Monday, June 22, 2009

IF THE SHOE FLITS, DUCK: A REAL-LIFE EXAMPLE OF HUMANS' DUAL VISION SYSTEM

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It's rare when real-world events perfectly mirror experiments that scientists are conducting. That's why neuroscientists at the University of Washington were delighted at the reactions of former President George W. Bush and Iraq's Prime Minister Nouri al-Maliki when an Iraqi reporter flung his shoes toward the two men during a Baghdad news conference.

When Bush ducked and Maliki didn't flinch as the first shoe sailed toward them, it was a real-world example supporting the theory that there are two independent pathways in the human visual system.

"The original idea proposed is that one system guides your actions and the other guides your perception. The interesting part is the 'action' system allows the brain to 'see' things your eyes do not perceive," said Jeffrey Lin, a UW psychology doctoral student and lead author of a paper appearing June 11 in the journal Current Biology. Co-authors are Scott Murray and Geoffrey Boynton, UW psychology assistant and associate professors, respectively.

"When we throw two balls at you with very similar trajectories, they may look the same to your perceptual system, but your brain can automatically calculate which one is more threatening and trigger a dodging motion before you've even realized what has happened," said Lin.

"If you look at the shoe-throwing video you will see that the prime minister doesn't flinch at all. His brain has already categorized the shoe as non-threatening which does not require evasive action. But Bush's brain has categorized the shoe as threatening and triggers an evasive dodge, all within a fraction of a second."

To explore how this dual visual system works, the researchers set up several experiments that were similar to what baseball players experience when they step into the batter's box and get ready for the pitcher to throw the ball. In a split second their action system determines if the ball is going to hit their body and whether to initiate a defensive bail out of the batter's box.

Instead of baseballs, college students participating in three experiments looked at a computer monitor and were instructed to quickly locate a target oval among a field of circular discs, determine its path and press a key when they had found the oval. The key manipulation in the study was that some of the trials began with a looming stimulus at the position of the target oval. When this looming motion was on a collision path with a student's head, the participant responded faster to the target than when the looming motion just missed the head. The experiment showed that a stimulus on a collision path with a student captured attention but one on a near-miss path did not. Critically, subjects could not differentiate between the subtly different collision and near-miss looming stimuli in a separate experiment.

The authors strongly believe the research supports the idea that the human visual system is composed of two independent systems.

"A major focus of neuroscience is understanding how we deal with sensory information," said Boynton. "There is no way the brain can possibly process and analyze everything we are exposed to. We have to select what is important. In the real world you are on your own and what you pay attention to is part of survival. This experiment shows that threatening stimuli grab your attention, even those we can't consciously identify. That this is more accurate than our conscious perception is pretty amazing."

(Photo: U. Washington)

University of Washington

MANATEES CAN PROBABLY HEAR WHICH DIRECTIONS BOATS APPROACH FROM

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The world is a perilous place for the endangered manatee. While the mammals are at risk from natural threats, human activity also poses a great danger to manatee numbers. Debborah Colbert, from the Association of Zoos and Aquariums, explains that many manatees die and are seriously injured in collisions with boats every year. However, little is known about how manatees perceive their environment.

Whether they can localize sounds, and specifically whether they can tell which direction a boat is approaching from, are crucial factors in the development of manatee protection programmes. Colbert and her colleagues from the Universities of Florida and South Florida and the New College of Florida decided to test whether the mammals can pinpoint sound sources. They publish their results on 12 June 2009 in the Journal of Experimental Biology at http://jeb.biologists.org/.

Working at the Mote Marine Laboratory and Aquarium in Florida, Colbert was able to work with two male manatees, Hugh and Buffett, when she initiated a research programme to find out more about these enigmatic creatures. Both young males had been trained previously to participate in a series of sensory studies, so Colbert, Joseph Gaspard 3rd, Gordon Bauer and Roger Reep trained the animals to swim to a specific stationing platform in their enclosure where they could listen to sounds played from one of four speakers arranged around their heads.

