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At least 955 records · Page 53Linked to original sources
PubMed1995

Perceptual consequences of ocular lens overshoot during saccadic eye movements.

In a previous paper we compared eye globe records of saccadic eye movements (recorded with a scleral eye coil) with lens reflection records of the same eye movements (recorded with a dual-Purkinje-image eyetracker); we found evidence for considerable dynamic deviations between the two during and immediately after saccades. We ascribed these deviations to the movements of the eye's lens relative to the optical axis of the eye. This paper quantifies a predicted psychophysical effect of lens displacements during and after saccades. Two small targets, one above the other, were flashed for 2 msec in total darkness, the bottom one exactly at the end of the saccade, the top one 30 msec later. The first target appears deviated horizontally relative to the other, in a direction opposite to the saccade. Magnitude of the relative mislocalization can be up to 0.03 deg for each degree of saccadic eye movement. The result shows that the position of the visual image on the retina is affected both by position of the globe and by deviations of the lens from its normal location.

Adult↗
PubMed1993

Influence of gravity on the eye movement response elicited by periodic lateral linear acceleration.

Periodic linear acceleration elicits eye movements in human beings. This is generally considered to be the result of the otolithic-ocular reflex (OOR). However, otolith organs respond not only to the resultant inertial force caused by head motion, but also to the gravitational force. We investigated the influence of the resultant gravito-inertial vector on the OOR using a linear acceleration sled. Subjects were 10 healthy volunteers. The sled moving back and forth parallel to the lateral head axis stimulated the subjects. We tested each subject in 7 different positions in the pitch plane. Horizontal eye movements with nystagmic patterns were elicited by these stimuli. The results indicate that the responses were larger in the forward tilted positions than in the backward tilted positions. It can be concluded that the horizontal OOR is influenced by the gravity vector. The cause of this phenomenon is still unclear. However, it could be closely related to the nystagmus observed during off vertical axis rotation.

Acceleration↗
PubMed1997

Deficits of gaze stability in multiple axes following unilateral vestibular lesions.

Abnormalities in the vestibulo-ocular reflex (VOR) after unilateral vestibular injury may cause symptomatic gaze instability. We compared five subjects who had unilateral vestibular lesions with normal control subjects. Gaze stability and VOR gain were measured in three axes using scleral magnetic search coils, in light and darkness, testing different planes of rotation (yaw and pitch), types of stimulus (sinusoids from 0.8 to 2.4 Hz, and transient accelerations) and methods of rotation (active and passive). Eye velocity during horizontal tests reached saturation during high-velocity/acceleration ipsilesional rotation. Rapid vertical head movements triggered anomalous torsional rotation of the eyes. Gaze instability was present even during active rotation in the light, resulting in oscillopsia. These abnormal VOR responses are a consequence of saturating nonlinearities, which limit the usefulness of frequency-domain analysis of rotational test data in describing these lesions.

Adaptation, Physiological↗
PubMed1984

Audio-ocular response: saccadic programming.

The eye movements elicited by auditory stimuli--the audio-ocular response (AOR)--differ from those made in response to a visual target. The movements consist of both monosaccadic and multiple saccadic refixations (MSR). In visual refixation, monosaccadic refixations are always accurate; in AOR, they rarely are. In MSR, many strategies were used in the attempt to find the target but they were not always successful. However, final amplitudes of the total refixation were quite accurate in both MSR and monosaccadic refixations. Velocity profiles of the AOR showed such anomalies as discrete decelerations and multiple, closely-spaced saccades. These data suggest that, without visual feedback, the location of acoustic targets is difficult. In the absence of visual afference, when vigilance may be decreased by the lack of arousal, the velocity profiles also became abnormal, even at small amplitudes. Thus, for cockpit warning devices, a combination of auditory and visual indicators should be used.

Acoustic Stimulation↗
PubMed1989

Studies of the horizontal vestibulo-ocular reflex in spaceflight.

Changes in the vestibulo-ocular reflex (VOR) during space flight have been suspected of contributing to space motion sickness. The horizontal VOR was studied in nine subjects on two space shuttle missions. Active unpaced head oscillation at 0.3 Hz was used as the stimulus to examine the gain and phase of the VOR with and without visual input, as well as the visual suppression of the reflex. No statistically significant changes were noted inflight in the gains or phase shifts of the VOR during any test condition, or between space motion sickness susceptible and nonsusceptible populations. Although VOR suppression was unaffected by spaceflight, the space motion sickness-susceptible group tended to exhibit greater error in the suppression than the nonsusceptible group. It is concluded that at this stimulus frequency, VOR gain is unaffected by spaceflight, and any minor individual changes do not seem to contribute to space motion sickness.

