Primate head-free saccade generator implements a desired (post-VOR) eye position command by anticipating intended head motion

Primate head-free saccade generator implements a desired (post-VOR) eye position command by anticipating intended head motion. J. Neurophysiol. 78: 2811-2816, 1997. When we glance between objects, the brain ultimately controls gaze direction in space. However, it is currently unclear how this is allocated into separate commands for eye and head movement. To determine the role of desired final eye position commands, and their coordination with intended head movement, we trained three monkeys to make large gaze shifts while wearing opaque goggles with a monocular 8 degrees aperture. Animals eventually developed a new set of context-dependent eye-head coordination strategies, in particular expanding the head range and compressing the eye-in-head range toward the aperture (while wearing the goggles). However, when we shifted the location of the aperture to a different subsection of the normal head-free oculomotor range (by covering the original aperture and creating a new one), eye-head saccades failed to acquire visual targets, because they continued to drive the eye ultimately toward the now occluded original aperture. Even when a head-stationary saccade acquired the new aperture, subsequent head-free saccades drove the eye eccentrically toward a point that anticipated the intended head movement, such that the subsequent vestibuloocular reflex slow phase brought the eye onto the location of the original aperture. Animals could only acquire the new aperture consistently after several days of retraining. These results suggest that 1) eye-head coordination is achieved by a plastic, context-dependent neural operator that uses information about initial eye/head position and intended movement to compute desired combinations of final eye/head position and 2) acquisition of these positions involves sophisticated anticipatory compensations for subsequent movement components, akin to those observed previously in complex oral and manual behaviors.     

Human oculomotor system accounts for 3-D eye orientation in the visual-motor transformation for saccades

A recent theoretical investigation has demonstrated that three-dimensional (3-D) eye position dependencies in the geometry of retinal stimulation must be accounted for neurally (i.e., in a visuomotor reference frame transformation) if saccades are to be both accurate and obey Listing's law from all initial eye positions. Our goal was to determine whether the human saccade generator correctly implements this eye-to-head reference frame transformation (RFT), or if it approximates this function with a visuomotor look-up table (LT). Six head-fixed subjects participated in three experiments in complete darkness. We recorded 60 degrees horizontal saccades between five parallel pairs of lights, over a vertical range of +/-40 degrees (experiment 1), and 30 degrees radial saccades from a central target, with the head upright or tilted 45 degrees clockwise/counterclockwise to induce torsional ocular counterroll, under both binocular and monocular viewing conditions (experiments 2 and 3). 3-D eye orientation and oculocentric target direction (i.e., retinal error) were computed from search coil signals in the right eye. Experiment 1: as predicted, retinal error was a nontrivial function of both target displacement in space and 3-D eye orientation (e.g., horizontally displaced targets could induce horizontal or oblique retinal errors, depending on eye position). These data were input to a 3-D visuomotor LT model, which implemented Listing's law, but predicted position-dependent errors in final gaze direction of up to 19.8 degrees. Actual saccades obeyed Listing's law but did not show the predicted pattern of inaccuracies in final gaze direction, i.e., the slope of actual error, as a function of predicted error, was only -0. 01 +/- 0.14 (compared with 0 for RFT model and 1.0 for LT model), suggesting near-perfect compensation for eye position. Experiments 2 and 3: actual directional errors from initial torsional eye positions were only a fraction of those predicted by the LT model (e. g., 32% for clockwise and 33% for counterclockwise counterroll during binocular viewing). Furthermore, any residual errors were immediately reduced when visual feedback was provided during saccades. Thus, other than sporadic miscalibrations for torsion, saccades were accurate from all 3-D eye positions. We conclude that 1) the hypothesis of a visuomotor look-up table for saccades fails to account even for saccades made directly toward visual targets, but rather, 2) the oculomotor system takes 3-D eye orientation into account in a visuomotor reference frame transformation. This transformation is probably implemented physiologically between retinotopically organized saccade centers (in cortex and superior colliculus) and the brain stem burst generator.     

