Depth perception is the mechanism in the human visual system which disambiguates the relative positions of two three-dimensional objects.

The visual system relies on both physiological and psychological cues to make accurate estimates of the depths of objects.

Physiological depth cues are formed by keeping track of the muscular adjustments the visual system makes. It's hard to establish just how much these cues contribute to depth perception, but they are assumed to be weak. They include the following:

• Accommodation - The change in the focal length of the crystalline lens in the eyes. This is a monocular cue.
• Convergence - Rotation of the eyeballs so that the directions of the gaze of the eyes intersects at the point of interest. This is a binocular cue.

Psychological depth cues are based on the analysis of the image on the retina by the visual cortex of the brain. They are usually much stronger than the physiological depth cues.

The following two psychological depth cues are based on parallax, which is the difference in the appearance of an object caused by a difference in viewpoint.

• Binocular Disparity - The difference in the positions of the same object in images viewed by the left and right eyes. Binocular disparity is the result of binocular parallax, which is the difference between the images viewed by the two eyes.
• Motion Parallax - A monocular cue that relies on the changes in the images viewed in an eye as the viewer moves.

Physiological and parallax depth cues are effective in perceiving depth of only objects near the viewer. This is because the farther away an object is, the larger the distance to the object is in comparison to the interocular distance or the distance moved by the viewer (in the case of motion parallax). For this reason, accommodation is effective only up to about 2 meters, and convergence up to about 10 meters.

The remaining psychological depth cues are based on the appearance of the object from one viewpoint. They are monocular cues, independent of parallax. They are effective at all distances and are the predominant depth cues for distant objects.

• Linear Perspective - The sizes of objects change in inverse proprotion to their depth. Objects which are farther appear smaller than objects which are closer. We compare the retinal image size of the object with the known size of object to judge how far it is. Furthermore, objects on the ground which are seen higher in the visual field are perceived to be farther. Similarly, objects in the sky which are seen lower in the visual field are perceived to be farther.
• Occlusion - If an object overlaps another, we perceive the blocker as being closer.
• Aerial Perspective - Objects which are far away often appear hazy or bluish. This is due to the atmospheric scattering of light.
• Texture Gradient - Textures on an object appear coarse if the object is close to the viewer, and fine if the object is far from the viewer.
• Shading and Shadowing - Known properties of surfaces and mechanisms of reflection are used by the visual system to estimate the orientation and shape of objects. Knowledge of the location of light sources helps us determine location of objects from their shadows.
• Chromostereopsis - Chromatic Aberration in the eyes results in objects colored with some wavelengths of light to appear closer or farther than objects colored with some other wavelength.

Since the majority of depth cues are monocular, vision loss in one eye does not result in the loss of the ability to perceive depth. In fact, only two depth cues -- convergence and binocular disparity -- require both eyes to operate. Therefore, loss of stereoscopic vision affects depth perception only near the viewer -- and even there, other cues make up for it to a great extent.

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Thanks to hobyrne for bug reports.

A biosurfactant is a natural surfactant: a chemical produced by a living organism that decreases the surface tension of water.

For instance, biosurfactants on the lungs of human babies and late-term fetuses keep their lungs from collapsing. Fetuses start producing lung surfactant in their last trimester.

Earlier-term fetuses have trouble surviving in part because their lungs tend to collapse due to the surface tension of water on their alveoli prevents them from expanding after the infant has exhaled. The medical name for this problem is respiratory distress syndrome (RDS). In 1990, the U.S. Food and Drug Administration approved a form of surfactant therapy that has saved the lives of many premature infants.

One of the surfactant agents for premature infants is marketed under the name Exosurf Neonatal. It comes as a powder that physicians mix with water and blow into the babies' lungs.

Other biosurfactants, which are often rhamnolipids, are used for a wide variety of industrial applications. They are sometimes produced by engineered bacteria and are used as emulsifiers and solvents in:

• hydrocarbon recovery
• concrete
• soil washing
• metal processing
• leather processing
• lubricants
• pulp and paper processing
• textiles
• detergents
• foods
• inks
• paints
• adhesives
• personal hygiene products
• agricultural adjuvants
• water treatment (especially removing metals from wastewater)
• pharmaceuticals
• cosmetics
• ophthalmic products

You have an older relative who calls, and they are short of breath. They have a wet cough. You take them to the emergency room and the doctors and staff do a lot of tests. You think that they must have pneumonia, because they can't breathe and are coughing. The doctor says that your relative will be hospitalized. The doctors talk about pneumonia but then later on they say, no, it is congestive heart failure. It is not a heart attack. You wonder why a heart problem shows up in the lungs.

