66() 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



to force accommodation to this level. A chart of 

 small letters is used to control fixation and to stimu- 

 late relaxation of accommodation. Plus lenses are 

 placed in front of the eye to relax accommodation and 

 then additional plus lenses are used to get below 

 the level to which accommodation can be relaxed. 

 This 'fogs' the eye. Once the eye is 'fogged' the plus 

 power is reduced to locate the level at which the 

 'fog' ends. This represents the 'zero lexel' of ac- 

 commodation. The measurement can be carried out 

 on the two eyes at the same time if they have pre- 

 viously been corrected for astigmatism and ani- 

 sometropia. The target is placed at a distance of 

 about 6 m to create an awareness of distance and 

 also to relax convergence if the test is binocular. 



In many experiments it is necessary to stimulate 

 accommodation to various degrees. For most pur- 

 poses it suffices to use a single target with fine detail 

 to control fixation and accommodation. If the fine 

 detail is clearly visible, it is assumed that the eye is 

 in focus for the target. On the other hand it is often 

 desirable to find out whether the amount of ac- 

 commodation in play is leading or lagging behind 

 the stimulus. One target is used for stimulating 

 fixation and accommodation and, if both eyes are 

 used, it also serves as a convergence stimulus. An 

 independent target must be used to measure the 

 amount of accommodation in play. This target is 

 usually presented to one eye and a beam splitter is 

 used to superimpose an image of the measuring 

 target upon the stimulus target. The image of the 

 measuring target must be adjustable so that it can 

 be made to fall in front of or behind the stimulus 

 target. A single bright point source is a good meas- 

 uring target because it cannot compete with the 

 more complex stimulus target in controlling ac- 

 commodation. This is the principle underlyins; the 

 stigmatoscope. 



The threshold principle (56; 57, p. 485; 73) may 

 be employed to avoid stimulating accommodation. 

 As long as the target is invisible, it cannot stimulate 

 accommodation and hence it is moved from the out- 

 of-focus position to the just-visible point. This is 

 clone from both directions to determine the limits of 

 the visible range. The mid-point is assumed to be 

 the point of best focus. 



It is of interest to know what happens to ac- 

 commodation when the stimulus to accommodation 

 is fixed and the stimulus to convergence is \aried. 

 The Scheiner principle can be used to good advantage 

 in this kind of experiment because the changes in 

 accommodation can be continuously tracked by 

 having the subject adjust the target as the stimulus 



to convergence is slowly decreased or increased CsO- 

 The Scheiner target avoids stimulating accommoda- 

 tion because the two beams entering the pupil are 

 narrow. 



The skiascope (32) or the coincidence optometer 

 (30) may also be used in this kind of experiment to 

 measure the amount of accommodation in play. 

 However, the objective methods not only present 

 criteria problems (34). but also the additional 

 stimuli applied to the retina may affect the amount 

 of accommodation. This, of course, is avoided in 

 the case of the infrared skiascope (16, 49). 



At low levels of illumination and in the presence 

 of an optically empty field of high luminance, ac- 

 commodation fluctuates. Westheimer (91) has been 

 successful in measuring these fluctuations using 

 intermittent exposure of a Scheiner target. The 

 subject reports at each exposure. This target not 

 only measures accommodation but also controls 

 fixation. Chin & Horn (16) observed these fluctu- 

 ations with an infrared skiascope. 



An optically empty field at high luminance is pro- 

 \ided by Knoll's 'blob' (47) which is a luminous 

 patch with diffuse edges upon which the subject can- 

 not focus. It is small enough, however, so that fixation 

 can be controlled by looking at the center of the 

 'blob.' 



Another problem is that of controlling convergence 

 without stimulating accommodation and at the same 

 time measuring the amount of accommodation in 

 play. The 'blob' described above or a blurred line 

 (58) can serve as a stimulus to convergence and the 

 threshold, momentary exposure or infrared principle 

 can be used in measuring the accommodation. 



Tracking rapid changes in accommodation pre- 

 sents a special problem. Changes in the ciliary muscle 

 potential (76) can be used as an index of changes in 

 accommodation. Photographic records of the changes 

 in size of the third Purkinje image have also been em- 

 ploved (i, 6, 45). Campbell's automatic recording 

 optometer (15) should be useful for this purpose. 



The amount of accommodation in play at a gi\en 

 moment following a stimulus to a change in accommo- 

 dation can be measured by presenting a momentary 

 exposure of a measuring stimulus at the selected time 

 (3. 4. ?)• 



MECH.\NISM OF .\CCOMMOD.\TION 



Intraucular Mechanism vj Aaommndiilion 



Of all the structures which might be manipulated 

 to focus the eye, the lens alone fulfills this role. Young 



