Biomedical subjects

C J Erkelens

Publications and source records attributed to C J Erkelens.

At least 19 recordsLinked to original sources

PubMed1997

Trajectories of the human binocular fixation point during conjugate and non-conjugate gaze-shifts.

This paper describes the spatial trajectories of the binocular fixation point (the intersection point of the two lines of sight) during gaze-shifts within a horizontal plane of regard. Gaze was voluntarily shifted between pairs of real, continuously visible LED targets that were either iso-vergent at 5-25 deg convergence (conjugate version saccades) or differed in vergence angle (by 5-20 deg) as well as in direction (by 5-60 deg; combined version and vergence). Orientations of both eyes were recorded by phase detection in a homogeneous magnetic field with scleral sensor coils. "Conjugate" saccades showed an outward-looping, curved trajectory as a result of transient divergence, typically associated with horizontal saccades. These outward loops were disproportionately larger for far than for near targets, due to the non-linear relation between vergence and distance. Transient divergence increased moderately in magnitude and duration when basic vergence increased from 5 to 25 deg. As a result, transient saccadic disparities increased in angular magnitude as targets got close. Increasing tonic vergence did not, however, slow down conjugate saccades, in contrast to the previously described dynamic slowing effects of vergence on version during gaze-shift involving simultaneous vergence and version changes. Convergent and divergent non-conjugate gaze-shifts each had characteristic trajectories; outward loops were much reduced in convergent and virtually absent in divergent movements. The saccadic component of non-conjugate gaze-shifts was preceded by a pre-saccadic vergence component in the direction of the imminent gaze-shift; its magnitude increased systematically with the increase in vergence demand and with the decrease in version demand. For both pre-saccadic convergence and divergence, this pre-saccadic part of the trajectory tended to follow an iso-direction line through the target of origin; directional change did not start until the saccade began. This suggests that for targets that differ in direction as well as distance, control of the vergence and version components of the gaze-shift can be dissociated to some degree. This seems to argue against models of binocular oculomotor control which assume that each eye responds primarily to its own target, and suggests rather that target vergence and target direction may be processed and responded to separately by ocular vergence and version, with a strong interaction between the two oculomotor activities whenever they occur at the same time.

Adult↗

PubMed1997

Capture of visual direction: an unexpected phenomenon in binocular vision.

Binocular perception of visual direction is based on laws which were formulated more than 100 years ago. These laws govern the directions in which human beings perceive objects visible to both eyes (binocular objects) and objects visible to only one eye (monocular objects). We report here that the laws do not hold for monocular objects adjacent to binocular objects. The perceived directions of these monocular objects are captured by those of nearby binocular objects. Capture of binocular visual direction is an unexpected phenomenon because it refutes the generally accepted notion that a particular retinal location gives rise to a particular subjective visual direction. The practical consequence is that the subjective techniques for measuring eye position which are widely used in fundamental research and clinical practice are unreliable if they are used in densely structured stimuli. We suggest that capture results from a mechanism of lateral interaction between adjacent visual directions. This mechanism ensures that, despite eye movements, objects have the same spatial order in monocular and binocular vision. This conservation of spatial order also explains why retinal blind spots are not manifest in binocular vision.

Convergence, Ocular↗

PubMed1997

Capture of the visual direction of monocular objects by adjacent binocular objects.

Investigations of binocular visual direction have concentrated mainly on stationary objects. Eye positions were generally not measured and binocular fixation was assumed to be perfect. During the viewing of stationary objects, vergence errors are not negligible but small. During the viewing of moving objects, however, errors in binocular fixation are much larger. Existing rules for binocular visual direction were examined under the latter, more demanding viewing conditions. Eye movements were measured objectively by the scleral coil technique. Subjects viewed a large stereogram in which the half-images oscillated in counterphase. The stereogram contained two square random-dot patterns placed side by side with a gap in between. A vertical line, visible only to one eye, oscillated in the gap. Subjects were asked to adjust the amplitude of line motion until the line was perceived to be stationary. In so doing, they set amplitudes equal to the amplitudes of half-image motion if the gap between the patterns was narrow. They set amplitudes significantly smaller in wider gaps. Subjects made considerable fixational errors in following the oscillations of the line and the random-dot patterns. The results of the settings and of the retinal errors together refute existing rules for binocular visual direction of monocular objects. Perceived directions of monocular objects cannot be specified by geometrical rules that include only the positions of the objects and of the two eyes. The results suggest that perceived directions of monocular objects are captured by the binocular visual directions of adjacent binocular objects. Capture of binocular visual direction was found to be effective for gaps as wide as 8 deg between the binocular objects. The phenomenon of binocular capture has negative consequences for the general use of nonius lines as indicators of eye position.

