Comment and Reply Why eye movements and perceptual factors have to be controlled in studies on "representational momentum”

In order to study memory of the final position of a smoothly moving target, Hubbard (e.g., Hubbard & Bharucha, 1988) presented smooth stimulus motion and used motor responses. In contrast, Freyd (e.g., Freyd & Finke, 1984) presented implied stimulus motion and used the method of constant stimuli. The same forward error was observed in both paradigms. However, the processes underlying the error may be very different. When smooth stimulus motion is followed by smooth pursuit eye movements, the forward error is associated with asynchronous processing of retinal and extraretinal information. In the absence of eye movements, no forward displacement is observed with smooth motion. In contrast, implied motion produces a forward error even without eye movements, suggesting that observers extrapolate the next target step when successive target presentations are far apart. Finally, motor responses produce errors that are not observed with perceptual judgments, indicating that the motor system may compensate for neuronal latencie

Mental and sensorimotor extrapolation fare better than motion extrapolation in the offset condition

Evidence for motion extrapolation at motion offset is scarce. In contrast, there is abundant evidence that subjects mentally extrapolate the future trajectory of weak motion signals at motion offset. Further, pointing movements overshoot at motion offset. We believe that mental and sensorimotor extrapolation is sufficient to solve the problem of perceptual latencies. Both present the advantage of being much more flexible than motion extrapolatio

Perceptual asynchronies between color and motion at the onset of motion and along the motion trajectory

A color change that is physically simultaneous with the onset of object motion may be perceived as occurring before the initial displacement. In contrast, a colored flash during object motion is displaced in the direction of motion, suggesting that it is perceived after the continuous position change. The aim of our study was to reconcile these apparently conflicting results. To this end, we reexamined color-motion asynchronies as a function of trajectory position. Our results indicate that an abrupt color change lags object motion along the trajectory, but no asynchrony was found when the abrupt color change occurred at motion onset. Even if the lag of color relative to motion decreased with increasing object size, we did not replicate a lag of motion relative to color in any of our experiments. Furthermore, judgments at motion onset were not correlated with judgments along the trajectory, suggesting that the underlying mechanisms or task demands were different. Temporal order may be judged at motion onset, whereas position is judged during ongoing motio

Evidence for an attentional component in saccadic inhibition of return

After presentation of a peripheral cue, facilitation at the cued location is followed by inhibition of return (IOR). It has been recently proposed that IOR may originate at different processing stages for manual and ocular responses, with manual IOR resulting from inhibited attentional orienting, and ocular IOR resulting form inhibited motor preparation. Contrary to this interpretation, we found an effect of target contrast on saccadic IOR. The effect of contrast decreased with increasing reaction times (RTs) for saccades, but not for manual key-press responses. This may have masked the effect of contrast on IOR with saccades in previous studies (Hunt and Kingstone in J Exp Psychol Hum Percept Perform 29:1068-1074, 2003) because only mean RTs were considered. We also found that background luminance strongly influenced the effects of gap and target contrast on IO

Involuntary cueing effects during smooth pursuit: facilitation and inhibition of return in oculocentric coordinates

Peripheral cues induce facilitation with short cue-target intervals and inhibition of return (IOR) with long cue-target intervals. Modulations of facilitation and IOR by continuous displacements of the eye or the cued stimuli are poorly understood. Previously, the retinal coordinates of the cued location were changed by saccadic or smooth pursuit eye movements during the cue-target interval. In contrast, we probed the relevant coordinates for facilitation and IOR by orthogonally varying object motion (stationary, moving) and eye movement (fixation, smooth pursuit). In the pursuit conditions, cue and target were presented during the ongoing eye movement and observers made a saccade to the target. Importantly, we found facilitation and IOR of similar size during smooth pursuit and fixation. The results suggest that involuntary orienting is possible even when attention has to be allocated to the moving target during smooth pursuit. Comparison of conditions with stabilized and moving objects suggest an oculocentric basis for facilitation as well as inhibition. Facilitation and IOR were reduced with objects that moved on the retina both with smooth pursuit and eye fixatio

