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However, previous experiments have not assessed whether or not this stimulus is subject to FTV bias when inverted. It could also be the case that inverted stimuli with explicit shape would not be subject to a facing-the-viewer bias but instead would be seen more often facing away from the viewer. This would be expected if observers prioritise the part of the body located in the lower visual field when making assessments of walker facing direction, given that inversion of the stimulus renders the arms in the lower half of the display and the legs in the upper half.
If FTV bias for upright stimuli is caused primarily by a preference for the lower half of the stimulus, we would expect to observe a facing away bias for inverted stick figures. We designed an experiment to assess whether stick figures, for which explicit shape is provided, are subject to FTV bias or facing away bias when inverted. We compared the FTV bias for stick figure walkers and point-light walkers in upright and inverted configurations.
Blue skies and cloud inversions: BMC volunteers bring walkers together on Kinder
We expected to observe a facing bias for inverted stick figures, either a FTV bias if the shape inversion effect is responsible for reducing the facing bias in point-light displays or a facing away bias if judgements of facing direction in inverted stick figures are influenced by a lower visual field preference. Participants were 40 undergraduate and graduate students at Queen's University, Kingston. Mean age was All had normal or corrected to normal vision.
Each participant gave informed written consent before the study in accordance with the Declaration of Helsinki. We presented walking biological motion figures that represented a bilaterally symmetric average of the gait patterns of 50 men and 50 women based on a Fourier representation as outlined by Troje , All walkers were rendered both as point-light displays and as stick figures. Half of the stimuli presented were upright and half were inverted. These projections represented figures that were either facing away or towards the viewer due to the depth ambiguity of the stimuli.
In addition, the inclusion of the stick figure stimuli doubled the number of trials in our study compared with the original, so we removed these lateral views to reduce the number of trials. The stimuli were presented using Matlab MathWorks Inc.
All stimuli were white on a black background. Example stimuli are shown in Figure 2. Examples of upright stimuli. Next, they were shown an example of a fronto-parallel projected depiction of upright and inverted point-light displays on paper. The experimenter then described the apparatus with which the participant would make their responses. The apparatus is depicted in Figure 3. This consisted of a circular shape drawn onto cardboard, with lines indicating one of six directions: the centre view, three-quarter views to the left and to the right and the mirror flipped versions of these directions about the image plane.
A moveable arrow was attached to the centre of the circle for the purpose of indicating which of the six directions the walker faced in each trial. Participants were instructed not to try to respond with an equal number of facing directions per direction. Participants were asked to fixate on the midpoint of the stimuli. Participants were instructed to maintain use of a chinrest throughout the experiment.
The red arrow was moveable to one of six directions which were drawn on the circle. Each trial consisted of two phases. First, the biological motion stimulus appeared and walked for 4 seconds. Second, there was a blank screen for 4 seconds during which the participant made a response by rotating the arrow on the response apparatus to denote the facing direction. The experiment progressed automatically without breaks, with the experimenter recording the responses of the participants in every trial. The inversion and rendering factors were blocked. The azimuth changed randomly from trial to trial.
The order of the four blocks was counterbalanced across participants using a Latin Square design. Each stimulus was repeated 20 times resulting in a total of trials. The total duration was approximately 45 minutes including instruction and debriefing.
We computed a measure of response accuracy from the proportion of correct identifications of the azimuth angles of walkers, regardless of whether it was perceived as facing towards or away. These responses would indicate facing towards or facing away, respectively. We call this the proportion FTV. Chance response levels would result in a proportion FTV of 0. Observers were highly accurate in all conditions. No interactions or main effects were observed for accuracy. Proportion FTV responses are plotted in Figure 4. Mean proportion FTV for upright figures was 0.
One sample two-tailed t tests compared with the 0. Dotted line indicates point of subjective equivalence between facing towards and away from the viewer. Error bars are standard errors of the means. Upright stimuli were seen FTV at a proportion of 0. We examined whether facing bias was significant for each condition separately using two-tailed one-sample t tests compared with the PSE of 0.
All upright stimulus conditions showed significant FTV biases, and four of the six inverted stimulus conditions produced significant facing away biases. The two conditions that did not show significant bias were the inverted stick figures in the central view and the right three-quarter view. Inspection of individual participant data revealed that many participants consistently observed the upright stimuli as FTV and the inverted stimuli as facing away.
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Individual data are plotted in Figure 5. Individual proportion FTV for upright and inverted stimuli. Dotted line indicates point of subjective equivalence between towards and away. Error bars are standard error of the means. The test revealed a main effect of azimuth, F 1.
Data for the azimuth conditions are plotted in Figure 6. Proportion FTV for each level of the azimuth factor. Here, we have found that both stick figure walkers and point-light walkers produce a significant facing away bias when inverted. We initially predicted that adding explicit shape information using connecting lines between points would produce FTV bias if observers tended to use the legs to infer facing direction or a facing away bias if the lower half of the display was dominant. The fact that we observed facing away bias for almost all inverted stimuli provides strong evidence that facing bias is primarily caused by a convexity bias operating at a low level of visual processing.
We also documented for the first time that there is a small but significant dependency of FTV bias on the viewpoint of the biological motion walkers. However, our data indicated a significant facing away bias in inverted biological motion stimuli which has not been previously identified. My hopes were high, but would I emerge from the cloud to perfect blue skies? All that hard work scrambling up alongside the ghyll in the warm and wet cloud had brought its reward as I stood and watched the cloud curve its way round the route of our popular Vale of Lorton walking holiday.
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Loweswater, Crummock and Buttermere all lie beneath the cloud. As I make my way down from Great Borne the cloud rolls in and Starling Dodd disappears from sight as my visibility is reduced. Time to head over beside the fence to keep me on track. Anyone walking alongside Crummock and Buttermere will be having a grey and wet time of it.
If only they knew! Perhaps my favourite view of the day. Red Pike from Starling Dodd. Look at the right of the picture to see the cloud rolling in over cols between Great Gable, Kirk Fell and Pillar. Ennerdale Side too looses its fight with the inversion as the cloud rolls over like breaking waves. I really like the summit cairns on Starling Dodd.
Time for a spot of lunch before heading back to Great Borne. The orientation of walkers was upright or inverted. Ten observers were asked to identify one approaching or deviating walker as accurately and quickly as possible. Identification of an approaching walker among deviating walkers was quicker than the opposite identification with small deviation 6 deg , but that of a deviating walker among approaching walkers was quicker with 30 and deg deviations for both upright and inverted walkers. Visual search of inverted walkers was less efficient than upright walkers especially with small deviations.
In Experiment 2, we conducted the same experiment using biological motion stimuli 18 point-lights on joints. We found that the reaction time was much longer than computer-graphics walkers with small deviations twice for 6-deg and 1. These results suggest that searching walkers with small deviations requires walker-specific social-cognitive processing in which approaching is more important than deviating, while the search with large deviations is related with ordinary object perception in which deviation properties are salient.
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