According to the nowadays widely accepted cognitive approach,
the human brain plays a key role to the emotional processing,
since it regards each experienced situation as either pleasant or
unpleasant. Therefore, the current emotional theory is based on
two brain mechanisms, which are the appetitive and the defensive
one [20]. They have evolved in order to assure the physical
survival and are implemented by neural circuits in the brain.
Consequently, emotions could be described by means of affective
valence and arousal. Valence refers to the direction of behavioural
activation according to the motivational system induced by the
stimulus. Arousal represents the activation level elicited by the
emotionally evocative stimulus [21].