Biomedical subjects

J T Enright

Publications and source records attributed to J T Enright.

At least 19 recordsLinked to original sources

PubMed2003

Are the electroencephalograms mainly rhythmic? Assessment of periodicity in wide-band time series.

To test the hypotheses that (i). electroencephalograms (EEGs) are largely made up of oscillations at many frequencies and (ii). that the peaks in the power spectra represent oscillations, we applied a new method, called the period specific average (PSA) to a wide sample of EEGs. Both hypotheses can be rejected. Although the principal peaks in the two spectra agree most of the time, quite often a peak in the power spectrum accompanies no periodicity peak and some periodicity peaks have no power spectral peak. The Fourier spectrum is not a reliable indication of rhythms. EEG samples from patients during waking, sleeping and seizure states, and volunteer healthy subjects doing cognitive tasks quite often show no significant rhythms, on an arbitrary, common sense definition. When clear rhythms are seen, they involve one or two, rarely up to four or five simultaneous non-harmonically related frequencies. Rhythms are special cases; most of the power spectrum most of the time is nonrhythmic. "Good" rhythms usually have quite narrow peaks, with frequency modulation of <5%, strengths of >2.5 up to >10 times the expectation from chance, and they often show fine structure by being quite local and brief. Most rhythms are quasisinusoidal but others are sharp-cornered recurrent events with <50% duty cycle. In the face of wide variability, we do not report any systematic differences in periodicity among EEGs from different parts of the brain or different brain states or species; it will take many more exemplars of each state, species or brain part to establish characteristic features. The PSA method may be the best so far proposed to demonstrate and quantify periodicity in wide-band time series with noise, but it has serious limitations. Discussion leads to the conclusion that it is time for a new paradigm or metaphor for brain waves.

Animals↗

PubMed1998

Monocularly programmed human saccades during vergence changes?

1. When binocular fixation is shifted to a new target located at a different distance and in a different direction from initial fixation, a binocularly unbalanced saccade occurs at or near the onset of the composite eye movement. Those saccades typically produce good post-saccadic foveation of the target by one eye or the other. 2. Following such saccades, the better-aligned eye is typically as well aimed at the target as after pure versional saccades, but the partner eye deviates much more, thus requiring asymmetrical post-saccadic vergence movement. 3. This phenomenon suggests that during binocular viewing, the retinal eccentricity of a new-target's image from one of the eyes can be used to programme that eye's own saccade, so that it arrives reliably on target; and that the images of that target from both eyes participate in generating the saccadic excursion of the partner eye. 4. The ecologically useful result is rapid achievement of a high-resolution monocular view of the new target, although full binocular foveation is achieved much later.

Dominance, Cerebral↗

PubMed1998

On the "cyclopean eye": saccadic asymmetry and the reliability of perceived straight-ahead.

If two targets are both on the visual axis of one eye or the other, and binocular fixation is shifted from the farther one to the nearer, the aligned eye consistently makes an initial, seemingly pointless saccade in a temporal direction. The size of those saccades typically differs markedly, depending on whether the targets are aligned with the observer's dominant or non-dominant eye. Pickwell [(1972) Vision Research, 12, 1499-1507] proposed that this binocular asymmetry in oculomotor performance reflects a subject-specific lateral displacement of the egocenter (the "binoculus" of Hering, which has traditionally been assumed to be on the midline). An empirical test of Pickwell's widely endorsed hypothesis has now been conducted and the proposal has been found wanting. In an otherwise darkened room, subjects were required repeatedly to set a small light to a perceived straight-ahead location in the horizontal plane, first for a target at 300 cm distance and then for one at 30 cm. Extrapolation of a line that connects the two averages of those settings to the inter-ocular axis provides an estimate of the subjective egocenter to which visual directions are referred. Contrary to Pickwell's proposal, those locations of the inferred egocenter were usually quite near the midline, and were completely uncorrelated with same-subject data on the extent of saccadic asymmetry at the onset of asymmetrical convergence. The data on perceived straight-ahead underlying this result indicate the availability of extraretinal information about eye orientation that is quite precise at a given moment (median standard deviation of 47 min arc) but conspicuously non-stationary over several-minute intervals (monotonic drifts in sequential settings being very common).

