Hack66.Trick Half Your Mind


Hack 66. Trick Half Your Mind

When it comes to visual processing in the brain, it's all about job delegation. We've got one pathway for consciously perceiving the worldrecognizing what's whatand another for getting involvedusing our bodies to interact with the world out there.

The most basic aspects of the visual world are processed altogether at the back of your brain. After that, however, the same visual information is used for different purposes by two separate pathways. One pathway flows forward from the back of your brain to the inferior temporal cortex near your ears, where memories are stored about what things are. The other pathway flows forward and upward toward the crown of your head, to the posterior parietal cortex, where your mental models of the outside world reside. Crudely speaking, the first pathway (the "ventral" pathway) is for recognizing things and consciously perceiving them, whereas the second (the "dorsal" pathway) is for interacting with them. (Well, that's according to the dual-stream theory of visual processing [Hack #13] .)

The idea was developed by David Milner and Melvyn Goodale in the 1990s, inspired in part by observation of neurological patients with damage to one pathway but not the other. Patients with damage to the temporal lobe often have difficulty recognizing thingsa toothbrush, saybut when asked to interact with the brush they have no problems. In contrast, patients with damage to the parietal lobe show the opposite pattern; they often have no trouble recognizing an object but are unable to reach out and grasp it appropriately.

Since then, psychologists have found behavioral evidence for this separation of function in people without neurological problems, using visual illusions.

6.6.1. In Action

In the mid-'90s, Salvatore Aglioti1 and colleagues showed that when people are presented with the Ebbinghaus illusion (see Figure 6-6) they find the disk surrounded by smaller circles seems larger than an identically sized disk surrounded by larger circles, and yet, when they reach for the central disks, they use the same, appropriate, finger-thumb grip shape for both disks. The brain's conscious perceptual system (the ventral pathway) appears to have been tricked by the visual illusion, whereas the brain's visuomotor (hand-eye) system (the dorsal pathway) appears immune.

Figure 6-6. The Ebbinghaus Illusion. Both central circles are the same size; although they don't look it to your perceptual system, your visuomotor system isn't fooled


There are many examples of situations in which our perception seems to be tricked while our brain's visuomotor system remains immune. Here's one you can try. You'll need a friend and a tape measure. Find a sandy beach so you can draw in the sand or a tarmac area where you can draw on the ground with chalk. Tell your friend to look away while you prepare things.

6.6.1.1 Part 1

Draw a line in the sand, between 2 and 3 meters long. Now draw a disk at the end, about 70 cm in diameter, as in Figure 6-7A. Ask your friend to stand so her toes are at the start of the line, with the disk at far end, and get her to estimate how long the line is, using whichever units she's happy with. Then blindfold her, turn her 90°, and get her to pace out how long she thinks the line is. Measure her "walked" estimate with your tape measure.

Figure 6-7. A draw-it-yourself visual illusion


6.6.1.2 Part 2

Tell your friend to look away again, get rid of the first line, and draw another one of identical length. (You could use another length if you think your friend might suspect what's going onit just makes comparing estimates easier if you use the same length twice.) This time, draw the disk at the end so that it overlays the line, as in Figure 6-7B. Now do exactly as before: get your friend to stand with her toes at the line start and guess the length verbally from where she is, blindfold her, and ask her to walk the same length as she thinks the line is.

6.6.1.3 Part 3

You should find that your friend's spoken estimate of the second line is less than her estimate of the first, even though both lines were the same length. That's the visual illusion. (If you used different length lines, this difference will be in relative terms.) And yet her walked-out estimates should be pretty much the same (i.e., not tricked by the illusion), or at least you should find she underestimates the second line's length far less when walking. That is, her conscious judgment should be tricked more by this illusion (a version of a famous illusion called the Muller-Lyer illusion), than her walked-out estimate, controlled by her dorsal stream.

6.6.2. How It Works

How it works depends upon whom you ask. Advocates of the dual-stream theory of visual processing argue that these demonstrations, of the immunity of our actions to visual illusions, are evidence for the separateness of the dorsal (action) and ventral (perception) streams. The ventral stream is susceptible, they argue, because it processes objects relative to their surroundings, assessing the current context in order that we might recognize things. The dorsal stream, by contrast, is invulnerable to such illusions because it processes objects of interest in egocentric coordinates, relative to the observer, so that we might accurately interact with them.

Doubters of the dual-stream theory take a different view. One reason we are sometimes duped by illusions, and sometimes not, they argue, is all to do with the type of task, far less to do with there being separate processing pathways in our brain. For instance, when we view the Ebbinghaus illusion (Figure 6-6), we are typically asked to compare the two central disks. Yet, when we reach for one of the disks, we are focused on only one disk at a time. Perceptual tasks tend to involve taking context and nearby objects into account, whereas motor tasks tend to involve focusing on one object at a time and, by necessity, using egocentric coordinates to interact accurately. When changing the task conditions reverses these tendencies, the visuomotor system can be found to be susceptible to illusion or the perceptual system invulnerable.

Which argument is right? Well, there's evidence both ways and the debate will probably roll on for some time yet.2,3 What is clear, is that this phenomenon provides yet another example [Hack #62] of how our illusory sense of a unified self keeps all these conflicting processes conveniently out of mind.

Does the world really appear as you're seeing it? Who cares? Just sit back and enjoy the view, accurate or not, while your neurons fight things out.

6.6.3. End Notes

  1. Aglioti, S. et al. (1995). Size contrast illusions deceive the eye but not the hand. Current Biology, 5, 679-685.

  2. Franz, V. H. (2001). Action does not resist visual illusions. Trends in Cognitive Sciences, 5, 457-459.

  3. Milner, D., & Dyde, R. (2003). Why do some perceptual illusions affect visually guided action, when others don't? Trends in Cognitive Sciences, 7, 10-11.

Christian Jarrett



    Mind Hacks. Tips and Tools for Using Your Brain
    Mind Hacks. Tips and Tools for Using Your Brain
    ISBN: 596007795
    EAN: N/A
    Year: 2004
    Pages: 159

    flylib.com © 2008-2017.
    If you may any questions please contact us: flylib@qtcs.net