Adult↗
PubMed2006

Incremental retinal-defocus theory of myopia development--schematic analysis and computer simulation.

Previous theories of myopia development involved subtle and complex processes such as the sensing and analyzing of chromatic aberration, spherical aberration, spatial gradient of blur, or spatial frequency content of the retinal image, but they have not been able to explain satisfactorily the diverse experimental results reported in the literature. On the other hand, our newly proposed incremental retinal-defocus theory (IRDT) has been able to explain all of these results. This theory is based on a relatively simple and direct mechanism for the regulation of ocular growth. It states that a time-averaged decrease in retinal-image defocus area decreases the rate of release of retinal neuromodulators, which decreases the rate of retinal proteoglycan synthesis with an associated decrease in scleral structural integrity. This increases the rate of scleral growth, and in turn the eye's axial length, which leads to myopia. Our schematic analysis has provided a clear explanation for the eye's ability to grow in the appropriate direction under a wide range of experimental conditions. In addition, the theory has been able to explain how repeated cycles of nearwork-induced transient myopia leads to repeated periods of decreased retinal-image defocus, whose cumulative effect over an extended period of time results in an increase in axial growth that leads to permanent myopia. Thus, this unifying theory forms the basis for understanding the underlying retinal and scleral mechanisms of myopia development.

Animals↗
PubMed1995

Ocular space exploration in the dark and its relation to subjective and objective body orientation in neglect patients with parietal lesions.

Eye movements of neglect patients with right parietal lesions were recorded during ocular searching for a (non-existent) target in complete darkness. With respect to the objective orientation of the sagittal midplane, ocular exploration was biased toward the ipsilesional side. However, in relation to the patients' subjective localization of the sagittal midplane in space, exploratory eye movements were symmetrically distributed to the subjective "left" and "right" as observed in non-brain-damaged controls. The present results further support the hypothesis that the essential aspect leading to spatial neglect is a disturbance of those cortical structures that are crucial for computing egocentric, body-centred coordinates that allow use to determine our body position in space and that are necessary for visuomotor coordination and exploration of space. In neglect patients the central coordinate transformation seems to work with a systematic error resulting in a deviation of the spatial reference frame to the ipsilesional side. Consequences of this deviation are a displacement of subjective localization of body orientation and--to the same degree--of the spatial area in which motor behavior (here exploratory eye movements) is executed.

Aged↗
PubMedDate not supplied

Cervico-ocular reflex in the normal adult.

(1) The cervico-ocular reflex (COR) in humans was measured while the subjects (n = 10) stood on a rotatable platform in a dark room with the head fixed by a stationary biteplate. Eye movements were measured in response to active and passive rotations about a vertical axis. (2) The COR gain (i.e., horizontal eye movement/amplitude of body rotation) was as great as 22% at low frequency of body rotation (0.025 Hz). With increasing frequency (e.g., at 0.4 Hz) the gain decreased to about 2%. (3) The phase angle of the eye movement ranged generally between -80 and -240 degrees with an average response around -180 degrees. (4) During active rotation, the COR response was similar to the responses measured during passive body rotation. (5) The principal conclusion drawn is that in normal adult humans the COR does not aid in stabilization of the image on the retina during passive or active body rotations. A theoretical function for the COR is presented and discussed.

Adult↗
PubMed2007

Eye, head, and body coordination during large gaze shifts in rhesus monkeys: movement kinematics and the influence of posture.

