Tag Archives: Visual cortex

Visual cortex implant allows the blind to ‘see’ letters and other shapes

This figure illustrates how dynamic stimulation to the visual cortex enables participants to ‘see’ shapes. Credit: Beauchamp et al./Cell.

Scientists at Baylor College of Medicine in Houston zapped the visual cortex of blind volunteers with specific patterns of electricity that allowed the participants to ‘see’ shapes that were simulated on a computer.

The operating principle of the new setup, which took years to perfect through countless instances of trial and error, is similar to how cochlear implants stimulate nerves of the inner ear in order to enhance hearing.

The researchers hope that this proof of concept might one day serve as a stepping stone for more advanced implants that could restore vision.

Basically, instead of trying to restore retina or other functions in the eyes, such an approach could restore vision by delivering visual information from a digital camera.

This video shows a blind participant drawing letters based on dynamic stimulation to the visual cortex. Credit: Beauchamp et al./Cell.

While there’s still a long way to go before something like that could happen, the proof of concept is impressive, to say the least.

“When we used electrical stimulation to dynamically trace letters directly on patients’ brains, they were able to ‘see’ the intended letter shapes and could correctly identify different letters,” senior author Daniel Yoshor says. “They described seeing glowing spots or lines forming the letters, like skywriting.”

This image shows different letter-like shapes (W and Z) created by different dynamic stimulation patterns, with the stimulation pattern on the left and the participant drawings on the right. Credit: Beauchamp et al./Cell.

The research team worked with five participants, three sighted and two blind individuals. All participants had previously undergone surgery that implanted electrodes in the visual cortex, which is responsible for processing information pertaining to light that hits the retina. The sighted individuals originally had electrodes implanted for an epilepsy trial.

In order to zap letters and other shapes into the field of vision of the participants, the researchers looked to generate phosphenes (from the Greek phos, light, and phainain, to show) — the illusion of light that occurs without light actually hitting the retina.

If you ever rubbed your eyes in a pitch-black room and then started seeing tiny pricks of light, those are phosphenes right there. This phenomenon is commonly known as “seeing stars”.

The area of the visual cortex where electrodes were implanted works similarly to a map, in the sense that different regions of the visual cortex correspond to different zones in our field of vision. Some regions, for instance, process information in the upper right corner of our field of vision. Some believe if you implant enough electrodes to cover the whole cortex, one might be able to turn it into a digital computer screen.

For their study, the authors swept an electrical current across several electrodes. This sweeping pattern of electricity was translated into shapes that the participants could ‘see’.

The generated shapes were very simple. Nothing crazy for now.

“Rather than trying to build shapes from multiple spots of light, we traced outlines,” says first author Michael Beauchamp. “Our inspiration for this was the idea of tracing a letter in the palm of someone’s hand.”

Writing in the journal Cell, the authors claim that their proof-of-concept could allow the blind to regain their ability to detect and recognize visual forms. Being able to see the shape of objects in the household or that of another person could prove to be a huge leap in the quality of life.

“The primary visual cortex, where the electrodes were implanted, contains half a billion neurons. In this study we stimulated only a small fraction of these neurons with a handful of electrodes,” Beauchamp says. “An important next step will be to work with neuroengineers to develop electrode arrays with thousands of electrodes, allowing us to stimulate more precisely. Together with new hardware, improved stimulation algorithms will help realize the dream of delivering useful visual information to blind people.”

How the eye works

 

Image via flickr. 

Doing some light reading

Touch interprets changes of pressure, texture and heat in the objects we come in contact with. Hearing picks up on pressure waves, and taste and smell read chemical markers. Sight is the only sense that allows us to make heads and tails of some of the electromagnetic waves that zip all around us — in other words, seeing requires light.

Apart from fire (and other incandescent materials), bioluminiscent sources and man-made objects (such as the screen you’re reading this on) our environment generally doesn’t emit light for our eyes to pick up on. Instead, objects become visible when part of the light from other sources reflects off of them.

Let’s take an apple tree as an example. Light travels in a (relatively) straight line from the sun to the tree, where different wavelengths are absorbed by the leaves, bark and apples themselves. What isn’t absorbed bounces back and is met with the first layer of our eyes, the thin surface of liquid tears that protects and lubricates the organ. Under it lies the cornea, a thin sheet of innervated transparent cells.

