The role of size and binocular information in guiding reaching: insights from virtual reality and visual form agnosia III (of III)

Reaching out to grasp an object requires information about the size of the object and the distance between the object and the body. We used a virtual reality system with a control population and a patient with visual form agnosia (DF) in order to explore the use of binocular information and size cues in prehension. The experiments consisted of a perceptual matching task in addition to a prehension task. In the prehension task, control participants modified their reach distance in response to step changes in vergence in the absence of any clear reference for relative disparity. Their reach distance was unaffected by equivalent step changes in size, even though they used this information to modify grasp and showed a size bias in a distance matching task. Notably, DF showed the same pattern of results as the controls but was far more sensitive to step changes in vergence. This finding complements previous research suggesting that DF relies predominantly on vergence information when gauging target distance. The results from the perceptual matching tasks confirmed previous findings suggesting that DF is unable to make use of size information for perceptual matching, including distance comparisons. These data are discussed with regard to the properties of the pathways subserving the two visual cortical processing streams. Electronic Publication     

Monocular and binocular distance cues: insights from visual form agnosia I (of III)

The human nervous system constructs a Euclidean representation of near (personal) space by combining multiple sources of information (cues). We investigated the cues used for the representation of personal space in a patient with visual form agnosia (DF). Our results indicated that DF relies predominantly on binocular vergence information when determining the distance of a target despite the presence of other (retinal) cues. Notably, DF was able to construct an Euclidean representation of personal space from vergence alone. This finding supports previous assertions that vergence provides the nervous system with veridical information for the construction of personal space. The results from the current study, together with those of others, suggest that: (i) the ventral stream is responsible for extracting depth and distance information from "monocular" retinal cues (i.e. from shading, texture, perspective) and (ii) the dorsal stream has access to binocular information (from horizontal image disparities and vergence). These results also indicate that DF was not able to use size information to gauge target distance, suggesting that intact temporal cortex is necessary for "learned size" to influence distance processing. Our findings further suggest that in neurologically intact humans, object information extracted in the ventral pathway is combined with the products of dorsal stream processing for guiding prehension. Finally, we studied the "size-distance paradox" in visual form agnosia in order to explore the cognitive use of size information. The results of this experiment were consistent with a previous suggestion that the paradox is a cognitive phenomenon. Electronic Publication     

Primate translational vestibuloocular reflexes. II. Version and vergence responses to fore-aft motion

To maintain binocular fixation on near targets during fore-aft translational disturbances, largely disjunctive eye movements are elicited the amplitude and direction of which should be tuned to the horizontal and vertical eccentricities of the target. The eye movements generated during this task have been investigated here as trained rhesus monkeys fixated isovergence targets at different horizontal and vertical eccentricities during 10 Hz fore-aft oscillations. The elicited eye movements complied with the geometric requirements for binocular fixation, although not ideally. First, the corresponding vergence angle for which the movement of each eye would be compensatory was consistently less than that dictated by the actual fixation parameters. Second, the eye position with zero sensitivity to translation was not straight ahead, as geometrically required, but rather exhibited a systematic dependence on viewing distance and vergence angle. Third, responses were asymmetric, with gains being larger for abducting and downward compared with adducting and upward gaze directions, respectively. As frequency was varied between 4 and 12 Hz, responses exhibited high-pass filter properties with significant differences between abduction and adduction responses. As a result of these differences, vergence sensitivity increased as a function of frequency with a steeper slope than that of version. Despite largely undercompensatory version responses, vergence sensitivity was closer to ideal. Moreover, the observed dependence of vergence sensitivity on vergence angle, which was varied between 2.5 and 10 MA, was largely linear rather than quadratic (as geometrically predicted). We conclude that the spatial tuning of eye velocity sensitivity as a function of gaze and viewing distance follows the general geometric dependencies required for the maintenance of foveal visual acuity. However, systematic deviations from ideal behavior exist that might reflect asymmetric processing of abduction/adduction responses perhaps because of different functional dependencies of version and vergence eye movement components during translation.     

