Flash-lag effect: differential latency, not postdiction

Spatial and temporal proximity of objects for maximal crowding

Crowding refers to the deleterious visual interaction among nearby objects. Does maximal crowding occur when objects are closest to one another in space and time? We examined how crowding depends on the spatial and temporal proximity, retinally and perceptually, between a target and flankers. Our target was a briefly flashed T-stimulus presented at 10° right of fixation (3-o'clock position). It appeared at different target-onset-to-flanker asynchronies with respect to the instant when a pair of flanking Ts, revolving around the fixation target, reached the 3-o'clock position. Observers judged the orientation of the target-T (the crowding task), or its position relative to the revolving flankers (the flash-lag task). Performance was also measured in the absence of flanker motion: target and flankers were either presented simultaneously (closest retinal temporal proximity) with different angular spatial offsets, or were presented collinearly (closest retinal spatial proximity) with different temporal onset asynchronies. We found that neither retinal nor perceptual spatial or temporal proximity could account for when maximal crowding occurred. Simulations using a model based on feed-forward interactions between sustained and transient channels in static and motion pathways, taking into account the differential response latencies, can explain the crowding functions observed under various spatio-temporal conditions between the target and flankers.

The implicit sense of agency is not a perceptual effect but is a judgment effect

The sense of agency (SoA) is characterized as the sense of being the causal agent of one's own actions, and it is measured in two forms: explicit and implicit. In the explicit SoA experiments, the participants explicitly report whether they have a sense of control over their actions or whether they or somebody else is the causal agent of seen actions; the implicit SoA experiments study how do participants' agentive or voluntary actions modify perceptual processes (like time, vision, tactility, and audition) without directly asking the participants to explicitly think about their causal agency or sense of control. However, recent implicit SoA literature reported contradictory findings of the relationship between implicit SoA reports and agency states. Thus, I argue that the purported implicit SoA reports are not agency-driven perceptual effects per se but are judgment effects, by showing that (a) the typical operationalizations in implicit SoA domain lead to perceptual uncertainty on the part of the participants, (b) under uncertainty, participants' implicit SoA reports are due to heuristic judgments which are independent of agency states, and (c) under perceptual certainty, the typical implicit SoA reports might not have occurred at all. Thus, I conclude that the instances of implicit SoA are judgments (or response biases)-under uncertainty-rather than perceptual effects.

Remembering the Past to See the Future

In addition to the role that our visual system plays in determining what we are seeing right now, visual computations contribute in important ways to predicting what we will see next. While the role of memory in creating future predictions is often overlooked, efficient predictive computation requires the use of information about the past to estimate future events. In this article, we introduce a framework for understanding the relationship between memory and visual prediction and review the two classes of mechanisms that the visual system relies on to create future predictions. We also discuss the principles that define the mapping from predictive computations to predictive mechanisms and how downstream brain areas interpret the predictive signals computed by the visual system. Expected final online publication date for the Annual Review of Vision Science, Volume 7 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

When predictions fail: Correction for extrapolation in the flash-grab effect

Motion-induced position shifts constitute a broad class of visual illusions in which motion and position signals interact in the human visual pathway. In such illusions, the presence of visual motion distorts the perceived positions of objects in nearby space. Predictive mechanisms, which could contribute to compensating for processing delays due to neural transmission, have been given as an explanation. However, such mechanisms have struggled to explain why we do not usually perceive objects extrapolated beyond the end of their trajectory. Advocates of this interpretation have proposed a "correction-for-extrapolation" mechanism to explain this: When the object motion ends abruptly, this mechanism corrects the overextrapolation by shifting the perceived object location backwards to its actual location. However, such a mechanism has so far not been empirically demonstrated. Here, we use a novel version of the flash-grab illusion to demonstrate this mechanism. In the flash-grab effect, a target is flashed on a moving background that abruptly changes direction, leading to the mislocalization of the target. Here, we manipulate the angle of the direction change to dissociate the contributions of the background motion before and after the flash. Consistent with previous reports, we observe that perceptual mislocalization in the flash-grab illusion is mainly driven by motion after the flash. Importantly, however, we reveal a small but consistent mislocalization component in the direction opposite to the direction of the first motion sequence. This provides empirical support for the proposed correction-for-extrapolation mechanism, and therefore corroborates the interpretation that motion-induced position shifts might result from predictive interactions between motion and position signals.

Inhibition of return modulates the flash-lag effect

Transient events are known to draw exogenous attention, and visual processing at the attended location is transiently facilitated, but after several hundred milliseconds, attentional processing at the cued location becomes poorer than processing elsewhere, resulting in a slower reaction to a target stimulus that subsequently appears at the cued location. Despite a number of previous studies on this effect, termed inhibition of return (IOR), it is still unclear whether a perceptual process related to the subjective onset time of the target stimulus is disrupted when IOR occurs. In the present study, we used a distinct visual phenomenon termed the flash-lag effect (FLE) as a tool to quantify IOR. The FLE is an illusion in which a flashed stimulus appears to lag behind a moving stimulus, despite being physically aligned. We used an identical stimulus configuration and asked observers to conduct two independent tasks in separate sessions. The first was a simple reaction task to measure the onset reaction time (RT) to an abruptly appearing target. The second was an orientation judgment task to measure the degree of the FLE. Both the RT and the FLE were found to be altered in accordance with IOR, and a significant correlation was demonstrated between the changes in the RT and those in the FLE. These results demonstrate that the perceptual process related to the stimulus onset can be compromised by IOR.

