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Acuity, Crowding, Feature Detection, and Fixation in Normal and Amblyopic Vision


Both functional and physical properties related to vision are known to be abnormal in amblyopia. The former include spatial and temporal visual deficits such as reduced letter acuity and temporal sensitivity, crowding, and decreased position acuity; the latter refers to eccentric and unstable fixational eye movements in strabismic amblyopia. This dissertation represents parallel studies that aim to characterize amblyopic vision from these angles using different approaches.

In the first series of experiments, we systematically explored the size and spacing requirements for identifying a letter among other letters by measuring flanked and unflanked letter acuities in normals and amblyopes, centrally and peripherally, while fixing the medium contrast and brief presentation duration. At fixation, for normals and non-strabismic amblyopes, no crowding has been found: legibility of a flanked letter is limited by overlap masking or acuity depending on the letter spacing. In normal peripheral vision and strabismic amblyopia, legibility is limited by crowding unless the letter spacing is very loose. We show that strabismic and non-strabismic amblyopia are much like normal vision at increased eccentricity or with added blur, respectively, as characterized by the crowding-acuity ratio. For clinical screening tests for strabismic and non-strabismic amblyopia, we recommend measuring both unflanked letter acuity and flanked letter acuity with tight spacing.

In the second series of experiments, we explored the effects of various factors on both flanked and unflanked visual acuities in normal observers. Specifically, we varied polarity, contrast, and duration of the letter stimuli, as well as the pupil size of the observer by adding a pinhole. We find that the critical spacing for crowding in the fovea is 0.09 ~ 0.1 deg. The main effects of and interactions between the above factors provide the context and supplement to the results from the above experiments, based on which we extend the letter legibility model developed from the above experiments by parameterization of each legibility limit to account for a wide range of conditions.

In the third series of experiments, the first experiment estimated the spatiotemporal mechanism for detecting a luminance increment of a bright bar embedded in spatiotemporal noise for normal and amblyopic observers. The normal template is characterized by a temporal summation zone surrounded by symmetric spatial inhibition zones and followed by a temporal inhibition zone. The abnormal amblyopic template lacks inhibition but with normal temporal summation. Neither blurring the stimuli in space and time nor varying the signal-to-noise ratio caused any significant change in the normal or amblyopic template. However, decreasing the fundamental frequency of the stimuli restored the normal template in the amblyopic eye. Furthermore, the normal periphery shares spatial properties with amblyopia but the temporal properties may be different. The second experiment mapped the dynamic of spatiotemporal interactions of crowded stimulus orientations in normal and amblyopic eyes. For normal eyes, crowding occurs at locations adjacent to the target in space and around or before the target presentation in time. Different types of cues may affect the spatiotemporal interaction map differently, but the strongest crowding never coincides with the target presentation. The amblyopic spatiotemporal interaction map is not substantially different from that of the normal eyes, except that in general the spatiotemporal interaction is more widely distributed around and after the target presentation. "Anti-crowding" - repulsion of perceived orientation that is induced by farther flankers is documented in both normal and amblyopic eyes.

In the fourth series of experiments, an adaptive optics scanning laser ophthalmoscope (AOSLO) was used to record fixational eye movements, allowing direct viewing of retinal movements and fixation locations. Fixational eye movements were modeled as a stochastic process governed by a potential function assumed to have the form of a quadratic polynomial for both normal and strabismic amblyopic observers. Our results confirm that microsaccades, on average, correct for fixation inaccuracy. Microsaccades occur more frequently and tend to move faster towards the target at relatively large displacements. Drift is modeled as a Brownian motion with constant rate over time plus an error-correcting component initially following a microsaccade. The unstable eccentric fixational eye movements in strabismic amblyopia are characterized by frequent intrusive saccades with large amplitudes and high speeds. The strabismic amblyopic fixation pattern on multiple loci is task dependent, with less eccentric but unstable fixation induced by challenging tasks, and relatively stable fixation by small highly visible static targets.

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