Effects of translational background motion on visual localization

Three subjects were asked to judge the position of small spots of light flashed before, during or after rapid translational motion of the background grating pattern. Mislocalization of the spots was observed when the background moved during or immediately after presentation of the spot. In both cases, mislocalization always occurred in the direction of the fixation point. Furthermore, this mislocalization occurred only when the background moved in the opposite direction to the visual half-field in which the spot appeared. That is to say, a spot to the right of the fixation point was mislocalized when its background moved to the left, but not when it moved to the right, and the converse was also true. This finding was interpreted as reflecting a long-term adaptation to the optokinetic stimulation that we experience during forward and backward locomotion.     

Localisation of auditory targets during optokinetic nystagmus

Previous studies have shown that the perceived location of visual stimuli briefly flashed during smooth pursuit, saccades, or optokinetic nystagmus (OKN) is not veridical. We investigated whether these mislocalisations can also be observed for brief auditory stimuli presented during OKN. Experiments were carried out in a lightproof sound-attenuated chamber. Participants performed eye movements elicited by visual stimuli. An auditory target (white noise) was presented for 5 ms. Our data clearly indicate that auditory targets are mislocalised during reflexive eye movements. OKN induces a shift of perceived location in the direction of the slow eye movement and is modulated in the temporal vicinity of the fast phase. The mislocalisation is stronger for look- as compared to stare-nystagmus. The size and temporal pattern of the observed mislocalisation are different from that found for visual targets. This suggests that different neural mechanisms are at play to integrate oculomotor signals and information on the spatial location of visual as well as auditory stimuli.     

Spatial but not temporal cueing influences the mislocalisation of a target flashed during smooth pursuit

Human subjects misjudge the position of a target that is flashed during a pursuit eye movement. Their judgments are biased in the direction in which the eyes are moving. We investigated whether this bias can be reduced by making the appearance of the flash more predictable. In the normal condition, subjects pursued a moving target that flashed somewhere along its trajectory. After the presentation, they indicated where they had seen the flash. The mislocalisations in this condition were compared to mislocalisations in conditions in which the subjects were given information about when or where the flash would come. This information consisted of giving two warning flashes spaced at equal intervals before the target flash, of giving two warning beeps spaced at equal intervals before the target flash, or of showing the same stimulus twice. Showing the same stimulus twice significantly reduced the mislocalisation. The other conditions did not. We interpret this as indicating that it is not predictability as such that influences the performance, but the fact that the target appears at a spatially cued position. This was supported by a second experiment, in which we examined whether subjects make smaller misjudgments when they have to determine the distance between a target flashed during pursuit and a reference seen previously, than when they have to determine the distance between the flashed target and a reference seen afterwards. This was indeed the case, presumably because the reference provided a spatial cue for the flash when it was presented first. We conclude that a spatial cue reduces the mislocalisation of targets that are flashed during pursuit eye movements. The cue does not have to be exactly at the same position as the flash.     

Perceived shifts of flashed stimuli by visible and invisible object motion

Perceived positions of flashed stimuli can be altered by motion signals in the visual field-position capture (Whitney and Cavanagh, 2000 Nature Neuroscience 3 954-959). We examined whether position capture of flashed stimuli depends on the spatial relationship between moving and flashed stimuli, and whether the phenomenal permanence of a moving object behind an occluding surface (tunnel effect; Michotte 1950 Acta Psychologica 7 293-322) can produce position capture. Observers saw two objects (circles) moving vertically in opposite directions, one in each visual hemifield. Two horizontal bars were simultaneously flashed at horizontally collinear positions with the fixation point at various timings. When the movement of the object was fully visible, the flashed bar appeared shifted in the motion direction of the circle. But this position-capture effect occurred only when the bar was presented ahead of or on the moving circle. Even when the motion trajectory was covered by an opaque surface and the bar was flashed after complete occlusion of the circle, the position-capture effect was still observed, though the positional asymmetry was less clear. These results show that movements of both visible and 'hidden' objects can modulate the perception of positions of flashed stimuli and suggest that a high-level representation of 'objects in motion' plays an important role in the position-capture effect.     

The spoke brightness illusion originates at an early motion processing stage

When a bright white disk revolves around a fixation point on a gray background, observers perceive a "spoke": a dark gray region that connects the disk with the fixation point. Our first experiment suggests that motion across the retina is both necessary and sufficient for spokes: The illusion occurs when a disk moves across the retina even though it is perceived to be stationary, but the illusion does not occur when the disk appears to move while remaining stationary on the retina. A second experiment shows that the strength of the illusion decreases with decreasing luminance contrast until subjective equiluminance, where little or no spoke is perceived. These results suggest that spokes originate at an early, predominantly luminance-based stage of motion processing, before the visual system discounts retinal motion caused by smooth pursuit.     

