Segment interdependency and gaze anchoring during manual two-segment sequences

Eye-hand coordination during sequential reaching to uncertain targets: the effect of task difficulty, target width, movement amplitude, and task scaling

When making reciprocal hand movements between two fixed/known targets, Fitts' law states that movement time (MT) is a linear function of the index of difficulty (ID) set by the ratio between movement amplitude (A) and target width (W). Crucially ID also impacts eye-hand coordination. However, because known/fixed targets limit the usefulness of eye movements, and because hand dynamics changes drastically with ID (either continuous or discrete), we reexamined this issue using a variant of the Fitts task in which the next target position was assigned randomly and unveiled only when the ongoing target was reached. Practically, hand and eye movements were recorded in participants (N = 25) who had to successively reach 12 circular targets of width W (0.3, 0.6, or 1.2 cm) separated by an amplitude A (5, 10, or 20 cm), allowing to examine IDs ranging from 2.36 to 6.08 bits. Introducing target uncertainty did not alleviate the linear relationship between MT and ID (R2 = 0.99), neither the impact of ID on hand kinematics. Importantly, the influence of ID on eye-hand coordination persisted. Notably, by cross-correlating eye and hand signals, we show a trade-off between its temporal and spatial aspects. Finally, we found that eye-hand coordination was influenced to a larger extent by A than W, making it prone to task scaling effects (differences in AW combinations resulting in similar ID). Altogether these results reinforce the critical role of task difficulty on eye-hand coordination and the need to simultaneously consider its temporal and spatial aspects.

The Effects of Constraining Head Rotation on Eye and Whole-Body Coordination During Standing Turns at Different Speeds

A limitation of the ability to rotate the head with respect to the upper body has been associated with turning problems; however, the extent of head constraints on whole-body coordination has not been fully determined. The aim of this study was to limit head on body rotation and observe the effects on whole-body coordination during standing turns at various speeds. Twelve participants completed standing turns at 180°. A Vicon motion system and a BlueGain Electrooculography system were used to record movement kinematics and measure horizontal eye movements, respectively. All participants were tested at 3 randomized speeds, and under 2 conditions with or without their head constrained using a head, neck, and chest brace which restricted neck movement. A repeated-measures analysis of variance found a significant main effect of turning speed on the onset latency of all segments, peak head-thorax angular separation, and step characteristics. Constraining the head rotation had multiple significant effects including delayed onset latency and decreased intersegmental coordination defined as peak head segmental angular separations, increased total step and step duration, and decreased step size. This indicates the contribution of speed, head, and neck constraints, which have been associated with falls during turning and whole-body coordination.

Delay of gaze fixation during reaching movement with the non-dominant hand to a distant target

The present study examined the effects of hand and task difficulty on eye-hand coordination related to gaze fixation behavior (i.e., fixating a gaze to the target until reach completion) in single reaching movements. Twenty right-handed young adults made reaches on a digitizer, while looking at a visual target and feedback of hand movements on a computer monitor. Task difficulty was altered by having three target distances. In a small portion of trials, visual feedback was randomly removed at the target presentation. The effect of a moderate amount of practice was also examined using a randomized trial schedule across target-distance and visual-feedback conditions in each hand. The results showed that the gaze distances covered during the early reaching phase were reduced, and the gaze fixation to the target was delayed when reaches were performed with the left hand and when the target distance increased. These results suggest that when the use of the non-dominant hand or an increased task difficulty reduces the predictability of hand movements and its sensory consequences, eye-hand coordination is modified to enhance visual monitoring of the reach progress prior to gaze fixation. The randomized practice facilitated this process. Nevertheless, variability of reach trajectory was more increased without visual feedback for right-hand reaches, indicating that control of the dominant arm integrates more visual feedback information during reaches. These results together suggest that the earlier gaze fixation and greater integration of visual feedback during right-hand reaches contribute to the faster and more accurate performance in the final reaching phase.

