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The human motor system and muscles are subject to fluctuations in the short and long term. Motor adaptation is classically thought of as a low-level process that compensates for the error between predicted and executed movements in order to maintain movement accuracy. Contrary to a low-level account, accurate movements might be only a means to support high-level behavioral and perceptual goals. To isolate the influence of high-level goals in adaptation of saccadic eye movements, we manipulated perceptual task requirements in the absence of low-level errors. Observers had to discriminate one character within a peripheral array of characters. Between trials, the location of this character within the array was changed. This manipulation led to an immediate strategic change and a slower, gradual adaptation of saccade amplitude and direction. These changes had a similar magnitude to classical saccade adaptation and transferred at least partially to reactive saccades without a perceptual task. These results suggest that a perceptual task can modify oculomotor commands by generating a top-down error signal in saccade maps just like a bottom-up visual position error. Hence saccade adaptation not only maintains saccadic targeting accuracy, but also optimizes gaze behavior for the behavioral goal, showing that perception shapes even low-level oculomotor mechanisms.

Prior to the onset of a saccadic eye movement, perception is facilitated at the saccade target location. This has been attributed to a shift of attention. To test whether presaccadic attention shifts are strictly dependent on saccade execution, we examined whether they are found when observers are required to cancel the eye movement. We combined a dual task with the stop-signal paradigm: Subjects made saccades as quickly as possible to a cued location while discriminating a stimulus either at the saccade target or at the opposite location. A stop signal was presented on a subset of trials, asking subjects to cancel the eye movement. The delay of the stop signal was adjusted to yield successful inhibition of the saccade in 50% of trials. Results show similar perceptual facilitation at the saccade target for saccades with or without a stop signal, suggesting that presaccadic attention shifts are obligatory for all saccades. However, there was facilitation only when saccades were actually performed, not when observers successfully inhibited them. Thus, preparing an eye movement without subsequently executing it does not result in an attention shift. The results speak to a difference between saccade preparation and saccade programming. In light of the strong dependence on saccade execution, we discuss the functional role and causes of presaccadic attention shifts.

Humans use saccadic eye movements to fixate different parts of their visual environment. While stimulus features that determine the location of the next fixation in static images have been extensively studied, temporal stimulus features that determine the timing of the gaze shifts received less attention. It is also unclear if stimulus features at the present gaze location can trigger gaze shifts to another location. To investigate these questions, we asked observers to switch their gaze between two blobs. In three different conditions, either the fixated blob, the peripheral blob, or both blobs were flickering. A time-frequency analysis of the flickering noise values, time locked to the gaze shifts, revealed significant phase locking in a time window 300 to 100 ms before the gaze shift at temporal frequencies below 20 Hz. The average phase angles at these time-frequency points indicated that observer's gaze was repelled by decreasing contrast of the fixated blob and attracted by increasing contrast of the peripheral blob. These results show that temporal properties of both, fixated, and peripheral stimuli are capable of triggering gaze shifts.

Newtonian physics constrains object kinematics in the real world. We asked whether eye movements towards tracked objects depend on their compliance with those constraints. In particular, the force of gravity constrains round objects to roll on the ground with a particular rotational and translational motion. We measured tracking eye movements towards rolling objects. We found that objects with rotational and translational motion that was congruent with an object rolling on the ground elicited faster tracking eye movements during pursuit initiation than incongruent stimuli. Relative to a condition without rotational component, we compared objects with this motion with a condition in which there was no rotational component, we essentially obtained benefits of congruence, and, to a lesser extent, costs from incongruence. Anticipatory pursuit responses showed no congruence effect, suggesting that the effect is based on visually-driven predictions, not on velocity storage. We suggest that the eye movement system incorporates information about object kinematics acquired by a lifetime of experience with visual stimuli obeying the laws of Newtonian physics.