Distinct mechanisms of temporal binding in generalized and cross-modal flash-lag effects

It remains unknown how the brain temporally binds sensory data across different modalities and attributes to create coherent perceptual experiences. To address this question, we measured what we see at the time we experience an event using a generalized version of the flash-lag effect (FLE) for combinations of visual attribute (bar orientation, face orientation, or face identity) and probe modality (visual or auditory). We asked participants to judge the content of rapidly and serially presented images seen at the same time as a briefly presented visual (flash) or auditory (click) probe and estimated the “time windows” contributing to decisions using reverse correlation analysis. We also used displays in which the visual attribute of a stimulus continuously changed and measured FLEs around abrupt flip in change direction and at the initiation and termination of a sequence. We consistently found clear latency-difference effects, which depended on visual attribute for the visual probe but did not for the auditory probe. The intra-modal FLE can be explained in terms of differential latency and temporal integration, but the cross-modal FLE is suggested to operate via a distinct mechanism; the content of a successive visual stream experienced after the awareness of a click is interpreted as simultaneous with the click.

Faster processing of moving compared with flashed bars in awake macaque V1 provides a neural correlate of the flash lag illusion

When the brain has determined the position of a moving object, because of anatomical and processing delays the object will have already moved to a new location. Given the statistical regularities present in natural motion, the brain may have acquired compensatory mechanisms to minimize the mismatch between the perceived and real positions of moving objects. A well-known visual illusion-the flash lag effect-points toward such a possibility. Although many psychophysical models have been suggested to explain this illusion, their predictions have not been tested at the neural level, particularly in a species of animal known to perceive the illusion. To this end, we recorded neural responses to flashed and moving bars from primary visual cortex (V1) of awake, fixating macaque monkeys. We found that the response latency to moving bars of varying speed, motion direction, and luminance was shorter than that to flashes, in a manner that is consistent with psychophysical results. At the level of V1, our results support the differential latency model positing that flashed and moving bars have different latencies. As we found a neural correlate of the illusion in passively fixating monkeys, our results also suggest that judging the instantaneous position of the moving bar at the time of flash-as required by the postdiction/motion-biasing model-may not be necessary for observing a neural correlate of the illusion. Our results also suggest that the brain may have evolved mechanisms to process moving stimuli faster and closer to real time compared with briefly appearing stationary stimuli. NEW & NOTEWORTHY We report several observations in awake macaque V1 that provide support for the differential latency model of the flash lag illusion. We find that the equal latency of flash and moving stimuli as assumed by motion integration/postdiction models does not hold in V1. We show that in macaque V1, motion processing latency depends on stimulus luminance, speed and motion direction in a manner consistent with several psychophysical properties of the flash lag illusion.

The Flash-Lag, Fröhlich and Related Motion Illusions Are Natural Consequences of Discrete Sampling in the Visual System

The Fröhlich effect and flash-lag effect, in which moving objects appear advanced along their trajectories compared to their actual positions, have defied a simple and consistent explanation. Here, I show that these illusions can be understood as a natural consequence of temporal compression in the human visual system. Discrete sampling at some stage of sensory perception has long been considered, and if it were true, it would necessarily lead to these illusions of motion. I show that the discrete perception hypothesis, with a single free parameter, the perceptual moment or sampling rate, can quantitatively explain all of the scenarios of the Fröhlich and flash-lag effect. I interpret discrete perception as the implementation of data compression in the brain, and our conscious perception as the reconstruction of the compressed input.

The haptic and the visual flash-lag effect and the role of flash characteristics

When a short flash occurs in spatial alignment with a moving object, the moving object is seen ahead the stationary one. Similar to this visual "flash-lag effect" (FLE) it has been recently observed for the haptic sense that participants judge a moving hand to be ahead a stationary hand when judged at the moment of a short vibration ("haptic flash") that is applied when the two hands are spatially aligned. We further investigated the haptic FLE. First, we compared participants' performance in two isosensory visual or haptic conditions, in which moving object and flash were presented only in a single modality (visual: sphere and short color change, haptic: hand and vibration), and two bisensory conditions, in which the moving object was presented in both modalities (hand aligned with visible sphere), but the flash was presented only visually or only haptically. The experiment aimed to disentangle contributions of the flash's and the objects' modalities to the FLEs in haptics versus vision. We observed a FLE when the flash was visually displayed, both when the moving object was visual and visuo-haptic. Because the position of a visual flash, but not of an analogue haptic flash, is misjudged relative to a same visuo-haptic moving object, the difference between visual and haptic conditions can be fully attributed to characteristics of the flash. The second experiment confirmed that a haptic FLE can be observed depending on flash characteristics: the FLE increases with decreasing intensity of the flash (slightly modulated by flash duration), which had been previously observed for vision. These findings underline the high relevance of flash characteristics in different senses, and thus fit well with the temporal-sampling framework, where the flash triggers a high-level, supra-modal process of position judgement, the time point of which further depends on the processing time of the flash.

