Final answer:
The investigation described is utilizing a double-blind procedure to ensure unbiased results by keeping both the researchers and participants unaware of who has received the actual drug and who has received a placebo.
Step-by-step explanation:
In the described memory study where neither researchers nor participants know who has received the drug or the placebo, the investigation uses a double-blind procedure. The double-blind methodology is the gold standard in clinical trials and experiments for reducing both participant and experimenter bias. It controls for the placebo effect, where expectations can influence outcomes, and experimenter bias, where researchers' expectations may skew results. Having both parties 'blind' to who is in each group ensures that the results of the study are not influenced by either's expectations or biases, thus providing a more objective assessment of the drug's efficacy on memory.