This is what is called a paired difference experiment.
Subtract one value from the other and do the one sample t-test
as you are used to doing.
So your data is now {5,-5,6,7,..} and test to see if the population mean (change in memory) is positive.

Here's a test procedure for it, the confidence interval is the same as before.
http://www.vias.org/tmdatanaleng/cc_test_diffttest.html
BUT I would test , since you want to prove that memory is increased after this.

A very good example of a paired difference experiment is weight reduction.
You can test to see if people do lose a certain amount of weight over a period of time.
Since you use the same subject before and after the samples are dependent.
You wouldn't want to use two different sets of data, that doesn't make sense.
Weigh 20 people before an 20 different people a month later is illogical.