|แสดงความคิดเห็นเมื่อวันที่: 09 May 2005 เวลา 21:37 | IP
· The horopter is the set of all points in visualspace that will stimulate pairs of coresponding retinal points. It is a three-dimensional structure, but the slice of it along the horizontal plane, the longitudinal horopter, is most important to the study of binocular vision
· The Vieth-Muller circle or geometric horopter is a theoretical horopter that is base on an equiangular arrangment of corresponding retinal points in each eye. It assumes that all pairs of corresponding retinal points occur at equal angles from the fovea. Inthis horopter, all objects lying anywhere on a circle that intersect the fixation point and the nodalpoints of each eye will stimulate pairs of corresponding point.
· Properties of corresponding retinal points provide criteria that can be used to measure the horopter. When both eyes fixation a given point in space, it is possible to map out all points in space that seem to come from the same visual direction (identical visual direction of nonius horopter). These points will appear to be located in a flat plane equidistant from the observer as the fixation point [stereoscopic depth matching, or apparent frontoparallel plane (AFPP) horopter]. Dofferent points along the horopter will not elicit fusional eye movements (zero vergence horopter). The horopter will lie within Panums fusional area, the basis for the singleness of haplopia horopter.
· The horopter represents the boundary between crossed and uncrossed disparities as we fixation a particular point. Objects lying on the horopterhave zero disparity, whereas objects closer to the observer than the horopter have crossed disparity and those farther away have uncrossed disparity. The horopter is the place in space where we are most sensitive to changes in depth because even a slight change will alter the percept of an object from closer to farther away (minimum stereoacuity threshold horopter).
· A FD will cause the horopter to be displaced inward (for as esoFD) or outward (for an exoFD) relative to where the intended physical fixation point lies, because the visual axes of the two eyes arent actually crossing at the physical fixation point. The horopter lies where the visual axes cross.
· The appearent frontoparallel plane method is precise and easy to do w/ untrained subjects. However, it is important to remember that the shape of the frontoparallel plane perceived by the subject will be the mirror image of the horopter settings.
· For subjects on the Vieth-Muller circle, the relative magnification between images formed on corresponding retinal points in the right eye and the left eye (R) is equal to 1, where the value R represents the tangents of the external longitudinal angles (a1 and a2) for the left and right eyes, respedtively. An R value of 1 is necessary for the right eye and left eye targets to be seen as lying at the same angle relative to the fixation point in perceived space. R is not equal to 1 for all points off the Vieth-Muller circle.
· Empirical measure have revealed that the horopter tends to be kess sharply curved than the Vieth-Muller circle is called the Hering-Hillebrand horopter deviation (H). the existence of this deviation shows that our perception of space is wraped a bit; our corresponding points are not laid out in an evenly spaced distribution between the two eyes. The Hering-Hillebrand deviation does not change w/ fixation distance.
· The analytical plot [R = H(tana2) + R0] tells us the degree of relative magnification via its y-intercept, R0. if R0 = 1 there is no skewing of the horopter. If R0 ¹ 1, ther is uniform relative magnification of one eyes image across the visual field, tilting the horopter and ones percept of the world. This skewing might be caused by anseikonia or by unequal spectacle lens correction. The value of the slope of the plot, H, is a measure of the horopter relative to the Vieth-Muller circle. This value is uasually positive, indicating the horopter is less curved than the Vieth-Muller circle. This may be attributed to a layout of corresponding points that creates a relative minification of the nasal retina relative to the temporal hemiretina.
· The shape of the empirical horopter curves more and more away from the observer as the fixation distance is increased. However, the curvature of the Vieth-Muller circle changes proportionately, so that the Hering-Hillebrand deviation (H) remains the same at all fixation distances. The abathic distance is the distance at w/c the horopter is flat. The abathic distance is calculated by:
H = 2a/b
Where 2a is the interpupillary distance and b is the fixation distance, at about six meters from the observer.
· The vertical horopter tends to be tilted w/ its superior portion rotated away from the observer and the inferior portion closer. However, the precise degree of tilt changes w/ viewing distance.
· In strbismic subjects, the horopter is be shifted toward the intersection of their visual axes, just as in fixation disparity.
