Biomedical subjects

C J Erkelens

Publications and source records attributed to C J Erkelens.

At least 19 recordsLinked to original sources

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.

Feedback↗

PubMed1991

Control of vergence: gating among disparity inputs by voluntary target selection.

The ability of human beings to shift their vergence between multiple disparity stimuli was investigated. The stimulus was a stereogram consisting of a single bar (1 x 0.3 degree) projected in the center of a larger circular pattern (28 degrees dia) of dots (0.3 degree dia). In the initial condition, the subjects perceived a single bar in the center of a cluster of dots, all lying in a single depth plane. The subjects were instructed to fixate either the bar or one of the dots close to the bar. Stepwise changes of target-vergence of either the bar, or of all dots, or of both configurations in opposite directions, were imposed in a random sequence. Under these conditions, ocular vergence was controlled by the target-vergence of the selected target at all times, even if this implied a loss of binocular fusion for the non-selected target. The effect of target selection per se, without concomitant changes in retinal position of the selected target, was studied in two experienced subjects with stabilized image techniques. The subjects viewed a configuration containing short vertical bars placed at either side of a long vertical bar. The short bars on different sides of the long bar had opposite (crossed or uncrossed) disparities. After stabilization of the configuration while the subject fixated the long bar, subjects attempted to fixate the short bars alternatingly. Due to the stabilization the ensuing eye movements did not affect the retinal positions, and thus the disparity, of the bars. Attempted fixation of a parafoveally viewed, short bar induced vergence responses in the appropriate direction.(ABSTRACT TRUNCATED AT 250 WORDS)

Attention↗

PubMed1991

Coordination of hand movements and saccades: evidence for a common and a separate pathway.

We studied the reaction times and initial directions of hand movements and saccades of human subjects who fixated and pointed as quickly as possible at eccentric targets which were presented unexpectedly. The targets were positioned on a horizontal bar which was placed in front of the subject. Different stimulus conditions were used in the experiments. Knowledge of the target position or the presence of an auditory co-stimulus slightly affected the reaction times of saccades in response to visual stimuli. Auditory co-stimuli reduced the reaction times considerably when the targets were presented after a delay of 200 ms after extinction of the central fixation point. Similar reductions were observed in the reaction times of the hand movements. However, these reductions were seen in hand responses to undelayed as well as delayed target presentations. The saccades were always made in the correct direction when the target was presented without delay. When the target was delayed about 50% of the saccades were made in the wrong direction. Even for undelayed targets the hand sometimes made mistakes. The number of mistakes increased to 35% when the target presentation was accompanied by the sound pulse. For delayed targets the proportion of wrong hand movements was about 50%. For such targets saccades and hand movements were practically always made in the same direction. If visual information is available, saccades and hand movements are generated independently of each other. However, if visual information is not present at the appropriate time and the target position has to be guessed, saccades and hand movements are generated on the basis of shared information.(ABSTRACT TRUNCATED AT 250 WORDS)

Acoustic Stimulation↗

PubMed1991

Dependence of autogenic and heterogenic stretch reflexes on pre-load activity in the human arm.

1. Subjects held their right arm in a horizontal plane. The angle of the elbow was 90 deg. They exerted forces in several directions in the plane of the arm, varying independently the pre-load torques about shoulder and elbow. We measured electromyographic (EMG) activity in several arm muscles in response to force perturbations which extended the shoulder, without changing the elbow angle. 2. The EMG activity in flexors of both shoulder and elbow showed reflex responses at short latency (approximately 25 ms). In all muscles the reflex activity increased with the pre-load activity of that muscle. 3. The short-latency reflex activity of m. brachialis, which was not stretched by the perturbations, was independent of the pre-load activity of the muscles acting over the shoulder. 4. From these results we conclude that the force resulting from the short-latency reflex, assessed from the EMGs, does not counteract the perturbations exactly. Having found that the short-latency reflex is dependent on the pre-load direction, we argue that this dependence makes the short-latency reflex suitable for correcting fast movements for misjudgements of load. 5. At longer latencies (greater than 50 ms) the direction of the force resulting from the reflex, assessed from the EMGs, was almost independent of the direction of the pre-load. In our experiment the force resulting from the long-latency reflex counteracted the perturbations quite well.

Arm↗

PubMed1990

Adjustments of fast goal-directed movements in response to an unexpected inertial load.

