Periodic change in phase relationship between target and hand motion during visuo-manual tracking task: Behavioral evidence for intermittent control

When one performs visuo-manual tracking tasks, velocity profile of hand movements shows discontinuous patterns even if the target moves smoothly. A crucial factor of this “intermittency” is considerable delay in the sensorimotor feedback loop, and several researchers have suggested that the cause is intermittent correction of motor commands. However, when and how the brain monitors task performance and updates motor commands in a continuous motor task is uncertain. We examined how tracking error was affected by the timing of target disappearance during a tracking task. Results showed that tracking error, defined as the average phase difference between target and hand, varied periodically in all conditions. Hand preceded target at one specific phase but followed it at another, implying that motor control was not performed in a temporally uniform manner. Tracking stability was evaluated by the variance in phase difference, and changed depending on the timing of target-removal. The variability was larger when target disappeared around turning points than that when it disappeared around the center of motion. This shows that visual information at turning points is more effectively exploited for motor control of sinusoidal target tracking, suggesting that our brain controls hand movements with intermittent reference to visual information.     

The Use of Prescanning in the Parameterization of Sequential Pointing and Reaching Movements

The accuracy of reaching movements improves when active gaze can be used to fixate on targets. The advantage of free gaze has been attributed to the use of ocular proprioception or efference signals for online control. The time course of this process, however, is not established, and it is unclear how far in advance gaze can move and still be used to parameterize subsequent movements. In this experiment, the authors considered the advantage of prescanning targets for both pointing and reaching movements. The authors manipulated the visual information and examined the extent to which prescanning of targets could compensate for a reduction in online visual feedback. In comparison with a conventional reaching/pointing condition, the error in pointing was reduced, the eye-hand lead decreased, and both the hand-closure time and the size of the maximum grip aperture in reaching were modulated when prescanning was allowed. These results indicate that briefly prescanning multiple targets just prior to the movement allows the refinement of subsequent hand movements that yields an improvement in accuracy. This study therefore provides additional evidence that the coordinate information arising from efference or ocular-proprioceptive signals can, for a limited period, be buffered and later used to generate a sequence of movements.     

The use of prescanning in the parameterization of sequential pointing and reaching movements

The accuracy of reaching movements improves when active gaze can be used to fixate on targets. The advantage of free gaze has been attributed to the use of ocular proprioception or efference signals for online control. The time course of this process, however, is not established, and it is unclear how far in advance gaze can move and still be used to parameterize subsequent movements. In this experiment, the authors considered the advantage of prescanning targets for both pointing and reaching movements. The authors manipulated the visual information and examined the extent to which prescanning of targets could compensate for a reduction in online visual feedback. In comparison with a conventional reaching/pointing condition, the error in pointing was reduced, the eye-hand lead decreased, and both the hand-closure time and the size of the maximum grip aperture in reaching were modulated when prescanning was allowed. These results indicate that briefly prescanning multiple targets just prior to the movement allows the refinement of subsequent hand movements that yields an improvement in accuracy. This study therefore provides additional evidence that the coordinate information arising from efference or ocular-proprioceptive signals can, for a limited period, be buffered and later used to generate a sequence of movements.     

Intermittency in human manual tracking tasks

We confirm Craik's (1947) observation that the human manual1y tracking a visual target behaves like an intermittent servo-control1er. Such tracking responses are indicative of "sampled" negative-feedback control but could be the result of other, continuous, mechanisms. Tracking performance therefore was recorded in a task in which visual feedback of the position of the hand-held joystick could be eliminated. Depriving the subjects of visual feedback led to smoother tracking and greatly reduced the signal power of their responses between 0.5-1.8 Hz. Their responses remained intermittent when they used feedback of their own position but not of the target to track a remembered (virtual) target. Hence, intermittency in tracking behavior is not exclusively a signature of visual feedback control but also may be a sign of feedback to memorized waveforms. Craik's (1947) suggestion that the intermittency is due to a refractory period following each movement was also tested. The errors measured at the start of each intermittent response, during tracking of slow waveforms, showed evidence of a small error deadzone (measuring 0.7 cm on the VDU screen or 0.80 degrees at the eye). At higher target speeds, however, the mean size of starting errors increased, and the upper boundary of the distribution of starting error was close to that expected of a refractory delay of approximately 170 ms between responses. We consider a model of the control system that can fit these results by incorporating an error deadzone within a feedback control loop. We therefore propose that the initiation of intermittent tracking responses may be limited by a positional error deadzone and that evidence for a refractory period between successive corrective movements can be satisfied without evoking an explicit timing or sampling mechanism.     