Updating target distance across eye movements in depth

We tested between two coding mechanisms that the brain may use to retain distance information about a target for a reaching movement across vergence eye movements. If the brain was to encode a retinal disparity representation (retinal model), i.e., target depth relative to the plane of fixation, each vergence eye movement would require an active update of this representation to preserve depth constancy. Alternatively, if the brain was to store an egocentric distance representation of the target by integrating retinal disparity and vergence signals at the moment of target presentation, this representation should remain stable across subsequent vergence shifts (nonretinal model). We tested between these schemes by measuring errors of human reaching movements (n = 14 subjects) to remembered targets, briefly presented before a vergence eye movement. For comparison, we also tested their directional accuracy across version eye movements. With intervening vergence shifts, the memory-guided reaches showed an error pattern that was based on the new eye position and on the depth of the remembered target relative to that position. This suggests that target depth is recomputed after the gaze shift, supporting the retinal model. Our results also confirm earlier literature showing retinal updating of target direction. Furthermore, regression analyses revealed updating gains close to one for both target depth and direction, suggesting that the errors arise after the updating stage during the subsequent reference frame transformations that are involved in reaching.     

The role of learned pictorial cues in the programming and control of grasping

Binocular information has been shown to be important for the programming and control of reaching and grasping. Even without binocular vision, people are still able to reach out and pick up objects accurately – albeit less efficiently. It remains unclear, which of the many available monocular depth cues humans use to calibrate manual prehension when binocular information is not available. In the present experiment, we examined whether or not subjects could use a learned relationship between the elevation of a goal object in the visual scene and its distance to help program and control the required grasp. The elevation of the goal object was systematically varied with distance in some blocks of trials by presenting the object at different positions along a horizontal plane 35 cm below eye level. In other blocks of trials, elevation did not vary with distance because the objects were always presented along the subject′s line of sight. When subjects viewed these two displays monocularly, they showed fewer on-line adjustments in the trajectory of the limb and the aperture of the fingers when the elevation of the target object in the visual scene could be used to help program the required movements. No such difference between performance on the two arrays was seen when subjects were allowed a full binocular view. This study confirms that subjects are indeed able to use a learned relationship between the elevation of an object and its distance as a cue for programming grasping movements when binocular information is not available. Together with evidence from work with neurological patients who have difficulty perceiving pictorial cues, these findings suggest that the visuomotor system might normally “prefer” to use binocular cues, but can fall back on learned pictorial information when binocular vision is denied. Received: 10 December 1997 / Accepted: 11 March 1998     

Latency of adaptive vergence eye movements induced by vergence-vestibular interaction training in monkeys

Clear vision of objects that move in depth toward or away from an observer requires vergence eye movements. The vergence system must interact with the vestibular system to maintain the object images on the foveae of both eyes during head movement. Previous studies have shown that training with sinusoidal vergence-vestibular interaction improves the frequency response of vergence eye movements during pitch rotation: vergence eye velocity gains increase and phase-lags decrease. To further understand the changes in eye movement responses in this adaptation, we examined latencies of vergence eye movements before and after vergence-vestibular training. Two head-stabilized Japanese monkeys were rewarded for tracking a target spot moving in depth that required vergence eye movements of 10°/s. This target motion was synchronized with pitch rotation at 20°/s. Both target and chair moved in a trapezoidal waveform interspersed with random inter-trial intervals. Before training, pitch rotation in complete darkness without a target did not induce vergence eye movements. Mean latencies of convergence and divergence eye movements induced by vergence target motion alone were 182 and 169 ms, respectively. After training, mean latencies of convergence and divergence eye movements to a target synchronized with pitch rotation shortened to 65 and 53 ms, and vergence eye velocity gains (relative to vergence target velocity) at the normal latencies were 0.68 and 1.53, respectively. Pitch rotation alone without a target induced vergence eye movements with similar latencies after training. These results indicate that vestibular information can be used effectively to initiate vergence eye movements following vergence-vestibular training.     