Gaze control during reaching is flexibly modulated to optimize task outcome

When reaching for an object with the hand, the gaze is usually directed at the target. In a laboratory setting, fixation is strongly maintained at the reach target until the reaching is completed, a phenomenon known as "gaze anchoring." While conventional accounts of such tight eye-hand coordination have often emphasized the internal synergetic linkage between both motor systems, more recent optimal control theories regard motor coordination as the adaptive solution to task requirements. We here investigated to what degree gaze control during reaching is modulated by task demands. We adopted a gaze-anchoring paradigm in which participants had to reach for a target location. During the reach, they additionally had to make a saccadic eye movement to a salient visual cue presented at locations other than the target. We manipulated the task demands by independently changing reward contingencies for saccade reaction time (RT) and reaching accuracy. On average, both saccade RTs and reach error varied systematically with reward condition, with reach accuracy improving when the saccade was delayed. The distribution of the saccade RTs showed two types of eye movements: fast saccades with short RTs, and voluntary saccade with longer RTs. Increased reward for high reach accuracy reduced the probability of fast saccades but left their latency unchanged. The results suggest that gaze anchoring acts through a suppression of fast saccades, a mechanism that can be adaptively adjusted to the current task demands.NEW & NOTEWORTHY During visually guided reaching, our eyes usually fixate the target and saccades elsewhere are delayed ("gaze anchoring"). We here show that the degree of gaze anchoring is flexibly modulated by the reward contingencies of saccade latency and reach accuracy. Reach error became larger when saccades occurred earlier. These results suggest that early saccades are costly for reaching and the brain modulates inhibitory online coordination from the hand to the eye system depending on task requirements.

A condition that produces sensory recalibration and abolishes multisensory integration

We examined the influence of extended exposure to a visuomotor rotation, which induces both motor adaptation and sensory recalibration, on (partial) multisensory integration in a cursor-control task. Participants adapted to a 30° (adaptation condition) or 0° (control condition) visuomotor rotation by making center-out movements to remembered targets. In subsequent test trials of sensory integration, they made center-out movements with variable visuomotor rotations and judged the position of hand or cursor at the end of these movements. Test trials were randomly embedded among twice the number of maintenance trials with 30° or 0° rotation. The biases of perceived hand (or cursor) position toward the cursor (or hand) position were measured. We found motor adaptation together with proprioceptive and visual recalibrations in the adaptation condition. Unexpectedly, multisensory integration was absent in both the adaptation and control condition. The absence stemmed from the extensive experience of constant visuomotor rotations of 30° or 0°, which probably produced highly precise predictions of the visual consequences of hand movements. The frequently confirmed predictions then dominated the estimate of the visual movement consequences, leaving no influence of the actual visuomotor rotations in the minority of test trials. Conversely, multisensory integration was present for sensed hand positions when these were indirectly assessed from movement characteristics, indicating that the relative weighting of discrepant estimates of hand position was different for motor control. The existence of a condition that abolishes multisensory integration while keeping sensory recalibration suggests that mechanisms that reduce sensory discrepancies (partly) differ between integration and recalibration.

Effects of auditory feedback on movements with two-segment sequence and eye-hand coordination: Using a short auditory contact cue

During sequential reaches to multiple targets, eye and hand movements are highly coordinated, and the gaze is anchored to each target until the reaching hand makes contact to each of them. Such contact events are monitored by multimodal (visual, proprioceptive) sensory systems, and one function of the gaze anchoring to each target is verification of successful target contact (reach completion). The present study focused on this verification function and examined how planning and control of eye and hand movements during two-segment eye-hand movements are affected by augmented auditory feedback of reach completion. Young adults made a reach to the first target with a saccade, and then made another saccade to the second target in blocked trials. An auditory target-contact cue condition delivered four short sounds during the initial reach, and the last sound was synchronized with target contact, whereas a control condition lacked the last target-contact sound. The results showed that saccadic reaction time increased with the target-contact cue, especially when the reaching accuracy demand was high. The reach also became slower with lower peak velocity and longer time to peak velocity with that cue, suggesting that the limb-motor system lower the preplanned speed of the reach in a top-down fashion for a better preparation toward reach completion. However, no auditory effects were found for the timing of gaze shift to the second target. These results were different from those seen in previous studies, indicating that the effects of the additional auditory contact feedback differ depending on behavioral tasks and cue characteristics.

