Basic Visual Functions

Demonstrations (Direct Links)

Demonstration 4.1 The Ganzfeld
Demonstration 4.2 Mach Bands
Demonstration 4.3 Involuntary Eye Movements
Demonstration 4.4 Filling in the Blind Spot
Demonstration 4.5 Lightness Constancy
Demonstration 4.6 Mach's Book
Demonstration 4.7 Retinal Location and Acuity
Demonstration 4.8 Accommodation
Demonstration 4.9 Convergence
Demonstration 4.10 Jumbled Text

Before You Start

• Perception is an active process. Change over space (edges) and time are both important for vision. To produce change over time, your eyes are constantly in motion as a result of involuntary eye movements (tremors, drifts, and microsaccades) as well as vergence and version movements. Given all that activity, you might give some thought to the fact that the world appears to be stable.

• You'll encounter a number of examples of the importance of context and the role of experience (consistent with Themes 2 and 4). Moreover, you'll learn that the absolute energy levels reaching the photoreceptors are less crucial than the relative energy levels. That fact emphasizes the ambiguity of the retinal information (consistent with Theme 3). At the same time, our visual system resolves the ambiguity and yields "absolute" information (e. g., "white" as opposed to "the lightest surface in the visual field"). How does it do so?

• How much of perception is driven by "what's out there" and how much is driven by what's inside you (brain/mind)? In examining Mach bands, Mach's book, filling-in processes at the blind spot, etc., you'll encounter examples of perceptual experiences that are not completely tied to the incoming perceptual information. How much of our perceptual experience is constructed? What role does experience play in determining our perceptions?

Prerequisites for Normal Vision

Edges Are Important

• Change over space is important for vision. We call such change in lightness, color, etc. an edge. As you look around you, you'll see all sorts of edges. The very text you're reading is, of course, comprised of edges (primarily from the white background to the black of the text and then back to the white background). To illustrate the importance of edges, you should experience a uniform field called a ganzfeld. A fancy ganzfeld would be most effective, but you can create one using spoons, as illustrated in the demonstration below.

Demonstration 4.1 The Ganzfeld. You can create a uniform white field by placing two white plastic spoons in front of your eyes.  You will look a bit strange, so you might want to do this in private. As you can see below, a classroom demonstration with everyone in class equipped with plastic spoons is a sight to behold! 

Photo by H. Foley

To make the field colored, simply tape colored cellophane to the back of the spoons, or color them with a marker, being sure not to create any visible lines or marks on the spoons.  You might as well sit down, and then all you have to do is look toward a source of light and wait.  After about 10 minutes, regardless of the color of light striking your retina, you will experience a uniform dark gray field — much like what you would experience with your eyes closed on a very dark night.  In fact, your visual experience is similar to having your eyes closed.
            What happens if you introduce an edge into the Ganzfeld?  You can have a friend place a piece of paper in front of your eyes so that a shadow falls over one of the spoons, or raise your foot until it comes between your light source and your spoons.  You will immediately see the field in its original color, but now with an edge falling across it.  As soon as you provide the visual system with a single edge, the uniform gray field disappears. 

• Given the importance of edges, you shouldn't be surprised to learn that our visual system makes great use of edges. Moreover, the lateral connections in the retina created by horizontal cells serve to enhance edges through a process called lateral inhibition. A byproduct of lateral inhibition is a Mach band. As illustrated in the demonstration below, you will perceive sharp edges at the transition from one shade of gray to another, and to either side of the edge a lighter and darker band that serves to highlight the edge.

Demonstration 4.2 Mach Bands You can create Mach bands with a desk lamp.  Turn on the lamp, turn off all the other lights in the room, and make sure that there are no other sources of light (close curtains, etc.).  Move the lamp so that it is about 1 foot from your desk.  Put a white sheet of paper down on your desk under the light. Hold a book a few inches away from the white sheet of paper under the lamp.  Observe the area surrounding the book’s shadow.  Notice how the edge of the shadow looks particularly dark and how the edge of the well-lit part of the white paper nearest the shadow looks particularly light.  Although we seldom notice them, Mach bands are actually common in everyday life.

Photo by H. Foley

Now look at the figure below, focusing on the region just to the right of the arrow.  Find another shade of gray in a different stripe that seems to match that shade as closely as possible. (Does it look more like the gray in 4 or 5 or 6?)

