Eyeing Myopia
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gered-species program for the Na - tional Audubon Soc iety, "is a balance between economic interests and en- vironmental 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 cre- ate can provide a decisive advantage in a surprise assault; by the same to- ken sonic booms have been used to confuse hijackers. As so often hap- pens, nature appears to offer a model for such innovations: recordings of dolphins and whales in the wild sug- gest that some of them generate in- tense pulses of sound that may stun fish, render ing the prey helpless. It is well known that several spe- cies of dolphins and other toothed whales find prey by echolocation, emitting ultrasonic "clicks " in rap id succession and listening for echoes retu rning from objects in the water. According to Kenneth Marten of the Long Marine Laboratory at the Un i- versity of California at Santa Cruz, the clicks may be "jet-engine loud" if the object is far away. Marten and his fellow worker Kenneth S. Norris won- dered whether the clicks might not also disturb the sens it ive lateral l ines of the prey: organs in fish that detect minute movements in the water. Fur- thermore, several anecdotal reports describe fish as appearing to be stunned immediately before be ing eaten, and whale stomachs often con- tain fish that seem undamaged. Yet Marten and Norris were not able to show that even very loud echolocation cl icks affect prey. Re- cently the fish-stunning hypothesis has reg ained ground. Several investi- gators, starting with Virginia L. Cass, formerly at the La Jolla Southwest Fisher ies Center of the Nat ional Ma- rine Fisheries Service, found that wild bottle-nosed dolph ins and kill- er whales produce banging noises while feed ing. Tape recordings of the bangs show that they are much low- er in frequency than clicks and so co- incide with the hearing range of the prey; they are also much louder and last about 1,000 times longer. The recordings feature ascending trills of cl icks followed by what sounds exactly like a gun fir ing-or a 36 stun grenade exploding. Sometimes the bangs sound like mach ine-gun fire. Similar noises are m a de by dol- phins in threatening social interac- tions, suggesting that for a dolph in a bang might be the equ ivalent of bared fangs for a dog. Analysis of the sounds rules out a nonvocal source, Marten believes. Sperm whales have also been recorded making bang- ing nO ises, although it is not known whether they were feeding. Marten points out that the record- ings do not prove the fish-stunning hypothesis. Bangs are not always produced when toothed whales and dolphins feed, and captive dolphins have not been heard to make the sounds (Marten speculates that they might be unbearably loud in a con- fined tank). He plans to invest igate the effect of recorded bangs on cap- tive prey fish. -T.MB. Eyeing Myopia Research suggests how reading could lead to nearsightedness C an myopia, or nearsightedness, be caused by too much reading? A long history of observat ions sug- gests that it can-including a report- ed increase in myopia among Eski- mos after the advent of compulso- ry education. Research has also sug- gested that myopia is associated with elongation of the eye: l ight from dis- tant images tends to focus in front of the retina instead of on it. But what is the link between reading and ocular elongation? A hypothesis based on recent experiments with ch icks has been put forward by a team led by Josh Wallman of the City College of the City Un ivers ity of New York. If the peripheral regions of the retina are understimulated, the workers re- port in Science, the resulting reduced neuronal activity there may cause parts of the eye to elongate. The en- tire eye becomes myopic as a result. To test the hypothesis the team ob- scured the vision of one eye of each recently hatched chick (the unob- scured eye served as the control). In one group the entire eye was cov- ered with a translucent plastic sh ield (imagine see ing through a Ping-Pong ball-perceiv ing light but not seeing shapes). In another group only part of the eye, and thus only part of the retina, was shielded. Measurements made at two and six weeks of age showed that all the ob- scured eyes had enlarged and be- come myopic. Yet there were major differences between the groups. In chicks whose eyes were part ially oc- cluded, elongation occurred only in the regions that had been visually de- prived. In contrast, the totally occlud- ed eyes enlarged uniformly. Myopia, then, may be caused by the retina's localized control of eye growth rath- er than by process es involving the entire eye, such as focus ing, Wall- man suggests. Similar results were achieved when ch icks' opt ic nerves were cut, which shows that "little patches of retina, without input from the brain," can influence eye growth. Why is local control significant? Both human infants and c hicks are hyperopic, or farsighted, at b irth: the eye is too short for the opt ical power of the lens, and so light rays con- verge behind the retina. During nor- mal development the length of the eye tends to increase, thereby mov- ing the image plane closer to the ret- ina. According to Wallman's theo- ry, different retinal regions indepen- dently control this increase in length. If there is a lot of neuronal activity in a region of the retina (that is, if clear vis ion has been attained), growth in- hib itors may slow the change. Con- versely, low activity may stimulate the secretion of growth promoters. Wallman suggests that neurons in peripheral regions of the ret ina are unstimulated by reading, just as the chicks' occluded eyes were unstimu- lated by seeing through white plas- tic. The neurons near the center of the retina, or fovea, Wallman says, look at small parts of the visual field, such as the dot of an i. The variation in letters and in the posit ion of white space along a page provides enough stimulation to fire many nerve im- pulses in this region. The neurons in the retina's per iphery, on the other hand, have receptive fields that are at least 10 times larger; hence they average the black type with the white background into a uniform and unchanging gray. The responses of these unstimulated neurons dwindle, that section of the eye elongates and the force 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-concoct ing witches al- ready know something that a worker at the National Institutes of © 1987 SCIENTIFIC AMERICAN, INC
Transcript of 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
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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
