Bistability in the perception of motion and stationarity: effects of temporal asymmetry.

Evidence for bistability in the perception of motion and stationarity was obtained for a displaced dot-figure embedded in a background of randomly moving noise dots. In the temporal symmetry condition, the figure was presented for the same duration in its two locations; either figure motion or random motion was perceived, depending on the number of noise dots. In the temporal asymmetry condition, the figure was presented for different durations in its two locations; figure motion, a single, stationary figure in a fixed position, or random noise was perceived, again depending on the number of noise dots. Competition between these percepts was established by an analysis of switching rates and by an experiment demonstrating the presence of hysteresis as noise levels were gradually increased and decreased across the figure-motion and figure-stationarity boundaries. This evidence for bistability in the perception of figure motion and figure stationarity (one or the other, but not both, was perceived for the same stimulus) suggested the presence of strong inhibitory competition between motion- and position-detecting mechanisms.     

An anxiety-induced bias in the perception of a bistable point-light walker

Human sensitivity for social cues is exquisite, as illustrated by the ease with which simplified point-light movements invoke social and emotional responses. Compared to faces, these biological motion stimuli only recently started to be used to explore questions regarding social cognition and anxiety. We presented human point-light walkers that could be perceived as facing towards or facing away from the observer, and tested whether participants with high social anxiety would perceive these bistable stimuli differently, because this type of stimuli has particular relevance for them. The results showed that observers with high social anxiety tended to see walkers as facing away more frequently than those with low social anxiety. This may mean that high socially anxious observers are biased towards the more positive perceptual alternative because they are motivated to protect themselves against threatening social experiences, but we also explore alternative explanations. The findings are in line with the evidence for a positivity bias in perception, also called wishful seeing, but in contrast with the attentional negativity bias often found in social anxiety. We discuss reasons for this divergence and possible limitations of the current study.     

Inertia and memory in ambiguous visual perception

Perceptual multistability during ambiguous visual perception is an important clue to neural dynamics. We examined perceptual switching during ambiguous depth perception using a Necker cube stimulus, and also during binocular rivalry. Analysis of perceptual switching time series using variance–sample size analysis, spectral analysis and time series shuffling shows that switching times behave as a 1/f noise and possess very long range correlations. The long memory feature contrasts sharply with the traditional satiation models of multistability, where the memory is not incorporated, as well as with recently published models of multistability and neural processing, where memory is excluded. On the other hand, the long memory feature favors the concept of “dynamic core” or coalition of neurons, where neurons form transient coalitions. Perceptual switching then corresponds to replacement of one coalition of neurons by another. The inertia and memory measures the stability of a coalition: a strong and stable coalition has to be won over by another similarly strong and stable coalition, resulting in long switching times. The complicated transient dynamics of competing coalitions of neurons may be addressable using a combination of functional imaging, measurement of frequency-tagged magnetoencephalography and frequency-tagged encephalography, simultaneous recordings of groups of neurons in many areas of the brain, and concepts from statistical mechanics and nonlinear dynamics theory.     

The recency effect in the perception of ambiguous figures

After a failure to obtain a recency effect in the perception of ambiguous figures three experiments were carried out to determine the conditions which govern the effect. It was found that the duration of the stimulus which precedes the ambiguous figure (the most recent stimulus) was important. Also of importance was the interval between the next-to-most-recent stimulus and the ambiguous figure. The effect of manipulating these variables was such that sometimes no recency effect was obtained, implying that their state is sometimes critical for the effect. Variations in the duration of the ambiguous stimulus did not cause the recency effect to vary. Recommendations are made for the demonstration of a reliable recency effect.     

Prior context and the perception of lexically ambiguous sentences

The perceptual complexity of lexically ambiguous and unambiguous sentences was compared in three experiments. In Experiment 1, the report of ambiguous words from rapidly presented ambiguous sentences was worse than the report of corresponding unambiguous words from unambiguous sentences. Results of Experiment 2 showed that the effect was not reduced by the presence of prior biasing context within the sentence. Experiment 3 repeated the finding with a sentence meaning classification task. It was concluded that both meanings of a lexically ambiguous sentence must be computed, even when prior context makes one meaning more plausible than the other.     

Working memory biasing of visual perception without awareness

Previous research has demonstrated that the contents of visual working memory can bias visual processing in favor of matching stimuli in the scene. However, the extent to which such top-down, memory-driven biasing of visual perception is contingent on conscious awareness remains unknown. Here we showed that conscious awareness of critical visual cues is dispensable for working memory to bias perceptual selection mechanisms. Using the procedure of continuous flash suppression, we demonstrated that “unseen” visual stimuli during interocular suppression can gain preferential access to awareness if they match the contents of visual working memory. Strikingly, the very same effect occurred even when the visual cue to be held in memory was rendered nonconscious by masking. Control experiments ruled out the alternative accounts of repetition priming and different detection criteria. We conclude that working memory biases of visual perception can operate in the absence of conscious awareness.     

