Emotions may be induced internally though imagination or self-generation, or externally in perceptually driven emotions. Kosslyn and colleagues (1996) reported that the negative emotional content of stimuli increased activity in the occipital cortex both during perception and imagery. Thus, the neural circuits underlying self-generated and perceptually driven emotion may overlap. While self-generated emotions unequivocally involve feelings, perceiving emotion in others may tax cognitive processes rather than generating an emotional experience. Hence, it is important not to equate perception with experience of emotion. Imaging studies of emotional perception have explored the neural networks involved in perceiving facial expressions of primary emotions like fear and disgust presented both consciously and unconsciously, that is, subliminally (Adolphs, 2002). Neuronal activity in the amygdala seems to undergo rapid habituation both in emotional perception and induction studies (for reviews see Whalen, 1998; Davis & Whalen, 2001). It has been argued that the amygdala respond to biologically significant stimuli but predominantly in ambiguous situations (Wahlen, 1998).
Studies of emotional induction have focused mainly on unpleasant emotions, often involving pharmacological probes (e.g., yohimbine, procaine, cholecystokinin tetrape-tide [CCK-4] administration) or sensory stimulation using visual (e.g., films or pictures) or auditory (e.g., scripts or tapes) stimuli. Only a small number of studies have extended into other domains such as the olfactory, gustatory, and somatosensory modalities. In an examination of 25 neuroimaging publications on brain and emotion in healthy individuals, Maddock (1999) noted that the inferior frontal and posterior cingulate cortex, in particular the right retro-splenial cortex, were the regions most frequently activated by emotional conditions.
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