Quantifying the interactions between allo- and egocentric representations of space

Under many circumstances, humans do not judge the location of objects in space where they really are. For instance, when a background is added to a target object, the judged position of a target with respect to oneself (egocentric position) is shifted in the opposite direction as the placement of such a background with respect to the body midline. It is an ongoing debate whether such effects are due to a uni- or bi-directional interaction between allo- and egocentric spatial representations in the brain, or reflect a response strategy, known as the perceived midline shift. In this study, the effects of allocentric stimulus coordinates on perceived egocentric position were examined more precisely and in a quantitative manner. Furthermore, it was investigated whether the judged allocentric position (with respect to a background) is also influenced by the egocentric position in space of that object. Allo- and egocentric coordinates were varied independently. Also, the effect of background luminance on the observed interactions between spatial coordinates was determined. Since background luminance had an effect on the size of the interaction between allocentric stimulus coordinates and egocentric judgments, and no reverse interaction was found, it seems that interactions between ego- and allocentric reference frames is most likely only unidirectional, with the latter affecting the former. This interaction effect was described in a quantitative manner.     

Transcranial direct current stimulation of the right temporoparietal junction impairs third-person perspective taking

Given the current debates about the precise functional role of the right temporoparietal junction (rTPJ) in egocentric and exocentric perspective taking, in the present study we manipulated activity in the rTPJ to investigate the effects on a spatial perspective-taking task. Participants engaged in a mental body transformation task, requiring them to mentally rotate their own body to the position of an avatar, while undergoing anodal, cathodal, or sham transcranial direct current stimulation (tDCS) of the rTPJ. As a control task, participants judged the laterality of a stimulus feature with respect to a fixation cross on the screen. For the first half of the experiment (only during online tDCS), a task-selective effect of tDCS was observed, reflected in slower reaction times following anodal than following cathodal and sham tDCS for the mental body transformation task, but not for the control task. The effects of tDCS were most pronounced for stimuli implying a more difficult mental body transformation. No effects of tDCS were observed during the second half of the experiment. The effects of tDCS were most pronounced for participants scoring low on aberrant perceptual beliefs and spiritual transcendence, suggesting a relation between third-person perspective taking and bodily and perceptual experiences. The finding that anodal stimulation of the rTPJ impairs third-person perspective taking indicates a key role of this region in exocentric spatial processing.     

Eye position affects audio-visual fusion in darkness

We investigated the frame of reference involved in audio-visual (AV) fusion over space. This multisensory phenomenon refers to the perception of unity resulting from visual and auditory stimuli despite their potential spatial disparity. The extent of this illusion depends on the eccentricity in azimuth of the bimodal stimulus (Godfroy et al, 2003 Perception 32 1233-1245). In a previous study, conducted in a luminous environment, Roumes et al 2004 (Perception 33 Supplement, 142) have shown that variation of AV fusion is gaze-dependent. Here we examine the contribution of ego- or allocentric visual cues by conducting the experiment in total darkness. Auditory and visual stimuli were displayed in synchrony with various spatial disparities. Subjects had to judge their unity ('fusion' or 'no fusion'). Results showed that AV fusion in darkness remains gaze-dependent despite the lack of any allocentric cues and confirmed the hypothesis that the reference frame of the bimodal space is neither head-centred nor eye-centred.     

Judgments of exocentric direction in large-scale space

Judgments of exocentric direction are quite common, especially when judging where others are looking or pointing. To investigate these judgments in large-scale space, observers were shown two targets in a large open field and were asked to judge the exocentric direction specified by the targets. The targets ranged in egocentric distance from 5 to 20 m with target-to-target angular separations of 45 degrees, 90 degrees, and 135 degrees. Observers judged exocentric direction using two methods: (i) by judging which point on a distant fence appeared collinear with the two targets, and (ii) by orienting their body in a direction parallel with the perceived line segment. In the collinearity task, observers had to imagine the line connecting the targets and then extrapolate this imagined line out to the fence. Observers indicated the perceived point of collinearity on a handheld 360 degrees panoramic cylinder representing their vista. The two judgment methods gave similar results except for a constant bias associated with the body-pointing response. Aside from this bias, the results of these two methods agree with other existing research indicating an effect of relative egocentric distance to the targets on judgment error--line segments are perceived as being rotated in depth. Additionally, verbal estimates of egocentric and exocentric distance suggest that perceived distance is not the cause for the systematic errors in judging exocentric direction.     

