compare the phosphenes patients saw to known neurons in the visual cortex, and pinpoint the exact reach of electrical pulses in this area of the brain.

So far, this kind of electrical stimulation has been done in animals—but mice can’t exactly tell us exactly what kind of pattern they see or feel. And although doctors can use electricity farther in the brain to to treat patients with advanced movement disorders, scientists have been unsure how this kind of electrical stimulation would work more superficially on a different set of neurons with and with more precision.

In this case, the researchers found that a greater charge led to larger phosphenes, but only up to a point: after certain point, subjects didn’t reporter larger phosphenes, but rather colors with brighter intensity.

It’s still too early to see much clinical applications with direct electrical stimulation on the brain—this study only involved four people, and would need to be replicated much more widely first. But Parvizi hopes that this dosing can be applied to future therapies for conditions that originate in the brain, like depression and obsessive compulsive disorder.