Emotion perception from dynamic and static body expressions in point-light and full-light displays

Research on emotion recognition has been dominated by studies of photographs of facial expressions. A full understanding of emotion perception and its neural substrate will require investigations that employ dynamic displays and means of expression other than the face. Our aims were: (i) to develop a set of dynamic and static whole-body expressions of basic emotions for systematic investigations of clinical populations, and for use in functional-imaging studies; (ii) to assess forced-choice emotion-classification performance with these stimuli relative to the results of previous studies; and (iii) to test the hypotheses that more exaggerated whole-body movements would produce (a) more accurate emotion classification and (b) higher ratings of emotional intensity. Ten actors portrayed 5 emotions (anger, disgust, fear, happiness, and sadness) at 3 levels of exaggeration, with their faces covered. Two identical sets of 150 emotion portrayals (full-light and point-light) were created from the same digital footage, along with corresponding static images of the 'peak' of each emotion portrayal. Recognition tasks confirmed previous findings that basic emotions are readily identifiable from body movements, even when static form information is minimised by use of point-light displays, and that full-light and even point-light displays can convey identifiable emotions, though rather less efficiently than dynamic displays. Recognition success differed for individual emotions, corroborating earlier results about the importance of distinguishing differences in movement characteristics for different emotional expressions. The patterns of misclassifications were in keeping with earlier findings on emotional clustering. Exaggeration of body movement (a) enhanced recognition accuracy, especially for the dynamic point-light displays, but notably not for sadness, and (b) produced higher emotional-intensity ratings, regardless of lighting condition, for movies but to a lesser extent for stills, indicating that intensity judgments of body gestures rely more on movement (or form-from-movement) than static form information.     

Estimation of lifted weight and produced effort through perception of point-light display

It has been shown that human observers can estimate the weight of a box from the observation of a point-light display of a lifting motion. We asked observers to report the weight of the box and the effort produced by five lifters ranging in size and strength to determine if observers can perceive lifter size. In experiment 1, five or six weights from each of five lifters were shown to fourteen observers in a random order. Observers showed less error in estimating the amount of effort each lifter produced than in estimating the actual weight of the box. In experiment 2, the lifters were presented individually to forty observers to remove any effect observing a previous lift might have had on estimating the subsequent lift by a different lifter. The results showed an improvement in estimated weight but not in estimated effort. In experiment 3, the actual size of the lifters was given to thirty-four observers, and the estimations of both weight and effort improved. In experiment 4, observers did not improve when observing practice trials and estimating either only weight or only effort. The results from the four experiments suggest that observers are more sensitive to lifter's effort than to the weight lifted, and that observers tend to use changes in the velocity profile of the lift when making their estimates.     

Bistability in the perception of motion and stationarity: Effects of temporal asymmetry

Evidence for bistability in the perception of motion and stationarity was obtained for a displaced dotfigure embedded in a background of randomly moving noise dots. In the temporal symmetry condition, the figure was presented for the same duration in its two locations; either figure motion or random motion was perceived, depending on the number of noise dots. In the temporal asymmetry condition, the figure was presented for different durations in its two locations; figure motion, a single, stationary figure in a fixed position, or random noise was perceived, again depending on the number of noise dots. Competition between these percepts was established by an analysis of switching rates and by an experiment demonstrating the presence of hysteresis as noise levels were gradually increased and decreased across the figure-motion and figure-stationarity boundaries. This evidence for bistability in the perception of figure motion and figure stationarity (one or the other, but not both, was perceived for the same stimulus) suggested the presence of strong inhibitory competition between motion- and position-detecting mechanisms.     

The perception of emotion from body movement in point-light displays of interpersonal dialogue

We examined whether it is possible to identify the emotional content of behaviour from point-light displays where pairs of actors are engaged in interpersonal communication. These actors displayed a series of emotions, which included sadness, anger, joy, disgust, fear, and romantic love. In experiment 1, subjects viewed brief clips of these point-light displays presented the right way up and upside down. In experiment 2, the importance of the interaction between the two figures in the recognition of emotion was examined. Subjects were shown upright versions of (i) the original pairs (dyads), (ii) a single actor (monad), and (iii) a dyad comprising a single actor and his/her mirror image (reflected dyad). In each experiment, the subjects rated the emotional content of the displays by moving a slider along a horizontal scale. All of the emotions received a rating for every clip. In experiment 1, when the displays were upright, the correct emotions were identified in each case except disgust; but, when the displays were inverted, performance was significantly diminished for some emotions. In experiment 2, the recognition of love and joy was impaired by the absence of the acting partner, and the recognition of sadness, joy, and fear was impaired in the non-veridical (mirror image) displays. These findings both support and extend previous research by showing that biological motion is sufficient for the perception of emotion, although inversion affects performance. Moreover, emotion perception from biological motion can be affected by the veridical or non-veridical social context within the displays.     

