Overview of confounds*
| Confounding factor | Effect | Comments/suggestions |
|---|---|---|
| Instructions to participant | Differing activity and FC with different instructions to participant. Particular effect in dmPFC. | Consistent instructions should be given to all participants. Care should be taken with groups that may respond differently to instructions. |
| Time of day | Spatial extent of RSNs generally consistent throughout day (greater variability is seen in some networks, e.g., hippocampal and occipital). Substantial variation in regional usage strength exists, except in midline regions. Global small-world properties of RSNs vary throughout the day. | Care should be taken when scheduling experiment times to reduce time of day effects. |
| Visual noise | Opening or closing eyes changes resting-state measures significantly, including EEG measures, ALFF and resting activity. Rhythmic visual noise may produce spurious patterns in resting-state measures. | Consistent instructions should be given and compliance may be monitored with a simple camera setup. Eyes closed may be the optimal condition. |
| Auditory noise | MRI noise reduces measured spatial extent of the DMN; MRI noise alters activity in the auditory cortex and other regions (including cingulate and insula); MRI noise may interfere with EEG data acquired using simultaneous MRI-EEG. | Noise attenuation should be used. Comparisons between modalities may consider noise differences. Systematic differences among groups is important. |
| Age | Reduced FC in the DMN and increased FC in the somatosensory cortex with age. Long range connectivity reduced with relatively small increases in age. EEG frequency changes throughout life. | Include participant age in analysis. |
| Body weight | Resting EEG measures vary with BMI, including reduced α activity in overweight. MEG Δ and β synchronicity increased in obese participants. Reduced FC in overweight participants. | BMI can be easily recorded and included in the analysis. |
| Sex | Women show higher resting-state measures, including FC, network efficiency, ReHo, in the left hemisphere; men in right. Greater α, β, γ power in women; greater EEG entropy in men. Greater frontal NIRS fluctuations in men. | Sex should be controlled for in resting-state studies. Particular care should be taken to match among groups. |
| Emotional state | Anxiety may alter FC patterns (e.g., increased coupling between insula and thalamus). | Where possible, participants should be familiarized with experimental environment. |
| Sleep | Transition between wakefulness and sleep alters FC and “anti- correlated” networks. Sleep deprivation reduces coherence and FC in DMN; also reduces “anti-correlations.” | Care should be taken with experiment scheduling and participants’ state of wakefulness noted. Sleeping during the experiment may be monitored by camera. |
| Movement | Greater mean movement reduces long-range inter-regional FC, increases short-range FC. Affects ICA and ALFF. Movement may produce neural activity in itself. | Particular care in analysis must be taken when comparing groups with systematic differences in movement. |
| Caffeine | Alters BOLD response; reduces measures of functional connectivity and ALFF; reduces global α power and increases mean α frequency. | Instruct participants not to consume caffeine in hours before experimental session. |
| Nicotine | Acute nicotine consumption alters activity and FC in DMN and other networks; increases EEG α2 power; reduces cerebral glucose metabolism. Craving leads to activations in frontal midline regions and deactivations in posterior ones, with EEG α band effects. Chronic use reduces gCBF and has effects on vasculature. | Smokers should abstain for 2–3 hours preceding experimental session. Smoking status and history should be taken for all participants. |
ALFF = amplitude of low frequency fluctuations; BMI = body mass index; BOLD = blood oxygen–level dependent; DMN = default-mode network; dmPFC = dorsomedial prefrontal cortex; EEG = electroencephalography; FC = functional connectivity; gCBF = global cerebral blood flow; ICA = independent component analysis; MEG = magnetoelectroencephalography; MRI = magnetic resonance imaging; NIRS = near infrared spectroscopy; ReHo = regional homogeneity; RSN = resting-state network.
↵* Confounding factors identified are listed along with their observed effects on resting state experiments and brief comments on how their effects can be minimized. References can be found in the relevant section of the main text.