ADULT NEUROGENESIS - Can We Grow New Brain Cells?

By Kendra Cherry, About.com Guide

Question: Adult Neurogenesis: Can We Grow New Brain Cells?

Conventional wisdom has long suggested that we cannot grow new brain cells; that we are born with all the brain cells we will ever have and that once those precious neurons expire, they're gone for good.

How true is this idea? Is it ever possible to grow new brain cells?

Answer:

While the vast majority of the brain's cells are formed during prenatal development, there are certain areas of the brain that continue to create new neural cells – a process known as neurogenesis – during infancy. Perhaps even more surprising, there is at least one important area of the brain that continues to create new cells throughout the entire lifespan.

During the 1990s, researchers conducted studies that involved injecting participants with a compound that is taken up into a cell's DNA. The researchers discovered that the hippocampus – a region of the brain associated with memories, learning, and emotions – continues to create new cells well into old age.

Later studies that involved looking at how nuclear fallout impacted human cells allowed researchers to further demonstrate that the human hippocampus generates new cells. Cells in the hippocampus are also continually dying. The formation of new nerve cells allows the hippocampus to maintain a balance, although the number of cells usually does begin to decline with age.

Why is the formation of new brain cells such important news? Researchers believe that it raises the possibility of treating degenerative brain diseases including Alzheimer's and Parkinson's. It may also mean that one day scientists can develop new treatments for brain trauma and injury.

While there have been indications that the brain grows new cells for decades, it wasn't until recently that research has confirmed these suspicions. Why did the idea that the brain was largely set in stone after birth persist for so long? Partly because many experts believed that in order to form lasting memories and store information over a long period of time, it was essential that the brain be relatively stable.

Today, scientists understand that adult neurogenesis is not only possible, it is a regular occurrence.

Factors That Might Influence the Formation of New Brain Cells

It turns out that exercise is not just good for your body; it might also be good for your brain. In experiments with rats, researchers also found that aerobic exercise could lead to increased cell production in the hippocampus. Not only did the exercising rats grow more brain cells than rats that were not exercised, they were also smarter as well. Some research with human participants also suggests that exercise can lead to increased neurogenesis in people.

In addition to exercise, researchers have also found that enriched learning environments can also help contribute to the survival and integration of new brain cells.

Not surprisingly, age does play a role in neurogenesis. Researchers have found that the formation of new cells is greatly reduced in the hippocampal region of older animals. Since neurons are regularly dying as well, reduced production of new cells can lead to a gradual depletion of hippocampal neurons. By the time many adults reach their 80s, they may have lost as much as 20 percent of the neural connections in this region of the brain. Those who exhibit such loss also tend to display lower performance on memory tests.

The Bottom Line: Your brain continues to produce new neurons even as you age.

References

Creer, D. J., Romberg, C., Saksida, L. M., van Praag, H., Bussey, T. J. (2010). Running enhances spatial pattern separation in mice. Proceedings of the National Academy of Sciences in the United States of America.doi: 10.1073/pnas.0911725107

Kolata, G. (1998, March 17). Studies find brain grows new cells. The New York Times. Retrieved from http://www.nytimes.com/1998/03/17/science/studies-find-brain-grows-new-cells.html

Lazarov, O., Robinson, J., Tang, Y. P., Hairston, I. S, Korade-Mirnics, Z., Lee, V. M., Hersh, L. B., Sapolsky, R. M., Mirnics, K., Sisodia, S. S. (2005). Environmental enrichment reduces Abeta levels and amyloid deposition in transgenic mice. Cell, 120(5), 701–13. doi:10.1016/j.cell.2005.01.015

PSYCHOLOGY - Top 10 Controversial Psychiatric Disorders

Impassioned fights

The proposed revisions of the Diagnostic and Statistical Manual of Mental Disorders (DSM) have spurred debate over what illnesses to include in the essential psychiatric handbook. Everything from gender identity disorder to childhood mood swings has come under fire, and it's not the first time. The history of psychiatry is littered with impassioned fights over controversial diagnoses.

Top 10 Controversial Psychiatric Disorders

1.  Gender identity disorder

Perhaps the most controversial of all current DSM disorders is gender identity disorder. Under the DSM-IV, people who feel that their physical gender does not match their true gender are diagnosed with gender identity disorder (GID). The DSM-5 proposals would revise the criteria for the disorder and change the name to the less-stigmatizing gender incongruence.

