Amnesia can be transient or permanent. There are two temporal categories of amnesia: retrograde amnesia and anterograde amnesia.
> Retrograde amnesia: it is the inability to recall old facts (comes from a Latin root meaning to go backwards).
Example of a symptom: inability to recall memories of vacations or places of life ten years ago.
> Anterograde amnesia: it is inability to remember recent events (comes from a Latin root meaning "to go forward"). The subject can no longer build new knowledge and learning.
Example of a symptom: unable to remember the name of someone just met.
One can also define the amnesia according to the type of memory concerned. Man has at least two qualitative types of information storage: declarative memory and non-declarative memory.
These memories are not located in the same areas of the brain. Depending on the brain region that will be damaged, there will be different types of amnesia:
1) Amnesia concerning declarative (or explicit) memory.
Declarative memory concerns the storage and retrieval of data that can emerge to consciousness and be expressed by language. This memory is located in a brain region called "the hippocampus". We distinguish between semantic declarative memory and episodic declarative memory:
> Semantic memory: it is the generalized memory.
Example of a symptom: forget the meaning of a word, the name of the president, etc.
> Episodic memory: it is the autobiographical memory.
Symptom example: no longer remember the date and location of the last family vacation.
2) Amnesia regarding non-declarative (or implicit) memory.
Non-declarative memory is not accessible in the details to consciousness. It has different types of memory, each located in specific areas of the brain. Among these: procedural memory. It represents the habits, the acts that we have learned and that we do without being aware of the precise way in which they must be realized. This memory is located among others in a deep zone of the brain called the "striatum".
Example: riding a bicycle, playing the piano, typing on a computer keyboard.
> Seeding or priming: an unconscious phenomenon that makes it easier to remember the latest data.
> Associative learning (Pavlovian): a dog whose sound of a bell is associated with the meal will, after a few days, salivate to the sound of the bell.
> Non-associative learning (reflex paths).
Any involvement of the cerebral region involved in one of these four categories of implicit memory may therefore lead to a particular amnesia relating to the faculties allowed by the type of memory damaged.