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Biomedical subjects

H J Wyatt

Publications and source records attributed to H J Wyatt.

At least 19 recordsLinked to original sources
PubMed2001

The role of target position in smooth pursuit deceleration and termination.

Subjects smoothly pursued a target moving horizontally at 15 deg/s. After pursuit for 1 s, the target jumped 3 deg ahead of the fovea. At the moment of the jump, target velocity became 0 and 'effective visual feedback' assumed a value of either 0 (target retinally stabilized), -0.2, -0.4, or -1.0 (target fixed in space). With 0 visual feedback the eye continued to move smoothly at a moderate velocity, an apparent response to target position relative to the fovea. When negative visual feedback was present eye velocity decreased. With -0.2 and -0.4 feedback, this decrease was not a simple exponential, but often consisted of an initial fast decrease followed by slower decrease. With -1.0 feedback, eye velocity quickly decreased in an approximately exponential manner, and stopped. We were able to simulate these pursuit responses using a simple model of the pursuit system. Key features of the model are: (a) a target-velocity channel whose output decreases with target offset from the fovea, and whose gain switches from high to low as pursuit velocity approaches zero; (b) a target-position channel with a saturation non-linearity at 1-3 deg; and (c) a positive feedback loop with gain of less than 1.0. All of these features are essential to simulate the pursuit responses, especially with visual feedback values of -0.2 and -0.4. Our results and model suggest that target position serves as an important stimulus in guiding smooth pursuit as pursuit velocity decreases, and especially during pursuit termination.

Feedback↗
PubMed2001

Probing anterior segment kinetics with focally applied mydriatics.

The purpose of this study was to examine the effects of convectional flow and anterior segment configuration on drug kinetics. Mydriatics were applied focally at the limbus in order to produce sector dilation of the pupil. Subjects were tested with either tropicamide or phenylephrine, applied at the superior, temporal, inferior, or nasal limbus (or as a conventional drop). Changes in pupil form were analyzed by means of photography, digitization, and circular Fourier series representation. Both tropicamide and phenylephrine were found to produce sector dilation; however, phenylephrine was approximately twice as effective. Applications at the superior limbus were significantly less effective than applications at the inferior limbus. The results are interpreted in terms of anterior segment convectional flow, which is believed to play a substantial role in pharmacokinetics of the anterior segment.

Adult↗
PubMed2000

A 'minimum-wear-and-tear' meshwork for the iris.

Large changes in pupil size occur frequently, yet the connective tissue components of the iris, consisting largely of relatively-inextensible collagen, last for many years. The concept of a minimum-wear-and-tear meshwork is developed in the context of the iris geometry, and an optimum form for such a meshwork is derived. A moderate improvement on the performance is obtained by allowing the iris to stretch nonlinearly. Comparison of the optimum mesh and stretch behavior with available data suggests that the iris may approximate a structure which minimizes wear and tear.

Biomechanical Phenomena↗
PubMed1998

Detecting saccades with jerk.

In studies of smooth eye movements, saccades are often detected and removed from eye movement records during analysis. A simple and effective method for saccade detection is described; the method uses jerk (the third derivative of eye position with respect to time).

Algorithms↗
PubMed1997

Offset dynamics of human smooth pursuit eye movements: effects of target presence and subject attention.

Subjects made smooth pursuit eye movements with a target moving horizontally at 15 deg/sec. At a specified location the target either: (1) suddenly vanished; or (2) jumped to the fovea with target retinal velocity and feedback becoming 0 (target stabilized at the fovea). In each type of trial, the subjects either: "looked" at the target, "pushed" the target, or "passively" gazed. When the target vanished, eye velocity decreased exponentially with a short time-constant (tau approximately 0.10 sec), regardless of whether the subjects were "looking," "pushing" or "passively" gazing. However, some subjects while "pushing" (using an imaginary target) did generate low velocity smooth movement (1-2.5 deg/sec) late in the offset. When the target was stabilized at the fovea, eye velocity also decreased, but with a relatively long time-constant (tau = 0.4-0.8 sec). The time-constant was the same with both "looking," and "pushing", but was shorter for some subjects with "passive" gazing (tau = 0.1-0.5 sec). These findings show that smooth pursuit offset is influenced by the presence of a target, but is relatively independent of attentional mode. All of the pursuit offset responses can be simulated using a model of the pursuit system with target velocity and position inputs, and an internal positive feedback loop enabled by target presence.

Attention↗
PubMed1996

Ocular pharmacokinetics and convectional flow: evidence from spatio-temporal analysis of mydriasis.