But to many, the DSM changes don't go far enough.

"To have gender incongruence in the DSM-5, as they've defined it, still leaves it open to a child being sent to be 'fixed' when a child doesn't have any problems," said Diane Ehrensaft, a clinical psychologist in Oakland, Calif., who specializes in gender identity issues in children.

In fact, the largest controversy over GID is about an area the DSM doesn't cover: treatment. Should kids who feel gender mismatched be allowed to define themselves, or should they be encouraged to identify with their physical gender? Those who argue for the latter see their role as helping kids get comfortable in their own skin. Those who argue for letting the child take the lead, like Ehrensaft, say that forcing a kid to live as an unwanted gender causes depression and anxiety.

Therapies like Ehrensaft's are relatively new, so there are no long-term studies comparing the two approaches. When it comes to examining differences in how the kids turn out, "We don't know," said Kenneth Zucker, a University of Toronto psychologist who advocates teaching kids to identify with their physical gender. "We should know in another 10 years."

2.  Sex addiction

According to the Society for the Advancement of Sexual Health, sex addiction is marked by a lack of control over one's sexual behavior. Sex addicts pursue sex despite negative consequences, can't set boundaries and obsess over sex even when they don't want to think about it. Self-described addicts report that they get no pleasure from their sexual behaviors, only shame.

This disorder has yet to darken the pages of the DSM, and it likely won't be included in the DSM-5, either. Instead, the APA is recommending the addition of a new sexual disorder called hypersexual disorder, which doesn't describe sex as an addiction, but could fit many people who are in treatment.

3.  Homosexuality

In perhaps the most famous psychiatric controversy of all, the APA did away with homosexuality as a mental disorder in 1973 after much protest by gay and lesbian activists. The change wasn't easy, but the weight of the scientific evidence suggested that same-sex attraction was a normal variant of sexuality among well-adjusted people.

Still, the APA included a diagnosis in the 1980 DSM-III called ego-dystonic homosexuality. This category was a compromise with psychiatrists who insisted that some gays and lesbians came to them looking for treatment. "This revision in the nomenclature provides the possibility of finding a homosexual to be free of psychiatric disorder, and provides a means to diagnose a mental disorder whose central feature is conflict about homosexual behavior," explained Robert Spitzer, a member of the APA's task force, in a 1973 position statement.

But ego-dystonic homosexuality was short-lived. The category didn't make sense to many psychiatrists, who argued that anxiety over sexual orientation could fit into already-existing categories, according to UC Davis psychologist Gregory Herek. In 1986, ego-dystonic homosexuality disappeared from the DSM.

4.  Asperger's disorder

In 1994, Asperger's disorder, which is marked by normal intelligence and language abilities but poor social skills, made the DSM-IV. When the DSM-5 is published in 2013, the disorder may get the boot.

The reason? Research on Asperger's and high-functioning autism has failed to find a difference between the two diagnoses. Overlap between the two disorders is rampant (up to 44 percent of kids diagnosed with Asperger's or "other autism spectrum" labels actually met the criteria for high-functioning autism, according to a 2008 survey). If the proposed changes are adopted, people with Asperger's will be reclassified as having high-functioning autism.
But some Asperger's advocates disapprove. The high-functioning autism label doesn't always fit people with Asperger's, said Dania Jekel, the executive director of the Asperger's Association of New England, which opposes the change.

"People with Asperger's are going to be missed," Jekel said.

5.  Childhood bipolar disorder

If diagnosing adults with a childhood disorder is controversial, so is diagnosing children with a disorder once thought to occur mainly in adults. Bipolar disorder, which is characterized by mood swings between depression and excitability, recently skyrocketed as a childhood disorder. Between 1994 and 2003, the number of doctor visits associated with childhood bipolar disorders went up 40-fold, according to a 2007 study in the journal Archives of General Psychiatry.

The problem, according to the APA, is that at least some of that increase is due to changes in the way psychiatrists diagnose bipolar in kids, not an actual increase in cases. To correct the issue, the APA is considering changes to the current bipolar criteria, as well as the addition of a new disorder, temper dysregulation with dysphoria. That disorder would apply to kids with persistent irritable moods and frequent temper tantrums, but has already drawn skepticism from some who believe it pathologizes normal kid behavior.

