Oscillatory neuronal synchronization in primary visual cortex as a correlate of stimulus selection.
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Spike and local field potential activity were recorded simultaneously from multiple sites in primary visual cortex of strabismic cats, while monocular stimulation alternated with dichoptic stimulation, inducing interocular rivalry. During interocular rivalry, there is competition between the two nonfusible stimuli presented to the two eyes, and only one stimulus is selected at any time. We biased this competition in three different ways: (1) we exploited the condition that in strabismic cats there is often one dominant eye that is selected for most of the time. (2) We presented the two stimuli with a temporal offset, which biases competition in favor of the newly appearing stimulus. (3) We presented the two stimuli with highly different contrasts, which biases competition in favor of the stimulus with higher contrast. Whenever competition was biased in favor of the stimulus activating the recorded neurons, gamma-frequency synchronization of the respective responses was enhanced, and vice versa. Firing rates showed some differences between stimulation conditions. However, when present, these changes were inversely related to a competitive advantage of the respective stimulus. We hypothesize that enhanced gamma-frequency synchronization in primary visual cortex is a correlate of stimulus selection. Synchronization is likely to be translated into firing rate changes at later processing stages. (+info)
The latency of saccades, vergence, and combined eye movements in children and in adults.
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PURPOSE: To examine the latency of eye movements in three-dimensional space (saccade, vergence, and combined saccade-vergence) in children and adults. METHODS: Fifteen normal children (4.5-12 years of age) and 15 normal adults (22-44 years of age) were tested. A standard paradigm was used to elicit pure lateral saccades at far and close viewing distance, pure vergence (convergence and divergence), and saccade combined with vergence movements. Horizontal eye movements from both eyes were recorded simultaneously by the oculometer, a photoelectric device. RESULTS: The mean latency in saccades, vergence, and combined eye movements was longer in children than in adults; the variability of such latency values was also larger in children. There was a progressive decrease with age in mean latency. All latencies approached or reached adult levels at approximately 10 to 12 years of age. Latency of saccades at close viewing distance was shorter than that at far in both adults and children. Convergence latency was longer than divergence latency in adults and most of the children. Latency of components of combined movements was longer than that of corresponding pure movements. Children initiated combined movements by triggering the vergence component first, whereas adults did not show a dominant pattern. The percentage of synchronous start of the two components was significantly higher in adults. CONCLUSIONS: Saccade- and vergence-triggering mechanisms are distinct and mature progressively and in parallel with age. The capacity for synchronization of the two components of combined eye movements develops more slowly and remains below adult level, even at the age of 12 years. (+info)
Effects of voluntary blinks on saccades, vergence eye movements, and saccade-vergence interactions in humans.
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Blinks are known to change the kinematic properties of horizontal saccades, probably by influencing the saccadic premotor circuit. The neuronal basis of this effect could be explained by changes in the activity of omnipause neurons in the nucleus raphe interpositus or in the saccade-related burst neurons of the superior colliculus. Omnipause neurons cease discharge during both saccades and vergence movements. Because eyelid blinks can influence both sets of neurons, we hypothesized that blinks would influence the kinematic parameters of saccades in all directions, vergence, and saccade-vergence interactions. To test this hypothesis, we investigated binocular eye and lid movements in five normal healthy subjects with the magnetic search coil technique. The subjects performed conjugate horizontal and vertical saccades from gaze straight ahead to targets at 20 degrees up, down, right, or left while either attempting not to blink or voluntarily blinking. While following the same blink instruction, subjects made horizontal vergence eye movements of 7 degrees and combined saccade-vergence movements with a version amplitude of 20 degrees. The movements were performed back and forth from two targets simultaneously presented nearby (38 cm) and more distant (145 cm). Small vertical saccades accompanied most vergence movements. These results show that blinks change the kinematics (saccade duration, peak velocity, peak acceleration, peak deceleration) of not only horizontal but also of vertical saccades, of horizontal vergence eye movements, and of combined saccade-vergence eye movements. Peak velocity, acceleration, and deceleration of eye movements were decreased on the average by 30%, and their duration increased by 43% on the average when they were accompanied by blinks. The blink effect was time dependent with respect to saccade and vergence onset: the greatest effect occurred 100 ms prior to saccade onset, whereas there was no effect when the blink started after saccade onset. The effects of blinks on saccades and vergence, which are tightly coupled to latency, support the hypothesis that blinks cause profound spatiotemporal perturbations of the eye movements by interfering with the normal saccade/vergence premotor circuits. However, the measured effect may to a certain degree but not exclusively be explained by mechanical interference. (+info)
Motor and sensory characteristics of infantile nystagmus.