Coordinated movements of the eye, head, and body are used to redirect the axis of gaze between objects of interest. However, previous studies of eye-head gaze shifts in head-unrestrained primates generally assumed the contribution of body movement to be negligible. Here we characterized eye-head-body coordination during horizontal gaze shifts made by trained rhesus monkeys to visual targets while they sat upright in a standard primate chair and assumed a more natural sitting posture in a custom-designed chair. In both postures, gaze shifts were characterized by the sequential onset of eye, head, and body movements, which could be described by predictable relationships. Body motion made a small but significant contribution to gaze shifts that were > or =40 degrees in amplitude. Furthermore, as gaze shift amplitude increased (40-120 degrees ), body contribution and velocity increased systematically. In contrast, peak eye and head velocities plateaued at velocities of approximately 250-300 degrees /s, and the rotation of the eye-in-orbit and head-on-body remained well within the physical limits of ocular and neck motility during large gaze shifts, saturating at approximately 35 and 60 degrees , respectively. Gaze shifts initiated with the eye more contralateral in the orbit were accompanied by smaller body as well as head movement amplitudes and velocities were greater when monkeys were seated in the more natural body posture. Taken together, our findings show that body movement makes a predictable contribution to gaze shifts that is systematically influenced by factors such as orbital position and posture. We conclude that body movements are part of a coordinated series of motor events that are used to voluntarily reorient gaze and that these movements can be significant even in a typical laboratory setting. Our results emphasize the need for caution in the interpretation of data from neurophysiological studies of the control of saccadic eye movements and/or eye-head gaze shifts because single neurons can code motor commands to move the body as well as the head and eyes.

Animals↗
PubMedDate not supplied

Limitations of pupil tracking in refractive surgery: systematic error in determination of corneal locations.

PURPOSE: The goal of this investigation was to show the theoretical limitations of pupil tracking in refractive surgery. The parallax error associated with localizing corneal positions by tracking the subjacent entrance pupil center was quantified. METHODS: Optical ray-tracing in a schematic model eye was performed to determine the geometric parallax error. The calculations required several assumptions regarding ocular geometry, eye movements, and eye tracker position. Various parameter combinations were evaluated to assess the potential range of error to be expected in clinical practice. RESULTS: Tracking error can amount to 30% (or more for eye trackers mounted closer than 500 mm to the eye) of the detected lateral shift. Thus, if the eye tracker registers a lateral shift of the entrance pupil of 0.2 mm away from the tracking reference axis, the point of interest located on the cornea would essentially be 0.26 mm away from this reference axis. A laser pulse fired at that moment would be systematically displaced by 60 microm. Our results depended on geometric parameters of the eye and the tracking device. Based on conservative assumptions regarding these geometric parameters, partial compensation could be realized by adding a certain percentage to the modulus of each eye tracker reading. CONCLUSIONS: The fact that corneal displacement was generally underestimated by up to 30% of the measured entrance pupil shift demonstrates the severity of the parallax effect.

Cornea↗
PubMed2004

Adaptations in horizontal head stabilization in response to altered vision and gaze during natural walking.

The purpose of this study was to determine adaptations in head stability resulting from altered gaze control and vision during over-ground walking. Using over-ground walking permitted adaptations in walking velocity and cadence that are otherwise not possible during treadmill walking or walking-in-place. Gaze control and vision were manipulated by having 20 young adult subjects 1) walk naturally, 2) view a distant, earth-fixed target to enhance the vestibulo-ocular reflex (VOR), 3) view a head-fixed target to suppress the VOR, and 4) walk in darkness. Horizontal head and trunk angular velocities in space, walking velocity and cadence were measured. Root-mean-square head and trunk angular velocities were calculated and frequency analyses determined head-trunk movement patterns. Results demonstrated that when given the opportunity, subjects slowed down and decreased cadence in response to challenging tasks. Despite strongly reduced walking velocity and cadence, walking in darkness proved most challenging for head stabilization, indicating the importance of vision during this process. Viewing the earth-fixed target demonstrated the greatest head stability thereby, facilitating gaze stabilization. However, comparisons between the earth-fixed and head-fixed target conditions suggest a reciprocal relationship where gaze stability also facilitates head stability. This contribution of gaze stability to head stability is more important than vision alone as the head stabilization response was diminished during the VOR suppressed condition.

Adaptation, Physiological↗
PubMed1976

Clinically relevant physiology of the vestibulo-ocular reflex.

