Perceptuomotor Compatibility in Pursuit Tracking of Two-Dimensional Movements

In a previous article, we reported an investigation of visuomanual pursuit tracking of unpredictable two-dimensional targets. This article extends the study to the tracking of predictable stimuli. In both investigations, the target trajectory was elliptical. The experimental factors we varied were the orientation of the major axis of the ellipses (horizontal or vertical), the period of the movement (9.65 to 1.61 s), and the law of motion (natural vs. transformed). In the natural condition (L), the motion results from the combination of harmonic functions, as would be the case if the target were generated by a human. In the transformed (T) condition, the law of motion departs systematically from this natural model. The main results of the study are as follows: (a) Satisfactory performance is achieved only in the natural condition. Pursuit movements obey the same constraints observed in spontaneous movements. (b) Predictability affects significantly the average delay between target and pursuit. (c) Each component of the pursuit movements depends on both components of the targets. Thus, two-dimensional tracking generalizes significantly the classical one-dimensional condition. (d) The simple model developed previously to describe performance with unpredictable targets can be generalized to cover the present case as well.     

Perceptuomotor compatibility in pursuit tracking of two-dimensional movements

In a previous article, we reported an investigation of visuomanual pursuit tracking of unpredictable two-dimensional targets. This article extends the study to the tracking of predictable stimuli. In both investigations, the target trajectory was elliptical. The experimental factors we varied were the orientation of the major axis of the ellipses (horizontal or vertical), the period of the movement (9.65 to 1.61 s), and the law of motion (natural vs. transformed). In the natural condition (L), the motion results from the combination of harmonic functions, as would be the case if the target were generated by a human. In the transformed (T) condition, the law of motion departs systematically from this natural model. The main results of the study are as follows: (a) Satisfactory performance is achieved only in the natural condition. Pursuit movements obey the same constraints observed in spontaneous movements. (b) Predictability affects significantly the average delay between target and pursuit. (c) Each component of the pursuit movements depends on both components of the targets. Thus, two-dimensional tracking generalizes significantly the classical one-dimensional condition. (d) The simple model developed previously to describe performance with unpredictable targets can be generalized to cover the present case as well.     

Is target movement as well as target displacement a stimulus for saccadic eye movements?

The effects of visual movement on saccadic eye movement have been examined. In a classic apparent-movement demonstration with two successively exposed line-segment targets the quality of the movement is dependent on the relative orientation of the line segments. If saccadic eye movements are elicited between the targets in this situation, the configuration leading to optimal apparent movement also leads to the shortest-latency saccades. When a single line segment is succeeded by two line segments flanking it on opposite sides, and if one of these has the same orientation as the initial one and the other a different orientation, then apparent motion is seen between the two lines with the same orientation. However, the direction of saccades elicited in this configuration is not influenced by the relative orientations of the line segments. The two results together suggest that the effect of visual movement on saccadic eye movement is nonspecific.     

The constancy of object orientation: Compensation for ocular rotation

When a thin horizontal line is displaced, either left or right of straight ahead, or when a vertical line is displaced up or down, systematic changes occur in the binocular disparity associated with the target. In threeexperiments, Ss matched the orientation of displaced targets with a variable comparison line. Estimates of apparent displacement with a pointing technique were also made. Since head position was fixed, apparent displacement was mediated by the angle of ocular rotation. Near perfect matches were made with vertical targets, but horizontal targets produced errors suggestive of underestimation of apparent displacement. However, the pointing data did not yield clear evidence for this view. Control data denien the possible role of the induced effect (IE) in matching the horizontal targets, and the result were discussed in the context of orientation constancy based upon compensation for displacement.     

A simple device for detecting departures from visual fixation

A device for detecting departures from visual fixation is described. The principle is to present control targets to one or both eyes at the projected position of the blind spot. When fixation is maintained the control target or targets are not seen. Departures from fixation manifest themselves by the reappearance of the target or targets in the visual field. The system allows approximately 1 deg of movement or less away from the fixation point in either direction.     