Visual and proprioceptive recalibrations after exposure to a visuomotor rotation

Adaptation to a visuomotor rotation in a cursor-control task is accompanied by proprioceptive recalibration, whereas the existence of visual recalibration is uncertain and has even been doubted. In the present study, we tested both visual and proprioceptive recalibration; proprioceptive recalibration was not only assessed by means of psychophysical judgments of the perceived position of the hand, but also by an indirect procedure based on movement characteristics. Participants adapted to a gradually introduced visuomotor rotation of 30° by making center-out movements to remembered targets. In subsequent test trials, they made center-out movements without visual feedback or observed center-out motions of a cursor without moving the hand. In each test trial, they judged the endpoint of hand or cursor by matching the position of the hand or of a visual marker, respectively, moving along a semicircular path. This path ran through all possible endpoints of the center-out movements. We observed proprioceptive recalibration of 7.3° (3.1° with the indirect procedure) and a smaller, but significant, visual recalibration of 1.3°. Total recalibration of 8.6° was about half as strong as motor adaptation, the adaptive shift of the movement direction. The evidence of both proprioceptive and visual recalibration was obtained with a judgment procedure that suggests that recalibration is restricted to the type of movement performed during exposure to a visuomotor rotation. Consequently, identical physical positions of the hand can be perceived differently depending on how they have been reached, and similarly identical positions of a cursor on a monitor can be perceived differently.

Effects of Hand and Hemispace on Multisensory Integration of Hand Position and Visual Feedback

The brain generally integrates a multitude of sensory signals to form a unified percept. Even in cursor control tasks, such as reaching while looking at rotated visual feedback on a monitor, visual information on cursor position and proprioceptive information on hand position are partially integrated (sensory coupling), resulting in mutual biases of the perceived positions of cursor and hand. Previous studies showed that the strength of sensory coupling (sum of the mutual biases) depends on the experience of kinematic correlations between hand movements and cursor motions, whereas the asymmetry of sensory coupling (difference between the biases) depends on the relative reliabilities (inverse of variability) of hand-position and cursor-position estimates (reliability rule). Furthermore, the precision of movement control and perception of hand position are known to differ between hands (left, right) and workspaces (ipsilateral, contralateral), and so does the experience of kinematic correlations from daily life activities. Thus, in the present study, we tested whether strength and asymmetry of sensory coupling for the endpoints of reaches in a cursor control task differ between the right and left hand and between ipsilateral and contralateral hemispace. No differences were found in the strength of sensory coupling between hands or between hemispaces. However, asymmetry of sensory coupling was less in ipsilateral than in contralateral hemispace: in ipsilateral hemispace, the bias of the perceived hand position was reduced, which was accompanied by a smaller variability of the estimates. The variability of position estimates of the dominant right hand was also less than for the non-dominant left hand, but this difference was not accompanied by a difference in the asymmetry of sensory coupling – a violation of the reliability rule, probably due a stronger influence of visual information on right-hand movements. According to these results, the long-term effects of the experienced kinematic correlation between hand movements and cursor motions on the strength of sensory coupling are generic and not specific for hemispaces or hands, whereas the effects of relative reliabilities on the asymmetry of sensory coupling are specific for hemispaces but not for hands.

Effects of auditory feedback on movements with two-segment sequence and eye-hand coordination