Eye-hand coordination during flexible manual interception of an abruptly appearing, moving target

As a vital skill in an evolving world, interception of moving objects relies on accurate prediction of target motion. In natural circumstances, active gaze shifts often accompany hand movements when exploring targets of interest, but how eye and hand movements are coordinated during manual interception and their dependence on visual prediction remain unclear. Here, we trained gaze-unrestrained monkeys to manually intercept targets appearing at random locations and circularly moving with random speeds. We found that well-trained animals were able to intercept the targets with adequate compensation for both sensory transmission and motor delays. Before interception, the animals' gaze followed the targets with adequate compensation for the sensory delay, but not for extra target displacement during the eye movements. Both hand and eye movements were modulated by target kinematics, and their reaction times were correlated. Moreover, retinal errors and reaching errors were correlated across different stages of reach execution. Our results reveal eye-hand coordination during manual interception, yet the eye and hand movements may show different levels of prediction based on the task context.

NEW & NOTEWORTHY Here we studied the eye-hand coordination of monkeys during flexible manual interception of a moving target. Eye movements were untrained and not explicitly associated with reward. We found that the initial saccades toward the moving target adequately compensated for sensory transmission delays, but not for extra target displacement, whereas the reaching arm movements fully compensated for sensorimotor delays, suggesting that the mode of eye-hand coordination strongly depends on behavioral context.

The Mechanical Representation of Temporal Delays

When we knock on a door, we perceive the impact as a collection of simultaneous events, combining sound, sight, and tactile sensation. In reality, information from different modalities but from a single source is flowing inside the brain along different pathways, reaching processing centers at different times. Therefore, interpreting different sensory modalities which seem to occur simultaneously requires information processing that accounts for these different delays. As in a computer-based robotic system, does the brain use some explicit estimation of the time delay, to realign the sensory flows? Or does it compensate for temporal delays by representing them as changes in the body/environment mechanics? Using delayed-state or an approximation for delayed-state manipulations between visual and proprioceptive feedback during a tracking task, we show that tracking errors, grip forces, and learning curves are consistent with predictions of a representation that is based on approximation for delay, refuting an explicit delayed-state representation. Delayed-state representations are based on estimating the time elapsed between the movement commands and their observed consequences. In contrast, an approximation for delay representations result from estimating the instantaneous relation between the expected and observed motion variables, without explicit reference to time.

Confusion of Space and Time in the Flash-Lag Effect

The apparent lagging of a short flash in the relation to a moving object, the flash-lag effect (FLE), has so far been measured mainly in terms of illusory spatial offset. We propose a method of measuring the perceived temporal asynchrony of the FLE separately from its perceived spatial offset. We presented a moving stimulus that changed its colour at a certain moment. The observer indicated, in two different tasks, where and when the colour change occurred in relation to a stationary reference flash. Results show that the perceived time of the colour change was not congruent with the perceived location of the colour change: the colour change is perceived simultaneously with the flash, but is shifted in position. The presentation of the reference in the form of a flash is not critical for the occurrence of the FLE, because the same effect was obtained with a constantly visible reference signal, the position of which or time when it changed its colour were varied. The observer was not able to ignore the irrelevant dimension of the reference signal: the apparent time of the colour change was influenced by the position of the reference signal, and the apparent location of the colour change was influenced by the presentation time of the reference signal. The observer's inability to separate the spatial and temporal aspects of the moving stimulus clearly imposes certain limits on theories that are attempting to explain the FLE exclusively in terms of the perceived space and time.

Predictability and the Dynamics of Position Processing in the Flash-Lag Effect

Several models have been proposed to account for the flash-lag effect. One criterion for evaluating alternative models is to consider the separate effects of motion predictability and flash predictability. We first established that flash predictability has an impact on the size of the perceived spatial offset in the flash-lag illusion. We then examined motion predictability by varying the consistency of the motion trajectory. Both manipulations affected the magnitude of the flash-lag illusion. These outcomes suggest that the perception of position is a dynamic process that can be modulated by explicit cues in advance of the flash and by the temporal integration of position information over a consistent motion trajectory. A complete explanation of the flash-lag effect must specify how flash predictability and motion predictability modulate position-processing mechanisms.

Time Slices: What Is the Duration of a Percept?

We experience the world as a seamless stream of percepts. However, intriguing illusions and recent experiments suggest that the world is not continuously translated into conscious perception. Instead, perception seems to operate in a discrete manner, just like movies appear continuous although they consist of discrete images. To explain how the temporal resolution of human vision can be fast compared to sluggish conscious perception, we propose a novel conceptual framework in which features of objects, such as their color, are quasi-continuously and unconsciously analyzed with high temporal resolution. Like other features, temporal features, such as duration, are coded as quantitative labels. When unconscious processing is "completed," all features are simultaneously rendered conscious at discrete moments in time, sometimes even hundreds of milliseconds after stimuli were presented.