Previous research has demonstrated that prior to saccade execution visual attention is imperatively shifted towards the saccade target (e.g., Deubel & Schneider, 1996; Kowler, Anderson, Dosher, & Blaser, 1995). Typically, observers had to make a saccade according to an arrow cue and simultaneously perform a perceptual discrimination task either at the saccade endpoint or elsewhere on the screen. Discrimination performance was poor if the location of the saccade target (ST) and the discrimination target (DT) did not coincide. However, those experiments only investigated shifts of spatial attention. In the current experiments, we examined how feature-based attention is deployed before a saccade. In Experiment 1, we randomly varied the colors of the ST and DT. Results showed that discrimination performance was better when the DT was shown in the same color as the ST. This color congruency effect was slightly larger and more reliable when ST color was relevant and constant across trials (Experiment 2). We conclude that selection of a colored ST can induce display-wide facilitative processing of stimuli sharing this color. Results are discussed in terms of saccade programming and saccade selection, color priming in visual search, color cuing, and color-based top-down contingent attentional capture. We also discuss basic mechanisms of spatial- and feature-based attention and predictive remapping of visual information across saccades.

Improvements of perceptual performance following the presentation of peripheral cues have been ascribed to accelerated accrual of information, enhanced contrast perception, and decision bias. We investigated effects of peripheral cues on the perception of Gabor and letter stimuli. Non-predictive, peripheral cues improved perceptual accuracy when the stimuli were masked. In contrast, peripheral cues degraded perception of low-contrast letters and did not affect the perception of low-contrast Gabors. The results suggest that involuntary attention accelerates accrual of information but are not entirely consistent with the idea that involuntary attention enhances subjective contrast. Rather, peripheral cues may cause crowding with single letter targets of low contrast. Further, we investigated the effect of the amount of uncertainty on involuntary attention. Cueing effects were (initially) larger when there were more possible target locations. In addition, cueing effects were larger when error feedback was absent and observers had no knowledge of results. Despite these strategic factors, location uncertainty was not sufficient to produce cueing effects, showing that location uncertainty paired with non-predictive cues reveals perceptual and not (only) decisional processes.

In a previous study (M. Carrasco, S. Ling, & S. Read, 2004), observers selected one of two Gabors that appeared to have higher contrast (comparative judgment). A peripheral cue preceded one of the Gabors by 120 ms. Results showed that the cue increased the perceived contrast of the adjacent Gabor. We replicated the experiment and found correlations between the precision of judgments and perceptual cueing effects. Larger cueing effects occurred in conditions with less precise judgments and in observers who saw less difference between the stimuli. Further, we asked observers to judge whether the two Gabors were equal or different (equality judgment). This method avoids decision biases but turned out to be less efficient. Cueing effects were absent for equality judgments and trained observers but present for untrained observers with poor ability to discriminate the stimuli. In another experiment, we showed that when observers' responses became more rapid over the course of the experiment, a cueing effect in brightness perception emerged that is unlikely to reflect perceptual changes. Overall, the results show that large effects of peripheral cues on appearance correlate with poor ability to discriminate the stimuli. We suggest that the cue biased observers' decision but did not affect their perception.

To investigate the mechanisms involved in automatic processing of facial expressions, we used the QUEST procedure to measure the display durations needed to make a gender decision on emotional faces portraying fearful, happy, or neutral facial expressions. In line with predictions of appraisal theories of emotion, our results showed greater processing priority of emotional stimuli regardless of their valence. Whereas all experimental conditions led to an averaged threshold of about 50 ms, fearful and happy facial expressions led to significantly less variability in the responses than neutral faces. Results suggest that attention may have been automatically drawn by the emotion portrayed by face targets, yielding more informative perceptions and less variable responses. The temporal resolution of the perceptual system (expressed by the thresholds) and the processing priority of the stimuli (expressed by the variability in the responses) may influence subjective and objective measures of awareness, respectively.

Observers reported the orientation of a tilted grating that was presented together with a vertical distractor grating. In the main experiments, target contrast was low. There was location uncertainty because target location varied randomly and differences between target and distractor were small. In contrast to a previous report (T. Liu, F. Pestilli, & M. Carrasco, 2005), our results showed that non-informative peripheral cues do not improve perceptual performance at the cued location. However, informative peripheral or central cues improved perceptual performance. When we changed the task from an unspeeded perceptual task to a speeded reaction time task, the absence of involuntary cueing effects persisted when a distractor was presented. Without distractors, involuntary cueing effects re-emerged. When target contrast was increased, involuntary cueing effects re-emerged with a distractor but were smaller than without. We suggest that more difficult perceptual tasks reduce or abolish involuntary cueing effects.

