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.     

Surrounding motion affects the perceived locations of moving stimuli

The perceived position of an object is determined not only by the retinal location of the object but also by gaze direction, eye movements, and the motion of the object itself. Recent evidence further suggests that the motion of one object can alter the perceived positions of stationary objects in remote regions of visual space (Whitney & Cavanagh, 2000). This indicates that there is an influence of motion on perceived position, and that this influence can extend over large areas of the visual field. Yet, it remains unclear whether the motion of one object shifts the perceived positions of other moving stimuli. To test this we measured two well-known visual illusions, the Fröhlich effect and representational momentum, in the presence of extraneous surrounding motion. We found that the magnitude of these mislocalizations was altered depending on the direction and speed of the surrounding motion. The results indicate that the positions assigned to stationary and moving objects are affected by motion signals over large areas of space and that both types of stimuli may be assigned positions by a common mechanism.     

Object-based anisotropies in the flash-lag effect

The relative visual position of a briefly flashed stimulus is systematically modified in the presence of motion signals. We investigated the two-dimensional distortion of the positional representation of a flash relative to a moving stimulus. Analysis of the spatial pattern of mislocalization revealed that the perceived position of a flash was not uniformly displaced, but instead shifted toward a single point of convergence that followed the moving object from behind at a fixed distance. Although the absolute magnitude of mislocalization increased with motion speed, the convergence point remained unaffected. The motion modified the perceived position of a flash, but had little influence on the perceived shape of a spatially extended flash stimulus. These results demonstrate that motion anisotropically distorts positional representation after the shapes of objects are represented. Furthermore, the results imply that the flash-lag effect may be considered a special case of two-dimensional anisotropic distortion.     

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.     

Presentation of a visual nearby moving object alters stream/bounce event perception

Two identical visual objects moving across each other in a two-dimensional display can be perceived as either streaming through or bouncing off each other. The bouncing event percept is promoted by the presentation of a brief sound at the point of coincidence of the two objects. In this study, we examined the effect of the presence of a moving object near the two objects as well as the brief sound on the stream/bounce event perception. When both the nearby moving object and brief sound were presented, a streaming event, not a bouncing event, was robustly perceived (experiment 1). The percentage of the streaming percept was also systematically affected by the proximity of the nearby object (experiment 2). These results suggest that the processing of intramodal grouping between a nearby moving object and either of the two objects in the stream/bounce display interferes with crossmodal (audiovisual) processing. Moreover, we demonstrated that, depending on the trajectory of the nearby moving object, the processing of intramodal grouping can promote the bouncing percept, just as crossmodal processing does (experiment 3).     

Visual processing of music notation: a study of event-related potentials

In reading music, the acquisition of pitch information depends mostly on the spatial position of notes, hence more spatial processing, whereas the acquisition of temporal information depends mostly on the visual features of notes and object recognition. This study used both electrophysiological and behavioral methods to compare the processing of pitch and duration in reading single musical notes. It was observed that in the early stage of note reading, identification of pitch could elicit greater N1 and N2 amplitude than identification of duration at the parietal lobe electrodes. In the later stages of note reading, identifying pitch elicited a greater negative slow wave at parietal electrodes than did identifying note duration. The sustained contribution of parietal processes for pitch suggests that the dorsal pathway is essential for pitch processing. However, the duration task did not elicit greater amplitude of any early ERP components than the pitch task at temporal electrodes. Accordingly, a double dissociation, suggesting involvement of the dorsal visual stream, was not observed in spatial pitch processing and ventral visual stream in processing of note durations.     

Neural delays,visual motion and the flash-lag effect

In the primate visual system, there is a significant delay in the arrival of photoreceptor signals in visual cortical areas. Since Helmholtz, scientists have pondered over the implications of these delays for human perception. Do visual delays cause the ' position of a moving object to lag its 'real' position? This question has recently been re-evaluated in the context of the flash-lag phenomenon, in which a flashed object appears to lag behind a moving object, when physically the two objects are co-localized at the instant of the flash. This article critically examines recent accounts of this phenomenon, assesses its biological significance, and offers new hypotheses.     

Appearance of an illusory object in the blind spot

We report a phenomenon showing new aspects of perceptual filling-in at the blind spot. When two and three discs were presented below the blind spot, observers perceived them as three and four discs, respectively, but when they were presented in the nasal visual field as a control, the perceived numbers were veridical. The phenomenon demonstrates that the visual system produces an illusory object in the blind spot even when the inducing objects presented on only one side of the blind spot are clearly discrete and countable.     

The open-object illusion: size perception is greatly influenced by object boundaries

This study presents a new powerful visual illusion, in which simple “open” objects—ones with missing boundaries—are perceived as bigger than the same size, fully “closed” objects. In a series of experiments that employed a continuous-response adjustment procedure, it was found that the lack of vertical boundaries inflated the perceived width of an object, whereas the lack of horizontal boundaries inflated its perceived length. The effect was highly robust and it was replicated across different stimulus types and experimental parameters, with almost all observers exhibiting a strong effect. In contrast to the overestimation of the size of an object due to missing boundaries, the inclusion of inner boundaries within an object caused observers to underestimate its size, suggesting that filled space sometimes shrinks, rather than inflates, the perceived size of an object. The open-object illusion bears practical implications for graphics and design as well as important theoretical implications. Specifically, it indicates that the perception of an object’s area is not veridical but rather critically depends on contour closure. It is suggested that the visual system extends the missing boundaries of open contour objects, which results in an overestimation of the object’s size.     

