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In the 1950s, two neuroscientists, James Olds and Peter Milner, were curious about the behavioral response of rats to electric stimulation of certain brain areas. They implanted the electrode into the nucleus accumbens (NAc) located deep in the brain (Berridge and Kringelbach, 2015, Figure 5). The rat was then placed in a box which contains a device that can automatically deliver current to the electrode whenever the rat walks to a corner of the box. If the electric stimulation makes the rat feels uncomfortable, it will avoid this corner. To their surprise, the rat came back soon after leaving the corner.
In subsequent experiments, Olds and Milner let rats self-stimulate NAc simply by pressing a lever. Amazingly, rats kept pressing the lever, up to seven hundred times per hour. Even the hungry rats preferred electric stimulation to nearby food. The researchers thought the electric stimulation should be able to produce pleasurable feeling and thus called nucleus accumbens the "pleasure center".
However, did stimulation of NAc really cause pleasure? The reason why rats kept pressing level could be due to another possibility: stimulation of NAc causes "wanting" to get the stimulation, rather than "liking" of the stimulation. Over the past two decades, Kent Berridge and his colleagues have published nearly a hundred papers, demonstrating that the feeling of "liking" is not the same as the action of "wanting", although the two are closely related. Chapter 11 will present the neural circuits showing that electric stimulation of NAc indeed causes "wanting", rather than "liking".
Pleasure is a feeling of the mind. Several hundred years ago, Descartes had already noticed that the mind is indivisible. It exists as a whole, with different states such as pleasure, pain, anxiety, angry, depression, etc. These mental states are produced by distributed networks in the brain. Each state does not correspond to any "center" in the brain. The nucleus accumbens is not a pleasure center, but it plays a key role in the action of "wanting" (Chapter 11).
According to the Geon Hypothesis, different mental states reflect distinct wave structures in the geon. As discussed in the previous chapter, pain could arise from elevated density of gravitational (GR) and electromagnetic (EM) waves in the geon. Then, could pleasure result from lower wave density?
Pleasure and Brain Activity
Orgasm can be considered as the climax of pleasure. The density of GR and EM waves is determined by neural activity in the brain: the more neurons fire, the more ions will pass through ion channels, producing more GR and EM waves (Chapter 6). Therefore, if pleasure results from lower wave density, brain activity should be generally reduced during orgasm. Activities in various regions of the brain can be observed using neuroimaging, such as PET and fMRI. It was found that genital stimulation did cause decrease in the activity of the prefrontal cortex (PFC) and temporal cortices (Georgiadis, 2012). Major findings are summarized below.
The notion that pleasure results from lower wave density in the geon is further supported by the study of major depressive disorder. The core symptom of depression is anhedonia, namely, the inability to feel pleasure in normally pleasurable activities. It has been well-established that depression correlates with excessive brain activity:
The next chapter will show that ACC could be the neural correlate of "conscience".
Author: Frank Lee