How the visual system detects changes in the direction of moving targets

To determine how the visual system represents information about change in target direction, we studied the detection of such change under conditions of varying stimulus certainty. Target direction was either held constant over trials or was allowed to vary randomly. When target direction was constant the observer could be certain about that stimulus characteristic; randomizing the target direction rendered the observer uncertain. We measured response times (RTs) to changes in target direction following initial trajectories of varying time and distance. In different conditions, the observer was uncertain about either the direction of the initial trajectory, or the direction of change or both. With brief initial trajectories in random directions, uncertainty about initial direction elevated RTs by 50 ms or more. When the initial trajectories were at least 500 ms, this directional uncertainty ceased to affect RTs; then, only uncertainty about the direction of change affected RTs. We discuss the implications of these results for (i) schemes by which the visual system might code directional change; (ii) the visual integration time for directional information; and (iii) adaptational processes in motion perception.     

Cues reduce direction uncertainty and enhance motion detection

Previous work has shown that detectability of motion is better when the observer knows ahead of time the direction of that motion (“certainty”) than when he does not know the direction (“uncertainty”). We now report attempts to reduce this performance decrement associated with direction uncertainty. In these experiments, a briefly flashed, oriented line cued the observer to the direction of motion that might occur. When the cue appeared before the moving target, detectability increased; when the cue appeared after the moving target, performance dropped below that for no cue at all. In addition, we examined the effect of cue reliability, varying the relation between cue orientation and actual direction of target motion. The more accurate the cue is, the larger the performance increment. When the cue indicated a direction more than 90 deg from the actual target direction, performance was worse than when there was no cue. Results are discussed with regard to the feasibility of reducing uncertainty in real-world situations.     

Effects of facial expression and gaze direction on approach–avoidance behaviour

Humans must coordinate approach–avoidance behaviours with the social cues that elicit them, such as facial expressions and gaze direction. We hypothesised that when someone is observed looking in a particular direction with a happy expression, the observer would tend to approach that direction, but that when someone is observed looking in a particular direction with a fearful expression, the observer would tend to avoid that direction. Twenty-eight participants viewed stimulus faces with averted gazes and happy or fearful expressions on a computer screen. Participants were asked to grasp (approach) or withdraw from (avoid) a left- or right-hand button depending on the stimulus face's expression. The results were consistent with our hypotheses about avoidance responses, but not with respect to approach responses. Links between social cues and adaptive behaviour are discussed.     

Effects of facial expression and gaze direction on approach-avoidance behaviour

Humans must coordinate approach-avoidance behaviours with the social cues that elicit them, such as facial expressions and gaze direction. We hypothesised that when someone is observed looking in a particular direction with a happy expression, the observer would tend to approach that direction, but that when someone is observed looking in a particular direction with a fearful expression, the observer would tend to avoid that direction. Twenty-eight participants viewed stimulus faces with averted gazes and happy or fearful expressions on a computer screen. Participants were asked to grasp (approach) or withdraw from (avoid) a left- or right-hand button depending on the stimulus face's expression. The results were consistent with our hypotheses about avoidance responses, but not with respect to approach responses. Links between social cues and adaptive behaviour are discussed.     

Novel-view scene recognition relies on identifying spatial reference directions

Five experiments investigated whether observer locomotion provides specialized information facilitating novel-view scene recognition. Participants detected a position change after briefly viewing a desktop scene when the table stayed stationary or was rotated and when the observer stayed stationary or locomoted. The results showed that 49° novel-view scene recognition was more accurate when the novel view was caused by observer locomotion than when the novel view was caused by table rotation. However such superiority of observer locomotion disappeared when the to-be-tested viewpoint was indicated during the study phase, when the study viewing direction was indicated during the test phase, and when the novel test view was 98°, and was even reversed when the study viewing direction was indicated during the test phase in the table rotation condition but not in the observer locomotion condition. These results suggest scene recognition relies on the identification of the spatial reference directions of the scene and accurately indicating the spatial reference direction can facilitate scene recognition. The facilitative effect of locomotion occurs because the spatial reference direction of the scene is tracked during locomotion and more accurately identified at test.     

