Amblyopia is a developmental spatiotemporal visual impairment caused by early abnormal vision. It is frequently associated with early childhood strabismus (ocular misalignment), anisometropia (unequal refractive error), or form deprivation. Amblyopia cannot be optically corrected immediately and it is not caused by an obvious change or defect in the eyes. Strabismic and anisometropic amblyopia are produced by a disruption of binocular input during the critical period in the development of binocularity.
1,2 A large body of research
3–10 has been dedicated to the sensory characteristics of patients with amblyopia, which include deficits in visual acuity, contrast sensitivity, form and motion perception, spatial and temporal crowding, and stereopsis. Deficits in saccadic eye movements and visuomotor behavior have recently been investigated.
11–14
During normal fixation, the eyes exhibit a series of involuntary movements ranging in amplitude from high-frequency tremors and microsaccades to slow drifts, the combination of which determines the precision or stability of the eyes.
15,16 A fourth kind of oscillatory eye movement of low amplitude (< 0.02°) and lower frequency (0.04–0.1 Hz) than any other movement has been recently discovered,
17 but requires long fixation trials to be detected. In patients with amblyopia, the stability of the eyes during attempted steady fixation has been shown to differ depending on the viewing eye, instructions, and viewing conditions. Ciuffreda and associates
18 have examined the fixation stability of patients with and without strabismus and/or amblyopia but without specific instructions to hold the gaze steadily. For the amblyopic eye, they found an increase in saccadic intrusions, which are associated with strabismus but not with amblyopia.
19 They also found drifts accounting for 75% of the total fixation time in amblyopia without strabismus, 50% of the total fixation time in constant strabismus amblyopia, and only 20% of the total fixation time in intermittent strabismus. From these data, they conclude that amblyopia rather than strabismus is the necessary condition producing an increase in drifts as people attempt to fixate.
20 In all three studies, references to normal viewing involves the fellow (nonamblyopic) eye, binocular viewing, or previous research findings with people with normal binocular vision, but no control group data were obtained under the same testing conditions.
During fixation of a stationary target, slips of retinal images stimulate the brain to generate eye movements that counter the slips in order to hold the gaze steady. This response to retinal image drifts caused by gaze instability during active fixation has been referred to as slow-control, or field-holding reflex.
21,22 Sporadic saccades away from fixation and their corrective counterparts (square-wave jerks) are also known to happen in pathological conditions and, in smaller numbers, in normal observers.
23 In this study, we use the term open loop as it is used in control theory where it refers to the removal of the visual feedback loop.
24 In this study, we used a quantitative measure of fixation stability in patients with amblyopia and in people with normal binocular vision, tested under binocular and monocular viewing conditions and under instructions to hold the gaze steadily. We hypothesized that the sensory impairments associated with amblyopia should be reflected in the patients' fixation control during binocular and monocular viewing with the fellow and amblyopic eye. The present study also differed from the previous literature in that we recorded the movements of the two eyes concurrently, even during monocular viewing; in other words, we recorded the covered eye's position behind the occluder in an open-loop condition. This method allowed us to obtain measures of the magnitude of binocular summation and offered some insight into the mechanisms that control eye position in the absence of corrective visual feedback. We use the term open loop as it is used in control theory where it refers to the removal of the visual feedback loop.
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