Biomedical subjects

F C Sun

Publications and source records attributed to F C Sun.

6 recordsLinked to original sources

PubMed1995

[Dual mode control of head movements during eye-head coordination].

Eye-head coordination during the shift of gaze is investigated. Dynamic trajectories of eye movements and head movements were measured for exploring the control mechanism of the head movement in eye-head coordination. The experimental results revealed the dual mode control of the head movement in eye-head coordination: a linear control for small amplitude movement (less than 30 degree), and a Bang-Bang control for larger amplitude.

Eye Movements↗

PubMed1995

[The central basis of discrete sampling behavior in the human pupillary control system].

The human pupillary control system was investigated with double-pulse light dichoptic stimulation to explore its discrete sampling behavior. In this dichoptic stimulating method, the first pulse of the double pulse light stimulated one eye, of which the pupil area was recorded, and the second pulse stimulated the other eye (the contralateral eye). The experiment data demonstrated that the pupillary control system requires a minimum time interval, approximately 0.6 s, to elicit next transient response to the second light pulse, even though the two pulses were presented respectively to different eyes of the subject. The result supports the suggestion that the discrete sampling process exists in the pupillary control system. Moreover, it indicates that the minimum time interval is not caused by the retina adaptation, but determined by the central nerve system.

Accommodation, Ocular↗

PubMed1990

Switching control of accommodation: experimental and simulation responses to ramp inputs.

The irregular staircase appearance of the accommodation response to a ramp stimulus, slow movement of a target towards the subject's eye, suggests a switching between different states of control. Experiments and model simulations indicate that blur processing by retina and cortex underlies threshold triggering of the switch. The ON state consists of a fairly high gain, closed-loop control system that minimizes error. The OFF state provides for a quasi-open-loop state and a slow drift to a bias level, under the influence of a leaky integrator.

Accommodation, Ocular↗

PubMed1990

[Sampling process in pupillary response to double pulse light stimuli].

Sampling data behavior of the pupillary control system was explored with experiments of pupillary responses to light stimuli in open loop condition (Maxwellian View). When double-pulse low intensity light stimuli with the separation longer than 0.6 seconds were applied, the pupillary responses normally resulted in a double transient contractions. When the separation between the two pulses was shorter than 0.6 s, a single transient contraction similar to those obtained for single pulse stimuli was found. The same results were obtained from different runs of experiments either in the same subject or from five different subjects. It means that a minimum period about 0.6 s is always required for the pupillary response to the second pulse light stimulus following the first response. It the reveals that dynamic behavior of the pupillary system is a discrete sampling control process. Furthermore, the pupillary responses were transient (AC) with low intensity of pulse stimuli, and were sustained (DC) with high intensity. Therefore the pupillary system control can be interpreted in terms of a dual mode control mechanism: the transient (AC) part exhibits a discrete sampling behavior, while the sustained (DC) part shows continuous feedback control.

Adult↗

PubMed1988

Changes in accommodation with age: static and dynamic.

Accommodative amplitude decreases with age, not with aging. The decrease is largely completed by age 40 years; only minor residual accommodation is present in most subjects after the mid-40s. Dynamical measurements show the accommodative response of subjects over 30 years of age to be significantly slowed (time constants of accommodation increases). Accommodation amplitude is less than 3 D by 30 years of age. Thus prepresbyopia is a sign of continual development, not of deterioration of the accommodative mechanism. Accommodation, or the change of clear vision with change in lens power, has been studied by many distinguished scientists including Descartes and Thomas Young. Helmholtz's "Theory of Accommodation" is a dual, indirect, active theory. There are both lenticular, including lens and capsule, and also extralenticular mechanisms, comprised of the zonule of Zinn or suspensory ciliary ligament and the ciliary muscle itself. The ciliary muscle does not act directly on the lens but indirectly through its action on the zonule of Zinn. Active contraction of the ciliary muscle, a unified muscle, produces accommodation; relaxation of the ciliary muscle permits relaxation of accommodation.

Accommodation, Ocular↗

PubMed1981

Dynamic pupillary response to positive differential of light stimulus.

Under low background illumination, the pupil responds only to an increase in light intensity by a transient constriction when the extent and rate of the increase exceed a certain values. We term this response the dynamic papillary response in order to differentiate it from the much more sluggish and much less pronounced response moiety that governs the static pupil size according to the level of light adaptation. The characteristics of the dynamic response are studied in some detail. This virtually unidirectional rate sensitivity of pupil to light renders it difficult to be subjected to frequency-response analysis even by small signal approximation. Analysis of the experimental results suggests that this transient response is a light-precipitated confluence of few consecutive hippus.

Adaptation, Ocular↗