Visual-motor transformations required for accurate and kinematically correct saccades

The goal of this study was to identify and model the three-dimensional (3-D) geometric transformations required for accurate saccades to distant visual targets from arbitrary initial eye positions. In abstract 2-D models, target displacement in space, retinal error (RE), and saccade vectors are trivially interchangeable. However, in real 3-D space, RE is a nontrivial function of objective target displacement and 3-D eye position. To determine the physiological implications of this, a visuomotor "lookup table" was modeled by mapping the horizontal/vertical components of RE onto the corresponding vector components of eye displacement in Listing's plane. This provided the motor error (ME) command for a 3-D displacement-feedback loop. The output of this loop controlled an oculomotor plant that mechanically implemented the position-dependent saccade axis tilts required for Listing's law. This model correctly maintained Listing's law but was unable to correct torsional position deviations from Listing' s plane. Moreover, the model also generated systematic errors in saccade direction (as a function of eye position components orthogonal to RE), predicting errors in final gaze direction of up to 25 degrees in the oculomotor range. Plant modifications could not solve these problems, because the intrisic oculomotor input-output geometry forced a fixed visuomotor mapping to choose between either accuracy or Listing's law. This was reflected internally by a sensorimotor divergence between input-defined visual displacement signals (inherently 2-D and defined in reference to the eye) and output-defined motor displacement signals (inherently 3-D and defined in reference to the head). These problems were solved by rotating RE by estimated 3-D eye position (i.e., a reference frame transformation), inputting the result into a 2-D-to-3-D "Listing's law operator," and then finally subtracting initial 3-D eye position to yield the correct ME. This model was accurate and upheld Listing's law from all initial positions. Moreover, it suggested specific experiments to invasively distinguish visual and motor displacement codes, predicting a systematic position dependence in the directional tuning of RE versus a fixed-vector tuning in ME. We conclude that visual and motor displacement spaces are geometrically distinct such that a fixed visual-motor mapping will produce systematic and measurable behavioral errors. To avoid these errors, the brain would need to implement both a 3-D position-dependent reference frame transformation and nontrivial 2-D-to-3-D transformation. Furthermore, our simulations provide new experimental paradigms to invasively identify the physiological progression of these spatial transformations by reexamining the position-dependent geometry of displacement code directions in the superior colliculus, cerebellum, and various cortical visuomotor areas.     

Gaze-related activity of putative inhibitory burst neurons in the head-free cat

1. Previous studies in the cat have demonstrated that output neurons of the superior collicular as well as brain stem omnipause neurons have discharges that are best correlated, not with the trajectory of the eye in the head but, with the trajectory of the visual axis in space (gaze = eye-in-head + head-in-space) during rapid orienting coordinated eye and head movements. In this study, we describe the gaze-related activity of cat premotor "inhibitory burst neurons" (IBNs) identified on the basis of their position relative to the abducens nucleus. 2. The firing behavior of IBNs was studied during 1) saccades made with the head stationary, 2) active orienting combined eye-head gaze shifts, and 3) passive movements of the head on the body. IBN discharges were well correlated with the duration and amplitude of saccades made when the head was stationary. In both head-free paradigms, the behavior of cat IBNs differed from that of previously described primate "saccade bursters". The duration of their burst was better correlated with gaze than saccade duration, and the total number of spikes in a burst was well correlated with gaze amplitude and generally poorly correlated with saccade amplitude. The behavior of cat IBNs also differed from that of previously described primate "gaze bursters". The slope of the relationship between the total number of spikes and gaze amplitude observed during head-free gaze shifts was significantly lower than that observed during head-fixed saccades. 3. These studies suggest that cat IBNs do not fit into the categories of gaze-bursters or saccade-bursters that have been described in primate studies.(ABSTRACT TRUNCATED AT 250 WORDS)     

When is it best to hear a verb? The effects of the timing and focus of verb models on children''s learning of verbs

Recent neurophysiological studies of the saccadic ocular motor system have lent support to the hypothesis that this system uses a motor error signal in retinotopic coordinates to direct saccades to both visual and auditory targets. With visual targets, the coordinates of the sensory and motor error signals will be identical unless the eyes move between the time of target presentation and the time of saccade onset. However, targets from other modalities must undergo different sensory-motor transformations to access the same motor error map. Because auditory targets are initially localized in head-centered coordinates, analyzing the metrics of saccades from different starting positions allows a determination of whether the coordinates of the motor signals are those of the sensory system. We studied six human subjects who made saccades to visual or auditory targets from a central fixation point or from one at 10 degrees to the right or left of the midline of the head. Although the latencies of saccades to visual targets increased as stimulus eccentricity increased, the latencies of saccades to auditory targets decreased as stimulus eccentricity increased. The longest auditory latencies were for the smallest values of motor error (the difference between target position and fixation eye position) or desired saccade size, regardless of the position of the auditory target relative to the head or the amplitude of the executed saccade. Similarly, differences in initial eye position did not affect the accuracy of saccades of the same desired size. When saccadic error was plotted as a function of motor error, the curves obtained at the different fixation positions overlapped completely. Thus, saccadic programs in the central nervous system compensated for eye position regardless of the modality of the saccade target, supporting the hypothesis that the saccadic ocular motor system uses motor error signals to direct saccades to auditory targets.     

