Clinical studies have also provided interesting preliminary evidence of oculomotor changes accompanying myopia development. Birnbaum[68] observed differences in clinical measures of accommodation and vergence during myopia development. He suggested that“incipient myopia”can be associated with lower positive relative accommodation and amplitude of accommodation，and greater esophoria or less exophoria at near. Drobe and de Saint－Andre[69] compared near phorias measured by French optometrists in children and young adults who become myopic and who remained emmetropic. They found that the difference between the two groups was 2.9 prism diopters（P＝0.08）with the pre－myopic group having more esophoria. Goss and Jackson[70] found that their became－myopic group had a more convergent（eso）phoria at near. In the children of this group，the near phoria showed an eso shift with time，beginning before and continuing after the onset of myopia. Their results are similar to the finding that young adults who developed LOM showed higher response AC／A ratios before and continuing after the onset of myopia[32]. In addition，the higher AC／A ratios are related to the higher dissociated near phorias[71]. This finding reveals the important role of the interaction between accommodation and vergence in the development of myopia. For the same vergence response，a subject with higher AC／A ratio has bigger lag in accommodative response to the near target than someone with a normal or low AC／A ratio. Furthermore，the accommodative lag potentially creates a blurred image on the retina that may serve as an error signal for the compensative growth of the eye that could lead to the development of myopia.

　　3　Discussion

　　Many hypotheses of myopia etiology have been suggested but none has been widely accepted. But，it is certain that one of environmental factors associated with myopia prevalence is nearwork. In this review，I concentrated on work which revealed how the oculomotor functions are altered by nearwork and whether these effects differ between emmetropes and myopes. Among oculomotor parameters，dark－focus has been identified as the tonic posture of accommodation. After sustained nearwork，dark－focus shifts to near，which is called accommodative adaptation or nearwork aftereffect. I have discussed the changes of accommodative parameters under 4 conditions，i.e.，static open－loop，static closed－loop，dynamic open－loop，and dynamic closed－loop. Because accommodation system can be thought of as a negative feedback control system，under the static closed－loop condition，dark－focus shift has a little effect on the accommodative response（i.e.，the output of this system）if the gain of the system is high. But，under the static and dynamic open－loop conditions and even in the dynamic closed－loop condition，the accommodative response shows a hysteresis phenomenon which is related to accommodative or ciliary spasm. In the clinic，ciliary spasm is defined as an excessive，unnecessary and inappropriate contraction of the ciliary muscle（Borish，1970）. This abnormal status of accommodative spasm associated with nearwork can be thought as a transient myopia and pseudomyopia if it becomes continuous. On the other hand，permanent myopia mainly shows axial elongation of the eye. Both animal model studies and work on human subjects lead to a hypothesis in which the accuracy of the retinal image and／or retinal image quality provides feedback for growth of the posterior segment of the eye. The reduced accommodative response to near stimuli，the high AC／A ratio，the near esophoria，and the high defocus threshold of the accommodative system described above result in a condition of a defocused image on the retina and support this hypothesis.

　　Much work has been done，but not enough yet to solve the mystery. We still face the puzzle why not all people who perform nearwork become myopic. Recent studies led us to further hypothesize an oculomotor theory of myopia development：“Near work causes changes in the oculomotor characteristics of susceptible individuals which begin even before the development of refractive error. This series of changes in oculomotor function apparently results in optical defocus，which has the potential to induce compensatory changes resulting in myopia”[69]. There still are gaps in bridging the relationship between nearwork and the development of myopia that prevent our complete formulation of the theory. Although transient myopia is not a necessary step for a person to develop refractive error，it is true that transient or pseudo－myopia can become permanent myopia. Therefore，a major question is how this change occurs. Is the change related to oculomotor functions？Further research in this area appears to be very critical and necessary.

　　Acknowledgments　The author is grateful to Dr. Harold Bedel and Dr. Stephen Morse for their helpful comments on the manuscript.

　　References

　　[39]　Fledelius HC. Ophthalmic changes from age 10 to 18 years. A longitudinal study of sequels to low birth weight. III. Ultrasound oculometry of keratometry of anterior eye segment. Acts Ophthal，1982a，60：393～402

　　[40]　Fledelius HC. Ophthalmic changes from age 10 to 18 years. A longitudinal study of sequels to low birth weight. IV. Ultrasound oculometry of vitreous and axial length. Acts Ophthal，1982，60（13）：403～411

　　[41]　Tokoro T，Suzuki K. Changes in ocular refractive components and development of myopia during seven years. Jpn J Ophthalmol，1969，13：27～34

　　[42]　Goss DA，Erickson P. Meridional corneal components of myopia progression in young adults and children. Am J Optom Physiol Opt，1987，64：475～481