Pupil shape and placement were studied during pharmacologic dilation, using a photographic technique. Dilation onset was highly asymmetric; typically, the inferior or inferior-nasal side of the pupil moved maximally, while the opposite side barely moved. Pupil shape tended to elongate at a large angle to the direction of maximum shift. Late in dilation, pupils returned to more circular shape and pupil centers returned to near their original position. Results when a subject's head was inverted during and immediately after drug administration were essentially unchanged. However, results when a subject's head was rolled on one side (and kept there) after administration were markedly different; the dilation pattern shifted roughly in accord with head tilt. It is proposed that anterior chamber convectional flow is responsible for the asymmetry. Supportive evidence was obtained from experiments in which cooled or heated gel-packs were applied to the lids. Analysis indicates that convectional flow makes a very substantial contribution to anterior segment pharmacokinetics and is also likely to be of general importance in anterior segment transport.

Anterior Chamber↗
PubMed1995

Visual fixation of a target and suppression of optokinetic nystagmus: effects of varying target feedback.

The ability to maintain foveal fixation of a target with either a stationary or moving background is often assumed to depend primarily on a difference (in velocity and/or position) between fovea and target. However, when subjects look at a target stabilized at the fovea presented against sinusoidal motion of an optokinetic stimulus field, optokinetic nystagmus (OKN) is suppressed. This suppression is not simply the absence of movement but instead most subjects show some amount of residual slow eye movements roughly counterphase to the field motion. We have varied the visual feedback of the target from 0 (stabilized) to -1 (stationary in space); as feedback increased, amplitude and phase lag of residual eye movements decreased systematically. The mechanism responsible for residual movements appears to operate for all feedback values (including the "real world" value of -1), which suggests a new view of the role played by retinal slip during fixation of a target and suppression of OKN.

Eye Movements↗
PubMed1995

The form of the human pupil.

The purpose of this study was to characterize the form of the pupil in normal human subjects. Using a modified slitlamp, photographs of pupils were taken in steady illumination and 10-20 sec after darkness. Transparencies were projected and digitized, and the pupil margin was represented as a circular Fourier series. Best-fit ellipses were also determined. The placement of the pupil relative to the limbus was determined in a number of subjects. Results from 23 subjects indicated that in both darkness and light, average pupil noncircularity was 0.0166. (A value of 0.0200 is easy to detect with the unaided eye from the photographs.) On average, the best-fit ellipse accounted for about half of the noncircularity (59.6% in darkness; 47.7% in light). Most of the contribution to shape was made by the first 4 or 5 harmonics. Shapes were usually stable within a session and could remain fairly stable for at least a year; however, shapes for different subjects were not very similar, especially in the light. (Average pairwise similarity: 0.106 in darkness; 0.034 in light; similarity can have values from -1 to 1.) For a given subject, shapes in light and dark were often fairly similar (average similarity 0.260), but there were systematic differences: in eyes where the ellipse contributed > 20% of noncircularity, ellipse major axes clustered around vertical in darkness, and horizontal in light, implying greater contraction near the vertical meridian. Even pupils with little elliptical contribution turned out to contract more near the vertical meridian. There was some tendency for left and right eyes of an individual to show mirror symmetry of shape. In the dark, pupils were located 0.27 +/- 0.09 mm nasal and 0.20 +/- 0.15 mm superior to the limbus center, and usually moved slightly further nasal or superior in the light. Noncircularity increased with age (0.0015/decade). It was concluded that pupils show individuality of shape, with significant regularities within and across subjects.

Adult↗
PubMed1995

Optokinetic nystagmus (OKN) suppression by fixation of a stabilized target: the effect of OKN-stimulus predictability.

In previous work, subjects looked at a target stabilized at the fovea, superimposed on a sinusiodally moving OKN stimulus. The stabilized target (no retinal-slip) suppressed OKN leaving residual eye movements that were often in counterphase with the OKN stimulus motion. In the present study we explored how this type of suppression of OKN is influenced by OKN stimulus predictability: OKN stimulus motion was either sinusoidal or a random walk of half-sinusoids. During fixation of a stabilized target with sinusoidal stimulus motion, OKN was suppressed leaving residual eye movement whose amplitude was typically less than OKN and with a phase lag of about 180 deg (roughly in counterphase with stimulus motion). With random-walk stimulus motion, the residual movement amplitude was even smaller, and at higher frequencies the phase lag decreased to become the same as for OKN. For both stimulus motions, OKN was suppressed when the target was present, but counterphase residual movements appear to depend on stimulus predictability.

Eye Movements↗
PubMed1995

A simple view of age-related changes in the shape of the lens of the human eye.

Data describing the radius of curvature of the anterior surface of the human lens have been re-examined for (i) far-accommodated lenses and (ii) near-accommodated or excised lenses. It is found that, with increasing age, the curvatures converge to an intermediate value. Taking this together with the body of work on the mechanics of accommodative lens changes, a perspective is suggested which affords a simple view of curvature changes with age. The only datum in the literature is found to fit reasonably well with this perspective.

