eyeing myopia

gered-species program for the Na­tional Audubon Society, "is a balance between e conomic interests and en­vironmental concerns . If we reduce the issue to owls versus jobs, we lose everything . " -Laurie Burnham

Sonic Punch

Dolphins and whales generate "bangs" that may stun prey

S tun grenades occupy a key place in the weaponry of commandos .

The disorientation the weapons cre­ate can provide a dec isive advantage in a surprise assault; by the same to­ken sonic booms have been used to confuse hij ackers . As so ofte n hap­pens, nature appears to offer a model for such innovations : re cordings of dolphins and whales in the wild sug­gest that some of them gene rate in­tense pulses of sound that may stun fish, rende ring the prey helpless .

It i s well known that several spe­c ies of dolphins and other toothed whales find prey by ec holoc ation, emitting ultrasonic "c licks" in rapid succession and listening for echoes returning from obj e cts in the wate r . Ac cording to Kenneth Marten of the Long Marine Laboratory at the Uni­versity of California at Santa Cruz, the clicks may be "j et-engine loud" if the obj e c t is far away. Marten and his fe l low worker Kenneth S . Norris won­dered whether the clicks might not also disturb the sensitive lateral l ines of the prey: organs in fish that dete ct minute movements in the water . Fur­thermore, several anecdotal reports describe fish as appe aring to be stunned imme diately before being e aten, and whale stomachs often con­tain fish that seem undamaged.

Yet Marten and Norris were not able to show that even very loud e c holocation cl icks affect prey. Re­c ently the fish-stunning hypothesis has regained ground. Seve ral investi­gators, starting with Virginia L . Cass, formerly at the La Jolla Southwest Fisheries Center of the National Ma­rine Fisheries Service, found that wild bottle -nosed dolphins and kill­er whales produc e banging noises whi le fe eding. Tape recordings of the bangs show that they are much low­er in fre quency than cl icks and so co­incide with the hearing range of the prey; they are also much louder and last about 1 ,000 times longer.

The recordings fe ature ascending tri l ls of c l icks followed by what sounds exactly like a gun firing-or a

36

stun grenade exploding. Sometime s the bangs sound like machine-gun fire. Similar noises are made by dol­phins in threatening social interac­tions, sugge sting that for a dolphin a bang might be the equivalent of bared fangs for a dog. Analysis of the sounds rules out a nonvoc al source, Marten believe s . Sperm whales have also been re corded making bang­ing nOises, although it is not known whether they were fe eding.

Marten points out that the re cord­ings do not prove the fish-stunning hypothesis . Bangs are not always produced when toothed whales and dolphins fe ed, and captive dolphins have not been he ard to make the sounds (Marten speculate s that they might be unbe arably loud in a con­fined tank). He plans to investigate the effect of recorded bangs on cap­tive prey fish . -T.M.B.

Eyeing Myopia

Research suggests how reading could lead to nearsightedness

Can myopia, or nearsightedness, be caused by too much re ading?

A long history of observations sug­gests that it can-including a report­ed increase in myopia among Eski­mos after the advent of compulso­ry educ ation . Research has also sug­gested that myopia is assoc iate d with e longation of the eye : l ight from dis­tant images tends to focus in front of the retina instead of on it . But what is the link between reading and ocular e longation? A hypothesis based on recent experiments with chicks has been put forward by a team led by Josh Wallman of the City College of the City University of New York. If the periphe ral re gions of the retina are understimulated, the workers re­port in Science, the re sulting reduc ed neuronal activity there may cause parts of the eye to e longate . The en­tire eye becomes myopic as a result .

To test the hypothesis the team ob­scured the vision of one eye of each recently hatc hed chick (the unob­scured eye served as the control ) . In one group the entire eye was cov­ered with a translucent plastic shield ( imagine seeing through a Ping-Pong ball-perceiving light but not seeing shape s) . In another group only part of the eye, and thus only part of the retina, was shielded.

Me asurements made at two and six weeks of age showed that all the ob­scured eyes had enlarged and be-

come myopic . Yet there were major differences betwe en the groups . In chicks whose eyes were partially oc­cluded, elongation occ urred only in the re gions that had been visually de­prive d. In contrast, the totally occlud­ed eyes enlarged uniformly. Myopia, then, may be caused by the retina's loc alized control of eye growth rath­er than by proc esses involving the e ntire eye, such as foc using, Wall­man suggests. Similar results were achieved when chicks' optic ne rve s were cut, which shows that " little patc hes of retina, without input from the brain," can influence eye growth.

Why is loc al control signific ant? Both human infants and chicks are hyperopic, or farsighted, at birth: the eye is too short for the optical power of the lens, and so light rays con­verge behind the retina. During nor­mal development the le ngth of the eye tends to incre ase, thereby mov­ing the image plane c loser to the ret­ina. Ac cording to Wallman's theo­ry, different retinal re gions indepen­dently control this increase in length . If there is a lot of ne uronal activity in a re gion of the retina (that is, if c lear vision has been attained), growth in­hibitors may slow the change. Con­versely, low activity may stimulate the secretion of growth promoters .

Wallman sugge sts that neurons in periphe ral re gions of the retina are unstimulated by re ading, j ust as the chicks ' occluded eyes were unstimu­lated by seeing through white plas­tic . The neurons near the center of the retina, or fovea, Wallman says, look at small parts of the visual fie ld, such as the dot of an i. The variation in letters and in the position of white space along a page provides enough stimulation to fire many nerve im­pulses in this region. The ne urons in the retina's periphery, on the other hand, have receptive fie lds that are at least 1 0 times larger ; hence they average the black type with the white background into a uniform and unchanging gray. The responses of these unstimulated ne urons dwindle, that sec tion of the eye elongate s and the forc e pulls the rest of the eye into myopia. -Elizabeth Collins

Skin of Frog ...

A newly revealed defense has a potent effect against microbes

D id potion-concocting witc hes al­ready know something that a

worker at the National Institute s of

© 1987 SCIENTIFIC AMERICAN, INC