Depth Perception and Attention 

5. Depth Perception and Attention 

 Unlocked: Monday, September 4, 2017 12:00 AM EDT – Sunday, September 17, 2017 11:59 PM EDT.

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Please respond to Question 1 or Question 2.

In this week’s learning resources, the follow-on page for the article on depth perception might escape notice. The link to the second page, on binocular depth cues, is at the bottom, labeled 1:77.

However, there’s a clearer account of depth perception in this textbook chapter.

As primates, we enjoy binocular vision*, or stereopsis. Our two eyes have overlapping visual fields that allow us to judge distance better than animals who lack binocular vision, such as rabbits. You can find much more about stereopsis (when you have time and if you have the inclination) at the following sites:

· http://www.vision3d.com/stereo.html

· http://mysite.du.edu/~jcalvert/optics/stereops.htm

· http://nzphoto.tripod.com/sterea/3dvision.htm

You have already encountered the notion that each object projects an ambiguous image onto the retina, which permits different interpretations, as in the illustration at left. To understand binocular depth perception, keep two further points in mind: Images in the two retinas are slightly different and these differences provide a cue to depth.

Binocular Disparity: The Most Powerful Depth Cue?

Binocular disparity is simply the notion that the eyes receive different views of near objects but not of distant objects, so the disparity between the retinal images of the left and right eye provide information about how far away something is. Understanding how it works is the hard part.

How Binocular Disparity Works

Recall that the visual cortex contains a map of each retina, whereby each point in the left retina has a corresponding retinal point in the right retina. Corresponding retinal points will appear in the same place in the cortical map. Further, any object that you fixate on will be projected onto corresponding retinal points.

For any object that we fixate on, there is a circle of objects that will also be imaged on corresponding retinal points. This circle is the horopter. Objects that are not on the horopter will be imaged on noncorresponding retinal points. The more distant these objects are from the horopter, the greater the difference will be between their images in the two eyes. This difference between the left and right retinal images is binocular disparity.

You can demonstrate it yourself. If you hold your thumb a few inches in front of you and close first one eye and then the other, leaving the other eye open, you will notice that the right eye sees more of the right side of your thumb and the left eye sees more of the left side of your thumb. This disparity decreases as you move your thumb farther away. Since you are fixating on your thumb it must lie on the horopter, yet corresponding points on your retinas do not show quite the same images. This produces an effect similar to what we experience with objects off the horopter.

Binocular disparity is a depth cue. If an object lies off the horopter it must be closer to you or farther from you than is your point of fixation. The degree of difference between the two retinal images for this object is binocular disparity. It can be measured as degrees of visual angle.

A Small Experiment

Here’s a quick way to demonstrate how important the correspondence of points on the two retinas is. Keep both eyes open. Gently, gently push on the outside of your right eye with your finger while you’re looking at something with pattern. You should feel no pain; if you feel pain, back off and press more gently. Do you see two images? If so, good. If not, try focusing on a closer object.

Do you see a double image? When objects are projected onto noncorresponding points in the two eyes, double vision is the result. Does it make sense that the double image disappears when you close either eye? The muscles that move the eye are among the fastest in the body. They’re called the extrinsic ocular muscles; there are six of them. They are controlled by three cranial nerves: the oculomotor [N. III], trochlear [N. IV], and abducens [N. VI].

Now suppose one of the cranial nerves that controls eye movements is damaged. What will the patient experience? Right–just what you experienced when you pushed on your eyeball. It’s called diplopia, or double vision. It occurs whenever the image of what you’re looking at no longer falls exactly on the same retinal location in each eye. (You may smell a rat here. You just produced double vision by creating non-corresponding points of vision, which should create stereopsis. Is this a contradiction? It turns out that the brain interprets small disparities as depth; large disparities are interpreted as double vision.)

Rembrandt was Stereoblind

Many people, perhaps as many as one in ten**, lack stereoscopic vision to some extent despite normal vision in both eyes. Such people are said to be stereoblind, but it’s not an all-or-none condition; the causes and the severity vary. Most affected people aren’t even aware of the deficit until they try to visualize depth in random dot stereograms. Instead, they rely on other depth cues to drive and reach for things. Babe Ruth may have been stereoblind! Binocular vision is useless as a depth cue for really distant objects, anyway. It’s fine for catching a football but not for judging the distance of a football 100 feet away. You can find out more at these sites:

· http://www.boston.com/news/nation/articles/2004/09/16/an_eye_on_rembrandt/

· http://www.abc.net.au/science/articles/2004/09/16/1198105.htm  

Question 1

To further evaluate the role of binocular disparity as a depth cue, consider that you are looking at your friend’s face in a photograph. You are thus viewing an object that produces retinal images with no binocular disparity. Does zero binocular disparity give you any information about the depth (or distance) of your friend the way that viewing your friend in real life does? Now imagine you are viewing your friend outside in real life. Would stereopsis help you distinguish a friend at 300 meters’ distance from another at 320 meters? Would it help to tell you that the moon is closer than the stars?

Attention

Attention magnifies important things and filters out trivial things. We don’t analyze everything we see fully, but anything that’s important or that appears unexpectedly gets extra analysis yet there are people who can’t pay attention to two things at the same time. Studying the resources that we devote to objects and people tells us about perception.

Eye movements provide a clue to what’s happening with attention. We move our eyes when we view a scene. Actually our eyes are always moving very quickly when we’re awake; ocular microtremor, 60-80 Hz movements of both eyes, prevents the formation of a stabilized retinal image that would overbleach the rods and cones and make us blind. When we scan a scene we use different eye movements called saccades. Shouldn’t saccades make the world seem to jump around by suddenly changing where the images are focused on the retina? Yet apparently these movements help to stabilize the scene. (You can read more about eye movements here).

When attention is directed at one object it must ignore another. We sacrifice understanding of some parts of a scene in order to understand another part more thoroughly. Here are some examples similar to what you will find in the readings:

· Change blindness. You can find different kinds of change blindness (see WARNING*) here and there.

· Inattentional blindness

· Emotion-induced blindness (also called attentional rubbernecking). Click on different components of the “story map” to progress to the demo.

· Attentional blink.

Question 2

Suggest an example of how others manipulate our attention. You might think of catching someone’s attention–or dampening it–by flirting, hijacking our emotions in film, fooling us with stage magic, or pick an example you prefer. What does your example show about attention?

*This demo may be disturbing or dangerous for people who are susceptible to seizures set off by flickering lights. If viewing the demo makes you feel euphoric or uncomfortable it is best to terminate viewing. The demo is not of worldshaking importance anyway, so don’t spend more than a few minutes at this!

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