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Biomedical subjects

J T Enright

Publications and source records attributed to J T Enright.

At least 19 recordsLinked to original sources
PubMed1992

The remarkable saccades of asymmetrical vergence.

The saccades that usually arise near the onset of asymmetrical changes in vergence, when one eye is aligned with both targets, are remarkably different from ordinary saccades: (1) the excursions of the two eyes are typically very unequal, often differing by several fold from each other; (2) mean excursion (version) is extremely variable across replicate tests with identical targets; (3) at the end of the saccades, eye orientation is usually not even briefly stable: the aligned eye immediately reverses its movement, indicating that the pulse in muscular forces is apparently not followed by a corresponding step; and (4) a second saccade in the opposite direction can immediately follow the initial saccade of asymmetrical divergence, with no sign of refractoriness. These phenomena suggest that the pulse and step components of saccadic motoneuron activity may be generated by largely independent processes; that the step component for each eye depends only on that eye's visual input; and that the pulse components generated for each eye depend on weighted averaging of visual stimuli that impinge on both eyes. This interpretation is incompatible with most current models of saccade generation, but was anticipated in its essentials by Ditchburn [(1973) Eye movements and visual perception. Oxford: Clarendon Press]. A corollary of this hypothesis is that disparity-evoked vergence changes can be viewed as the general-case output from that system which produces fully conjugate saccades as a special case.

Adolescent↗
PubMed1992

Unexpected role of the oblique muscles in the human vertical fusional reflex.

1. If a weak vertically oriented prism is inserted before one eye, binocular single vision is restored by vertically divergent eye movements (one eye turning upward, the other downward); and it is usually assumed that the vertical rectus muscles mediate that fusional reflex. 2. When vertically divergent eye movements occur, both eyes also systematically rotate in parallel around their lines of sight (conjugate cyclotorsion). The direction of these unexpected eye movements demonstrates that they must be due to the oblique muscles, not the vertical recti. 3. The magnitude of these conjugate torsional movements is large enough to imply that the oblique muscles, in producing such torsion, would simultaneously effect all the divergent vertical re-orientation of the eyes required by the targets. 4. The cyclotorsion is accompanied by systematic translation of the eye along a nasal-temporal axis; the direction and extent of that non-rotational displacement indicate that the eye movements of the fusional reflex may well be mediated exclusively by the superior oblique muscles, acting against fixed tone in the inferior oblique muscles. 5. This revised understanding of the oculomotor co-ordination involved in the vertical fusional reflex has significant implications for both neurophysiology and oculomotor surgery.

Adolescent↗
PubMed1991

Exploring the third dimension with eye movements: better than stereopsis.

Eye movements are usually presumed to be irrelevant for (or detrimental to) stereoacuity. When targets of interest are not adjacent, however, better discrimination of distance can be achieved by looking back and forth between them. In order to exclude ordinary stereopsis and examine this viewing strategy in isolation, judgements of apparent equidistance have been obtained for pairs of small targets separated horizontally by the angular spacing that corresponds to the fovea-to-blind-spot distance. Precise, stereopsis-like evaluations of relative distance can be made by fixating each of those targets in turn, even if they are not simultaneously presented but are instead shown in alternation. Sequential comparisons of stimuli are thus involved in this form of distance discrimination, but direct utilization of oculomotor information (vergence) is rendered unlikely because very brief target presentation is sufficient. Hence, the evidence argues for "sequential stereopsis": comparisons of the disparities of targets, both seen foveally, before and after saccades. This interpretation makes stringent demands on oculomotor coordination during saccades, but measurements of vergence "noise" indicate that this requirement can probably be fulfilled.

Adolescent↗
PubMed1991

Stereo-thresholds: simultaneity, target proximity and eye movements.

Stereo-thresholds are much higher when adjacent targets are presented without temporal overlap than when they are shown simultaneously. Sequentially presented adjacent targets also evoke small involuntary eye movements toward the newly presented target. Neither of these phenomena is evident with widely separated targets; for sequential presentation of targets 10 degrees apart, stereo-thresholds are only slightly higher (a factor of about 1.5) than for simultaneous presentation; and stable fixation can be maintained. If the differing influence of simultaneity on stereoacuity for adjacent and for widely separated targets arises because adjacent alternating targets evoke eye movements, that effect is apparently not mediated exclusively by displacement of retinal images due to the measured eye movements. It could, however, be due to a general long-term instability of fixation associated with repetitive small involuntary eye movements.