The underestimation of egocentric distance: evidence from frontal matching tasks

There is controversy over the existence, nature, and cause of error in egocentric distance judgments. One proposal is that the systematic biases often found in explicit judgments of egocentric distance along the ground may be related to recently observed biases in the perceived declination of gaze (Durgin & Li, Attention, Perception, & Psychophysics, in press), To measure perceived egocentric distance nonverbally, observers in a field were asked to position themselves so that their distance from one of two experimenters was equal to the frontal distance between the experimenters. Observers placed themselves too far away, consistent with egocentric distance underestimation. A similar experiment was conducted with vertical frontal extents. Both experiments were replicated in panoramic virtual reality. Perceived egocentric distance was quantitatively consistent with angular bias in perceived gaze declination (1.5 gain). Finally, an exocentric distance-matching task was contrasted with a variant of the egocentric matching task. The egocentric matching data approximate a constant compression of perceived egocentric distance with a power function exponent of nearly 1; exocentric matches had an exponent of about 0.67. The divergent pattern between egocentric and exocentric matches suggests that they depend on different visual cues.     

Attenuation of alignment effect with exocentric encoding of location

An object's location is best retrieved from the orientation in which it was learned. Otherwise, retrieval necessitates a mental effort to restore the original perspective. In this case there is a cost to speed and accuracy of location responses known as the alignment effect. We hypothesised that one can attenuate this alignment effect by systematically referring objects in an exocentric frame of reference during learning. Sixteen male students were asked to learn the location of five objects disposed in a totally new environment either by locating the objects in an egocentric or in an exocentric spatial frame of reference. After the learning phase, the participants were asked to imagine orienting themselves to an object in the scene and to point to another object. The analysis of pointing accuracy, orientation, and pointing times showed that the performances of participants engaged in the exocentric condition remained insensitive to the augmentation of the angle between their actual position on the path and the imagined orientation. On the other hand, the participants engaged in egocentric learning were disoriented when the difference between their actual orientation and the imagined orientation was great. We conclude that when an object's location is intentionally referred to in an exocentric reference frame, alignment effect can be significantly reduced.     

Do reference surfaces influence exocentric pointing?

All elements of the visual field are known to influence the perception of the egocentric distances of objects. Not only the ground surface of a scene, but also the surface at the back or other objects in the scene can affect an observer's egocentric distance estimation of an object. We tested whether this is also true for exocentric direction estimations. We used an exocentric pointing task to test whether the presence of poster-boards in the visual scene would influence the perception of the exocentric direction between two test-objects. In this task the observer has to direct a pointer, with a remote control, to a target. We placed the poster-boards at various positions in the visual field to test whether these boards would affect the settings of the observer. We found that they only affected the settings when they directly served as a reference for orienting the pointer to the target.     

Spatial working memory activity of the caudate nucleus is sensitive to frame of reference

We used event-related fMRI to test the hypothesis that the caudate nucleus is preferentially recruited by a spatial working memory task employing egocentrically defined stimuli, which are amenable to transformation into a motor code, as contrasted with allocentrically defined stimuli, which are not. Our results revealed greater delay-epoch activity in egocentric than in allocentric trials in the caudate nucleus and trends in the same direction in the putamen and the lateral premotor cortex (PMC). Response-related activity was greater for egocentric trials in the lateral PMC. We propose that the neostriatum, possibly interacting with the PMC, may contribute to the sensory-motor transformation necessary to establish a prospective motor code (e.g., the representation of a saccade or a grasp). In addition, the PMC may participate in decision-making processes, prompted by the onset of the probe stimulus, that employ this prospective motor information. This model accounts for the empirical evidence that motor distraction disrupts spatial working memory performance.     