Saccades reveal that allocentric coding of the moving object causes mislocalization in the flash-lag effect

The flash-lag effect is a visual misperception of a position of a flash relative to that of a moving object: Even when both are at the same position, the flash is reported to lag behind the moving object. In the present study, the flash-lag effect was investigated with eye-movement measurements: Subjects were required to saccade to either the flash or the moving object. The results showed that saccades to the flash were precise, whereas saccades to the moving object showed an offset in the direction of motion. A further experiment revealed that this offset in the saccades to the moving object was eliminated when the whole background flashed. This result indicates that saccadic offsets to the moving stimulus critically depend on the spatially distinctive flash in the vicinity of the moving object. The results are incompatible with current theoretical explanations of the flash-lag effect, such as the motion extrapolation account. We propose that allocentric coding of the position of the moving object could account for the flash-lag effect.     

The flash-lag phenomenon: object motion and eye movements

An object flashed briefly in a given location, the moment another moving object arrives in the same location, is perceived by observers as lagging behind the moving object (flash-lag effect). Does the flash-lag effect occur if the retinal image of the moving object is rendered stationary by smooth pursuit of the moving object? Does the flash-lag effect occur if the retinal image of a stationary object is caused to move by smooth-pursuit eye movements? A disk was briefly flashed in the center of a moving ring such that the ring center was completely 'filled' by the disk. In this display, observers perceived the flashed disk to lag such that it appeared only to partially 'fill' the ring center. The 'unfilled' portion (perceived void) of the moving ring was seen in the color of the background. With smooth pursuit of the ring, the flash-lag effect was eliminated, and observers saw the flashed disk centered on the moving ring. A strong flash-lag effect was observed when observers smoothly pursued a moving point target past a continuously visible stationary ring. Once again, the flashed disk appeared to only partially fill the center of the continuously visible stationary ring, yielding a vivid 'perceived void'. These results are discussed in terms of neural delays and their compensation.     

Lateral masking in cycling displays: the relative importance of separation, flanker duration, and interstimulus interval for object-mediated updating

A central bar repeatedly presented in alternation with two flanking bars can lead to the disappearance of the central bar. Recently it has been suggested that this masking effect could be explained by object-mediated updating: the information from the central bar is integrated into the representation of the flankers, leading not only to the disappearance of the central bar as a separate object, but also to the perception of the flankers in apparent motion between their real position and the position of the central bar. This account suggests that the visibility of the central bar should depend on the same factors as those that influence the construction and maintenance of object representations. Therefore separation between central bar and flankers should not influence visibility as long as the time interval between them is adequate to make an interpretation of the scene in terms of one object moving from one location to the other possible location. We found that if the time interval between the central bar and the flankers is neither too short nor too long, the central bar becomes invisible even at large separations. These findings are inconsistent with traditional accounts of the cycling lateral masking displays in terms of local inhibitory mechanisms.     

The position of moving objects.

Moving objects occupy a range of positions during the period of integration of the visual system. Nevertheless, a unique position is usually observed. We investigate how the trajectory of a stimulus influences the position at which the object is seen. It has been shown before that moving objects are perceived ahead of static objects shown at the same place and time. We show here that this perceived position difference builds up over the first 500 ms of a visible trajectory. Discontinuities in the visual input reduce this buildup when the presentation frequency of a stimulus with a duration of 42 ms falls below 16 Hz. We interpret this relative mislocalization in terms of a spatiotemporal-filtering model. This model fits well with the data, given two assumptions. First, the position signal persists even though the objects are no longer visible and, second, the perceived distance is a 500 ms average of the difference of these position signals.     

Coherence determines speed discrimination

The visual system must determine which elements in a scene to regard as parts of a single object and which to regard as different objects. We can create stimuli that are ambiguous, ie consistent with more than one interpretation, and ask in what situations the stimulus elements are interpreted as part of a single object and when they are interpreted as multiple objects. The ambiguous stimuli in this study were moving plaid patterns--the sum of two drifting gratings with different orientations. Observers may see a rigid coherent plaid object moving in one direction, or may see two gratings moving in different directions sliding over one another. When the gratings have similar contrasts they appear to cohere and only the plaid speed is perceptually available; when the gratings have different contrasts they appear to slide and only the speeds of the gratings are perceived. Coherence thus determines what speed information is passed to higher stages of motion processing. A two-stage model of plaid motion perception is presented which agrees with the model proposed by Adelson and Movshon and extends it, detailing the relationship between coherence and speed discrimination.     