Adolescent↗
PubMed1990

Stereopsis, cyclotorsional "noise" and the apparent vertical.

In principle, stereopsis can be used to evaluate the subjective vertical in a sagittal plane, but temporal variation in cyclotorsion should degrade that ability. Video recordings of eye orientation during steady fixation were used to evaluate long-term instability in cyclotorsion. Torsion was measured simultaneously in each eye at 1-sec intervals during about 30 sequential fixations (5-sec duration) on the same target. For each eye separately, the standard deviation of torsion around its mean value averaged about 18 min arc. Some of this variation was conjugate, but the variability in torsional difference between the eyes averaged 17 min arc. Most of this second-to-second variation arose between fixations (average SD = 15 min arc). Such low-frequency, inter-fixational variation in torsional difference between the eyes must produce spurious horizontal disparities in the upper and lower visual fields, and should thereby limit the precision with which the vertical horopter can be evaluated. All subjects exceeded those theoretical limits on precision, however, in performance tests requiring that two vertically separated targets be adjusted to apparent equidistance--but only when permitted to shift fixation back and forth between the upper and lower targets. That latter result provides a challenge to current understanding of stereopsis.

Adolescent↗
PubMed1989

Manipulating stereopsis and vergence in an outdoor setting: moon, sky and horizon.

A simple stimulus generator has been constructed that permits a small illuminated target to be seen with variable inter-ocular disparity, when superimposed upon the binocular view of an outdoor landscape. This device was applied to several questions involving perception of size, distance and orientation, with the following results: (1) when the apparent distance to an "artificial moon", as perceived through stereopsis, is decreased by about 50-fold (from near horizon to about 60 m), its apparent size is reduced by only a miniscule amount (8% on average); hence, the moon illusion is probably not due to compensation--conscious or subconscious--for its apparent distance; (2) those changes in apparent size known as convergence micropsia vary as a function of the visual surround; for a vergence change of 1 deg, greater perceived change in size of a small target arises when a landscape is seen nearby than with empty sky as surround; (3) when a target is shown somewhat above the horizon against an empty sky, it must be viewed with divergence of the visual axes (image positions for "hyper-infinite" distance), in order to be perceived as vertically above objects on the skyline; this effect implies a strong backward tilt to the apparent vertical and probably reflects an attempt to "null out" the perceptual consequences of the convergence that typically occurs during downward saccades.

Depth Perception↗
PubMed1989

Convergence during human vertical saccades: probable causes and perceptual consequences.

1. When a downward saccade is made between equidistant targets, convergence consistently occurs during the saccade: about 1 deg overconvergence after an 8 deg saccade, with either binocular or monocular viewing, with either far (3 m) or near (30 cm) viewing distance. 2. During binocular viewing, this unnecessary convergence is corrected by divergence movement with a half-time of about 200 ms. During monocular viewing of far targets, similar post-saccadic divergence occurs, but for monocularly-seen near targets, recovery is considerably slower. 3. Vergence changes associated with upward saccades are much smaller and typically more variable among subjects. 4. The up-down asymmetry of intrasaccadic vergence changes can be accounted for by superposition of two plausible adventitious processes: co-contraction of the vertical recti, and tension increase (upward saccades) or tension release (downward saccades) in the superior oblique muscles. 5. During the 1000 ms after an upward saccade, constriction of the pupil consistently occurs; it apparently represents a near-triad response, for which concurrent convergence is masked. Such near-triad activation during upward gaze would presumably be necessary to counterbalance residual steady-state torques from the superior oblique muscles. 6. The up-down asymmetry of intrasaccadic vergence changes also arises when targets require both a vergence change and a vertical shift of gaze, thereby accelerating refixation for the typical natural spatial configuration, in which nearer objects are lower in the visual field. 7. During binocular viewing of equidistant targets, the convergence resulting from downward saccades produces large transient disparities, which can be expected to lead to biased evaluations of relative distances to targets. Several up-down illusions involving apparent distance may well be due to these disparities, including (a) backward tilt of the apparent vertical and of the vertical horopter, (b) the 'soup-bowl sky' illusion, and (c) the 'diverging sunbeams' illusion.