Control strategies when intercepting slowly moving targets

In 3 experiments, the authors investigated and described how individuals control manual interceptive movements to slowly moving targets. Participants (N = 8 in each experiment) used a computer mouse and a graphics tablet assembly to manually intercept targets moving across a computer screen toward a marked target zone. They moved the cursor so that it would arrive in the target zone simultaneously with the target. In Experiment 1, there was a range of target velocities, including some very slow targets. In Experiment 2, there were 2 movement distance conditions. Participants moved the cursor either the same distance as the target or twice as far. For both experiments, hand speed was found to be related to target speed, even for the very slowly moving targets and when the target-to-cursor distance ratios were altered, suggesting that participants may have used a strategy similar to tracking. To test that notion, in Experiment 3, the authors added a tracking task in which the participants tracked the target cursor into the target zone. Longer time was spent planning the interception movements; however, there was a longer time in deceleration for the tracking movements, suggesting that more visually guided trajectory updates were made in that condition. Thus, although participants scaled their interception movements to the cursor speed, they were using a different strategy than they used in tracking. It is proposed that during target interception, anticipatory mechanisms are used rather than the visual feedback mechanism used when tracking and when pointing to stationary targets.     

Progression-regression effects in tracking repeated patterns

Subjects used a position control system to perform compensatory tracking of a repeated input pattern. Tracking error was roughly proportional to the velocity of the input signal. Error magnitude decreased with practice and increased with the addition of a concurrent memory task. These effects can be modeled as progressive and regressive changes in how well subjects used control movement velocity and displayed error velocity to anticipate the input pattern and thereby reduce their effective time delay. The weighting of velocity cues in this model progressed with practice and regressed with the secondary task, even though the secondary task required no concurrent visual scanning or simultaneous motor response. This regression effect appears to indicate cognitive interference with the anticipation process. Stationary linear models provide a good approximation to the movement patterns; however, these models do not account for episodes of rapid pulse-like movements that were revealed in the ensemble-averaged trajectories.     

Audiovisual integration increases the intentional step synchronization of side-by-side walkers

When people walk side-by-side, they often synchronize their steps. To achieve this, individuals might cross-modally match audiovisual signals from the movements of the partner and kinesthetic, cutaneous, visual and auditory signals from their own movements. Because signals from different sensory systems are processed with noise and asynchronously, the challenge of the CNS is to derive the best estimate based on this conflicting information. This is currently thought to be done by a mechanism operating as a Maximum Likelihood Estimator (MLE). The present work investigated whether audiovisual signals from the partner are integrated according to MLE in order to synchronize steps during walking. Three experiments were conducted in which the sensory cues from a walking partner were virtually simulated. In Experiment 1 seven participants were instructed to synchronize with human-sized Point Light Walkers and/or footstep sounds. Results revealed highest synchronization performance with auditory and audiovisual cues. This was quantified by the time to achieve synchronization and by synchronization variability. However, this auditory dominance effect might have been due to artifacts of the setup. Therefore, in Experiment 2 human-sized virtual mannequins were implemented. Also, audiovisual stimuli were rendered in real-time and thus were synchronous and co-localized. All four participants synchronized best with audiovisual cues. For three of the four participants results point toward their optimal integration consistent with the MLE model. Experiment 3 yielded performance decrements for all three participants when the cues were incongruent. Overall, these findings suggest that individuals might optimally integrate audiovisual cues to synchronize steps during side-by-side walking.     

Visual attention to color: parvocellular guidance of attentional resources?

Although transient changes in luminance have been well documented to automatically attract attention to their location, experiments looking at abrupt changes in color have failed to find similar attentional capture. These results are consistent with current theories of the role of the magnocellular (M) and parvocellular (P) streams that postulate that the M stream, which is "color-blind," plays the dominant role in guiding attention and eye movements. The experiment reported here used stimuli that contained only information defined by color, and masked residual luminance information with dynamic noise, to assess the capacity of purely chromatic cues to automatically guide spatial attention. Such stimuli were as effective as those containing large luminance signals in guiding attention. To the extent that these purely chromatic signals isolated the P stream, these results suggest that this stream is also capable of automatic attentional capture. Hence, color vision not only aids target identification but also is a strong aid for target detection and localization.     

