Perception Laboratory: Basic Visual Processing

Purpose: The purpose of this laboratory is to familiarize you with some basic visual processes. You will learn about aspects of lightness perception as well as filling-in processes.

1. The Ganzfeld
The first lab exercise doesn't require the computer, but instead another sophisticated technological device — the plastic spoon! You will need to assume a relaxed position in your chair, with your head slightly back. Then place a spoon over each eye so that it rests comfortably over the eye, with the handle toward your ear. You'll need to remain in this position for about 10 minutes, so get comfortable. It's very important that you not dislodge the spoon, or else you will harm the effect. Holding the spoon in place gets tiring, however, so try to find a good balance point. And, of course, keep your eyes open!

Keep in mind that your retina is being exposed to a fairly constant amount of light throughout the exercise. Thus, you might expect that your perceptual experience would remain unchanged. Instead, something interesting should happen after about 10 minutes. What does your experience tell you about the relationship between your perception and the stimulation of your receptors (eyes in this case)?

The spoon eliminates edges from your perceptual experience. What might this exercise tell you about the importance of edges?

2. Mach Bands
Look at the figure below. It is comprised of 5 achromatic colors (black, grays, white). Focus on the edges that mark the transition from one color to the next. You should notice a faint darker stripe to the dark side (left) of the edge and a faint lighter stripe to the light (right) side of the edge. Those faint stripes are called Mach Bands after Ernst Mach. And, of course, those faint stripes are not present in the stimulus, but are a byproduct of your visual system — particularly lateral inhibition due to horizontal connections in the retina.

For example, in the figure below, which is similar to the two bars on the far right in the figure above, can you determine how each ganglion cell would fire (given the receptive fields illustrated)? (Can you rank the firing patterns of the five receptive fields (A-E)?) When you compare the respective firing patterns, do they seem compatible with the experience of Mach bands? (You may want to focus your comparison on A-B and D-E.)

3. Lightness in Context
Your perceptual experience is driven by the stimulus impinging on your receptors. But is that the only contributor to your perceptual experience? As you saw with Mach Bands, your perceptual systems contribute to the ultimate perceptual experience. Another example of a perceptual experience that differs from the level of stimulation on the retina is found in simultaneous lightness contrast. Look at the three figures below. As you might imagine, the two gray squares are identical. However, your perceptual experience of the gray rectangles should differ substantially. In which setting do the two squares seem most similar? Most different? Why? (Reflect on the nature of the background as you consider these questions.)

A.

B.

C.

It may help to think in terms of what's happening at the edges. However, you should realize that it's unlikely that a complete explanation for simultaneous lightness contrast can emerge from thinking about light patterns in the stimulus (cf. Purves & Lotto, 2003).

You can also experience lightness contrast in dynamic form.

From these three examples of simultaneous lightness contrast (A, B, & C), you may reach the conclusion that lightness is determined by the immediately surrounding context. That's not likely the case, as the following examples (D, E, & F) illustrate. Articulate why the lightnesses in these examples are not determined by the immediately surrounding context.

D.

E.

F.

Our perception of lightness is also determined by our perception of the amount of light falling on the scene. For the scene below (courtesy of Ted Adelson), the two squares (A and B) are identical in lightness. Why do you perceive them as so different in lightness?

In the room pictured below, each of the gray ellipses is identical, but you will not likely perceive them as equal in lightness. Explain why not?

4. A Variety of Lightness Illusions and Demonstrations

4a. Go to R. Beau Lotto's site for Brightness Illusions. You'll see a set of 15 choices as seen below (without the gray letters):

I think that you'll find most of these illusions fascinating, so you may want to explore all of them. However, let's focus on just a small number of them for the purposes of this lab. On your Lab Response Sheet, try to explain the following illusions as best you can [C, H, I (think of Adelson's demonstration above), L]. What principles seem to be operating in producing the experiences that emerge?

b. Go to Adelson's site to experience a variety of lightness illusions and demonstrations. Again, you should find all the demonstrations interesting, but for this lab you can focus on the following illusions: Simultaneous Contrast, Craik-O'Brien-Cornsweet Effect, and the Vasarely Illusion.