An acceleration illusion caused by underestimation of stimulus velocity during pursuit eye movements: Aubert-Fleischl revisited

When the eyes pursue a fixation point that sweeps across a moving background pattern, and the fixation point is suddenly made to stop, the ongoing motion of the background pattern seems to accelerate to a higher velocity. Experiment I showed that this acceleration illusion is not caused by the sudden change in (i) the relative velocity between background and fixation point, (ii) the velocity of the retinal image of the background pattern, or (iii) the motion of the retinal image of the rims of the CRT screen on which the experiment was carried out. In experiment II the magnitude of the illusion was quantified. It is strongest when background and eyes move in the same direction. When they move in opposite directions it becomes less pronounced (and may disappear) with higher background velocities. The findings are explained in terms of a model proposed by the first author, in which the perception of object motion and velocity derives from the interaction between retinal slip velocity information and the brain's 'estimate' of eye velocity in space. They illustrate that the classic Aubert-Fleischl phenomenon (a stimulus seems to be moving slower when pursued with the eyes than when moving in front of stationary eyes) is a special case of a more general phenomenon: whenever we make a pursuit eye movement we underestimate the velocity of all stimuli in our visual field which happen to move in the same direction as our eyes, or which move slowly in the direction opposite to our eyes.     

Visual transients reveal the veridical position of a moving object

The position of a moving object is often mislocalised in the direction of movement. At the input stage of visual processing, the position of a moving object should still be represented veridically, whereas it should become closer to the mislocalised position at a later processing stage responsible for positional judgment. Here, we show that visual transients expose the veridical position of a moving object represented in early visual areas. For example, when a ring is flashed on a moving bar, the part of the bar within the ring is perceived at the veridical position, whereas the part outside the ring is perceived to be ahead of the ring as in the flash-lag effect. Our observations suggest that a filling-in process is triggered at the edges of the flash. This indicates that, in early cortical areas, moving objects are still represented at their veridical positions, and the perceived location is determined by the higher visual areas.     

Haidinger's brushes phenomenon for control of eye-fixation in a laterality experiment

Haidinger's brushes phenomenon, as a fixation point, was examined in an experiment on laterality. 9 subjects were asked to recognize a Japanese letter presented in a hemifield and to estimate the accuracy of eye-fixation. A central digit was presented for 500 msec. before a letter. The subjects had twofold control of eye movement in the condition in which the brushes were presented. Most were able to maintain eye fixation and/or to detect eye movement more easily in the condition in which the brushes were presented. Left visual-field dominance in letter recognition was observed in both conditions both when Haidinger's brushes were presented and when they were not. The phenomenon significantly decreased the mean percent correct letters recognized in both right and left visual fields.     

Effect of visual surrounding motion on body sway in a three-dimensional environment

Unidirectional motion of a uniplanar background induces a codirectional postural sway. It has been shown recently that fixation of a stationary foreground object induces a sway response in the opposite direction (Bronstein & Buckwell, 1997) when the background moves transiently. The present study investigated factors determining this contradirectional postural response. In the experiments presented, center of foot pressure and head displacements were recorded from normal subjects. The subjects faced a visual background of 2 x 3 m, at a distance of 1.5 m, which could be moved parallel to the interaural axis. Results showed that when the visual scene consisted solely of a moving background, the conventional codirectional postural response was elicited. When subjects were asked to fixate an earth-fixed foreground (window frame) placed between them and the moving background, a consistent postural response in the opposite direction to background motion was observed. In addition, we showed that this contradirectional postural response was not transient but was sustained for the 11 sec of background motion. We investigated whether this contradirectional postural response was the consequence of the induced movement of the foreground by background motion. Although induced movement was verbally reported by subjects when viewing an earth-fixed target projected onto the moving background, the contradirectional sway did not occur. These results indicate that foreground-background separation in depth was necessary for the contradirectional postural response to occur rather than induced movement. Another experiment showed that, when the fixated foreground was attached to the head of the observer, the contradirectional sway was not observed and was therefore unrelated to vergence. Finally, results showed that the contradirectional postural response was, in the main, monocularly mediated. We conclude that the direction of the postural sway produced by a moving background in a three-dimensional environment is determined primarily by motion parallax.     

Lateral differences in tachistoscopic recognition of bilaterally presented verbal material.