In three experiments, we examined effects of target-distractor similarity in the remote distractor effect (RDE). Observers made saccades to peripheral targets that were either gray or green. Foveal or peripheral distractors were presented at the same time. The distractors could either share the target's defining property (congruent) or be different from the target (incongruent). Congruent distractors slowed down saccadic reaction times more than incongruent distractors. The increase of the RDE with target-distractor congruency depended on task demands. The more participants had to rely on the target property to locate the target, the larger the congruency effect. We conclude that the RDE can be modulated in a top-down manner. Alternative explanations such as persisting memory traces for the target property or differences in stimulus arrangement were considered but discarded. Our claim is in line with models of saccade generation which assume that the structures underlying the RDE (e.g. the superior colliculus) receive bottom-up as well as top-down information.

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 ~200 ms 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 error.

Many studies indicate that saccades are necessarily preceded by a shift of attention to the target location. There is no direct evidence for the same coupling during smooth pursuit. If smooth pursuit and attention were coupled, pursuit onset should be delayed whenever attention is focused on a stationary, non-target location. To test this hypothesis, observers were instructed to shift their attention to a peripheral location according to a location cue (Experiments 1 and 2) or a symbolic cue (Experiment 3) around the time of smooth pursuit initiation. Attending to static targets had only negligible effects on smooth pursuit latencies and the early open-loop response but lowered pursuit velocity substantially about the onset of closed-loop pursuit. Around this time, eye velocity reflected the competition between the to-be-tracked and to-be-attended object motion, entailing a reduction of eye velocity by 50% compared to the single task condition. The precise time course of attentional modulation of smooth pursuit initiation was at odds with the idea that an attention shift must precede any voluntary eye movement. Finally, the initial catch-up saccades were strongly delayed with attention diverted from the pursuit target. Implications for models of target selection for pursuit and saccades are discussed.

When we view the world around us, we constantly move our eyes. This brings objects of interest into the fovea and keeps them there, but visual sensitivity has been shown to deteriorate while the eyes are moving. Here we show that human sensitivity for some visual stimuli is improved during smooth pursuit eye movements. Detection thresholds for briefly flashed, colored stimuli were 16% lower during pursuit than during fixation. Similarly, detection thresholds for luminance-defined stimuli of high spatial frequency were lowered. These findings suggest that the pursuit-induced sensitivity increase may have its neuronal origin in the parvocellular retino-thalamic system. This implies that the visual system not only uses feedback connections to improve processing for locations and objects being attended to, but that a whole processing subsystem can be boosted. During pursuit, facilitation of the parvocellular system may reduce motion blur for stationary objects and increase sensitivity to speed changes of the tracked object.

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 motion.

In the flash-lag effect, a flash displayed at the same position as a moving object is perceived to lag the moving object. Current accounts of the illusion make different predictions about how the size of the lag would change if participants compared the position of a moving object to the onset position of a moving probe instead of a flash. We compared the lag effect with a moving probe relative to a flashed probe at motion onset, during ongoing motion, and at motion offset. At motion onset and offset, the lag effect was larger with a moving than with a flashed probe, but there was no difference during ongoing motion. Our results are best explained by the assumption that abrupt changes are erroneously bound to continuous changes following the occurrence of the abrupt change. Typically, the abrupt onset of the flash is misbound to continuous target motion, resulting in the flash-lag effect. With moving probes, abrupt changes of the target (onset, offset) may also be misbound to continuous motion of the probe which increases the lag.

A number of studies have shown that stationary backgrounds compromise smooth pursuit eye movements. It has been suggested that poor attentional selection of the pursuit target was responsible for reductions of pursuit gain. To quantify the detrimental effects of attention, we instructed observers to either pay attention to background objects or to ignore them. The to-be-attended object was indicated by peripheral or central cues. Strong reductions of pursuit gain occurred when the following conditions were met: (a) the subject payed attention to the object (b) a salient event was present, for instance the onset of the target or cue and (c) the attended target produced retinal motion. Removing any of the three conditions resulted in no or far smaller decreases of pursuit gain. Further, decreases in pursuit gain were present with perceptual discrimination and simple manual detection.