Depth Perception↗

PubMed1996

Binocular alignment in different depth planes.

A generally accepted notion in binocular vision is that we see the world as if viewed by a single eye, the cyclopean eye. A consequence of seeing the world from a single point in space is that the outlines of occluding and occluded surfaces have the same shape. We designed stereograms in which subjects aligned binocularly visible lines to each other. The lines were lying in different depth planes. In the vicinity of occluded areas, binocular alignment was achieved by alignment of the lines in the eye that viewed the monocularly visible details. Stereograms in which shapes of surfaces lying in different depth planes were compared to each other show that occluding and occluded surfaces do not have the same shape: a square surface occludes rectangular surfaces in other depth planes of which the horizontal widths are smaller than the vertical widths. This difference is perceived shape is not possible if the centre of binocular direction has a fixed position in the head.

Depth Perception↗

PubMed1996

Anisotropy in Werner's binocular depth-contrast effect.

We investigated Werner's binocular depth-contrast effect. Subjects viewed stereograms consisting of a test pattern and an inducing pattern. The half-images of the inducing pattern were either horizontally scaled or sheared relative to each other. Subjects judged the (induced) perceived slant of the test pattern. We were interested in what influence the spatial configuration of the test pattern and the inducing pattern had on the depth-contrast effect. We conclude that the depth-contrast effect is a global effect. In other words, it is not restricted to the location of the inducing pattern. The effect decreases with distance, however, in an anisotropic way. The depth-contrast effect was present most prominently when the test pattern was positioned in the direction along the slant (rotation) axis of the inducing pattern. We suggest that Werner's depth-contrast effect can be explained by the (previously reported) findings that: (1) stereopsis is relatively insensitive to whole-field horizontal scale and shear; and (2) stereopsis is very sensitive to horizontal scale and shear of two stimuli relative to each other.

Adult↗

PubMed1996

Stability of binocular depth perception with moving head and eyes.

We systematically analyse the binocular disparity field under various eye, head and stimulus positions and orientations. From the literature we know that certain classes of disparity which involve the entire disparity field (such as those caused by horizontal lateral shift, differential rotation, horizontal scale and horizontal shear between the entire half-images of a stereogram) lead to relatively poor depth perception in the case of limited observation periods. These classes of disparity are found to be similar to the classes of disparities which are brought about by eye and head movements. Our analysis supports the suggestion that binocular depth perception is based primarily (for the first few hundred milliseconds) on classes of disparity that do not change as a result of ego-movement.

Depth Perception↗

PubMed1996

Control of fixation duration in a simple search task.

To obtain insight into the control of fixation duration during visual search, we had 4 subjects perform simple search tasks in which we systematically varied the discriminability of the target. The experiment was carried out under two conditions. Under the first condition (blocked), the discriminability of the target was kept constant during a session. Under the second condition (mixed), the discriminability of the target varied per trial. Under the blocked condition, fixation duration increased with decreasing discriminability. For 2 subjects, we found much shorter fixation durations in difficult trials with the mixed condition than in difficult trials with the blocked condition. Overall, the subjects fixated the target, continued to search, and then went back to the target in 5%-55% of the correct trials. In these trials, the result of the analysis of the foveal target was not used for preparing the next saccade. The results support a preprogramming model of the control of fixation duration. In a simple search task, control of fixation duration appears to be indirect.

Adult↗

PubMed1996

Temporal aspects of binocular slant perception.

We investigate temporal aspects of binocular slant perception in the presence and absence of a visual reference. Subjects judge slant induced by large-field stereograms of which one half-image is either horizontally scaled or sheared relative to the other half-image. Each stimulus is presented for different observation periods ranging from 0.1 to 19.2 sec. We quantitatively corroborate earlier findings that perceived slant develops significantly faster and to higher levels with visual reference than without it. In daily life, when we are active, there will not be much time for slant to develop. We find that if observation periods are brief (a few seconds or less) slant is poorly perceived if there is no visual reference. We conclude that the visual system is relatively insensitive to large-field horizontal scale and shear.