When subjects have to report verbal material presented tachistoscopically simultaneously on both sides of a fixation point, right visual field superiority has been obtained in several experiments using a central task technique, i.e. where a stimulus presented at the fixation point had to be identified before reporting the rest of the material. Without a central task, left visual field superiority has generally been obtained. It has been suggested to attribute the difference to control of eye fixation by the central task. An alternative interpretation, that the central task modifies the order of attentional scanning was put to the test in Experiment I. With two normally printed words, right visual field superiority was obtained with a central task and left visual field superiority without it. It was predicted that with mirrored words, the opposite pattern would be obtained, yet here right visual field superiority was obtained both without and with a central task. Experiment II shows that the latter result is nevertheless dependent on scanning order, for it can be completely inverted through recall order instructions. It is concluded that lateral differences observed with bilateral presentation cannot be explained without taking account of optional processing priorities, but that the factors on which the latter depend are not yet fully understood.     

The whereabouts of visual attention: Involuntary attentional bias toward the default gaze direction

This study proposed and verified a new hypothesis on the relationship between gaze direction and visual attention: attentional bias by default gaze direction based on eye-head coordination. We conducted a target identification task in which visual stimuli appeared briefly to the left and right of a fixation cross. In Experiment 1, the direction of the participant’s head (aligned with the body) was manipulated to the left, front, or right relative to a central fixation point. In Experiment 2, head direction was manipulated to the left, front, or right relative to the body direction. This manipulation was based on results showing that bias of eye position distribution was highly correlated with head direction. In both experiments, accuracy was greater when the target appeared at a position where the eyes would potentially be directed. Consequently, eye–head coordination influences visual attention. That is, attention can be automatically biased toward the location where the eyes tend to be directed.     

Experimental note on fading of briefly flashed lines

A peripheral uniform stimulus on a uniform background tends to fade when a ring enclosing the stimulus is flashed briefly, indicating that neural signals external to the stimulus such as those from the ring or those from the background tend to inhibit the onset-response or after-discharge of the stimulus. In this study, a peripheral achromatic line was flashed briefly on an achromatic background while the subject was steadily fixating on a point on the background. It was predicted that the stimulus line would fade due to background inhibition of the onset-response or afterdischarge of the stimulus. On a reference line, subjects indicated two points delimiting the part of the stimulus line which looked uniform in intensity of color. These points converged toward the center of the reference line as the luminance of the stimulus line approached that of the background. Compared to the central uniform part, the nonuniform parts near the endpoints of the stimulus line looked more faded. These results support the idea that the background surrounding a stimulus tends to inhibit the onset-response of after-discharge of the stimulus.     

Visual-field asymmetries in letter recognition: evidence for asymmetry in early visual registration

Two experiments on visual-field differences in tachistoscopic letter recognition are described. In the first, a bright pre-exposure field with a black fixation point was used, and the conventionally expected dominance of the right visual field was found. However, a large number of "blank" trials were observed, in which subjects completely failed to detect the presence of the flashed target. These "blanks" were themselves significantly asymmetric between visual fields, suggesting that asymmetry in early stimulus registration may play an unsuspected role in typical measures of cerebral asymmetry in recognition accuracy. This was confirmed in a second experiment in which use of dark pre-exposure fields eliminated "blanks" and led to higher over-all accuracy, with no visual-field differences. Implications for interpretation of laterality data with normal subjects are discussed.     

Automatic visual bias of perceived auditory location

Studies of reactions to audiovisual spatial conflict (alias “ventriloquism”) are generally presented as informing on the processes of intermodal coordination. However, most of the literature has failed to isolate genuine perceptual effects from voluntary postperceptual adjustments. A new approach, based on psychophysical staircases, is applied to the case of the immediate visual bias of auditory localization. Subjects have to judge the apparent origin of stereophonically controlled sound bursts as left or right of a median reference line. Successive trials belong to one of two staircases, starting respectively at extreme left and right locations, and are moved progressively toward the median on the basis of the subjects’ responses. Response reversals occur for locations farther away from center when a central lamp is flashed in synchrony with the bursts than without flashes (Experiment 1), revealing an attraction of the sounds toward the flashes. The effect cannot originate in voluntary postperceptual decision, since the occurrence of response reversal implies that the subject is uncertain concerning the direction of the target sound. The attraction is contingent on sound-flash synchronization, for early response reversals did no longer occur when the inputs from the two modalities were desynchronized (Experiment 2). Taken together, the results show that the visual bias of auditory localization observed repeatedly in less controlled conditions is due partly at least to an automatic attraction of the apparent location of sound by spatially discordant but temporally correlated visual inputs.     