The present study investigated the effect of auditory feedback on planning and control of two-segment reaching movements and eye-hand coordination. In particular, it was examined whether additional auditory information indicating the progression of the initial reach (i.e., passing the midway and contacting the target) affects the performance of that reach and gaze shift to the second target at the transition between two segments. Young adults performed a rapid two-segment reaching task, in which both the first and second segments had two target sizes. One out of three auditory feedback conditions included the reach-progression information: a continuous tone was delivered at a consistent timing during the initial reach from the midway to the target contact. Conversely, the other two were control conditions: a continuous tone was delivered at a random timing in one condition or not delivered in the other. The results showed that the initial reach became more accurate with the auditory reach-progression cue compared to without any auditory cue. When that cue was available, movement time of the initial reach was decreased, which was accompanied by an increased peak velocity and a decreased time to peak velocity. These findings suggest that the auditory reach-progression feedback enhanced the preplanned control of the initial reach. Deceleration time of that reach was also decreased with auditory feedback, but it was observed regardless of whether the sound contained the reach-progression information. At the transition between the two segments, the onset latencies of both the gaze shift and reach to the second target became shorter with the auditory reach-progression cue, the effect of which was pronounced when the initial reach had a higher terminal accuracy constraint. This suggests that the reach-progression cue enhanced verification of the termination of initial reach, thereby facilitating the initiation of eye and hand movements to the second target. Taken together, the additional auditory information of reach-progression enhances the planning and control of multi-segment reaches and eye-hand coordination at the segment transition.

Eye-hand coordination during visuomotor adaptation: effects of hemispace and joint coordination

We previously examined adaptive changes of eye-hand coordination during learning of a visuomotor rotation. Gazes during reaching movements were initially directed to a feedback cursor in early practice, but were gradually shifted toward the target with more practice, indicating an emerging gaze anchoring behavior. This adaptive pattern reflected a functional change of gaze control from exploring the cursor-hand relation to guiding the hand to the task goal. The present study further examined the effects of hemispace and joint coordination associated with target directions on this behavior. Young adults performed center-out reaching movements to four targets with their right hand on a horizontal digitizer, while looking at a rotated visual feedback cursor on a computer monitor. To examine the effect of hemispace related to visual stimuli, two out of the four targets were located in the ipsilateral workspace relative to the hand used, the other two in the contralateral workspace. To examine the effect of hemispace related to manual actions, two among the four targets were related to reaches made in the ipsilateral workspace, the other two to reaches made in the contralateral workspace. Furthermore, to examine the effect of the complexity of joint coordination, two among the four targets were reaches involving a direct path from the start to the target involving elbow movements (simple), whereas the other two targets were reaches involving both shoulder and elbow movements (complex). The results showed that the gaze anchoring behavior gradually emerged during practice for reaches made in all target directions. The speed of this change was affected mainly by the hemispace related to manual actions, whereas the other two effects were minimal. The gaze anchoring occurred faster for the ipsilateral reaches than for the contralateral reaches; gazes prior to the gaze anchoring were also directed less at the cursor vicinity but more at the mid-area between the starting point and the target. These results suggest that ipsilateral reaches result in a better predictability of the cursor-hand relation under the visuomotor rotation, thereby prompting an earlier functional change of gaze control through practice from a reactive to a predictive control.

The effects of constraining vision and eye movements on whole-body coordination during standing turns

Turning the body towards a new direction is normally achieved via a top-down synergy whereby gaze (eye direction in space) leads the upper body segments, which in turn lead the feet. These anticipatory eye movements are observable even in darkness and constraining the initial eye movements modifies the stereotyped top-down reorientation sequence. Our aim was to elucidate the relative contributions of vision and eye movements to whole-body coordination during large standing turns by observing the effects of separately removing visual information or suppressing eye movements throughout the turn. We predicted that constraining eye movements would modify the steering synergy, whereas removing vision would have little effect. We found that preventing eye movements modified both timing and spatial characteristics of axial segment and feet rotation. When gaze was fixed, gait initiation, but not axial segment rotation, was delayed in comparison to both full vision and no vision turns. When eye movements were prevented, the predictable relationship between the extent head rotation led the body and peak head angular velocity was abolished suggesting that anticipatory head movements normally subserve gaze behaviour. In addition, stepping frequency significantly reduced during the gaze fixation condition but not during the no-vision condition, suggesting that oculomotor control is linked to stepping behaviour.

