There’s just one problem with this straightforward view of the perceptual process. It’s wrong. It may be radically wrong all the way down, but it is quite definitely wrong in its suggestion of a hierarchy where information flows only one way, up from the simple to the complex. In fact, in the brain information flows every which way. Tonight we will look at some of the effects of both lateral (sideways) and top-down information flow in your brain.

First, the lateral flow: In general, both eyes see almost the same thing, and the disparities between the points of view of the two eyes are not only compensated for by the brain but used as an important indicator of depth. But what happens if the two eyes happen to encounter radically different visual worlds? Now the brain cannot fuse the two visions into a single image. The conflict between the two eyes is called binocular rivalry. Rivalry is easy to observe by using a mirror to redirect the gaze of one eye from the gaze of the other. You might expect the brain to simply superimpose the two images, but in fact what happens is more complex: One eye dominates for a time, and then the other. We are seeing the effects of a form of lateral inhibition, as neurons driven by one eye inhibit neurons driven by the other eye. This then is a sideways or lateral influence on the steps of visual processing. Occasionally dominance is partial, and some patches of your visual image will be from one eye, and some from the other. In part this rivalry can be controlled deliberately. Think about the implication: A decision we make can control what we actually see, at this most basic level. This then is an example of a top-down influence, from a "higher" part of the brain (the top) down to a lower one.

So we have a complex rivalry between the eyes. You are probably wondering: Under what conditions does one eye dominate the other? Nice question! Have we got a lab for you!

Two kinds of attention: To formulate clear hypotheses tonight, you’ll need to understand a distinction between two meanings of "attention" in cognitive science:

1. Attention as noticing, or "bottom-up" attention: Some features of the visual world grab our attention. They are the features we notice. In a field of distractors, these features "pop out" without any effort on our part, and draw our eyes to them. Because this notice is driven by feature-detection originating at the sensory input level, we might also call it bottom-up.

2. Attention as heeding, or "top-down" attention: When we pay attention to something we engage in a deliberate effort of focusing our perceptual awareness -- we heed the stimulus that we attend to. This deliberate, "executive" act might also be called top-down.

Notice that noticing is something that grabs us whether we are paying attention or not. Also notice that we can pay attention to any feature of stimulus, including some that are very subtle, not something we might ordinarily notice.

These two senses of attention are thus quite different. It turns out that they are mediated by very different parts of the brain. In our lab, we won’t worry about the underlying brain function, but we will try to pull apart two questions about binocular rivalry:

1. What features grab notice-attention, as indicated by the grabbed eye becoming dominant in situations of binocular rivalry?

2. To what extent can deliberate heedful-attention also control which eye is dominant?

Start by grabbing exactly no more than two partners.

1. Take one mirror for your team. Hold it vertically, with the edge against your nose. Imagine your nose as a hinge, with the mirror swinging like a door in the middle of your face. You’ll quickly see as the mirror swings toward your face that each eye sees a very different scene. Hold the mirror steady for a minute at an angle that affords each eye different views, and wait for rivalry effects to occur. After a time, you should witness the dominance shift from one eye to the other. See if you can switch the dominance by deliberate, heedful attention. By this exercise, you’ll be able to generate and recognize binocular rivalry. Practice until you can deliberately shift from one eye to the other. You may find that switching which eye views the reflected field makes a difference in you ability to switch attention back and forth.

2. Now position yourself as follows: Find a blank patch of wall or blackboard and stand with the wall on your left, about three feet away. Your partner then stands facing you, about three feet away from you (and with the wall about three feet to her right). Use your mirror as before to divide your two eyes. In this case, swing the mirror so that you can see your partner clearly with your right eye, and hold the mirror so that your left eye sees only the blank wall. To make sure you’ve set this up right, close each eye in turn and check that the right sees your partner, and the left just the blank wall.