The illusion of control: Sequential dependencies underlie contingent attentional capture

The degree to which humans have top-down control over which information they process remains a central debate within the attention literature. Most of the evidence supporting the top-down control of visuospatial attention has come from cueing paradigms in which target stimuli are preceded by cues that are similar or dissimilar from the target. These studies find that the cues similar to targets capture attention, but dissimilar cues do not, suggesting the top-down control of attention. Here, we used a modified cueing paradigm to investigate an alternative possibility that the cue type differences are due to sequential dependency effects occurring between cue and target processing rather than the top-down control of attention. When individuals searched for color targets, we replicated contingent capture effects in RTs, which are susceptible to sequential dependencies, but memory performance was always best at the cued locations, regardless of the cue’s identity. When individuals searched for onset targets, we observed contingent capture in both tasks. These results demonstrate the utility of the memory probe paradigm and suggest an asymmetry between how strongly onsets and color defined cues capture attention.     

Sustained effect of auditory entrainment on sequential tapping: The role of movement path complexity

The effects of auditory-motor entrainment have generally been investigated with periodic movements. Previous research has focused on how auditory-motor entrainment is influenced by the temporal structure of rhythms. The present study aimed to investigate whether auditory entrainment improved timing performance of sequential movements with varied path structures, and whether path complexity would affect any possible sustained effect of auditory entrainment. We also investigated whether the sustained effect was moderated by hearing single- vs. multiple-pitch audio prompts. Thirty participants were enrolled to perform a sequential finger-tapping task with discrete targets, in which the algebraic ratio relation of path lengths was manipulated as path complexity. Participants completed three stages per trial: initiation (to introduce the path sequence), entrainment (tapping along with the auditory and visual cues), and timekeeping (repeating the sequence without cues). We found timing improvement in terms of mean asynchronies and absolute interval error decrease after auditory entrainment. Only interval accuracy performance during timekeeping and entrainment was affected by path complexity. Moreover, no clear difference was observed between the rhythm sets in terms of single vs. multiple pitches. In conclusion, we found that phase and interval duration accuracy of predefined isochronous sequential movements with varied path complexity can be improved by auditory entrainment, and that auditory entrainment affects our performance beyond the actual presence of the auditory cue.     

Effects of correct and transformed visual feedback on rhythmic visuo-motor tracking: tracking performance and visual search behavior

The effects of correct and transformed visual feedback on rhythmic unimanual visuo-motor tracking were examined, focusing on tracking performance (accuracy and stability) and visual search behavior. Twelve participants (reduced to 9 in the analyses) manually tracked an oscillating visual target signal in phase (by moving the hand in the same direction as the target signal) and in antiphase (by moving the hand in the opposite direction), while the frequency of the target signal was gradually increased to probe pattern stability. Besides a control condition without feedback, correct feedback (representing the actual hand movement) or mirrored feedback (representing the hand movement transformed by 180 degrees) were provided during tracking, resulting in either in-phase or antiphase visual motion of the target and feedback signal, depending on the tracking mode performed. The quality (accuracy and stability) of in-phase tracking was hardly affected by the two forms of feedback, whereas antiphase tracking clearly benefited from mirrored feedback but not from correct feedback. This finding extends previous results indicating that the performance of visuo-motor coordination tasks is aided by visual feedback manipulations resulting in coherently grouped (i.e., in-phase) visual motion structures. Further insights into visuo-motor tracking with and without feedback were garnered from the visual search patterns accompanying task performance. Smooth pursuit eye movements only occurred at lower oscillation frequencies and prevailed during in-phase tracking and when target and feedback signal moved in phase. At higher frequencies, point-of-gaze was fixated at a location that depended on the feedback provided and the resulting visual motion structures. During in-phase tracking the mirrored feedback was ignored, which explains why performance was not affected in this condition. Point-of-gaze fixations at one of the end-points were accompanied by reduced motor variability at this location, reflecting a form of visuo-motor anchoring that may support the pick up of discrete information as well as the control of hand movements to a desired location.     

