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PubMed1980

The perceived spatial frequency, contrast, and orientation of illusory gratings.

Illusory vertical gratings (V) and diagonal gratings (D) can be seen on a uniform field after inspection of a vertical grating. When using simultaneous and successive matching techniques the spatial frequencies of the V effect were found to be about 2 octaves below and 1-2 octaves above the adapting spatial frequency, but to be invariant with temporal frequency. At high adapting frequencies the D effect dominated, and was about 0.8 octave below the adapting spatial frequency, oriented about +/-35 degrees from vertical. The apparent contrast of V was about twice the value of the contrast threshold at its apparent spatial frequency. D effects seen during adaptation were about 60 degrees from vertical and 3 octaves below the adapting frequency. The results are interpreted in terms of inhibition and disinhibition in an organized matrix of tuned channels, and the dominant pattern of inhibition in the matrix is inferred. Supporting evidence from neurophysiology, neuroanatomy, and psychophysics is briefly reviewed. An appendix deals with the question of interocular transfer of the aftereffect.

Adaptation, Ocular↗
PubMed1984

Exorcising the double-nail illusion: giving up the ghost.

The principles of visual direction explain the double-nail illusion, without the concept of a ghost image. Also, the stimulus arrangement for the illusion is not perceptually ambiguous for the visual system as Krol and van de Grind claim.

Convergence, Ocular↗
PubMed1992

Reduced spatial sensitization on nonuniform backgrounds.

Spatial sensitivity (Westheimer) functions, when measured on nonuniform backgrounds made up of light dots of 12 min arc, were found to differ in shape, depending on the polarity of the central area on which the test spot was placed. When thresholds were measured on the dark centre between light dots, ie on the adapting-field illumination, the resulting curve was similar to the control curve, measured on a uniform background equated for flux. In comparison, thresholds measured on a central light dot, serving as a pedestal, peaked at larger background diameters and showed much less sensitization compared to the control function.

Adaptation, Ocular↗
PubMed1980

Accommodative and fusional vergence in anomalous correspondence.

Strabismic subjects who have anomalous correspondence often exhibit covariation in their angle of deviation and angle of anomaly. One hypothesis is that these covariation responses represent a neurological link between binocular correspondence and vergence innervation; an alternative is that they are neurologically independent. A description of these two interpretations is followed by certain implications and predictions.

Accommodation, Ocular↗
PubMed1984

Adaptive changes with prolonged effect of comitant and incomitant vergence disparities.

When comitant and incomitant vergence disparities are induced for prolonged periods of time, the adaptive state becomes more stable. When the change is complete, the adaptive ability of the oculomotor system to a further disparity should be the same as that before any induced effect. Comitant change was investigated in five subjects who wore a 2 delta base-up Fresnel prism for prolonged periods of time (1 to 3 days). Adaptation was periodically monitored with a further 2 delta base-up prism. In three subjects the rate of adaptation to the additional prism approached that which occurred before the experiment. In two subjects, both of whom suffered asthenopic symptoms with the initial disparity, adaptation to the additional prism remained slow and incomplete. Incomitant change was investigated in two subjects who wore a -3.00 contact lens-spectacle lens (CL-SL) system for 2 to 3 days. Adaptation was monitored with a further -1.50 CL-SL system at the start and the end of the prolonged wear. Both these subjects had a much faster rate of adaptation at the end of the prolonged period than they had at the beginning.

Adaptation, Ocular↗
PubMed1988

Long-term endurance of adaptive shifts in tonic accommodation.

After an 8 min inducing period of sustained focusing on a target at the near point (NP) of accommodation, tonic accommodation was monitored at 8 min intervals over a 48 min period in darkness. A laser optometer provided evidence for asymptotically increasing tonic levels. Accordingly, eyes closed in darkness (EC) was not a condition for the relaxation or dissipation of enhanced tonic levels. Furthermore, refixating the NP target for 6 min, 24 min after the initial focusing period, provided no significant enhancement of tonic accommodation over EC. On the other hand, focusing at far point (FP) or walking within a lighted hallway (WH) significantly reduced the tonic after-effect to pre-adaptative levels, but only transiently. Tonic after-effects induced by NP focusing thus appear to be remarkably resistant either to long-term attenuation or enhancement. A model based on the decay of accumulated sympathetic and parasympathetic neurotransmitters was developed to account for the contrasting fact that focusing at near targets for periods up to about 2-3 min as well as sustained focusing at far targets tends to yield complete decay of tonic after-effects.

Accommodation, Ocular↗
PubMed2005

Distance vergence adaptation is abnormal in subjects with convergence insufficiency.

It is well known that in convergence insufficiency (CI) prism adaptation is reduced in response to base-out (BO) prisms at near. There have also been some suggestions in the literature that adaptation is reduced at other distances as well. The present data show that in CI adaptation is not only reduced in response to BO at near, but also in response to base-in (BI) at near and for both BI and BO at distance. This raises the interesting question whether distance adaptation becomes reduced because of reduced near adaptation or whether these subjects have a generally reduced horizontal adaptation mechanism.