Allocation of attention: Uncertainty effects when monitoring one or two visual gratings of noncontiguous spatial frequencies

When an observer is visually presented with a sinusoidal grating, he will often do worse in detecting a given grating when he is uncertain about its spatial frequency than when he is certain. Theoretical explanations of such uncertainty effects assume that the observer has attentional control over multiple spatial-frequency channels. This attentional control can be selectively allocated. If one grating is presented on most of the trials, randomly intermixed with trials of gratings of other spatial frequencies, an experienced observer will use a stationary single-band attention strategy. If two gratings, separated in spatial frequency by four octaves, are randomly presented on most of the intermixed trials, an experienced observer will use a more complex attention strategy; he can monitor the spatial frequencies of the two extreme stimuli with little or no monitoring of intermediate spatial frequencies.     

Accuracy of head orientation perception in triadic situations: experiment in a virtual environment

Research has revealed high accuracy in the perception of gaze in dyadic (sender- receiver) situations. Triadic situations differ from these in that an observer has to report where a sender is looking, not relative to himself. This is more difficult owing to the less favourable position of the observer. The effect of the position of the observer on the accuracy of the identification of the sender's looking direction is relatively unexplored. Here, we investigate this, focusing exclusively on head orientation. We used a virtual environment to ensure good stimulus control. We found a mean angular error close to 5 degrees. A higher observer viewpoint results in more accurate identification. Similarly, a viewpoint with a smaller angle to the sender's midsagittal plane leads to an improvement in identification performance. Also, we found an effect of underestimation of the error in horizontal direction, similar to findings for dyadic situations.     

'Perceiving the present' as a framework for ecological explanations of the misperception of projected angle and angular size

An implicit, underlying assumption of most Helmholtzian/Bayesian approaches to perception is the hypothesis that the scene an observer perceives is the probable source of the proximal stimulus. There is, however, a nontrivial latency (on the order of 100 ms) between the time of a proximal stimulus and the time a visual percept is elicited. It seems plausible that it would be advantageous for an observer to have, at any time t, a percept representative of what is out there at that very time t, not a percept of the recent past. If this is so, it implies a modification to the implicit hypothesis underlying most existing probabilistic approaches to perception: the new hypothesis is that, given the proximal stimulus, the scene an observer perceives is the probable scene present at the time of the percept. That is, the hypothesis is that what an observer perceives is not the probable source of the proximal stimulus, but the probable way the probable source will be when the percept actually occurs. A model of an observer's typical movements in the world is developed, and it is shown that projected angles are perceived in a way consistent with the way the probable source will project to the eye after a small time period of forward movement by the observer. The predicted and actual direction of projected-angle misperception is sometimes toward 90 degrees and sometimes away from 90 degrees, depending on whether the probable source angle is lying in a plane parallel or perpendicular to the probable direction of motion, respectively. The perception of angular size for lines in a figure with cues they are lying in a plane perpendicular to the direction of motion is also shown to fit the predictions of the model.     

Pointing with the eyes: the role of gaze in communicating danger

Facial expression and direction of gaze are two important sources of social information, and what message each conveys may ultimately depend on how the respective information interacts in the eye of the perceiver. Direct gaze signals an interaction with the observer but averted gaze amounts to "pointing with the eyes", and in combination with a fearful facial expression may signal the presence of environmental danger. We used fMRI to examine how gaze direction influences brain processing of facial expression of fear. The combination of fearful faces and averted gazes activated areas related to gaze shifting (STS, IPS) and fear-processing (amygdala, hypothalamus, pallidum). Additional modulation of activation was observed in motion detection areas, in premotor areas and in the somatosensory cortex, bilaterally. Our results indicate that the direction of gaze prompts a process whereby the brain combines the meaning of the facial expression with the information provided by gaze direction, and in the process computes the behavioral implications for the observer.     

Visual and nonvisual information disambiguate surfaces specified by motion parallax.

Motion parallax has been shown to be an effective and unambiguous source of information about the structure of three-dimensional (3-D) surfaces, both when an observer makes lateral movements with respect to a stationary surface and when the surface translates with respect to a stationary observer (Rogers & Graham, 1979). When the same pattern of relative motions among parts of the simulated surface is presented to a stationary observer on an unmoving monitor, the perceived corrugations are unstable with respect to the direction of the peaks and troughs. The lack of ambiguity in the original demonstrations could be due to the presence of (1) non-visual information (proprioceptive and vestibular signals) produced when the observer moves or tracks a moving surface, and/or (2) additional optic flow information available in the whole array. To distinguish between these two possibilities, we measured perceived ambiguity in simulated 3-D surfaces in situations where either nonvisual information or one of four kinds of visual information was present. Both visual and nonvisual information were effective in disambiguating the direction of depth within the simulated surface. Real perspective shape transformations affecting the elements of the display were most effective in disambiguating the display.     