Convergence, Ocular↗

PubMed1996

Sequential stereopsis: a simple demonstration.

Most people strongly prefer to use back-and-forth eye movements in order to discriminate 3-dimensional distances among targets that are widely separated from each other in direction. This viewing strategy permits sequential stereopsis: a comparison between the foveally-seen pre-saccadic disparity of one target with post-saccadic disparity of the other. This note describes a simple and qualitatively compelling demonstration of the usefulness of sequential stereopsis, in a situations in which classical stereopsis, with steady fixation, is greatly degraded. Targets of high-spatial-frequency texture are used, with details that can be resolved foveally before and after saccades, but that are unresolvable in peripheral vision. Back-and-forth eye movements between such textured targets, separated by 8-10 deg from each other, led to estimates of threshold that average less than 45 sec arc disparity (corresponding to about 0.18% of viewing distance): some of the best performances ever reported for targets so widely separated.

Adolescent↗

PubMed1996

Slow-velocity asymmetrical convergence: a decisive failure of "Hering's law".

With targets aligned in the midsagittal plane, six of seven subjects tested were often able to make smooth symmetrical convergence movements in which no detectable saccade occurred during the initial 300 msec of the eye movement (12-95% of their trials). With targets located in a plane parallel to, but appreciably to one side of the midsagittal plane, those same six subjects were also often able to make smooth, slow, "saccade-free" asymmetrical convergence movements that were appropriate in magnitude and velocity to the target location (ratio of excursions about 2 to 1). Vergence movements are thus more versatile than can be accounted for by a single generator of binocularly symmetrical input to the eye muscles (Hering's Law of Equal Innervation). The occurrence of "saccade-free" asymmetrical convergence suggests instead that during binocular viewing, each eye can respond independently to that eye's view of the target, resulting in binocularly simultaneous slow-velocity nasalward eye movements--which will represent symmetrical or asymmetrical convergence depending on the arrangement of the targets. A similar interpretation is also apparently demanded by recent data on the initiation of disjunctive smooth-pursuit movements in the monkey [King & Zhou (1995) Vision Research, 35, 3389-3400].

Adolescent↗

PubMed1995

The non-visual impact of eye orientation on eye-hand coordination.

When a peripheral visual stimulus is briefly presented in an empty surround, and an observer is required, after a delay of a few seconds, to point toward the remembered location of that target, the responses are strongly influenced by eye orientation at the time of pointing. Remembered locations, as indicated in total darkness, are typically more precise (more reproducible across trials) when the subject's eyes are aimed toward the target before pointing, than when initial fixation (straight ahead) is maintained during pointing. Furthermore, when the eyes are aimed toward the target, the indicated directions are usually biased toward less eccentric locations than those indicated with eyes aimed straight ahead. These differences in scatter and in bias arise regardless of whether the eye movement toward target location, which precedes pointing, is made while the target is visible or occurs thereafter in total darkness, thus demonstrating that non-visual stimuli associated with eye orientation affect the spatial memory used by the skeletal muscle system.

Adolescent↗

PubMed1994

To stare or to scrutinize: "grasping" the eye for better vision.

Words of a rich variety exist for describing the different ways of "looking" at a target; and one of the major dimensions for such differentiation is the spectrum between staring and scrutinizing, between gazing and peering. This dimension seems primarily to reflect the level of attention given to details of the visual stimuli: a "cognitive" distinction. Evidence now demonstrates that staring and scrutiny also differ in the net muscular forces exerted on the eye; during scrutiny, opposing pairs of rectus muscles apparently jointly contract pulling the eye somewhat backward into its orbit, while it stays aimed in the same direction.

Adolescent↗

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↗