Subjects made fast goal-directed elbow flexion movements against an inertial load. Target distance was 8 or 16 cm, randomly chosen. To exert a force in the direction of the movement subjects had to activate flexors of both shoulder and elbow, but shoulder flexors did not change appreciably in length during the movement. In 20% of the trials the inertial load was increased or decreased without knowledge of the subjects. Until 90-110 ms after the onset of the agonist muscle activity (about 65-85 ms after the start of movement) EMG activity was very similar in all conditions tested. The changes that occurred in the EMG from that moment on were effectively a later cessation of the agonist activity and a later start of the antagonist activity if the load was increased unexpectedly. If the load was reduced unexpectedly, the agonist activity ceased earlier and the antagonist activity began earlier. The latency at which EMGs started to change was the same for muscles around shoulder and elbow, for agonists and antagonists and for both distances. All adjustments had the same latency (37 ms) relative to the point where the angular velocity of the elbow in the unexpectedly loaded movements differed by 0.6 rad/s from the expected value. We discuss why simple reflex- or servo-mechanisms cannot account for the measured EMG changes. We conclude that appropriate adjustments of motor programmes for fast goal-directed arm movements start within 40 ms of the detection of misjudgment of load.

Arm↗

PubMed1990

Differences in coordination of elbow flexor muscles in force tasks and in movement tasks.

Motor-unit activity in m. biceps brachii, m. brachialis and m. brachioradialis during isometric contractions has been compared with motor-unit activity during slow voluntary (extension and flexion) movements made against external loads. During these slow movements the recruitment threshold of m. biceps motor units is considerably lower than it is during isometric contractions but the recruitment threshold of both m. brachialis and m. brachioradialis motor units is considerably higher. For all three elbow flexor muscles the motor-unit firing frequency seems to depend on the direction of movement: the firing frequency is higher during flexion movements (3 deg/s) and lower during extension movements (-3 deg/s) than during isometric contractions. The relative contribution of the biceps to the total exerted flexion torque during slow voluntary movements is estimated to increase from 36% to about 48% and that of the brachialis/brachioradialis is estimated to decrease from 57% to about 45% compared to the relative contribution of these muscles during isometric contractions. This difference in the relative contribution of the three major elbow flexor muscles is shown to be caused by differences in the central activation in force tasks and movement tasks.

Elbow↗

PubMed1989

Ocular vergence under natural conditions. I. Continuous changes of target distance along the median plane.

Horizontal binocular eye movements of four subjects were recorded with the scleral sensor coil--revolving magnetic field technique while they fixated a natural target, whose distance was varied in a normally illuminated room. The distance of the target relative to the head of the subject was changed in three ways: (a) the target was moved manually by the experimenter; (b) the target was moved manually by the subject; (c) the target remained stationary while the subject moved his upper torso towards and away from the target. The rate of change of target distance was varied systematically in four levels, ranging from 'slow' to 'very fast', corresponding to changes in target vergence from about 10 degrees s-1 to about 100 degrees s-1. The dynamics of ocular vergence with regard to delay and speed were, under all three conditions, considerably better than could be expected from the literature on ocular vergence induced by disparity and/or blur. When 'very fast' changes in the distance of the target were made, subjects achieved maximum vergence speeds of up to about 100 degrees s-1. Delays of these fast vergence responses were generally smaller than 125 ms. Negative delays, i.e. ocular vergence leading the change in target distance, were observed. The eyes led the target (i.e. predicted target motion) by about 90 ms on average, when the subject used his hand to move the target. Vergence tracking was almost perfect when changes in distance were produced by moving the upper torso. In this condition, the eye led the target by about 5 ms. In the 'slow' and 'medium' conditions (stimulus speeds about 10-40 degrees s-1) tracking was accurate to within 1-2 degrees, irrespective of the way in which the target was moved. In the 'fast' and 'very fast' conditions (stimulus speeds about 40-100 degrees s-1), the accuracy of vergence tracking was better for self-induced than for experimenter-induced target displacements, and accuracy was best during voluntary movements of the upper torso. In the last case, ocular vergence speed was within about 10% of the rate of change of the vergence angle formed by the eyes and the stationary target. The dynamics of convergent and divergent vergence responses varied considerably. These variations were idiosyncratic. They were consistent within, but not between, subjects. Ocular vergence associated with attempted fixation of an imagined target, changing distance in darkness, could only be made by two of the four subjects.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗

PubMed1989

Ocular vergence under natural conditions. II. Gaze shifts between real targets differing in distance and direction.