Gaze as depicted in Vermeer's Girl with a Pearl Earring

Ten observers viewed Vermeer's painting Girl With a Pearl Earring and estimated her direction of gaze when her eyes were exposed together and separately. The observers also viewed the painting when her eyes were digitally replaced by those of a real person. The authors found that Vermeer painted the girl's eyes with a precision that was near the limits of visual acuity. Also, Vermeer included 3 gaze illusions, none of which researchers have documented as known in Vermeer's time: (a) when a model whose head is turned to one side gazes at an observer, there is an illusion that she is looking to the side of the observer away from the direction of her head turn; (b) when a model's eyes are viewed separately, they appear to gaze outward relative to each other; and (c) when both of a model's eyes are viewed, the perceived direction of gaze follows that of the nearer eye.     

Visual and nonvisual information disambiguate surfaces specified by motion parallax

Motion parallax has been shown to be an effective and unamhiguous:source of information about the structure of three-dimensional (3-D) surfaces, both when an observer makes lateral movementswith respect to a stationary surface and when the surface translates with respect to a stationary observer (Rogers & Graham, 1979). When the same pattern of relative motions among parts of the simulated surface is presented to a stationary observer on an unmoving monitor, the perceived corrugations are unstable with respect to the direction of the peaks and troughs. The lack of ambiguity in the original demonstrations could be due to the presence of (1) non-visual information (proprioceptive and vestibular signals) produced when the observer moves or tracks a moving surface, andlor (2) additional optic flow information available in the whole array. To distinguish between these two possibilities, we measured perceived ambiguity in simulated 3-D surfaces in situations where either nonvisual information or one of four kinds of visual information was present. Both visual and nonvisual information were effective in disambiguating the direction of depth within the simulated surface. Real perspective shape transformations affecting the elements of the display were most effective in disambiguating the display.     

Temporal and channel uncertainty in detection: A multiple observation analysis

A detection model is generalized to apply to certain multiple observation tasks and data from two auditory experiments are presented which suggest its validity. An observer’s ability to discriminate trials on which tone signals were present during a randomly selected t of n observations, from trials on which no signals were present, was shown to be consistent with the model over a wide range of n and t values. Special emphasis is placed on the performance decrement produced by increasing n relative to t. This “temporal uncertainty” effect is then related to “channel uncertainty” in experiments designed to study attentional processes.     

Emotional expression modulates perceived gaze direction

Gaze perception is an important social skill, as it portrays information about what another person is attending to. Gaze direction has been shown to affect interpretation of emotional expression. Here the authors investigate whether the emotional facial expression has a reciprocal influence on interpretation of gaze direction. In a forced-choice yes-no task, participants were asked to judge whether three faces expressing different emotions (anger, fear, happiness, and neutral) in different viewing angles were looking at them or not. Happy faces were more likely to be judged as looking at the observer than were angry, fearful, or neutral faces. Angry faces were more often judged as looking at the observer than were fearful and neutral expressions. These findings are discussed on the background of approach and avoidance orientation of emotions and of the self-referential positivity bias.     

Unity of perception

Perceptual scientists have recently enjoyed success in constructing mathematical theories for specific perceptual capacities, capacities such as stereovision, auditory localization, and color perception. Analysis of these theories suggests that they all share a common mathematical structure. If this is true, the elucidation of this structure, the study of its properties, the derivation of its consequences, and the empirical testing of its predictions are promising directions for perceptual research. We consider a candidate for the common structure, a candidate called an "observer". Observers, in essence, perform inferences; each observer has a characteristic class of perceptual premises, a characteristic class of perceptual conclusions, and its own functional relationship between these premises and conclusions. If observers indeed capture the structure common to perceptual capacities, then each capacity, regardless of its modality or manner of instantiation, can be described as some observer. In this paper we develop the definition of an observer. We first consider two examples of perceptual capacities: the measurement of visual motion, and the perception of depth from visual motion. In each case, we review a formal theory of the capacity and abstract its structural essence. From this essence we construct the definition of observer. We then exercise the definition in discussions of transduction, perceptual illusions, perceptual uncertainty, regularization theory, the cognitive penetrability of perception, and the theory neutrality of observation.     