Exaggerating temporal differences enhances recognition of individuals from point light displays

Humans are very good at perceiving each other's movements. In this article, we investigate the role of time-based information in the recognition of individuals from point light biological motion sequences. We report an experiment in which we used an exaggeration technique that changes temporal properties while keeping spatial information constant; differences in the durations of motion segments are exaggerated relative to average values. Participants first learned to recognize six individuals on the basis of a simple, unexaggerated arm movement. Subsequently, they recognized positively exaggerated versions of those movements better than the originals. Absolute duration did not appear to be the critical cue. The results show that time-based cues are used for the recognition of movements and that exaggerating temporal differences improves performance. The results suggest that exaggeration may reflect general principles of how diagnostic information is encoded for recognition in different domains.     

Visual attention amplifies response priming of pointing movements to color targets

We studied the influence of spatial visual attention on the time course of primed pointing movements. We measured pointing responses to color targets preceded by color stimuli priming either the same response as the target or the opposite response. The effects of visual attention at the prime and target locations were studied by varying both the cue-prime and prime-target intervals when presenting either exogenous attentional cues or, in a separate experiment, endogenous cues whose processing was a precondition for performing the task. Pointing trajectories revealed large priming effects in which pointing responses were first controlled by prime signals and then captured in midflight by target signals. Priming effects were strongly amplified when the relevant prime locations were visually attended at optimal cue-prime SOAs, with attention modulating the entire time course of the primed pointing movements. We propose that visual attention amplifies the earliest waves of visuomotor feedforward information, elicited in turn by primes and by targets.     

Arm-tracking performance with and without visual feedback in children and adults: developmental changes

Tracking performance was investigated in children (aged 6-7 and 10-11) and in adult subjects. Target signals, moving unpredictably along a straight line, were tracked with the preferred arm, alternately with and without visual feedback. Qualitative observations indicate that tracking is based on continuous adjustments of the ongoing response to the continuously changing target position. No step-and-hold strategy could be detected in any of the three age groups. Tracking performance was described with four simple parameters, derived from linear systems analysis: (a) the delay between target signal and tracking movement (DL); (b) performance at the low-frequency range (LF), (c) performance at the high-frequency range (HF); and (d) a measure of tracking quality or overall similarity in the shape of target signal and tracking movement (Q). There was a considerable improvement in tracking performance with age, even after the age of 10-11, which was mainly demonstrated by a decrease in DL and increases in HF and Q. Tracking performance decreased only to a small extent when visual feedback was withdrawn. Age-related differences in the contribution of visual feedback to tracking performance could not be demonstrated.     

Visual control of hand action

Recent investigations in normal and brain-damaged individuals have begun to identify the types of visual information used to plan and guide reaches. Binocular visual cues have been shown to be important for both movement planning and on-line guidance of hand movements, while emerging evidence suggests that dynamic visual analysis of the moving limb may provide a rich source of information for precise control of the hand in flight. Reaching movements appear to be planned to follow what is perceived to be a straight trajectory in peripersonal space. Furthermore, the process of selecting visual targets appears to influence hand trajectories, with hand movements curving away from non-target objects. This behaviour may be explained most effectively by a dynamic representation of space which is sculpted by attentional mechanisms into selected (target) and inhibited (non-target) regions. The role of attention in movement planning in individuals with attentional disorders is controversial. Patients with visual neglect have impairments of visuomotor control including reaches that, under certain conditions, are significantly more curved than those of normal individuals. The representations of space that neglect patients use to plan reaches may be distorted by impairments in the mechanisms that normally act to select target regions and inhibit non-target zones.     