Horizontal binocular eye movements of three subjects were recorded with the scleral sensor coil--revolving magnetic field technique during voluntary shifts of gaze between pairs of stationary, real, continuously visible targets. The target pairs were located either along the median plane (requiring symmetrical vergence), or on either side of the median plane (requiring asymmetrical vergence). Symmetrical vergence was primarily smooth, but it was often assisted by small, disjunctive saccades. Peak vergence speeds were very high; they increased from about 50 degrees s-1 for vergence changes of 5 degrees to between 150 and 200 degrees s-1 for vergence changes of 34 degrees. Differences between convergence and divergence were idiosyncratic. Asymmetrical vergence, requiring a vergence of 11 degrees combined with a version of 45 degrees, was largely saccadic. Unequal saccades mediated virtually all (95%) of the vergence required in the divergent direction, whereas 75% of the vergence required in the convergent direction was mediated by unequal saccades, with the remaining convergence mediated by smooth vergence, following completion of the saccades. Peak divergence speeds during these saccades were very high (180 degrees s-1 for a change of vergence of 11 degrees); much faster than the smooth, symmetrical vergence change of comparable size (14 degrees). Peak convergent saccadic speeds were about 20% lower. This difference in peak speed was caused by an initial, transient divergence, observed at the beginning of all horizontal saccades. The waveform of disjunctive saccades did not have the same shape as the waveform of conjugate saccades of similar size. The smaller saccade of the disjunctive pair was stretched out in time so as to have the same duration as its larger, companion saccade. These results permitted the conclusion that the subsystems controlling saccades and vergence are not independent. Vergence responses were relatively slow and incomplete with monocular viewing, which excluded disparity as a cue. Monocularly stimulated vergence decreased as a function of the increasing presbyopia of our three subjects. Subjects were able to generate some vergence in darkness towards previously seen and remembered targets. Such responses, however, were slow, irregular and evanescent. In conclusion, vergence shifts between targets, which provided all natural cues to distance, were fast and accurate; they appeared adequate to provide effective binocular vision under natural conditions. This result could not have been expected on the basis of previous observations, all of which had been made with severely reduced cues to depth.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗

PubMed1989

Asymmetrical adaptation of human saccades to anisometropic spectacles.

We report the extent to which effective asymmetrical saccadic adaptation was achieved by a myopic subject, who was exposed to "long-term" adaptation as he wore anisometropic corrective spectacles for about 40 years and also the extent of "short-term" adaptation in this subject and two other subjects, who initially made conjugate saccades, when they wore newly fitted anisometropic spectacles for about 8 hr. Two-dimensional binocular eye positions were measured with an accurate and precise revolving magnetic field-sensor coil technique. We found that long-term adaptation of vertical saccades was virtually perfect (almost 100% of the asymmetry introduced by the spectacles was corrected). Long-term adaptation of horizontal saccades was less complete and increased with target separation from about 40% for saccadic amplitudes of 5 degrees to about 75% for amplitudes of 60 degrees. Short-term adaptation of vertical saccades was virtually complete (100%) in one newly fitted subject and only partially complete (40%) in the other two subjects. The persistence of the adaptive asymmetry of saccades during monocular viewing showed that adaptation derived from plasticity in the programming of saccades and not from modification of vergence responses. Without the anisometropic spectacles, 30 min of self-paced, one per second changes in binocular fixation between two targets, which required a version change of 45 degrees in combination with a vergence change of 11 degrees, did not induce any asymmetrical adaptation. This result shows that a specific repeated association of version and vergence eye movements was not sufficient to induce asymmetrical adaptation, leading us to suggest that the transient fixation disparities at saccade-offset might be the necessary stimulus for the asymmetrical saccadic adaptation we observed.

Adaptation, Physiological↗

PubMed1988

Fusional limits for a large random-dot stereogram.

