Accommodative Disfacility Presenting as Intermittent Exotropia

244

187

When a young observer attempts to accommodate steadily on a fixed stimulus, the norminally steady‐state response shows small instabilities or fluctuations (sometimes termed microfluctuations or oscillations). These fluctuations typically have an amplitude of a few tenths of a dioptre and a frequency spectrum extending up to a few Hertz. The properties of these fluctuations are described for various viewing conditions: pupil diameter, target vergence, target form, target contrast, and target luminance all influence the frequency spectra of the oscillations, as may anomalies of vision such as amblyopia. The possible roles that the fluctuations might play in the function of the accommodative system are discussed. It is suggested that the higher frequency components around 2 Hz may arise from the mechanical and elastic characteristics of the lens, zonule and ciliary body. Components at lower frequencies (<0.5 Hz) may be of more significance in the function of the accommodative control system.

Section: Accommodation Fluctuation In Arnblyopesmentioning

confidence: 52%

Fluctuations in accommodation: a review

Charman

Heron

1988

244

187

“…Changes in position of the image of the target on the retina vary due to small rapid continuous fluctuations ( * 0.10 D; 2 Hz) in crystalline lens shape/ power even before the target begins to move (Campbell and Westheimer, 1960;Arnulf, Santamaria and Bescos, 1981). Also, small fixational eye movements may be used to help detect, interpret and process the blur (Finchman, 1951;Crane, 1966;Stark et al, 1984). Inadvertent reduction or suppression of microsaccades during intent fixation on the target may delay the accommodative response to blur (Fincham, 1951).…”

Section: Discussionmentioning

confidence: 99%

“…modelling of the accommodation system with continuous feedback control alone. For example, Stark, Takahashi and Zames (1965) required an unrealistic 100 ms latency in their non-linear model to maintain stability in their simulations. Although O'Neill et ul.…”

mentioning

confidence: 99%

Dual‐mode behaviour in the human accommodation system

Hung

Ciuffreda

1988

Self Cite

The human accommodation system was investigated using a dynamic infra-red optometer to determine whether its feedback control process exhibited dual-mode behaviour. Ramp changes of target accommodative demand presented monocularly at various velocities (0.5-0.6 D s-1) with a fixed 2 D amplitude elicited two modes of behaviour. Smooth ramp movements were observed for low-velocity stimuli, whereas step responses predominated for high-velocity stimuli. At intermediate velocities, combined step-ramp responses were more prevalent. The characteristic of these step-ramp responses showed two important properties that suggest a pre-programming mechanism. The amplitude of the step portion of a step-ramp response approximated the value of the ramp stimulus at the time corresponding to the end of the step movement. Also, the initial step amplitude increased with increasing target ramp velocity. It appears that the amplitude of the preprogrammed accommodative movement was based on an estimate of the expected ramp target position at the end of the response. These findings are consistent with a recent unifying theory of oculomotor control in which the various oculomotor subsystems (versional and vergence eye movements and accommodation) each has fast and slow components. The fast component exhibits preprogramming, while the slow component shows continuous feedback control.

“…40 and 340 ? 115 msec, respectively, which represent normal values (Stark et al, 1984). In addition, some vergence overshooting was evident.…”

Section: S N E L L E N S N E L L E N ( W H O L E C H a R T ) ( W H O mentioning

confidence: 74%

Different Rates and Amounts of Vision Function Recovery During Orthoptic Therapy in an Older Strabismic Amblyope

Hokoda

Ciuffreda

1986

Self Cite

Orthoptic therapy was instituted in an 11-year-old patient having deep amblyopia, constant small-angle esotropia with anomalous retinal correspondence, and a past history of minimal success with such therapy. This combination of factors pointed toward a poor prognosis for substantial recovery of vision function. Rate of recovery of several monocular and binocular vision functions was monitored during the course of 18 months of intensive orthoptic therapy. Results showed marked improvement in several monocular vision functions, suggesting presence of considerable residual neural plasticity of multiple sites in the visual pathways of this older amblyope.