Limits...
Effects of Prism Eyeglasses on Objective and Subjective Fixation Disparity.

Schroth V, Joos R, Jaschinski W - PLoS ONE (2015)

Bottom Line: This study investigates effects of wearing prisms constantly for about 5 weeks in daily life.Repeated measurements were made without the prisms and with the prisms after about 5 weeks of wearing these prisms.This response pattern was related to the vergence adaptability, i.e. the individual fusional vergence reserves.

View Article: PubMed Central - PubMed

Affiliation: University of Applied Sciences and Arts Northwestern Switzerland, Olten, Switzerland.

ABSTRACT
In optometry of binocular vision, the question may arise whether prisms should be included in eyeglasses to compensate an oculomotor and/or sensory imbalance between the two eyes. The corresponding measures of objective and subjective fixation disparity may be reduced by the prisms, or the adaptability of the binocular vergence system may diminish effects of the prisms over time. This study investigates effects of wearing prisms constantly for about 5 weeks in daily life. Two groups of 12 participants received eyeglasses with prisms having either a base-in direction or a base-out direction with an amount up to 8 prism diopters. Prisms were prescribed based on clinical fixation disparity test plates at 6 m. Two dependent variables were used: (1) subjective fixation disparity was indicated by a perceived offset of dichoptic nonius lines that were superimposed on the fusion stimuli and (2) objective fixation disparity was measured with a video based eye tracker relative to monocular calibration. Stimuli were presented at 6 m and included either central or more peripheral fusion stimuli. Repeated measurements were made without the prisms and with the prisms after about 5 weeks of wearing these prisms. Objective and subjective fixation disparity were correlated, but the type of fusion stimulus and the direction of the required prism may play a role. The prisms did not reduce the fixation disparity to zero, but induced significant changes in fixation disparity with large effect sizes. Participants receiving base-out prisms showed hypothesized effects, which were concurrent in both types of fixation disparity. In participants receiving base-in prisms, the individual effects of subjective and objective effects were negatively correlated: the larger the subjective (sensory) effect, the smaller the objective (motor) effect. This response pattern was related to the vergence adaptability, i.e. the individual fusional vergence reserves.

No MeSH data available.


Related in: MedlinePlus

Conditions of measuring fixation disparity.Without prisms, the objective fixation disparity (oFD) is the difference between the observed vergence angle V during binocular recording and the stimulus vergence angle V0, which is geometrically given by the viewing distance D and the interpupillary distance p, i.e. V0 = 2 arctan (p/2D). In this example of an over-convergent (eso) oFD, V is larger than V0. The monocular components of V0 are measured during the eye tracker calibration that is made separately for the left and right eye: the eye position during monocular fixation represents the zero position for the subsequent binocular recording period. The covered eye assumes the heterophoria resting state. To correct an eso fixation disparity (as in this example), base-out prisms are applied. These prisms turn the visual axes optically outward (drawn lines), which requires the eye muscles to converge more (broken lines) to maintain fusion (see Fig 2). When prisms are applied, V0P = Prismpower + V0 is the stimulus vergence angle and Vp is the vergence angle. The fixation disparity with prisms is measured relative to the reference condition of monocular fixation when the prisms are worn. The subjective fixation disparity (sFD = arctan (dNon/D)) is illustrated by the amount of nonius offset dNon, which is typically smaller than the objective fixation disparity. Note that both types of fixation disparity are smaller with prisms than without prisms, as suggested by the study results. The graphs show the case of visual axes that intersect in front of the fixation point; this over-convergent state is referred to as eso fixation disparity with a positive sign. In the opposite under-convergent state, the visual axes intersect behind the fixation point (exo fixation disparity with a negative sign); in the latter case, base-in prisms are applied.
© Copyright Policy
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4592239&req=5