Adult↗

PubMed1995

The function of visual search and memory in sequential looking tasks.

UNLABELLED: Eye and head movements were recorded as unrestrained subjects tapped or only looked at nearby targets. Scanning patterns were the same in both tasks: subjects looked at each target before tapping it; visual search had similar speeds and gaze-shift accuracies. Looking, however, took longer and, unlike tapping, benefitted little from practice. Looking speeded up more than tapping when memory load was reduced: memory was more efficient during tapping. CONCLUSION: eye movements made when only looking are different from those made when tapping. Visual search functions as a separate process, incorporated into both tasks: it can be used to improve performance when memory load is heavy.

Eye Movements↗

PubMed1995

Voluntary binocular gaze-shifts in the plane of regard: dynamics of version and vergence.

We studied the dynamics of voluntary, horizontal, binocular gaze-shifts between pairs of continuously visible, real three-dimensional targets. Subjects were stabilized on a biteboard to allow full control of target angles, which were made to differ only in distance (pure vergence), only in direction (pure version; conjugate saccades) or in both distance and direction (disjunctive saccades). A wide range of changes in vergence (0-25 deg) and version (0-65 deg) was recorded to study the dynamics of disjunctive saccades, described until now for limited ranges, throughout the horizontal oculomotor range within manual working space, and to study the velocity-duration-amplitude relations ("main sequence") of disjunctive vs conjugate saccades. Pure vergence was almost never observed; divergence, especially, was always associated with saccades. Likewise, horizontal saccades were never strictly conjugate, they always contained a transient divergence-convergence sequence. The amplitude and velocity of these transient components varied systematically with saccadic size. In combined version-vergence movements, vergence was, in general, accelerated and shortened as a function of increasing version. This effect was fairly uniform for divergence, which appeared to increase in velocity by about as much as the transient peak divergent velocity of the version saccade. The intrasaccadic fraction of divergence increased from about 50% to close to 100% as a function of increasing version. For convergence, saccades up to about 20 deg were also accelerating; in this case it appeared as if the transient peak convergent velocity of the version saccade was added to the basic convergence velocity. For larger saccades this effect was partly counteracted by the penetration of an initial divergence associated with the saccade. This initial divergence delayed and slowed down convergence. The intrasaccadic fraction of convergence varied between about 40% and 70%. In disjunctive saccades the individual eyes did not follow the main-sequence parameters of conjugate saccades of comparable sizes, except for the eye that moved with the combination "abduction and divergence". For all other combinations of vergence and version, disjunctive saccades had lower peak velocities and longer durations than conjugate saccades. As a consequence, disjunctive version was also slower than conjugate version. Thus, while version accelerates vergence, vergence slows down version: in the generalized case of three-dimensional gaze-shifts, peak velocities and durations are in between those of the limiting cases of pure version and pure vergence. We conclude that, within manual working space, binocular gaze-shifts are effected by the highly integrated action of conjugate and disjunctive mechanisms, both of which are expressed preferentially in fast, saccadic movements.

Adult↗

PubMed1995

Binocular correspondence and visual direction.

Two classic theories of direction vision, one by Hering, the other by Wells, are expressed in mathematical form and compared. The Hering disparity field differs considerably from the Wells disparity field, but if both are scaled for the change of acuity with eccentricity their differences are much more subtle. This explains why it is hard to determine which theory predicts direction perception best, although the tests favour Hering's theory. It is proved that Wells's construction (his rule 3) follows directly from his first two rules and Aguillonius's assumption that the horopter in the fixation plane is a frontoparallel line. Wells's theory is clearly outdated and does not mesh well with modern three-dimensional geometry of binocular vision, which Hering's theory does. Moreover, Wells inextricably mixes distance and direction vision right from the start, whereas Hering properly treats the two-dimensional manifold of directions and the depth-gauging principles separately. The use of terms such as 'Wells-Hering' rules should be discouraged and both Wells and Hering should be remembered separately for their clearly distinct and independent contributions. The work of Hering is still relevant to modern theory and praxis of binocular vision. The extension of Hering's approach to vertical disparities is treated for stimuli in frontoparallel planes. It is shown that acuity-scaled vertical-disparity information sampled at a single glance is below resolution beyond about arm's length. It can only be used if eye movements are allowed. Throughout, the simplest derivations of the geometrical relations that it was possible to find are given, so that the review of binocular geometry might also be of some didactical use. Finally it is indicated in which direction it might be necessary to modernise the concept of binocular correspondence.