Are you looking at me? Measuring the cone of gaze

The processing of gaze cues plays an important role in social interactions, and mutual gaze in particular is relevant for natural as well as video-mediated communications. Mutual gaze occurs when an observer looks at or in the direction of the eyes of another person. The authors chose the metaphor of a cone of gaze to characterize this range of gaze directions that constitutes "looking at" another person. In 4 experiments using either a real person or a virtual head, the authors investigated the influences of observer distance, head orientation, visibility of the eyes, and the presence of a 2nd head on the perceived direction and width of the gaze cone. The direction of the gaze cone was largely affected by all experimental manipulations, whereas its angular width remained comparatively stable.     

A test of Bayesian observer models of processing in the Eriksen flanker task

Two Bayesian observer models were recently proposed to account for data from the Eriksen flanker task, in which flanking items interfere with processing of a central target. One model assumes that interference stems from a perceptual bias to process nearby items as if they are compatible, and the other assumes that the interference is due to spatial uncertainty in the visual system (Yu, Dayan, & Cohen, 2009). Both models were shown to produce one aspect of the empirical data, the below-chance dip in accuracy for fast responses to incongruent trials. However, the models had not been fit to the full set of behavioral data from the flanker task, nor had they been contrasted with other models. The present study demonstrates that neither model can account for the behavioral data as well as a comparison spotlight-diffusion model. Both observer models missed key aspects of the data, challenging the validity of their underlying mechanisms. Analysis of a new hybrid model showed that the shortcomings of the observer models stem from their assumptions about visual processing, not the use of a Bayesian decision process.     

Individualism and the field viewpoint: Cultural influences on memory perspective

Two perspectives from which memories can be retrieved have been distinguished: field resembles the view from the first-person vantage point of original experience, whereas observer resembles the view from the third-person vantage point of a spectator. There is evidence that the incidences of the two types of perspective differ between at least two different cultural groups. It is hypothesised here that this is a special case of a more general relation between memory perspective and cultural individualism, such that field and observer perspectives are more prevalent among people from, respectively, relatively individualist and relatively collectivist societies. Memory perspectives adopted by participants from a range of different countries were recorded, and were found to vary in the predicted manner. Regression analysis showed that the potential effects of three other cultural variables - uncertainty avoidance, masculinity and, to a lesser extent, power distance - were eclipsed by the influence of individualism, and the implications are discussed.     

Imitative learning in Japanese quail (Coturnix japonica) using the bidirectional control procedure

In the bidirectional control procedure, observers are exposed to a conspecific demonstrator responding to a manipulandum in one of two directions (e.g., left vs. right). This procedure controls for socially mediated effects (the mere presence of a conspecific) and stimulus enhancement (attention drawn to a manipulandum by its movement), and it has the added advantage of being symmetrical (the two different responses are similar in topography). Imitative learning is demonstrated when the observers make the response in the direction that they observed it being made. Recently, however, it has been suggested that when such evidence is found with a predominantly olfactory animal, such as the rat, it may result artifactually from odor cues left on one side of the manipulandum by the demonstrator. In the present experiment, we found that Japanese quail, for which odor cues are not likely to play a role, also showed significant correspondence between the direction in which the demonstrator and the observer push a screen to gain access to reward. Furthermore, control quail that observed the screen move, when the movement of the screen was not produced by the demonstrator, did not show similar correspondence between the direction of screen movement observed and that performed by the observer. Thus, with the appropriate control, the bidirectional procedure appears to be useful for studying imitation in avian species.     

Effects of circular motion on judgment of rotation direction and depth order in visual motion

Abstract:  The rotation direction and depth order of a rotating sphere consisting of random dots often reverses while it is viewed under orthographic projection. However, if a short viewing distance is simulated under perspective projection, the correct rotation direction can be perceived. There are two motion cues for the rotation direction and depth order. One is the speed cue; points with higher velocities are closer to the observer. The other is the vertical motion cue; vertical motion is induced when the dots recede from or approach the observer. It was examined whether circular motion, which does not have any depth information but induces vertical velocities, masks the vertical motion cue. In Experiment 1, the effects of circular motion on the judgment of the rotation direction of a rotating sphere were examined. The magnitude of the two cues (the speed cue and the vertical velocity cue) as well as the angular speed of circular motion was varied. It was found that the performance improved as the vertical velocity increased and that the speed cue had slight effects on the judgment of the rotation direction. It was also found that the performance worsened as the angular speed of the circular motion was increased. In Experiment 2, the effects of circular motion on depth judgment of a rotating half sphere were investigated. The performance worsened as the angular speed of the circular motion increased, as in Experiment 1. These results suggest that the visual system cannot compensate perfectly for circular motion for the judgment of the rotation direction and depth order.