Heart failure, or congestive heart failure, is a confusing name for an illness. Heart failure is uncommon among the young but becomes more and more common as people age. Understanding heart failure starts by remembering that the heart is a pump. Heart failure means the heart is not pumping right. What happens when a pump is failing?

"Something backs up," reply my patients. "Something floods."

Exactly! And where is the flooding? This depends on what part of the heart pump is not working. The heart has four chambers. The right side of the heart pumps blood in to the lungs, to pick up oxygen. The oxygenated blood goes to the left heart. The left side of the heart pumps blood to everything else: heart, brain, body, kidneys, organs, skin and so forth. Then the blood returns through veins to the right heart to pick up oxygen again.

With right sided heart failure, the blood backs up into the body. With left sided heart failure the blood backs up in to the lungs. Both can occur, so that the whole system is sluggish.

What causes heart failure? The two most common causes are coronary artery disease and hypertension. Coronary artery disease is partially blocked arteries that are supposed to take oxygen and nutrition to the heart. If the arteries are partially blocked, the heart muscle cannot get enough oxygen or nutrition and can't pump well. If the artery is fully blocked, that part of the heart muscle can die: that is a heart attack, or myocardial infarction. Hypertension is high blood pressure. If the heart pump is always pumping against high pressure, what does it do? The wall of the heart thickens, the muscle getting thicker. We think bigger muscles are better, but they aren't always. When the wall of the left heart thickens too much, the amount of blood that the heart can pump with each beat drops. Normally, the left heart pumps 55-70% of the blood into the body with each beat. In heart failure, the amount drops. If the left heart can only pump 20%, this is clearly bad. Some mild heart changes are normal with aging: as people reach their 70s and 80s, there is usually some right sided mild heart pump failure. This is not usually a problem. There are other more rare causes of heart failure and it can happen to someone young.

What are the symptoms of heart failure? If the right heart, that pumps to the lungs, is not pumping well, blood backs up in the body. Usually in the legs, because over time gravity and the pooling of blood cause swelling. The swelling can be enormous and frightening. If the left heart backs up, the lungs have too much fluid. The body tries to cope with this and there is often fluid backed up in the legs as well.

How do we prevent heart failure? Stay as healthy as possible, avoid coronary artery disease, and have your blood pressure checked yearly. I see people who boast that they have not seen a doctor for twenty years. If they have had high blood pressure for 15 of those years, the damage may already be done to their heart. We all know the recommendations for avoiding coronary artery disease and heart attacks: don't smoke or stop smoking, exercise, eat a diet high in whole grains and vegetables and low in fat and sweets, drink small amounts or no alcohol, avoid being overweight, and drugs like cocaine and methamphetamines can have bad heart effects as well.

"But doctor, I've lived a healthy life and now you say my blood pressure is high?" Yes, the high blood pressure experts say that 90% of people in the United States will develop high blood pressure. Sometimes not until age 90, but most people will! With exercise, not smoking, a healthy diet and so forth, you can delay this as long as possible: but when your doctor tells you your blood pressure is high, take your medicine. Then you delay heart failure!

How do medicines for heart failure work? There are many medicines that can help with heart failure. Diuretics, such as HCTZ and lasix, remove extra fluid through the kidneys and both lower blood pressure and the amount of fluid backing up the system. Beta blockers, like atenolol and propranolol, help the heart to pump more strongly and lower blood pressure. Calcium channel blockers, like diltiazem, also work on the heart muscle and lower blood pressure. If the cause is coronary artery disease, nitrates help to keep the arteries open and aspirin helps keep clots from forming.

If I have heart failure, what else can I do besides take medicine? Again, stop smoking and so forth. Eating salt can make heart failure worse, because the salt tends to keep the kidneys from removing fluid. If your doctor recommends a low salt diet, avoiding the potato chips and most canned soups and using less salt will all help. When someone's heart failure is getting out of control, their weight increases as fluid backs up: so get a good scale, know your goal weight and contact your doctor if you have a sudden weight gain, 4-5 pounds within a couple of days. The other big symptom is shortness of breath and a wet cough, if fluid is backing up in the lungs. Know when to ask for help.

This appeared in The Peninsula Daily News Healthy Living section, June, 2010.
For Science Quest 2012.