The structural and functional connectivity neural underpinnings of body image

Abstract

How we perceive our bodies is fundamental to our self-consciousness and our experience in the world. There are two types of interrelated internal body representations-a subjective experience of the position of a limb in space (body schema) and the subjective experience of the shape and size of the limb (body image). Body schema has been extensively studied, but there is no evidence of the brain structure and network dynamics underpinning body image. Here, we provide the first evidence for the extrastriate body area (EBA), a multisensory brain area, as the structural and functional neural substrate for body shape and size. We performed a multisensory finger-stretch illusion that elongated the index finger. EBA volume and functional connectivity to the posterior parietal cortex are both related to the participants' susceptibility to the illusion. Taken together, these data suggest that EBA structure and connectivity encode body representation and body perception disturbances.

Keywords: body image; body representation; extrastriate cortex; gray matter; magnetic resonance imaging; posterior parietal cortex; visual illusions.

FIGURE 1

Behavioral experiment to determine whether the finger‐stretch illusion can be performed within an MRI scanner with a different egocentric setup. (a) The finger‐stretch illusion is a visuotactile illusion in which participants experience their index finger elongating in a computer‐mediated augmented reality system with congruent sensory feedback from the experimenter. The progression of the illusion is shown from the left panel to the right. As the image of the finger is elongated, the experimenter pulls on the tip to add tactile feedback. (b) The illusion was tested in two experimental setups: (1) proximal (which is the original setup) and (2) distal (n = 12). The distal setup is similar to that which will be performed in the MRI. In the distal setup, the participant is watching a digital image of their finger undergoing the finger‐stretch illusion. The screen is 2 m away from the participant. (c). Mean and SEM ratings of distal (magenta) and proximal (blue) setup are depicted. There were no significant differences between ratings for the proximal and distal setups, indicating that the feeling that the participant's own finger was stretched is similar and equally effective in the distal as the proximal setup (all related‐samples Wilcoxon signed‐rank tests p > .05. See Figure S1 and Table S1)

FIGURE 2

Susceptibility to illusion and neural correlates. (a) Individual participant ratings of the finger stretch illusion and control condition in the MRI scanner. These ratings (between 0 and 5) represent the susceptibility of the participants to the illusions. Mean (±SE) of ratings to the statement “The extent to which you feel that your finger is actually being stretched” (n = 18). Participant ratings were based on 16 trials of each condition and were averaged across all trials for each condition. Susceptibility scores were calculated by subtracting the control ratings from the finger‐stretch illusion ratings. Ratings for the illusion were significantly higher than the control condition (p = .00009; Cohen's d = 1.19671). (b) Bilateral temporo‐occipital gray matter volumes correlate with finger‐stretch illusion susceptibility (the difference between the illusion and control conditions). Statistical images are cluster‐corrected p < .05 family‐wise error, with a cluster‐forming height threshold of p < .001. (c) Significant correlation of the right extrastriate body area gray matter volume with susceptibility ratings (r ² = .74). Note that both EBA were significantly correlated with susceptibility, and the right EBA is shown for simplicity. Blue lines represent 95% confidence intervals. LOC, lateral occipital cortex; R EBA, right extrastriate body area

FIGURE 3

Contrast and conjunction analyses between finger‐stretch and control conditions. (a) Brain activations in response to the finger‐stretch illusion compared to the control illusion (n = 18). (b) Conjunction map showing overlap of regions that show activation in both the finger‐stretch illusion and the control illusion. Statistical images are cluster‐corrected p _FWE < .05 (cluster‐forming height threshold Z > 3.1). EBA, extrastriate body area; FBA, fusiform body area; LOC, lateral occipital cortex; PMv, ventral premotor cortex; PPC, posterior parietal cortex. Images are shown in radiological convention

FIGURE 4

Functional connectivity during illusion with susceptibility scores. (a) Psychophysiological interaction of the right extrastriate body area (EBA; shown in green) during the illusion condition. The EBA was significantly connected to the primary somatosensory cortex/posterior parietal cortex (S1/PPC), the supplementary motor area (SMA) and the ventrolateral prefrontal cortex (vlPFC). The statistical threshold includes a cluster corrected threshold of p < .05 (p < .007, Bonferroni‐corrected for psychophysiological interactions [PPIs] performed) using a cluster forming threshold Z > 3.1. Note that bilateral EBA showed functional connectivity to these regions, but the right is shown for simplicity, and because it is contralateral to the illusion. (b) Positive correlation between right EBA–PPC task‐based functional connectivity during the illusion condition and susceptibility scores (R ² = .32, p < .017; Bonferroni‐corrected p < .05 divided by three tests). Blue lines represent 95% confidence intervals. a.u., arbitrary units. Images are shown in radiological convention