Perceptual representation and effectiveness of local figure–ground cues in natural contours

A contour shape strongly influences the perceptual segregation of a figure from the ground. We investigated the contribution of local contour shape to figure-ground segregation. Although previous studies have reported local contour features that evoke figure-ground perception, they were often image features and not necessarily perceptual features. First, we examined whether contour features, specifically, convexity, closure, and symmetry, underlie the perceptual representation of natural contour shapes. We performed similarity tests between local contours, and examined the contribution of the contour features to the perceptual similarities between the contours. The local contours were sampled from natural contours so that their distribution was uniform in the space composed of the three contour features. This sampling ensured the equal appearance frequency of the factors and a wide variety of contour shapes including those comprised of contradictory factors that induce figure in the opposite directions. This sampling from natural contours is advantageous in order to randomly pickup a variety of contours that satisfy a wide range of cue combinations. Multidimensional scaling analyses showed that the combinations of convexity, closure, and symmetry contribute to perceptual similarity, thus they are perceptual quantities. Second, we examined whether the three features contribute to local figure-ground perception. We performed psychophysical experiments to judge the direction of the figure along the local contours, and examined the contribution of the features to the figure-ground judgment. Multiple linear regression analyses showed that closure was a significant factor, but that convexity and symmetry were not. These results indicate that closure is dominant in the local figure-ground perception with natural contours when the other cues coexist with equal probability including contradictory cases

Perception of Border Ownership by Multiple Gestalt Factors

Gestalt factors such as convexity, closure, and size promote the perception of the direction of figure. Although recent studies have investigated the effects of the individual factors, it has not been clarified how these factors interact with each other, when the features coexist in a local contour. In the present study, we examined psychophysically the interaction of convexity and closure that coexist in a local contour of natural image. First, we established the quantification of convexity and closure (indices to represent the factors) for contours of natural images. By using these quantities, we selected a set of local contours that includes a uniform and wide variation of convexity and closure. Second, we performed a psychophysical experiment to measure the apparent direction of figure for the set of local contours. The result showed that the perceived direction of figure tends to agree with the direction of closure when both convexity and closure coexist in a local contour. The result also showed that while the direction of closure dominates the perception of the direction of figure, the index of closure does not correlate linearly with the perceptual judgment. These results suggest that the perceptual judgment of the direction of figure depends mostly on closure when it coexists with convexity, and that the dependence is dichotomic; two contours with different degrees of closure do not evoke distinct perception. Closure appears to be a dominant and strong cue for the direction of figure

Analysis of Facial Expressions to Estimate the Level of Engagement in Online Lectures

The present study aimed to develop a method for estimating students’ attentional state from facial expressions during online lectures. We estimated the level of attention while students watched a video lecture by measuring reaction time (RT) to detect a target sound that was irrelevant to the lecture. We assumed that RT to such a stimulus would be longer when participants were focusing on the lecture compared with when they were not. We sought to estimate how much learners focus on a lecture using RT measurement. In the experiment, the learner’s face was recorded by a video camera while watching a video lecture. Facial features were analyzed to predict RT to a task-irrelevant stimulus, which was assumed to be an index of the level of attention. We applied a machine learning method, light Gradient Boosting Machine (LightGBM), to estimate RTs from facial features extracted as action units (AUs) corresponding to facial muscle movements by an open-source software (OpenFace). The model obtained using LightGBM indicated that RTs to the irrelevant stimuli can be estimated from AUs, suggesting that facial expressions are useful for predicting attentional states while watching lectures. We re-analyzed the data while excluding RT data with sleepy faces of the students to test whether decreased general arousal caused by sleepiness was a significant factor in the RT lengthening observed in the experiment. The results were similar regardless of the inclusion of RTs with sleepy faces, indicating that facial expression can be used to predict learners’ level of attention to video lectures

Differential relationship between decreased muscle oxygenation and blood pressure recovery during supraventricular and ventricular tachycardia

Abstract Vasoconstriction during tachyarrhythmia contributes to maintenance of arterial pressure (AP) by decreasing peripheral blood flow. This cross-sectional observational study aimed to ascertain whether the relationship between peripheral blood flow and AP recovery occurs during both paroxysmal supraventricular (PSVT, n = 19) and ventricular tachycardias (VT, n = 17). Peripheral blood flow was evaluated using forearm tissue oxygen index (TOI), and mean AP (MAP) was measured using a catheter inserted in the brachial or femoral artery during an electrophysiological study. PSVT and VT rapidly decreased MAP with a comparable heart rate (P = 0.194). MAP recovered to the baseline level at 40 s from PSVT onset, but not VT. The forearm TOI decreased during both tachyarrhythmias (P ≤ 0.029). The TOI response was correlated with MAPrecovery (i.e., MAP recovery from the initial rapid decrease) at 20–60 s from PSVT onset (r = -– 0.652 to – 0.814, P ≤ 0.0298); however, this association was not observed during VT. These findings persisted even after excluding patients who had taken vasoactive drugs. Thus, restricting peripheral blood flow was associated with MAP recovery during PSVT, but not VT. This indicates that AP recovery depends on the type of tachyarrhythmia: different cardiac output and/or vasoconstriction ability during tachyarrhythmia