The effect of perceived length on visuomotor localization.

PURPOSE: The purpose of this experiment was to study visuomotor localization in the presence of either a horizontal array of equally spaced dots or a thin horizontal line. METHODS: Pointing behavior was used to assess directional localization. In experiment 1, subjects were made myopic using a contact lens and then corrected with a spectacle lens. Subjects were tested in the presence and absence of a regularly spaced, horizontal array of dots with and without the contact lens/spectacle combination. In experiment 2, subjects wore the contact lens/spectacle in all cases. Some subjects were tested in the presence and then in the absence of a regularly spaced, horizontal array of dots while the order of conditions was reversed for other subjects. In experiments 3 and 4, subjects were tested without the contact lens/spectacle combination. In experiment 3, subjects were tested in the absence and then in the presence of a regularly spaced, horizontal arrays of dots. In experiment 4, subjects were tested in the absence and then in the presence of a thin horizontal line. RESULTS: In experiment 1, in the absence of the array of dots, subjects undershot targets with the contact lens/spectacle combination. When the array was present, pointing with the contact lens/spectacle combination was accurate. In experiment 2, subjects undershot targets in the absence of the array of dots if this condition was performed first. If the array was present in the initial condition, the pointing undershoot in the second condition (array absent) was reduced. In all cases, the pointing undershoot was reduced in the presence of the array. In experiments 3 and 4, a pointing overshoot was found in the presence of an array of dots or a thin line. CONCLUSIONS: It is concluded that extraretinal eye position information is not the primary determinant of visuomotor localization in the presence of a horizontal contour. The overshoot produced by a horizontal contour may be related to a length illusion brought about by spatial filtering in the visual system or inaccurate distance judgments.

On determining the relationship between blood alcohol concentration and smooth pursuit eye movements.

Residual impairment after alcohol consumption implies that the relationship between blood alcohol concentration (BAC) and deficits in performance of some task is not the same when becoming intoxicated as it is when returning to sobriety. A pilot study was performed to determine the feasibility of and the appropriate methodology for studies on residual impairment of smooth pursuit performance. Four subjects consumed alcohol for 2 h. Measurements of BAC and smooth pursuit eye movements were made every 30 min during drinking and for 4 h after drinking. Pursuits were elicited by having subjects track a sinusoidal target (0.40 Hz and 0.60 Hz) for 10 s. Impairment of smooth pursuit was quantified with frequency analysis scores. Frequency analysis scores declined as BAC increased. As BAC decreased, frequency analysis scores tended to increase toward pre-drink levels. The relationship between BAC and frequency analysis score was not significantly different on the ascending and descending limbs of the blood alcohol curve. However, the idea that residual impairment does not occur could not be conclusively demonstrated for several reasons. First, as BAC returned to pre-drink levels, frequency analysis scores were inconsistent for each subject perhaps because of boredom and fatigue. Second, the relationship between BAC and frequency analysis score varied between subjects. Finally, because the recording periods were short, the effect of alcohol on sustained attention could not be assessed. In this paper, potential artifacts in studies of residual impairment of pursuits are discussed and potential solutions to the methodological problems encountered in the pilot study are provided.

The effect of saccades and brief fusional stimuli on phoria adaptation.

PURPOSE: The purpose of this experiment was to determine whether brief fusional stimuli and saccades similar to those seen in the alternate cover test affect phoria disadaptation. METHODS: Three cover test conditions were performed randomly. Before each test condition, subjects fused for 2 min at an angle convergent to the subjective phoria. In one test condition, subjects viewed monocularly. In another condition, subjects alternately fixated with each eye (no binocular time). In a third condition, subjects alternately fixated with each eye, and there was a 100-ms period of binocular viewing between alternations. The ocular vergence angle was monitored using scleral search coils. RESULTS: Vergence angle was plotted against time for each condition. The area under this plot was determined using a computer program. The area reflected the rate at which ocular vergence returned to the original phoria position. The mean area for the monocular condition was 300.9, the mean area for alternate fixation with no binocular time was 300.3, and the mean area for alternate fixation with binocular time was 205.2. CONCLUSIONS: Saccadic alternations do not affect phoria adaptation. However, short periods in which binocular disparate images are viewed significantly increase the rate at which phoria adaptation declines for some subjects.