6.  Adult ADHD

Attention deficit hyperactivity disorder is a well-known childhood diagnosis. Kids with ADHD have trouble sitting still, paying attention and controlling their impulses. Only recently, however, have psychiatrists begun to diagnose ADHD in adults.

But just as ADHD in children was criticized as over-diagnosed, so is adult ADHD. A common accusation is that psychiatrists are conspiring with pharmaceutical companies to sell more ADHD drugs, writes New York University psychiatrist Norman Sussman in a March 2010 editorial in Psychiatry Weekly. However, adult ADHD is here to stay, Sussma writes: "The benefits of pharmacologic and behavioral therapies are well-established."

7.  Dissociative identity disorder

Once known as multiple personality disorder, dissociative identity disorder was made famous by the book "Sybil" (Independent Pub Group, 1973), which was made into a movie of the same name in 1976. The film and book told the story of Shirley Mason, pseudonym Sybil, who was diagnosed as having 16 separate personalities as a result of physical and sexual abuse by her mother.

The book and the movie were hits, but the diagnosis soon came under fire. In 1995, psychiatrist Herbert Spiegel, who consulted on Mason's case, told the "New York Review of Books" that he believed Mason's "personalities" were created by her therapist, who — perhaps unwittingly — suggested that Mason's different emotional states were distinct personalities with names. Likewise, critics of the dissociative identity diagnosis argue that the disorder is artificial, perpetuated by well-meaning therapists who convince troubled and suggestible patients that their problems are due to multiple personalities.

Nonetheless, dissociative identity disorder has weathered this criticism and won't undergo any major changes in the DSM-5.

8.  Narcissistic personality disorder

Someone with an inflated ego, need for constant praise and lack of empathy for others might sound like a shoe-in for psychotherapy. But the introduction of narcissistic personality disorder into the DSM in 1980 was not without controversy.

The biggest problem was that no one could agree on who had the disorder. Up to half of people diagnosed with a narcissistic personality also met the criteria for other personality disorders, like histrionic personality disorder or borderline personality disorder, according to a 2001 review in the Journal of Mental Health Counseling. Which diagnosis they got seemed almost arbitrary.

To solve the problem, the American Psychiatric Association has proposed big changes to the personality disorder section of the DSM-5 in 2010. The new edition would move away from specific personality disorders to a system of dysfunctional types and traits. The idea, according to the APA, is to cut out the overlap and create categories that would be useful for patients who have personality problems, not just full-blown disorders.

9.  Penis envy

Sigmund Freud revolutionized psychiatry in the late 1800s and early 1900s with his theories on the unconscious state, talk therapy and psychosexual development. Nowadays, many of these theories — like his conclusion that young girls' sexual development is driven by jealousy over lack of a penis and sexual desire for their father — seem outdated.

But not everyone has consigned Freud to the dust heap. Organizations like the American Psychoanalytic Association still practice and promote Freudian-style psychoanalysis, and groups like the International Neuropsychoanalysis Society try to combine cutting-edge neuroscience research with Freud's century-old theories. How successful they'll be is unknown: A 2008 study in the Journal of the American Psychoanalytic Association found that today's psychology departments rarely teach psychoanalysis.

10.  Hysteria

In the Victorian era, hysteria was a catch-all diagnosis for women in distress. The symptoms were vague (discontentment, weakness, outbursts of emotion, nerves) and the history sexist (Plato blamed the wanderings of an "unfruitful" uterus).

The treatment for hysteria? "Hysterical paroxysm," also known asorgasm. Physicians would massage their patients' genitals either manually or with a vibrator, a task they found tedious but surprisingly uncontroversial. More contentious was the practice of putting "hysterical" women on bed rest or demanding that they not work or socialize, a treatment that often worsened anxiety or depression.

According to a 2002 editorial in the journal Spinal Cord, the diagnosis of hysteria gradually petered out throughout the 20th century. By 1980, hysteria disappeared from the DSM in favor of newer diagnoses like conversion and dissociative disorders.

OPTICAL ILLUSION - 10 Cool Optical Illusions

10 Cool Optical Illusions

A Selection of Fun and Fascinating Illusions

By Kendra Cherry, About.com Guide

Optical illusions can be fun and fascinating, but they can also tell us a great deal of information about how the brain and perceptual system function. There are countless optical illusions out there, but here is a sampling of some of the most fun and interesting.

1.   Hermann Grid Illusion

What Do You See?