pone.0138871.g001: Conditions of measuring fixation disparity.Without prisms, the objective fixation disparity (oFD) is the difference between the observed vergence angle V during binocular recording and the stimulus vergence angle V0, which is geometrically given by the viewing distance D and the interpupillary distance p, i.e. V0 = 2 arctan (p/2D). In this example of an over-convergent (eso) oFD, V is larger than V0. The monocular components of V0 are measured during the eye tracker calibration that is made separately for the left and right eye: the eye position during monocular fixation represents the zero position for the subsequent binocular recording period. The covered eye assumes the heterophoria resting state. To correct an eso fixation disparity (as in this example), base-out prisms are applied. These prisms turn the visual axes optically outward (drawn lines), which requires the eye muscles to converge more (broken lines) to maintain fusion (see Fig 2). When prisms are applied, V0P = Prismpower + V0 is the stimulus vergence angle and Vp is the vergence angle. The fixation disparity with prisms is measured relative to the reference condition of monocular fixation when the prisms are worn. The subjective fixation disparity (sFD = arctan (dNon/D)) is illustrated by the amount of nonius offset dNon, which is typically smaller than the objective fixation disparity. Note that both types of fixation disparity are smaller with prisms than without prisms, as suggested by the study results. The graphs show the case of visual axes that intersect in front of the fixation point; this over-convergent state is referred to as eso fixation disparity with a positive sign. In the opposite under-convergent state, the visual axes intersect behind the fixation point (exo fixation disparity with a negative sign); in the latter case, base-in prisms are applied.

Mentions: The objective fixation disparity (oFD) refers to the oculomotor position of the eyes, i.e. the vergence angle V between the visual axes, which is measured with eye trackers using a monocular calibration procedure (Fig 1): targets fixated by the left or right eye alone are assumed to be projected onto the centre of the foveola and the corresponding left and right eye positions define a theoretical vergence angle V0, which is assumed to represent the optimal vergence state, i.e. zero objective fixation disparity (oFD = 0); thus monocular calibrations of the eye tracker define the reference for objective fixation disparity. Any deviating vergence state (vergence error) represents an objective fixation disparity which can be up to about 60 min arc. Nevertheless, double vision does not occur, rather the binocular targets appear as single objects, i.e. the left and right retinal images are fused and receive the same visual direction in space. Thus, despite of vergence errors, neural mechanisms with different stages provide sensory fusion of the binocular targets [1, 34]. However, even if the binocular stimuli are fused, two physically aligned monocular test stimuli for each eye (nonius lines) may nevertheless be perceived in different visual directions: this psychophysically measured nonius offset is referred to as subjective fixation disparity (sFD). A nonius test result is not a psychophysical equivalent of the oculomotor vergence error measured with eye trackers. Rather, the validity of nonius lines as vergence indicators is questionable [12, 35–37]. The subjective fixation disparity seems to be affected by two processes: the oculomotor adjustment of the visual axis (objective fixation disparity) and the mapping of visual directions by sensory fusion [38, 39]. The conditions of fusion play an important role. In non-fusion conditions, as in measurements of heterophoria or resting vergence (dark vergence), nonius lines give a very similar vergence angle as eye trackers [40, 41]. In fusion however, the visual directions in the vicinity of the binocular target are modified in a way to facilitate sensory fusion. This shift in retinal correspondence, however, can affect the visual directions of dichoptic nonius lines in the following way [1, 30, 31, 35, 38, 42]. When nonius lines are more distant from a local central fusion stimulus by about 4 deg, the subjective and objective fixation disparity are similar in amount. However, the closer the nonius lines are located near the fusion stimulus, the more the visual direction of the fusion stimulus is transferred to the nonius lines and the smaller is the subjective fixation disparity. Thus, the spatial arrangement of binocular and monocular objects in the test are important, but individuals differ in the effect of the gap between nonius lines and fusion object [43]. Some studies reported that objective fixation disparity can be 10 times larger than subjective fixation disparity, other studies found similar amounts (summarized in [33]). Given these physiological differences, both types of fixation disparity are only weakly correlated [33]. Still they have some common properties: they both are correlated with the resting vergence position, measured as heterophoria or dark vergence [20, 21, 33, 44]; they both increase as the viewing distance shortens [13, 44].


