Cortical activity during painful and non-painful stimulation over four lower limb body sites: a functional near-infrared spectroscopy study

Figures

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  Fig. 1. (a) Schematic illustration of the stimulation at the various body sites. The electrical stimulation was applied at left and right groins and both knees via an anode and cathode at a distance of about one centimeter between them. The blue line represents the anode. The red line represents the cathode. And the lightning bolt represents the electrical stimulation. (b) Representation of one stimulation block and a rest block. Each stimulation modality consisted of 6 stimulation blocks of 5 s, each separated by 6 rest blocks of pseudo-randomised duration (25 to 29 s). The lightning bolt represents the electrical stimulation.

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  Fig. 2. (a) fNIRS optode and channel placement over the bilateral primary somatosensory cortex and prefrontal cortex within the international 10–10 standard system. The red and blue dots represent the location of light sources and detectors, respectively. The yellow squares represent the location of the channels. Sources 03, 04, and 05 represent C3, C4, and Fz, respectively. (b) The co-registration of the 49 channels on a standard human brain template.

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  Fig. 3. Stimulation intensity (mA) of each body site under the two stimulation modalities. Boxplots are used to display the painful stimulus values (represented by pink boxes) and non-painful stimulus values (represented by blue boxes) at each of the four locations. The central line within each box represents the median. The edges of the box correspond to the 25th and 75th percentiles, defining the interquartile range (IQR). Whiskers extend to the most extreme data points within 1.5 times the IQR from the first and third quartiles. Outliers are depicted as circles outside the boxes.

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  Fig. 4. The visual analogue scale (VAS) ratings of each body site under the two stimulation modalities. Boxplots are used to display the VAS ratings under the painful stimulation (represented by pink boxes) and non-painful stimulation (represented by blue boxes) at each of the four locations. The central line within each box represents the median. The edges of the box correspond to the 25th and 75th percentiles, defining the interquartile range (IQR). Whiskers extend to the most extreme data points within 1.5 times the IQR from the first and third quartiles. Outliers are depicted as circles outside the boxes. The (a, b) show the different rating times (a before the stimulation modality; b after the stimulation modality).

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  Fig. 5. Bar plots showing the mean visual analogue scale (VAS) rating across body sites. Error bars represent the standard error of the mean (SE).

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  Fig. 6. (a) Topographic images of group-averaged cortical deactivations and/or activations during painful (left) and non-painful (right) stimulation modalities given on each body site (left groin, right groin, left knee and right knee). (b) Comparisons of the group-averaged β values of different brain regions when stimulating different body sites. The pink bar represents the painful stimuli, and the blue bar represents the non-painful stimuli. Left S1 means left primary somatosensory area, right S1 means right primary somatosensory area, and PFC means prefrontal area. Error bars represent the standard error of the mean. (*p < 0.05, **p < 0.01 and ***p < 0.001).