The Hermann grid was first discovered by a physiologist named Ludimar Hermann in 1870. When the viewer looks at the grid, the white dots and the center of each 'corridor' seem to shift between white and gray. When the viewer focused his or her attention on a specific dot, it is obvious that it is white. But as soon as attention is shifted away, the dot shifts to a gray color.

How Does the Hermann Grid Illusion Work?

So why do people see gray where there should be white? Why do we see something so different from reality?

Researchers have traditionally used what is known as lateral inhibition to explain why people see these gray areas. This phenomena demonstrates a very important principle of perception: we don't always see what's really there. Our perceptions depend upon how our visual system responds to environmental stimuli and how our brain then interprets this information.

However, there is evidence suggesting that this explanation is likely inaccurate. The fact that the illusion is not dependent upon size, can be seen with contrast reversal and can be negated by slightly distorting the lines have been cited as reasons why the classic theory is wrong. One possible explanation that has been proposed is known as the S1 simple-cell theory.

2. The Spinning Dancer Illusion

What Do You See?

In this image, you see the silhouette of a woman spinning. Which direction is she turning? You may be surprised to learn that it is possible to see her spinning both clockwise and counterclockwise. How? While it may be very difficult, you can probably get her to switch directions spontaneously. Try looking at the figure and then blink; she may appear to change directions immediately after you blink. Another strategy is to focus on a specific part of the figure.

How Does the Spinning Dancer Illusion Work?

After it was initially created by Nobuyuki Kayahara, the illusion was mistakenly referred to as a scientific personality test of right brain/left brain dominance by numerous websites and blogs. In reality, the spinning dancer illusion is related to bistable perception in which an ambiguous 2-dimensional figure can be seen in from two different perspectives. Because there is no third dimension, our brains try to construct space around the figure. Similar illusions include the Necker Cube and the Reversible Face/Vase Illusion.

In a New York Times column, Thomas C. Toppino, chair of the department of psychology at Villanova University suggested, "What's happening here to cause the flip is something happening entirely within the visual system. If we can understand why it is these figures reverse then we're in a position to understand something pretty fundamental to how the visual system contributes to the conscious experience."

3.   Zöllner illusion

What Do You See?

The Zöllner illusion is another commonly demonstrated optical illusion. First discovered in 1860 by a German astrophysicist named Johann Karl Friedrich Zöllner, this illusion presents a series of oblique lines crossed with overlapping short lines. The oblique lines look as if they are crooked and will diverge. In reality, all of the oblique lines are parallel.

How Does It Work?

Much like the Muller-Lyer and Herring illusions, this optical illusion demonstrates how the background of an image can distort the appearance of straight lines. Several different explanations for the Zöllner illusion have been suggested. First, the angle of the short lines compared to the longer lines creates an impression of depth. One of the lines appears to be nearer to us; the other farther away. Another possible explanation is that the brain attempts to increase the angles between the long and short lines. This results in a distortion as the brain tries to bend the lines away and towards each other.

Interestingly, if the color of the lines are switched to green and the background to red, the effect completely disappears as long as the two colors are of equal brightness.

4.   The Ames Room Illusion

How Does the Ames Room Illusion Work?

The effect works by utilizing a distorted room to create the illusion of a dramatic disparity in size. While the room appears square-shaped from the viewers perspective, it is actually has a trapezoidal shape. The woman on the right hand side of the image above is actually standing in a corner that is much further away than the woman on the left.

The illusion leads the viewer to believe that the two individuals are standing in the same depth of field, when in reality the subject is standing much closer. The woman on the left in the image above appears at a much greater visual angle, but the fact that she appears to be at the same depth of field as the figure on the right makes the closer individual look much larger.

The effect can be observed in a number of films, including The Lord of the Rings trilogy. Note the early scenes in The Fellowship of the Ring where the effect is prominently used to make Gandalf appear larger than the hobbits.

5.   The Ponzo Illusion

What Do You See?

In the image above illustrating the Ponzo illusion, the two yellow lines are the exact same size. Because they are placed over parallel lines that seem to converge in the distance, the top yellow line actually appears to be longer than the bottom one.

How Does the Ponzo Illusion Work?

The Ponzo illusion was first demonstrated in 1913 by an Italian psychologist named Mario Ponzo. The reason the top horizontal line looks longer is because we interpret the scene using linear perspective. Since the vertical parallel lines seem to grow closer as they move further away, we interpret the top line as being further off in the distance. An object in the distance would need to be longer in order for it to appear the same size as a near object, so the top "far" line is seen as being longer than the bottom "near" line, even though they are the same size.