Behind them, there’s a body of liquid named the aqueous humor. This clear fluid keeps a constant pressure applied to the cornea so it doesn’t wrinkle and maintains its shape. This is a pretty important role, as that layer provides two-thirds of the eye’s optical power.

Anatomy of the eye.
Image via flikr

The light is then directed through the pupil. No, there’s no schoolkids in your eye; the pupil is the central, circular opening of the iris, the pretty-colored part of our eyes. The iris contracts or relaxes to allow an optimal amount of light to enter deeper into our eyes. Without it working to regulate exposure our eyes would be burned when it got bright and would struggle to see anything when it got dark.

The final part of our eye’s focusing mechanism is called the crystalline lens. It only has half the focusing power of the cornea but its most important function is that it can change how it does this. The crystalline is attached to a ring of fibrous tissue on its equator, that pull on the lens to change its shape (a process known as accommodation), allowing the eye to focus on objects at various distances.

Fun fact: You can actually observe how the lens changes shape. Looking at your monitor, hold your up hands some 5-10 centimeters (2-4 inches) in front of your eyes and look at them till the count of ten. Then put them down; those blurry images during the first few moments and the weird feeling you get in your eyes are the crystalline stretching to adapt to the different focal vision.
Science at its finest.

After going through the lens, light passes through a second (but more jello-like) body of fluid and falls on an area known as the retina. The retina lines the back of the eye and is the area that actually processes the light. There are a lot of different parts of the retina working together to keep our sight crispy clear, but three of them are important in understanding how we see.

  • First, the macula. This is the “bull’s eye” of the retina. At the center of the macula there’s a slight dip named the fovea centralis (fovea is latin for pit). As it lies at the focal point of the eye, the fovea is jam-packed with light sensitive nerve endings called photoreceptors.
  • Photoreceptors. These differentiate in two categories: rods and cones. They’re structurally and functionally different, but both serve to encode light as electro-chemical signals.
  • Retinal pigment epithelium. The REP is a layer of dark tissue whose cells absorb excess light to improve the accuracy of our photoreceptors’ readings. It also delivers nutrients to and clears waste from the retina’s cells.

So far you’ve learned about the internal structure of your eyes, how they capture electromagnetic light, focus it and translate it into electro-chemical signals. They’re wonderfully complex systems, and you have two of them. Enjoy!

There’s still something I have to tell you about seeing, however. Don’t be alarmed but….

The images are all in your head

While eyes focus and encode light into the electrical signals our nervous system uses to communicate, they don’t see per se. Information is carried by the optical nerves to the back of the brain for processing and interpretation. This all takes place in an area of our brain known as the visual cortex.

Brain shown from the side, facing left. Above: view from outside, below: cut through the middle. Orange = Brodmann area 17 (primary visual cortex)
Image via wikipedia

Because they’re wedged in your skull a short distance apart from each other, each of your eyes feeds a slightly different picture to your brain. These little discrepancies are deliberate; by comparing the two, the brain can tell how far an object is. This is the mechanism that ‘magic eye’ or autostereogram pictures attempt to trick, causing 2D images to appear three dimensional.  Other clues like shadows, textures and prior knowledge also help us to judge depth and distance.

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The neurons work together to reconstruct the image based on the raw information the eyes feed them. Many of these cells respond specifically to edges orientated in a certain direction. From here, the brain builds up the shape of an object. Information about color and shading are also used as further clues to compare what we’re seeing with the data stored in our memory to understand what we’re looking at. Objects are recognized mostly by their edges, and faces by their surface features.

Brain damage can lead to conditions that impair object recognition (an inability to recognize the objects one is seeing) such as agnosia.  A man suffering from agnosia was asked to look at a rose and described it as ‘about six inches in length, a convoluted red form with a linear green attachment’. He described a glove as ‘a continuous surface infolded on itself, it appears to have five outpouchings’. His brain had lost its ability to either name the objects he was seeing or recognize what they were used for, even though he knew what a rose or a glove was. Occasionally, agnosia is limited to failure to recognize faces or an inability to comprehend spoken words despite intact hearing, speech production and reading ability.

The brain also handles recognition of movement in images. Akinetopsia, a movement-recognition impairing condition is caused by lesions in the posterior side of the visual cortex. People suffering from it stop seeing objects as moving, even though their sight is otherwise normal. One woman, who suffered such damage following a stroke, described that when she poured a cup of tea the liquid appeared frozen in mid-air, like ice. When walking down the street, she saw cars and trams change position, but not actually move.