Dog Physics

From the NYTimes.
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Fly Ball or Frisbee, Fielder and Dog Do the Same Physics
By YUDHIJIT BHATTACHARJEE

hen a dog goes after a sailing Frisbee — now racing, now turning, head cocked skyward — it looks like nothing so much as an outfielder chasing a fly ball. The resemblance is impossible to miss. Now researchers say it is laden with deeper similarities.

The scientists, at Arizona State University, had previously shown that outfielders navigated by keeping the ball's image moving along a straight line against its background.

Now, after a study involving a cheerful springer spaniel, Dr. Dennis M. Shaffer and his colleagues say that dogs use the same instinctive arithmetic, and they say that the similarity, while not unexpected, could shed light on questions about instinct and learning.

Of course, neither dogs nor baseball players use the strategy consciously.

Their brains take in the image of the moving target, perform split-second computations to estimate their required speed and direction at any instant, and make them act accordingly. These computations are what lie beneath the outfielder's grace and reflexive magic.

In a paper in the journal Science in 1995, the researchers proposed that their straight-line model (they called it LOT, for linear optical trajectory, offered a more accurate description of ball-catching mechanics than an earlier model that said fielders navigated by keeping the ball's image moving at a constant speed in their fields of vision.

That 1995 paper, by Dr. Michael K. McBeath, his colleague Dr. Shaffer and others, argued that LOT explained certain baseball behaviors that the earlier model could not, including the reason fielders run along an arc rather than straight toward the ball.

The roundabout path enables fielders to keep the ball's image rising in a straight line.

To see if the new model worked for another species and for a more complicated trajectory than that of a fly ball, Dr. Shaffer and Dr. McBeath, along with Dr. Scott Krauchunas at St. Anselm College in New Hampshire, studied a Frisbee-catching dog in action on a basketball court.

(They tried two dogs before they found a cooperative subject.)

Even with a tiny camera attached to his head and a battery and transmitter strapped to his back, the spaniel, Romeo, proved to be "a good Frisbee dog," said Dr. Krauchunas, who borrowed him from a student.

"He may have felt a little restricted at first, but as soon as you showed him the Frisbee, he just wanted to go catch it," Dr. Krauchunas said.

The camera recorded a moment-to-moment snapshot of the Frisbee as seen by Romeo at different points on his running course. This image stream was transmitted to a nearby computer.

From an analysis of recordings for 37 throws, most of them caught successfully, the researchers found that the dog's path closely followed the expectations from the LOT model. When the Frisbee changed course in midflight, the dog seemed to reset his navigational system and proceeded to keep the image moving along a new straight line.

In a poster presentation in November at the Psychonomics Society in Orlando, the researchers showed that the dog was using one straight-line trajectory to track the Frisbee from the thrower's hand to the point where it curved in the air, and then a second from this turning point to the landing spot.

"Dogs not only appear to use the same principle as baseball players, but they do so with a target that has a more complicated flight path than a baseball," Dr. McBeath said. "What this suggests is that a simple rule of thumb for intercepting targets may work across different species and for different trajectories."

For animals, tracking a target through space is more than sport. For a multitude of organisms, from bats and bees to flies and fish, the ability to intercept and avoid moving objects is essential to survival.

Male hoverflies, for instance, need to estimate an interception course while pursuing females during mating.

The teleost fish catches its prey by keeping a constant angle between its line of motion and its target.

A question that interests scientists is whether navigational strategies are a product of evolution or experience.

Researchers say there is no clear evidence to show whether organisms have an instinct for the calculus involved in pursuing a target, or whether they learn it unconsciously, by trial and error.

Some believe that while the neural mechanisms used in computing an interception course are hard-wired in the brain, specific tasks must be learned.

"Both dogs and humans seem to have the innate ability to track an object flying through three-dimensional space by using information in the two-dimensional image on their retina," Dr. Shaffer said. "Through experience, they learn to apply this instinct to catch a ball or a Frisbee."

It is not surprising that the same strategy seems to be used by dogs and baseball players, said Dr. Michael Land, a researcher at the University of Sussex in England.

"There is a lot of convergent evolution in sensory-motor tasks among very distantly related animals," Dr. Land said. "For example, flies and primates can track targets with their eyes using similar basic strategies, though those of primates are more elaborate."

Dr. Land's colleague Dr. Thomas Collett, who has worked with him on studies of male flies' intercepting females, thinks that navigational behavior involves a combination of instinct and experience. "The mix depends on where you are in the animal kingdom," he said. "In flies, it is likely that the behavior is mostly preprogrammed. In mammals, learning plays a larger role."

Studies with infants have shown that the basic ability to predict object motion is present very early, before the infant can successfully reach for objects.

Dr. Claes von Hofsten and his colleagues at Uppsala University in Sweden found that 2-month-old babies were able to hold their gaze on a moving target — a task that requires making eye movements in anticipation of the target's position at a future instant.

"The ability to make smooth pursuit eye movements appears very rapidly over just a few weeks of development," Dr. von Hofsten said. "Such rapid emergence indicates that the ability for predictive tracking is a result of new connections being established in the cerebral cortex rather than something that the infant learns from experience."

In other experiments, Dr. von Hofsten and his colleagues found that 6-month-olds successfully caught stuffed toys sliding slowly across a board, at 0.9 miles per hour.

Eight-month-olds were able to catch objects traveling three times as fast.

"Catching skills can be refined using a principle like LOT," Dr. von Hofsten said, "but they rely on the more fundamental ability to predict object motion."