The five interior edges of Demonstration 4.2 all seem to be distinct and appear to have a faint lighter border to the right and a faint darker border to the left of each edge. Keep in mind, however, that all six stripes are simply solid rectangles filled with a single shade of gray, becoming increasingly dark from right to left. The top part of the figure below illustrates the actual changes in intensity.

Mach band fig

In other words, that slightly lighter line on the left side of Rectangles 2 through 6 isn't really there! Nor is the slightly darker line on the right side of Rectangles 1 through 6. (The dark line you see to the right of Rectangle 6 occurs because of the white background. Similarly, except that it's already so dark, you would see a thin dark line to the left of Rectangle 1.) The lower part of the figure above illustrates the presence of these Mach bands.

Can you think of a way to demonstrate that the Mach bands aren't really there? Given what you know about lateral inhibition, what should happen if you placed a white piece of paper to the left side of a gray rectangle? Yes, the lighter strip to the left side of the rectangle should become darker! Try it.

Change Is Important

• Edges represent change over space. But change over time is also crucial for vision. Without change, even if edges are present, an image will fade. To illustrate this principle, researchers have conducted stabilized retinal image research. These researchers have found many creative means of stabilizing an image on the retina, but the take-home message is basically the same -- the image disappears if it remains at the same retinal location.

• Ordinarily we don't have to worry about images being stabilized on our retinas because our eyes are constantly in motion. There are three types of involuntary eye movements: tremors, drifts, and microsaccades. The tremor (physiological nystagmus) is the smallest movement (on the order of the diameter of one cone). Drifts are slow movements and might range over a dozen photoreceptors. Microsaccades are fast (25 ms) jerky motions that cover a wider area (dozens of photoreceptors or more). Microsaccades may correct for changes of eye fixation caused by drifts.

Demonstration 4.3 Involuntary Eye Movements You can actually convince yourself that your eyes are constantly in motion in a couple of different ways. One way takes a bit of work. You'll need to create a single pinpoint of light in a completely dark room. The light cannot be so bright that it illuminates its surroundings. A little LED would work well. Or you could use a small flashlight with cardboard covering the lamp except for a small hole punched in the cardboard. Now, mount the light so that it cannot move and stand about 10 feet away from it. After you have watched the light for a brief while, it will appear to begin moving around slightly. Without other stimuli around the light to serve as points of reference (which would all shift simultaneously with involuntary eye movements), the visual system interprets the shifting position of the light on the retina as being produced by movement of the light rather than movement of the eyes. At one time, people thought that you could use the "moving light" as a kind of projective test, with some people reporting that the moving light spelled out a complex message!

Link - You can also demonstrate that your eyes are in motion when you look at stable images that nonetheless produce a sense of movement. Akiyoshi Kitaoka is a Japanese psychologist who has developed a number of compelling demonstrations of apparent movement that is produced, in part, by the small involuntary movements of your eyes. Check out the rotating acorns of "Dungururin" (among his earliest works). His most widely viewed work is probably the "Rotating Snakes" seen on his opening page. You can also see movement in some op art, such as that produced by Bridget Riley (e.g., Suspension). Once again, what you're actually seeing is a product of involuntary eye movements.

Link - Ikuya Murakami has produced a couple of demonstrations of motion that arises due to involuntary eye movements. In the aftereffect demonstration, it appears that the subsequent motion of the stable inner disc is due to compensation for image slip on the retina. In the more recent demonstration, no afterimage is needed. As you watch the display, the static inner disc appears to move in a random coherent fashion.

Link - Susana Martinez-Conde (Barrow Neurological Institute) has provided an interesting illustration of the role of small involuntary eye movements. When stationary objects are in our peripheral vision, they can fade while we focus on a central fixation point (Troxler's effect). However, once we move our eyes, the object will again be visible.

• With the Ganzfeld or with the entire retinal image stabilized, you can illustrate what the visual system does if there is no change at all on the retina. However, what happens if there is only a localized area with no change? Filling-in or completion processes arise at the blind spot, at scotomas, or when parts of the retinal image are stabilized. It is interesting that our visual systems are capable of filling-in the missing information with quite complex information. The demonstrations below illustrate how your visual system must interpret the context surrounding the blind spot before it can determine how to fill in the missing information.