This review attempts to explain those aspects of the physiology of the vestibulo-ocular reflex (VOR) which could be of future clinical value. The literature cited has been selected for its didactic worth to readers with limited time, preferring concise reviews to detailed reports wherever possible. Physiological data provide the background for the following possible improvements in clinical diagnosis: 1) In gaze analysis, coordination of the VOR with other motor patterns can be analyzed. 2) Precision of vestibular tests can be improved by selecting stimuli within the range of natural movements.3) Resolution of the caloric test can be imporved when the change of temperature at the semicircular canal mimics endolymph pressure changes during natural movements. 4) A direct test of the three neuron VOR pathways is possible, but not practicable. 5) Integration of the input signal (transformation from head acceleration to eye position information) can be directly tested. 6) Plasticity (the adaptation to visual requirements) of the VOR can be examined. 7) It is possible to quantify vestibular damage and detect the side of lesion in one test analyzing gain and binocular symmetry of the vertical VOR.

Cerebellum↗
PubMed1993

Ocular torsion and perceived vertical in oculomotor, trochlear and abducens nerve palsies.

Ocular torsion (OT) and subjective visual vertical (SVV) were determined in acute and chronic oculomotor (n = 6), trochlear (n = 21) and abducens (n = 7) palsies separately for each eye in the primary position with the head upright. Ocular torsion measured by fundus photographs was not only within normal range in all abducens palsies, but unexpectedly also in 68% of third and fourth nerve palsies which involve oblique eye muscles. Pathological OT, when measurable, was slight (2 degrees - 8 degrees), monocular and occurred either in the paretic or in the nonparetic eye. Subjective visual vertical tilts were more frequent (67% of third and fourth nerve palsies) although mostly small in amplitude (1 degree - 6 degrees). They were confined either to the paretic or the nonparetic eye depending on the duration of the palsy. Determinations of SVV were always normal under binocular viewing conditions. The dissociated occurrence of OT and SVV tilts in the paretic or the nonparetic eye was dependent on the acuteness of the palsy and reflected sensory and/or motor compensation mechanisms. Third and fourth nerve palsies cause only minor and unpredictable monocular OT and SVV tilts as distinct from the frequent binocular and conjugate tilts seen in patients with acute unilateral brainstem lesions.

Abducens Nerve↗
PubMed2000

Adaptation of the phase of the human linear vestibulo-ocular reflex (LVOR) and effects on the oculomotor neural integrator.

The phase of the translational linear VOR (LVOR) can be adaptively modified by exposure to a visual-vestibular mismatch. We extend here our earlier work on LVOR phase adaptation, and discuss the role of the oculomotor neural integrator. Ten subjects were oscillated laterally at 0.5 Hz, 0.3 g peak acceleration, while sitting upright on a linear sled. LVOR was assessed before and after adaptation with subjects tracking the remembered location of a target at 1 m in the dark. Phase and gain were measured by fitting sine waves to the desaccaded eye movements, and comparing sled and eye position. To adapt LVOR phase, the subject viewed a computer-generated stereoscopic visual display, at a virtual distance of 1 m, that moved so as to require either a phase lead or a phase lag of 53 deg. Adaptation lasted 20 min, during which subjects were oscillated at 0.5 Hz/0.3 g. Four of five subjects produced an adaptive change in the lag condition (range 4-45 deg), and each of five produced a change in the lead condition (range 19-56 deg), as requested. Changes in drift on eccentric gaze suggest that the oculomotor velocity-to-position integrator may be involved in the phase changes.

Adaptation, Physiological↗
PubMed1997

Gaze stabilization by optokinetic reflex (OKR) and vestibulo-ocular reflex (VOR) during active head rotation in man.

Vestibulo-ocular reflex (VOR)-optokinetic reflex (OKR) interaction was studied in normal human subjects during active sine-like head movements in the horizontal plane for a variety of vestibular-optokinetic stimulus combinations (frequency range, 0.05-1.6 Hz). At low to mid frequencies (< 0.2 Hz) the eyes tended to be stabilized on the optokinetic pattern, independently of whether the head, the pattern, or both were rotated. At higher frequencies, the OKR gain was attenuated and, in each of the differing stimulus combinations, the eyes became increasingly stabilized in space. Qualitatively similar results were obtained when, for the same visual-vestibular combinations, the head was passively rotated at 0.05 and 0.8 Hz. The data could be simulated by a model which assumes a linear interaction of vestibular and optokinetic signals. It considers the OKR with its negative feedback loop of primordial importance for image stabilization on the retina and the VOR only as a useful addition which compensates for the limited bandwidth of the OKR during high frequency/velocity head rotations in a stationary visual environment.

Adult↗
PubMedDate not supplied

Eye movements during active head turning with different vestibular and cervical input.