Neural correlates of the dependence of compensatory eye movements during translation on target distance and eccentricity

To stabilize objects of interest on the fovea during translation, vestibular-driven compensatory eye movements [translational vestibulo-ocular reflex (TVOR)] must scale with both target distance and eccentricity. To identify the neural correlates of these properties, we recorded from different groups of eye movement-sensitive neurons in the prepositus hypoglossi and vestibular nuclei of macaque monkeys during lateral and fore-aft displacements. All neuron types exhibited some increase in modulation amplitude as a function of target distance during high-frequency (4 Hz) lateral motion in darkness, with slopes that were correlated with the cell's pursuit gain, but not eye position sensitivity. Vergence angle dependence was largest for burst-tonic (BT) and contralateral eye-head (EH) neurons and smallest for ipsilateral EH and position-vestibular-pause (PVP) cells. On the other hand, the EH and PVP neurons with ipsilateral eye movement preferences exhibited the largest vergence-independent responses, which would be inappropriate to drive the TVOR. In addition to target distance, the TVOR also scales with target eccentricity, as evidenced during fore-aft motion, where eye velocity amplitude exhibits a "V-shaped " dependence and phase shifts 180 degrees for right versus left eye positions. Both the modulation amplitude and phase of BT and contralateral EH cells scaled with eye position, similar to the evoked eye movements during fore-aft motion. In contrast, the response modulation of ipsilateral EH and PVP cells during fore-aft motion was characterized by neither the V-shaped scaling nor the phase reversal. These results show that distinct premotor cell types carry neural signals that are appropriately scaled by vergence angle and eye position to generate the geometrically appropriate compensatory eye movements in the translational vestibulo-ocular reflex.     

The effects of pretectal and superior collicular lesions on binocular vision

Summary Monkeys with mid-brain lesions involving the pre-tectum and superior colliculi often have an odd stare, as if the visual axes were parallel and the animal was looking into the distance. Such a visuo-motor abnormality could lead to double-vision for objects close to the animal. The experiments reported here were designed to test this hypothesis of diplopia in monkeys with combined bilateral ablations of the colliculi and frontal eye-fields (area 8). These animals performed better on a task of visually-guided reaching, and in a series of visual pattern discriminations, when they viewed stimuli monocularly rather than binocularly. Monkeys with other cortical lesions showed no such monocular superiority. We propose that an abnormality of vergence eye movements provides a simple explanation for some of the so-called perceptual impairments that follow damage to the mid-brain visual pathways in monkeys     

Human gaze stabilization during active head translations.

This study investigated how binocular gaze is controlled to compensate for self-generated translational movements of the head where geometric requirements dictate that the ideal gaze signal needs to be modulated by the inverse of target distance. Binocular gaze (eye plus head) was measured for visual and remembered targets at various distances in six human subjects during active head translations at frequencies of 0.25, 0.5, 1.0, and 1.5 Hz. We found that, during head translations, gaze changes were achieved by a combination of eye and head rotations. Accordingly, stabilization performance was characterized by the gaze response parameters sensitivity and phase, where sensitivity is defined as the ratio of gaze velocity and translational eye velocity and where phase refers to the phase delay of gaze velocity relative to translational eye velocity. In the analysis, we used a binocular coordinate system yielding a version and a vergence component. We examined how frequency and target distance, estimated from the vergence angle, affected sensitivity and phase of the version component of the gaze signal and compared the results to the requirements for ideal performance. The relation between gaze sensitivity and the inverse of distance was characterized by a linear regression analysis. The ratio of the slope of the linear regression and the slope required for ideal stabilization provided a measure for the degree of "distance compensation." The results show that distance compensation was better for a visual target than for remembered targets in darkness, and behaved according to low-pass characteristics in both target conditions. It declined from 1.00 to 0.84 for visual targets and from 0.87 to 0.57 for remembered targets in the frequency range 0.25-1.5 Hz. The intercept obtained from the regression yielded the gaze response at zero vergence and specified a "default sensitivity" of gaze compensation. Default sensitivity increased with frequency from 0.02 at 0.25 Hz to 0.10 degrees/cm at 1.5 Hz for visual targets and from 0.04 to 0.16 degrees/cm in darkness. The phase delays of the gaze response increased on average from 2 to 7 degrees in the frequency range 0.25-1.5 Hz. In comparison with earlier passive studies, active translation compensation in the dark is superior at all frequencies where comparison was possible. We conclude that a nonvestibular signal with low-pass characteristics contributes to gaze during active head translations.     