　　[43]　Adams AT. Axial length elongation，not corneal curvature，as a basis of adult onset myopia. Am J Optom Physiol Opt，1987，64：150～152

　　[44]　McBrien NA，Millodot M. A biometric investigation of late onset myopic eyes. Acta Ophthal，1987，65：461～468

　　[45]　Grosvenor T，Scott R. Three－year changes in refraction and its components in youth－onset and early adult－onset myopia. Optom Vis Sci，1993，70：677～683

　　[46]　Jiang B，Woessnor WM. Increase in axial length is responsible for late－onset myopia. Optome Vis Sci，1996，73：231～234

　　[47]　Wiesel TN，Raviola E. Myopia and eye enlargement after neonatal lid fusion in monkeys. Nature，1977，266：66～68

　　[48]　Crawford MLJ，Marc RE. Light transmission of cat and monkey eyelids. Vis Res，1976，16：323～324

　　[49]　Loop MS，Sherman S. Visual discrimination during eyelid closure in the cat. Brain Res，1977，128：329～339

　　[50]　Irving EL，Sivak TG，Callender MG. Refractive plasticity of the developing chick eye. Ophthal Physiol Opt，1992，12：448～456

　　[51]　Irving EL，Sivak JG，Callender MG. Defocus effects on the developing chick eye. Invest Ophthal Vis Sci，1993，（Suppl）34：880

　　[52]　Rohrer B，Schaeffel F，Zrenner E，Longitudinal chromatic aberration and emmetropization：results from the chicken eye. J Physiol，1992，449：363～376

　　[53]　Schaeffel F，Glasser A，Howland HC.Accommodation，refractive error and eye growth in chickens. Vis Res，1988，28：639～657

　　[54]　Schaeffel F，Howland HC. Mathematical model of emmetropization in the chicken. J Opt Soc Am A，1988，5：2080～2086

　　[55]　Schaeffel F，Howland HC. Properties of the feedback loops controlling eye growth and refractive state in the chicken. Vis Res，1991，31：717～734

　　[56]　Schaeffel F，Troilo D，Wallman J，Howland HC. Developing eyes that lack accommodation growth to compensate for imposed defocus. Vis Neurosci，1990，4：177～183

　　[57]　Schmid KL，Wildsoet CF. The sensitivity of the chick eye to refractive defocus. Ophthal Physiol Opt，1997，17：61～67

　　[58]　Hung LF，Crawford MLJ，Smith EL. Spectacle lenses alter eye growth and the refractive status of young monkeys. Nature Medicine，1995，1：761～765

　　[59]　Troilo D，Wallman J. The regulation of eye growth and refractive state：an experimental study of emmetropization. Vis Res，1991，31：1237～1250

　　[60]　McBrien NA，Millodot M. The effect of refractive error on the accommodative response gradient. Ophthal Physiol Opt，1986，6：145～149

　　[61]　Rosenfield M，Gilmartin B. Assessment of the CA／C ratio in a myopic population. Am J Optom Physiol Opt，1988，65：168～173

　　[62]　Bullimore MA，Gilmartin B，Royston JM. Steady－state accommodation and ocular biometry in late－onset myopia. Doc Ophthal，1992，80：143～155

　　[63]　Gwiazda J，Bauer T，Thorn F，Held R. A dynamic relationship between myopia and blur－driven accommodation in school－aged children. Vis Res，1995，35：1299～1304

　　[64]　Abbott ML，Schmid KL，Strang NC. Difference in the accommodation stimulus response curves of adwlt muopes and emmetropes. Ophthal Physiol Opt，1988，18∶13～20

　　[65]　Jiang B. A modified control model for steady－state accommodation. The proceedings of the first international symposium on accommodation／vergencemechanisms in the visual system，Stockholm：in press，1998.130

　　[66]　Jiang B. Integration of a sensory component into the accommodation model reveals differences between emmetropes and late－onset myopes. Invest Ophthalmol Vis Sci，1997，38：1511～1516

　　[67]　Birnbaum MH. Clinical management of myopia. Am J Optom Physiol Opt，1981，58：554～559

　　[68]　Drobe B，de Saint－Andre R. The premyopic syndrome. Ophthal Physiol Opt，1995，15：375～378

　　[69]　Jiang B，Morse SE. Oculomotor functions and late－onset myopia. Ophthal Physiol Opt，1998

　　[70]　Goss DA，Tackson TW，Clinical findings before the onset of myopia in youth：3. Heterophoria. Optom Vis Sci，1966，73∶269～278

　　[71]　Kent PR. Acquired myopia of maturity. Am J Optom Arch Am Acad Optom，1963，40：247～256

　　from College of Optometry，University of Houston，USA

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