The thickness of the human precorneal tear film: evidence from reflection spectra.

PURPOSE: Interferometric methods have considerable potential for studying the thickness of layers of the human tear film and cornea because of their ability to make noninvasive, accurate, and rapid measurements. However, previous interferometric studies by Prydal and Danjo yielded tear thickness values near 40 and 11 microm, respectively, considerably greater than estimates made by invasive methods of 4 to 8 microm. Using a modified version of Danjo's method, interference effects from the tear film and cornea were studied, with the aim of correlation with known structure and optical properties of the cornea and hence determining the most probable value of tear film thickness. METHODS: Reflectance spectra from the human cornea were measured at normal incidence. These spectra show oscillations whose maxima correspond to constructive interference between light reflected from the air surface and from some deeper surface. The frequency of these spectral oscillations is proportional to the thickness of the layer between the air surface and the second surface. Therefore, Fourier analysis of reflectance spectra can be used to determine the thickness of layers of the tear film and cornea. In the main experiment, 36 low-resolution spectra were obtained from six normal eyes for measuring thickness up to 100 microm. Control experiments included measurements of the time course of thickness changes and high-resolution spectra for measuring thickness up to 1000 microm. RESULTS: For the main experiment, in the thickness range 1 to 100 microm, the strongest peak in the Fourier transform was near 3 microm (range, 1.5-4.7 microm) beneath the air surface. In the range 20 to 100 microm, the strongest peak was near 55 microm (range, 50-59 microm) for all 36 spectra; none were in Prydal's range near 40 microm. This 55-microm peak is consistent with a reflection from the basement membrane of the epithelium. Time course measurements after a blink show that the 3-microm peak is not an artifact. High-resolution spectra gave a peak near 510 microm, corresponding to the complete thickness of the cornea (plus tear film). This peak had a contrast similar to that of the 3-microm peak. CONCLUSIONS: These studies did not confirm Prydal's estimate of approximately 40 microm. Nor were there prominent peaks near Danjo's value of approximately 11 microm, except in cases of probable reflex tears. Because the reflection at the aqueous-mucus boundary would be expected to be weaker than that from the epithelial surface, the 3-microm peak is unlikely to correspond to the aqueous layer (rather than the complete tear film). The proposal that the 3-microm peak corresponds to a reflection from the front of the cornea is supported by the demonstration of a peak of similar contrast from the back of the cornea. Thus, the current evidence consistently supports a value of approximately 3 microm for the thickness of the human precorneal tear film.

The negative directional aftereffect associated with adaptation to the prismatic effects of spectacle lenses.

PURPOSE: To determine whether subjects would demonstrate a negative directional aftereffect in a sparse visual environment after being trained to point accurately through a spectacle lens. METHODS: Subjects were made myopic using a contact lens and then the myopia was corrected with a spectacle lens. Pointing behavior was used to assess directional localization. Training was carried out by showing subjects their pointing errors. RESULTS: Seven of 10 subjects demonstrated a negative directional aftereffect after spectacle lens adaptation. CONCLUSIONS: The presence of a negative directional aftereffect indicates that some patients who switch between spectacles and contact lenses can accurately localize objects only in one condition. Patients who do not demonstrate a negative directional aftereffect may be able to correctly localize objects with both spectacles and contact lenses.

The effect of experience on the detection of small eye movements.

PURPOSE: The purpose of this study was to determine whether experience improves detection of small eye movements similar to those seen in the cover test during ideal conditions. METHODS: Three groups of examiners with varying amounts of experience in eye movement detection tasks were studied. Eight examiners were 1st-year optometry students, six were 4th-year optometry students, and six were practicing optometrists. To determine thresholds for eye movement detection, examiners indicated the horizontal direction in which the eyes of one of the experimenters moved. The direction and extent of each eye movement was randomly selected. The extent of the eye movement was one of eight logarithmic steps from a maximum that was determined through an initial session where the approximate sensitivity of the examiners was found (generally +/- 1.75 prism diopters). RESULTS: Detection thresholds were determined by fitting the data with a cumulative Gaussian function. The mean horizontal eye movement eye movement necessary to obtain 99% correct judgments was 2.65 prism diopters for the 1st-year students, 2.47 prism diopters for the 4th-year students, and 2.40 prism diopters for the practicing optometrists. CONCLUSIONS: The results indicate that during ideal conditions, little or no training is required for efficient detection of small eye movements.

