The Adaptive After-effect Mechanism Of Body Size Illusion

We are living in a social world where at any time we are perceiving the body figures of others and ourselves.With the rapid development of the mass media,people have been receiving visual information from the mass media all the time.Therefore,the mass media should have a significant impact on people's mind,especially when the media follows the aesthetic standards of "thinness is beauty".The preference for lean body shapes has its evolutionary significance.Compared to fat bodies,slim bodies usually have good physical health and suffer less from obesity-related diseases.But there is another question worth thinking within the framework of this aesthetic preference.That is,whether the aesthetic standards of “thinness is beauty” promoted by the media have a negative impact on our lives and health? If there are negative effects,what should we do to get rid of them?Body Size Misperception is one of the potential negative effects.Body size misperception is defined as the phenomenon of over-or under-estimating someone's body size.Misperception of our body figures may result in consequences such as excessive dieting,anxiety,or depression.Body size misperception is very similar to a common phenomenon in vision perception – the Visual Adaptation Aftereffect.Visual adaptation aftereffect usually manifests as perceiving a subsequently presented stimulus with an opposite direction from the attributes of the stimulus people firstly being exposed to.This perception bias is considered to be the result of adaptive adjustment and re-calibration that happen in the visual system according to the current environment.Here,this article focuses on body size misperception and attempts to explain the cause of the body size misperception through the visual phenomenon of visual adaptation aftereffect.Experiment 1 investigated whether participants' perception of their own body shape could be biased after viewing a slim body shape or a fat body shape for a long while.By applying the visual adaptation paradigm,together with EEG recordings,we used admirable body sizes from the social media as adaptors,while a series of morphed body sizes from participants themselves were adopted as test stimuli.The adaptation paradigm includes two phases: the adaptation phase and the testing phase.During the adaptation phase,a slim body or a fat body(that is,the adaptor)synthesized from 2 pictures was presented for 4 s.At this stage,participants were asked to concentrate on the adaptor.Then,during the test phase,a picture of the participant's own body shape flashed(0.2 s)(that is,the test stimulus)on the computer screen.Participants'task was to judge whether their own body shapes displayed on the computer screen were fatter or thinner than their real body size.The results of the experiment 1 exhibited significant perceptual biases induced by visual adaptation.It was found that when exposed to admirable body sizes,participants biased their perception of their body sizes.To be specific,after adapting to the slim body size,participants perceived their real body shapes to be fatter;on the contrary,after adapting to the fat body shape,the participants perceived their real body shapes to be thinner.Experiment 1 also collected the EEG responses.As revealed by the results of Event Related Potentials(ERPs),it was found that the Late Positive Potential(LPP),which peaked around 400-500 ms and distributed in the parietal region of the scalp,was the ERP component that was closely related to the behavioral adaptation aftereffect.Specifically,the EEG results of experiment 1 found that the degree of similarity between the adaptor and the test stimulus significantly modulated the amplitude of LPP.We speculated that the modulation effect on the LPP amplitudes represents the underlying neural mechanism of the generation of body misperception induced by adaptation.On the basis of Experiment 1,experiment 2 explored whether visual crowding could be adopted as an effective way to reduce the misperception observed in experiment 1.The experimental procedure was similar to that of Experiment 1.The only difference was that in order to induce the crowding effect,the adaptor was flanked with 4 pictures of different body sizes.It was found that when those admirable bodies were flanked with other different body sizes(i.e.,crowding),the perception bias which was formed previously disappeared.That is,with the crowding effect,the behavioral body size adaptation aftereffect was weakened.Furthermore,experiment 1 demonstrated that the similarity in body size between the adaptor from the mass media and the test from participants themselves could modulate the amplitudes of the LPP.However,this modulation effect vanished in experiment 2.The absence of both the behavioral aftereffect and the modulation effect in the crowding condition again proved that the modulation effect of adaptor-test similarity on the LPP amplitude might serve as the neural mechanism of the body size biases observed in experiment 1.The purpose of experiment 3 was to exclude the possibility that the results obtained in experiment 1 was attributed to low-level adaptation aftereffects.The adaptors in experiment 3 were pictures of body contours composed of low-level information such as grayscale and contour shapes.Except for the above-mentioned difference,the materials and procedures of experiment 3 were identical to those of experiment 1.However,the results showed that in the condition of slim body adaptation,low-level information could induce significant high-level body size adaptation aftereffect.But in the condition of fat body adaptation,no such cross-level adaptation aftereffect was found.This study,as a whole,investigated the adaptation mechanism of self-body perception bias and found a way to eliminate this bias.The results obtained from the three interrelated experiments,firstly,proved that the perceptional bias induced by visual adaptation aftereffect could account for the generation of body size misperception in the behavioral level.The modulation effect of the similarity between the adapting stimulus and the test stimulus on the LPP amplitude might be the neural mechanism of body size misperception induced by visual adaptation.Secondly,it was found that visual crowding could be used as an effective manner to reduce the generation of adaptation-induced biases of body size perception.These results and speculations might indicate how mass media could try to present models' pictures without bias the audience's perception of their own body shape.For example,try to advocate the beauty of different body shapes as much as possible,or try to present models' pictures against crowded background instead of monotonous backgrounds.Finally,the present study observed the contribution of low-level visual adaptation to high-level body size aftereffects,and this effect is asymmetric in different body sizes processing.That is,the processing of slim body shapes depends more on the low-level visual information of body contour than does the processing of fat body shapes.