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A V Van den Berg

Publications and source records attributed to A V Van den Berg.

9 recordsLinked to original sources
PubMed2003

Different visual search strategies in stationary and moving radial patterns.

This study compared visual search strategies in patterns of radially moving dots (simulating self-motion) to those used in matched stationary displays (radial patterns of lines). To control for differences in target visibility, 75% detection thresholds for deviating motion direction and line orientation, respectively, were determined as a function of eccentricity in Experiment 1. These individual thresholds were used to study saccadic parameters in Experiment 2, when subjects searched for targets in the stationary and moving patterns. Despite similarities in search performance, visual search in moving radial patterns was characterised by fewer saccades, longer initial fixation times, and shorter saccadic amplitudes after the initial saccade than during search in a matched stationary radial pattern. These results suggest that detection performance alone cannot explain saccadic search behaviour, and that different search strategies may be used in moving compared to stable environments.

Adult↗
PubMed2002

The timing of sequences of saccades in visual search.

According to the LATER model (linear approach to thresholds with ergodic rate), the latency of a single saccade in response to target appearance can be understood as a decision process, which is subject to (i) variations in the rate of (visual) information processing; and (ii) the threshold for the decision. We tested whether the LATER model can also be applied to the sequences of saccades in a multiple fixation search, during which latencies of second and subsequent saccades are typically shorter than that of the initial saccade. We found that the distributions of the reciprocal latencies for later saccades, unlike those of the first saccade, are highly asymmetrical, much like a gamma distribution. This suggests that the normal distribution of the rate r, which the LATER model assumes, is not appropriate to describe the rate distributions of subsequent saccades in a scanning sequence. By contrast, the gamma distribution is also appropriate to describe the distribution of reciprocal latencies for the first saccade. The change of the gamma distribution parameters as a function of the ordinal number of the saccade suggests a lowering of the threshold for second and later saccades, as well as a reduction in the number of target elements analysed.

Humans↗
PubMed1997

Torsion during saccades between tertiary positions.

We systematically studied the effect of saccade direction and saccade starting position on the velocity profile of the saccade. Saccades were made between targets placed at optical infinity by dichoptic presentation. This arrangement was chosen to evoke conjugate eye movements. Eye movements were recorded binocularly, including torsion. Horizontal and vertical movements of the eyes are strongly correlated (r > or = 0.95) during the saccade, torsional movements are much less so (r approximately 0.67). Listing's law would predict that the three-dimensional versional velocity of the eye would be located in a plane that is tilted out of Listing's plane by an amount that depends on the saccade's starting position (half angle rule). Taking together all saccades that started from the same initial position a plane could be fitted through the velocity vectors. However, this plane was tilted less relative to Listing's plane than predicted by the half angle rule. The deviation was especially large for the yaw component of the tilt (56% of predicted). For the pitch component the prediction was better (81% of predicted). In addition, we find that the torsional velocity during the fast "intrasaccadic" part of the motion can be unequal in the two eyes. The implications for three-dimensional models of saccadic control are discussed.

Humans↗
PubMed1987

A direct test of Listing's law--I. Human ocular torsion measured in static tertiary positions.

The validity of Listing's law was reinvestigated by means of a direct test. Horizontal, vertical and torsional eye movements were measured simultaneously with a recently developed scleral induction coil. Either eye of 4 subjects was measured monocularly. Eye position were measured in Fick coordinates and ocular torsion values were compared to the theoretical ones predicted by Listing's law. During consecutive measurements in the primary position torsion values were close to zero although considerable fluctuations of torsion were seen. Torsion values in the secondary positions were also close to zero. In the tertiary positions torsion in the direction as predicted by Listing's law and increasing with eccentricity was recorded. In the temporal quadrants mean torsion was quantitatively in agreement with Listing's law; torsion values in the nasal quadrants however showed systematically larger values and this discrepancy increased with eccentricity to more than 50%. Statistical support for this finding however, was seen only in 4 out of 8 eyes. Symmetry could be obtained by shifting the chosen horizontal primary position (gaze parallel to the midplane) in the temporal direction; as a consequence all measured torsion values would exceed the ones specified by Listing's law. Torsion values varied idiosyncratically among subjects and among the left and right eyes of any one subject. It is concluded that Listing's law specifies ocular torsion only approximately: physiological eye movements show considerable stochastical as well as systematical deviations from this law.

Eye Movements↗
PubMed1987

A direct test of Listing's law--II. Human ocular torsion measured under dynamic conditions.

