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Biomedical subjects

J T Enright

Publications and source records attributed to J T Enright.

35 records · Page 2Linked to original sources
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↗
PubMed1984

Saccadic anomalies: vergence induces large departures from ball-and-socket behavior.

The configuration of muscular forces, which maintains a given orientation of the eye, varies with vergence state. As a consequence, changes in vergence produce both static and dynamic violations of simple ball-and-socket behavior: during strong convergence, the entire eye is displaced temporally within its orbit at steady state by as much as 200 microns; and the axis of ocular rotation for small horizontal saccades is consistently displaced forward within the globe by an average of about 1 mm. These phenomena occur regardless of whether vergence is maintained by accommodation or by binocular disparity. Hence, systematic errors of as much as a full degree can arise in measurement of vergence movement, unless monitoring methods are used which are insensitive to translational motion. The observed displacement on the axis of rotation for saccades may be involved in subjective shrinkage of visual targets during convergence ("experimental micropsia ").

Accommodation, Ocular↗
PubMed1984

A new random-dot stereo illusion and its application to the Anstis-Howard-Rogers effect.

When the dynamic visual noise of an untuned television set is viewed with image defocusing (positive lenses) and with a narrow vertical obstruction partially blocking the pupil of one eye, the video 'snow' seems to separate into two stable surfaces at different depths, divided by a vertical discontinuity. The main features of this illusion can be quantitatively accounted for in terms of the optics of defocused images and the retinal disparities predicted from blur circles. A residual component of the illusion, however, which was perceived by a majority of subjects, cannot be readily explained by geometrical optics; it apparently reflects a more subtle aspect in the processing of visual images, corresponding to the Anstis-Howard-Rogers stereo-effect, in which local depth configurations can bias global stereopsis. Several novel aspects of that effect are described, based on use of this obstructed-pupil illusion as the evoking stimulus.

Adult↗
PubMed1984

Changes in vergence mediated by saccades.

When binocular fixation is shifted between two targets which require change in vergence as well as an equivalent or greater alteration in the mean visual direction, the observed eye motions do not--as asserted by Yarbus (1957) and widely accepted today--consist of slow symmetrical change in vergence, upon which a conjugate (binocularly balanced) saccade is additively superimposed. In all tested target configurations, an unexpectedly large fraction of the total change in vergence occurred during the saccades; observed values ranged from about 40% in certain tasks, to essentially 100% when large version (4 degrees) was combined with small vergence change (less than 1 degree). In these latter situations, binocular congruence can be restored within about 50 ms by appropriately unbalanced saccades, rather than about 500 ms, as expected if slow fusional vergence movement were required. When larger vergence changes are demanded, additivity between vergence movement and conjugate saccade is also violated in that the rate of vergence change during the saccades is several-fold larger than the rate before the saccade or during subsequent completion of the required change in vergence. Furthermore, the residual fusional vergence movement observed in these tests was usually strongly asymmetrical, and often almost entirely monocular. Vertical saccades are nearly as effective as horizontal saccades in mediating a large fraction of an intended change in vergence. In saccades, which contributed strongly to (or fully mediated) an intended vergence change, target-specific binocular differences in saccadic excursion of as much as 40-50% were observed; hence, these eye movements are not fully yoked, as the term 'conjugate' implies. Instead, the eyes behave in such situations as though visual information from each eye is processed separately prior to the saccade, in order to generate the neural signals which control open-loop saccadic movement of the eye.

Adult↗
PubMed1980

Temporal precision in circadian systems: a reliable neuronal clock from unreliable components?

Mutual coupling among oscillators of an ensemble has been proposed to explain the precision of some circadian rhythms. Reciprocal triggering is one of the most familiar forms of mutual coupling in nervous systems, but it can at best produce only modest improvement in temporal precision. Nevertheless, models with an elementary elaboration of such coupling show that circadian precision could be derived from oscillators that are intrinsically "sloppy"; sufficient conditions are that output of the individual components be summed and that mutual triggering be mediated by a nonlinear phenomenon, such as a threshold.

Animals↗
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↗