In the Fröhlich effect, the perceived onset of a moving stimulus is displaced in the direction of motion. Previously, we observed that pointing movements produced a Fröhlich effect only when the onset position was highly predictable. Here, we show that relative judgments are not affected by spatial predictability if the relative judgment task is performed in isolation. However, when the two tasks vary randomly from trial to trial, effects of spatial predictability carry over to the perceptual task. Thus, observers' intentions before stimulus onset determine the way position signals are processed. An account in terms of sensory and motor maps is discussed.

During smooth pursuit eye movements, stimuli other than the pursuit target move across the retina, and this might affect their detectability. We measured detection thresholds for vertically oriented Gabor stimuli with different temporal frequencies (1, 4, 8, 12, 16, 20, and 24 Hz) of the sinusoids. Observers kept fixation on a small target spot that was either stationary or moved horizontally at a speed of 8 deg/s. The sinusoid of the Gabor stimuli moved either in the same or in the opposite direction as the pursuit target. Observers had to indicate whether the Gabor stimuli were displayed 4 degrees above or below the target spot. Results show that contrast sensitivity was mainly determined by retinal-image motion but was slightly reduced during smooth pursuit eye movements. Moreover, sensitivity for motion opposite to pursuit direction was reduced in comparison to motion in pursuit direction. The loss in sensitivity for peripheral targets during pursuit can be interpreted in terms of space-based attention to the pursuit target. The loss of sensitivity for motion opposite to pursuit direction can be interpreted as feature-based attention to the pursuit direction.

The pathways controlling motor behavior are believed to exhibit little selectivity for color, but there is growing evidence suggesting that color signals can be used to guide actions. We investigated this by having observers make a saccade or a rapid pointing movement to a small, peripherally flashed (100 ms) Gaussian target (SD=0.5 degrees) defined exclusively by luminance (maximum contrast) or color (from cardinal DKL red-green or blue-yellow axes, at maximum saturation). We found no difference in saccadic or pointing accuracy for luminance or color targets. The same was true using shutter goggles during pointing (to minimize the use of external cues), and when the luminance contrast of color targets was varied by up to +/-10%. In terms of response times, both eye and hand latencies increased with target eccentricity for R-G targets only, in a manner consistent with the sensitivity of this channel across eccentricity. We found little difference in response latencies between luminance and color targets once matched in terms of cone contrast. While RTs were longer when coupled with a goal directed pointing movement (versus a simple reaction without pointing), the difference was the same for color or luminance targets, suggesting that the spatial coding for the movements was also the same. In a final experiment we compared the accuracy of pointing to color-naming performance in a 4AFC procedure. The psychometric functions relating pointing accuracy (% correct quadrant) to color-naming (% correct color-name) were identical. Taken together, the results show that human observers can efficiently use pure chromatic signals to guide actions.

When 2 targets for pursuit eye movements move in different directions, the eye velocity follows the vector average (S. G. Lisberger & V. P. Ferrera, 1997). The present study investigates the mechanisms of target selection when observers are instructed to follow a predefined horizontal target and to ignore a moving distractor stimulus. Results show that at 140 ms after distractor onset, horizontal eye velocity is decreased by about 25%. Vertical eye velocity increases or decreases by 1 degrees /s in the direction opposite from the distractor. This deviation varies in size with distractor direction, velocity, and contrast. The effect was present during the initiation and steady-state tracking phase of pursuit but only when the observer had prior information about target motion. Neither vector averaging nor winner-take-all models could predict the response to a moving to-be-ignored distractor during steady-state tracking of a predefined target. The contributions of perceptual mislocalization and spatial attention to the vertical deviation in pursuit are discussed., (Copyright 2006 APA.)

