Body posture aftereffects - Does viewing slouched bodies change people's perception of normal posture?

People increasingly lead sedentary lifestyles and spend extended periods sitting in slouched and head-forward positions, which can lead to health issues. People are so accustomed to seeing slouched posture that they may perceive it as normal and fail to notice their own slouched posture. We aim to investigate this possibility using the visual adaptation paradigm, which has provided insights into the perception of body size and shape in the context of exposure to thin bodies in the media. The experiment was conducted in three phases. First, participants established the posture they perceived normal by manipulating body stimuli shown in profile view. In the second phase, the adaptation phase, participants viewed bodies with extremely upright or slouched postures before establishing their perceived normal posture again in the third phase. Perceived normal posture differed significantly before versus after adaptation, demonstrating a visual aftereffect. However, this only applied if test and adaptation bodies were presented in the same orientation, suggesting that our representation of posture is retina-centred rather than object-centred. This result reduces the likelihood that visual adaptation influences the increase in slouched posture in the population. These results contribute to understanding visual influences on people’s perception of body posture. 

Methods

The study used a 2x2 mixed ANOVA. The between-subjects variable was Adaptation Posture, with two levels: slouched and upright. The within-subjects variable was Congruency between the test and adaptor view, with two levels: congruent (i.e., facing the same way), and incongruent (i.e., facing the opposite direction).

The dependent variable was Change in the Point of Subjective Normality (ΔPSN) for posture. The PSN indicates the posture that a participant perceives as normal, averaged across all test stimuli both before (baseline test) and after (adaptation test) exposure to the adaptation stimuli. High PSNs indicate that upright postures appear normal, while lower PSNs mean more slouched postures are considered normal. Changes in PSN from baseline to adaptation are calculated by subtracting the baseline test score from the adaptation test score. Negative scores indicate that adaptation has caused an aftereffect such that stimuli appear more upright than they really are, and hence, more slouched bodies are perceived as normal. Conversely, positive scores indicate an aftereffect wherein stimuli appear more slouched after adaptation, and hence more upright bodies are perceived as normal.

当下人群的生活方式愈发趋于久坐不动，会长时间维持含胸驼背、头部前伸的坐姿，此举可能引发一系列健康问题。由于大众早已对含胸体态司空见惯，往往会将其视作常态，甚至无法察觉自身的含胸姿态。本研究借助视觉适应范式（visual adaptation paradigm）开展相关探究——该范式此前曾被用于解析媒体中瘦体型曝光对人体尺寸与形态感知的影响机制。

实验共分为三个阶段：第一阶段，参与者通过调整侧视图呈现的人体体态刺激物，确定自身感知为正常的体态；第二阶段为适应阶段，参与者需观看极度挺拔或含胸的人体体态刺激物；第三阶段，参与者再次评定自身感知的正常体态。适应前后，参与者感知的正常体态存在显著差异，这证实了视觉后效的存在。但该效应仅在测试刺激与适应刺激朝向一致时出现，这表明人类对体态的表征是以视网膜为中心，而非以客体为中心。该结果表明，视觉适应不太可能是人群中含胸体态愈发普遍的诱因。本研究结果有助于进一步理解视觉因素对人体体态感知的调控作用。

方法
本研究采用2×2混合方差分析（ANOVA）进行实验设计。组间变量为适应体态，包含含胸与挺拔两个水平；组内变量为测试刺激与适应刺激的视角一致性，分为一致（即朝向相同）与不一致（即朝向相反）两个水平。

因变量为体态主观正常点（Point of Subjective Normality, PSN）的变化量（ΔPSN）。PSN指参与者在适应刺激暴露前（基线测试）与暴露后（适应测试），基于所有测试刺激平均后判定为正常的体态。PSN数值越高，代表越挺拔的体态会被感知为正常；反之PSN数值越低，则意味着越含胸的体态会被视作正常。从基线测试到适应测试的PSN变化量，通过适应测试得分减去基线测试得分计算得到。若变化量为负值，表明适应刺激引发了视觉后效：此时测试刺激看起来比实际更挺拔，因此更含胸的体态会被判定为正常；反之，若变化量为正值，则代表适应后测试刺激看起来更含胸，因此更挺拔的体态会被感知为正常。