Abreaction is the name for the thing carbon dioxide therapy was supposed to induce: the sudden recollection of a traumatic memory, which would then be powerless to hurt you anymore. The very concept as used here shows the weird relationship in Walter's time between Freudian gospel and the idea that maybe we could do something to the brain itself? Just a little? Not to challenge Freud in any way, of course, we wouldn't dream of that, just to prime the pump. Walter used to tell psychoanalysts that lobotomy would make their patients more amenable to therapy, which was, I think, his sly way of pretending not to challenge them while actually saying that their model was a load of crap -- which, in my opinion, it mostly was. You can say what you want about Walter but he never told any of his patients that they wanted to fuck their parents. I mean, I'm pretty sure. I can't absolutely assert that he never did that because those files are confidential.
I do find Imaginary Walter comforting, and I was thinking about why this might be. Partially it's because he reminds me of my achievements, which I can look around and see on the walls of my office/bedroom -- this particular part of the book is now nineteen pages long, and contains a very detailed funeral wreath, Dr. Keen standing on a box, girls with pompadours, and me yelling about a clown suit. But I also find him comforting because he and I are so intimate and I imagine we understand each other pretty well. Also, I know that no thought or feeling I could express would disturb him: he dealt with terror and pain and insanity all day long, and needed no protection from the worst of the human mind, perfectly safe as he was within the armor of his own vast egotism.
Try it yourself! Imagine Walter with you when you're having a tough time. He'll be the wiry, grinning fellow telling you that you need to get out and get some more exercise. And is he so far off the mark, hm?