Dendrite spine density, should it be higher or lower?

The answer is always a balance. No one aspect of memory and cognition can be considered objectively better. You want strong lasting connections for long term memories. However, you also want to be able to creating shorter fleeting memories, and discard irrelevant information, so that you can efficiently direct the processing power of your brain. Being able to remember every little detail sounds nice. However, you need to be able to discard irrelevant information, just as much as you need to store pertinent information longer.

Imagine remembering what you ate for dinner every night for the past year. It would be useless. You've probably had some good dinners, bad dinners, so so dinners, or gotten food poisoning. Remembering where you got food poisoning is useful, so your brain will create a stronger connection for that memory. If you had many weaker connections, you would eventually overwrite the pertinent information about the food poisoning, with your crazy aunt Amy's mediocre meatloaf. Then you might stroll back into that shady Chinese place that gave you food poisoning in the first place. That's not a good evolutionary advantage! Of course this is a drastic oversimplification, and your aunt Amy's meatloaf could just as well have given you food poisoning as well. But you get the idea.

So you need a balance of stronger connections, and weaker ones. The brain regions those connections are in also makes a big difference. Being able to interpret information, discard the irrelevant parts, and store the crucial data, is the basis for how we think. Shorter dendritic spines that have larger heads, have stronger synapses. Longer dendritic spines have synapses that are less mature, but more able to be modified. Having all one or the other will make normal memory processes cease to work properly. All large headed dendtritic spines would make learning new things difficult. All longer and less mature spines would make creating new memories quick and easy, but make storing them for long term impossible. Alzheimer's disease actually reduces the density of dendritic spines, due to a buildup of amyloid plaques. One interesting thing that does happen with Alzheimer's, is that as spine density reduces, the head diameter of the remaining spines increases. So the brain attempts to compensate for the loss in spine density by trying to strengthen the remaining ones. It also seems to only be half of the dendrites that are affected by Aβ. So as one loses memories for some things, other memories might get more vivid.

My girlfriend's grandfather has Alzheimer's, and he will talk to me for hours about his younger days, and how he started the Arizona mountain rescue program. He remembers is all in great detail. However, he then sometimes forgets who his son is, and tries to escape at night, thinking he has been kidnapped. Having only only more mature, wider diameter dendritic spines, is not ideal, just like having all thinner and less mature spines is not good.

I'm going to go eat some meatloaf.