Neural network of body representation differs between transsexuals and cissexuals

Abstract

Body image is the internal representation of an individual's own physical appearance. Individuals with gender identity disorder (GID), commonly referred to as transsexuals (TXs), are unable to form a satisfactory body image due to the dissonance between their biological sex and gender identity. We reasoned that changes in the resting-state functional connectivity (rsFC) network would neurologically reflect such experiential incongruence in TXs. Using graph theory-based network analysis, we investigated the regional changes of the degree centrality of the rsFC network. The degree centrality is an index of the functional importance of a node in a neural network. We hypothesized that three key regions of the body representation network, i.e., the primary somatosensory cortex, the superior parietal lobule and the insula, would show a higher degree centrality in TXs. Twenty-three pre-treatment TXs (11 male-to-female and 12 female-to-male TXs) as one psychosocial group and 23 age-matched healthy cissexual control subjects (CISs, 11 males and 12 females) were recruited. Resting-state functional magnetic resonance imaging was performed, and binarized rsFC networks were constructed. The TXs demonstrated a significantly higher degree centrality in the bilateral superior parietal lobule and the primary somatosensory cortex. In addition, the connectivity between the right insula and the bilateral primary somatosensory cortices was negatively correlated with the selfness rating of their desired genders. These data indicate that the key components of body representation manifest in TXs as critical function hubs in the rsFC network. The negative association may imply a coping mechanism that dissociates bodily emotion from body image. The changes in the functional connectome may serve as representational markers for the dysphoric bodily self of TXs.

Figure 1. The between-group difference in the degree centrality of the node of interests (NOIs).

All NOIs showed an increased degree centrality (one-tailed two-sample t-test, TX>CIS) across a range of network densities (1–12%). An asterisk denotes p<0.05.

Figure 2. The between-group difference in the degree centrality of all nodes.

Only the nodes with significantly increased (red) or decreased (blue) degree centrality in the TX group vs. the CIS group are shown (one-tailed two-sample t-test, alpha = 0.05). The network density is 3%.

Figure 3. Connectional pattern of the node of interests (NOIs).

The nodes with stronger connections (i.e., group connectivity >10%) within each of the NOIs are displayed for the TX and the CIS groups. The nodal size and edge color denote the strength of the group connectivity between a node and the NOI. Stronger group connectivity indicates that a larger number of participants shared the same edge in their binary networks.

Figure 4. Regression analysis.

The connectivity (i.e., the weight of the edge and z score) between the right Post and IC as well as the left Post and right IC are plotted against the selfness score that the TX participants rated for the characters with their identified genders. In the TX group, the connectivity showed a significant negative correlation with the selfness score (right PostC: r = −.57, p<.005; left PostC: r = −.60, p<.005).