In A Nutshell
In some cases, sight can be partially restored to people who have been blind for a long time. However, the brain rewires the senses to compensate for blindness, meaning that most long-term blind people may never fully regain their sight. In the blind, the visual cortex can route sound for echolocation and touch for reading Braille. But the blind can still mentally picture objects like tables, except for color.
The Whole Bushel
Blind people can see. They mentally picture objects like tables, except for color. That’s why sighted people have such a hard time understanding the perceptions of the blind. Sighted people detect objects visually by sensing the differences in wavelengths of light, which we perceive as color, to determine where the borders are between different areas or objects. In other words, we’re just defining objects visually by differences in color.
If you don’t believe it, try imagining a table in your mind that has absolutely no color. If your mental image uses black or white, you’re cheating. When we say no color, we mean there can’t be any borders differentiated by color at all. If you can’t do it—which you probably can’t—it’s because in a strange twist, your ability to see has blinded you.
However, if you ask a blind person to mentally picture a table without color, he can do it easily. His mental image of objects aren’t visual, but they are spatial. “My image of the table is exactly the same as a table,” said Paul Gabias, who was blinded shortly after birth. “It has height, depth, width, texture; I can picture the whole thing all at once. It just has no color.” He constructs his images through touch and echolocation, a way of figuring out where objects are by listening to the reflected sounds when he taps his cane or clicks his tongue. Echolocation is the same technique bats use to navigate and track prey. Not all blind people are able to echolocate.
However, the ability of blind people to see in these ways has also rewired some parts of their brains that perceive senses. To understand the difference, let’s look at how sighted people use their brains to see. First, the visual information is routed to the back of the brain in an area called the visual cortex. Next, the information travels to the parietal lobe, which lets us sense where the object is. Then the information goes to the temporal lobe, which tells us what the object is.
Through brain-imaging experiments using fMRI machines, scientists discovered that blind people see by using the same parts of the brain to process touch and sound that sighted people use for vision. For example, the visual cortex processes the touch data that blind people receive from reading Braille. But their perception is spatial—they determine where the dots are located relative to one another in a type of map—rather than visual. The visual cortex also processes the sound information that some blind people use to echolocate. Again, it’s a spatial perception rather than a visual one. However, these maps may be detailed enough at times to allow the blind to hike, ride mountain bikes, and play basketball. There’s even (at least) one blind man who uses echolocation to successfully go hunting.
But there can be a downside to this. In some cases, sight can be partially restored to people who have been blind for a long time. However, after the brain rewires the senses to compensate for blindness, most long-term blind people may never fully regain their sight. “This important brain reorganization represents a challenge for people encountering eye surgery to recover vision, because the deprived and reorganized occipital cortex [where the visual cortex is located] may not be capable of seeing anymore after having spent years in the dark,” said Giulia Dormal, who led a study on sight restoration for the long-term blind. Using fMRI images to compare how a patient processes sight and sound before and after vision surgery, the researchers found that the brain doesn’t totally recover its visual processing. Some overlap in visual and sound processing may remain in the brain even months after surgery, leaving some aspects of vision below normal.