Demonstration 4.4 Filling in the Blind Spot You experienced the blind spot in Demonstration 3.3. Now I want to make the point that what you fill in at the blind spot is actually quite complex. If you'd like to learn more about the complexities involved, you should read V. S. Ramachandran's (1992) fun Scientific American article on the blind spot.

First, close your left eye and focus on the X below. You'll need to adjust your distance from the screen, but eventually, you should be able to get the red spot to disappear. Note what happens when you do so. You see a continuous blue line, right?

Now, let's add a few more blue lines to the display and see how that affects your perception at the blind spot.

Of course, the display isn't all that different -- just some more blue lines added. Nonetheless, you may well now perceive a solid white line running over the space where the red spot used to be. Note that your visual system appears to be determining what's surrounding the blind spot and filling in with a plausible "filler." In one case a horizontal blue line makes the most sense, so that's what you see. In the other case, a solid white line makes the most sense, so that's what you see. And you can fill in at the blind spot with a filler that's a lot more complex than a simple blue or white line, as seen in the example below:

• Given the importance of change, you'd think that people would be able to detect changes in the world quite readily. And, of course, most of the time we do. But not always! On occasion, we don't notice changes that are quite extreme. Researchers refer to this effect as change blindness. Among the many researchers who work on this effect are Ronald Rensink (University of British Columbia) and Daniel Simons (University of Illinois). Clicking on the researchers' names will bring you to some interesting illustrations of the effect. Some of Rensink's demonstrations are also available here. You'll also find a clever illustration from Richard Wiseman (University of Hertfordshire). (You may also want to check out his site).

Higher-Level Processes and Experience Are Important

• One way to assess the importance of visual experience is to study people who lost their vision early in life, but whose vision was later restored. Three relatively well-known cases are S. B., Virgil, and Michael May. You may even have seen the 1999 movie based on Oliver Sacks' study of Virgil, At First Sight. (Quite appropriately, it didn't win any Academy Awards.)

Link - You can learn more about Michael May by visiting the web site of his company, Sendero Group. A web search will also yield a number of related articles.

Link - S. R. D. was born with congenital cataracts, but had them removed when she was 12. When her vision was tested 20 years later, she exhibited remarkable visual abilities, suggesting that the 20 years of experience had been of great benefit. To learn more about how S. R. D. was tested, check out Pawan Sinha's web site (next to the original paper by Ostrovsky, Andalman, & Sinha).

Perceiving Light Energy

Lightness Energy and Lightness Perception

• Although it is basically true that photoreceptors fire more rapidly when stimulated by light of greater intensity, rate of photoreceptor firing is not sufficient to explain how we perceive lightness. A surface reflects a certain percentage of the incident light, which is the albedo of that surface. White paper might have an albedo of 90% and black print on that paper might have an albedo of 3%. Under very bright light, the black print might actually reflect more light to your eye than would the white paper under very dim light. Nonetheless, under both the very bright and very dim lights, you would describe the paper as white and the print as black. Thus, in order to explain how we perceive lightness, we need a theory that can explain how a surface is perceived as equally light even though it reflects different amounts of light energy under a variety of lighting situations. (And, no, unfortunately we cannot perceive albedo directly.)

• Hermann von Helmholtz (1821-1894) theorized that we perceive lightness by taking the illumination into account (via unconscious inference). If you knew how bright the ambient light was, then it's possible that you could conduct some internal calculations to determine the percentage of the light being reflected from each surface. Unfortunately, Helmholtz did not fully articulate how the process might work. Note that Helmholtz is proposing the importance of context, which was the ambient light in which the stimulus is bathed.

• Hans Wallach (1904-1998) proposed that we determine lightness by considering the ratios of lightness in the visual field. As you look around you, you'll note that some surfaces are very light and some are very dark, with others in between. Note that Wallach was also proposing that context was important (in his case, the surrounding surfaces provided the context). This relative approach could well lead us to notice that one surface is lighter than another, however, it's not at all clear how one arrives at the perception of "white." If confronted with an equally illuminated array of light grays and blacks, we wouldn't typically call the light grays "white" even though they are the lightest surfaces in the array.

• Dale Purves (Duke University) and his colleagues emphasize the essential ambiguity of the information on the retina, but argue that experience allows us to disambiguate the lightnesses involved. If you visit his site, you'll be able to experience a number of compelling demonstrations of the complexities of lightness perception. Pay particular attention to his illustration of the Craik-O'Brien-Cornsweet effect!

