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ACCOMMODATION: MADDOX COMPONENTS AND ACCURACY Key words: Maddox, tonic, proximal, blur, convergence, focus Outline Part I: Accommodation (contí): Maddox Components and Accuracy
The proximal or perceived distance cues are used to shift
focus from one target of interest to another much like a saccade
changes foveal fixation from one target to another. The
blur cue is used to maintain clarity of a fixated image as the
head and or target move about. The tonic cue is a set point
or resting bias that represents the balance of all intrinsic
innervations in the absence of an external stimulus. Finally,
the cross-coupled stimulus is an associated response between
accommodation and convergence that allows a stimulus to either
motor system to activate both responses. This coupling helps
coordinate actions of motor systems that are responding to a
common target. |
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There are optical and sensory components of night myopia Optical components include pupil dilation and spherical aberration. Normally the lens has positive spherical aberration, i.e. it is more powerful in the periphery than along the optical axis. The periphery focuses images in front of the retina and this myopic error is more pronounced as the pupil diameter increases. In addition there is a sensory phenomenon called the Purkinje shift in which the dark adapted eye is more sensitive to short (blue 505) than medium (yellow 555) wavelengths of light. Thus the eye has a shift in luminance sensitivity toward shorter wavelengths in the dark. Because the eye has positive chromatic aberration, the long wavelengths are focused behind the short ones. Accommodation is presumed to focus the most sensitive wavelength components of the image. Accordingly refractions with multiwavelength photopic stimuli result in myopic errors for shorter wavelength stimuli. In darkness, the Purkinje shift combined with chromatic aberration of the eye causes a change in refractive error under scotopic condition. Taken together the sensory and optical factors, this amounts to 0.75 D of myopia. This leaves another 0.75 D of myopia that results from imbalance of the autonomic nervous system caused by innate imbalance, drugs or motor aftereffects of prolonged near work that produces accommodative spasm. The latter condition can affect any of the myopias and is very common. Accommodative spasm aftereffects can be relieved by periods of rest every 20-30 minutes during which patients are encouraged to get up, stretch their legs, and focus on distant targets. Space or empty field myopia is attributed to the balance of sympathetic and parasympathetic autonomic components and after-effects of prior accommodative responses when an accommodative stimulus is absent, such as in a uniform field. A related effect to space myopia is called plus lens-fog myopia. During a refraction, many clinicians like to relax accommodation of both eyes by placing plus lenses of 0.5D before the eyes and then measure the balance of the proposed refractive correction. Also when we refract we tend to fog vision, and then reduce the lens value until the patient has peak visual acuity. This technique produces a plus bias to the refractive correction. It prescribes the minimum minus or maximum plus lens to avoid over stimulation of accommodation. One potential problem with this technique is that occasionally plus lenses can trigger a space myopia and cause an increase rather than relaxation of accommodation in as many as 40% of your patients. There is no strong familiar distance cue to indicate the correct distance of the target and the patient can perceive the near point card as closer than it really is. This happens with children. You may find that greater minus lens power is needed for the patient to clearly see their acuity limit than had been needed prior to the plus lens fog. If this happens, take a break and use the chromatic duo chrome test. Occasionally, plus lenses will manifest a latent hyperopia, in which patients continue to relax accommodation in the presence of plus lenses. Proximal accommodation There have been several studies that illustrate the effect of proximal distance cues on accommodation. Ames used relative size and overlap and recently Kruger used loom or changing size while blur was eliminated by pin hole pupils. It is clear from these experiments that accommodation does indeed respond to apparent distance. Blur accommodation (optical reflex) What are the additional stimuli that make the initial correct responses to smaller amounts of blur possible? There are several that are used, including chromatic aberration and astigmatism. The role of chromatic aberration becomes apparent during difficulty in accommodating correctly with monochromatic light. With chromatic aberration we can sense a myopic-overaccommodated state by the higher contrast of red than blue chromatic fringes and for a hyperopic-underaccommodated state we sense the higher contrast of blue than red chromatic fringes. Chromatic fringes are utilized even though they are not visible on a conscious perceptual level. It is possible to experimentally modulated chromatic fringes independently