SUMMARY1. 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 (1X25-80 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 H. COLLEWIJN, C. J. ERKELENS AND R. M. STEINMAN 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. The result of postsaccadic drift was that the fovea of each eye was guided towards the target. The net post-saccadic drift of the abducting eye was smaller than that of the adducting eye, a result appropriate to reduce fixation errors remaining at saccadic offset.9. A tight relationship between skewness and saccade duration was not found. For instance, velocity profiles of centrifugal saccades were more skewed than ...

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Over the past decade several research groups have taken a renewed interest in the special role of a type of small eye movement, called ‘microsaccades’, in various visual processes, such as the activation of neurons in the central nervous system, or the prevention of image fading. As the study of microsaccades and their relation to visual processes goes back at least half a century, it seems appropriate to review the more recent reports in light of the history of research on maintained oculomotor fixation, in general, and on microsaccades in particular. Our review shows that there is no compelling evidence to support the view that microsaccades (or, fixation saccades more generally) serve a necessary role in improving oculomotor control or in keeping the visual world visible. The role of the retinal transients produced by small saccades during fixation needs to be evaluated in the context of both the brisk image motions present during active visual tasks performed by freely moving people, as well as the role of selective attention in modulating the strength of signals throughout the visual field.

Static and dynamic components of ocular counterroll as well as cyclorotatory optokinetic nystagmus were measured with a scleral search coil technique. Static counterroll compensated for about 10% of head roll when the head was tilted to steady positions up to 20 deg from the upright position. The dynamic component of counterroll, which occurs only while the head is moving, is much larger. It consists of smooth compensatory cyclorotation opposite to the head rotation, interrupted frequently by saccades moving in the same direction as the head. During voluntary sinusoidal head roll, cyclorotation compensated from 40% to more than 70% of the head motion. In the range 0.16 to 1.33 Hz, gain increased with frequency and with the amount of visual information. The lowest values were found in darkness. The gain increased in the presence of a visual fixation point and a further rise was induced by a structured visual pattern. Resetting saccades were made more frequently in the dark than in the light. These saccades were somewhat slower than typical horizontal saccades. Cyclorotatory optokinetic nystagmus could be induced by a patterned disk rotating around the visual axis. It was highly variable even within a same subject and had in general a very low gain (mean value about 0.03 for stimulus velocities up to 30 deg/s). It is concluded that cyclorotational slip velocity on the retina is considerably reduced by counterroll during roll of the head, although the residual cyclorotation after the head has reached a steady position is very small.

SUMMARY1. Horizontal and vertical eye movements of ten human subjects were recorded with a scleral induction-coil technique during voluntary pursuit of sinusoidal, triangular and pseudo-random target motions of different frequency, amplitude and dimensionality upon a dark, diffuse or structured background.2. Data processing included separation of the composite eye movement into a cumulative smooth and saccadic displacement, computation of gain and phase of the composite and smooth eye movements with respect to the target movement and analysis of retinal position error.3. Pursuit eye movements were never completely smooth. Smooth pursuit gain was always lower than 0-95 and saccades were used to supplement the smooth eye movements in pursuing the target with the proper amplitude.4. The gain of composite eye movements was about unity for sinusoidal target motions and ramps; it exceeded unity for the highest frequency components in a pseudo-random motion.5. The gain of the smooth eye movements decreased monotonously whenever target velocity increased. It was higher for single sine waves than for a pseudo-random motion, however, with pseudo-random motion it was relatively higher for the higher frequency components.6. Phase lags were in general smaller for single sine waves than for pseudo-random motion, but for the latter a phase lead of the smooth component was consistently found for the lower frequency components.7. During pursuit of a rhomboid trajectory, the eye movements showed directional errors which are interpreted as anticipatory behaviour.8. The distribution of the retinal error was symmetrical around zero. Its standard deviation varied between about 0-2 and 1.30; it was about proportional to target velocity and inversely proportional to smooth pursuit gain. It was limited by the insertion of saccades which were in general corrective.9. The influence of a diffusely illuminated background was minimal. A structured background inhibited smooth pursuit in the horizontal direction by about 10 % and

We studied the dynamics of voluntary, horizontal, binocular gaze-shifts between pairs of continuously visible, real three-dimensional targets. Subjects were stabilized on a biteboard to allow full control of target angles, which were made to differ only in distance (pure vergence), only in direction (pure version; conjugate saccades) or in both distance and direction (disjunctive saccades). A wide range of changes in vergence (0-25 deg) and version (0-65 deg) was recorded to study the dynamics of disjunctive saccades, described until now for limited ranges, throughout the horizontal oculomotor range within manual working space, and to study the velocity-duration-amplitude relations ("main sequence") of disjunctive vs conjugate saccades. Pure vergence was almost never observed; divergence, especially, was always associated with saccades. Likewise, horizontal saccades were never strictly conjugate, they always contained a transient divergence-convergence sequence. The amplitude and velocity of these transient components varied systematically with saccadic size. In combined version-vergence movements, vergence was, in general, accelerated and shortened as a function of increasing version. This effect was fairly uniform for divergence, which appeared to increase in velocity by about as much as the transient peak divergent velocity of the version saccade. The intrasaccadic fraction of divergence increased from about 50% to close to 100% as a function of increasing version. For convergence, saccades up to about 20 deg were also accelerating; in this case it appeared as if the transient peak convergent velocity of the version saccade was added to the basic convergence velocity. For larger saccades this effect was partly counteracted by the penetration of an initial divergence associated with the saccade. This initial divergence delayed and slowed down convergence. The intrasaccadic fraction of convergence varied between about 40% and 70%. In disjunctive saccades the individual eyes did not follow the main-sequence parameters of conjugate saccades of comparable sizes, except for the eye that moved with the combination "abduction and divergence". For all other combinations of vergence and version, disjunctive saccades had lower peak velocities and longer durations than conjugate saccades. As a consequence, disjunctive version was also slower than conjugate version. Thus, while version accelerates vergence, vergence slows down version: in the generalized case of three-dimensional gaze-shifts, peak velocities and durations are in between those of the limiting cases of pure version and pure vergence. We conclude that, within manual working space, binocular gaze-shifts are effected by the highly integrated action of conjugate and disjunctive mechanisms, both of which are expressed preferentially in fast, saccadic movements.

SUMMARY1. 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 1P5 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.H. COLLEWIJN, C. J. ERKELENS AND R. M. STEINMAN 8. These changes in vergence were followed by converging horizontal postsaccadic drift after upward saccades, and in diverging horizontal drift after downward saccades. Vertical post-saccadic drift consisted mainly of a conjugated component, directed towards the target position. We conclude that post-saccadic drift on the vertical meridian is effective in decreasing binocular fixation errors in a way similar to error reduction following horizontal saccades.

eye movements made when only looking are different from those made when tapping. Visual search functions as a separate process, incorporated into both tasks: it can be used to improve performance when memory load is heavy.