• Alan Gilchrist (Rutgers University) and Paolo Bressan (Universita di Padova), among others, have proposed anchoring theories, which add anchors within areas of similar illumination (e.g., "call the lightest surface "white") in an effort to explain lightness perception. There is evidence for such anchoring because Gilchrist and his colleagues have demonstrated that we tend to perceive the lightest surface in the field of view as white. He presented observers with only two surfaces, one gray and one black. However, the observers tended to see the gray surface as white and the black surface as gray.


Lightness Constancy

• Lightness constancy refers to the fact that a surface will be perceived as equally light regardless of illumination. There are a number of different constancies (e.g., size, shape). In order to talk about constancies, it helps to first define a couple of terms. The object "out in the world" is referred to as the distal stimulus. The energy from the object falls on the receptor system (e.g., the retina) to produce the proximal stimulus. Thus, we can refer to a constancy as arising when the perception of the distal stimulus remains roughly the same in spite of changes in the proximal stimulus. So lightness constancy arises due to the fact that in spite of changes in the proximal stimulus (in terms of light energy), we perceive the distal stimulus as staying roughly the same lightness.

Demonstration 4.5 Lightness Constancy To conduct this demonstration, you'll first need to print out the jpeg image below.

You'll note that there is a gray box on the left and then five gray boxes on the right. Your task is to match the box on the left with the box of the same lightness on the right. However, you need to provide the box on the left with different illumination than the ones on the right. Place some thick cardboard or a magazine over the straight line then shine a lamp on one side or the other. Let's shine a light on the left square. (You could also shine the light on the right side.)

Photo by H. Foley

Now your task is to pick out which of the five squares on the right seems equal in lightness to the left square. You know that there is more light energy being reflected from the gray square on the left, but will that lead you to match it with one of the lighter gray squares on the right? If you choose to match the left square with the equally gray square from the right (#3) -- in spite of the different amounts of energy being reflected into your eyes -- then you're exhibiting lightness constancy.

• Ted Adelson (MIT) has provided an illustration of the role of shadows in determining lightness. Look at the display below. What is your perception of the lightness of the two squares labeled A and B?

Obviously A is a dark square in light and B is a light square in shadow, right? Can you believe that they are sending the identical level of light energy to your retina? You can look at the "proof" in the figure below. Both A and B are identical shades of gray! Note that the organization of the scene into lighted areas and shadowed areas must play a role in determining the perceived lightness of the squares. In the "real world," white squares in shadow actually do reflect relatively less light into your eyes than do white squares in bright light. Given what you know about lightness constancy, it should not surprise you to learn that Square B reflects less light than a white square in direct light (such as the one to the left of Square A). Nor would it surprise you that you see both squares as white. What does surprise you is the fact that Square B is reflecting so little light that it is reflecting the same amount of light as a black square in direct light (Square A). They look so very different!

Explanations for Lightness Perception

Perceptual Organization Precedes Lightness Perception

• It is certainly the case that context is important for lightness perception. One way to experience the importance of context is to look at a display of simultaneous lightness contrast. Note in the image below that the identical gray square appears darker against the white background than it does against the black background. The two gray squares should look fairly different to you, even though they are the same lightness.

In the above display, the gray square is lighter than the black background and darker than the white background.What happens when the backgrounds change so that the gray square is lighter than both backgrounds? Do the two small gray squares in the display below seem as different to you?

Now let's look at two small gray squares that are darker than their backgrounds. How different do the two gray squares appear to be now?

It should be clear to you that the background against which a surface is seen can have an impact on the lightness of that surface. Sometimes the impact of the context is more dramatic than at other times. In all these cases, however, the gray squares and their backgrounds seem to be co-planar (at the same distance from you). What happens when you have surfaces at different depths?

• Alan Gilchrist and others have emphasized the importance of organization in determining lightness. That is, before you can determine the lightness of a surface, you need to know where the surface is located. Some of Gilchrist's early research illustrates this point. He had people look through a peep-hole into two adjoining rooms that were illuminated differently. The front room was dimly lit and the rear room was brightly lit. On the wall in the rear room was a white square. Mounted in the doorway separating the two rooms were two pieces of paper, one white and one black. Thus, those two pieces of paper were illuminated by the dim light.