of true focus errors and bias the accommodative response by the fringes (Mathiews and Kruger). Astigmatism could also provide error information. The focus of the major and minor astigmatic meridians cause blur distortion of the image which could be used to indicate whether the eye was over or under accommodated. Utilization of this cue would require learning the axis of astigmatism and the distal and proximal limits of the interval of Sturm. Finally, the eye may actually make continuous trial and error adjustments of accommodation (hunting cycle) by using the unstable fluctuations of accommodation that are analogous to fixation drift of the eyes. During attempted steady fixation the lens has slow and abrupt variations in power. The slow variations are on the order of 0.5 diopters and occur once every 2 seconds (0.5 Hz) whereas the fast variations are small 0.1 D and occur twice a second. |
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During the hunting cycle, if the eye kept track of the variations of retinal image clarity and the direction of accommodative change it could focus in the direction that made the image clearer. All of these stimuli are thought to assist the fine adjustment mechanism of accommodation. These stimuli are called odd-error stimuli because they indicate the correct direction in which to accommodate or unaccommodate. | |||||||||||||||||||||||||||
The DOF is inversely related to pupil diameter. When the pupil becomes small enough, only paraxial rays enter the eye and images of objects at all viewing distances appear equally clear. However small pupils under 2 mm reduce acuity because they produce diffraction blur of all images. Larger pupils allow non-paraxial rays to enter the eye, which make blur of nonconjugate images more visible. As a result, the DOF gets smaller. Eventually, at sizes greater than 4 mm, peripheral spherical and chromatic aberrations degrade the retinal image and acuity. Thus there is a balance between best visual acuity and smallest depth of focus. The optimal size is about 2 mm where the DOF is large and visual sensitivity is still minimally affected by either peripheral aberrations or diffraction blur. Chromatic aberration will also increase the DOF by degrading visual acuity. This is demonstrated with achromatizing lenses or monochromatic light. The DOF is smallest at the wavelength at the peak of the spectral luminosity function or at 555nm. Sensory factors also influence the DOF by their impact on contrast sensitivity. Blur detection is basically a contrast decrement task. Anything that impairs contrast sensitivity such as peripheral viewing, baseline refractive error, and subnormal vision will increase the depth of focus. In addition it is mainly the high spatial frequency detail that is affected by small amounts of blur. Blur is essentially a low-pass filter of the image plane and it will impair detection of small objects or test letters more than large ones. This is why we refine refractive corrections with the small letters near the patientís acuity limit. |
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It is possible to make the accommodative response lead the
stimulus by the DOF by starting with a large accommodative response
and reducing the stimulus. The lead represents a hysteresis
or spasm aftereffect from the prior accommodative response and
shows that accommodation lags the change in stimulus value rather
than always being smaller than the current stimulus.
Fig 17.5 Step response of accommodation. Stepped stimuli (bottom) illustrates latency,velocity,andexponential nature of accommodative response (top).
Accommodation has difficulty following periodic variations of blur, such as pendular or sinusoidal changes, and requires the .4 second latency. Therefore the response lags behind the periodic stimulus by .4 sec. Accommodation is not a good predictor to slow variations in blur. For this reason it has a limited ability in keeping up with abrupt high frequency changes grater than 0.5 Hz.
Dynamic accommodation is assessed clinically using a lens-flipper apparatus with which the patient views printed material through alternate views of plus 2D and minus 2D lenses. The objective is to clear the near target as quickly as possible following each lens flip. The number of complete plus-minus cycles/minute is the measure of accommodative facility. A normal value is 20 cycles/minute which corresponds to an accommodative response time of 1.5 seconds. Slower rates are easily increased with practice. Binocular Accommodation:
Figure 17.7 above illustrates an experiment which demonstrates the conjugacy of accommodation. Yoking is indicated by comparing the accommodative response of the covered eye when accommodation is stimulated in the open eye that is normal or cyclopelged. Unequal stimuli for accommodation also occur in asymmetric convergence. Approximately 1D aniso accommodation is stimulated at near distance of 15 cm in 45 degrees eccentric gaze because the target is closer to one eye than the other. The aniso accommodation response produces unequal resting foci aftereffects which suggests that tonic accommodation can adapt unequally in the two eyes.
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