Visual grouping by motion precedes the relative localization between moving and flashed stimuli

A flashed stimulus is perceived as spatially lagging behind a moving stimulus when they are spatially aligned. When several elements are perceptually grouped into a unitary moving object, a flash presented at the leading edge of the moving stimulus suffers a larger spatial lag than a flash presented at the trailing edge (K. Watanabe. R. Nijhawan. B. Khurana, & S. Shimojo. 2001). By manipulation of the flash onset relative to the motion onset, the present study investigated the order of perceptual operations of visual motion grouping and relative visual localization. It was found that the asymmetric mislocalization was observed irrespective of physical and/or perceptual temporal order between the motion and flash onsets. Thus, grouping by motion must be completed to define the leading-trailing relation in a moving object before the visual system explicitly represents the relative positions of moving and flashed stimuli.     

Perceptual localization of visual stimuli flashed during saccades

Subjects were asked to make a saccade to a visual target flashed in the dark during a prior primary saccade, and to report its apparent position by moving an adjustable light spot to that position. When targets were presented at the beginning of the primary saccade, subjects perceptually mislocated them in the direction of the saccade, whereas when targets were presented immediately before the end of the primary saccade, the flashed targets were mislocated in the opposite direction. The perceptually localized position of the target was primarily determined by its retinal position. However, at all actual and retinal positions of the target, the localized position shifted from the position that would be predicted if the location of the target was determined only by its retinal position to the prior primary saccade direction. The results were discussed in relation to extraretinal eye position signals. Subjects moved their eyes not to the actual position of the target, but to its apparent position. In some trials, there was a discrepancy between perceptual and oculomotor localization, which was interpreted as having been caused by the imprecise localization ability of the oculomotor system.     

Figure-ground formation by movement: influences and effects

A twelve-letter array was segmented into figure and ground by moving some of its letters. Moved letters were shifted one letter-width left of right, independently of each other, in apparent movement. Since the figure of a display attracts attention, identification of letters of the figure segment should show an advantage over letters of the background segment. Three results are of interest. First, moved letters were identified more accurately and faster than stationary letters when only one or two letters moved. Stationary letters showed the advantage when eight of the twelve letters moved. This result suggests that the segment seen as figure is determined by both rapidly encodable letter movement and by the number of moved letters within the display. Second, segmentation of the visual display acids identification of moved letters in less than 90 ms, or well before the eye can move to the selected letter position. Third, letters in the figure segment which are closer to fixation are more likely to be identified than more eccentric letters.     

Rapid categorization of foveal and extrafoveal natural images: associated ERPs and effects of lateralization

Humans are fast and accurate at performing an animal categorization task with natural photographs briefly flashed centrally. Here, this central categorization task is compared to a three position task in which photographs could appear randomly either centrally, or at 3.6 degrees eccentricity (right or left) of the fixation point. A mild behavioral impairment was found with peripheral stimuli with no evidence in support of hemispheric superiority; but enlarging the window of spatial attention to three possible stimuli locations had no behavioral cost on the processing of central images. Performance in the central categorization task has been associated with a large difference between the potentials evoked to target and non-target correct trials, starting about 150 ms after stimulus onset on frontal sites. Present results show that this activity originates within extrastriate visual cortices and probably reflects perceptual stimuli differences processed within areas involved in object recognition. Latencies, slopes, and peak amplitudes of this differential activity were invariant to stimulus position and attentional load. Stimulus location uncertainty and lateralization did not affect speed of visual processing.     

Spatial attentional distribution is modulated by head direction during eccentric gaze

Visual perception during eccentric gaze can be facilitated when a visual stimulus appears in front of the head direction. This study investigated the relative effects of gaze location and head direction on visual perception in central and peripheral vision. Participants identified the orientation of a T-shaped figure presented in the centre of a monitor and simultaneously localised a dot appearing in the periphery, while head direction relative to gaze location was to the left, right or centre. Effects of head direction were found only when the dot appeared far from the gaze fixation point, such that dot detection was superior when it appeared to the left (right) of fixation in the left (right) head direction. Experimental results indicated this was not due to a small shift of gaze location. Thus this study suggests that head direction influences visual perception particularly in peripheral vision where visual acuity decreases.     

Motion induction from biological motion

A new type of motion illusion is described in which ambiguous motion becomes unidirectional on superimposition of a human figure walking on a treadmill. A point-light walker in profile was superimposed on a vertical counterphase grating backdrop. Eleven na?ve observers judged the apparent direction of motion against the grating as left or right in a two-alternative forced-choice task and found that the grating appeared to drift in a direction opposite to the walking. The illusion disappeared when the point lights moved in scrambled configurations. This indicates that the illusion is caused by biological motion that provides recognition of gaits. A human figure walking backwards produced no illusion because of the difficulty in identifying the gait. This indicates that the illusion is determined by translational motion rather than form represented from biological motion.