Analysis of saccadic short-term plasticity in three dimensions

The capacity for short-term adaptation is a well-established property of the horizontal (H) and vertical (V) components of saccades. It allows these directional components, which clearly serve the goal of foveation, to maintain their precision even under changing circumstances. Torsional (T) saccade components, on the other hand, which deal with the orientation of the target on the fovea, have hardly been investigated in adaptation experiments. They appear to be severely restricted by Listing's law during fixations and saccades. The main purpose of Listing's law is far from obvious but could be visual or oculomotor. Better knowledge of the adaptive capacity of the saccadic system in the torsional direction could throw new light on the functional significance of this interesting neural strategy. To study short-term plasticity in the torsional components of saccades, binocular 3D-eye positions were measured, using magnetic search coils. Five normal human subjects were instructed to make uni-directional refixation saccades, while they viewed a large visual scene. To induce a change in the torsional component, the complete stimulus was rapidly rotated during these saccades. We thoroughly investigated the torsional responses of the saccadic system, to see if any short-term adaptive response in torsional direction was induced, in which case the notion of a visual purpose for Listing's law would be strengthened. In none of our experiments, however, did we find any clear adaptive response in torsional direction. To further investigate the reliability of this result and to ascertain that our experimental conditions allowed classical gain adaptation, we also did experiments designed to achieve a combination of torsional adaptation and classic gain shortening in one of the directional components. While gain adaptation was very obvious, none of the experiments provided evidence for a short-term effect in torsion. We conclude that our experiments do not support a purely visual basis for Listing's law.     

Properties of memory-guided saccades toward targets flashed during smooth pursuit in human subjects

PURPOSE: This study in human subjects investigated whether or not the saccade system can monitor smooth changes of the eye position in total darkness. METHODS: The authors studied the properties of memory-guided saccades toward targets flashed during pursuit eye movements (target velocities of 15 degrees/s, 30 degrees/s, and 45 degrees/s) in four normal human subjects. Subjects were instructed to execute memory-guided saccades toward the position of the flashed target in total darkness when the pursuit target was extinguished. RESULTS: The vector of the saccade was more highly correlated with the vector of "spatial error" (the vector from the position of the eye at the time of the saccade to the position of the flashed target in space) than with the vector of "retinal error" (the vector from the position of the eye at the time of the presentation of the flashed target to the position of the flashed target). The amplitude and direction errors of memory-guided saccades were correlated with the amplitude of the retinal error but not with amplitude of eye deviation after the presentation of the flashed target. Pursuit velocity did not affect the error of the saccade. CONCLUSIONS: These findings suggest that the saccade system can monitor smooth changes of the eye position in total darkness, regardless of the velocity of pursuit, and that the accuracy of memory-guided saccades is dependent only on the amplitude of the retinal error.     

Control of vertical eye alignment in three-dimensional space

A target that is nearer to one eye than the other subtends a larger visual angle in the closer eye. Consequently, when making saccades between vertically separated targets that are closer to one eye, there is a vertical retinal disparity that must be overcome by a change in the relative alignment of the eyes. We recorded eye movements in three normal subjects and showed that in such viewing circumstances subjects made unequal vertical saccades that led to a rapid change (peak velocity up to 30 deg/sec) in vertical eye alignment. On average, 81% of the required change in alignment occurred within the saccade for downward movements and 47% for upward movements. Such unequal vertical saccades occurred independently of immediate disparity cues; saccades remained unequal when refixing to the remembered locations of the vertically-oriented targets, or even when the natural vertical disparity was nullified by a prism. On the other hand, when subjects wore the nullifying prism in front of the inferior visual field of the left eye for 8-20 hr, they showed a decrease in saccade disconjugacy (to 12-35% of the preadaptation value) to targets closer to the left eye in the inferior but not in the superior visual field. We suggest that the brain develops a three-dimensional map (horizontal, vertical, depth) for vertical saccade yoking, which is under adaptive control, and which is used to preprogram automatically the relative excursions of the eyes during vertical saccades as a function of the current and the desired point of regard.     