The extent to which compensation for the prismatic effects of spectacle lenses is maintained.

PURPOSE: The ability of subjects to compensate for the visual displacement caused by the prismatic effects of spectacle lenses was studied over 4 days. METHODS: Subjects were made myopic using contact lenses and then corrected with spectacles. Pointing behavior was used to assess directional localization. Compensation for the refractive lenses was induced by showing subjects their pointing errors. RESULTS: Compensation was maintained for 4 days in 3 of 4 subjects despite 24 h of normal viewing between testing sessions. One subject demonstrated a negative directional aftereffect which has not been previously reported in studies of spectacle lens adaptation. CONCLUSIONS: Compensation for the prismatic effects of spectacle lenses can be maintained for several days in some subjects. This compensation may be functionally useful for some patients who switch between spectacles and contact lenses.

Three interferometric methods for measuring the thickness of layers of the tear film.

The thickness of different layers of the tear film has been measured by three types of interference method, namely, wavelength-dependent fringes (WDFs), thickness-dependent fringes (TDFs), and angle-dependent fringes (ADFs). This review begins with a discussion of characteristics which are common to all these methods--high-, intermediate-, and low-index layers, phase, optical path difference, and contrast. For each of the three methods, we present a figure showing constructive and destructive interference, derive equations for calculating tear layer thickness, describe a typical optical system, and show representative results. The particular advantages and limitations of each method are discussed. Given the clinical importance of the tear film in dry eye syndrome and contact lens wear, it is unfortunate that there are considerable discrepancies among the results of interferometric and other methods for measuring tear film thickness; further development of these noninvasive, interferometric methods should help to provide a clearer picture of the thickness of different layers of the tear film, in normal and dry eyes, and in contact lens wear.

The effect of slant on visuomotor localization.

Visuomotor localization in the presence of slant was measured. Five subjects pointed at the center LED of an array of five LEDs viewed monocularly. Pointing was open-loop. The LEDs were separated by 1.5, 3.0, or 4.5 cm. The array was rotated about a vertical axis coincident with the center LED to slant the array from -50 degrees to +50 degrees. LED separation had no effect. A small linear relationship was found between errors in localization and slant (slope = -0.02). The errors were as expected if the perceived straight ahead was in the direction of the normal to the surface, but these errors were so small as to be functionally insignificant. It is concluded that extraretinal eye position information dominates over slant for visuomotor localization.

The effect of forced vergence on retinal correspondence.

The spatial characteristics of the changes in retinal correspondence produced by forced convergence were studied. The vertical extent of lateral shifts in binocular correspondence were quantified by comparing the convergence of the eyes measured with binocular search coils to the convergence of the eyes as determined using nonius lines having vertical separations (gaps) between the nonius lines of 0.5-4.8 degrees. Lateral shifts in binocular correspondence only occurred for nonius gaps < 3-4 degrees. The effects of horizontal retinal eccentricity on lateral shifts in correspondence were determined by measuring the nonius horopter of the subject under forced convergence, using 11 nonius line eccentricities between 4.5 degrees left and right. The nonius horopter was shifted toward the fusion target maximally near the fixation point. There was no shift beyond 3 degrees of eccentricity. We conclude that the nonius horopter is 'dimpled' vertically and horizontally, facilitating local fusion by shifting the line horopter and the region of single binocular vision toward the point of regard over a region of 3 degrees around the fixation point.

Comparison of fixation disparities obtained by objective and subjective methods.

