had been conducted on topics related to HSPs' high sensitivity. The First International Scientific Conference on High Sensitivity or Sensory Processing Sensitivity was held at the Vrije Universiteit Brussel. By 2015, more than a million copies of The Highly Sensitive Person had been sold. Earlier research Research pre-dating the Arons' coining of the term "high sensitivity" includes that of German medicine professor Wolfgang Klages, who argued in the 1970s that the phenomenon of sensitive and highly sensitive humans is "biologically anchored" and that the "stimulus threshold of the thalamus" is much lower in these persons. As a result, said Klages, there is a higher permeability for incoming signals from afferent nerve fibers so that they pass "unfiltered" to the cerebral cortex. The Arons (1997) recognized psychologist Albert Mehrabian's (1976, 1980, 1991) concept of filtering the "irrelevant", but wrote that the concept implied that the inability of HSPs' (Mehrabian's "low screeners") to filter out what is irrelevant would imply that what is relevant is determined from the perspective of non-HSPs ("high screeners"). Attributes, characteristics and prevalence Boterberg et al. (2016) describe high SPS as a "temperamental or personality trait which is present in some individuals and reflects an increased sensitivity of the central nervous system and a deeper cognitive processing of physical, social and emotional stimuli". People with high SPS report having a heightened response to stimuli such as pain, caffeine, hunger, and loud noises. According to Boterberg et al., these individuals are "believed to be easily overstimulated by external stimuli because they have a lower perceptual threshold and process stimuli cognitively deeper than most other people." This deeper processing may result in increased reaction time as more time is spent responding to cues in the environment, and might also contribute to cautious behavior and low risk-taking. The HSP Scale, initially (1997) a questionnaire designed to measure SPS on a unidimensional scale, was subsequently decomposed into two, three, or four factors or sub-scales. Most components have been associated with traditionally accepted negative psychological outcomes including high stress levels, being easily overwhelmed, increased rates of depression, anxiety, and sleep problems, as well as autistic traits; the diathesis–stress model focused on increased vulnerability to negative influences. However, the differential susceptibility theory (DST) and biological sensitivity to context theory (BSCT) and sensory processing sensitivity (SPS) suggest increased plasticity in terms of responsiveness to both positive and negative influences; and the vantage sensitivity (VS) concept