Postsaccadic enhancement of initiation of smooth pursuit eye movements in monkeys

Step-ramp target motion evokes a characteristic sequence of presaccadic smooth eye movement in the direction of the target ramp, catch-up targets to bring eye position close to the position of the moving target, and postsaccadic eye velocities that nearly match target velocity. I have analyzed this sequence of eye movements in monkeys to reveal a strong postsaccadic enhancement of pursuit eye velocity and to document the conditions that lead to that enhancement. Smooth eye velocity was measured in the last 10 ms before and the first 10 ms after the first saccade evoked by step-ramp target motion. Plots of eye velocity as a function of time after the onset of the target ramp revealed that eye velocity at a given time was much higher if measured after versus before the saccade. Postsaccadic enhancement of pursuit was recorded consistently when the target stepped 3 degrees eccentric on the horizontal axis and moved upward, downward, or away from the position of fixation. To determine whether postsaccadic enhancement of pursuit was invoked by smear of the visual scene during a saccade, I recorded the effect of simulated saccades on the presaccadic eye velocity for step-ramp target motion. The 3 degrees simulated saccade, which consisted of motion of a textured background at 150 degrees/s for 20 ms, failed to cause any enhancement of presaccadic eye velocity. By using a strategically selected set of oblique target steps with horizontal ramp target motion, I found clear enhancement for saccades in all directions, even those that were orthogonal to target motion. When the size of the target step was varied by up to 15 degrees along the horizontal meridian, postsaccadic eye velocity did not depend strongly either on the initial target position or on whether the target moved toward or away from the position of fixation. In contrast, earlier studies and data in this paper show that presaccadic eye velocity is much stronger when the target is close to the center of the visual field and when the target moves toward versus away from the position of fixation. I suggest that postsaccadic enhancement of pursuit reflects activation, by saccades, of a switch that regulates the strength of transmission through the visual-motor pathways for pursuit. Targets can cause strong visual motion signals but still evoke low presaccadic eye velocities if they are ineffective at activating the pursuit system.     

Listing's plane dependence on alternating fixation in a strabismus patient

Listing's law of the eye is one of the best studied findings in motor control, but its functional meaning is still incompletely understood and its status in neurological disorders and in strabismus is almost entirely unknown. We investigated the mechanisms underlying Listing's law and its possible clinical relevance. The dual magnetic search coil technique was used to record three-dimensional binocular eye movements in a stereoblind strabismic patient with good visual acuity in both eyes and capable of voluntarily alternating fixation. This technique yielded an accurate, objective and simultaneous measure of ocular misalignment in three dimensions and showed that the squint angle depended on which eye was fixating. Saccadic eye movement data throughout the oculomotor range were used to fit Listing's plane. Listing's primary position and the thickness of the plane for each eye were calculated for three different fixation conditions. For comparison, control measurements were taken from four normals. In the patient, no large deviations from normal values for the thickness of Listing's plane and the confidence limits of the Listing primary position were found. The most remarkable abnormality was that the orientation of Listing's plane depended on which eye was fixating. Both the change in ocular misalignment and the shift of Listing's primary positions observed when changing fixation are probably linked to accommodation-related vergence. Despite repeated surgery at early age, the patient had well-defined Listing planes for both eyes, but their alignment during left-eye fixation was abnormal. The obedience to Listing's law may reflect a strategy which minimizes muscular effort in each eye separately. The abnormal fixation-condition dependence is probably due to an aberrant coupling with vergence.     

Contribution of the rostral fastigial nucleus to the control of orienting gaze shifts in the head-unrestrained cat