Fixation disparities (FD) were measured as a function of forced vergence using binocular scleral search coils and simultaneously with nonius lines. The slope of the objective FD curve was significantly greater than the subjective FD curve for three of five subjects. This indicates an alteration in retinal correspondence of up to one degree, that shifts Panum's area to avoid the diplopia normally present with large disparities. This process allows for fusion in the presence of large objective fixation disparities which would normally cause diplopia. The shift in correspondence enhances the range of forced vergence, since the larger objective FDs serve as more effective stimuli to fusional vergence. The remaining subjects who lacked this effect had "flat" FD curves indicative of high vergence adaptation.

Interferometric measurement of tear film thickness by use of spectral oscillations.

A method of measuring the tear film thickness is described in which interference causes oscillations in the reflectance spectrum from the tears. Strong oscillations were usually observed when a contact lens was worn. Measurement of modulation and phase of these oscillations confirmed that they were associated with the tear layer in front of the contact lens. Calculated thickness of this layer averaged 2.7 microns. In one out of five subjects, weak oscillations were sometimes observed without a contact lens. These oscillations probably arose from the aqueous layer of the tears with a thickness of approximately 3 microns. The relative merits of three interference methods of measuring the tear film are discussed.

Comparison of the monocular occlusion and a direct method for objective measurement of fixation disparity.

PURPOSE: Previous studies have measured objective fixation disparity using a monocular occlusion method, by which the monocular components of misalignment are revealed by measuring the shift in the position of the fixating eye after occlusion of the other eye. METHODS: We assessed the use of the monocular occlusion method by comparing it to direct measurements of binocular vergence. RESULTS: The direct method required precise calibration, but was found to be reliable. Several problems were discovered with the monocular occlusion method: (1) when using a central fusion stimulus, fixation was difficult to control accurately enough to measure the small monocular shifts in eye position expected at lower vergence demands; (2) with a peripheral fusion target, objective fixation disparity could not be measured directly; and (3) upon occlusion, a variable saccade occurred in the fixating eye. CONCLUSION: Objective measurement of fixation disparity with the direct method is quicker and more reliable than with the monocular occlusion method.

The effect of refractive lenses on perceived direction.

The accuracy of directional localization with refractive lenses was determined by open-loop pointing for four myopic habitual contact lens wearers. The target was a single point of light at random locations (dark), or was embedded in a horizontal array of lights (light). In the dark, the regression slope of indicated vs actual target position was significantly less for spectacles than for contact lenses, as predicted from the prismatic effects of the spectacles. In the light, slopes for spectacles and contacts were not significantly different. These results suggest that spectacle wearers do not utilize eye position information in making direction judgments in the light.

The treatment of superior oblique myokymia utilizing the Harada-Ito procedure.

A woman with superior oblique myokymia (SOM) was cured of her condition by performing a Harada-Ito procedure. This procedure involves transposing the anterior portion of the superior oblique tendon, which is responsible for cyclorotation, nasally to create an effective weakening of the anterior portion of the tendon instead of temporal displacement utilized for superior oblique paresis. We measured the patient's eye movements before and after surgery, using the magnetic search coil technique, and confirmed that (1) the SOM was abolished and (2) vertical eye movements, including saccades, were unaffected.

The syndrome of 'pure akinesia' and its relationship to progressive supranuclear palsy.

Five patients developed a stereotyped syndrome of progressive akinesia of gait, speech, and hand-writing without rigidity, tremor, or dementia. The symptoms did not improve with levodopa. These clinical findings conform to a syndrome described repeatedly in Japan since 1974 as "pure akinesia." Evidence has indicated that pure akinesia often represents a pre-ocular motor, and in some cases an ocular motor-sparing, form of progressive supranuclear palsy (PSP). Although we found disorders of eyelid movements, none of the patients demonstrated a gaze palsy on clinical examination. Four patients underwent eye movement recordings. The two patients with the longest disease duration had slow or small vertical saccades. These findings support the notion that patients with pure akinesia may develop a vertical gaze palsy, similar to that in PSP, late in their course. Our patients show that pure akinesia occurs in North America as well as in Japan. Recognition of the syndrome of pure akinesia may suggest the diagnosis of PSP before the development of abnormalities of ocular movement.