History, 18th Century↗

PubMed1995

Binocular perception of slant about oblique axes relative to a visual frame of reference.

From the literature it is known that the processing of disparity for slant is different in the presence and in the absence of a visual frame of reference. The experimental finding that vertical disparity is not processed for slant perception in the presence of a visual reference is elaborated. This theoretical analysis results in a reduction of the three basic first-order transformations between the retinal half images (divergence, rotation, and deformation) to only two basic orthogonal transformations. The first of these, horizontal scale, results in slant perception about the vertical axis, whereas the second, horizontal shear, results in slant perception about the horizontal axis. These transformations are based primarily on horizontal disparity. It is shown experimentally that in the presence of a frame of reference the amount of vertical transformation that is added to the two basic transformations (horizontal scale and shear) of a random-dot stimulus is indeed irrelevant for slant perception. It is suggested that, in the presence of a visual reference, slant perception about oblique axes is based solely on linear combinations of the horizontal-scale and horizontal-shear transformations. Subjects are able to reproduce slants about oblique axes experimentally merely by combining horizontal scale and shear.

Adult↗

PubMed1994

Task-dependent differences between mono- and bi-articular heads of the triceps brachii muscle.

We studied motor-unit recruitment and decruitment thresholds in the three heads of the human elbow extensor, the triceps brachii muscle (caput mediale and laterale, both mono-articular heads, and caput longum, the bi-articular head) by means of intramuscular electromyographic-recordings. Two experiments were performed: an 'isometric' and a 'movement' experiment. In the isometric experiment, subjects were asked to increase the elbow extension torque isometrically to a specific level, keep the torque at the level for 10 s, and then decrease the torque again to zero. In the movement experimental subjects moved their forearm from 90 degrees to 110 degrees extension against an increasing flexion torque, kept the latter position for 10 s and then moved their forearm back while the torque decreased. Results for caput longum showed that recruitment thresholds were higher than decruitment thresholds, whereas in caput mediale and laterale no difference in thresholds was found. In caput longum recruitment thresholds were found to be lower in movement conditions than in isometric conditions. The reverse effect was observed in caput mediale, whereas no difference in recruitment thresholds was found in caput laterale. Our results point to a transfer of force from mono-articular muscles in isometric conditions to bi-articular muscles in movement conditions. A similar transfer is found when recruitment and decruitment are compared. This means that the transfer is not only a property of the elbow-flexor muscles, but is a more common trait. A qualitative analysis of firing frequencies at recruitment and at decruitment in both conditions supports our findings.

Action Potentials↗

PubMed1994

Binocular visual direction.

We examine whether the rules of cyclopean visual direction, as expressed by Hering and others, correctly predict the percept of visual direction for structured visual stimuli. Theoretical inspection of the rules of cyclopean visual direction shows a paradox for the binocular visual directions of stimuli in which objects partly occlude each other. We investigate how the rules of cyclopean direction are violated during viewing of structured random-dot stereograms with different depth planes. The directions of monocular and binocular visual elements are determined in an alignment task. Subjects align a monocular/binocular slider with a monocular/binocular test line present in the random-dot stereograms. The results show that the available rules of cyclopean direction are not sufficient for human vision in this general situation. The available rules can only be used for alignment of two binocularly visible lines or two monocular lines presented to the same eye. Alignment in these cases is predicted by almost any set of rules that transforms visible lines to a cyclopean eye. Stimulus conditions, in which either one line is presented to one eye and the other line to the other eye or one line is presented to one eye and the other line to two eyes, provide a more critical test for validity of the cyclopean rules. Our results show that the rules of cyclopean direction fail to predict alignment precisely in these conditions. Inspection of the data shows that binocular alignment is achieved by alignment of two monocular lines presented to a single eye.

Humans↗

PubMed1993

The relationship between absolute disparity and ocular vergence.

