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Figure 2-6. A typical blind spot pattern

 

You may need to move the book back and forth a little. Try to notice when the black circle reappears as you increase the distance, then move the book closer again to hide the circle totally. It's important you keep your right eye fixed on the cross, as the blind spot is at a fixed position from the center of vision and you need to keep it still to find it.

Now that you've found your blind spot, use Jeffrey Oristaglio and Paul Grobstein's Java applet at the web site Serendip (http://serendip.brynmawr.edu/bb/blindspot; Java) to plot its size.

The applet shows a cross and circle, so, as before, close your left eye, fix your gaze on the cross, and move your head so that the circle disappears in your blind spot. Then click the Start button (at the bottom of the applet) and move your cursor around within the blind spot. While it's in there, you won't be able to see it, but when you can (only just), click, and a dot will appear. Do this a few times, moving the cursor in different directions starting from the circle each time.

Again, be careful not to move your head, and keep focused on the cross. You'll end up with a pattern like Figure 2-7. The area inside the ring of dots is your blind spot.

Figure 2-7. Matt's blind spot mapped

 

Here's a fun way of playing with your blind spot. In a room of people, close one eye and focus on your index finger. Pick a victim and adjust where your finger is until your blind spot makes his head disappear and the background takes its place. Not very profitable, but fun, and not as obvious as making as if to crush his head between your thumb and index finger.

T.S.

How It Works

The blind spot for each eye corresponds to a patch on the retina that is empty of photoreceptors. With no photoreceptors, there's nothing to detect light and turn it into information for use by the visual system, hence the blind spot.

Each receptor cell is connected to the brain via a series of cells that aggregate the signal before reporting it to the brain by an information-carrying fiber called an axon (see [Hack #9]). Bizarrely, the part of the photoreceptor responsible for detecting light is behind the fibers for carrying the information into the brain. That's rightthe light-sensitive part is on the side furthest from the light. Not only does this seem like bad design, but also it means that there has to be a gap in surface of the retina where the fibers gather together to exit the eyeball and run to the brainand that exit point is the blind spot.

At first sight, there doesn't appear to be any particular reason for this structure other than accident. It doesn't have to be this way. If the light-detecting parts of the cells were toward the light, you wouldn't need a blind spot; the fibers could exit the eye without interrupting a continuous surface of photoreceptors on the retina.

Can we be sure that this is a bug and not a feature? One bit of evidence is that in the octopus eye it was done differently. The eye evolved independently in octopuses, and when it did, the retinal cells have the photoreceptors in front of the nerve fibers, not behind, and hence no blind spot.



Conversely, there are benefits to the arrangement of the human retina: it allows a good blood supply close to the retina to both nourish the photoreceptors and help metabolize debris that accumulates there. Both orientations of the retina have their advantages.

 

We don't normally notice these two great big holes in our field of vision. Not only do our eyes move around so that there's no one bit of visual space we're ignoring, but the blind spots from the two eyes don't overlap, so we can use information from one eye to fill in the missing information from the other.

However, even in situations in which the other eye isn't providing useful information and when your blind spot is staying in the same place, the brain has evolved mechanisms to fill in the hole.1 This filling in is why, in the demostration above, you see a continuous grey background rather than a black hole.

Hacking the Hack

The Cheshire Cat experiment (http://www.exploratorium.edu/snacks/cheshire_cat.html; full instructions) shows a really good interaction of the blind spot, the filling-in mechanisms and our innate disposition to notice movement competing against our innate disposition to pay attention to faces. With a blank wall, a mirror, and a friend, you can use your blind spot to give yourself the illusion that you can slowly erase your friend's head until just her smile remains.

End Note

1. "Seeing More Than Your Eye Does" (http://serendip.brynmawr.edu/bb/blindspot1.html) is a fun tour through the capabilities of your blind spot (the link at the bottom of each page's article will lead you to the next page). It demonstrates how your brain uses colors and patterns in the area surrounding the blind spot to make a good guess of what should be in the blind spot itself and will report that to your conscious mind.

See Also

· Ramachandran, V. S. "Blind Spots." Scientific American, May 1992, 86-91.

· Ramachandran, V. S., & Gregory, R. L. (1991). Perceptual filling in of artificially induced scotomas in human vision. Nature, 350, 699-702.