We compared the spatio-temporal tuning of perception to the mechanisms that drive saccadic eye movements. Detection thresholds were measured for Gabor-targets presented left or right of fixation (4 or 8deg eccentricity), at one of four spatial frequencies (1, 2, 4 or 8cpd) oscillating at one of three temporal frequencies (1, 8 or 16Hz). We then measured saccade latency to each target presented at various multiples of detection threshold. Consistent with previous research, latency decreased as a function of contrast. However, at equal detection performance, we found no systematic difference in saccadic latency and no difference in average oculometric performance (% correct saccade direction) across the different target spatio-temporal frequencies. Furthermore, position error remained fairly constant across all conditions. The results are consistent with the idea that the spatio-temporal signals used for perception are the same as those used by the mechanisms driving saccadic eye movements.

In Eriksen's flanker paradigm, participants' responses are slower and more error-prone when task-relevant and simultaneously available task-irrelevant cues are incongruent. The influence of task-irrelevant information decreases as its distance from the task-relevant information increases. Here, we manipulated the quantity of task-irrelevant information while keeping the distance constant. We asked whether when the impact on response selection processes was stronger the more incongruent information was available, or whether the impact on response selection depended only on its presence or absence. We conducted an experiment, in which subjects had to discriminate the direction of motion of a central point-light-walker that was flanked by two, four, or eight point-light-walkers at an equal distance from the center. The experiment showed that reaction times increased with the number of incongruent walkers. This effect was modulated by the total number of walkers, showing that the effect of incongruent information saturates when the display is cluttered.

Targets that are briefly flashed during smooth pursuit eye movements are mislocalized in the direction of motion (forward shift) and away from the fovea (spatial expansion). Hansen [Hansen, R. M. (1979). Spatial localization during pursuit eye movements. Vision Research 19(11), 1213-1221] reported that these errors are not present for fast motor responses in the dark, whereas Rotman et al. [Rotman, G., Brenner, E., Smeets, J. B. (2004). Quickly tapping targets that are flashed during smooth pursuit reveals perceptual mislocalizations. Experimental Brain Research 156(4), 409-414] reported that they are present for fast motor responses in the light. To evaluate whether the lighting conditions are the critical factor, we asked observers to point to the positions of flashed objects during smooth pursuit either in the dark or with the room lights on. In a first experiment, the flash, which could appear at 1 of 15 different positions, was always shown when the eye had reached a certain spatial position. We found a forward bias and spatial expansion that were independent of the target and ambient luminance. In a second experiment, the flash was always shown at the same retinal position, but the spatial position of the eye at the moment of flash presentation was varied. In this case we found differences between the luminance conditions, in terms of how the errors depended on the velocity and position on the trajectory. We also found specific conditions in which people did not mislocalize the target in the direction of pursuit at all. These findings may account for the above-mentioned discrepancy. We conclude that although the lighting conditions do influence the localization errors under some circumstances, it is certainly not so that such errors are absent whenever the experiment is conducted in the dark.

In order to study memory of the final position of a smoothly moving target, Hubbard (e.g., Hubbard and Bharucha, 1988) presented smooth stimulus motion and used motor responses. In contrast, Freyd (e.g., Freyd and 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 latencies.

It has been suggested that the remote distractor effect is the result of nontarget stimulation of a central region representing a collicular fixation zone near the time of target onset. The distributed network of the cells responsible for this effect is believed to extend over a large area, responding to distractors < or =10 deg in the periphery. Several studies also implicate the superior colliculus as the substrate behind an inhibited saccadic response arising from a display change. We investigated this further by using a patch of pink noise of various sizes as a nontarget stimulus. We show that the onset of a small patch (2.3 x 2.3 deg) of centrally displayed pink noise can produce a significant increase in saccadic latency to a simultaneously presented peripheral Gabor target. In contrast, a large patch (36 x 36 deg) of pink noise did not increase latency despite the fact that it also stimulated the region representing the fixation zone. Furthermore, only the large patch of noise facilitated latency when presented before target onset. We also examined the effect of patch sizes between these two extremes and found a steady decrease in latency as patch size increased. This confirms that nontarget stimulation of the region representing the fixation zone near the time of target onset is not in itself sufficient to produce the increase in latency typically found with remote distractors. The results are consistent with the idea that only a spatially confined object leads to a discharge of collicular fixation neurons.