Under one viewing condition (a), Gilchrist arranged the two pieces of paper so that the white piece would appear to be in front of the black piece of paper (and thus in the front room). Under the other viewing condition (b), Gilchrist notched the white piece of paper so that it appeared to be behind not only the black piece of paper in the front room, but also behind the white piece of paper in the rear room. The two viewing conditions are illustrated below.

Keep in mind that the actual position of the white test patch remains the same, which means that it's illuminated by the dim light. That also means that the amount of light from the white test patch falling on the retina is always the same. Can you guess how light the white test patch appeared to viewers under the two conditions? This study illustrates that we need to determine the location of a surface before we can determine its lightness. Thus, depth perception must occur before lightness perception.


• Mach's Book is another illustration of the importance of depth perception for lightness perception. Try the demonstration illustrated below to show yourself how lightness perception changes with changes in perceived depth.

Demonstration 4.6 Mach's Book Ernst Mach developed an interesting demonstration of the impact of "perceived" depth on lightness perception. All you need for this demonstration is a light and an index card. As seen below, fold the index card in half so that it looks like the roof of a house and then tilt it so that one side is exposed to the light and the other side is in shadow.

Photo by H. Foley

Now, close one eye and look at the card. Try to keep the apparent lightness of the two sides in mind as you proceed to the next step. While looking with one eye, imagine that instead of a roof, it's actually a book! That is, in your mind, push the middle away from you, so that it's like the spine of a book and mentally drag the two ends toward you. Once you're able to achieve this mental inversion of the index card, you should have a very different perception of the lightness of the two sides of the index card. The side in shadow should now appear to be darker and the side in light should now appear to be lighter (almost glowing). Note that all that has changed is your "mental set" of what's in your visual field -- an illustration of the power of higher-level processes. [One possible explanation for this effect is that you know that the light comes from the right, so if it were actually a book, then the right side should be in shadow and the left side might be in the light. But because the left side isn't light, but dark, you perceive it as even darker than when it isn't mentally inverted. Likewise, because the right side is lighter than expected, you perceive it as even lighter than when it isn't mentally inverted. It may even seem to glow!]

Dan Kersten (University of Minnesota) and his colleagues have a clever variant of Mach's Book. You can download the file to create the demo for yourself. The red side of the paper will reflect some pinkish light onto the white side of the paper. When seen in the actual spatial relationships, the pink may be quite faint. However, once you invert the image (aided by the construction of the figure), the pink side will appear to be much less faint. Of course, when seen in inverted fashion, the red side could not reflect any light onto the "white" side, so that side must actually be colored pink!

Photo by H. Foley

Note that you don't need mental inversions to convince you that depth is important for lightness perception. As Gilchrist and others have noted, you can often see a column in a room such that one side is in light and the other side in shadow. Though you can perceive a lightness difference in the two sides, it isn't great. That is, your visual system seems to say "yep, it's the same color paint, but on one side it's in the light and on the other side it's in shadow." But what happens if you remove the depth cues? If you look through a reduction screen, so that the depth cues are removed, the differences in lightness are greater. You should look around to experience this phenomenon for yourself. The photograph below doesn't completely capture the difference, but it gives you a sense of what you should look for.

Photo by H. Foley

• Okay, so what do we know about the effects of context (specifically background) on lightness perception? From simple simultaneous lightness contrast displays, etc., we know that when seen against a dark background, a gray surface seems lighter. And when seen against a white background, the same gray surface seems darker. Not exactly simple, but fairly simple, eh? What, then, do you make of the demonstrations below?

First, consider the two triangles in the display below (after Benary/Wertheimer). Does one appear darker than the other?


Shouldn't the two triangles appear to be equally light? After all, the surfaces next to the triangles are identical (two sides surrounded by black and one side surrounded by white). Thus, the only difference between the two triangles is whether or not we organize the triangle as part of the cross or not. When we organize the triangle as part of the black cross, we perceive it against that black background (so it seems lighter). When we organize the triangle as outside the black cross, we perceive it against the white background (so it seems darker).

Okay, then let's consider the display below (after Paola Bressan's dungeon illusion). Which gray squares seem darker?

As you know, when seen against a white background, the gray squares should be darker. Then why are the gray squares seen against the black background darker? Can you see the complex way in which our visual system determines lightness? When the gray squares are seen as members of a group of small white squares, they seem darker than when seen as members of a group of small black squares (even though they're seen against a black background). So, it's not necessarily the immediate background that's crucial, but instead the way in which our visual system has organized the stimuli.