Simultaneous perception of both interpretations of ambiguous figures

When viewing ambiguous figures like the classic duck/rabbit, people alternately perceive one interpretation and then the other, but not both interpretations at once. When two identical ambiguous figures appear together, the majority of observers perceive them as identical, and they typically alternate in unison. Just as most observers cannot see a single figure as both a duck and a rabbit, most cannot see one figure in a pair as a duck and the other as a rabbit even though the two figures and their features are spatially distinct. Is this inability to see both interpretations an inherent limitation of the visual system or is it just due to differences in top-down processing? We show that a simple prompt immediately allows people to see both interpretations, to their own surprise.     

The first glimpse determines the perception of an ambiguous figure

The ambiguous rat-man figure was tachistoscopically presented to 36 subjects in successive segments to test the hypothesis that the starting segment would determine the perception of the figure. Starting segments were selected which were expected to produce the perception of a rat, a man, or either a rat or man. The remaining segments came from figures evaluated in a preliminary study. The selected figures differed in drawn bias and tended to be seen as a rat, a man, or either a rat or a man. The three starting segments were combined factorially with the three levels of drawn bias of the remaining segments. The effect of the starting segment was significant; the effect of drawn bias was not. A further experiment showed that presentation of the rat vs. man starting segments by themselves did not produce a reliable difference. The results support a constructive model of form perception in which the stimulus material first presented establishes a hypothesis which is used to interpret the remaining material.     

Spatiotemporal context influences perception of an ambiguous target in visual search

The two-stage model of amodal completion or TSM (Sekuler & Palmer, 1992), and the ambiguity theory (Rauschenberger, Peterson, Mosca, & Bruno, 2004) provide conflicting accounts of the phenomenon of amodal completion in 2-D images. TSM claims that an initial mosaic (2-D) representation gives way to a later amodally completed (3-D) representation. Furthermore, the 2-D representation is accessible only prior to formation of the 3-D representation. On the other hand, the ambiguity theory claims that the 2-D and 3-D representations develop in parallel and that preference for one of the coexisting representations over the other may be subject to the influence of spatiotemporal context provided by other elements in the visual display. Our experiments support the claim that, once formed, both representations coexist, with spatiotemporal context potentially determining which representation is perceived.     

The biasing effects of selection and attrition on estimating the mean

Organizational and validation researchers often work with data that has been subjected to selection on the predictor and attrition on the criterion. These researchers often use the data observed under these conditions to estimate either the predictor or criterion's restricted population means. We show that the restricted means due to direct or indirect selection are a function of the population means plus the selection ratios. Thus, any difference between selected mean groups reflects the population difference plus the selection ratio difference. When there is also attrition on the criterion, the estimation of group differences becomes even more complicated. The effect of selection and attrition induces measurement bias when estimating the restricted population mean of either the predictor or criterion. A sample mean observed under selection and attrition does not estimate either the population mean or the restricted population mean. We propose several procedures under normality that yield unbiased estimates of the mean. The procedures focus on correcting the effects of selection and attrition. Each procedure was evaluated with a Monte Carlo simulation to ascertain its strengths and weaknesses. Given appropriate sample size and conditions, we show that these procedures yield unbiased estimators of the restricted and unrestricted population means for both predictor and criterion. We also show how our findings have implications for replicating selected group differences.     

Textural effects in perceived pointing of ambiguous triangles

Five experiments examined the influence of textural stripes on perceived pointing of ambiguous (equilateral) triangles. Reaction times for discriminating specific directions of pointing were measured for plain triangles and for triangles containing stripes that were either coincident with or perpendicular to one of the three possible directions of pointing. Perceptual interference occurred when the stripes biased a direction other than one specified by the discrimination task. The magnitude of the interference decreased (a) as the environmental salience of the specified direction increased and (b) as the spatial frequency of the stripes increased. Further results indicated that this frequency effect is relative to the size of the triangle; it is not determined by absolute (retinal) frequency. Stripes in the ground region surrounding the triangle also produced interference. However, this interference decreased as the region around the triangle was cleared of stripes. The results are discussed in terms of factors that determine the qualitative and quantitative nature of the interference effect, particularly symmetry and spatial frequency.     

Frequency and recency effects on response biasing in a pseudo-perceptual task

The pseudo-recognition technique provides an ingenious method of evaluating the influence of response biasing variables. This study tests the effects of recency on low and medium frequency words using the pseudo-recognition procedure. It was found that under short recency conditions the thresholds of medium and low frequency words did not differ, while under longer recency conditions the low frequency words had significantly lower thresholds than medium frequency words.