It is well known that moving stimuli can appear to move more slowly when contrast is reduced (P. Thompson, 1982). Here we address the question whether changes in stimulus contrast also affect smooth pursuit eye movements. Subjects were asked to smoothly track a moving Gabor patch. Targets varied in velocity (1, 8, and 15 deg/s), spatial frequency (0.1, 1, 4, and 8 c/deg), and contrast, ranging from just below individual thresholds to maximum contrast. Results show that smooth pursuit eye velocity gain rose significantly with increasing contrast. Below a contrast level of two to three times threshold, pursuit gain, acceleration, latency, and positional accuracy were severely impaired. Therefore, the smooth pursuit motor response shows the same kind of slowing at low contrast that was demonstrated in previous studies on perception.

The position of a drifting sine-wave grating enveloped by a stationary Gaussian is misperceived in the direction of motion. Previous research indicated that the illusion was larger when observers pointed to the center of the stimulus than when they indicated the stimulus position on a ruler. This conclusion was reexamined. Observers pointed to the position of a small Gabor patch on the screen or compared its position to moving patches, stationary lines, or flashed lines. With moving patches, the illusion was larger with probe than with motor judgments; with stationary lines, the illusion was about the same size; and with flashed lines, the illusion was smaller with probe than with motor judgments. Thus, the comparison between perceptual and motor measures depended strongly on the methods used. Further, the target was mislocalized toward the fovea with motor judgments, whereas the target was displaced away from the fovea relative to line probes.

Five experiments were carried out to test whether (task-irrelevant) motion information provided by a stimulus changing its position over time would affect manual left-right responses. So far, some studies reported direction-based Simon effects whereas others did not. In Experiment 1a, a reliable direction-based effect occurred, which was not modulated by the response mode--that is, by whether participants responded by pressing one of two keys or more dynamically by moving a stylus in a certain direction. Experiments 1a, 1b, and 2 lend support to the idea that observers use the starting position of target motion as a reference for spatial coding. That is, observers might process object motion as a shift of position relative to the starting position and not as directional information. The dominance of relative position coding could also be shown in Experiment 3, in which relative position was pitted against motion direction by presenting a static and dynamic stimulus at the same time. Additionally, we explored the role of eye movements in stimulus-response compatibility and showed in Experiments 1b and 3a that the execution or preparation of saccadic eye movements--as proposed by an attention-shifting account--is not necessary for a Simon effect to occur.

Visual short-term memory (VSTM) was probed while observers performed smooth pursuit eye movements. Smooth pursuit keeps a moving object stabilized in the fovea. VSTM capacity for position was reduced during smooth pursuit compared with a condition with eye fixation. There was no difference between a condition in which the items were approximately stabilized on the retina because they moved with the pursuit target and a condition in which the items moved across the retina because they were stationary in space. The reduction of capacity for position was eliminated when miniature items were presented on the pursuit target. Similarly, VSTM capacity for color did not differ between smooth pursuit and fixation. The results suggest that visuospatial attention is tied to the target during smooth pursuit, which impairs VSTM for the position of peripheral objects. Sensory memory during smooth pursuit was only slightly impaired., (((c) 2005 APA, all rights reserved).)

Localization of the onset and offset of a moving target is subject to a number of errors that have to be attributed to events following or preceding the target event. Apparently, observers are unable to ignore the spatiotemporal context surrounding the target event. In two experiments, observers' attention was directed toward a single position along a trajectory, two positions along a single trajectory, or two positions along two different trajectories. In the latter condition, attention to details of a single trajectory was reduced. At the same time, motion type was manipulated by varying the temporal interval between successive target presentations. The localization error was not affected by attentional load; however, effects of motion type were eliminated when two trajectories had to be attended to. It may be sufficient to notice that the target has moved for localization errors to occur, while specifics of the trajectory are ignored.

In the Fröhlich illusion, the first position of a moving target is mis-localized 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.