Ocular responses to head rotations during mirror viewing.

The gain of the human vestibuloocular reflex (VOR) is influenced by the proximity of the object of regard. In six human subjects, we measured the eye rotations induced by passive, sinusoidal, horizontal head rotations at 2.0 Hz during binocular fixation of a stationary far target at 7 m; a stationary target close to the subject's near point of fixation (<15 cm); and the bridge of the subject's own nose, viewed through a mirror positioned so that, for each subject, the angle of vergence was similar to that during viewing of the near target. The median gain of compensatory eye movements for the group of subjects during far viewing was 0.99 (range 0.80-1.04), during near viewing was 1.21 (range 0.88-1.47), and during mirror viewing was 0.85 (range 0.71-1.01). The gain during near and mirror viewing was significantly different for each subject (P < 0.001) even though the vergence angles were similar. The lower gain values during mirror viewing can be attributed to the geometric relationship between the head rotation, the position of the eyes in the head, and the movement of the subject's virtual image in the mirror. To determine whether visually mediated eye movements were responsible for the observed gain values, we conducted a control experiment in which subjects were rotated using a sum-of-sines stimulus that minimized the effects of predictive visual tracking; differences of gain values between near- and mirror-viewing conditions were similar to those during rotation at 2 Hz. We conclude that, in these experiments, target proximity and vergence angle were not the key determinants of gain of the visuo-vestibular response during head rotation while viewing a near target but that contextual cues from motion vision were more important in generating the appropriate response.     

Common organization for unimanual and bimanual reach-to-grasp tasks

 In two experiments comparisons between characteristics of performance of a unimanual and a bimanual reach-to-grasp (prehension) task were made on an individual subject basis. The unimanual prehension task used required that the object be grasped by finger and thumb pad opposition, the bimanual task required that the grasp be made by opposing the pads on the two index fingers. Experiment 1 examined adaptation of prehension movements to objects of different size (width) but equal grasp surface area placed at different distances. Experiment 2 examined adaptation of movements to objects of different grasp surface areas. It was found that the aperture and transport components of the two prehension tasks developed over time in very similar fashion in all subjects. Movements were adapted to different task constraints in the same way as has previously been reported in the literature and were very similar in both tasks: maximum aperture increases with increasing object size and occurs later in the movement for larger objects; movement time increases with target distance; time of maximum aperture occurs earlier in the movement for targets with smaller grasp surface areas; movement times are longer for such objects, largely due to increases in the deceleration phase of the movement. These results support the notion that there is an effector independent level of organization that governs the coordination of movements during performance of reaching and grasping tasks. Received: 24 May 1996 / Accepted: 9 December 1996     

Short-latency vergence eye movements induced by radial optic flow in humans: dependence on ambient vergence level

