This study has shown for the first time that viewing blocks of emotionally charged images results in a significant increase in SSNA, measured as total burst count as well as burst amplitude, yet no significant changes in other physiological parameters, such as blood pressure, heart rate, or respiration. Moreover, that there were no significant changes in skin blood flow or sweat release indicates that recording the nerve activity directly provides a more sensitive means of assessing sympathetic outflow to the skin than does measurement of indirect markers of cutaneous sympathetic activity. Overall, similar increases in SSNA were evoked for both positively and negatively charged emotional stimuli, suggesting that increases in SSNA can be evoked by visual emotional stimuli (regardless of valence).
Indirect measurements of the sympathetic nervous system in response to emotion have been widely utilized, with picture viewing, film clips, personalized recall, and threat of shock being just some of the methods used to elicit a wide range of emotions (Lang et al., 1993
; Christie and Friedman, 2004
; Blechert et al., 2006
; Rochman and Diamond, 2008
). While cardiovascular, respiratory, and electrodermal measures can be useful indicators of the state of the sympathetic nervous system, the physiological responses reported in previous studies were variable, and the exclusive use of such measures can be misleading. In addition to the varied results, the comparison of studies can be difficult as different studies often utilized a variety of methodologies and physiological measurements. The IAPS is one method used in an attempt to elicit emotions such as disgust and anticipatory pleasure. Yet this standardized approach can still yield conflicting results: Lang et al. (1993
) found an increased skin conductance response (i.e., an increase in sweat release) when viewing erotic images, while Ritz et al. (2005
) reported little to no change in this same parameter. Furthermore, while viewing mutilation-related images of disgust, Lang et al. (1993
) observed a decrease in heart rate and an increase in skin conductance yet, despite using images of similar valence and arousal scores, Ritz et al. (2005
) found an increase in heart rate and no change in skin conductance. Increased respiratory cycle time while viewing images of disgust has also been reported (Ritz et al., 2005
), yet Sokhadze (2007
) observed no changes in respiration in the same context. Likewise, Christie and Friedman (2004
) and Kunzmann et al. (2005
) reported increased blood pressure in response to film clips of disgust, while Rohrmann and Hopp (2008
) observed no changes in blood pressure while presenting similar film clips.
This variation in effector-organ responses to emotional stimuli that share similar valence and arousal scores emphasizes the need for direct measurements of sympathetic nerve activity. While microneurographic studies have been conducted that have directly recorded sympathetic nerve traffic in response to emotional stimuli, the results are varied and usually involved evoking a cognitive stress (mental arithmetic) rather then an emotional stress (Hallin and Torebjork, 1974
; Callister et al., 1992
; Carter et al., 2005
). However, Carter et al. (2008
) did examine the effects of presenting negatively charges images from the IAPS set on sympathetic outflow to muscle, but found no increases in either sympathetic nerve activity or any other physiological responses. This is, in part, in agreement with the present study: we also found no physiological responses to negatively charged images. In spite of this, there are no reported studies on the effects of viewing any emotionally charged images (negative or positive) on sympathetic outflow to skin
We know that bursts of SSNA directed to hairy skin are composed of cutaneous vasoconstrictor and sudomotor impulses, and that arousal leads to co-activation of these cutaneous vasoconstrictor and sudomotor neurones, as evidenced by the early studies of Delius and colleagues (1972b
) and Hagbarth and colleagues (1972
). These studies also found that arousal stimuli can induce bursts of SSNA that are highly responsive, but with no correlation to blood pressure changes or any coupling to heart rate. These studies, however, used cognitive stress and threat of shock to induce an increase in sympathetic outflow, unlike the present study that used passive viewing of emotional images. Nonetheless, like these studies, the present study found that not only were bursts of SSNA highly responsive to emotional stimuli, but also these responses could be accompanied by complex vasomotor and sudomotor responses that often had a long delay that did not correlate with the bursts of SSNA and were sluggish in nature.