To clarify whether motion information per se has a separable influence on action control, the authors investigated whether irrelevant direction of motion of stimuli whose overall position was constant over time would affect manual left-right responses (i.e., reveal a motion-based Simon effect). In Experiments 1 and 2, significant Simon effects were obtained for sine-wave gratings moving in a stationary Gaussian window. In Experiment 3, a direction-based Simon effect with random-dot patterns was replicated, except that the perceived direction of motion was based on the displacement of single elements. Experiments 4 and 5 studied motion-based Simon effects to point-light figures that walked in place--displays requiring high-level analysis of global shape and local motion. Motion-based Simon effects occurred when the displays could be interpreted as an upright human walker, showing that a high-level representation of motion direction mediated the effects. Thus, the present study establishes links between high-level motion perception and action., (((c) 2004 APA, all rights reserved))

It is known that observers make localization errors in the direction of motion when asked to localize the perceived onset position of a moving target (Fröhlich effect). However, recent studies also revealed the contrary: In the onset repulsion effect, the error is opposite to the direction of motion. In four experiments we demonstrate that the conflict between these findings is resolved by considering the trial context: when the stimuli appeared at predictable positions to the left or right of fixation, pointing responses to the perceived onset position were displaced in movement direction. In contrast, when the stimuli appeared at unpredictable positions in the visual field, pointing errors were displaced opposite to motion or at least drastically reduced. Thus, localization of the perceived onset position varies with the trial context.

Moving 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. 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. 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. However, moving objects do not appear ahead of their final or reversal points. 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.

Some accounts hold that the position of moving objects is extrapolated either in visual perception or visual short-term memory ("representational momentum"). However, some studies did not find forward displacement of the final position when smooth motion was used, whereas reliable displacement was observed with implied motion. To resolve this conflict, the frequency of position changes was varied to sample motion types between the extreme cases of implied and smooth motion. A continuous function relating frequency of target change and displacement was found: Displacement increased when the frequency of position changes was reduced. Further, the response mode was varied. Probe judgments produced less forward displacement than motor judgments such as mouse or natural pointing movements. Also, localization judgments were susceptible to motion context, but not to variations of probe shape or expectancy about trajectory length. It is suggested that forward displacement results from the extrapolation of the next step in the observed motion sequence.

Observers were asked to indicate when a target moving on a circular trajectory changed its luminance. The judged position of the luminance change was displaced from the true position in the direction of motion, indicating differences between the times-to-consciousness of motion and luminance change. Motion was processed faster than luminance change. The latency difference was more pronounced for a small (116-134 ms) than for a large luminance decrement (37 ms). The results show that first-order motion is perceived before an accurate representation of luminance is available. These findings are consistent with current accounts of the flash-lag effect. Two control experiments ruled out that the results were due to a general forward tendency. Localization of the target when an auditory signal was presented did not produce forward displacement, and the judged onset of motion was not shifted in the direction of motion.

Many observers believe that a target will continue on a curved trajectory after exiting a spiral tube. Similarly, when observers were asked to localize the final position of a target moving on a circular orbit, displacement of the judged position in the direction of forward motion ("representational momentum") and toward the center of the orbit was observed (cf. T. L. Hubbard, 1996). The present study shows that memory displacement of targets on a circular orbit is affected by eye movements. Forward displacement was larger with ocular pursuit of the target, whereas inward displacement was larger with motionless eyes. The results challenge an account attributing forward and inward displacement to mental analogues of momentum and centripetal force, respectively.

Observers' judgments of the final position of a moving target are typically shifted in the direction of implied motion ("representational momentum"). The role of attention is unclear: visual attention may be necessary to maintain or halt target displacement. When attention was captured by irrelevant distractors presented during the retention interval, forward displacement after implied target motion disappeared, suggesting that attention may be necessary to maintain mental extrapolation of target motion. In a further corroborative experiment, the deployment of attention was measured after a sequence of implied motion, and faster responses were observed to stimuli appearing in the direction of motion. Thus, attention may guide the mental extrapolation of target motion. Additionally, eye movements were measured during stimulus presentation and retention interval. The results showed that forward displacement with implied motion does not depend on eye movements. Differences between implied and smooth motion are discussed with respect to recent neurophysiological findings.