Accommodation, Ocular↗
PubMed1989

The parallactic view, statistical testing, and circular reasoning.

A "parallactic view" (i.e., subjectivity in interpreting data) is an important and perhaps essential tool for formulating hypotheses, but it also represents a hazardous contaminant to be avoided in testing hypotheses. Computer simulations demonstrate that statistical testing of data that are contaminated by even a modest level of such parallax can be very misleading; probability levels are greatly distorted. An even more insidious influence of the parallactic view arises when the fundamental assumptions for a statistical test are not adequately respected. Single-cosinor analysis, which has been used to "demonstrate" circaseptan rhythms (tau = about 7 days), lends itself to such abuse: The statistical test of the zero-amplitude hypothesis assumes that if any serial correlation is present in the data, it is due to a sinusoidal oscillation with period that is known a priori. One cannot, therefore, legitimately use this method to demonstrate the existence of such a rhythm.

Humans↗
PubMed1988

The cyclopean eye and its implications: vergence state and visual direction.

The cyclopean illusion (Hering, 1861) is an anomalous lateral shift in the apparent direction to a monocularly seen target, which arises when a change in vergence is made by the opposite (nonobserving) eye. Surprisingly, this directional illusion does not arise, when the observed target is an afterimage or an intermittently illuminated (4-8 Hz) object. Instead, during convergence, a monocularly imprinted afterimage seems to move toward the observer, and a stroboscopically illuminated target seems to remain fully stationary. The apparent displacement of an afterimage in depth is particularly puzzling. Since binocular interactions in the persistence of monocularly induced afterimages can be demonstrated, it is conceivable that long-persistent afterimages arise, in part, from binocularly driven neurons in the visual cortex, and that a monocularly induced afterimage can thereby become the perceptual equivalent of a binocularly fused target.

Afterimage↗
PubMed1987

Perspective vergence: oculomotor responses to line drawings.

When a perspective drawing is viewed monocularly, changes in fixation point are accompanied by changes in steady-state vergence; their direction is usually appropriate for the distance relationships implied in the illustration. The absolute magnitude of these responses varies appreciably among subjects; it can be consistently enhanced or reduced by modest changes in the drawing. Similar configurations of stimuli from three-dimensional objects would presumably also contribute to normal vergence movements during binocular viewing; it appears that their importance would increase with target distance. Corresponding changes in pupil diameter, as expected for the "near reflex", were not observed with perspective stimuli. Consistent, directionally appropriate vergence changes, paralleling perception, were also made by most subjects during monocular viewing of a Necker cube, but there, exceptionally large pupillary responses arose.

Adult↗
PubMed1987

Art and the oculomotor system: perspective illustrations evoke vergence changes.

When a painting or drawing is viewed monocularly and fixation alternated between points that are at different implied distances from the observer, the covered eye usually makes vergence movements that are directionally appropriate for the indicated depth differences. These vergence changes evoked by perspective artwork vary greatly in magnitude and consistency from one illustration to the next: some drawings and paintings lead to convergence-divergence changes smaller than would be appropriate for the illustrated content, if seen from the implied viewing distance; others are supernormal stimuli, evoking inappropriately large vergence changes in all observers tested.

Adolescent↗
PubMed1986

The aftermath of horizontal saccades: saccadic retraction and cyclotorsion.

During horizontal saccades from either nasal or temporal direction, the eye is retracted into its orbit (about 100 micron for 8 degrees saccades), presumably due to co-contraction of the recti. That translational displacement of the eye thereafter slowly decays, with a half-time of about 100 msec. Transient cyclotorsion (up to 1 degree) also often arises during a saccade, with a direction of movement which depends upon pre-saccade position of the eye. When present, it decays with a half-time of about a full second, frequently leaving residual torsion, the direction of which also depends on where the saccade originated (static hysteresis). These two sorts of slow recovery process greatly extend the "duration" of a saccade, compared with presently accepted values.