Vernier acuity with opposite-contrast stimuli

Vernier acuity has usually been tested with stimuli of the same contrast polarity (SC). This traditional vernier acuity was compared to that obtained with stimuli of opposite-contrast (OC) in which one target was brighter than the background and the other was darker. For both bar and dot targets vernier acuity with OC stimuli was about half as good as with SC stimuli. There were large individual differences in the size of the disadvantage with OC stimuli, although thresholds remained within the hyperacuity range. There were also individually-differing biases to see a dark vernier stimulus on one or the other side of a bright stimulus. Differences between OC and SC vernier acuities persisted over a wide range of interstimulus spacings, widths, and contrasts. At extremes of these spatial manipulations acuities became similar, but only because SC acuities were degraded to the level of OC acuities. Subjects showed little improvement in OC vernier acuity, even after 50,000 trials. It is concluded that finest judgements of spatial position arise in a level of the visual system at which light and dark stimuli are treated independently.     

Misperception of exocentric directions in auditory space

Previous studies have demonstrated large errors (over 30 degrees ) in visually perceived exocentric directions (the direction between two objects that are both displaced from the observer's location; e.g., Philbeck et al. [Philbeck, J. W., Sargent, J., Arthur, J. C., & Dopkins, S. (2008). Large manual pointing errors, but accurate verbal reports, for indications of target azimuth. Perception, 37, 511-534]). Here, we investigated whether a similar pattern occurs in auditory space. Blindfolded participants either attempted to aim a pointer at auditory targets (an exocentric task) or gave a verbal estimate of the egocentric target azimuth. Targets were located at 20-160 degrees azimuth in the right hemispace. For comparison, we also collected pointing and verbal judgments for visual targets. We found that exocentric pointing responses exhibited sizeable undershooting errors, for both auditory and visual targets, that tended to become more strongly negative as azimuth increased (up to -19 degrees for visual targets at 160 degrees ). Verbal estimates of the auditory and visual target azimuths, however, showed a dramatically different pattern, with relatively small overestimations of azimuths in the rear hemispace. At least some of the differences between verbal and pointing responses appear to be due to the frames of reference underlying the responses; when participants used the pointer to reproduce the egocentric target azimuth rather than the exocentric target direction relative to the pointer, the pattern of pointing errors more closely resembled that seen in verbal reports. These results show that there are similar distortions in perceiving exocentric directions in visual and auditory space.     

Spatial judgments of exocentric extents in an open-field situation: familiar versus unfamiliar size.

Carlson and Tassone (1971) found that an object of familiar size presented in an outdoor setting and viewed at an appreciable distance is judged to be more distant than an unfamiliar object. Three experiments examined whether object familiarity also affects spatial judgments of exocentric extents presented under conditions comparable to those of Carlson and Tassone's experiments. The markers of the extents were either familiar or unfamiliar objects. In Experiment 1, subjects reproduced the perceived extents of depth intervals by adjusting a comparison egocentric extent, and in Experiment 2, subjects also compared the relative depths of two equally long extents. In Experiment 3, the two equally long extents were presented in the subjects' frontoparallel plane, and the subjects reported which of these two extents appeared longer and farther away. The results of these experiments indicate that familiar size does not affect the perceived depths or lengths of exocentric extents viewed under naturalistic conditions.     

Induced motion: isolation and dissociation of egocentric and vection-entrained components.

Induced motion (IM) is illusory motion of a stationary test target opposite to the direction of the real motion of the inducing stimulus. We define egocentric IM as an apparent motion of the test target relative to the observer, and vection-entrained IM as an apparent motion of a stationary object along with an apparent motion of the self (vection) induced by the same stimulus. These two forms of IM are often confounded, and tests for distinguishing between them have not been devised. We have devised such tests. Our test for egocentric IM relies on evidence that this form of IM is due mainly to a misregistration of eye movements when optokinetic nystagmus (OKN) is inhibited, and on evidence that OKN is evoked only by stimuli in the plane of convergence. Our test for vection-entrained IM relies on evidence that vection is evoked only by the more distant of two superimposed inducing stimuli. Thus we found egocentric IM to be induced without vection or vection-entrained IM when subjects converged on a foreground moving display with a stationary display in the background, and vection-entrained IM to be induced without egocentric IM when subjects converged on a stationary-foreground display with a moving display in the background. The two types of IM were evoked in opposite directions at the same time when subjects converged on a foreground moving display while a background display moved in the opposite direction. The two forms of IM showed no signs of interaction, and we conclude that they rely on independent motion mechanisms that operate within distinct frames of reference. A control experiment suggested that the depth adjacency effect in IM is determined by the depth adjacency of the inducing stimulus to convergence, not just to the test target.     