Perceived localizations and eye movements with action‐generated and computer‐generated vanishing points of moving stimuli

When observers localize the vanishing point of a moving target, localizations are reliably displaced beyond the final position, in the direction the stimulus was travelling just prior to its offset. We examined modulations of this phenomenon through eye movements and action control over the vanishing point. In Experiment 1 with pursuit eye movements, localization errors were in movement direction, but less pronounced when the vanishing point was self‐determined by a key press of the observer. In contrast, in Experiment 2 with fixation instruction, localization errors were opposite movement direction and independent from action control. This pattern of results points at the role of eye movements, which were gathered in Experiment 3. That experiment showed that the eyes lagged behind the target at the point in time, when it vanished from the screen, but that the eyes continued to drift on the targets' virtual trajectory. It is suggested that the perceived target position resulted from the spatial lag of the eyes and of the persisting retinal image during the drift.     

Evidence for frames of reference based on pursuit eye movements

Lighted points that moved as if located on the rim of a rolling wheel were displayed to subjects whose task was to describe the pattern they perceived. The perceived patterns could be classified into one of four categories ranging from cycloidal to circular motion. Pursuit eye movements were controlled by having subjects track a fixation point that moved in the direction of the rolling wheel on a path just above the wheel’s rim. With respect to the translatory velocity of the rolling wheel, the velocity of the fixation point was 100%, 67%, 33%, or 0% (i.e., stationary). The patterns traced out by the points on the wheel were perceived to become increasingly circular as pursuit eye movements more closely matched the translatory speed of the rolling wheel. This is taken to support Stoper’s hypothesis that pursuit eye movements can establish a frame of reference for motion analysis.     

The perception of briefly exposed point-sources of light

Point-sources of light (dots) were exposed for 10 to 50 msec, before five dark-adapted subjects in a dimly illuminated room. During voluntary fixation with one eye, the target was exposed some 10° on the nasal side of the optic axis. The intensity X duration of all targets was 2 X threshold and they consisted of either a single dot, or a pair of dots separated by a distance that was less than that required for two-point discrimination. In two-thirds of trials both the single-dot and the two-dot targets were perceived as short thin lines of various orientation. Although individual percepts were unpredictable, there was a preferred or most likely orientation for responses to the single-dot target; this was near to the horizontal for all five subjects. There was no significant difference between the preferred orientations for single-dot targets tested at sites more than 1° apart in the visual field. When two single-dot targets, separated by about 1°, were exposed simultaneously, the orientations of the perceived lines sometimes differed by as much as 80°; occasionally, one target was reported as a dot while the other was seen as a thin line. If the single-dot was briefly exposed between two continuously visible and parallel straight lines, the target usually appeared as a thin line, parallel to the framing lines. Some of these results appear to be consistent with the hypothesis that the human visual cortex, like that of the cat and monkey, contains neurones that are orientation specific.     

Catching oriented objects

We have investigated how participants match the orientation of a line, which moves on a vertical screen towards the subject. On its path to the participant, the line could disappear at several positions. Participants were instructed to put a bar on a predefined interception point on the screen, such that the bar touched the screen with the same orientation as the moving line at the very moment when the line passed through the interception point or (in case of line disappearance) when the hidden line would pass through the interception point (like in catching). Participants made significant errors for oblique orientations, but not for vertical and horizontal orientations of the moving line. These errors were small or absent when the moving line was visible all the way along its path on the screen. However, these errors became larger when the line disappeared farther away from the interception point. In the second experiment we tested whether these errors could be related to errors in visual perception of line orientation. The results demonstrate that errors in matching of the bar do not correspond to the last perceived orientation of the line, but rather to the perceived orientation of the moving line near the beginning of the movement path. This corresponds to earlier observations that participants shortly track a moving target and then make a saccadic eye movement to the interception point.     

