TABLE 1:

Summary of neuroimaging studies of the gut-brain axis

Species	Imaging Modality	Methodology Altering GM	Outcome	Reference
Human	rsfMRI	None – prospective study	GM composition was negatively associated with activity of the cerebellar vermis	[38]
Human	rsfMRI	None – prospective study	GM composition was associated with GI sensorimotor function and FC	[39]
Human	rsfMRI task-based fMRI	Consumption study	Consumption of fermented milk products resulted in altered activity within a periaqueductal, gray-seeded network; decreased BOLD in primary viscerosensory and somatosensory cortices	[5]
Human	rsfMRI	Consumption study	Consumption of probiotics resulted in FC differences in the DMN, salience network, visual network, and middle and superior frontal gyrus network	[40]
Human	rsfMRI	None – prospective study	GM composition and cognitive outcomes in 2-year-olds was associated with FC between the supplementary motor area and the inferior parietal lobule	[41]
Human	rsfMRI	None – prospective study	Patients with IBS were associated with higher FC in the left DMN to left basal ganglia and from the right somatosensory network to the right basal ganglia	[42]
Human	Task-based fMRI	None – prospective study	Prevotella abundance was associated with decreased hippocampal BOLD response to negative images	[43]
Human	Task-based fMRI	Consumption study	Probiotic treatment of IBS patients shown emotional faces showed a reduced engagement of the amygdala and frontal and temporal cortices, and heightened engagement of the occipital regions	[44]
Human	Task-based fMRI, DTI	Consumption study	Probiotic consumption resulted in a subtle GM composition change and in response to tasks, differences in the activity of the cingulum, precuneus, inferior parietal lobule, thalamus, and parahippocampal gyrus were observed; no differences in FA or MD were observed	[45]
Human	Structural brain imaging	None – prospective study	GM-dependent grey matter volume differences in the right superior occipital gyrus were noted and after a year, GM-specific differences were observed in the left and right caudate nucleus	[47]
Human	Structural brain imaging	None – prospective study	GM-specific groups of patients with IBS showed increased volumes in sensory regions and decreased volumes in insula, right globus pallidus, and motor cortex	[48]
Human	DTI	None – prospective study	Decreased GM diversity in patients with obesity and an abundance of Actinobacteria was positively correlated with FA in the amygdala and thalamus	[57]
Human	18F-Florbetapir Amyloid PET	None – prospective study	Increased amyloid uptake in the frontal, anterior cingulate and precuneus cortex was associated with an increase in bacterial byproducts in plasma	[62]
Mouse	Structural brain imaging, DTI	Germ-free mice	GF mice showed regional expansion of the olfactory bulbs and prefrontal cortex; FA was increased in the fimbria, anterior commissure, corpus callosum, optic tract, internal capsule, and the periventricular white matter	[49]
Mouse	DTI	Fecal matter transplant to germ-free mice	Fecal matter transplant from patients with ADHD to GF mice showed decreased FA in the right and left hippocampus and in the right internal capsule and right optic tract, and increased MD in the right hippocampus and fornix	[56]
Mouse	18F-FDG PET-CT	High fat diet consumption	Maternal high-fat diet consumption resulted in brain insulin resistance and reduction of memory and exploratory behavior in offspring with an age-dependent change in bacterial composition	[63]
Rat	DTI	None – prospective study	Diet-dependent FA increases were observed in the left frontal neocortex	[58]
Rhesus macaques	Structural brain imaging	Transition to solid food in developing macaques	Nursing-associated increases in Proteobacteria was negatively correlated with total brain volume, specifically cortical grey matter of both hemispheres	[50]