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BACKGROUND/AIMS: Past studies have explored some of the associations between particular motor and sensory characteristics and specific categories of non-neurological infantile nystagmus. The purpose of this case study is to extend this body of work significantly by describing the trends and associations found in a database of 224 subjects who have undergone extensive clinical and psychophysical evaluations. METHODS: The records of 224 subjects with infantile nystagmus were examined, where 62% were idiopaths, 28% albinos, and 10% exhibited ocular anomalies. Recorded variables included age, mode of inheritance, birth history, nystagmus presentation, direction of the nystagmus, waveform types, spatial and temporal null zones, head postures and nodding, convergence, foveation, ocular alignment, refractive error, visual acuity, stereoacuity, and oscillopsia. RESULTS: The age distribution of the 224 patients was between 1 month and 71 years, with the mean age and mode being 23 (SD 16) years and 16-20 years respectively. By far the most common pattern of inheritance was found to be autosomal dominant (n = 40), with the nystagmus being observed by the age of 6 months in 87% of the sample (n = 128). 139 (62%) of the 224 subjects were classified as idiopaths, 63 (28%) as albinos, and 22 (10%) exhibited ocular anomalies. Conjugate uniplanar horizontal oscillations were found in 174 (77.7%) of the sample. 32 (14.3%) had a torsional component to their nystagmus. 182 (81.2%) were classed as congenital nystagmus (CN), 32 (14.3%) as manifest latent nystagmus (MLN), and 10 (4.5%) as a CN/MLN hybrid. Neither CN nor MLN waveforms were related to any of the three subject groups (idiopaths, albinos, and ocular anomalies) MLN was found in idiopaths and albinos, but most frequently in the ocular anomaly group. The most common oscillation was a horizontal jerk with extended foveation (n = 49; 27%). The amplitudes and frequencies of the nystagmus ranged between 0.3-15.7 degrees and 0.5-8 Hz, respectively. Periodic alternating nystagmus is commonly found in albinos. Albino subjects did not show a statistically significantly higher nystagmus intensity when compared with the idiopaths (p>0.01). 105 of 143 subjects (73%) had spatial nulls within plus or minus 10 degrees of the primary position although 98 subjects (69%) employed a compensatory head posture. Subjects with spatial null zones at or beyond plus or minus 20 degrees always adopted constant head postures. Head nodding was found in 38 subjects (27% of the sample). Horizontal tropias were very common (133 out of 213; 62.4%) and all but one of the 32 subjects with MLN exhibited a squint. Adult visual acuity is strongly related to the duration and accuracy of the foveation period. Visual acuity and stereoacuity were significantly better (p<0.01) in the idiopaths compared to the albino and ocular anomaly groups. 66 subjects out of a sample of 168 (39%) indicated that they had experienced oscillopsia at some time. CONCLUSIONS: There are strong ocular motor and sensory patterns and associations that can help define an infantile nystagmus. These include the nystagmus being bilateral, conjugate, horizontal uniplanar, and having an accelerating slow phase (that is, CN). Decelerating slow phases (that is, MLN) are frequently associated with strabismus and early form deprivation. Waveform shape (CN or MLN) is not pathognomonic of any of the three subject groups (idiopaths, albinos, or ocular anomalies). There is no one single stand alone ocular motor characteristic that can differentiate a benign form of infantile nystagmus (CN, MLN) from a neurological one. Rather, the clinician must consider a host of clinical features. (+info)
Combined influence of vergence and eye position on three-dimensional vestibulo-ocular reflex in the monkey.