Ocular torsion was recorded with a scleral search coil technique in five normal subjects. The dynamic aspects of torsion were investigated during monocular fixation, blinking, smooth pursuit and saccades. Torsion near the primary position showed considerable short-term (SD about 0.25 deg) and a much larger long-term fluctuation (SD about 2.3 deg). During saccades between diagonally opposite tertiary positions torsion transiently reached values approximating those in the sustained primary position. During smooth pursuit across the primary position, the minimal values of torsion varied with the direction and the trajectory of pursuit, in violation of Donder's law. Changes in torsion associated with horizontal and vertical saccades and during the aftermath of blinks often had a sluggish, exponential time course. During eye movements around a circular or square trajectory torsion showed hysteresis. During clockwise pursuit the right eye showed relative intorsion compared to counterclockwise pursuit. It is proposed that central nervous control of torsion is usually imprecise, and that the eye follows Listing's and Donder's laws only approximately.

Blinking↗
PubMed1987

Human gaze stability in the horizontal, vertical and torsional direction during voluntary head movements, evaluated with a three-dimensional scleral induction coil technique.

The stability of gaze in three dimensions (horizontal, vertical and torsion) was measured with a new type of scleral search coil in eight emmetropic observers. Subjects held the head still or oscillated it at 0.16-0.67 Hz (amplitude about 10 deg) in the horizontal, vertical or torsional plane while fixating a point target at optical infinity. Veridical gaze and head coordinates were calculated with full correction for non-linear goniometric relations and for cross-coupling artifacts due to misalignments of the coil on the eye. The amount of gaze instability in the horizontal and vertical direction was virtually identical. With the head still, in either of these directions the mean standard deviation of gaze position (inclusive saccades) was about 7 min arc; mean non-saccadic retinal image speeds were 20-30 min arc/sec. During head oscillation these values increased to about 16 min arc and 1 deg/sec; a mean of about 2.5% of the head motion remained uncorrected by the compensatory eye movements. These findings agree well with our earlier results for the horizontal plane; the effect of the corrections was relatively small because the adventitious cross-coupling of horizontal and vertical to torsional head movements proved to be usually smaller than 10%. However, the corrections were important when head torsion was deliberately produced. Gaze stability in the torsional plane was considerably inferior to that in the horizontal and vertical plane. With the head held still, the mean SD of torsional gaze position was about 17 min arc; mean torsional non-saccadic retinal image speed was about 46 min arc/sec. Gain of the torsional compensatory eye movements was frequency dependent and rose from about 0.26 in static conditions (0 Hz) to about 0.42 at 0.16 Hz and 0.64 at 0.67 Hz. Accordingly, position instability and speed of the retinal image in torsion were about an order of magnitude larger than in the horizontal and vertical direction.

Eye Movements↗
PubMedDate not supplied

Binocular eye orientation during fixations: Listing's law extended to include eye vergence.

Any eye position can be reached from a position called the primary position by rotation about a single axis. Listing's law states that for targets at optical infinity all rotation axes form a plane; the so-called Listing plane. Listing's law is not valid for fixation of nearby targets. To document these deviations of Listing's law we studied binocular eye positions during fixations of point targets in the dark. We tested both symmetric (targets in a sagittal plane) and asymmetric vergence conditions. For upward fixation both eyes showed intorsion relative to the position that would have been taken if each eye followed Listing's law. For downward fixation we found extorsion. The in- or extorsion increased approximately linearly with the vergence angle. The direction of the Listing axis and the turn angle about this axis can be described by rotation vectors. Our observations indicate that for fixation of nearby targets the rotation vectors of the two eyes become different and are no longer located in a single plane. However, we find that it is possible to decomose the rotation vector of each eye into the sum of a symmetric and an anti-symmetric part, each with its own properties. (1) The symmetric part is associated with eye version. This component of the rotation vector is identical for both eyes and lies in Listing's plane. In contrast to the classical form of Listing's law, this part of the rotation vector lies in Listing's plane irrespective of the fixation distance. (2) The anti-symmetric part of the rotation vector is related to eye vergence. This component is of equal magnitude but oppositely directed in each eye. The anti-symmetric part lies in the mid-sagittal plane, also irrespective of fixation distance. For fixation of targets at optical infinity the anti-symmetric part equals zero and the eye positions obey the classical form of Listing's law. Thus, the symmetric and anti-symmetric parts of the rotation vectors are restricted to two perpendicular planes. Combining these restrictions in a model, with the additional restriction that the vertical vergence equals zero during fixation of point targets, we arrive at the prediction that the cyclovergence is proportional to the product of elevation and horizontal vergence angles. This was well born out by the data. The model allows to describe the binocular eye position for fixation of any target position in terms of the bipolar coordinates of the target only (i.e. using only three degrees of freedom instead of the six needed for two eyes).

Eye Movements↗