Eye movements were measured in 15 volunteers during vestibulo-ocular reflex (VOR), cervico-ocular reflex with the head fixed from the ceiling (passive COR), during voluntary stabilization of the head in space while the trunk was moved sinusoidally (active COR) and active head movements with and without additional vestibular or cervical stimuli. The subjects were sitting with eyes covered on a rotating chair swinging sinusoidally at 40 degrees peak to peak amplitude at 0.05, 0.1 and 0.2 Hz. The saccadic activity during passive COR is below the VOR and increases slightly during active COR. During voluntary head movements it shows a marked increase and is further activated if cervical or vestibular stimuli are added. The amplitudes of eye shifts of passive and active COR are not different. During active head movements and more with additional cervical or vestibular input, they increase significantly. The phase of the maximum eye shifts to head position is anticompensatory during passive COR and compensatory during VOR. The phase lead of about 45 degrees during active head movements is less during active COR but is larger with additional cervical and vestibular stimuli reaching 90 degrees.

Adult↗
PubMed1994

Effects of oxazepam on eye movements and performance in vigilance tasks with static and dynamic stimuli.

The aim of the present study was to determine whether in a task with stimuli inducing frequent saccadic eye movements, ingestion of oxazepam impairs performance more than in a task in which the stimuli remained fixed at the same location, due to effects of oxazepam on the ocular system. Eighteen males performed a vigilance task with static and dynamic stimuli under the influence of oxazepam (20 and 40 mg) in a placebo-controlled, double blind, crossover design. Oxazepam (40 mg) had a larger effect on vigilance performance in the first part of the dynamic task, relative to its static counterpart. Oxazepam also had an effect on oculomotor behavior, but this effect was unrelated to impaired performance. There were dose-dependent effects of oxazepam on absolute, overall level of performance but not on the decrement with time. The non-dose-dependent aggravation of the decrement in correct detections, caused by the drug, could only partly be accounted for by pharmacokinetics and increased eyelid closures, and was also caused by pharmacodynamic effects of the drug, such as those on attention. Different effects were noted for the two signal detection measures of response behavior, B" and RI.

Adult↗
PubMed1989

Factors affecting the predictability of pseudo-random motion stimuli in the pursuit reflex of man.

1. Experiments have been performed on human subjects to determine the principal mechanisms underlying the break-down in performance during ocular pursuit of pseudo-random target motion stimuli composed of a mixture of two, four or six sinusoids. As observed in a previous experiment there was a reduction in the ratio of eye velocity to target velocity (eye velocity gain) for lower-frequency components of the stimulus whenever the highest frequency exceeded 0.4 Hz, but the following effects were also observed. 2. Using a combination of four sinusoids in which the three lowest frequencies (0.11, 0.24 and 0.37 Hz) had a constant peak velocity (3 or 6 deg/s) it was shown that an increase in the velocity of the highest frequency (0.78 or 1.56 Hz) caused a progressive decline in gain of the low frequencies and a significant reduction in phase lag for the highest-frequency component. 3. Using a combination of two sinusoids (0.44 and 1.56 Hz), in which the peak velocity was varied over a wide range (4-32 deg/s), it was shown that the reduction in low-frequency gain was dependent on the velocity ratio between the frequency components rather than their absolute velocity. 4. Experiments using a combination of either four or six sinusoids in which the two highest frequencies were very close have revealed a true enhancement in the gain of the highest-frequency component in relation to other frequency components of the stimulus. 5. In the same experiments the phase relationships in the response were shown to vary according to the frequency range of the stimulus in such a way that phase advance was normally present at the lowest frequency even when this ranged up to 0.89 Hz. 6. When the oculomotor system was passively stimulated by allowing the subject to fixate a tachistoscopically illuminated stationary target, pseudo-random target motion induced a response which exhibited characteristics similar to those of active pursuit; that is, enhancement of the gain of the highest frequency and phase advance at the lowest frequency. 7. During passive stimulation the changes in gain of the low frequencies with increasing frequency of the highest-frequency component were not consistent with those of active pursuit. However, increasing the velocity of the highest-frequency component to simulate the retinal velocity error conditions of normal active pursuit caused a significant decrease in low-frequency gain and a subjective effect of high-frequency dominance similar to that observed during active pursuit.(ABSTRACT TRUNCATED AT 400 WORDS)

Eye Movements↗