The relationship between disparity and ocular vergence was investigated under closed-loop as well as under open-loop viewing conditions. First we examined whether vergence responded similarly to disparity presented under open-loop and closed-loop conditions. Similar response were observed in both conditions. The direct relationship between disparity and vergence was examined by presenting constant disparities between 0.2 degrees and 4 degrees under open-loop viewing conditions. Such vergence responses are described as the outputs of first-order low-pass filters with different filter characteristics for each amplitude of disparity. By analyzing the latency of vergence responses induced by constant disparities with help of the transfer function of disparity-controlled vergence, the time delay of disparity processing in the vergence loop was estimated. We suggested that the time delay was approximately between 80 and 120 ms instead of 160 ms as is generally assumed. The relationship between the rate of disparity change and vergence was examined by comparing responses to ramp and stepwise changes in target vergence. From the similar responses to ramp and staircase changes in disparity we concluded that vergence is not sensitive to the velocity of target vergence as such. On the basis of these findings we developed a model of disparity-controlled vergence. In this model disparity is processed through several parallel, imperfect integrators with slightly different low-pass filter characteristics, each of them susceptible to a limited range of disparities. Gains as well as phase lags of vergence responses to sinusoidal disparities are accurately simulated by this model.(ABSTRACT TRUNCATED AT 250 WORDS)

Convergence, Ocular↗

PubMed1993

Selective adaptation of internally triggered saccades made to visual targets.

We examined whether internally triggered saccades made to a nonjumping target (I-saccades) could be adapted independently from externally triggered saccades induced by a jumping target (E-saccades). Five subjects made I-saccades between two fixed targets, one placed straight ahead and the other one positioned at an eccentricity of 17.5 degrees. The peripheral target was displaced to an eccentricity of 8.75 degrees during the saccadic movements toward this target. Amplitudes of the I-saccades made from the central to the peripheral target before and after adaptation were compared with each other. Saccadic amplitudes after adaptation were between 10% and 42% smaller than those before adaptation. E-saccades induced by a single target which jumped from straight ahead to the same peripheral target position as was used for the I-saccades were also measured before and after the adaptation of I-saccades. Amplitudes of E-saccades before and after adaptation were hardly different from each other except in one subject. The mean decreases in amplitude of the two types of saccades, averaged over all subjects, were 21% for I-saccades and 5% for E-saccades. These results show that I-saccades can be adapted to changed visual conditions while E-saccades remain unadapted. We conclude from this finding that I-saccades and E-saccades are generated by at least partially different neural mechanisms.

Adaptation, Physiological↗

PubMed1993

Simultaneous hand tracking does not affect human vergence pursuit.

In order to find out whether human vergence eye movements are influenced by simultaneous hand tracking movements, vergence was studied when sinusoidal (expressed in vergence angles) target movements were tracked. The target motion was externally generated and the target actually moved in depth. Tracking was done by the eyes alone or by the eyes and hand together, in both light and dark viewing conditions. Our data show that the target motion was tracked by the eyes with a short delay (on average 48 ms), independent of the tracking condition. This suggests that vergence modeling should include some predictive mechanism similar to that proposed for the smooth pursuit subsystem. Furthermore, in contrast to effects on smooth pursuit, simultaneous hand tracking movements did not influence vergence eye movements. From this, we argue that the balance between smooth pursuit and saccadic eye movements is adjustable and can be adapted to the requirements of different tasks.

Darkness↗

PubMed1992

Influences of hand movements on eye movements in tracking tasks in man.

We investigated horizontal smooth pursuit eye movements and hand movements in tracking tasks in order to find out whether hand movements influence eye movements and if so, in what ways. Externally controlled target movements were tracked either by the eyes alone or by the eyes and right hand together. Because a possible influence might depend on the stimulus, we used two classes of target movements: sinusoidal target movements (predictable target movements) and pseudo-random target movements (unpredictable target movements). Our data show that the eye movements contained only a few small saccades when sinusoidal target movements with frequencies higher than about 1 Hz were tracked by eyes and hand together. More and larger saccades were made when the same target movements were tracked by the eyes alone. The difference in smoothness of eye movements was highly significant between the two tracking conditions. Such a difference was not found during the tracking of a pseudo-random target motion. This suggests that the influence of hand movements is related to the predictability of the stimulus. In contrast to the gain of the smooth pursuit eye movements and the maximum of the cross-correlation function, the gain of the composite eye movements did not depend on the tracking condition. The delay of the eye movements with respect to the (sinusoidal) target movements also showed no dependence on the tracking condition. Visual feedback from the tracking hand was found not to play a role in the difference in eye movements for the two tracking conditions.

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