High temporal frequency adaptation compresses time in the Flash-Lag illusion

Previous research finds that 20 Hz temporal frequency (TF) adaptation causes a compression of perceived visual event duration. We investigate if this temporal compression affects further time-dependent percepts, implying a further functional role for duration perception mechanisms. We measure the effect of 20 Hz flicker adaptation on Flash-Lag, an illusion whereby an observer perceives a moving object displaced further along its trajectory compared to a spatially localized briefly flashed object. The illusion scales with object speed; therefore, it has a fixed temporal component. By comparing adaptation at 5 Hz and 20 Hz we show that 20 Hz TF adaptation reduces perceived Flash-Lag magnitude significantly, with no effect at 5 Hz, whereas the opposite pattern of adaptation was seen on perceived speed. There is a significant effect of 20 Hz adaptation on the perceived duration of a moving bar. This suggests that 20 Hz TF adaptation has compressed the fixed temporal component of the Flash-Lag illusion, implying the mechanism underlying duration perception also has effects on judging spatial relationships in dynamic stimuli.

Modulation of perceived contrast in the brightness comparison of asynchronous stimuli

Comparative judgment is a crucial task in ecological settings, as well as in many experimental studies about basic aspects of perceptual processes. It has long been known that sequential comparison is prone to order effects. This phenomenon has received little attention and has often been discounted as a type of response bias. In the present study, we investigated brightness discrimination of two brief (100 ms) spatially disjoint luminance stimuli. In the first and second experiments, stimuli were presented against a dark background with a stimulus onset asynchrony (SOA) from 0 to 200 ms, in a paradigm controlling for response bias. In the third experiment, stimuli were presented against a bright background. We demonstrate that the time interval between stimuli modulates and even inverts their perceived brightness difference, enhancing the second stimulus relative to the first. When the background is brighter than the target stimuli, the sign of the effect is inverted, suggesting that the underlying mechanism operates on contrast rather than brightness. The magnitude of this effect is shown to depend on SOA and average luminance level of the target stimuli. Hypotheses in terms of neural and attentional dynamics are proposed.

Turning the corner with the flash-lag illusion

Previous attempts to measure localization bias around a right-angle turn (L-trajectory) have found either no spatial bias off the trajectory (Whitney, Cavanagh, & Murakami, 2000) or a bias, in different experiments, both 'inside' and 'outside' the trajectory (Nieman, Sheth, & Shimojo, 2010). However, Eagleman and Sejnowski (2007) presented data showing that the perceived location of a brief feature on two moving stimuli could be predicted from the vector sum of their directions after the feature appeared. Such a vector sum with an L-trajectory could predict that the perceived position before the turn should be biased 'sideways' off the trajectory, in the direction of the final motion. With stimuli that particularly facilitated accurate vernier judgments, and measuring bias via the flash-lag illusion, this is indeed what we observed. Our data thus favour Eagleman and Sejnowski's (2007) supposition. Further, the bias occurred before the change in direction, rather than after it, supporting the contention that it is motion after a point being sampled that affects its perception (Bachmann et al., 2003; Eagleman & Sejnowski, 2007; Krekelberg & Lappe, 2000; Nieman, Sheth, & Shimojo, 2010).

Macaque monkeys perceive the flash lag illusion

Transmission of neural signals in the brain takes time due to the slow biological mechanisms that mediate it. During such delays, the position of moving objects can change substantially. The brain could use statistical regularities in the natural world to compensate neural delays and represent moving stimuli closer to real time. This possibility has been explored in the context of the flash lag illusion, where a briefly flashed stimulus in alignment with a moving one appears to lag behind the moving stimulus. Despite numerous psychophysical studies, the neural mechanisms underlying the flash lag illusion remain poorly understood, partly because it has never been studied electrophysiologically in behaving animals. Macaques are a prime model for such studies, but it is unknown if they perceive the illusion. By training monkeys to report their percepts unbiased by reward, we show that they indeed perceive the illusion qualitatively similar to humans. Importantly, the magnitude of the illusion is smaller in monkeys than in humans, but it increases linearly with the speed of the moving stimulus in both species. These results provide further evidence for the similarity of sensory information processing in macaques and humans and pave the way for detailed neurophysiological investigations of the flash lag illusion in behaving macaques.

Inferring sense of agency from the quantitative aspect of action outcome

The sense of agency refers to an experience in which one's own action causes a change in environment. It is strongly modulated by both the contingency between action and its outcome and the consistency between predicted and actual action outcomes. Recent studies have suggested that the action outcome can retrospectively modulate action awareness. We suspect that the sense of agency can also be retrospectively modulated. This study examined whether the quantity of action outcome could influence the sense of agency. The participants' task was to trigger dot motion in a display and rate the extent to which they could control the initiation of dot motion. Independently of both the temporal contiguity between action and its outcome and the consistency between predicted and actual action outcomes, the speed of dot motion as an action's outcome strongly influenced the sense of agency rating. The present study suggests that the sense of agency stems partly from the inference of action efficiency based on the quantitative aspect of action outcome.