Fusional limits of absolute retinal disparity between the two half-images of a large random-dot stereogram have been measured. The stereogram was viewed with disparity clamped at selected values (vergence loop opened), but without stabilization against conjugate eye movements, allowing the subject to look freely to different parts of the stereogram. Movements of both eyes were measured with a scleral coil technique. Limits for acquisition and retention of fusion were similar for the large stereogram. Total fusional ranges between 128 and 175 min arc were found in the different subjects. Limits for acquisition of fusion were smaller when fusion was preceded by prolonged stimulation with a large disparity (exceeding the fusional range) of the same direction (crossed or uncrossed). Thus, hysteresis between acquisition and retention was absent and the hysteresis present between loss of fusion and refusion was due to a reduction of the refusional limit. Probably this reduction is related to the recent history of binocular rivalry. Inspection of ocular vergence movements made during stimulation with constant or slowly changing disparity shows that neural remapping of retinal correspondence is rather unlikely.

Convergence, Ocular↗

PubMed1988

Binocular co-ordination of human horizontal saccadic eye movements.

1. The binocular co-ordination of human horizontal saccades was analysed for the first time systematically over the full oculomotor range with a precise and accurate scleral sensor coil technique. Effects of amplitude (1.25-80 deg), direction (adduction vs. abduction and centrifugal vs. centripetal) and eccentricity (symmetrical about primary or between primary and secondary positions) were systematically investigated in three subjects). 2. To minimize extraneous effects of stimulus presentation on the programming of saccades, subjects were instructed to voluntarily change their gaze between two continuously visible targets. These were positioned on an iso-vergence locus, and thus contained no stimulus for disjunctive eye movements. 3. Under these conditions the amplitudes of the primary saccades of the two eyes were remarkably accurate; undershooting of the target by about 0.5 deg (independent of amplitude in the range 10-70 deg) was typical. This finding contrasts with the undershooting by about 10% described in the literature as characteristic for other stimulus conditions. 4. Saccadic peak velocities saturated at a mean asymptotic level of 502 +/- 32 (S.D.) deg/s for saccades of 40 deg and larger. The duration was linearly related to amplitude for saccades up to 50 deg; for saccades of larger sizes the duration increased progressively more steeply. Skewness values (acceleration time as a fraction of total saccadic duration) decreased from about 0.45 for saccades up to 10 deg to about 0.20 for saccades of 50 deg and larger. 5. Binocular saccades showed an abduction-adduction asymmetry and were not well yoked dynamically. The saccades of the abducting eye consistently had a larger size, a higher peak velocity, a shorter duration and were more skewed than the concomitant adducting saccades of the fellow eye. As a result, the eyes diverged transiently by as much as 3 deg during horizontal saccades. 6. Saccades also showed a marked centrifugal-centripetal asymmetry. Peak velocities of saccades towards the primary position were about 10% higher than peak velocities of corresponding centrifugal saccades. 7. These directional asymmetries were the main source of variability in the pool of saccades. In comparison, intra- and intersubject variability was minor in our sample. 8. Post-saccadic drift consisted of a vergence and a version component. The vergence component of this drift was a continuation of the vergence movement occurring during saccades. The version component, generally smaller than the vergence component, was directed towards the target position.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗

PubMed1988

Binocular co-ordination of human vertical saccadic eye movements.

1. The binocular co-ordination of human vertical saccades was analysed systematically over the full oculomotor range, with a precise and accurate scleral sensor coil technique. Effects of amplitude (1.25-70 deg), direction (upward vs. downward and centripetal vs. centrifugal), as well as position (upper or lower sector of vertical oculomotor range), were investigated systematically in three subjects. 2. All saccades were made voluntarily between continuously presented pairs of targets, which subtended equal angles of target vergence. 3. Vertical saccades were less accurate than horizontal saccades (as described by Collewijn, Erkelens & Steinman, 1988). For target distances between 10 and 70 deg, upward saccades undershot the target by about 10%, whereas downward saccades tended to overshoot the target. Downward saccades were about 1.5 deg larger than upward saccades between the same targets. 4. Peak velocities continued to increase monotonically with saccadic amplitude up to 513 +/- 27 (S.D.) deg/s for 70 deg saccades; a distinct asymptotic level was not reached. 5. Velocity profiles of upward and downward saccades, made symmetrically about the primary (straight-ahead) position, were very similar for amplitudes up to 30 deg. At larger amplitudes, velocity profiles of upward saccades remained single peaked, whereas those of downward saccades invariably developed a second velocity peak. 6. Parameters of upward saccades depended heavily on the position of the eye. In the upper oculomotor range such saccades had lower maximum speeds, longer durations, and were more skewed than similar saccades in the lower oculomotor range (below primary). Downward saccades were almost independent of eye position. 7. Vertical eye movements during vertical saccades were virtually identical in the two eyes. In contrast, disjunctive horizontal components were systematically present. Upward saccades, at all amplitudes, were associated with diverging eye movements. Converging eye movements occurred during downward saccades. These systematic effects suggest that the vergence subsystem is not turned off during saccades. 8. These changes in vergence were followed by converging horizontal post-saccadic drift after upward saccades, and in diverging horizontal drift after downward saccades.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗

PubMed1987

Adaptation of ocular vergence to stimulation with large disparities.

Ocular vergence movements were measured with a scleral coil technique under stabilized viewing conditions for disparity. Crossed disparity steps, ranging between 0.25 and 10 deg, of three different targets were imposed. Ocular vergence responses consisted of converging movements with an initially constant velocity. This velocity increased with the magnitude of disparity up to about 4 deg and decreased for larger disparities. For disparities up to 2 deg the responses saturated at the limit of convergence. For larger disparities responses were transient, i.e. after large converging movements the angle of convergence gradually declined to about its initial value. For disparities larger than 5 deg amplitudes of the transient responses decreased and occasionally responses were completely absent. The transient character of responses was apparently due to adaptation of the vergence system to a specific disparity, since responses to different disparities could still be induced. Probing of the vergence system with two successive disparity steps of different magnitudes showed that adaptation was selective for a limited range of disparities around the adapting disparity stimulus. Stabilized disparity pulses with durations ranging from 100 to 800 ms induced ocular vergence movements following the time integral of disparity rather than momentary disparity. This indicated that the part of the vergence system sensitive to disparity has mainly integrative properties for large as well as small disparities. Stimulation with trains of shortlasting disparity pulses showed that adaptation also occurred under normal viewing conditions. They further revealed that adaptation is erased during a short period after each blink.

Adaptation, Ocular↗

PubMed1986

Human ocular vergence movements induced by changing size and disparity.

Human subjects viewed an electronically generated bright square. Horizontal movements of the two eyes were recorded with the scleral coil method. The dynamic properties of vergence movements induced by movement of the bright square were investigated for the following three kinds of stimulus motion: (a) both the size and the binocular disparity of the square changed together, in such a way as to exactly mimic the retinal image changes produced by a real object's motion in depth; (b) the changing-size component in (a) was present with no disparity component; (c) the changing-disparity component in (a) was present with no size component. The gain and phase of the ocular vergence responses to these three stimuli were computed. Ocular vergence movements were induced by changing size in all five subjects. Responses during binocular viewing were higher and less variable than responses during monocular viewing. Size oscillations induced ocular vergence oscillations with a phase lead of up to 65 deg relative to target size for frequencies of stimulation below 1.0 Hz. Vergence oscillation amplitudes were of the order of 10 min of arc and maximal for frequencies of 0.4-0.7 Hz. Ocular vergence movements were not induced by changes in target size in one dimension nor by flickering a stationary square. Ocular vergence movements induced by size changes were entirely transient with no sustained component: vergence responses to disparity were sustained. When the stimulus combined size change with disparity change in the ratio characteristic of a real moving object, vergence tracking was more accurate and less noisy than when the eyes were stimulated with the disparity component alone. The ocular vergence response induced by the combination of size change with disparity change was accurately predicted by linearly adding the vergence response produced by the size change alone to the vergence response produced by the disparity change alone: combined stimulation produced no evidence of non-linear interaction between responses to size change and to disparity change. The properties of vergence responses induced by changing size and by changing disparity showed several close correlations with the corresponding data on psychophysical sensitivity for motion-in-depth sensation. We suggest that responses to changing size contribute to the accuracy with which ocular vergence tracks real objects moving in depth.

Adult↗

PubMed1986

Visual field defects for vergence eye movements and for stereomotion perception.