When observers are asked to judge the first position of a moving object, displacements of the judged onset in the direction of and in the direction opposite to the motion have been reported. These errors have been referred to as the Fröhlich illusion and the onset repulsion effect, respectively. To resolve the apparent contradiction between these results, a number of experimental parameters were investigated. Displacement in the direction opposite to motion was only observed when observers pointed to the onset of a slowly moving target. At higher velocities, no displacement with pointing was observed. In contrast, relative judgements of motion onset were accurate at slow velocities, but displaced in the direction of motion at fast velocities. Whether the target moved on a linear or circular trajectory did not alter the results. In one experiment, a dissociation between perceptual and memory-based judgements was found. Overall, the experimental task determined whether displacement in the direction of or in the direction opposite to motion occurred.

The present study investigated compatibility effects between written and spoken syllables. Participants saw the syllables "Ba" or "Da" printed on a speaker's mouth that was articulating either /b wedge / or /d wedge /. Participants classified either the printed syllable or the mouth movement by pressing a left or right key. Responses were faster when mouth movement and letters were congruent regardless of imperative stimulus dimension. As the two stimulus dimensions (mouth movements and letters) showed dimensional overlap, but did not overlap with the response, stimulus-response compatibility was ruled out according to some models. It is argued that the compatibility effect was due to the competition of phonological codes at a stage preceding response selection. Also, the results lend support to the view that Stroop-like tasks are ambiguous with regard to the locus of compatibility effects. Stimulus-response and stimulus-stimulus compatibility may be observed.

When observers are asked to localize the final position of a moving target, the judged position is usually displaced from the actual position. It has been suggested that mental processes derived from a number of invariant and noninvariant principles produce the mislocalization in memory. In this study, the effects of velocity, expectation, friction, memory averaging, and weight were reconsidered, and evidence was accumulated that supports the alternative view that the distortions arise to a large degree at a perceptual level. Effects of velocity and expectation were present when observers still perceived a persisting image of the target. It is suggested that the active reorienting of the perceptual organs explains the distortions. Furthermore, distortions of the perceived center of a visible stimulus may explain effects that have previously been attributed to memory averaging and mental analogues of weight. Thus, the locus of memory displacement is at least partially perceptual.

When observers are asked to localize the final position of a moving stimulus, judgements may be influenced by additional elements that are presented in the visual scene. Typically, judgements arc biased toward a salient non-target element. It has been assumed that the non-target element acts as a landmark and attracts the remembered final target position. The present study investigated the effects of briefly flashed non-target elements on localization performance. Similar to landmark attraction, localization was biased toward these elements. However, an influence was only noted if the distractor was presented at the time of target disappearance or briefly thereafter. It is suggested that memory traces of distracting elements are only averaged with the final target position if they are highly activated at the time the target vanishes.

The perceived and remembered position of stationary target objects is subject to a large number of distortions. Objects are localized toward the fovea, and when an additional object (distractor) is presented, a tendency to average target and distractor position was observed. These distortions in visual short-term memory have been referred to as foveal bias and memory averaging, respectively. Because most studies on memory averaging did not monitor eye fixation, foveal bias and memory averaging may have been confounded. That is, observers may have fixated the distractor. To disentangle these factors, target and distractor were presented in the periphery, and fixation was monitored. Memory averaging was not observed. Rather a bias away from the distractor occurred when the distractor was briefly presented during the retention interval, or when it was visible throughout the trial. In contrast, a foveal bias was observed regardless of whether an additional object was present.

In the Fröhlich illusion, judgements of the first position of a moving object are typically displaced in the direction of motion. The illusion has been obtained with linear motion of a small target, and with rotary motion of a spatially extended line. We compared judgements of the initial orientation of a small dot and a line that rotated around the point of fixation. The illusion was absent with the dot, whereas it was reliably obtained with the line. When the density of the line was reduced to two dots, the illusion persisted. However, the illusion was absent when a half-line extending to only one side from fixation was presented. We discuss the results with respect to two attentional accounts of the Fröhlich illusion and an account based on spatiotemporal integration.