Perceived path of oblique motion: horizontal-vertical and stimulus-orientation effects

Subjects adjusted the path of moving stimuli to produce apparent slopes of 45 degrees with respect to horizontal. The stimulus was either a single moving dot or a vertical or horizontal bar. In separate experiments either the stimuli were tracked or fixation was maintained on a stationary fixation target positioned 8 deg to the right of the center of stimulus motion. In both experiments the selected path slopes were in general more horizontal than 45 degrees. This pattern indicates that subjects overestimate the vertical component of motion along an oblique path, and is interpreted as a manifestation of the spatial anisometropy generally termed the 'horizontal-vertical illusion'. Additionally, paths selected for horizontal bars were more vertical than those for vertical bars. This finding is interpreted in the context of a previous report of the influence of stimulus orientation on perceived velocity.     

Spatial judgments of exocentric extents in an open-field situation: Familiar versus unfamiliar size

Carlson and Tassone (1971) found that an object of familiar size presented in an outdoor setting and viewed at an appreciable distance is judged to be more distant than an unfamiliar object. Three experiments examined whether object familiarity also affects spatial judgments of exocentric extents presented under conditions comparable to those of Carlson and Tassone’s experiments. The markers of the extents were either familiar or unfamiliar objects. In Experiment 1, subjects reproduced the perceived extents of depth intervals by adjusting a comparison egocentric extent, and in Experiment 2, subjects also compared the relative depths of two equally long extents. In Experiment 3, the two equally long extents were presented in the subjects’ frontoparallel plane, and the subjects reported which of these two extents appeared longer and farther away. The results of these experiments indicate that familiar size does not affect the perceived depths or lengths of exocentric extents viewed under naturalistic conditions.     

听障和听力正常人群空间主导性和空间参照框架的交互作用

通过要求被试分别在近处空间和远处空间完成空间参照框架的判断任务, 考察了听障和听力正常人群空间主导性和空间参照框架的交互作用。结果表明：(1)相对于听力正常人群, 听障人群完成自我参照框架判断任务的反应时更长, 而在完成环境参照框架判断任务无显著差异; (2)听障人群和听力正常人群空间主导性和空间参照框架交互作用呈现出相反模式。研究表明, 听障人群在听力功能受损后, 其空间主导性和空间参照框架的交互作用也产生了变化。     

Is ‘object-centred neglect’ a homogeneous entity?

The nature of object-centred (allocentric) neglect and the possibility of dissociating it from egocentric (subject-centred) forms of neglect are controversial. Originally, allocentric neglect was described by  and  in patients who reproduced all the elements of a multi-object scene, but left unfinished the left side of one or more of them. More recently, however, Karnath, Mandler, and Clavagnier (2011) have claimed that the severity of allocentric neglect worsens when a complex ‘object’ shifts from an ipsilesional to a contralesional egocentric position. On the basis of these and of other clinical data, showing that allocentric and egocentric neglect are strongly associated, they have questioned the possibility of dissociating these two forms of neglect, suggesting that egocentric and allocentric neglect constitute different manifestations of the same disturbed system. Since these statements were inconsistent with the clinical findings which had prompted the construct of object-centred neglect, we checked in a group of right brain-damaged patients, who had copied the original multi-object scene, if the degree of neglect for the left side of figures varied as a function of their position on the horizontal axis. Furthermore, we reviewed all papers where copies of other multi-object scenes had been reported. Results of both studies failed to confirm the assumption of a relationship between spatial location of the stimulus and severity of object-centred neglect. This discrepancy between our data and those obtained by Karnath et al. (2011) could be due to the characteristics of stimuli and of procedures used to evaluate ‘object-centred’ neglect. If the stimulus is complex and the task requires its thorough exploration, the spatial location of the stimulus will influence the severity of ‘object-centred neglect’. If, on the contrary, the stimulus is simple and can be identified with few eye fixations, the spatial location of the stimulus should not influence the severity of ‘object-centred neglect’. In any case, our data confirm the possibility of dissociating allocentric from egocentric neglect.     