Parts, cavities, and object representation in infancy

Part representation is not only critical to object perception but also plays a key role in a number of basic visual cognition functions, such as figure-ground segregation, allocation of attention, and memory for shapes. Yet, virtually nothing is known about the development of part representation. If parts are fundamental components of object shape representation early in life, then the infant visual system should give priority to parts over other aspects of objects. We tested this hypothesis by examining whether part shapes are more salient than cavity shapes to infants. Five-month-olds were habituated to a stimulus that contained a part and a cavity. In a subsequent novelty preference test, 5-month-olds exhibited a preference for the cavity shape, indicating that part shapes were more salient than cavity shapes during habituation. The differential processing of part versus cavity contours in infancy is consistent with theory and empirical findings in the literature on adult figure-ground perception and indicates that basic aspects of part-based object processing are evident early in life.     

Visual impressions of penetration in the perception of objects in motion

Observers viewed visual stimuli in which one object moved to a position of partial occlusion by another. The objects were presented as two-dimensional profiles moving in an undefined space, so the partial occlusion supports several different physical interpretations. In fact some stimuli reliably gave rise to a perceptual impression that the moving object penetrated or pierced the stationary one. This kind of interaction impression has not previously been reported. The impression was maximized by rapid deceleration to a halt with minimal occlusion. If the object decelerated more slowly, so that it was completely occluded or projected from the far side of the stationary object, it was perceived as moving behind the stationary object. The shape of the moving object and its speed prior to occlusion had significant but small effects.     

Amodal representation of occluded surfaces: role of invisible stimuli in apparent motion correspondence

A series of demonstrations were created where the perceived depth of targets was controlled by stereoscopic disparity. A closer object (a cloud) was made to jump back and forth horizontally, partially occluding a farther object (a full moon). The more distant moon appeared stationary even though the unoccluded portion of it, a crescent, changed position. Reversal of the relative depth of the moon and cloud gave a totally different percept: the crescent appeared to flip back and forth in the front depth plane. Thus, the otherwise-robust apparent motion of the moon crescents was completely abolished in the cloud-closer case alone. This motion-blocking effect is attributed to the 'amodal presence' of the occluded surface continuing behind the occluding surface. To measure the effect of this occluded 'invisible' surface quantitatively, a bistable apparent motion display was used (Ramachandran and Anstis 1983a): two small rectangular-shaped targets changed their positions back and forth between two frames, and the disparity of a large centrally positioned rectangle was varied. When the perceived depths supported the possibility of amodal completion behind the large rectangle, increased vertical motion of the targets was found, suggesting that the amodal presence of the targets behind the occluder had effectively changed the center position of the moving targets for purposes of motion correspondence. Amodal contours are literally 'invisible', yet it is hypothesized that they have a neural representation at sufficiently early stages of visual processing to alter the correspondence solving process for apparent motion.     

The neural basis of haptic object processing.

We review the organization of the neural networks that underlie haptic object processing and compare that organization with the visual system. Haptic object processing is separated into at least two neural pathways, one for geometric properties or shape, and one for material properties, including texture. Like vision, haptic processing pathways are organized into a hierarchy of processing stages, with different stages represented by different brain areas. In addition, the haptic pathway for shape processing may be further subdivided into different streams for action and perception. These streams may be analogous to the action and perception streams of the visual system and represent two points of neural convergence for vision and haptics.     

From objects to names: A cognitive neuroscience approach

To name an object, we need both to recognize it and to access the associated phonological form, and phonological retrieval itself may be constrained by aspects of the visual recognition process. This paper reviews evidence for such constraints, drawing on data from experimental psychology, neuropsychology, functional imaging, and computational modelling. Data on picture identification in normal observers demonstrate that the speed of name retrieval processes differs for natural objects and artifacts, due at least in part to differences in visual similarity between exemplars within these categories. Also, effects of variables on early and late stages of object identification combine in an interactive rather than an additive manner, consistent with object processing stages operating in a continuous rather than a discrete manner. Neuropsychological evidence supports this proposal, demonstrating that subtle perceptual deficits can produce naming problems, even when there is good access to associated semantic knowledge. Functional activation studies further show increased activity in visual processing areas when conditions stress object naming relative to the recognition of familiar object structures. These studies indicate that object naming is based on a series of continuous processing stages and that naming involves increased visual processing relative to recognition tasks. The data can be modelled within an interactive activation and competition framework. Received: 25 November 1997 / Accepted: 16 May 1998     

Distinct mechanisms for the representation of moving and static objects

The visual system has been suggested to integrate different views of an object in motion. We investigated differences in the way moving and static objects are represented by testing for priming effects to previously seen ("known") and novel object views. We showed priming effects for moving objects across image changes (e.g., mirror reversals, changes in size, and changes in polarity) but not over temporal delays. The opposite pattern of results was observed for objects presented statically; that is, static objects were primed over temporal delays but not across image changes. These results suggest that representations for moving objects are: (1) updated continuously across image changes, whereas static object representations generalize only across similar images, and (2) more short-lived than static object representations. These results suggest two distinct representational mechanisms: a static object mechanism rather spatially refined and permanent, possibly suited for visual recognition, and a motion-based object mechanism more temporary and less spatially refined, possibly suited for visual guidance of motor actions.