The implication of the caudal part of the fastigial nucleus (cFN) in the control of saccadic shifts of the visual axis is now well established. In contrast a possible involvement of the rostral part of the fastigial nuceus (rFN) remains unknown. In the current study we investigated in the head-unrestrained cat the contribution of the rFN to the control of visually triggered saccadic gaze shifts by measuring the deficits after unilateral muscimol injection in the rFN. A typical gaze dysmetria was observed: gaze saccades directed toward the inactivated side were hypermetric, whereas those with an opposite direction were hypometric. For both movement directions, gaze dysmetria was proportional to target retinal eccentricity and could be described as a modified gain in the translation of visual signals into eye and head motor commands. Correction saccades were triggered when the target remained visible and reduced the gaze fixation error to 2.7 +/- 1.3 degrees (mean +/- SD) on average. The hypermetria of ipsiversive gaze shifts resulted predominantly from a hypermetric response of the eyes, whereas the hypometria of contraversive gaze shifts resulted from hypometric responses of both eye and head. However, even in this latter case, the eye saccade was more affected than the motion of the head. As a consequence, for both directions of gaze shift the relative contributions of the eye and head to the overall gaze displacement were altered by muscimol injection. This was revealed by a decreased contribution of the head for ipsiversive gaze shifts and an increased head contribution for contraversive movements. These modifications were associated with slight changes in the delay between eye and head movement onsets. Inactivation of the rFN also affected the initiation of eye and head movements. Indeed, the latency of ipsiversive gaze and head movements decreased to 88 and 92% of normal, respectively, whereas the latency of contraversive ones increased to 149 and 145%. The deficits induced by rFN inactivation were then compared with those obtained after muscimol injection in the cFN of the same animals. Several deficits differed according to the site of injection within the fastigial nucleus (tonic orbital eye rotation, hypermetria of ipsiversive gaze shifts and fixation offset, relationship between dysmetria and latency of contraversive gaze shifts, postural deficit). In conclusion, the present study demonstrates that the rFN is involved in the initiation and the control of combined eye-head gaze shifts. In addition our findings support a functional distinction between the rFN and cFN for the control of orienting gaze shifts. This distinction is discussed with respect to the segregated fastigiofugal projections arising from the rFN and cFN.     

Modeling control of eye orientation in three dimensions. I. Role of muscle pulleys in determining saccadic trajectory

This study evaluates the effects of muscle axis shifts on the performance of a vector velocity-position integrator in the CNS. Earlier models of the oculomotor plant assumed that the muscle axes remained fixed relative to the head as the eye rotated into secondary and tertiary eye positions. Under this assumption, the vector integrator model generates torsional transients as the eye moves from secondary to tertiary positions of fixation. The torsional transient represents an eye movement response to a spatial mismatch between the torque axes that remain fixed in the head and the displacement plane that changes by half the angle of the change in eye orientation. When muscle axis shifts were incorporated into the model, the torque axes were closer to the displacement plane at each eye orientation throughout the trajectory, and torsional transients were reduced dramatically. Their size and dynamics were close to reported data. It was also shown that when the muscle torque axes were rotated by 50% of the eye rotation, there was no torsional transient and Listing's law was perfectly obeyed. When muscle torque axes rotated >50%, torsional transients reversed direction compared with what occurred for muscle axis shifts of <50%. The model indicates that Listing's law is implemented by the oculomotor plant subject to a two-dimensional command signal that is confined to the pitch-yaw plane, having zero torsion. Saccades that bring the eye to orientations outside Listing's plane could easily be corrected by a roll pulse that resets the roll state of the velocity-position integrator to zero. This would be a simple implementation of the corrective controller suggested by Van Opstal and colleagues. The model further indicates that muscle axis shifts together with the torque orientation relationship for tissue surrounding the eye and Newton's laws of motion form a sufficient plant model to explain saccadic trajectories and periods of fixation when driven by a vector command confined to the pitch-yaw plane. This implies that the velocity-position integrator is probably realized as a subtractive feedback vector integrator and not as a quaternion-based integrator that implements kinematic transformations to orient the eye.     

Eye-head coordination in labyrinthine-defective humans

Eye-head coordination during saccadic gaze shifts normally relies on vestibular information. A vestibulo-saccadic reflex (VSR) is thought to reduce the eye-in-head saccade to account for current head movement, and the vestibulo-ocular reflex (VOR) stabilizes postsaccadic gaze while the head movement is still going on. Acute bilateral loss of vestibular function is known to cause overshoot of gaze saccades and postsaccadic instability. We asked how patients suffering from chronic vestibular loss adapt to this situation. Eye and head movements were recorded from six patients and six normal control subjects. Subjects tracked a random sequence of horizontal target steps, with their heads (1) fixed in primary position, (2) free to move, or (3) preadjusted to different head-to-target offsets (to provoke head movements of different amplitudes). Patients made later and smaller head movements than normals and accepted correspondingly larger eye eccentricities. Targeting accuracy, in terms of the mean of the signed gaze error, was better in patients than in normals. However, unlike in normals, the errors of patients exhibited a large scatter and included many overshoots. These overshoots cannot be attributed to the loss of VSR because they also occurred when the head was not moving and were diminished when large head movements were provoked. Patients' postsaccadic stability was, on average, almost as good as that of normals, but the individual responses again showed a large scatter. Also, there were many cases of inappropriate postsaccadic slow eye movements, e.g., in the absence of concurrent head movements, and correction saccades, e.g., although gaze was already on target. Performance in patients was affected only marginally when large head movements were provoked. Except for the larger lag of the head upon the eye, the temporal coupling of eye and head movements in patients was similar to that in normals. Our findings show that patients with chronic vestibular loss regain the ability to make functionally appropriate gaze saccades. We assume, in line with previous work, three main compensatory mechanisms: a head movement efference copy, an active cervico-ocular reflex (COR), and a preprogrammed backsliding of the eyes. However, the large trial-to-trial variability of targeting accuracy and postsaccadic stability indicates that the saccadic gaze system of patients does not regain the high precision that is observed in normals and which appears to require a vestibular head-in-space signal. Moreover, this variability also permeates their gaze performance in the absence of head movements.     

