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A lot of people try to scare you from taking drugs by showing you pictures of brains with lower brain activities on PET scans. But if meditating does the same exact thing and creates PET scans that have lower brain activity. Why is it good when meditating and bad when taking drugs?

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Lowered brain activity in a PET scan isn't necessarily bad. It depends on what part of the brain is less active than usually and whether this is a permanent or transient state. Our brains are less active during sleep - that isn't a bad thing.

Anyway, brain activity isn't just generally lowered during meditation. On the contrary.

Functional brain mapping of the relaxation response and meditation found (emphasis mine):

Significant (p <10−7) signal increases were observed in the group-averaged data in the dorsolateral prefrontal and parietal cortices, hippocampus/parahippocampus, temporal lobe, pregenual anterior cingulate cortex, striatum, and pre- and post-central gyri during meditation. Global fMRI signal decreases were also noted, although these were probably secondary to cardiorespiratory changes that often accompany meditation. The results indicate that the practice of meditation activates neural structures involved in attention and control of the autonomic nervous system.

Long-term meditators self-induce high-amplitude gamma synchrony during mental practice.

In addition, the ratio of gamma-band activity (25-42 Hz) to slow oscillatory activity (4-13 Hz) is initially higher in the resting baseline before meditation for the practitioners than the controls over medial frontoparietal electrodes. This difference increases sharply during meditation over most of the scalp electrodes and remains higher than the initial baseline in the postmeditation baseline.

Basically, it's a lot more complicated than meditation decreasing brain activity.

As for recreational drugs, I am not sure where you got a generally decreased activity from, but for example long-term use of meth decreases the amount of dopamine receptors in the brain. Dopamine is important for feelings of pleasure and reward and building memories.

Studies in methamphetamine abusers have also documented significant loss of DA transporters (used as markers of the DA terminal) that are associated with slower motor function and decreased memory. The extent to which the loss of DA transporters predisposes methamphetamine abusers to neurodegenerative disorders such as Parkinsonism is unclear and may depend in part on the degree of recovery.

Long term use of cocaine decreases the actual amount of grey brain matter.

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