Conscious updating is a rhythmic process

As the visual world changes, its representation in our consciousness must be constantly updated. Given that the external changes are continuous, it appears plausible that conscious updating is continuous as well. Alternatively, this updating could be periodic, if, for example, its implementation at the neural level relies on oscillatory activity. The flash-lag illusion, where a briefly presented flash in the vicinity of a moving object is misperceived to lag behind the moving object, is a useful tool for studying the dynamics of conscious updating. Here, we show that the trial-by-trial variability in updating, measured by the flash-lag effect (FLE), is highly correlated with the phase of spontaneous EEG oscillations in occipital (5-10 Hz) and frontocentral (12-20 Hz) cortices just around the reference event (flash onset). Further, the periodicity in each region independently influences the updating process, suggesting a two-stage periodic mechanism. We conclude that conscious updating is not continuous; rather, it follows a rhythmic pattern.

Misperceptions in the trajectories of objects undergoing curvilinear motion

Trajectory perception is crucial in scene understanding and action. A variety of trajectory misperceptions have been reported in the literature. In this study, we quantify earlier observations that reported distortions in the perceived shape of bilinear trajectories and in the perceived positions of their deviation. Our results show that bilinear trajectories with deviation angles smaller than 90 deg are perceived smoothed while those with deviation angles larger than 90 degrees are perceived sharpened. The sharpening effect is weaker in magnitude than the smoothing effect. We also found a correlation between the distortion of perceived trajectories and the perceived shift of their deviation point. Finally, using a dual-task paradigm, we found that reducing attentional resources allocated to the moving target causes an increase in the perceived shift of the deviation point of the trajectory. We interpret these results in the context of interactions between motion and position systems.

Motion extrapolation in the central fovea

Neural transmission latency would introduce a spatial lag when an object moves across the visual field, if the latency was not compensated. A visual predictive mechanism has been proposed, which overcomes such spatial lag by extrapolating the position of the moving object forward. However, a forward position shift is often absent if the object abruptly stops moving (motion-termination). A recent "correction-for-extrapolation" hypothesis suggests that the absence of forward shifts is caused by sensory signals representing 'failed' predictions. Thus far, this hypothesis has been tested only for extra-foveal retinal locations. We tested this hypothesis using two foveal scotomas: scotoma to dim light and scotoma to blue light. We found that the perceived position of a dim dot is extrapolated into the fovea during motion-termination. Next, we compared the perceived position shifts of a blue versus a green moving dot. As predicted the extrapolation at motion-termination was only found with the blue moving dot. The results provide new evidence for the correction-for-extrapolation hypothesis for the region with highest spatial acuity, the fovea.

Feature binding of a continuously changing object

Consider a feature of a stimulus (such as color, luminance, or spatial frequency) that changes over time along a continuum. When a second stimulus is briefly pulsed with the same feature value as the first stimulus, the two stimuli are not perceived to match. Instead, the continuously changing stimulus is perceived to be further ahead on the feature continuum than the pulsed stimulus [Nat. Neurosci. 3, 489 (2000)]. This shift is quantified by the amount of time ahead on the changing continuum, which is different for various types of features. A basic question is how our percepts are affected when an object has two continuously changing features (such as color and orientation) with different magnitudes of time ahead. This was addressed using a bar continuously changing in both color and orientation. Even though the two features were part of the same object, each feature maintained a distinctly different time ahead. This implies that observers perceived at each moment a combination of color and orientation that never was presented to the eye.

Reduction of the flash-lag effect in terms of active observation

In the present study, we investigated how observers' control of stimulus change affects temporal and spatial aspects of visual perception. We compared the illusory flash-lag effects for automatic movement of the stimulus with stimulus movement that was controlled by the observers' active manipulation of a computer mouse (Experiments 1, 2, and 5), a keyboard (Experiment 3), or a trackball (Experiment 4). We found that the flash-lag effect was significantly reduced when the observer was familiar with the directional relationship between the mouse movement and stimulus movement on a front parallel display (Experiments 1 and 2) and that, although the unfamiliar directional relationship between the mouse movement and stimulus movement increased the flash-lag effect at the beginning of the experimental session, the repetitive observation with the same unfamiliar directional relationship reduced the flash-lag effect (Experiment 5). We found no consistent reduction of the flash-lag effect with the use of a keyboard or a trackball (Experiments 3 and 4). These results suggest that the learning of a specific directional relationship between a proprioceptive signal of hand movements and a visual signal of stimulus movements is necessary for the reduction of the flash-lag effect.

Neurophysiology of compensation for time delays: Visual prediction is off track

Speculation by Nijhawan that visual perceptual mechanisms compensate for neural delays has no basis in the physiological properties of neurons known to be involved in motion perception and visuomotor control. Behavioral and physiological evidence is consistent with delay compensation mediated primarily by motor systems.