Knowing that the manatees' hearing was most sensitive to sounds ranging from 10 to 20kHz, while the animals' calls range from 2.5 to 6kHz, Colbert and David Mann designed three sounds ranging from 0.2 to 20kHz, 6 to 20kHz and 0.2 to 2kHz to play to the animals. The team also selected two single frequency (tonal) sounds at 4kHz and 16kHz to test how the manatees responded to less complex sounds. Having trained the manatees to swim to the speaker that they thought the sound came from, the team then played the broadband sounds, of 0.2, 0.5, 1 to 3s, from each speaker at random while monitoring the animals' responses.

One of the manatees, Buffett, successfully identified the source of the broadband sounds with almost 90% accuracy, while Hugh did slightly less well. The team was also surprised that the manatees were able to locate the sources of both the 4kHz and 16kHz tones, although the team only tested the animals with the longest of the two tonal notes, as the animals had shown signs of frustration when they heard these sounds.

So how are the animals able to localize sounds? Colbert explains that many terrestrial animals use the time difference between a sound arriving at their two ears to find the source. However, this time difference is probably extremely short in aquatic animals, as sounds travel 5 times faster in water than in air. Animals also use the intensity difference as the sound arrives at each ear, which is more pronounced in high-pitched noises, to pinpoint the source. Colbert suspects that the manatees use combinations of these and other cues to help them localize sounds, as they were able to locate the sources of high- and low-pitched sounds equally well.

Crucially, the animals can probably hear approaching speed boats and tell which direction they are coming from, which is an essential piece of information for conservation organisations as they battle to save this gentle giant.

The Company of Biologists

STRESS MAKES YOUR HAIR GO GRAY

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Those pesky graying hairs that tend to crop up with age really are signs of stress, reveals a new report in the June 12 issue of Cell, a Cell Press publication.

Researchers have discovered that the kind of "genotoxic stress" that does damage to DNA depletes the melanocyte stem cells (MSCs) within hair follicles that are responsible for making those pigment-producing cells. Rather than dying off, when the going gets tough, those precious stem cells differentiate, forming fully mature melanocytes themselves. Anything that can limit the stress might stop the graying from happening, the researchers said.

"The DNA in cells is under constant attack by exogenously- and endogenously-arising DNA-damaging agents such as mutagenic chemicals, ultraviolet light and ionizing radiation," said Emi Nishimura of Tokyo Medical and Dental University. "It is estimated that a single cell in mammals can encounter approximately 100,000 DNA damaging events per day."

Consequently, she explained, cells have elaborate ways to repair damaged DNA and prevent the lesions from being passed on to their daughter cells.

"Once stem cells are damaged irreversibly, the damaged stem cells need to be eliminated to maintain the quality of the stem cell pools," Nishimura continued. "We found that excessive genotoxic stress triggers differentiation of melanocyte stem cells." She says that differentiation might be a more sophisticated way to get rid of those cells than stimulating their death.

Nishimura's group earlier traced the loss of hair color to the gradual dying off of the stem cells that maintain a continuous supply of new melanocytes, giving hair its youthful color. Those specialized stem cells are not only lost, they also turn into fully committed pigment cells and in the wrong place.

Now, they show in mice that irreparable DNA damage, as caused by ionizing radiation, is responsible. They further found that the "caretaker gene" known as ATM (for ataxia telangiectasia mutated) serves as a so-called stemness checkpoint, protecting against MSCs differentiation. That's why people with Ataxia-telangiectasia, an aging syndrome caused by a mutation in the ATM gene, go gray prematurely.

The findings lend support to the notion that genome instability is a significant factor underlying aging in general, the researchers said. They also support the "stem cell aging hypothesis," which proposes that DNA damage to long-lived stem cells can be a major cause for the symptoms that come with age. In addition to the aging-associated stem cell depletion typically seen in melanocyte stem cells, qualitative and quantitative changes to other body stem cells have been reported in blood stem cells, cardiac muscle, and skeletal muscle, the researchers said. Stresses on stem cell pools and genome maintenance failures have also been implicated in the decline of tissue renewal capacity and the accelerated appearance of aging-related characteristics.

"In this study, we discovered that hair graying, the most obvious aging phenotype, can be caused by the genomic damage response through stem cell differentiation, which suggests that physiological hair graying can be triggered by the accumulation of unavoidable DNA damage and DNA-damage response associated with aging through MSC differentiation," they wrote.

Cell

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