The same "unexpected" result is seen in the display below, which is similar to one produced by Economou, Annan & Gilchrist.

• Bart Anderson (University of Sydney) and his colleagues have explored the importance of organizing space prior to determining lightness. When you look at a scene, you detemine the illumination and lightness of the surface at different layers of a scene. This approach also leads to some very interesting illusions, which you can explore at Anderson's site (go to demos).

Further Complexities in Lightness Perception

• When you look at the world, there are often many different levels of illumination in a single scene. In the example below, you can see the bright sunlight streaming into the room, a lit lamp, and parts of the room that are in shadow. Superimposed on the scene are a bunch of ellipses of exactly the same shade of gray (same lightness). Do all the ellipses seem identical in lightness? Why does the ellipse on the floor in shadow seem lighter than the ellipse in the sunlight? Are your experiences consistent or inconsistent with lightness constancy?

Photo by H. Foley

Link - Note the illusory "folds" that occur in Susana Martinez-Conde's Alternating Brightness Star. They're not there! The artist Victor Vasarely produced similar illusions using squares.


Measuring Acuity

• Central to many visual topics, including acuity, is the notion of visual angle. As you can see in the illustration below, knowing a particular visual angle allows you understand the effect of distance on the size of a target that would fit within that visual angle. Consider the rod-free area of the fovea, which has a visual angle of about 2°. A target with a diameter of 2.4 cm (roughly the size of a quarter) held at 70 cm (roughly arm's length) would have a visual angle of 2°. A target with a diameter of 1.2 cm at 35 cm would also have a visual angle of 2°. So, too, would a target with a diameter of 4.8 cm at 140 cm. Given the interplay of object size and distance, both the sun and the moon subtend roughly the same visual angle (about half a degree), even though the sun has a diameter almost 400 times greater than the moon.

It should also be obvious to you that the same object would subtend different visual angles depending on its distance from you. That is, a quarter would have a very large visual angle if it's near your eye and a very small visual angle if it's at a great distance.

The visual angle is actually measured from a point about 7 mm behind the cornea (or roughly 17 mm in front of the retina). If you're interested in computing visual angles, you need the formula: Tangent (visual angle) = size of object / distance of object.

How does visual angle relate to acuity? People with normal vision (i.e., 20/20) would be able to detect two distinct dots if they are separated by 1' (minute) of arc (with 60' per degree). In terms of the typical Snellen chart, people with normal vision can discriminate letters when the letters subtend 5' of arc. Note that speaking of acuity in terms of visual angle provides useful information about discrimination abilities regardless of the distance of the object from the viewer.

Link - The WebVision site has a useful page about acuity, including examples of different types of acuity that one can measure.


Characteristics of the Eye that Affect Acuity

• A number of factors will affect acuity. Let's first examine factors related to the eye itself. These factors include where on the retina the image is focused, the ability of the lens to accommodate, and the shape of the eye.

• As you may recall from Chapter 3, you see most clearly when an image is focused on your fovea. As you move to the periphery of your retina, the photoreceptors are more likely to be interconnected in a many-to-one fashion. As a result, as the image is focused toward the periphery, it cannot be seen as clearly. The demonstration below is repeated from Chapter 3 to remind you of the fact that objects in the periphery are not seen as clearly.

Demonstration 4.7  Retinal Location and Acuity You probably already saw this demonstration in Chapter 3 (Demonstration 3.2). In case you didn't, here's what to do. Move your head until you're about five inches from the screen and cover your left eye with your left hand. When you focus on the plus sign below, you'll see it clearly because it is focused on your fovea. However, as you remain focused on the plus sign note that the letters to the left become increasingly difficult to read as they become more peripheral. Because the letters to the right become larger toward the periphery, you may still be able to read them.

• It's also the case that you need to focus the image of the object on the retina in order to see it clearly. As you learned in Chapter 3, most of the incoming light is bent (by a fixed amount) by the cornea, but the lens serves to bend the light by a variable amount. In order to see nearby objects clearly, your lens needs to accommodate (become fatter) -- which yields more optical power. The demonstration below illustrates the role of accommodation.

