Emotions are not determined by distinctive stimuli, but by the meaning the stimulus has aquired over time. Recently, Damasio (80) introduced in this context the term "emotionally competent stimulus," referring to the object or event whose presence, actual or in mental recall, triggers emotion. While there are biologically relevant stimuli that are innately pleasurable or aversive, most stimuli will acquire meaning through classical conditioning. As a consequence, meanings of stimuli depend on the individual's past experience, and may differ from one individual to another. Stimuli may have conveyed several meanings, and meanings relevant for different emotions may be present at the same time. Moreover, the value of a stimulus may differ over time since it will be influenced by the current internal state of the organism. Thus, the rewarding value of a stimulus is dependent on the current internal state, and on prior experience with that stimulus.
There is an increasing notion that emotional responses are automatic and precede feelings (80,81). Damasio stresses that all living organisms are born with devices designed to solve automatically, without proper reasoning required, the basic problems of life. He calls this equipment of life governance the "homeostasis machine." At the basis of the organization of that machine are simple responses like approach or withdrawal of the organism relative to some object, and increases or decreases in activity. Higher up in the organization there are competitive or cooperative responses. The simpler reactions are incorporated as components of the more elaborated and complex ones. Emotion is high in the organization, with more complexity of appraisal and response. According to Damasio, an emotion is a complex collection of chemical and neural responses forming a distinctive pattern. When the brain detects an emotionally competent stimulus, the emotional responses are produced automatically. The result of the responses is a temporary change in the state of the body, and in the brain structures that map the body and support thinking. Damasio (80) and LeDoux (81), and a long time before them James (82), stress that the conscious experience of emotion, what we call feelings, is the result of the perception of these changes. In this view, feelings are based on the feedback of the emotional bodily and brain responses to the brain; they are the end result of the whole "machinery of emotion."
Recently, functional imaging studies showed that the subjective experience of various emotions such as anger, disgust, anxiety, and sexual arousal is associated with activation of the insula and the orbitofrontal cortex (83-86). It has been suggested that the insula is involved in the representation of peripheral autonomic and somatic arousal that provides input to conscious awareness of emotional states. It appears that the feedback of autonomic and somatic responses are integrated in a so-called meta-representation in the right anterior insula, and this meta-representation seems to provide the basis for "the subjective image of the material self as a feeling entity, that is emotional awareness" (83).
In men and women alike, meanings of a sexually competent stimulus will automatically generate a genital response, granted the genital response system is intact. The difference between men and women in experienced sexual feelings have to do with the relative contribution of two sources. The first source is the awareness of this automatic genital response (peripheral feedback), which will be a more important source for men's sexual feelings than for women's sexual feelings (87). For women, a stronger contribution to sexual feelings will come from a second source, the meanings generated by the sexual stimulus. In other words, women's sexual feelings will be determined to a greater extent by all kinds of (positive and negative) meanings of the sexual stimulus than by actual genital response.
Canli et al. (88) found support for the idea that emotional stimuli activate explicit memory more readily in women than in men. They asked 12 women and 12 men, during functional MRI, to rate the intensity of their emotional arousal to 96 pictures ranging from neutral to negative. After 3 weeks, they were given an unexpected memory task. It was found that women rated more pictures as highly negatively arousing than did men. The memory task revealed that women had better memory for the most intensely negative pictures. Exposure to the emotional stimuli resulted in left amygdala activation in both sexes, the central brain structure for implicit memory (77). In women only, the left amygdala and right hippocampus were activated during the most emotionally arousing stimuli that were also recognized 3 weeks later. Explicit memory is situated in the neocortex and is mediated by the hippocampus (89). These findings may suggest that in processing emotional stimuli, explicit memory is more readily accessible in women. If these findings would hold for sexual stimuli, we may have a neural basis for our suggestion that sexual stimuli activate explicit memory in women, and that the different meanings sexual stimuli may have, influence sexual feelings.
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