Evidence for an error deadzone in compensatory tracking

Humans and monkeys show intermittent arm movements while tracking moving targets. This intermittency has been explained by postulating either a psychological refractory period after each movement and/or an error deadzone, an area surrounding the target within which movements are not initiated. We present a technique to detect and quantify the size of this deadzone, using a compensatory tracking paradigm that distinguishes it from a psychological refractory period. An artificial deadzone of variable size was added around a visual target displayed on a computer screen. While the subject was within this area, he received visual feedback that showed him to be directly on target. The presence of this artificial deadzone could affect tracking performance only if it exceeded the size of his intrinsic deadzone. Therefore, the size of artificial deadzone at which performance began to be affected revealed the size of the intrinsic deadzone. Measured at the subjects' eye, the deadzone was found to vary between 0.06 and 0.38 degrees, depending on the tracking task and viewing conditions; on the screen, this range was 1.3 mm to 3.3 mm. It increased with increasing speed of the target, with increasing viewing distance, and when the amplitude of the movement required was reduced. However, the deadzone size was not significantly correlated with the subjects' level of performance. We conclude that an intrinsic deadzone exists during compensatory tracking, and we suggest that its size is set by a cognitive process not simply related to the difficulty of the tracking task.     

Sex differences and the effect of instruction on reorientation abilities by humans

This study examined whether differences in the amount of information provided to men and women, in the form of verbal instruction, influenced their encoding during a reorientation task. When a navigator needs to orient, featural (e.g., colour or texture) and geometry (e.g., metric information) are used to determine which direction to begin traveling. The current study used a spatial reorientation task to examine how men and women use featural and geometric cues and whether the content of the task’s instructions influenced how these cues were used. Participants were trained to find a target location in a rectangular room with distinctive objects situated at each corner. Once the participants were accurately locating the target, various tests manipulating the spatial information were conducted. We found both men and women encoded the featural cues, and even though the features provided reliable information, participants generally showed an encoding of geometry. However, when participants were not provided with any information about the spatial aspects of the task in the instructions, they failed to encode geometry. We also found that women used distant featural cues as landmarks when the featural cue closest to the target was removed, whereas men did not. Yet, when the two types of cues were placed in conflict, both sexes weighed featural cues more heavily than geometric cues. The content of the task instructions also influenced how cues were relied upon in this conflict situation. Our results have important implications for our understanding of how spatial cues are used for reorientation.     

Smooth pursuit eye movements in normal and dyslexic children

This paper describes a detailed study of horizontal eye movements associated with visual tracking of a smoothly moving target. Essentially all children, even at target velocities as low as 5 degrees/sec., show some saccadic eye movements superimposed on smooth tracking movements. Detailed analysis of pursuit eye-movements from a group of 26 poor readers and 34 normal controls (8 to 13 yr.) showed that about 25% of poor readers have an abnormally raised saccadic component in smooth pursuit. This suggests that studies of eye movements during tracking of smoothly moving targets at low velocity, combined with a quantitative approach to data analysis, may be useful for early detection of a significant proportion of poor-reading children.     

Arm-Tracking Performance With and Without Visual Feedback in Children and Adults

Tracking performance was investigated in children (aged 6-7 and 10-11) and in adult subjects. Target signals, moving unpredictably along a straight line, were tracked with the preferred arm, alternately with and without visual feedback. Qualitative observations indicate that tracking is based on continuous adjustments of the ongoing response to the continuously changing target position. No step-and-hold strategy could be detected in any of the three age groups. Tracking performance was described with four simple parameters, derived from linear systems analysis: (a) the delay between target signal and tracking movement (DL); (b) performance at the low-frequency range (LF), (c) performance at the high-frequency range (HF); and (d) a measure of tracking quality or overall similarity in the shape of target signal and tracking movement (Q). There was a considerable improvement in tracking performance with age, even after the age of 10-11, which was mainly demonstrated by a decrease in DL and increases in HF and Q. Tracking performance decreased only to a small extent when visual feedback was withdrawn. Age-related differences in the contribution of visual feedback to tracking performance could not be demonstrated.     

When causality does not imply correlation: more spadework at the foundations of scientific psychology

Experimental research in psychology is based on an open-loop causal model which assumes that sensory input causes behavioral output. This model was tested in a tracking experiment where participants were asked to control a cursor, keeping it aligned with a target by moving a mouse to compensate for disturbances of differing difficulty. Since cursor movements (inputs) are the only observable cause of mouse movements (outputs), the open-loop model predicts that there will be a correlation between input and output that increases as tracking performance improves. In fact, the correlation between sensory input and motor output is very low regardless of the quality of tracking performance; causality, in terms of the effect of input on output, does not seem to imply correlation in this situation. This surprising result can be explained by a closed-loop model which assumes that input is causing output while output is causing input.