Effects of Prism Eyeglasses on Objective and Subjective Fixation Disparity.

Schroth V, Joos R, Jaschinski W - PLoS ONE (2015)

Conditions of measuring fixation disparity.Without prisms, the objective fixation disparity (oFD) is the difference between the observed vergence angle V during binocular recording and the stimulus vergence angle V0, which is geometrically given by the viewing distance D and the interpupillary distance p, i.e. V0 = 2 arctan (p/2D). In this example of an over-convergent (eso) oFD, V is larger than V0. The monocular components of V0 are measured during the eye tracker calibration that is made separately for the left and right eye: the eye position during monocular fixation represents the zero position for the subsequent binocular recording period. The covered eye assumes the heterophoria resting state. To correct an eso fixation disparity (as in this example), base-out prisms are applied. These prisms turn the visual axes optically outward (drawn lines), which requires the eye muscles to converge more (broken lines) to maintain fusion (see Fig 2). When prisms are applied, V0P = Prismpower + V0 is the stimulus vergence angle and Vp is the vergence angle. The fixation disparity with prisms is measured relative to the reference condition of monocular fixation when the prisms are worn. The subjective fixation disparity (sFD = arctan (dNon/D)) is illustrated by the amount of nonius offset dNon, which is typically smaller than the objective fixation disparity. Note that both types of fixation disparity are smaller with prisms than without prisms, as suggested by the study results. The graphs show the case of visual axes that intersect in front of the fixation point; this over-convergent state is referred to as eso fixation disparity with a positive sign. In the opposite under-convergent state, the visual axes intersect behind the fixation point (exo fixation disparity with a negative sign); in the latter case, base-in prisms are applied.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4592239&req=5

pone.0138871.g001: Conditions of measuring fixation disparity.Without prisms, the objective fixation disparity (oFD) is the difference between the observed vergence angle V during binocular recording and the stimulus vergence angle V0, which is geometrically given by the viewing distance D and the interpupillary distance p, i.e. V0 = 2 arctan (p/2D). In this example of an over-convergent (eso) oFD, V is larger than V0. The monocular components of V0 are measured during the eye tracker calibration that is made separately for the left and right eye: the eye position during monocular fixation represents the zero position for the subsequent binocular recording period. The covered eye assumes the heterophoria resting state. To correct an eso fixation disparity (as in this example), base-out prisms are applied. These prisms turn the visual axes optically outward (drawn lines), which requires the eye muscles to converge more (broken lines) to maintain fusion (see Fig 2). When prisms are applied, V0P = Prismpower + V0 is the stimulus vergence angle and Vp is the vergence angle. The fixation disparity with prisms is measured relative to the reference condition of monocular fixation when the prisms are worn. The subjective fixation disparity (sFD = arctan (dNon/D)) is illustrated by the amount of nonius offset dNon, which is typically smaller than the objective fixation disparity. Note that both types of fixation disparity are smaller with prisms than without prisms, as suggested by the study results. The graphs show the case of visual axes that intersect in front of the fixation point; this over-convergent state is referred to as eso fixation disparity with a positive sign. In the opposite under-convergent state, the visual axes intersect behind the fixation point (exo fixation disparity with a negative sign); in the latter case, base-in prisms are applied.
Mentions: The objective fixation disparity (oFD) refers to the oculomotor position of the eyes, i.e. the vergence angle V between the visual axes, which is measured with eye trackers using a monocular calibration procedure (Fig 1): targets fixated by the left or right eye alone are assumed to be projected onto the centre of the foveola and the corresponding left and right eye positions define a theoretical vergence angle V0, which is assumed to represent the optimal vergence state, i.e. zero objective fixation disparity (oFD = 0); thus monocular calibrations of the eye tracker define the reference for objective fixation disparity. Any deviating vergence state (vergence error) represents an objective fixation disparity which can be up to about 60 min arc. Nevertheless, double vision does not occur, rather the binocular targets appear as single objects, i.e. the left and right retinal images are fused and receive the same visual direction in space. Thus, despite of vergence errors, neural mechanisms with different stages provide sensory fusion of the binocular targets [1, 34]. However, even if the binocular stimuli are fused, two physically aligned monocular test stimuli for each eye (nonius lines) may nevertheless be perceived in different visual directions: this psychophysically measured nonius offset is referred to as subjective fixation disparity (sFD). A nonius test result is not a psychophysical equivalent of the oculomotor vergence error measured with eye trackers. Rather, the validity of nonius lines as vergence indicators is questionable [12, 35–37]. The subjective fixation disparity seems to be affected by two processes: the oculomotor adjustment of the visual axis (objective fixation disparity) and the mapping of visual directions by sensory fusion [38, 39]. The conditions of fusion play an important role. In non-fusion conditions, as in measurements of heterophoria or resting vergence (dark vergence), nonius lines give a very similar vergence angle as eye trackers [40, 41]. In fusion however, the visual directions in the vicinity of the binocular target are modified in a way to facilitate sensory fusion. This shift in retinal correspondence, however, can affect the visual directions of dichoptic nonius lines in the following way [1, 30, 31, 35, 38, 42]. When nonius lines are more distant from a local central fusion stimulus by about 4 deg, the subjective and objective fixation disparity are similar in amount. However, the closer the nonius lines are located near the fusion stimulus, the more the visual direction of the fusion stimulus is transferred to the nonius lines and the smaller is the subjective fixation disparity. Thus, the spatial arrangement of binocular and monocular objects in the test are important, but individuals differ in the effect of the gap between nonius lines and fusion object [43]. Some studies reported that objective fixation disparity can be 10 times larger than subjective fixation disparity, other studies found similar amounts (summarized in [33]). Given these physiological differences, both types of fixation disparity are only weakly correlated [33]. Still they have some common properties: they both are correlated with the resting vergence position, measured as heterophoria or dark vergence [20, 21, 33, 44]; they both increase as the viewing distance shortens [13, 44].