6.   The Kanizsa Triangle Illusion

The Kanizsa Triangle illusion was first described in 1955 by an Italian psychologist named Gaetano Kanizsa. In the illusion, a white equilateral triangle can be seen in the image even though there is not actually a triangle there. The effect is caused by illusory or subject contours.

Gestalt psychologists use this illusion to describe the law of closure, one of the gestalt laws of perceptual organization. According to this principle, objects that are grouped together tend to be seen as being part of a whole. We tend to ignore gaps and perceive the contour lines in order to make the image appear as a cohesive whole.

7.   The Müller-Lyer Illusion

In the Müller-Lyer illusion, two lines of the same length appear to be of different lengths.

What Do You See?

In the image above, which line appears the longest? For most people, the line with the fins of the arrow protruding outward appears to be the longest while the line with the arrow fins pointing inwards appears shorter. In reality, the shafts of both lines are exactly the same length.

First discovered in 1889 by F.C. Müller-Lyer, the illusion has become of the subject of considerable interest and different theories have emerged to explain the phenomenon.

How Does It Work?

Optical illusions can be fun and interesting but they also serve as an important tool for researchers. By looking at how we perceive these illusions, we can learn more about how the brain and perceptual process work. However, experts do not always agree on exactly what causes optical illusions, as is the case with the Müller-Lyer illusion.

According to psychologist Richard Gregory, this illusion occurs because of a misapplication of size constancy scaling. In most cases, size constancy allows us to perceive objects in a stable way by taking distance into account. In the three dimensional world, this principle allows us to perceive a tall person as tall whether they are standing next to us or off in the distance. When we apply this same principle to two-dimensional objects, Gregory suggests, errors can result.

Other researchers contend that Gregory's explanation does not sufficiently explain this illusion. For example, other versions of the Müller-Lyer illusion utilize two circles at the end of the shaft. While there are no depth cues, the illusion still occurs. It has also been demonstrated that the illusion can even occur when viewing three-dimensional objects.

An alternative explanation proposed by R. H. Day suggests that the Müller-Lyer illusion occurs because of conflicting cues. Our ability to perceive the length of the lines depends upon the actual length of the line itself and the overall length of the figure. Since the total length of one figure is longer than the length of the lines themselves, it causes the line with the outward facing fins to be seen as longer.

Researchers from the University of London suggest that the illusion demonstrates how the brain reflexively judges information about length and size before anything else. "Many visual illusions might be so effective because they tap into how the human brain reflexively processes information. If an illusion can capture attention in this way, then this suggests that the brain processes these visual clues rapidly and unconsciously. This also suggests that perhaps optical illusions represent what our brains like to see," explained researcher Dr. Michael Proulx.

8.  The Moon Illusion

Have you ever noticed how the moon looks bigger when it is on the horizon than it does when it is high in the sky? This phenomenon is known as the moon illusion. While the moon illusion is well known through human history and culture, researchers are still debating explanations for why it happens.

Possible Explanations for the Moon Illusion

• Apparent distance theory: According to this possible explanation for the moon illusion, depth perception plays an important role in how we see the moon at the horizon versus high in the sky. This theory is centered on the idea that when you view the moon at the horizon, you are seeing it in the presence of depth cues such as trees, mountains, and other scenery. When the moon has moved higher into the sky, those depth cues disappear. Because of this, the apparent distance theory suggests, we tend to see the moon as further away on the horizon than we see it when it elevated in the sky.
  
  Researchers have found evidence supporting the apparent distance explanation. In one experiment, participants perceived the moon as farther away and 1.3 times larger when it was viewed over natural terrain. Experimenters then masked off the terrain by having participants view the moon through a hole in a piece of cardboard, which caused the moon illusion to vanish.


• Angular size-contrast theory: This explanation focuses instead on the visual angle of the moon in comparison to surrounding objects. When the moon is on the horizon and surrounded by smaller objects, it appears larger. At its zenith, the moon appears much smaller because it is surrounded by the large expanse of the sky.

While these are just two of the most prominent theories, there have been many different explanations proposed over the years and no true consensus exists. Part of the reason is that there are a number of factors that appear to influence the occurrence of this optical phenomenon, including:

• Color: When the moon appears red (due to smoke or dust in the air), it appears larger.


