The cocktail party effect: Fish and human brains perform 'auditory scene analysis' when looking for love in all the loud places

Midshipman fish
Midshipman fish.

SEATTLE -- It's a problem faced by people joining noisy parties and by midshipman fish seeking mates: How to cut through the racket and find Mr. Right?

Now Cornell University biologists, who became underwater disc jockeys to study a homely fish that hums, say they have a clue as to how mate selection works. The auditory portion of the midbrain uses the acoustic qualities of all the noise to isolate one signal it is programmed to recognize as potentially interesting.

The biologists' research applies only to midshipman fish, but it could, they say, also be relevant to people.

"Neuroscientists call this auditory scene analysis," says Andrew H. Bass, Cornell professor of neurobiology and behavior who will present his group's findings June 25 at the International Congress on Acoustics-Acoustical Society of America meeting in Seattle. "It's really very similar to the cocktail party effect."

In a way, midshipman fish have more problems than people at loud parties. Only some of the male midshipman hum (See "Humming Fish Facts," below), and those males are hiding in cavitylike nests they have excavated under rocks. All the humming males together sound like a huge hive of bees or a squadron of motor boats, and a female midshipman fish has to choose one nest in which to deposit her eggs. When a humming male succeeds in attracting a female, he fertilizes her eggs, which adhere to the rocky ceiling of his nest. The female leaves forever, and the male resumes humming in hopes of attracting another female with more eggs.

Wondering how the female fish find the right males, the Cornell biologists examined the structure and function of midshipman brains. From earlier studies with Robert Baker at the New York University Medical School, Bass knew that a part of the midshipman male brain, called the hindbrain, contains neurons that constitute a kind of vocal pacemaker. Like a rhythm generator, the pacemaker tells the sound-generating muscles on the male's swim bladder to contract rhythmically and produce a hum averaging 100 Hz in frequency. In part of the midshipman female brain known as the midbrain (and humans have midbrains and hindbrains, too), Baker and Bass found neurons that respond to a 100-Hz hum.

Whenever the hums of two neighboring and competing males overlap, the Cornell biologists observed, the sounds form what is known as an acoustic beat. And because the tone of a midshipman's hum is so pure and simple, computer synthesizers can easily reproduce it. That's why the biologists were able to play disc jockey at a fish party, complete with underwater loudspeakers.

"Just as we expected, two or more synthesized fish hums played together produce the rhythmic, acoustic beats," Bass reports. "And sure enough, the females were able to directly localize one of the humming speakers. Their midbrain neurons form a code of the beats that helps in their calculations to locate the hum of interest from all the rest."

The brain side of the story recently was reported in the Journal of Neuroscience by Bass and by Deana Bodnar, a Cornell senior research associate in neurobiology and behavior. Details of the playback studies by Bass and by Jessica McKibben, a postdoctoral researcher in neurobiology and behavior at Cornell, will be published in the Journal of the Acoustical Society of America.

Meanwhile, experiments supported by the National Science Foundation and National Institutes of Health continue with midshipman fish along the California and Washington state coasts as well as in Cornell laboratories. Field studies led by Margaret Marchaterre, a research associate in the Bass group, use hydrophones (underwater microphones) to eavesdrop on fish gossip at night. Together, the Cornell "midshipman crew" hopes to learn how courtship signals are encoded in the brain and what it is about one love hum that makes it more attractive than another.

"Midshipman are regarded as some of the ugliest fish in the sea and a nuisance because they hum almost incessantly," Bass comments. "But they have thrived for hundreds of thousands of years, so they must be doing something right. We'd like to find out what."


Humming Fish Facts

Source: Section of Neurobiology and Behavior, Cornell University

NOT THE UNIFORM. Called midshipman fish because some varieties of Porichthys notatus have bioluminescent spots that resemble rows of uniform buttons, they are also known as California singing fish and canary bird fish. Females probably aren't attracted to the uniform, but rather to the male's song.

PICENE BOOMBOXES. When midshipmen migrate from the deep Pacific waters to the west coast of North America to mate each summer, the intertidal zone becomes a noisy place. Courting males hum to attract egg-laying females. The love song, described as a motorboat-like drone, comes from rapidly contracting muscles on the male's swim bladder and proves irresistible to female midshipman. Each female deposits all her eggs for that season in one nest and swims away. Hoping to lure more females to the nest, the male resumes singing, all the while remaining on guard until the offspring hatch and mature.

SNEAKER MALES. Not guys in Air Jordans but morphs with an attitude, the so-called Type II or sneaker males are smaller and less vocally talented than Type I males. Unable to attract females because they can't sing, sneaker males hang around nests where Type I males are humming and quickly dart in to fertilize some eggs. They frequently get away with this behavior, probably because they closely resemble the smaller females.

THAT TAKES TESTES. What sneaker males lack in vocal repertoire and size, they more than make up in reproductive capacity. Studies at Cornell have shown that the gonads of Type II sneaker males comprise up to 15 percent of their body weight, compared with only 1 percent in Type I males. The equivalent in a 150-pound human male would be 22.5-pound testes.

BIG VOCABULARY. Don't think humming fish hum because they don't know the words. They produce two others kinds of vocalizations that, if not especially eloquent, do get the message across: A series of grunts that bioacoustic scientists call a "grunt train" and a low growl both come from Type I males guarding their nests. Type II males have been heard making an occasional grunting sound but always in non-spawning situations.

ALL EARS. Only certain fish species are capable of vocalizing, but virtually all fish have ears to keep them in tune with their sound-filled environment. Cornell biologists who conduct acoustic playback experiments with midshipman fish in outdoor aquariums report that female midshipman are easily attracted to their computer-synthesized hums. But simulated grunts, they say, "seem to do nothing for the females."

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