Levitation jumping off the ring at the right point and floating forwards, it is possible to see a slide such as the ones used for the credits on the main menu, saying "Your Name Here".
Upon entering this area, any mental worlds before entering the Lair of the Lungfish excluding Basic Braining will be available after you entered them for the first time. This shows Basic Braining was actually not his true mental world.
Though, technically it's probably also to prevent the player from obtaining Oleander's Shame before its intended availability, as the player can legitimately get the Cobweb Duster surprisingly early in the game with enough skill.
Invisible Walls prevent Raz from walking off the level, but it is possible to Levitation jump over the doors, or to walk off the platform where the Brain Tumbler is when entering via the Tumbler. Falling off the level has the same effect as walking off the Brain Tumbler's platform: when entering via the Tumbler, you return to Sasha's Lab, otherwise, you teleport back onto the walkway.
The game appears to incorrectly display the location as being "Sasha's Underground Lab" in the journal. While this makes sense if the Collective Unconscious is accessed via the Brain Tumbler, it does not if you've passed the point of no return.
Actually, at a technical standpoint the Collective Subconscious is a subarea of the laboratory, as entering a new full on area (e.g. when going from the lake to the main cabin) a loading screen occurs. This loading screen does not occur when Raz uses the Brain Tumbler, meaning it's actually within the Laboratory, not its own separate world.
This can also be noticed by the fact that Raz can still use his real world items, such as the Dowsing Rod, despite the fact that he's supposed to be in a mental world.
Completing all the levels 100% in the game will cause massive text saying "ALL LEVELS 100% COMPLETE! CONGRATULATIONS PSYCHOMASTER!" to float around the area.