especially with Exterior?

We know for sure it can't be fixed. she's the strongest "telepath", yes, but even the greatest cook on earth needs ingredients to make a dish.
Some of Touma's brain cells were destroyed. It's not like his memories are there but are just inaccessible to him, they were reduced to nothing.

"Memory" isn't something that is stored uniformly in our brain. There are different types of memory and each of these type are most of the time processed/registered by different parts of our brain, and/or is retained/consolidated in different parts of our brain.

There is what you call short term and long term memory.

1. Short term memory is when you look at this number : 4891427664231618. For a few of seconds you will remember it, but then you'll quickly forget it.

2. Long term memory is when something that was initially short term memories is deemed by your brain worth remembering, and is in consequence registered correctly then consolidated in parts of your brain

Long term memories can also be divided in two : explicit and implicit

2.1 Implicit memory : it's the memory regarding processed that doesn't require your thought = memory that is retrieved and processed subconsciously. For example : Acquired skills or memories of actions that you perform during your daily life (how to hold a gun, how to ride a bike, how to walk, how to write), this one is also called procedural memory.
For example : reflexes acquired through experiences but you might lose if you don't practice enough (Pavlov's dog type of reflex in general)
The parts of the brain involves with that are according to some scientist the limbic system and the cerebellum regions. Touma's implicit memories are intact, so there's no need to go further into this one.

2.2 explicit memory : it's the type of memory that requires your consciousness and your thoughts to remember = memories that your retrieve by explicitly wanting to remember it with your will. There are also two types of Explicit memory : semantic memory and episodic memory.

2.2.1 semantic memory : is the type of memory where all your general knowledges of the world are stored.
Example : definition of "square", the capital of Canada, Abraham Lincoln, Innocentius, etc
Touma's semantic memories are also intact (mostly)

2.2.2 Episodic memory : also called autobiographical memory sometimes, is the type of memory that involves our personal experiences or things that we took part ourselves.
Example : what I ate yesterday, my trip to Hawaii, who my mother is, what my name is, where I live, who thought me and where I learned what a square is, who thought me and where I learned who Abraham Lincoln is, who thought me and where I learned what Innocentius is.
It's this kind of memory that Touma Lost.

Things you have to know about explicit memory is that, the scientist themselves aren't exactly sure what parts of the brain are involved with this, especially with the semantic memory. They are almost certain that episodic memory is consolidated in the temporal lobe region of the cortex. Some say semantic memories is stored in the same region, some says it's stored on other region of the cortex, some says it's stored all over the brain like you said.
The reason for that is that most case of amnesia are about people being unable to retrieve their episodic memories yet, for some cases, have no trouble with their basic knowledges (and their implicit memories, obviously).

Touma had his cortex damaged by a magic spell, resulting in the loss of his episodic memories. Frog faced doctor himself told him the damage were impressive. It isn't just some problem with his synapse that unable him to access those memories, no, his brain cells were destroyed for good.
But he still retains all of his other long term memories (implicit and some of his semantic ones).

http://i.imgur.com/vf47icL.jpg

Notice the eyes^^ She is possesed.