53 research outputs found
Saccadic latency in amblyopia.
We measured saccadic latencies in a large sample (total n = 459) of individuals with amblyopia or risk factors for amblyopia, e.g., strabismus or anisometropia, and normal control subjects. We presented an easily visible target randomly to the left or right, 3.5° from fixation. The interocular difference in saccadic latency is highly correlated with the interocular difference in LogMAR (Snellen) acuity-as the acuity difference increases, so does the latency difference. Strabismic and strabismic-anisometropic amblyopes have, on average, a larger difference between their eyes in LogMAR acuity than anisometropic amblyopes and thus their interocular latency difference is, on average, significantly larger than anisometropic amblyopes. Despite its relation to LogMAR acuity, the longer latency in strabismic amblyopes cannot be attributed either to poor resolution or to reduced contrast sensitivity, because their interocular differences in grating acuity and in contrast sensitivity are roughly the same as for anisometropic amblyopes. The correlation between LogMAR acuity and saccadic latency arises because of the confluence of two separable effects in the strabismic amblyopic eye-poor letter recognition impairs LogMAR acuity while an intrinsic sluggishness delays reaction time. We speculate that the frequent microsaccades and the accompanying attentional shifts, made while strabismic amblyopes struggle to maintain fixation with their amblyopic eyes, result in all types of reactions being irreducibly delayed
Unconstrained stereoscopic matching of lines
The computation of horizontal binocular disparities used in stereoscopic depth perception depends
upon the identification of corresponding features in the two retinal images. In principle, binocular
matching is a two-dimensional problem that considers matches in all possible meridians. Normally,
constraints such as end points or crossing points limit the direction and magnitude of matches. If
matching is unconstrained, such as is the case with long lines, it is completely ambiguous. Under these
conditions the default match will be determined by the operating range, or upper disparity limit, of
matchable vertical and horizontal disparities. We computed the operating range of vertical matches for
stereoscopic depth as a function of line orientation. Our results suggest that the two-dimensional
operating range is anisotropic for vertical and horizontal disparity and that unconstrained matches are
not based upon either epipolar geometry or nearest neighbor constraints, but rather the mean of
disparity estimates within the operating range for binocular matches. This operating range can be
extended vertically when matches are constrained by image primitives
Initial destination of the disaccommodation step response
AbstractPeak velocity and peak acceleration of disaccommodation step responses remain invariant of response magnitude for a constant starting position and they increase linearly with proximity of starting position. This suggests that disaccommodation response is initiated towards an initial (default) destination and is switched mid-flight to attain the desired final destination. The dioptric location of initial destination was estimated from the x-intercept of regression of peak velocity on response starting position. The x-intercept correlated well with subject’s cycloplegic refractive state and poorly with their dark focus of accommodation. Altering the dark focus by inducing fatigue in the accommodative system did not alter the x-intercept. These observations suggest that cycloplegic refractive state is a good behavioral correlate of initial destination of disaccommodation step responses
Context-specific adaptation of vertical vergence to correlates of eye position
AbstractVertical phoria (vertical vergence in the absence of binocular feedback) can be trained to vary with non-visual cues such as vertical conjugate eye position, horizontal conjugate eye position and horizontal vergence. These prior studies demonstrated a low-level association or coupling between vertical vergence and several oculomotor cues. As a test of the potential independence of multiple eye-position cues for vertical vergence, context-specific adaptation experiments were conducted in three orthogonal adapting planes (midsagittal, frontoparallel, and transverse). Four vertical disparities in each of these planes were associated with various combinations of two specific components of eye position. Vertical disparities in the midsagittal plane were associated with horizontal vergence and vertical conjugate eye position; vertical disparities in the frontoparallel plane were associated with horizontal and vertical conjugate eye position; and vertical disparities in the transverse plane were associated with horizontal vergence and horizontal conjugate eye position. The results demonstrate that vertical vergence can be adapted to respond to specific combinations of two different sources of eye-position information. The results are modeled with an association matrix whose inputs are two classes of eye position and whose weighted output is vertical vergence
Human stereo matching is not restricted to epipolar lines
AbstractComputational approaches to stereo matching have often taken advantage of a geometric constraint which states that matching elements in the left and right eye images will always fall on “epipolar lines”. The use of this epipolar constraint reduces the search space from two dimensions to one, producing a tremendous saving in the computation time required to find the matching solution. Use of this constraint requires a precise knowledge of the relative horizontal, vertical and torsional positions of the two eyes, however, and this information may be unavailable in many situations. Experiments with dynamic random element stereograms reveal that human stereopsis can detect and identify the depth of matches over a range of both vertical and horizontal disparity. Observers were able to make accurate near/far depth discriminations when vertical disparity was as large as 45 arcmin, and were able to detect the presence of correlation over a slightly larger range. Thus, human binocular matching sensitivity is not strictly constrained to epipolar lines
Short-term adaptation of accommodation, accommodative vergence and disparity vergence facility