Radial patterns of optic flow, such as those experienced by moving observers who look in the direction of heading, evoke vergence eye movements at short latency. We have investigated the dependence of these responses on the ambient vergence level. Human subjects faced a large tangent screen onto which two identical random-dot patterns were back-projected. A system of crossed polarizers ensured that each eye saw only one of the patterns, with mirror galvanometers to control the horizontal positions of the images and hence the vergence angle between the two eyes. After converging the subject's eyes at one of several distances ranging from 16.7 cm to infinity, both patterns were replaced with new ones (using a system of shutters and two additional projectors) so as to simulate the radial flow associated with a sudden 4% change in viewing distance with the focus of expansion/contraction imaged in or very near both foveas. Radial-flow steps induced transient vergence at latencies of 80-100 ms, expansions causing increases in convergence and contractions the converse. Based on the change in vergence 90-140 ms after the onset of the steps, responses were proportional to the preexisting vergence angle (and hence would be expected to be inversely proportional to viewing distance under normal conditions). We suggest that this property assists the observer who wants to fixate ahead while passing through a visually cluttered area (e.g., a forest) and so wants to avoid making vergence responses to the optic flow created by the nearby objects in the periphery.     

Peripheral vision for perception and action

Anatomical and physiological evidence suggests that vision-for-perception and vision-for-action may be differently sensitive to increasingly peripheral stimuli, and to stimuli in the upper and lower visual fields (VF). We asked participants to fixate one of 24 randomly presented LED arranged radially in eight directions and at three eccentricities around a central target location. One of two (small, large) target objects was presented briefly, and participants responded in two ways. For the action task, they reached for and grasped the target. For the perception task, they estimated target height by adjusting thumb-finger separation. In a final set of trials for each task, participants knew that target size would remain constant. We found that peak aperture increased with eccentricity for grasping, but not for perceptual estimations of size. In addition, peak grip aperture, but not size-estimation aperture, was more variable when targets were viewed in the upper as opposed to the lower VF. A second experiment demonstrated that prior knowledge about object size significantly reduced the variability of perceptual estimates, but had no effect on the variability of grip aperture. Overall, these results support the claim that peripheral VF stimuli are processed differently for perception and action. Moreover, they support the idea that the lower VF is specialized for the control of manual prehension. Finally, the effect of prior knowledge about target size on performance substantiates claims that perception is more tightly linked to memory systems than action.     

Dissociation between vergence and binocular disparity cues in the control of prehension

Binocular vision provides important advantages for controlling reach-to-grasp movements. We examined the possible source(s) of these advantages by comparing prehension proficiency under four different binocular viewing conditions, created by randomly placing a neutral lens (control), an eight dioptre prism (Base In or Base Out) or a low-power (2.00–3.75 dioptre) Plus lens over the eye opposite the moving limb. The Base In versus Base Out prisms were intended to selectively alter vergence-specified distance (VSD) information, such that the targets appeared beyond or closer than their actual physical position, respectively. The Plus lens was individually tailored to reduce each subject’s disparity sensitivity (to 400–800 arc s), while minimizing effects on distance estimation. In pre-testing, subjects pointed (without visual feedback) to mid-line targets at different distances, and produced the systematic directional errors expected of uncorrected movements programmed under each of the perturbed conditions. For the prehension tasks, subjects reached and precision grasped (with visual feedback available) cylindrical objects (two sizes and three locations), either following a 3 s preview in which to plan their actions or immediately after the object became visible. Viewing condition markedly affected performance, but the planning time allowed did not. Participants made the most errors suggesting premature collision with the object (shortest ‘braking’ times after peak deceleration; fastest velocity and widest grip at initial contact) under Base In prism viewing, consistent with over-reaching movements programmed to transport the hand beyond the actual target due to its ‘further’ VSD. Conversely, they produced the longest terminal reaches and grip closure times, with multiple corrections just before and after object contact, under the Plus lens (reduced disparity) condition. Base Out prism performance was intermediate between these two, with significant increases in additional forward movements during the transport end-phase, indicative of initial under-reaching in response to the target’s ‘nearer’ VSD. Our findings suggest dissociations between the role of vergence and binocular disparity in natural prehension movements, with vergence contributing mainly to reach planning and high-grade disparity cues providing particular advantages for grasp-point selection during grip programming and application.     

