The very strong relationship between electrodermal hyporeactivity and suicide and other suicidal behaviors in depressed patients (Edman et al., 1986; Thorell, 1987, 2009; Eriksson et al., 2008; Thorell et al., 2013, 2014; Sarchiapone et al., 2017, 2017 (not published yet); Sarchiapone, 2017 (oral)) indicate a common explanatory factor.
Electrodermal hyporeactivity is the loss of specific orienting responses to insignificant tone stimuli in an electrodermal habituation test that may lead to a failure in creating detailed memory models of neutral stimuli and a loss of normal emotional reactions to it, i.e. a failure in the essential ability of “learning the usual”. The neurons evoking the orienting reactions have been found in rabbit to be located in the CA1 and CA3 neurons of hippocampus (Sokolov et al. 2002).
Also, the depressive suicide are two objectively observable behaviors that may be linked to hippocampal dysfunction. It is known that suicide is related to deviances in hippocampal anatomy and function which in turn have several causes. One important cause seems to be long lasting high steroid levels that affect hippocampus in various ways (see for example review by Dwivedi 2012).
Electrodermal hyporeactivity and the depressive suicide are two objectively observable behaviors. The repeated unanimously findings of the valid and powerful results are much dependent on the objective nature and robustness of these two variables.
Many observations that have been made regarding behavioral and biological concomitants in relation to suicidal behavior, such as biological conditions, cognitive dysfunctions, and potential treatment measures can be theoretically related to orienting hyporeactivity in hippocampus.