Reinstating salience effects over time: the influence of stimulus changes on visual selection behavior over a sequence of eye movements

Recently, we showed that salience affects initial saccades only in a static stimulus environment; subsequent saccades were unaffected by salience but, instead, were directed in line with task requirements (Siebold, van Zoest, & Donk, PLoS ONE 6(9): e23552, 2011). Yet multiple studies have shown that people tend to fixate salient regions more often than nonsalient ones when they are looking at images—in particular, when salience is defined by dynamic changes. The goal of the present study was to investigate how oculomotor selection beyond an initial saccade is affected by salience as derived from changing, as opposed to static, stimuli. Observers were presented with displays containing two fixation dots, one target, one distractor, and multiple background elements. They were instructed to fixate on one of the fixation dots and make a speeded eye movement to the target, either directly or preceded by an initial eye movement to the other fixation dot. In Experiment 1, target and distractor differed in orientation contrast relative to the background, such that one was more salient than the other, whereas in Experiments 2 and 3, the orientation contrast between the two elements was identical. Here, salience was implemented by a continuous luminance flicker or by a difference in luminance contrast, respectively, which was presented either simultaneously with display onset or contingent upon the first saccade. The results showed that in all experiments, initial saccades were strongly guided by salience, whereas second saccades were consistently goal directed if the salience manipulation was present from display onset. However, if the flicker or luminance contrast was presented contingent upon the initial saccade, salience effects were reinstated. We argue that salience effects are short-lived but can be reinstated if new information is presented, even when this occurs during an eye movement.     

Parametric adjustment in saccadic eye movements

During a change-of-fixation eye movement, the target toward which S was shifting his gaze was displaced 1° toward the original point of fixation so that the eye made an overshoot with respect to the new target position. When this was repeated several times in succession, the eye movement control system made an adjustment such that the overshoot gradually diminished. Ihe end-result of this “parametric adjustment” was that a visual target 10° from the fovea elicited an eye movement of only 9.1°.     

Reversals of visual depth caused by motion parallax

The role of the velocity and direction of retinal movement in the determination of apparent depth from motion parallax was examined. Motion parallax was produced either by linking the movement of random-dots to head movement or by making this motion independent of the head movement. The results show that apparent depth was largely estimated from the velocity difference between the stimuli. The direction of retinal movement in the absence of head movement did not determine whether the pattern appeared to protrude or recede. Information about direction linked to head movement was able to stabilize protrusion/recession by providing a cue for the location of the fixation point. Depth reversal occurred less frequently in the presence than in the absence of head movement. When the fixation point shifted from the apparently protruding pattern to the apparently receding pattern, in both the presence and absence of head movement, depth reversal was readily observed.     

Eye movements in response to real and apparent motions of acoustic targets.

Monitoring of eye movements resulting from the tracking of sound displacements in total darkness confirmed the generally accepted idea that smooth pursuit cannot be induced in the absence of a real visible target. Exclusively saccadic movements were obtained with real and apparent displacements of a constant frequency source and with frequency variations associated to spatially calibrated positions through training for 5 Ss. Smooth pursuit eye movements were only observed if S was allowed to point and follow with his hand the perceived position of acoustic targets.     

A new horizontal eye movement calibration method: Subject-controlled “smooth pursuit” and “zero drift”

A subject-controlled smooth pursuit method of calibration of eye movement apparatus is proposed for the horizontal axis which is more rapid, requires no interpolation and no linearity assumption, and is as accurate as the fixation point method usually used. The zero drift method of verification proposed then permits accuracy to be estimated in a way that distinguishes apparatus errors from fixation errors. This is only possible in the fixation point method by asking the subject to make repeated long fixations.     

No emotional "pop-out" effect in natural scene viewing

It has been shown that attention is drawn toward emotional stimuli. In particular, eye movement research suggests that gaze is attracted toward emotional stimuli in an unconscious, automated manner. We addressed whether this effect remains when emotional targets are embedded within complex real-world scenes. Eye movements were recorded while participants memorized natural images. Each image contained an item that was either neutral, such as a bag, or emotional, such as a snake or a couple hugging. We found no latency difference for the first target fixation between the emotional and neutral conditions, suggesting no extrafoveal "pop-out" effect of emotional targets. However, once detected, emotional targets held attention for a longer time than neutral targets. The failure of emotional items to attract attention seems to contradict previous eye-movement research using emotional stimuli. However, our results are consistent with studies examining semantic drive of overt attention in natural scenes. Interpretations of the results in terms of perceptual and attentional load are provided.     