The effects of saliency on manual reach trajectories and reach target selection

AbstractReaching trajectories curve toward salient distractors, reflecting the competing activation of reach plans toward target and distractor stimuli. We investigated whether the relative saliency of target and distractor influenced the curvature of the movement and the selection of the final endpoint of the reach. Participants were asked to reach a bar tilted to the right in a context of gray vertical bars. A bar tilted to the left served as distractor. Relative stimulus saliency was varied via color: either the distractor was red and the target was gray, or vice versa. Throughout, we observed that reach trajectories deviated toward the distractor. Surprisingly, relative saliency had no effect on the curvature of reach trajectories. Moreover, when we increased time pressure in separate experiments and analyzed the curvature as a function of reaction time, no influence of relative stimulus saliency was found, not even for the fastest reaction times. If anything, curvature decreased with strong time pressure. In contrast, reach target selection under strong time pressure was influenced by relative saliency: reaches with short reaction times were likely to go to the red distractor. The time course of reach target selection was comparable to saccadic target selection. Implications for the neural basis of trajectory deviations and target selection in manual and eye movements are discussed

Neuronal Processing Delays Are Compensated in the Sensorimotor Branch of the Visual System

AbstractMoving objects change their position until signals from the photoreceptors arrive in the visual cortex. Nonetheless, motor responses to moving objects are accurate and do not lag behind the real-world position [1]. The questions are how and where neural delays are compensated for. It was suggested that compensation is achieved within the visual system by extrapolating the position of moving objects [2]. A visual illusion supports this idea: when a briefly flashed object is presented in the same position as a moving object, it appears to lag behind [3, 4]. However, moving objects do not appear ahead of their final or reversal points [5–7]. We investigated a situation where participants localized the final position of a moving stimulus. Visual perception and short-term memory of the final target position were accurate, but reaching movements were directed toward future positions of the target beyond the vanishing point. Our results show that neuronal latencies are not compensated for at early stages of visual processing, but at a late stage when retinotopic information is transformed into egocentric space used for motor responses. The sensorimotor system extrapolates the position of moving targets to allow for precise localization of moving targets despite neuronal latencies

Spatial distortions and processing latencies in the onset repulsion and Fröhlich effects

AbstractIn the Fröhlich illusion, the first position of a moving target is mislocalized in the direction of motion. In the onset repulsion effect, the opposite error occurs. To reconcile these conflicting error patterns, we improved previous methods by using natural pointing movements and a large range of target velocities. Displacement was found to increase in the direction of motion, but the linear function relating velocity and displacement was shifted opposite to the direction of target motion. The results suggest that onset localization may be determined by two independent factors: first, an (attentional) delay that accounts for the increase of displacement in the direction of motion with increasing velocity. This delay is visible in motor and probe judgments and explains the Fröhlich illusion. Second, motor judgments are offset opposite to the direction of target motion. This bias is unique to motor judgments (pointing) and may be partially explained by attentional repulsion

Local motion inside an object affects pointing less than smooth pursuit

During smooth pursuit eye movements, briefly presented objects are mislocalized in the direction of motion. It has been proposed that the localization error is the sum of the pursuit signal and the retinal motion signal in a ~200ms interval after flash onset. To evaluate contributions of retinal motion signals produced by the entire object (global motion) and elements within the object (local motion), we asked observers to reach to flashed Gabor patches (Gaussian-windowed sine-wave gratings). Global motion was manipulated by varying the duration of a stationary flash, and local motion was manipulated by varying the motion of the sine-wave. Our results confirm that global retinal motion reduces the localization error. The effect of local retinal motion on object localization was far smaller, even though local and global motion had equal effects on eye velocity. Thus, local retinal motion has differential access to manual and oculomotor control circuits. Further, we observed moderate correlations between smooth pursuit gain and localization erro

Evidence for a dissociation between the control of oculomotor capture and disengagement

The current study investigated whether capture of the eyes by a salient onset distractor and the disengagement of the eyes from that distractor are driven by the same or by different underlying control modes. A variant of the classic oculomotor capture task was used. Observers had to make a saccade to the only gray circle among red background circles. On some trials, a green (novel color), red (placeholder color) or gray (target color) distractor square was presented with sudden onset. Results showed that when participants reacted fast, oculomotor capture was primarily driven by bottom-up pop-out: both types of distractors (green and gray) that popped out among the red background elements showed more capture than a red distractor that did not pop-out. In contrast to initial capture, disengagement of the eyes from the distractor was driven by top-down target-distractor similarity effects. We also examined the time-course of this effect. The distractor could change from green to either the target or placeholder color. When the color change was early in time (30-40ms after its onset), dwell times were strongly affected by the change, whereas the effect on oculomotor capture was weak. Importantly, a change occurring as early as 60-80ms after distractor onset did neither affect capture nor dwell times, corroborating the assumption of parallel programming of saccade