Accommodation, Ocular↗
PubMed1994

Smooth pursuit eye movements with imaginary targets defined by extrafoveal cues.

Initiation of smooth pursuit eye movements was studied using imaginary targets defined by pairs of small spots ("cues") moving together in step-ramp motion. The arrangement of cues and imaginary target were either "two-sided"--cues far apart and imaginary target midway between, or "one-sided"--cues 4 deg apart and imaginary target to one side of the cue pair. (1) Pursuit of two-sided stimuli was much better than pursuit of one-sided stimuli and was often as good as pursuit of a real foveal target. (2) Experiments with various cue configurations suggest an attribute required for vigorous pursuit: "foveal enclosure". Configuration information is available to the pursuit system immediately after the onset of motion. (3) Pursuit of imaginary targets is markedly degraded by a background; the effect is strongest for a background at the fovea and weaker for an eccentric background, including one at the cue location.

Cues↗
PubMed1992

Suppression of optokinesis by a stabilized target: effects of instruction and stimulus frequency.

Subjects viewed a foveally stabilized target presented against a background field of dots moving sinusoidally. Several different modes of viewing the target were used (subjects were instructed to gaze, look, or hold), and the frequency of sinusoidal field motion was varied from 1/32 to 2 Hz. In line with previous findings, the presence of a stabilized target resulted in substantial suppression of optokinesis. The characteristics of this suppression (gain and phase of slow residual eye movements) were dependent on both the mode of viewing the target and the frequency of field motion. When subjects used an imaginary target, little suppression occurred. These findings provide an overall profile of dynamic characteristics of mechanisms involved in the suppression of optokinesis. They support the view that this suppression is significantly determined by the presence of a target against a moving background (even without retinal slip), and by the mode of attending to the target.

Adult↗
PubMed1991

Modeling the kinetics of topically applied ophthalmic agents.

The equations for the ocular pharmacokinetics of topically applied agents are developed in a matrix form which is well suited to numerical solution on small computers. Some sample computations are presented. The technique is relatively simple to extend to additional compartments.

Administration, Topical↗
PubMed1991

Anisocoria. Variation and clinical observation with different conditions of illumination and accommodation.

Variations in anisocoria in light and dark conditions are used to help diagnose normal and pathologic conditions; however, there have been few observations of anisocoria in different lighting and accommodative conditions. The authors measured pupil size photographically in a group of normal subjects examined in six conditions that were controlled for illumination and accommodation. Greater variation and average extent of anisocoria were found in conditions that resulted in larger pupil size. A subset of subjects repeated several sessions. For this group, the average value of anisocoria and variability tended to be greater in dark conditions. These results show that the observation of anisocoria varies under different conditions, and they suggest careful consideration of conditions used clinically to assess pupil equality. Our analysis shows that for a given observation threshold, conditions that produce even modest changes in variability can cause dramatic changes in the probability of observing anisocoria.

Accommodation, Ocular↗
PubMed1989

The oculomotor "twitch"--a transient response to target motion.

The oculomotor "twitch" is a small, transient eye movement that occurs in response to onset of target motion during both passive and active regard of the target (Wyatt and Pola 1985 a, 1987). Among the properties described here, primarily tested with subjects remaining passive, the twitch (i) is elicited extrafoveally, but maximally at and near the fovea, (ii) often persists as a distinct early transient response even when target motion is onset of a simple ramp, and (iii) remains approximately constant over a wide range of target motions, varying only with direction. We suggest that the twitch reflects the working of a mechanism analyzing the direction in which a target starts to move.

Adult↗
PubMed1989

The perception of target motion during smooth pursuit eye movements in the open-loop condition: characteristics of retinal and extraretinal signals.

During smooth pursuit eye movement, the perception of target motion appears to come from retinal and extraretinal influences. To explore this, two open-loop conditions (experimental stimuli stabilized at the retina) were used: one to look at the combined effect of retinal and extraretinal signals on perception (using sinusoidal target motion); and the other to look at the characteristics of an extraretinal signal alone (using a complex target and square-wave motion). In both conditions subjects tracked target motion in the dark, and subsequently compared it to motion of a similar target in the light. The main findings of the study are that the magnitude of the extraretinal signal decreases with frequency, and that the retinal and extraretinal signals combine additively. This system appears to involve a transport-time, which could be in the form of a time advance. These features of perception have a variety of implications for motor control.

Eye Movements↗
PubMed1988

Predictive behavior of optokinetic eye movements.

Optokinetic nystagmus is often thought of as a "primitive" oculomotor response, while smooth pursuit is thought of as a "higher" one. We have used conditions that are usually thought of as eliciting optokinetic responses; i.e., large-field stimuli confined to the retinal periphery, and instructions to subjects to respond passively. In spite of this, the responses showed predictive behavior similar to that described for smooth pursuit.

Eye Movements↗