· Esotropic subjects have a large notch in the horopter near the fixation point (Floom notch), w/c suggests a regional spatial distortion under binocular conditions. Horror fusionis may be associated w/ the Flom notch.
· Aniseikonia is a difference in magnification between the two eyes, w/c may be optical or neural in origin. Optical aniseikonia results from a difference in retinal image size caused by internaloptical factors such as axial aniseikonia or refractive aniseikonia. In addition, unilateral aphakia, intraocular lenses (IOLs) in one eye alone, or monocular refractive surgery may cause aniseikonia. Induced aniseikonia is a form of optical aniseikonia caused by external optical factors such as size lenses or high astigmatic correction. Neural (essential) aniseikonia is a small degree of nonoptical aniseikoniain w/c the retinal images are of identical size, yet are perceived to be of different sizea.
· A size lens is an afocal magnifier that changes the overall magnification of an image w/o having any dioptric power. The total magnification of a size lens is determined by the same magnifying effects as any thrick lens: (a) the power factor induced by the refractive power of the lens, and (b) the shape factor related to the thickness and base curve of the lens. A meridional size lens changes the image magnification in only one meridian
· Placing an afocal meridional magnifier axis 90 in front of one eye will magnify the horizontal meridian, rotating the horopter toward the magnified eye and the perceived apparent frontoparallel would then be rotated in the opposite directionaway from the magnified eye. This is called the geometric effect. The stronger the magnification, the greater the degree of perceived ratation or tilting of the visual world. The degree of perceived ratation is determined by:
tana = [(M-1)/(M+1)][d/a)
where M is the magnification of the size lens, d is the viewing distance, and a is one-half the interpupillary distance.
· In vertical magnification w/ an axis 180 meridional size lens, the world will seem tilted in the same way as if an axis 90 magnifier were placed in front of the fellow eye. This is called the induced effect.
· Oblique magnification produces disparities that are opposite in sign for the upper and lower visual fields, resulting in an inclination or declination effect in w/c the world seems tilted about the horizontal meridian.
· Uniform magnification less than 4% produces both geometric and induced effects, w/c will cancel each other out, resulting in little or no effect on the orientation of the apparent frontoparallelplane. However, magnification greater than approximately 5% to 7% breaks down the induced effect leaving an uncorrected geometric effect in high anisometropic patients w/ its accompanying rotation of visual space.
· To correct the geometric effect, axial anisometropes should be corrected w/ spectacle lenses to offset their existing aniseikonia, and refractive anisometropes should be fitted w/ contact lens to avoid introducing an aniseikonia (Knapps law)
· Astigmatic lenses have unequal power in different meridians and can produced geometric and induced effects.
· When the image of one eye is magnified relative to that of the fellow eye, patients will make unequal saccadic and pursuit eye movements.
· We can demonstrate tilted percepts secondary to magnification effects w/ a leaf room, w/c minimizes monocular cues to depth, a space eikonometer, or telebinocular stereoscotopic cards. Magnification differences between the two eyes can also be detected w/ the Brecher Maddox rod technique.
· Prisms as well can cause nonuniform magnification distortions of visual space with more magnification at the apex than at the base. W/ base-out prisms, the horopter bows out away from you, whereas base-in prisms cause the horopter to curve toward you. As w/ lenses, visual space appears to move in the opposite direction from the horopter.
· The visual system can adapt to distortions of visual space. Short-term adaptation to aniseikinia begins quite rapidly, after only 20 minutes. The geometric effect is neutralized w/in 3 to 4 days and the induced effect take 5 to 6 days. Only minimal adaptation occurs for oblique magnification. Greater adaptation occurs in free space and natural viewing, where factors such as monocular depth cues, contours, and motor feedback have a greater effect. There is no change in the nonius horopter w/ adaptation, suggesting that there is no physiological recalibration of corresponding retinal points and visual directionality w/ adaptation.
· Small degree of aniseikonia (1% to 2%) can still produce clinical symptoms of headache of asthenopia. Aniseikonia beyond 5% will effect stereoscopic thresholds, and aniseikonia above 20% will eliminate binocular vision.
ขอขอบคุณ ข้อมูลที่ได้บ่มสอนมา จาก Dr. ประเสริฐ
แก้ไขโดย prestige เมื่อวันที่ 09 May 2005 เวลา 22:36
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