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This study examined two kinematical features of the rotational vestibulo-ocular reflex (VOR) of the monkey in near vision. First, is there an effect of eye position on the axes of eye rotation during yaw, pitch and roll head rotations when the eyes are converged to fixate near targets? Second, do the three-dimensional positions of the left and right eye during yaw and roll head rotations obey the binocular extension of Listing's law (L2), showing eye position planes that rotate temporally by a quarter as far as the angle of horizontal vergence? Animals fixated near visual targets requiring 17 or 8.5 degrees vergence and placed at straight ahead, 20 degrees up, down, left, or right during yaw, pitch, and roll head rotations at 1 Hz. The 17 degrees vergence experiments were performed both with and without a structured visual background, the 8.5 degrees vergence experiments with a visual background only. A 40 degrees horizontal change in eye position never influenced the axis of eye rotation produced by the VOR during pitch head rotation. Eye position did not affect the VOR eye rotation axes, which stayed aligned with the yaw and roll head rotation axes, when torsional gain was high. If torsional gain was low, eccentric eye positions produced yaw and roll VOR eye rotation axes that tilted somewhat in the directions predicted by Listing's law, i.e., with or opposite to gaze during yaw or roll. These findings were seen in both visual conditions and in both vergence experiments. During yaw and roll head rotations with a 40 degrees vertical change in gaze, torsional eye position followed on average the prediction of L2: the left eye showed counterclockwise (ex-) torsion in down gaze and clockwise (in-) torsion in up gaze and vice versa for the right eye. In other words, the left and right eye's position plane rotated temporally by about a quarter of the horizontal vergence angle. Our results indicate that torsional gain is the central mechanism by which the brain adjusts the retinal image stabilizing function of the VOR both in far and near vision and the three dimensional eye positions during yaw and roll head rotations in near vision follow on average the predictions of L2, a kinematic pattern that is maintained by the saccadic/quick phase system. (+info)
Compensatory and orienting eye movements induced by off-vertical axis rotation (OVAR) in monkeys.
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Nystagmus induced by off-vertical axis rotation (OVAR) about a head yaw axis is composed of a yaw bias velocity and modulations in eye position and velocity as the head changes orientation relative to gravity. The bias velocity is dependent on the tilt of the rotational axis relative to gravity and angular head velocity. For axis tilts <15 degrees, bias velocities increased monotonically with increases in the magnitude of the projected gravity vector onto the horizontal plane of the head. For tilts of 15-90 degrees, bias velocity was independent of tilt angle, increasing linearly as a function of head velocity with gains of 0.7-0.8, up to the saturation level of velocity storage. Asymmetries in OVAR bias velocity and asymmetries in the dominant time constant of the angular vestibuloocular reflex (aVOR) covaried and both were reduced by administration of baclofen, a GABA(B) agonist. Modulations in pitch and roll eye positions were in phase with nose-down and side-down head positions, respectively. Changes in roll eye position were produced mainly by slow movements, whereas vertical eye position changes were characterized by slow eye movements and saccades. Oscillations in vertical and roll eye velocities led their respective position changes by approximately 90 degrees, close to an ideal differentiation, suggesting that these modulations were due to activation of the orienting component of the linear vestibuloocular reflex (lVOR). The beating field of the horizontal nystagmus shifted the eyes 6.3 degrees /g toward gravity in side down position, similar to the deviations observed during static roll tilt (7.0 degrees /g). This demonstrates that the eyes also orient to gravity in yaw. Phases of horizontal eye velocity clustered ~180 degrees relative to the modulation in beating field and were not simply differentiations of changes in eye position. Contributions of orientating and compensatory components of the lVOR to the modulation of eye position and velocity were modeled using three components: a novel direct otolith-oculomotor orientation, orientation-based velocity modulation, and changes in velocity storage time constants with head position re gravity. Time constants were obtained from optokinetic after-nystagmus, a direct representation of velocity storage. When the orienting lVOR was combined with models of the compensatory lVOR and velocity estimator from sequential otolith activation to generate the bias component, the model accurately predicted eye position and velocity in three dimensions. These data support the postulates that OVAR generates compensatory eye velocity through activation of velocity storage and that oscillatory components arise predominantly through lVOR orientation mechanisms. (+info)
Development of the response AC/A ratio over the first year of life.
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This study investigated the development of the link from accommodation to vergence in infants by occluding one eye thus removing binocular cues. Occluded adults continue to converge partially demonstrating that the accommodative drive to vergence (the AC/A link) and proximal cues are sufficient to drive vergence. For infants of all ages, AC/A ratios were found to be in the normal adult range. We conclude that infants can use monocular cues to drive vergence and that this occurs before the age when there is a substantial increase in the accuracy of oculomotor processes. There is flexibility in the developing visual system which is able to produce early vergence responses by relying upon alternative cues. (+info)
Coordinating one hand with two eyes: optimizing for field of view in a pointing task.
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We previously found that subjects switched 'ocular dominance' as a function of horizontal gaze direction in a reaching task [Vision Res. 41 (14) (2001) 1743]. Here we extend these findings to show that when subjects pointed to targets across the horizontal binocular field, they aligned the fingertip with a vertical plane located between the eyes and the target. This eye-target plane gradually shifted from aligning with the left eye (leftward targets) to between the two eyes (intermediate targets) to the right eye (rightward targets). We suggest that this occurs to optimize eye-hand alignment towards the eye with the best overall field of view. (+info)