Eye-Hand Coordination during Visuomotor Adaptation with Different Rotation Angles: Effects of Terminal Visual Feedback

This study examined adaptive changes of eye-hand coordination during a visuomotor rotation task under the use of terminal visual feedback. Young adults made reaching movements to targets on a digitizer while looking at targets on a monitor where the rotated feedback (a cursor) of hand movements appeared after each movement. Three rotation angles (30°, 75° and 150°) were examined in three groups in order to vary the task difficulty. The results showed that the 30° group gradually reduced direction errors of reaching with practice and adapted well to the visuomotor rotation. The 75° group made large direction errors of reaching, and the 150° group applied a 180° reversal shift from early practice. The 75°and 150° groups, however, overcompensated the respective rotations at the end of practice. Despite these group differences in adaptive changes of reaching, all groups gradually adapted gaze directions prior to reaching from the target area to the areas related to the final positions of reaching during the course of practice. The adaptive changes of both hand and eye movements in all groups mainly reflected adjustments of movement directions based on explicit knowledge of the applied rotation acquired through practice. Only the 30° group showed small implicit adaptation in both effectors. The results suggest that by adapting gaze directions from the target to the final position of reaching based on explicit knowledge of the visuomotor rotation, the oculomotor system supports the limb-motor system to make precise preplanned adjustments of reaching directions during learning of visuomotor rotation under terminal visual feedback.

Gaze locations affect explicit process but not implicit process during visuomotor adaptation

The role of vision in implicit and explicit processes involved in adaptation to novel visuomotor transformations is not well-understood. We manipulated subjects' gaze locations through instructions during a visuomotor rotation task that established a conflict between implicit and explicit processes. Subjects were informed of a rotated visual feedback (45° counterclockwise from the desired target) and instructed to counteract it by using an explicit aiming strategy to the neighboring target (45° clockwise from the target). Simultaneously, they were instructed to gaze at either the desired target (target-gaze group), the neighboring target (hand-target-gaze group), or anywhere (free-gaze group) during aiming. After initial elimination of behavioral errors caused by strategic aiming, the subjects gradually overcompensated the rotation in the early practice, thereby increasing behavioral errors (i.e., a drift). This was caused by an implicit adaptation overriding the explicit strategy. Notably, prescribed gaze locations did not affect this implicit adaptation. In the late practice, the target-gaze and free-gaze groups reduced the drift, whereas the hand-target-gaze group did not. Furthermore, the free-gaze group changed gaze locations for strategic aiming through practice from the neighboring target to the desired target. The onset of this change was correlated with the onset of the drift reduction. These results suggest that gaze locations critically affect explicit adjustments of aiming directions to reduce the drift by taking into account the implicit adaptation that is occurring in parallel. Taken together, spatial eye-hand coordination that ties the gaze and the reach target influences the explicit process but not the implicit process.

Eye-hand coordination during visuomotor adaptation with different rotation angles

This study examined adaptive changes of eye-hand coordination during a visuomotor rotation task. Young adults made aiming movements to targets on a horizontal plane, while looking at the rotated feedback (cursor) of hand movements on a monitor. To vary the task difficulty, three rotation angles (30°, 75°, and 150°) were tested in three groups. All groups shortened hand movement time and trajectory length with practice. However, control strategies used were different among groups. The 30° group used proportionately more implicit adjustments of hand movements than other groups. The 75° group used more on-line feedback control, whereas the 150° group used explicit strategic adjustments. Regarding eye-hand coordination, timing of gaze shift to the target was gradually changed with practice from the late to early phase of hand movements in all groups, indicating an emerging gaze-anchoring behavior. Gaze locations prior to the gaze anchoring were also modified with practice from the cursor vicinity to an area between the starting position and the target. Reflecting various task difficulties, these changes occurred fastest in the 30° group, followed by the 75° group. The 150° group persisted in gazing at the cursor vicinity. These results suggest that the function of gaze control during visuomotor adaptation changes from a reactive control for exploring the relation between cursor and hand movements to a predictive control for guiding the hand to the task goal. That gaze-anchoring behavior emerged in all groups despite various control strategies indicates a generality of this adaptive pattern for eye-hand coordination in goal-directed actions.