Demonstration 4.8  Accommodation
• First, close your left eye, using only your right eye for this demonstration.  Next, move your left index finger toward your eye until you can see the details of your fingernail clearly.  If you move your finger any closer, it should become blurry. That's your near point (the closest point at which you can see clearly). As you remain focused on your left finger, accommodation has caused your lens to thicken. Now, while maintaining focus on your left index finger, extend your right index finger as far as you can. Notice how blurry your right finger is (as well as any more distant objects that might be in view). With your thickened lens, light rays from distant objects focus on a point in front of your retina, so you can't see them clearly.
• Now focus on your right finger, which will force your lens to become thinner.  Now your left finger will look blurry.  With a thin lens, light rays from nearby objects focus on a point that would be in back of the retina.
• Recall our discussion of a camera's depth of field in Chapter 3. With a lot of light, a small aperture allows a wide depth of field, which means that objects at varying distances from the camera may all be in focus simultaneously. However, with less light and a wider aperture, depth of field narrows. As a result, some objects may be out of focus while others are in focus. When focused on your left index finger, your eyes have a certain depth of field so that only objects within that field will be in focus. Because your right index finger is beyond the field, it is not in focus. When you shift your focus to your right index finger, you shift your field of focus so that your left index finger is no longer within that field. Under identical lighting conditions, how does your depth of field differ as you focus on nearby as opposed to distant objects?

• One accommodation problem that can arise is presbyopia. As you learned in Chapter 3, presbyopia causes your lens to become less capable of accommodating, largely due to a decrease in flexibility. Typical focusing problems, being nearsighted (myopia) or farsighted (hypermetropic), are often due to a change in shape of the eye itself.

Link - You can learn more about myopia at The Eye Digest (affiliated with the University of Illinois at Chicago).

Characteristics of the Stimulus that Affect Acuity

• Characteristics of the stimulus also affect acuity. In order to see clearly, the object needs to be sufficiently large (that is, have a sufficiently large visual angle). The contrast between the object and its background is also important. The greater the contrast (up to a point) between the object and its background, the easier it is to see the object. Finally, the amount of light falling on the object is important. Under very dim lights, even if sufficient for your cones to function, you won't see as clearly as when the luminance is greater (up to about 80 cd per square meter).

Eye Movements

• Your eyes are constantly in motion due to involuntary eye movements (tremors, drift, and microsaccades). However, your extraocular muscles also cause your eyes to produce vergence and version movements.

Link - Paul Knox (Liverpool) has developed pages that help explain eye movements, including the role played by the extraocular muscles that control eye movements. Be sure to check out the demonstration of the role played by each muscle in producing eye movements.

Vergence Movements

• In vergence movements, the line of sight of each eye moves toward your nose (convergence) or away from your nose (divergence). Convergence allows you to see nearby objects more clearly. Divergence allows you to shift from looking at a nearby object to look at a distant object. Try the demonstration below to illustrate convergence.

Demonstration 4.9 Convergence Hold both index fingers together as far away as possible from your body, and fixate both your eyes on your fingers.  Continue to focus on your right index finger, while slowly bringing your left finger in toward your nose.  Do not use convergence to keep your left finger in focus.  As you move your left finger inward, stop about 4 inches from your nose.  Notice that you will have double vision for your left finger.  Now converge on your left finger, bringing it into focus.  Notice that now you will have double vision for your right finger.

Version Movements

• Your eyes move together in version movements. One type of version movement is pursuit movement (often called smooth pursuit movement). You use this type of movement to track an object moving in your visual field. The other type of version movement is a saccade. Unlike pursuit movements, saccades are not smooth, but jerky and ballistic.

Natural Scenes

Everyday Tasks


Demonstration 4.10 Jumbled Text The following email made its way around the internet in 2003:

Aoccdrnig to rscheearch at Cmabrigde uinervtisy, it deosn't mttaer waht oredr the ltteers in a wrod are, the olny iprmoetnt tihng is taht the frist and lsat ltteres are at the rghit pclae. The rset can be a tatol mses and you can sitll raed it wouthit a porbelm. Tihs is bcuseae we do not raed ervey lteter by it slef but the wrod as a wlohe.

