White says when you're at a BAC of 0.08 to 0.1, the alcohol starts to affect the hippocampus, which is the area of the brain that’s in charge of consolidating information from your sensory organs, your body, your thoughts, and your feelings to create memories.
As your BAC climbs, there are two main reasons your memory gets cloudy: the hippocampus isn’t getting as much rich information from the brain about what's happening, and the alcohol is blocking certain receptors, so that even information that gets into the hippocampus is unlikely to be stored as a memory.
So if you're a heavy drinker, you may develop a tolerance that helps you speak and keep balance, but not for your memory. White says that’s probably why alcoholics are so likely to report blackouts. Because they can get to very high BAC levels and function, but aren’t able to make new memories.