Single mechanism, divergent effects; multiple mechanisms, convergent effect

It is commonplace for a single physiological mechanism to seed multiple phenomena, and for multiple mechanisms to contribute to a single phenomenon. We propose that the flash-lag effect should not be considered a phenomenon with a single cause. Instead, its various aspects arise from the convergence of a number of different mechanisms proposed in the literature. We further give an example of how a neuron's generic spatio-temporal response profile can form a physiological basis not only of “prediction,” but also of many of the other proposed flash-lag mechanisms, thus recapitulating a spectrum of flash-lag phenomena. Finally, in agreeing that such basic predictive mechanisms are present throughout the brain, we argue that motor prediction contributes more to biological fitness than visual prediction.

Moving backward through perceptual compensation

In the target article Nijhawan speculates that visual perceptual mechanisms compensate for neural delays so that moving objects may be perceived closer to their physical locations. However, the vast majority of published psychophysical data are inconsistent with this speculation.

A Psychophysical and Computational Analysis of the Spatio-Temporal Mechanisms Underlying the Flash-Lag Effect

Several accounts put forth to explain the flash-lag effect (FLE) rely mainly on either spatial or temporal mechanisms. Here we investigated the relationship between these mechanisms by psychophysical and theoretical approaches. In a first experiment we assessed the magnitudes of the FLE and temporal-order judgments performed under identical visual stimulation. The results were interpreted by means of simulations of an artificial neural network, that was also employed to make predictions concerning the FLE. The model predicted that a spatio-temporal mislocalisation would emerge from two, continuous and abrupt-onset, moving stimuli. Additionally, a straightforward prediction of the model revealed that the magnitude of this mislocalisation should be task-dependent, increasing when the use of the abrupt-onset moving stimulus switches from a temporal marker only to both temporal and spatial markers. Our findings confirmed the model's predictions and point to an indissoluble interplay between spatial facilitation and processing delays in the FLE.

The perceived position of a moving object is not the result of position integration

The flash-lag effect is a robust visual illusion in which a flash appears to spatially lag a continuously moving stimulus, even though both stimuli are actually precisely aligned. Some research has been done to test how visual information has been integrated over time. The position integration model suggests motion integration is a form of interpolation of past positions, and predicts that we cannot perceive the reversal point at its actual position on the trajectory of a moving object which reverses abruptly. In current research, we demonstrate that subjects could perceive the reversal point accurately while the psychometric function measured by a flash does not pass through the actual turning point. These results do not support the position integration model. We propose that the flash-lag effect is more likely to be a temporal illusion.

Cortical processing and perceived timing

As of yet, it is unclear how we determine relative perceived timing. One controversial suggestion is that timing perception might be related to when analyses are completed in the cortex of the brain. An alternate proposal suggests that perceived timing is instead related to the point in time at which cortical analyses commence. Accordingly, timing illusions should not occur owing to cortical analyses, but they could occur if there were differential delays between signals reaching cortex. Resolution of this controversy therefore requires that the contributions of cortical processing be isolated from the influence of subcortical activity. Here, we have done this by using binocular disparity changes, which are known to be detected via analyses that originate in cortex. We find that observers require longer stimulus exposures to detect small, relative to larger, disparity changes; observers are slower to react to smaller disparity changes and observers misperceive smaller disparity changes as being perceptually delayed. Interestingly, disparity magnitude influenced perceived timing more dramatically than it did stimulus change detection. Our data therefore suggest that perceived timing is both influenced by cortical processing and is shaped by sensory analyses subsequent to those that are minimally necessary for stimulus change perception.

Stimulus dependence of the flash-lag effect

When two moving objects are presented in perfect alignment, but are not visible for the same amount of time, the briefer object will often be perceived as "lagging" the object of greater duration. Most investigations of this flash-lag effect (FLE) employ high velocity broadband stimuli, such as lines or dots with sharp boundaries and flashes with rapid onset and offset. We introduce a stimulus paradigm with narrow-band stimuli and measure the stimulus dependence of the FLE when basic stimulus parameters of spatio-temporal frequency and temporal duration are varied. We suggest that this dependence is consistent with the involvement of early visual mechanisms and interpret our results in the context of existing theories of the FLE.

Attraction of flashes to moving dots

Motion is known to distort visual space, producing illusory mislocalizations for flashed objects. Previously, it has been shown that when a stationary bar is flashed in the proximity of a moving stimulus, the position of the flashed bar appears to be shifted in the direction of nearby motion. A model consisting of predictive projections from the sub-system that processes motion information onto the sub-system that processes position information can explain this illusory position shift of a stationary flashed bar in the direction of motion. Based on this model of motion-position interactions, we predict that the perceived position of a flashed stimulus should also be attracted towards a nearby moving stimulus. In the first experiment, observers judged the perceived vertical position of a flash with respect to two horizontally moving dots of unequal contrast. The results of this experiment were in agreement with our prediction of attraction towards the high contrast dot. We obtained similar findings when the moving dots were replaced by drifting gratings of unequal contrast. In control experiments, we found that neither attention nor eye movements can account for this illusion. We propose that the visual system uses predictive influences from the motion processing sub-system on the position processing sub-system to overcome the temporal limitations of the position processing system.