AbstractPrevious studies have found that subjects can increase the velocity of accommodation using visual exercises such as pencil push ups, flippers, Brock strings and the like and myriad papers have shown improvement in accommodation facility (speed) and sufficiency (amplitude) using subjective tests following vision training but few have objectively measured accommodation before and after training in either normal subjects or in patients diagnosed with accommodative infacility (abnormally slow dynamics). Accommodation is driven either directly by blur or indirectly by way of neural crosslinks from the vergence system. Until now, no study has objectively measured both accommodation and accommodative–vergence before and after vision training and the role vergence might play in modifying the speed of accommodation. In the present study, accommodation and accommodative–vergence were measured with a Purkinje Eye Tracker/optometer before and after normal subjects trained in a flipper-like task in which the stimulus stepped between 0 and 2.5diopters and back for over 200 cycles. Most subjects increased their speed of accommodation as well as their speed of accommodative vergence. Accommodative vergence led the accommodation response by approximately 77ms before training and 100ms after training and the vergence lead was most prominent in subjects with high accommodation and vergence velocities and the vergence leads tended to increase in conjunction with increases in accommodation velocity. We surmise that volitional vergence may help increase accommodation velocity by way of vergence–accommodation cross links
A virtual ophthalmotrope illustrating oculomotor coordinate systems and retinal projection geometry
Eye movements are kinematically complex. Even when only the rotational component is considered, the noncommutativity of 3D rotations makes it hard to develop good intuitive understanding of the geometric properties of eye movements and their influence on monocular and binocular vision. The use of at least three major mathematical systems for describing eye positions adds to these difficulties. Traditionally, ophthalmotropes have been used to visualize oculomotor kinematics. Here, we present a virtual ophthalmotrope that is designed to illustrate Helmholtz, Fick, and rotation vector coordinates, as well as Listing's extended law (L2), which is generalized to account for torsion with free changing vergence. The virtual ophthalmotrope shows the influence of these oculomotor patterns on retinal projection geometry
Effects of partial occlusion on perceived slant difference
When two slanted surfaces are placed in proximity, the perceived slant difference between them is exaggerated. This effect has been called slant contrast. When a partial occluder is presented in front of the gap between them, the perceived slant difference between the surfaces is reduced. We refer to this reduction in perceived slant difference as stereo-slant assimilation. We investigated two properties of the occluder that might affect perceived stereo-slant difference. Three vertically aligned random-dot patterns were presented either with a partial occluder over the gaps between them or without it. Observers judged the perceived slant difference between the center pattern and two surround random-dot patterns that had the same slant. The perceived slant difference was reduced when the partial occluder was present. We varied stereodepth and slant of the occluder and found that the decreased perceived slant difference was not due to either of these. Note that the surfaces were all simulated and presented on a computer screen and the results may not apply to real surfaces. The effect of the occluder on perceived slant differences could have resulted from either a reduction of slant contrast or an increase of slant assimilation
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Does ethnicity influence the short-term adaptation to first reading correction?
PurposeEthnic variations in accommodative amplitude (AA) are not uncommon. Accommodation can become reduced in response to short-term wear of first near spectacles. Whether ethnicity has an influence on the magnitude of this adaptation is not well understood. We investigated the impact of first near spectacles on changes in AA and on convergence cross-link interactions in incipient presbyopes of Chinese and Caucasian ethnicities.MethodsForty-one subjects (22 Caucasians and 19 Chinese) aged 36 to 44 years completed the study. Accommodative stimulus response function, AA, and AC/A and CA/C ratios were measured before and after single vision reading spectacles were used for near tasks over a 2-month period and then again 2 months after discontinuing near spectacle wear.ResultsAfter wearing reading spectacles for 2 months, the accommodative stimulus response slopes and AC/A and CA/C ratios remained invariant irrespective of ethnicity. The accommodative, but not vergence, bias decreased (p < 0.05). The nearpoint of accommodation shifted distally producing an average decrease in AA of 0.52 D from baseline (p < 0.05). Recovery to near baseline values occurred after discontinuing the reading glasses for 2 months. Differences based on ethnicity were not significant. The baseline AA vs. age plots showed steeper slopes for Chinese than the Caucasian subjects in the sample.ConclusionsThe pattern of adaptation by accommodation and cross-link interactions to short-term first reading spectacles is not influenced by ethnicity
Variation of Binocular-Vertical Fusion Amplitude with Convergence
PURPOSE. The maximum binocular vertical disparity that can be fused with disparity vergence (vertical-fusion amplitude or VFA), varies with convergence angle. VFA is larger for convergence responses to near than to far viewing distances; however, the clinical norms for changes in VFA with convergence have not been established. VFA at several convergence angles was measured to obtain a quantitative description of the changes in VFA with convergence. METHODS. Fifty-six adults took part in the study. Horizontal and vertical disparity stimuli were presented on a computer monitor by using the red-green anaglyphic technique. Stimulus to convergence was altered either by changing horizontal disparity on the computer monitor (experiment I: nine horizontal disparities: 1.2-22.5 PD [⌬]) or by changing the binocular viewing distance (experiment II: five viewing distances: 25-300 cm). Convergence was held constant during an experimental session, while vertical disparity was incremented in steps of 0.05 ⌬ after a subjective report of fusion, until the subject reported diplopia. The maximum vertical disparity that could be fused was defined as the VFA. RESULTS. VFA increased linearly over the range of convergence stimuli (y Ď 0.10x Ď© 1.62) and intersubject variability of VFA increased marginally with the amount of convergence. Linear regression equations with similar slopes and y-intercepts were observed in experiments I and II. CONCLUSIONS. The results of the experiments provide a quantitative description of a linear relationship between VFA and convergence. The linear regression equation could be used in a clinical setting to establish norms and to screen for vertical vergence abnormalities. (Invest Ophthalmol Vis Sci
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