Reference frames for orientation anisotropies in face recognition and biological-motion perception

Both face recognition and biological-motion perception are strongly orientation-dependent. Recognition performance decreases if the stimuli are rotated with respect to their normal upright orientation. Here, the question whether this effect operates in egocentric coordinates or in environmental coordinates is examined. In addition to the use of rotated stimuli the observers were also rotated and tested both with a same-different face-recognition task and with a biological-motion detection task. A strong orientation effect was found that depended only on the stimulus orientation relative to the observer. This result clearly indicates that orientation effects in both stimulus domains operate in an egocentric frame of reference. This finding is discussed in terms of the particular requirement of extracting sophisticated information for social recognition and communication from faces and biological motion.     

The role of stimulus predictability in the allocation of attentional resources: an eye-tracking study

By allocating less attention to predictable events we are able to focus on novel, unpredictable and unexpected events that require more extensive processing. This strategy should result in improved performance by optimizing the use of brain’s limited resources. Participants’ task was to look at two types of stimuli presented simultaneously at the opposite sides of a computer screen: “static” stimuli, i.e. emotionally neutral photographs; and “dynamic” stimuli, i.e. video clips presenting a moving dot. The dot moved along a predictable, semi-predictable or random trajectory. This was followed by a memory test of the static stimuli. Participants spent more time looking at the dynamic stimuli when its trajectory was less predictable. Additionally, participants who readily adjusted their dwell time allocation to the dot trajectory performed better in the memory test, as demonstrated by a positive correlation between memory test sensitivity and the rate of eye movement patterns adjustment to stimulus predictability. This suggests that people adjust gaze duration to stimulus predictability and that doing so optimizes attentional resource allocation and improves performance. However, study design did not allow to distinguish between spatial and temporal predictability, so it is impossible to estimate the impact of each type of predictability specifically.     

Effect of viewing plane on perceived distances in real and virtual environments

Three experiments examined perceived absolute distance in a head-mounted display virtual environment (HMD-VE) and a matched real-world environment, as a function of the type and orientation of the distance viewed. In Experiment 1, participants turned and walked, without vision, a distance to match the viewed interval for both egocentric (viewer-to-target) and exocentric (target-to-target) extents. Egocentric distances were underestimated in the HMD-VE while exocentric distances were estimated similarly across environments. Since egocentric distances were displayed in the depth plane and exocentric distances in the frontal plane, the pattern of results could have been related to the orientation of the distance or to the type of distance. Experiments 2 and 3 tested these alternatives. Participants estimated exocentric distances presented along the depth or frontal plane either by turning and walking (Experiment 2) or by turning and throwing a beanbag to indicate the perceived extent (Experiment 3). For both Experiments 2 and 3, depth intervals were underestimated in the HMD-VE compared to the real world. However, frontal intervals were estimated similarly across environments. The findings suggest anisotropy in HMD-VE distance perception such that distance underestimation in the HMD-VE generalizes to intervals in the depth plane, but not to intervals in the frontal plane. (PsycINFO Database Record (c) 2012 APA, all rights reserved).     

Role of familiar size in spatial judgments under natural viewing conditions.

The purpose of the present experiment was to examine the role of familiar size in judgments of size and egocentric distance under natural (non-reduced) viewing conditions. Independent groups (ns = 14) judged normal and large off-size chairs and control objects presented at a distance of 25.6 m. Subjects also judged the size of unfamiliar objects (adjacent stimuli) attached to the chairs and stakes. The results indicate that the effects of familiar size are specific to spatial judgments of the familiar object itself since judgments of the adjacent stimuli were unaffected by familiar size.