Spatial and temporal properties of the illusory motion-induced position shift for drifting stimuli

The perceived position of a stationary Gaussian window of a Gabor target shifts in the direction of motion of the Gabor's carrier stimulus, implying the presence of interactions between the specialized visual areas that encode form, position, and motion. The purpose of this study was to examine the temporal and spatial properties of this illusory motion-induced position shift (MIPS). We measured the magnitude of the MIPS for a pair of horizontally separated (2 or 8deg) truncated-Gabor stimuli (carrier=1 or 4cpd sinusoidal grating, Gaussian envelope SD=18arc min, 50% contrast) or a pair of Gaussian-windowed random-texture patterns that drifted vertically in opposite directions. The magnitude of the MIPS was measured for drift speeds up to 16deg/s and for stimulus durations up to 453ms. The temporal properties of the MIPS depended on the drift speed. At low velocities, the magnitude of the MIPS increased monotonically with the stimulus duration. At higher velocities, the magnitude of the MIPS increased with duration initially, then decreased between approximately 45 and 75ms before rising to reach a steady-state value at longer durations. In general, the magnitude of the MIPS was larger when the truncated-Gabor or random-texture stimuli were more spatially separated, but was similar for the different types of carrier stimuli. Our results are consistent with a framework that suggests that perceived form is modulated dynamically during stimulus motion.

Motion signals bias localization judgments: a unified explanation for the flash-lag, flash-drag, flash-jump, and Frohlich illusions

In the flash-lag illusion, a moving object aligned with a flash is perceived to be offset in the direction of motion following the flash. In the "flash-drag" illusion, a flash is mislocalized in the direction of nearby motion. In the "flash-jump" illusion, a transient change in the appearance of a moving object (e.g., color) is mislocalized in the direction of subsequent motion. Finally, in the Frohlich illusion, the starting position of a suddenly appearing moving object is mislocalized in the direction of the subsequent motion. We demonstrate, in a series of experiments, a unified explanation for all these illusions: Perceptual localization is influenced by motion signals collected over approximately 80 ms after a query is triggered. These demonstrations rule out "latency difference" and asynchronous feature binding models, in which objects appear in their real positions but misaligned in time. Instead, the illusions explored here are best understood as biases in localization caused by motion signals. We suggest that motion biasing exists because it allows the visual system to account for neural processing delays by retrospectively "pushing" an object closer to its true physical location, and we propose directions for exploring the neural mechanisms underlying the dynamic updating of location by the activity of motion-sensitive neurons.

Perceptual compression of space through position integration

The mechanism of positional localization has recently been debated due to interest in the flash-lag effect, which occurs when a briefly flashed stationary stimulus is perceived to lag behind a spatially aligned moving stimulus. Here we report positional localization observed at motion offsets as well as at onsets. In the 'flash-lead' effect, a moving object is perceived to be behind a spatially concurrent stationary flash before the two disappear. With 'reverse-repmo', subjects mis-localize the final position of a moving bar in the direction opposite to the trajectory of motion. Finally, we demonstrate that simultaneous onset and offset effects lead to a perceived compression of visual space. By characterizing illusory effects observed at motion offsets as well as at onsets, we provide evidence that the perceived position of a moving object is the result of an averaging process over a short time period, weighted towards the most recent positions. Our account explains a variety of motion illusions, including the compression of moving shapes when viewed through apertures.

Attention 'capture' by the flash-lag flash

We report data from eight participants who made alignment judgements between a moving object and a stationary, continuously visible 'landmark'. A reversing object had to overshoot the landmark by a significant amount in order to appear to reverse aligned with it. In addition, an adjacent flash irrelevant to the judgment task reliably increased this illusory 'foreshortening'. This and other results are most simply explained by a model in which the flash causes attentional capture, complemented by processes of temporal integration, or backward inhibition, and object representation. A flash used to probe the perception of a moving object's position disrupts that very perception.

Manual control of the visual stimulus reduces the flash-lag effect

We investigated how observers' control of the stimulus change affects temporal aspects of visual perception. We compared the flash-lag effects for motion (Experiment 1) and for luminance (Experiment 2) under several conditions that differed in the degree of the observers' control of change in a stimulus. The flash-lag effect was salient if the observers passively viewed the automatic change in the stimulus. However, if the observers controlled the stimulus change by a computer-mouse, the flash-lag effect was significantly reduced. In Experiment 3, we examined how observers' control of the stimulus movement by a mouse affects the reaction time for the shape change in the moving stimulus and flash. Results showed that the control reduced the reaction time for both moving stimulus and flash. These results suggest that observers' manual control of the stimulus reduces the flash-lag effect in terms of facilitation in visual processing.