First of all, the email is a hoax, in that no researchers at Cambridge University were actually doing such research. (And, presumably, researchers at Cambridge would spell "important" with an a and not an e.) Second, it's not entirely accurate. Keith Rayner (University of Massachusetts) and Rebecca Johnson (Skidmore College) and their colleagues have shown that there is a cost to reading such jumbled words, though the email does illustrate the importance of end letters in processing the "words." [Rayner, White, Johnson, & Liversedge (2006)]


Test Yourself

1. What are the prerequisites for vision, and how do they illustrate clear differences between our visual system and the functioning of an automatic camera?

2. Review the connection between lateral inhibition and Mach bands. Then try to integrate the information on lateral inhibition with information from Chapter 3 on receptive fields. How might receptive fields give rise to lateral inhibition?

3. A frog does not have involuntary eye movements. Discuss the implications of this deficit for the frog’s visual perception. What would you predict would happen if a frog’s head were immobilized?

4. Discuss theoretical approaches to lightness constancy. What role does albedo play in determining lightness constancy? Describe several instances in which constancy appears to fail, and indicate how each of these instances might be addressed by one of the theories of lightness constancy.

5. Many of your friends may think of perception as a passive process. Using information from different sections of this chapter, construct an argument for the active nature of perception.

6. Think about all the characteristics of the visual system and of a stimulus that could influence acuity. Combining all these factors, describe a situation in which acuity would be the best possible. Then describe a situation in which acuity would be the worst possible.

7. What are the four kinds of eye movements discussed in this chapter? What is their function? Identify the kind of eye movement(s) represented in each of these situations: (a) You watch a bird fly away from you in a diagonal direction, so that its flight is also from right to left. (b) You are staring into someone’s pupils, but your eyes move slightly. (c) You are carefully examining a painting in art class.

8. In Chapter 3 we discussed many of the structures that are important for visual perception. Draw on that information to discuss various topics from this chapter. Here are some questions you might try to answer: (a) How does your knowledge of the retina, particularly the fovea, help you to understand saccades? (b) What area of the visual cortex would process information important to directing pursuit movements? (c) What visual pathway would be important for lightness perception?

9. Describe the accommodation process that takes place in normal eyes when viewing a nearby object, and then summarize how this focusing is abnormal in nearsightedness and farsightedness. How can each of these disorders be corrected?

10. Contrast the focusing performed by the eye muscles during accommodation with the other eye movements mentioned in this chapter. Be certain to mention location of the muscle and speed of the movement.

Teaching Materials

Link - Ilya Rybak has an interesting page describing a behavioral model of perception and recognition.

Thomson Higher Education has published two very useful CD-ROMs. John Baro (Polyhedron Learning Media) has developed Insight: A Media Lab in Experimental Psychology [see Contrast Sensitivity and Mach Bands] and Colin Ryan (James Cook University) has developed Exploring Perception [see Module 4].

Lafayette Instrument Co. produces the Light Discrimination Apparatus (Model 14011), which is useful for illustrating our sensitivity to small changes in brightness.

Vision Research Group has created Vision Lab, a PC-based package of 30 demonstrations for sensation and perception.

Link - Dale Purves (Duke) has provided a number of compelling lightness/brightness demonstrations.

Link - Edward H. "Ted" Adelson (MIT) has also provided a page filled with lightness/brightness demonstrations. He also has some text to accompany a chapter (Ch 24) that he wrote for the hefty Gazzaniga-edited text The New Cognitive Neurosciences.

Link - Hans Irtel (Univeristy of Mannheim) has produced an amazing teaching device for perception called PXLab (including Vision Demonstrations). You'll find a number of relevant demonstrations for lightness perception.

Link - The Smith-Kettlewell Eye Research Institute maintains a research page that contains relevant links about eye movements.

Recommended Readings

Ebenholtz, S. M. (2001). Oculomotor systems and perception. Cambridge.

Findlay, J. M. & Gilchrist, I. D. (2003). Active vision: The psychology of looking and seeing. Oxford.

Gilchrist, A. (Ed.) (1994). Lightness, brightness, and transparency. Erlbaum.

Gilchrist, A. (2006). Seeing black and white. Oxford.

Henderson, J. M. & Ferreira, F. (Eds.) (2004). The interface of langauge, vision, and action: Eye movements and the visual world. Psychology Press.

Purves, D. & Lotto, B. (2003). Why we see what we do: An empirical theory of vision. Sinauer.

Rayner, K. (Ed.) (1992). Eye movements and visual cognition: Scene perception and reading. Springer-Verlag.