3. Keep the mirror in this position, and keep both eyes open. You should see your partner’s face -- it dominates the rivalry. Look just at her smile. As you look at her smile, pass a moving object through your left eye field of view -- wave something around in the space between you and the wall to your left. The easiest way to do this is simply to wave your left arm forward and backward so that the left eye can see it. Your partner should remain stationary. Keep looking at her smile while you wave to your left eye. Everything but the smile should disappear. This is the "Cheshire Cat Effect," after the cat in Alice in Wonderland who disappears, except for his smile.

Take turns trying this. Try both eyes, and play around with the effect until you are proficient at generating it.

How does it work? Nobody knows! Yet. Perhaps you can generate a few hypotheses.

With your partner(s), prepare a report addressing this question:

The Cheshire Cat Effect, as you have set it up, may suggest various possibilities. Formulate a hypothesis that predicts the winner in a rivalry. The hypothesis may take account of several factors. Here are a few factors you might want to consider:

1. The rules for central vision may be different than those for peripheral vision. For example, in the Cheshire cat case, everything in the dominant field vanishes (the face) except for the item you focus on in central vision -- the smile or nose. This may lead you to pursue two separate sets of hypotheses, one predicting winners in peripheral vision, and one predicting winners in central vision.

2. Top-down heedful attention clearly tilts a rivalry, and so is a "force" to be factored into your hypotheses. However, some stimuli in the non-heeded eye field may be so compelling of notice that the overwhelm your concentrated heedful efforts.

3. The Cheshire Cat effect obviously suggests that motion is a factor in pulling a rivalry one way or another. You might explore variations on this, looking at speed of movement, extent of it and extent of its influence, or other variables.

4. But static stimuli can also have an effect. For example, are faces especially "magnetic" for attention? How about familiar objects? Etc.

5. You could even explore rivalry when two similar stimuli are in the rivalrous fields. For example, if both eyes see (different) faces, do they fuse into an intermediate face?

6. For any hypotheses you test, at least confirm that it holds with all the members of your team. Note that some people simply never observe the Cheshire Cat effect. If you don’t see it, this does not imply that something is wrong with you! Most perceptual experiments affect people differently, and some people not at all.

This is a fun lab and explores some of the weirdest aspects of our lovely minds. Feel free to be creative in your exploration.

State your hypotheses and describe the most effective experiments in support of it. Two or three good experiments will be sufficient. Don’t forget: Seek disconfirming evidence, too. For example, try out more than one variation that conforms to your hypothesis and make sure that your prediction holds for the variation as well as for the original.

Appendix: Laurie Anderson's Brain

Just for fun, I include a nice expression of the sophomore theory of mind, from Laurie Anderson’s rambunctious rock song, "Baby Doll." Once your students are comfortable with your other eccentricities, they might enjoy listening to it and discussing the theory of brain function it presupposes.

"Baby Doll" from Strange Angels (1989)

I don't know about your brain, but mine is really bossy

I come home from a day on the golf course and all I find are these messages

Scribbled on wrinkled up scraps of paper

And they say things like:

Why don't you get a real job?

Or: You and what army?

Or: Get a horse.

And then I hear this voice coming from the back of my head, Uh huh

Yep! It's my brain again

And when my brain talks to me, he says:

Take me out to the ballgame, take me out to the park

Take me to the movies

'Cause I love to sit in the dark

Take me to Tahiti

'Cause I love to be hot

And take me out on the town tonight

'Cause I know the new hot spot. He says:

Babydoll... I love it when you come when I call

Babydoll... You don't have to talk, I know it all

Well I'm sitting around trying to write a letter

I'm wracking my brains trying to think of another word for horse

I ask my brain for some assistance

And he says: Huh... let's see... how about "cow"?

That's close. He says:

Take me out to the ballgame, take me out to the park

Take me to the movies

'Cause I love to sit in the dark

Take me to your leader

And I say: Do you mean George?

And he says: I just want to meet him

And I say: Come on I mean I don't even know George!

And he says: Babydoll (etc.)