Adaptation, 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↗
PubMed2002

Conjugate and vergence oscillations during saccades and gaze shifts: implications for integrated control of binocular movement.

Saccades made between targets at optical infinity require both eyes to rotate by the same angle. Nevertheless, these saccades are consistently accompanied by transient vergence eye movements. Here we have investigated whether the dynamics of these vergence movements depend on the trajectory of the coincident conjugate movement, and whether moving the head during eye-head gaze shifts modifies vergence dynamics. In agreement with previous reports, saccades with more symmetric (i.e., "bell-shaped") conjugate velocity profiles were accompanied by stereotyped biphasic vergence transients (i.e., a divergence phase immediately followed by a convergence phase). However, we found that saccades with more asymmetric, oscillatory-like dynamics (characterized by a typical conjugate reacceleration of the eyes following the initial peak velocity) were systematically accompanied by more complex vergence movements that also exhibited oscillatory-like dynamics. These findings could be extended to conditions where the head was free to move: comparable conjugate and vergence oscillations were observed during head-restrained saccades and combined eye-head gaze shifts. The duration of the vergence oscillation increased with gaze shift amplitude, such that as many as four vergence phases (divergence-convergence-divergence-convergence) were recorded during 55 degrees gaze shifts (approximately 240 ms). To quantify these observations, we first determined whether conjugate and vergence peak velocities were systematically correlated. Conjugate peak velocity was linearly related to the peak velocity of the initial divergence phase for saccades and gaze shifts of all amplitudes, regardless of their dynamics. However, for more asymmetric saccades and gaze shifts, the subsequent convergence and divergence peak velocities were not correlated with either the initial peak conjugate velocity or the peak velocity of the conjugate reacceleration. Next, we determined that the duration of the different conjugate and vergence oscillation phases remained relatively constant across all saccades and gaze shifts, and that the conjugate and vergence profiles oscillated together at approximately 7.5-10 Hz. Using computer simulations, we show that a classic feed-forward model is unable to reproduce vergence oscillations based solely on peripheral mechanisms. Furthermore, we demonstrate that small modifications to the gain and delay of a simple feedback model for saccade generation can generate conjugate oscillations, and propose that such changes reflect the influence of lowered alertness on the tecto-reticular pathways. We conclude that peripheral mechanisms can only account for the initial divergence that accompanies all saccades, and that the conjugate and vergence oscillations observed during asymmetric movements arise centrally from an integrative binocular controller.

Animals↗
PubMed2005

Gaze position corrective eye movements in normal subjects and in patients with vestibular deficits.

Eye movements in response to high-acceleration head rotations (thrusts) in the horizontal plane from patients with unilateral (UVD) or bilateral vestibular loss (BVD) were recorded. The rapid, gaze-position corrections (GPCs) that appeared when vestibulo-ocular reflex (VOR) slow phases were undercompensatory were characterized. For comparison, eye movements from normal subjects who were asked to generate saccades in the direction opposite head rotation (in the same direction as slow phases) were recorded. This normal-subject model produced responses with spatial and temporal characteristics similar to those from GPCs in patients as follows: When head rotations were generated actively, compared with passively, gaze-position errors and corresponding GPCs were smaller and occurred earlier. During passively generated head thrusts, GPCs still occurred when head rotations were made in total darkness, though their accuracy decreased as the requirement for maintaining gaze on a specific location in space was relaxed. Time of onset of GPCs was not rigidly tied to head kinematics (peak velocity or peak acceleration). Speeds of GPCs, however, were lower than speeds of similar-sized, head-fixed saccades. Finally, during passive and active head thrusts in patients, sustained, high-frequency (20 to 30 Hz) oscillations that appeared as tiny saccades were occasionally observed, one immediately following the other, resembling a compensatory slow-phase response. Taken together, the results suggest that one strategy for overcoming a VOR deficit is to enlist the saccadic system to produce an oculomotor response that is required to compensate for head rotation. This response may come in the form of high-velocity GPCs or smaller-amplitude oscillations.

Accommodation, Ocular↗
PubMed1991

[A simple description of color adaptation in the red-green system].

The red/green-system is examined experimentally under adaptation to green and to magenta. The theoretical basis consists of Grassmann's laws and of a new linear opponent colors theory. Besides brightness, this opponent colors theory specifies the two chromatic axes yellow-blue and green-magenta within color space. An axiom is formulated for the description of color adaptation for the red/green-system. Two pairs of color vectors are assumed to be equivalent with respect to the red/green-system if the quotients of the respective red/green-coordinates of test vector and adaptation vector are identical. For both vectors the control of the other two opponent colors systems is presupposed. The axiom is tested for four green and four magenta adaptation vectors. Three observers take part in the experiment. For each observer an individual plane of constant brightness and his or her individual opponent colors axes are estimated experimentally. For both magenta and green adaptation the data agree well with the theoretical predictions. Thus they provide empirical support for the axiom. For the red/green-system color adaptation can be described very simply if the other two opponent colors systems are controlled.

Adaptation, Ocular↗