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PubMed2005

Phoria adaptation after sustained symmetrical convergence: Influence of saccades.

We recorded divergence eye movements after short (4 s) and long (36 s) periods of sustained symmetrical convergence (30 degrees) in nine normal human subjects using the search coil technique. Following the long period of convergence, alignment after the initial 1,250 ms of divergence was more converged than after the short period of convergence, showing short-term "phoria adaptation". The first 1,000 ms of divergence, however, could be slower, faster or relatively unchanged, depending upon the subject. A change in the timing and/or amplitude of associated saccades (which accelerate ongoing vergence) between the long and short stimuli accounted for much of the difference in the rate of divergence. The differences in saccade pattern during early divergence following the long and short periods of convergence may reflect changes in attentional focus (to near or to far).

Adaptation, Physiological↗
PubMed1991

Phoria, Hering's laws, and monocular perception of direction.

By recording eye movements while subjects fixated a stationary stimulus and reported on its apparent position, four experiments tested whether Hering's laws of visual direction describe monocular perception of egocentric direction. Experiments 1, 2, and 3 alternately covered each eye. Experiments 1 and 2 showed that the stimulus appeared to displace leftward when the left eye was occluded, and vice versa. The apparent displacement was correlated with lateral heterophoria in the occluded eye, and phoria was necessary for the illusory displacement. As a descriptive model, these results expand the range covered by Hering's laws, but they leave open several process models, including ones based on retinal image displacements without reference to extraretinal information. Experiment 4 eliminated retinal image displacements and found apparent target displacements that imply processing of extraretinal information.

Adult↗
PubMed1977

Phoria, vergence, and fixation disparity in oculomotor problems.

Measures of horizontal phoria, vergence, and fixation disparity were obtained for 2 samples: 28 optometry students without symptoms and 32 orthoptic patients with symptoms associated with binocular oculomotor difficulties. Discriminant analysis was used to determine which tests or group of tests best discriminated between the 2 samples. Analysis was done for both samples and for exophoric and esophoric subsamples. Sheard's criterion was the best discriminator for the exophoric group, and amount of heterophoria was the best discriminator for the esophoric group. Fixation disparity was the next best discriminator for both groups.

Accommodation, Ocular↗
PubMed1981

The range of zero-associated phoria in an asymptomatic clinical population.

The clinical concept of the range of zero-associated phoria is developed, and normative data for this range are presented based on a clinical study of 201 asymptomatic patients. The relation of patient age, sex, and amount of near lens addition to this range is examined. The range at far is not related to patient age or sex. The range at near appears to bae independent of patient age, sex, and the amount of near addition required by presbyopes.

Adolescent↗
PubMed1983

Common clinical configuration of zone of zero-associated phoria.

The four common configurations of the zone of zero-associated phoria are presented based on the results of a clinical study of 156 asymptomatic patients. These configurations and the relative frequencies, constitute normative data against which we can compare similar zones and frequencies in patient populations having certain binocular defects.

Accommodation, Ocular↗
PubMed1983

Zone of zero-associated phoria in patients with convergence insufficiency.

The zone of zero-associated phoria (ZZAP) was determined for patients with convergence insufficiency. The ZZAP for these patients differs from that of asymptomatic patients. It is concluded that the ZZAP may be used as a supplemental test to help the clinician detect convergence insufficiency and possibly other binocular deficiencies.

Accommodation, Ocular↗
PubMed2000

Effect of sustained cyclovergence on eye alignment: rapid torsional phoria adaptation.

PURPOSE: To describe adaptive changes in torsional alignment that follow sustained cyclovergence in healthy humans. METHODS: Eye movements were recorded binocularly from four healthy subjects using dual-coil scleral annuli. Cyclovergence movements were evoked over periods of 30 to 150 seconds using a stereoscopic display, presenting gratings of lines arranged horizontally, vertically, or at 45 degrees, subtending angles of up to 48 degrees. In- and excyclodisparities of 5 degrees were introduced and removed in a single-step fashion. After stimulation, the time course and magnitude of the decay in cyclovergence was compared with the subject either in darkness or viewing a baseline stimulus of zero cyclodisparity. RESULTS: As reported previously, the cyclovergence response to incyclodisparities was greater than to excyclodisparities. After sustained excyclovergence, however, in all subjects and in response to all orientations of the gratings, the decay in darkness was incomplete, implying an adaptive change in torsional alignment. In response to the horizontal gratings, for incyclovergence there was also an incomplete decay in darkness but to a lesser degree than in response to excyclovergence, and in only three of four subjects. The incyclovergence evoked by the oblique and vertical gratings was of small magnitude, and its decay was unaffected by the presence or absence of a visual stimulus. CONCLUSIONS: After sustained cyclovergence, its decay in the absence of a visual stimulus may be incomplete. The residual component may be interpreted, by analogy with horizontal and vertical vergence, as reflecting so-called phoria adaptation for torsional alignment.

Adaptation, Ocular↗
PubMed2002

Phoria variation secondary to cover test technique at near.

BACKGROUND: This study examined variations in near-point phoria measurements as a result of cover test technique. Two observing positions by the clinician were assessed: (1) a midline position, with the clinician directly behind the near-point accommodative target and 12) an off-midline position, so as to not be within the patient's central field of vision. METHODS: One hundred thirteen subjects (55 men and 58 women) were classified using the off-midline cover test technique. A second set of measurements was taken with the examiner on the midline. All other testing aspects were held constant. RESULTS: No significant differences were observed between techniques for patients who were orthophoric or esophoric. Significant differences (P < 0.00001) were observed in subjects who were exophoric. The midline positioning of the examiner was associated with a decrease in the measured exophoria by an average of more than four prism diopters. CONCLUSIONS: While either measuring position by the clinician is comparable for patients with esophoria or orthophoria, the findings suggest that larger amounts of heterophoria will be measured if the cover test at near is performed from an off-midline position when patients with exophoria are examined.

Adult↗
PubMed1998

[Effects of presbyopia on clinical phoria].

BACKGROUND: Due to accommodation-vergence cross-link we can expect that presbyopia will affect vergence as well. From investigations of cross-link as function of age one may assess strain of visual system due to presbyopia. Furthermore, this observation will give hints on innervation of ciliary muscle. MATERIAL AND METHOD: Vergence is assessed as function of stimulus of accommodation in 27 subjects (24-65 years). RESULTS: Presbyops tend to be exophoric for near vision. Vergence is linked to accommodation by means of a quadratic polynome rather than by a linear function. CONCLUSIONS: We may expect, that missing vergence strains visual system of presbyops. Results support Hess-Gullstrand theory of presbyopia and indicate a loss of innervation of ciliary muscle with increasing age.

Accommodation, Ocular↗