Autonomic markers of emotional processing: Skin sympathetic nerve activity during exposure to emotionally-charged images in humans

The sympathetic innervation of the skin primarily subserves thermoregulation, but the system has also been commandeered as a means of expressing emotions. While it is known that the level of skin sympathetic nerve activity (SSNA) is affected by anxiety, the majority of brain imaging studies of emotion have utilised the galvanic skin response (GSR) as a means of inferring increases in SSNA. The purpose of the present study was to characterise the changes in SSNA when showing subjects neutral or emotionally-charged images from the International Affective Picture System (IAPS). SSNA was recorded via tungsten microelectrodes inserted into cutaneous fascicles of the common peroneal nerve in 12 subjects. Neutral images, positively-charged images (erotica) or negatively-charged images (mutilation) were presented on a screen in blocks of 15 images of a specific type, each block lasting 2 minutes. Images of erotica or mutilation were presented in a quasi-random fashion, each 2-min block following a 2-min block of neutral images. Both images of erotica or images of mutilation caused significant increases in SSNA, coupled with sweat release and cutaneous vasoconstriction, but the increases in SSNA were greater for mutilation. Interestingly, viewing images of mutilation had no effect on muscle sympathetic nerve activity (Carter et al., 2008), further emphasising the differential control of sympathetic outflow to muscle and skin. We conclude that SSNA, comprising cutaneous vasoconstrictor and sudomotor activity, increases with both positively-charged and negatively-charged emotional images. Measurement of SSNA provides a more comprehensive assessment of sympathetic outflow to the skin than does the use of sweat release alone as a marker of emotional processing.