In MPS I, a buildup of certain compounds in connective tissue leads to frequent infections, joint deformities and impaired heart function.

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It may also cause impaired lung function and increased respiratory infections.

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The researchers suggest a "small but significant" negative effect of 4% linked to the combined effects of smoking and impaired lung function - itself linked to smoking.

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But they also recorded a similar fall in 24% of patients with more mild symptoms of impaired brain function, who might have been in the very earliest stages of Alzheimer's.

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It has been suggested in previous studies that there could be a link between impaired lung function and a negative effect on the thinking processes, but it is not clear what the mechanism for that might be.

BBC: NEWS | Health | Smoking 'is bad for your brain'

Also, elderly people because their kidney function may be impaired.

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Some studies have found that people with internet addiction have fewer dopamine receptors in certain areas of the brain, and others have suggested additional ways in which dopamine function might be impaired.

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And the stronger that family history, the greater a person's risk of having recurrent bouts of the condition and to report that it impaired his or her ability to function.

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The next step is to figure out if stimulation also can help when recalling old memories, because that function can also be impaired with dementia, according to Nanthia Suthana, the first author on the study and a UCLA postdoctoral researcher.

WSJ: Memory Gets Jolt in Brain Research

And their function did not seem to be impaired.

FORBES: Magazine Article