Primary Motor Cortex Mediates the Association between Body Mass Index and Reaction Times to Visual Food‐cues in Subjects with Obesity: An fMRI Food‐cue Reactivity Study
Authors: Kahathuduwa Chanaka Nadeeshan , Davis Tyler , O'Boyle Michael , Binks Martin 
Published: November 2023

BackgroundPrevious studies have shown that functional magnetic resonance imaging (fMRI) food‐cue reactivity of the primary motor cortex is associated with body mass index (BMI) and the duration of absolute fasting (FAST). In our previous studies, we showed that BMI and FAST predict faster reaction times to visual food‐cues, suggesting that increased BMI and FAST may be associated with enhanced motor readiness to ingest.ObjectiveTo examine whether associations between BMI and FAST and the reaction times to visual food‐cues are mediated by left and right primary motor areas as quantified by fMRI visual food‐cue reactivity.MethodsWe analyzed the baseline data of the first 24 subjects who participated in a larger clinical fMRI trial. We measured BMI in an initial screening visit and instructed the subjects to fast for 8 hours before they presented for a baseline fMRI scan. fMRI scans were performed while subjects rated 120 images each of food and matched objects on a 1–7 Likert scale according to the question “How appealing is this to you?”. Subjects were given 5s to respond using fiber‐optic buttons manipulated by the right hand. Images were presented randomly in a jittered event‐related design.fMRI data were preprocessed and analyzed using FSL, and the mean BOLD responses for the food>object contrast of the precentral gyrus was extracted using the Featquery tool in FSL. Mediation analysis was performed using the Sobel method for mediation of the associations between fasting and BMI with reaction times to visual food‐cues by primary motor cortex.ResultsBMI predicted faster mean reaction times to food‐cues when controlling for FAST (β= − 15.99ms; SE=4.35; p<0.001). BMI also predicted both right and left precentral gyrus food‐cue reactivity when controlling for FAST (β=0.88; SE=0.06; p<0.001 and β=0.46; SE=0.06; p<0.001 respectively). Right and left precentral gyrus food‐cue reactivity predicted slower reaction times when controlled for BMI and FAST (β=7.67ms; SE=0.95; p<0.001 and β=10.5ms; SE=0.90; p<0.001 respectively). Right and left precentral gyri significantly mediated the association between BMI and reaction times to food‐cues (Z=7.06; p<0.001 and Z=6.24; p<0.001 respectively). The relationship between FAST and reaction time was not significantly mediated by left or right precentral gyrus food‐cue reactivity.ConclusionBilateral precentral gyri appear to mediate the negative association between BMI and reaction times to visual food‐cues. A positive association between precentral gyri, food‐cue reactivity and reaction times suggests that individuals with higher BMI may have higher motor readiness to respond to visual food stimuli, likely via increased efficiency of neural resource allocation in bilateral primary motor cortices. The latter may reflect an increased readiness to ingest. Further research is warranted.Support or Funding InformationFunding: Nestle Health Science.