Computational neurobiology of the flash-lag effect

In the flash-lag effect (FLE) a moving object is perceived ahead of a stationary stimulus flashed in spatial alignment. Several explanations have been proposed to account for the FLE and its dependence on a variety of psychophysical attributes. Here, we show that a simple feed-forward network reproduces the standard FLE and several related manifestations, such as its modulation by stimulus luminance, trajectory, priming, and spatial predictability. A minimal set of elements, based on plausible neuronal mechanisms, yields a unified account of these visual illusions and possibly other perceptual phenomena.

Differential latencies and the dynamics of the position computation process for moving targets, assessed with the flash-lag effect

To investigate the dynamics of the position computation process for a moving object in human vision, we measured the response to a continuous change in position at a constant velocity (ramp-response) using the flash-lag illusion. In this illusion, flashed and moving objects appear spatially offset when their retinal images are physically aligned. The steady-state phase of the ramp-response was probed using the "continuous-motion" (CM) paradigm, in which the motion of the moving object starts long before the occurrence of the flash. To probe the transient phase of the ramp-response, we used the "flash-initiated cycle" (FIC) paradigm, in which the motion of the moving object starts within a short time window around the presentation of the flash. The sampling instant of the ramp-response was varied systematically by changing the luminance or the presentation time of the flashed stimulus. We found that the perceived flash misalignments in the FIC and CM paradigms were approximately equal when sampling of the ramp-response occurred after a relatively long delay from the onset of motion and, were significantly different when sampling of the ramp-response occurred at a relatively short delay. The systematic variations in the perceived misalignment between the moving and flashed stimuli as a function of stimulus parameters are compared to the predictions of our differential latency and to alternative models of position computation.

Temporal dependence of local motion induced shifts in perceived position

It has been shown that a moving visual pattern can influence the perceived position of outlying, briefly flashed objects. Using a rotating bar as an inducing stimulus we observed a shift, in the direction of motion, of the perceived position of small bars flashed together on either side of the moving bar. The greatest shift occurred when the 13 ms flashes were presented 60 ms before the rotating bar came closest to their locations. By varying rotation speed we showed that the peak effect was determined by the temporal rather than the spatial interval. The motion induced shift could be attenuated by introducing background flickering dots. The perceived shift decreased with distance from motion when the eccentricity of the flashes was kept constant. We conclude that the shift reflects feedback to primary visual cortex from motion selective cells in extrastriate cortex with receptive fields that overlap the retinal location of the flash.

The line-motion illusion can be reversed by motion signals after the line disappears

In the line-motion illusion, a briefly flashed line appears to propagate from the locus of attention, despite being physically presented on the screen all at once. It has been proposed that the illusion reflects low-level visual information processing that occurs faster at the locus of attention (Hikosaka et al 1993 Vision Research 33 1219-1240; Perception 22 517-526). Such an explanation implicitly embeds the assumption that speeding or slowing of neural signals will map directly onto perceptual timing. This 'online' hypothesis presupposes that signals which arrive first are perceived first. However, other evidence suggests that events in a window of time after the disappearance of a visual stimulus can influence the brain's interpretation of that stimulus (Eagleman and Sejnowski 2000 Science 287 2036-2038; 289 1107a; 290 1051a; 2002 Trends in Neuroscience 25 293). If the online hypothesis were true, we should expect that events occurring after the flashing of the line would not change the illusion. Consistent with our hypothesis that awareness is an a posteriori reconstruction, we demonstrate that the perceived direction of illusory line-motion can be reversed by manipulating stimuli after the line has disappeared.

Effects of stimulus size and luminance on oscillopsia in congenital nystagmus

Although the absence of oscillopsia is a common feature of congenital nystagmus (CN), it is occasionally noted by patients under poor viewing conditions and has been provoked in laboratory settings with stabilised images. In the present study, the effects of reductions in background stimulus size and luminance on perceptual stability in CN were examined. Sixteen CN subjects were first interviewed using a structured questionnaire about whether they ever experienced oscillopsia and, if so, under what circumstances and with what perceptions. They next fixated an LED centred in projected images of three sizes (21x14 degrees, 10x6 degrees and 7x4 degrees) and four luminance levels (115.5, 24.5, 2.7 and 0.1 cd/m2, with contrasts from 96 down to 20%). Eye movements were recorded with a limbal tracker. They were asked after viewing each image "whether anything happened to the image while they watched it." Occasional oscillopsia was reported by 12/16 of the CN subjects on the questionnaire. In the laboratory, 13/16 subjects experienced oscillopsia in some manner for at least one of the stimuli. 8/13 CN subjects experienced it for the dimmest and smallest slides. 11/13 perceived certain parts (either the LED or background) of the visual stimuli as moving, with the perception of LED movement most pronounced at low background luminance. Foveation did not differ when trials with and without reported oscillopsia were compared (independent samples t-test, p>0.05). Oscillopsia may occur in CN with normal viewing of bright fixation targets against dim backgrounds. Under these conditions, the oscillopsia may be spatially inhomogeneous. Luminance differences between the fixation point and surround may have caused transmission time differences as the image moved across the retina, therefore leading to the perception of motion in one portion of the scene and not the other.