Electrical microstimulation distinguishes distinct saccade-related areas in the posterior parietal cortex

Electrical microstimulation (0.1-ms bipolar pulses at 500 Hz, current strength usually between 100 and 200 microA) was used to delineate saccade-related areas in the posterior parietal cortex of monkeys. Stimulation-induced saccades were found to be restricted to the lateral intraparietal area (area LIP) in the intraparietal sulcus (IPS) and a region on the medial aspect of the parietal lobe (area MP, medial parietal area), close to the caudal end of the cingulate sulcus, whereas stimulation of area 7a did not evoke eye movements. Two different types of evoked saccades were observed. Modified vector saccades, whose amplitude was modified by the position of the eyes at stimulation onset were the hallmark of sites in area LIP and area MP. The same sites were characterized by a propensity of single units active in the memory and presaccadic response segments of the memory saccade paradigm. Goal-directed saccades driving the eyes toward a circumscribed region relative to the head were largely restricted to a small strip of cortex on the lateral bank and the floor of the IPS (the intercalated zone), separating the representation of upward and downward directed saccades in LIP. Unlike stimulation in LIP or MP, stimulation in the intercalated zone gave rise to head, pinnae, facial, and shoulder movements accompanying the evoked saccades. We propose that the amplitude modification of vector saccades characterizing LIP and MP may reflect a spatially distributed head-centered coding scheme for saccades. On the other hand, the goal-directed saccades found in the intercalated zone could indicate the use of a spatially much more localized representation of desired location in head-centered space.     

Programming saccadic eye movements.

This article addresses questions about the preparatory processes that immediately precede saccadic eye movements. Saccade latencies were measured in a task in which subjects were provided partial advance information about the spatial location of a target fixation. In one experiment, subjects were faster in initiating saccades when they knew either the direction or amplitude of the required movement in advance compared to a condition with equal uncertainty about the number of potential saccade targets but without knowledge of the parameters required to execute the movement. These results suggest that the direction and amplitude for an upcoming saccade were calculated separately, and not in a fixed serial order. In another experiment, subjects appear to have programmed the saccades more holistically—with computations of direction and amplitude parameters occurring simultaneously. The implications of these results for models of eye movement preparation are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Eye-hand interactions during goal-directed pointing movements

Saccadic eye and hand movements made to step displacements in target position were measured under conditions designed to dissociate the output of the ocular and manual motor systems. This was accomplished by having subjects look and point, either with or without vision of the hand (closed or open loop, respectively) at peripheral targets starting from independent initial positions. The results showed that the amplitude of open loop pointing responses increased in size when accompanied by saccades that were larger than the required hand movement. Providing the subject with visual feedback of the hand during the response or asking them to visually fixate caused this effect to disappear. Taken together, this pattern of results suggests that when vision of the hand is unavailable the programming of saccade metrics influences the control of simultaneously produced pointing movements in an on-line manner.     

