Independent Aftereffects of Fat and Muscle: Implications for neural encoding, body space representation, and body image disturbance

Abstract

Although research addressing body size misperception has focused on socio-cognitive processes, such as internalization of the “ideal” images of bodies in the media, the perceptual basis of this phenomenon remains largely unknown. Further, most studies focus on body size per se even though this depends on both fat and muscle mass – variables that have very different relationships with health. We tested visual adaptation as a mechanism for inducing body fat and muscle mass misperception, and assessed whether these two dimensions of body space are processed independently. Observers manipulated the apparent fat and muscle mass of bodies to make them appear “normal” before and after inspecting images from one of four adaptation conditions (increased fat/decreased fat/increased muscle/decreased muscle). Exposure resulted in a shift in the point of subjective normality in the direction of the adapting images along the relevant (fat or muscle) axis, suggesting that the neural mechanisms involved in body fat and muscle perception are independent. This supports the viability of adaptation as a model of real-world body size misperception, and extends its applicability to clinical manifestations of body image disturbance that entail not only preoccupation with thinness (e.g., anorexia nervosa) but also with muscularity (e.g., muscle dysmorphia).

Experimental manipulation of visual attention affects body size adaptation but not body dissatisfaction

“…Participants who were instructed to direct overt visual attention to low (high) adiposity body stimuli showed a medium‐large visual adaptation effect, such that their PSN became lower (higher) adiposity. This is in line with previous studies showing visual adaptation to low (high) adiposity bodies presented in isolation (Brooks et al, , ; Sturman et al, ; Stephen et al, ). This is also in agreement with a previous study showing that, when participants are presented with low and high adiposity adaptation stimuli simultaneously, and are allowed to view freely, the amount of time that participants spend fixating on low versus high adiposity bodies predicts the magnitude and direction of the adaptation effect (Stephen et al, ).…”

“…It has previously been shown that body size adaptation effects can be induced in both adiposity and muscle dimensions independently (Sturman et al, ). There is increasing awareness that a significant proportion of young men desire a more muscular physique (Mitchison & Mond, ), and it has recently been shown that men at high risk of muscle dysmorphia show an attentional bias toward muscular bodies (Jin et al, ).…”

“…Apparent adiposity of test stimuli was manipulated using Psychomorph (Tiddeman, Burt, & Perrett, ; following Sturman et al, ) to form a series of 13 frames (labeled 0–12), in which frame 0 was reduced by 12 kg of apparent fat mass, increasing in equidistant increments through the original (frame 6) to frame 12, which was increased by 12 kg of apparent fat mass. This is equivalent to a change of ±5 BMI units for a typical 165 cm woman, meaning that the low and high adiposity stimuli had a BMI of approximately 17 (in the underweight range) and 27 (in the overweight range), respectively.…”

“…This is equivalent to a change of ±5 BMI units for a typical 165 cm woman, meaning that the low and high adiposity stimuli had a BMI of approximately 17 (in the underweight range) and 27 (in the overweight range), respectively. Manipulations of this magnitude have previously been successful in producing body size aftereffects (Stephen et al, ; Sturman et al, ). This technique allows apparent adiposity to be varied whilst maintaining identity cues (Figure ).…”

“…In bodies, exposure to narrow (wide) body shapes causes subsequently viewed bodies to appear wider (narrower) than they are. Hence, there is a change in the body width that is perceived to be normal (also known as the point of subjective normality [PSN]) toward the adaptation stimulus (Brooks, Mond, Stevenson, & Stephen, ; Challinor et al, ; Sturman, Stephen, Mond, Stevenson, & Brooks, ). These body size aftereffects have been shown to transfer across identities (Brooks et al, ).…”