• Atmospheric perspective: When it is hazy or smoky outside, the moon appears larger on the horizon.


• Visual factors: Convergence of the eyes when viewing things on the horizon also causes objects to appear larger.

As with other visual phenomena, it is possible that no single variable can adequately explain the moon illusion. Instead, it is possible that many different factors might play a role.

9.   The Lilac Chaser Illusion

The lilac chaser is a type of visual illusion that was first discovered by vision expert Jeremy Hinton in 2005. In order to view the illusion, start by clicking here to open the image in a new window. Stare at the black center cross for a minimum of 30 seconds and see what happens. Want to learn more? Continue reading to discover how this fascinating illusion works and what it reveals about the brain and perception.

What Do You See?

In the lilac chaser illusion, the viewer sees a series of lilac colored blurry dots arranged in a circle around a focal point. As the viewer stares as the focal point, a few different things are observed. At first, there will appear to be a space running around the circle of lilac discs. After about 10 to 20 seconds, the view will then see a green disc moving around the circle instead of the space. With longer observation, the lilac discs will disappear altogether and the viewer will only see the green disc moving around in a circle.

How Does the Lilac Chaser Illusion Work?

According to its inventor Jeremy Hinton, "the illusion illustrates Troxler fading, complementary colours, negative after-effects, and is capable of showing colours outside the display gamut." What exactly does this mean? Let's break it down a bit further.

• Why do the lilac discs appear to move around the circle?
  This is an example of what is known as apparent movement or beta movement. When we see something in one spot and then again in a slightly different spot, we tend to perceive movement. Motion pictures and neon signs operate work based upon this principle.


• Why do we begin to see green discs in place of the gray spaces?
  This is an example of a negative afterimage effect. When a color is presented in the visual field for an extended period of time, an afterimage results. In the case of this illusion, we see a green afterimage in place of the lilac discs. We generally don't notice afterimages because we move our eyes frequently enough that they rarely occur in day-to-day experience.


• Why do all of the lilac discs eventually disappear?
  This is an example of what is known as Troxler fading, which occurs when blurry objects that are located in the periphery of our visual field disappear while we have our eyes fixated on a certain spot.


• Why does the green disc appear to fly around in a circle?
  After fixating on the center cross for about 30 seconds or so and the lilac discs have disappeared, it seems as if the green disc is now flying around the circle by itself. This can be explained by a Gestalt effect known as the phi phenomenon. The sequential movement of the retinal afterimage (aka, the green disc) causes the illusion of movement.

10.   The Negative Photo Illusion

Did you think that you needed a darkroom to process a negative photo into a full-color image? In this fun optical illusion, you can see how your visual system and brain are actually able to briefly create a color image from a negative photo.

How to Perform the Illusion

1. Stare at the dots located at the center of the woman's face below for about 30 seconds to a minute.


2. Then turn your eyes immediately to the center x of the white image on the right.


3. Blink quickly several times.

What do you see? If you've followed the directions correctly, you should see an image of a woman in full-color. If you are having trouble seeing the effect, try staring at the negative image a bit longer or adjusting how far you are sitting from your computer monitor.

Explanations

How does this fascinating visual illusion work?

What you are experiencing is known as a negative afterimage. This happens when the photoreceptors, primarily the cone cells, in your eyes become overstimulated and fatigued causing them to lose sensitivity. In normal everyday life, you don't notice this because tiny movements of your eyes keep the cone cells located at the back of your eyes from becoming overstimulated.

If, however, you look at a large image, the tiny movements in your eyes aren't enough to reduce overstimulation. As a result, you experience what is known as a negative afterimage. As you shift your eyes to the white side of the image, the overstimulated cells continue to send out only a weak signal, so the affected colors remain muted. However, the surrounding photoreceptors are still fresh and so they send out strong signals that are the same as if we were looking at the opposite colors. The brain then interprets these signals as the opposite colors, essentially creating a full-color image from a negative photo.

According to the opponent process theory of color vision, our perception of color is controlled by two opposing systems: a magenta-green system and a blue-yellow system. For example, the color red serves as an antagonistic to the color green so that when you stare too long at a magenta image you will then see a green afterimage. The magenta color fatigues the magenta photoreceptors so that they produce a weaker signal. Since magenta's opposing color is green, we then interpret the afterimage as green.