"Change in sense of humour 'a sign of impending dementia.'"

"The University College London study involved patients with frontotemporal dementia, with the results appearing in the Journal of Alzheimer's Disease."

Nearly all of the [friends or relatives of the patients] said, with hindsight, that they had noticed a shift in the nine years before the dementia had been diagnosed. Many of the patients had developed a dark sense of humour — for example, laughing at tragic events in the news or in their personal lives. The dementia patients also tended to prefer slapstick to satirical humour, when compared with 21 healthy people of a similar age.

Dr [Camilla] Clark said: "These were marked changes - completely inappropriate humour well beyond the realms of even distasteful humour. For example, one man laughed when his wife badly scalded herself."

What does this say about people who begin with a "dark" sense of humor and laugh at the pain of others?