Hand deviations away from visual cues: indirect evidence for inhibition

Previous research has demonstrated that when a stimulus is to be ignored, the path of motion towards a target (saccade or manual reach) deviates away from the to-be-ignored stimulus. Path deviations in saccade and reaching tasks have, however, been observed in very different situations. In the saccade tasks subjects initially attended to a cue, then disengaged attention while saccading to a target. By contrast, in the selective reaching tasks attention was continuously withdrawn from the to-be-ignored stimulus, as this was irrelevant throughout the experiment. In the two experiments reported here, cues similar to those studied in saccade tasks are examined with selective reaching procedures. Experiment 1 shows that when a coloured light-emitting diode cue, upon which subjects engage and then subsequently disengage attention, is close to the responding hand, the hand deviates away from the cue. Experiment 2 confirms this cue avoidance by showing that, compared with central fixation alone, the hand veers away from a central cue. These results confirm that the path deviations observed in saccades can also be obtained in manual reaching movements. Such findings support the notion that eye and hand movements are both affected by inhibitory mechanisms of attention.     

Target predictability influences the distribution of coordinated eye-head gaze saccades in patients with homonymous hemianopia

In hemianopic patients target predictability plays a differential role for the distribution of different types of initial saccades. A linear correlation was shown to exist between the adaptive state of reading capability and the probability of a correct initial eye-head gaze saccade that hit the target accurately. A constant target frequency of 0.8 Hz was found to be the optimum frequency for hemianopic patients to follow square step targets with an eccentricity of +/- 20 degrees. Variability of the compensatory eye movement velocity gain was lowest at this frequency. We were able to use this target frequency as a test for a clinical classification of the status of adaptation of hemianopic patients in combination with simple reading tests. The repetition of this manoeuvre could correct the synkinesis and the balance of the VOR during active gaze of hemianopic patients.     

Mechanism of block by 4-aminopyridine of the transient outward current in rat ventricular cardiomyocytes

Binocular information has been shown to be important for the programming and control of reaching and grasping. But even without binocular vision, people are still able to reach out and pick up objects accurately - albeit less efficiently. As part of a continuing investigation into the role that monocular cues play in visuomotor control, we examined whether or not subjects could use retinal motion information, derived from movements of the head, to help program and control reaching and grasping movements when binocular vision is denied. Subjects reached out in the dark to an illuminated sphere presented at eye-level, under both monocular and binocular viewing conditions with their head either free to move or restrained. When subjects viewed the display monocularly, they showed fewer on-line corrections when they were allowed to move their head. No such difference in performance was seen when subjects were allowed a full binocular view. This study, combined with previous work with neurological patients, confirms that the visuomotor system "prefers" to use binocular vision but, when this information is not available, can fall back on other monocular depth cues, such as information produced by motion of the object (and the scene) on the retina, to help program and control manual prehension.     

Saccadic gain modification: visual error drives motor adaptation

The brain maintains the accuracy of saccadic eye movements by adjusting saccadic amplitude relative to the target distance (i.e., saccade gain) on the basis of the performance of recent saccades. If an experimenter surreptitiously moves the target backward during each saccade, thereby causing the eyes to land beyond their targets, saccades undergo a gradual gain reduction. The error signal driving this conventional saccadic gain adaptation could be either visual (the postsaccadic distance of the target from the fovea) or motoric (the direction and size of the corrective saccade that brings the eye onto the back-stepped target). Similarly, the adaptation itself might be a motor adjustment (change in the size of saccade for a given perceived target distance) or a visual remapping (change in the perceived target distance). We studied these possibilities in experiments both with rhesus macaques and with humans. To test whether the error signal is motoric, we used a paradigm devised by Heiner Deubel. The Deubel paradigm differed from the conventional adaptation paradigm in that the backward step that occurred during the saccade was brief, and the target then returned to its original displaced location. This ploy replaced most of the usual backward corrective saccades with forward ones. Nevertheless, saccadic gain gradually decreased over hundreds of trials. Therefore, we conclude that the direction of saccadic gain adaptation is not determined by the direction of corrective saccades. To test whether gain adaptation is a manifestation of a static visual remapping, we decreased the gain of 10 degrees horizontal saccades by conventional adaptation and then tested the gain to targets appearing at retinal locations unused during adaptation. To make the target appear in such "virgin territory," we had it jump first vertically and then 10 degrees horizontally; both jumps were completed and the target spot extinguished before saccades were made sequentially to the remembered target locations. Conventional adaptation decreased the gain of the second, horizontal saccade even though the target was in a nonadapted retinal location. In contrast, the horizontal component of oblique saccades made directly to the same virgin location showed much less gain decrease, suggesting that the adaptation is specific to saccade direction rather than to target location. Thus visual remapping cannot account for the entire reduction of saccadic gain. We conclude that saccadic gain adaptation involves an error signal that is primarily visual, not motor, but that the adaptation itself is primarily motor, not visual.