· There is an interesting discussion of the blind spot, filling in, and what that implies for the nature of experience in Daniel Dennett's Consciousness Explained, 344-366. Boston: Little, Brown and Co., 1991.

 

 


 

 

Hack 17. Glimpse the Gaps in Your Vision Our eyes constantly dart around in extremely quick movements called saccades. During each movement, vision cuts out. Despite the fact that the eye has a blind spot, an uneven distribution of color perception, and can make out maximal detail in only a tiny area at the center of vision, we still manage to see the world as an uninterrupted panorama. The eye jumps about from point to point, snapshotting high-resolution views, and the brain assembles them into a stunningly stable and remarkably detailed picture. These rapid jumps with the eyes are called saccades, and we make up to five every second. The problem is that while the eyes move in saccade all visual input is blurred. It's difficult enough for the brain to process stable visual images without having to deal with motion blur from the eye moving too. So, during saccades, it just doesn't bother. Essentially, while your eyes move, you can't see. 2.6.1. In Action Put your face about 6 inches from a mirror and look from eye to eye. You'll notice that while you're obviously switching your gaze from eye to eye, you can't see your own eyes actually movingonly the end result when they come to rest on the new point of focus. Now get someone else to watch you doing so in the mirror. They can clearly see your eyes shifting, while to you it's quite invisible. With longer saccades, you can consciously perceive the effect, but only just. Hold your arms out straight so your two index fingers are at opposite edges of your vision. Flick your eyes between them while keeping your head still. You can just about notice the momentary blackness as all visual input from the eyes is cut off. Saccades of this length take around 200 ms (a fifth of a second), which lies just on the threshold of conscious perception. What if something happens during a saccade? Well, unless it's really bright, you'll simply not see it. That's what's so odd about saccades. We're doing it constantly, but it doesn't look as if the universe is being blanked out a hundred thousand times a day for around a tenth of a second every time. Saccadic suppression may even be one of the ways some magic tricks work. We know that sudden movements grab attention [Hack #37] . The magician's flourish with one hand grabs your attention, and as your eyes are moving, you aren't able to see what he does with the other hand to pull off the trick. N.H. 2.6.2. How It Works Saccadic suppression exists to stop the visual system being confused by blurred images that the eye gets while it is moving rapidly in a saccade. The cutout begins just before the muscles twitch to make the eyes move. Since that's before any blur would be seen on the retina, we know the mechanism isn't just blurred images being edited out at processing time. Instead, whatever bit of the brain prepares the eyes to saccade must also be sending a signal that suppresses vision. Where exactly does that signal come from? That's not certain yet. One recent experiment proves that suppression definitely occurs before any visual information gets to the cortex. This isn't the kind of experiment that can be done at home, unfortunately, as it requires transcranial magnetic stimulation (TMS). TMS [Hack #5] essentially lets you turn on, or turn off, parts of the brain that are close enough to the surface to be affected by a magnet. The device uses rapid electromagnetic pulses to affect the cells carrying signals in the brain. Depending on the frequency of the pulses, you can enhance or suppress neuronal activity. Kai Thilo and a team from Oxford University1 used TMS to give volunteers small illusionary spots, called phosphenes, in their vision. When phosphenes were made at the retina, by applying TMS to the eye, saccadic suppression worked as normal. During a saccade, the phosphenes disappeared, as would be expected. The phosphenes were being treated like normal images on the retina. But when the spots were induced later in visual processing, at the cortex, saccades didn't affect them. They appeared regardless of eye movements. So, suppression acts between the retina and the cortex, stopping visual information before the point where it would start entering conscious experience. Not being able to see during a saccade isn't the same kind of obstruction as when you don't see because your attention is elsewhere. That is what happens during change blindness [Hack #40] you don't notice changes because your attention is engaged by other things, but the changes are still potentially visible. Instead, saccadic suppression is a more serious limitation. What happens during a saccade makes it nowhere near awareness. It's not just that you don't see it, it's that you can't. 2.6.3. End Note 1. Thilo, K. V., Santoro, L., Walsh, V., & Blakemore, C. (2004). The site of saccadic suppression. Nature Neuroscience, 7(1), 13-14. 2.6.4. See Also · Saccadic suppression also lies behind the stopped clock illusion [Hack #18] .

 

 


 

 


Date: 2015-12-11; view: 759


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