An objective visual field can be mapped in terms of stimulus-induced eye movement. The authors used the scleral coil technique to record vergence and conjugate eye movements while stimulating different visual field locations with a 3 X 3 deg target whose image vergence was oscillated. For each of three subjects tested there was a visual field location where vergence eye movements were much weaker than in a control location of equal retinal eccentricity. On the other hand, conjugate eye movements driven from these two locations by lateral motion were similar. Field defects for ocular vergence coincided with regions in which oscillating retinal disparity failed to produce a sensation of motion in depth, although visual responses to static disparity were normal, and psychophysical thresholds for lateral motion showed no defect with either binocular or monocular viewing. It was concluded, therefore, that the perceptual stereomotion scotomata were not due to a monocular loss, but to a defective binocular interaction between motion signals from the left and right eyes, and that this defective interaction was specific for opposed rather than parallel motion in the two eyes. Furthermore, the visual loss was specific for motion rather than for position. The correlation between the field defects for ocular vergence and stereomotion perception leads the authors to suggest that the same defect in binocular interaction is responsible for both the eye movement and sensory abnormalities. Two candidate hypotheses are proposed: one is framed in terms of a single population, and the other in terms of two populations of cortical neurons.

Blindness↗

PubMed1986

Necessary conditions for the perception of motion in depth.

This study investigated the relation between the perception of motion in depth and ocular vergence movements for a single foveally viewed dot, and for a 30 deg X 30 deg pattern of many dots. When the target's disparity was changed, it appeared to move in depth relative to stationary reference marks, but removing the reference marks completely abolished the sensation of motion in depth for the multi-dot target and left only a weak sensation of motion in depth for the single dot target. However, it is not the case that motion-in-depth sensation per se depends on the presence of reference marks; motion in depth generated by changing-size stimulation was unaffected by removing reference marks. Possible explanations for the loss of motion-in-depth sensation include ocular vergence exactly tracked stimulus motion; vergence changes and disparity changes, though unequal, produced equal and opposite motion-in-depth signals; vergence changes, though producing no motion-in-depth signals, suppressed the signals produced by disparity changes; motion-in-depth sensation requires relative motion. Explanation is rejected because vergence tracking errors were large. Explanation is rejected because vergence changes do not in themselves induce a sensation of motion in depth. Explanation is rejected because motion-in-depth threshold is not affected by vergence changes. Conclusions are as follows. For a single-dot target, visual sensitivity to motion in depth is much higher for changes in relative retinal disparity than for changes in absolute retinal disparity, while for a multi-dot target any residual sensitivity is abolished by an interaction between neighboring coherently moving dots. The authors suggest that the relative velocity elements proposed to explain sensitivity to changing size feed the stereomotion mechanism also.

Depth Perception↗

PubMed1985

Influences of periodontal and mandibular-joint receptors on reflex sensitivity of human jaw-closing muscles.

Reflex sensitivity from stimulating the chin or the dental arch of the mandible and the influences of periodontal and mandibular joint receptors on jaw muscles were investigated by experiments with and without anaesthesia. The findings showed that periodontal receptors exert an inhibitory influence on motoneural excitability of jaw-closing muscles and facilitate alpha-motoneurones of the digastric muscle and that the silent period following a jaw jerk is partly caused by inhibition of alpha-motoneurones of jaw-closing muscles due to activity of periodontal receptors. The ratio of reflex activity in temporal and masseter muscles appeared to depend on the position of the applied stimulus. The mandibular joint receptors appear to play a major part in distributing active muscle-force over these muscles.

Adult↗

PubMed1985

Eye movements and stereopsis during dichoptic viewing of moving random-dot stereograms.

The dynamic properties of the version and vergence system were studied in relation to stereopsis for movements of the whole visual scene. Large random-dot stereograms (30 X 30 deg arc), moving laterally, were viewed dichoptically by human subjects without a fixed visual frame of reference. Sinusoidal movements in counterphase of the two half-images constituting the stereogram induced sinusoidal ocular vergence movements. The gain of vergence depended on the frequency as well as the amplitude of stimulus movement, while the phase lag depended only on the frequency. Fusion and stereopsis were retained up to a maximal velocity of change in relative position of the two half-images between 6 and 13.5 deg/sec. Sinusoidal movement of one half-image while the other one remained stationary induced sinusoidal ocular version as well as vergence movements. For version gains were higher and phase lags were smaller than for vergence. At the retinal level, residual overall binocular disparities between the two half-images up to 2 deg arc were tolerated without loss of stereopsis. The presence of sinusoidally varying overall binocular disparities and ocular vergence movements without perception of motion in depth suggests that these variables are not adequate cues for perception of (change in) depth.

Convergence, Ocular↗