Bottom Line: This study investigates effects of wearing prisms constantly for about 5 weeks in daily life.Repeated measurements were made without the prisms and with the prisms after about 5 weeks of wearing these prisms.This response pattern was related to the vergence adaptability, i.e. the individual fusional vergence reserves.

View Article: PubMed Central - PubMed

Affiliation: University of Applied Sciences and Arts Northwestern Switzerland, Olten, Switzerland.

ABSTRACT
In optometry of binocular vision, the question may arise whether prisms should be included in eyeglasses to compensate an oculomotor and/or sensory imbalance between the two eyes. The corresponding measures of objective and subjective fixation disparity may be reduced by the prisms, or the adaptability of the binocular vergence system may diminish effects of the prisms over time. This study investigates effects of wearing prisms constantly for about 5 weeks in daily life. Two groups of 12 participants received eyeglasses with prisms having either a base-in direction or a base-out direction with an amount up to 8 prism diopters. Prisms were prescribed based on clinical fixation disparity test plates at 6 m. Two dependent variables were used: (1) subjective fixation disparity was indicated by a perceived offset of dichoptic nonius lines that were superimposed on the fusion stimuli and (2) objective fixation disparity was measured with a video based eye tracker relative to monocular calibration. Stimuli were presented at 6 m and included either central or more peripheral fusion stimuli. Repeated measurements were made without the prisms and with the prisms after about 5 weeks of wearing these prisms. Objective and subjective fixation disparity were correlated, but the type of fusion stimulus and the direction of the required prism may play a role. The prisms did not reduce the fixation disparity to zero, but induced significant changes in fixation disparity with large effect sizes. Participants receiving base-out prisms showed hypothesized effects, which were concurrent in both types of fixation disparity. In participants receiving base-in prisms, the individual effects of subjective and objective effects were negatively correlated: the larger the subjective (sensory) effect, the smaller the objective (motor) effect. This response pattern was related to the vergence adaptability, i.e. the individual fusional vergence reserves.

No MeSH data available.


Related in: MedlinePlus