Evidence for nonretinal feedback in combined version-vergence eye movements

Recently, a quantitative model for the generation of rapid eye movements in direction and depth was proposed. In this scheme, the saccadic and the vergence system share a common initiation system and are controlled by local feedback loops based on efference copy signals. We have used a remembered-target double-step paradigm to test the idea that both subsystems are guid ed by extraretinal signals. The subject was instructed to move the binocular point of fixation to the remembered positions indicated by a double-step movement of the target, in direction and depth. Since both binocular refixations were made in complete darkness, correct execution of this task requires information about both the stored visual coordinates of the final target and the co-ordinates of the first movement. Binocular eye movements from five subjects were compared with predictions from two feed-forward models and a feedback model. Analysis of the pooled direction data showed that the feedback model performed best and fitted well. Qualitatively the same result was obtained in the vergence component, but in this case the goodness of fit was considerably less. These results, confirmed in each individual subject, show that the saccadic and vergence subsystem can use nonretinal information about a prior movement in direction and depth. Further analysis showed that the gain of the direction response of the second movement was, on average, roughly correct. By contrast, the vergence component of these responses was only about 60% of the required amplitude. Since the fit procedure gave the same weighting factors to the second target and to the first movement, we propose that the low vergence gain reflects mechanisms operating after the calculation of the motor error signal, possibly at the execution stage. Finally, we discuss the possibility of a central control stage keeping track of the ongoing movement sequence, based on a comparison of desired and current eye position signals.     

The effect of obstacle position on reach-to-grasp movements

Numerous everyday tasks require the nervous system to program a prehensile movement towards a target object positioned in a cluttered environment. Adult humans are extremely proficient in avoiding contact with any non-target objects (obstacles) whilst carrying out such movements. A number of recent studies have highlighted the importance of considering the control of reach-to-grasp (prehension) movements in the presence of such obstacles. The current study was constructed with the aim of beginning the task of studying the relative impact on prehension as the position of obstacles is varied within the workspace. The experimental design ensured that the obstacles were positioned within the workspace in locations where they did not interfere physically with the path taken by the hand when no obstacle was present. In all positions, the presence of an obstacle caused the hand to slow down and the maximum grip aperture to decrease. Nonetheless, the effect of the obstacle varied according to its position within the workspace. In the situation where an obstacle was located a small distance to the right of a target object, the obstacle showed a large effect on maximum grip aperture but a relatively small effect on movement time. In contrast, an object positioned in front and to the right of a target object had a large effect on movement speed but a relatively small effect on maximum grip aperture. It was found that the presence of two obstacles caused the system to decrease further the movement speed and maximum grip aperture. The position of the two obstacles dictated the extent to which their presence affected the movement parameters. These results show that the anticipated likelihood of a collision with potential obstacles affects the planning of movement duration and maximum grip aperture in prehension. Electronic Publication     

Vergence-dependent adaptation of the vestibulo-ocular reflex

The gain of the vestibulo-ocular reflex (VOR) normally depends on the distance between the subject and the visual target, but it remains uncertain whether vergence angle can be linked to changes in VOR gain through a process of context-dependent adaptation. In this study, we examined this question with an adaptation paradigm that modified the normal relationship between vergence angle and retinal image motion. Subjects were rotated sinusoidally while they viewed an optokinetic (OKN) stimulus through either diverging or converging prisms. In three subjects the diverging prisms were worn while the OKN stimulus moved out of phase with the head, and the converging prisms were worn when the OKN stimulus moved in-phase with the head. The relationship between the vergence angle and OKN stimulus was reversed in the fourth subject. After 2 h of training, the VOR gain at the two vergence angles changed significantly in all of the subjects, evidenced by the two different VOR gains that could be immediately accessed by switching between the diverged and converged conditions. The results demonstrate that subjects can learn to use vergence angle as the contextual cue that retrieves adaptive changes in the angular VOR.     

