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The speed of human sight, Second champ migrant, How terns fly farther

Q: What is the speed of human sight? The speed of sound and light has always interested me but I have not been able to find out the speed of human sight. A dedicated reader of yours, --Howard A., Jasper, Alabama

[NASA] The eyes are an outgrowth of the brain

A: What a perceptive question! We don't see at light speed--that's for sure. Sight signals take 50 milliseconds to zip from eye to brain and for the brain to see. Much happens on the way and we don't know how it all works.

However, we've discovered some mechanisms in the last year or two that illuminate the process.

Sight is a four-step operation:

• light enters the eye
• the retina senses the image, changes the light signals to electrical ones, and extracts the essential information to pass to the brain
• nerves carry the electrical signal to the brain
• the brain decodes the sketchy information, fills in the blanks probably from memory, and perceives the image.

Light enters the cornea, passes through the pupil, through the aqueous humor, through the lens, through a maze of blood vessels and nerves, and finally reaches the retina. Only about 10 % of the light makes it to the retina. But don't worry-- the brain makes up the difference and can see even without light entering the eyes.

Blind people can see a candle flicker and catch a rolling ball for the first time in their lives, using a video camera worn on their forehead. They sense the image with their tongue. A 12 x 12 array of dot-electrodes "displays" a picture on the tongue. Each "dot" is an "on" tingle. (Related: figure of a dot-electrode circle and a tongue touching the device) "A nerve spike is a nerve spike," says Paul Bach-y-Rita, professor of rehabilitation medicine and biomedical engineering at the University of Wisconsin and inventor of the seeing device. "The brain doesn't give a damn where the information is coming from."

Usually, though, the retina senses the image and changes the light signals to electrical ones. But the retina is much more than a sensor and transducer. Layers of retinal cells process the image data to extract its essence. It shoots the sketch to the brain. That's all it is--a sketch: outlines, "hints, edges in space and time," says Frank S. Werblin, professor of molecular and cell biology in the College of Letters & Science at University of California at Berkeley.

One group of ganglion cells (nerve center cells) in the retina fires only when it detects a moving edge. Another sends its message only after a stimulus stops. Another reacts only to large uniform areas, yet another only to the area around a figure. The conglomeration sends 12 different pictures along 12 different paths to the brain--an edge, blob, movement...

The brain receives the information kernels and fills in the rest of the picture by merging the sketch with memory images. Weird? But effective and streamlined.

The entire process takes 50 milliseconds--1/6th of an eye blink.

Further Surfing:

WonderQuest: Why cat eyes glow in the dark

Wired Magazine: Eyes in the back of your mouth

University of California, Berkeley, UniSci: To see, brain assembles sketch images eyes feed it

Cornell University: Colors are composed by brain not eyes

Q: After the Arctic tern, what's the next farthest migrating animal? -- Kevin A., San Francisco, California

[NOAA] Gray whale at Scammons Lagoon in the Bering Sea off the Alaskan coast

A: The Gray Whale comes in second with up to 12,430 miles (20,000 km) round-trip travel each year. A gray whale may live 40 years or more. Over her lifetime, she swims far: to the Moon-- and back again!

Whales swim to cold waters to feed and to warm waters to breed and give birth. The Grays feed off Alaska in the Bering and the Beaufort Seas from April to November. In early winter they move south to warm, shallow lagoons in Mexico to breed. They mosey along (at most 5 miles an hour), stay close to the shore, and travel a couple of months. The whales eat little while migrating and calving. Some may go without food for three to five months.

In the spring, they head north to Alaska. The whales swim close to headlands. We suspect they navigate by listening to waves crash on the beach. They keep the surf sound on their right side if headed north and on their left when headed south.

Further Surfing:

Whale Route: Whale migration

San Diego Unified School District: Gray whale

Q: The longest migrating animal--Arctic terns-- go north and south instead of taking advantage of Westerly trade winds. Is there some other atmospheric factor they take advantage of? -- Kevin A., San Francisco, California

Q: What is the number one reason birds migrate? --"Why birds migrate", Boone, Iowa

A: Arctic Terns, like almost all North American birds, migrate north and south (instead of east and west) in search of summer's sustenance. In Europe and Asia, some migratory routes are more easterly-westerly but the routes change latitudes significantly, too.

Birds migrate to avoid harsh seasons and a foodless land. They breed in warm, fertile areas--like the Arctic during its brief summer-- to gobble up food piñatas. In fact, migration evolved to increase the number of chicks a bird can produce over his or her lifetime. If they go where the food is, they can breed earlier. The Arctic tern carries this good idea to an extreme: by going from pole to pole, following summer.

So, few birds get a migratory boost from the trade winds. However, an Arctic tern, encountering headwinds, follows strategies to maximize range. She flies at the altitude where winds are favorable. In general, she flies high. This has the added benefit of avoiding dehydration from the warmer air near the ground. (Birds don't sweat but rather pant to shed excess heat. Panting loses air from the respiratory system and can cause dehydration.) Birds migrating over the Caribbean like to fly about 10,000 feet although some fly half as high and others twice that altitude.

Long-distant migrants start at 5,000 and ease up to 20,000 feet, flying higher and higher the leaner they get. It's less work to gain altitude as they lose weight.

See the book, The Birder's Handbook by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye, for more information about bird flight.

Further Surfing:

WonderQuest: Migrant champ

(Answered Jan. 17, 2003)