5. Hermann Grid, etc.
Another example of perceptual experience that differs from the actual stimulation is found in the Hermann Grid. Look at the stimulus below and try to think of a reason that you see the dark intersections. Of course, the dark intersections are not really present, because when you go to look at them, they turn into white intersections. What's happening?

Here's a very similar stimulus. What happens when you look at this stimulus? Why does your experience of this stimulus differ from the one above?

OK, take a look at this stimulus. What happens as you look at the stimulus? How might you explain the effects that you experience? You may even get the sense of motion from this stimulus, even though it is clearly static.

6. Filling-In Processes: The Blind Spot

As you know, no photoreceptors are present at the optic disk (the place where the ganglion cells comprising the optic nerve leave the eye). Thus, you simply cannot see any object that falls on that point of the retina. At the same time, we don't walk around with the sense that we have two little holes in our visual field. Many people think that because the blind spots are located in different parts of each eye, they compensate for one another to provide us a sense of a world without holes. But if you close one eye, you still don't see a hole, so you don't need to use both eyes to eliminate the "hole." Instead you "fill in" the hole through a fairly sophisticated process (Ramachandran, 1992).

Try out the demonstrations below to see how the filling-in process works with various stimuli. To avoid using one eye's blind spot exclusively, some demonstrations will use the right eye and some will use the left eye. Focus on the X with the eye appropriate for the demonstration. Then move closer and farther from the screen in order to place the black circle within your blind spot. If you wear glasses, it may help to remove them as you do these exercises. As you go through the demonstrations, try to keep in mind what each tells you about the functioning of the filling-in processes.

a. [Close LEFT eye for this demonstration.] In the simple figure below, what happens when you place the circle in your blind spot? What is your perceptual experience?

b. [Close RIGHT eye for this demonstration.] OK, now the figure below is essentially the same as the one above (straight vertical line). However, you may have a different perceptual experience. What do you see? (Focus particularly on what happens right at the blind spot. Do you fill in with red or green?)

c. [Close LEFT eye for this demonstration.] Let's try one more simple stimulus (a straight vertical line). Once again, focus on your perceptual experience right at the blind spot as you describe your perceptual experience.

d. [Close RIGHT eye for this demonstration.] Now let's try a slightly more complex stimulus. What happens at the blind spot in this demonstration? Look carefully at the stimulus. Can you explain why you get the particular effect? Do you start to get a sense of the level of sophistication to these filling-in processes?

e. [Close LEFT eye for this demonstration.] Now here's another interesting stimulus. What happens at the blind spot this time? Why might that be?

f. [Close RIGHT eye for this demonstration.] Look carefully at the stimulus below with both eyes first. The vertical and horizontal lines don't line up, right? OK, now place the circle on your blind spot and see what happens. Why might that be?

g. [Close LEFT eye for this demonstration.] Look at the stimulus below. Note that the stimulus is getting more complex than preceding stimuli. What happens when you place the circle on the blind spot?

h. [Close RIGHT eye for this demonstration.] Now let's add a bit more complexity to the stimulus. Note that this stimulus is largely the same as the preceding stimulus (just more horizontal lines added to the right). Does your perceptual experience change? Why might your perceptual experience differ from that produced by the preceding stimulus?

i. [Close RIGHT eye for this demonstration.] OK, let's modify the above stimulus just a bit by removing information. What happens when you focus the circle below on your blind spot? Again, think about why your perceptual experience may have differed for this stimulus compared to the preceding one.

j. [Close RIGHT eye for this demonstration.] OK, let's remove even more information. Now what happens?

k. [Close LEFT eye for this demonstration.] The use of oblique lines has no impact on the filling-in processes. What happens when you look at the figure below?

l. [Close LEFT eye for this demonstration.] Notice the importance of the "local" information. What happens at the blind spot in the stimulus below? Why don't you see horizontal lines at the blind spot?

m. [Close LEFT eye for this demonstration.] Finally, what happens to Mickey when he's placed on your blind spot? What does that tell you about the complexity and sophistication of the filling-in process?