Eye Movements↗
PubMed1986

Facilitation of vergence changes by saccades: influences of misfocused images and of disparity stimuli in man.

When confronted with randomly presented targets demanding change in mean visual direction (version of 4.5-7 deg) as well as a change in vergence (1.5-2.5 deg) both naive and experienced subjects can make short-latency saccades which differ markedly and appropriately in the excursion of the two eyes, and which thereby achieve a large fraction of the required change in vergence. Mean values for the portion of vergence change occurring during the saccades, over all target positions, ranged from 41 to 70% for the five subjects tested, but subject-specific directional biases in performance were also conspicuous. When such targets were presented randomly during monocular viewing, so that only accommodation cues could induce vergence change, a consistent fraction (13-48%) of the resulting vergence movement also occurred during the saccades. These data indicate that on average about one-quarter of the intra-saccadic vergence change achieved during binocular viewing could have been due to accommodation stimuli perceived by one eye. The monocular results indicate that the direction of image misfocus (nearer vs. farther), which guides accommodation-vergence, can be correctly distinguished for targets which are about 6 deg from the centre of the fovea, and that this assessment can be made within the 200-300 ms latency for saccadic eye movements, and hence without trial-and-error refocusing. The binocular inequality of saccades during monocular viewing involved conspicuous subject-specific right-left asymmetries, as well as consistently positive within-subject correlations between intra-saccadic vergence change and finally realized accommodation-vergence movement, and consistent correlations between phoria before the saccade and intra-saccadic vergence change.

Accommodation, Ocular↗
PubMed1985

On Pulfrich-illusion eye movements and accommodation vergence during visual pursuit.

When the Pulfrich illusion is perceived with stationary fixation, and visual pursuit of the pendulum is then initiated, rapid vergence changes occur which correspond to the illusory elliptical path. During steady-state visual tracking of the illusion, however, the eyes move along a planar path without systematic changes in vergence. These latter pursuit movements with monocular filter involve large fixation disparities relative to unobstructed vision (0.5 degree to 1 degree divergence); hence, it is proposed that the planar tracking path probably results from strong dominance of the oculomotor system by stimuli from the unobstructed eye. During visual tracking with monocular filter and a target moving along a nonillusory elliptical path in depth, appropriate changes in vergence occur, but comparable vergence changes also arise when the target is fully hidden from one eye. This response apparently represents a superposition of accommodation vergence upon smooth pursuit movements; similar responses also occur during monocular tracking of a target moving around a circular path in depth.

Accommodation, Ocular↗
PubMed1970

Distortions of apparent velocity: a new optical illusion.

To an observer whose one eye is covered with a relatively strong filter (approximately 90 percent extinction) and who views a landscape from the side window of a moving automobile, the velocity of the vehicle appears to be markedly reduced when the uncovered eye is in the forward or leading position (in the sense of motion of the vehicle); the velocity seems to be increased when the covered eye is in the leading position. The illusion of reduced velocity is accompanied by an apparent dwarfing of objects near the roadside and an apparent foreshortening of the distance between object and observer; the illusion of increased velocity is accompanied by an apparent increase in size of objects and an increase in their apparent distance. These illusions can be understood as corollaries of the well-known Pulfrich phenomenon.

Humans↗
PubMed1967

Temperature compensation in short-duration time-measurement by an intertidal amphipod.

The duration of the swimming response of an intertidal amphipod to increases in hydrostatic pressure apparently serves to measure the timing of wave uprush on the beach. Experiments have demonstrated that this response to a standard pressure-increase stimulus varies in duration only slightly with temperature over the range from 10 degrees to 28 degrees C, with estimated Q(10) values of 1.3 to 1.5. Relative insensitivity to temperature, such as here described, seems to be an essential component of biological time-measuring systems (including endogenous circadian, tidal, and lunar rhythms) that are ecologically keyed to the timing of temperature-independent environmental factors.

Analysis of Variance↗