Coordination between the transport and the grasp components during prehension movements

In this study, the possible influence of the transport on the grasp component of prehension movements was investigated. The first phase of the transport (acceleration phase) and of the grasp (finger aperture phase) kinematics were studied under conditions of visual and non-visual object presentation (prehension experiment). In the non-visual condition, object size was estimated by haptics and object position was estimated by proprioception. Eight subjects were required to reach and grasp three objects of different size located at two distances. An additional experiment (matching experiment) was carried out to control the scaling of object size in the two conditions. The results showed that in the matching experiment size estimation for large objects was similar in the two conditions, whereas small stimuli were underestimated in the haptic condition. In the prehension experiment, maximal finger aperture and velocity of finger aperture were greater in the non-visual than in the visual condition, and the difference was greater for small than for large stimuli. Moreover, in both conditions, finger opening was larger for prehension movements directed to the far than to the near objects, but only for smaller stimuli. Hand trajectory variability increased in the non-visual condition and with the distance, whereas finger opening variability was only affected by the non-visual condition. For smaller stimuli, increased finger opening with distance was positively correlated with the increase in wrist variability in the visual condition, but not in the non-visual condition. Furthermore, increased finger opening between visual and non-visual conditions was correlated with the increase in wrist variability, for smaller objects at the near object location. No positive correlations were found between finger opening and grip variability. These results are interpreted in favour of the dependence of finger opening on transport, when control requirements during reaching increase.     

Reaching for virtual objects: binocular disparity and the control of prehension

Although, in principle, binocular cues provide veridical information about the three-dimensional shape of objects, our perception on the basis of these cues is distorted systematically. The consequences of these distortions may be less serious than they first appear, however, since in everyday life we rarely are required to judge the absolute shape, size or distance of objects. An important exception to this is in the control of prehension, where veridical information about an object to be grasped is required to plan the transport of the hand and to select the most appropriate grip. Here we investigate whether binocular cues provide accurate depth information for the control of prehension using disparity-defined, virtual objects and report that whilst binocular disparity can support prehensile movements, the kinematic indices, which reflect distance-reached and perceived size, show clear biases. These results suggest that accurate metric depth information for the control of prehension is not available from binocular cues in isolation. Electronic Publication     

Adaptive changes in vergence eye movements induced by vergence-vestibular interaction training in monkeys

Clear vision of objects moving in three-dimensional space near an observer is attained by a combination of smooth-pursuit and vergence eye movements. The two systems must interact with the vestibular system to maintain the image of the object on the fovea. Previous studies showed that training with smooth-pursuit vestibular interactions resulted in adaptive changes in the smooth-pursuit response. Although vergence and smooth-pursuit systems are thought to have separate neural substrates, recent studies indicate that the caudal parts of the frontal eye fields that receive vestibular inputs contain neurons that discharge in response to combinations of smooth-pursuit and vergence. This combination of discharge sensitivities suggests the possibility that adaptive changes may be induced in the vergence system by vestibular inputs during vergence-pursuit training. To explore this possibility, we examined the effects of training with conflicting vestibular and vergence tracking in four head-stabilized monkeys. Animals were rewarded for tracking a laser spot that moved towards or away from them at 1 Hz in phase with sinusoidal whole-body rotation (±5°) in the pitch plane; the spot moved closer when the monkeys nose moved downward. From the monkeys point of view, the spot moved sinusoidally 10–66 cm in front of them along the mid-sagittal plane, requiring symmetrical vergence eye movements of 4.8° for each eye. Eye movements induced by equivalent spot motion at 0.3–1.0 Hz with or without chair rotation were examined before and after training for each session (0.5–1.0 h). Before training, pitch rotation alone in complete darkness did not induce vergence eye movements in any of the monkeys tested. Vergence tracking without chair rotation showed decreased gain and increased phase lag (re vergence target velocity) at frequencies above 0.5 Hz. After training, the vergence response during chair rotation with the spot showed significantly higher gains and smaller phase lags at 0.3–1.0 Hz in all monkeys. Pitch rotation alone in complete darkness induced vergence eye movements with gains (eye vergence/chair) of 0.15–0.35 after training in two monkeys. These results suggest that vestibular information can be used effectively to modify vergence tracking.