A Voxel-Based Morphometric Study of Gray Matter in Specific Phobia

Abstract

The objective of this study was to analyze the neurostructural abnormalities of brain areas responsible for the acquisition and maintenance of fear in small animal phobia by comparing gray matter volume (GMV) in individuals with phobia and non-fearful controls. Structural magnetic resonance imaging was obtained from 62 adults (79% female) assigned to one of two groups: 31 were diagnosed with small animal phobia and 31 were non-fearful controls. To investigate structural alterations, a whole-brain voxel-based morphometry analysis was conducted to compare the GMV of the brain areas involved in fear between both groups. The results indicated that individuals with a small animal specific phobia showed smaller GMV in cortical regions, such as the orbitofrontal (OFC) and medial frontal cortex, and greater GMV in the putamen than non-fearful controls. These brain areas are responsible for avoidant behavior (putamen) and emotional regulation processes or inhibitory control (prefrontal cortex (PFC)), which might suggest a greater vulnerability of phobic individuals to acquiring non-adaptive conditioned responses and emotional dysregulation. The findings provide preliminary support for the involvement of structural deficits in OFC and medial frontal cortex in phobia, contributing to clarify the neurobiological substrates for phobias.

The phobic brain: morphometric features correctly classify individuals with small animal phobia

“…When the phobia subgroups were combined and compared to the control group, a significant volume increase was observed in the right OFC (Hilbert et al, 2015). Last but not least, a more recent study compared SAP individuals and healthy controls and found smaller GMV (gray matter volume) (in SAP individuals in the right and left insula, in the orbitofrontal cortex, in the left superior medial frontal, right superior frontal, and right anterior cingulate cortex (Rivero et al, 2023). SAP individuals also displayed larger GMV in the left putamen.…”

“…To our knowledge, this marks the first attempt of applying such a technique in this context. We hypothesize that specific brain structures, previously identified in separate studies as crucial in emotional regulation and the fear response mechanismsuch as the orbitofrontal cortex, amygdala, and basal gangliacan effectively differentiate these two groups (Mourao-Miranda et al, 2012;Rivero et al, 2023;Rondina et al, 2018;Vai et al, 2020). By employing BSVM, we can precisely quantify the contribution of each brain region to our classification models, identifying the most critical areas, assigning a weight to each, and highlighting the neurobiological distinctions between SAP individuals and CTRL ones (control individuals).…”

“…Despite these findings, prior studies investigating the neurological basis of anxiety disorders have faced considerable challenges that limit the depth and applicability of their findings. For example, previous studies frequently relied on limited sample sizes (Rauch et al, 2004;Rivero et al, 2023;Straube et al, 2006;Wright et al, 2003). Some of them exhibited an imbalance in participant gender distribution, with a preponderance of female subjects, leading to a possible bias in the neural findings.…”