Unintentional movements induced by sequential transient perturbations in a multi-joint positional task

We explored the phenomenon of unintentional movements of a multi-joint effector produced by multiple transient changes in the external force. The subjects performed a position-holding task against a constant bias force produced by a robot and were instructed not to intervene voluntarily with arm movements produced by changes in the robot force. The robot produced a smooth force increase leading to hand movement from the trunk, followed by a dwell time. Then, the force dropped to its initial value leading to hand movement toward the initial position but with an undershot. Such perturbation episodes were repeated four times in a row. The accumulated perturbation and undershoot distances kept increasing without saturation within the sequence of four perturbation episode. The limb apparent stiffness before dwell time increased over sequential perturbations while apparent stiffness after dwell time decreased. We interpret the results as consequences of a drift of the hand referent coordinate (RC) caused by a hypothesized RC-back-coupling mechanism and a coupled drift of the apparent stiffness. The results show that RC-back-coupling continues to lead to unintentional movements over repeated perturbations and is accompanied by a relatively slow re-setting process.     

场景情境和目标模板在视觉搜索中的交互作用

以真实场景图像中的物体搜索为实验任务, 操纵场景情境和目标模板, 采用眼动技术将搜索过程分为起始阶段、扫描阶段和确认阶段, 考察场景情境和目标模板对视觉搜索过程的影响机制。结果发现, 场景情境和目标模板的作用方式及时间点不同, 二者交互影响搜索的正确率和反应时, 仅场景情境影响起始阶段的时间, 随后二者交互影响扫描阶段和确认阶段的时间及主要眼动指标。作者由此提出了场景情境和目标模板在视觉搜索中的交互作用模型。     

A new illusion: The underestimation of distance during pursuit eye movements

A new visual illusion is reported. The apparent distance through which a displaced target appears to move is significantly shorter when pursuit tracked than when that same distance is observed by means of a saccadic eye movement. This misperception of distance seems to be related to the Aubert-Fleischl paradox, the underestimation of the velocity of a tracked target, and to reveal a consistent characteristic of the pursuit eye movement system.     

Effects of directional uncertainty on visually-guided joystick pointing

Reaction times generally follow the predictions of Hick's law as stimulus-response uncertainty increases, although notable exceptions include the oculomotor system. Saccadic and smooth pursuit eye movement reaction times are independent of stimulus-response uncertainty. Previous research showed that joystick pointing to targets, a motor analog of saccadic eye movements, is only modestly affected by increased stimulus-response uncertainty; however, a no-uncertainty condition (simple reaction time to 1 possible target) was not included. Here, we re-evaluate manual joystick pointing including a no-uncertainty condition. Analysis indicated simple joystick pointing reaction times were significantly faster than choice reaction times. Choice reaction times (2, 4, or 8 possible target locations) only slightly increased as the number of possible targets increased. These data suggest that, as with joystick tracking (a motor analog of smooth pursuit eye movements), joystick pointing is more closely approximated by a simple/choice step function than the log function predicted by Hick's law.     

Induced motion of a fixated target: influence of voluntary eye deviation.

Induced motion (IM) was observed in a fixated target in the direction opposite to the real motion of a moving background. Relative to a fixation target located straight ahead, IM decreased when fixation was deviated 10 degrees in the same direction as background motion and increased when fixation was deviated 10 degrees opposite background motion. These results are consistent with a "nystagmus-suppression" hypothesis for subjective motion of fixated targets: the magnitude of illusory motion is correlated with the amount of voluntary efference required to oppose involuntary eye movements that would occur in the absence of fixation. In addition to the form of IM studied, this explanation applies to autokinesis, apparent concomitant motion, and the oculogyral illusion. Accounts of IM that stress visual capture of vection, afferent mechanisms, egocenter deviations, or phenomenological principles, although they may explain some forms of IM, do not account for the present results.