P.S. My apologies for a somewhat irrelevant picture, but I found it worth sharing and suitable for the purpose of JungMinded website!
The Pixar story-tellers did not bother with further technical details of Dory's condition: she just forgot everything, from her own life story to an event from 10 seconds ago. In reality, however, amnesia(s) can be divided in numerous categories, depending on the cause of a condition as well as which memory system is affected.
Amnesia may be provoked by a number of reasons. The most common one is Alzheimer's disease and other dementias; if the brain cells in hippocampus are affected, it might lead to a memory disruption. Other reasons include brain tumours, head injuries, Thalamic stroke and epilepsy. Another reason worth mentioning is limbic encephalitis: a condition in which anti-bodies produced to fight a tumour start attacking limbic system (including hippocampus). The causes listed above are organic causes: physical changes in the brain which lead to memory impairments. However, amnesia can also be non-organic, or functional. Non-organic amnesia means it occurred for psychological reasons such as traumatic experience, stress, etc.
As is the case with many neuropsychological diseases, amnesia is an umbrella term for a variety of memory disorders. The type of amnesia often depends on which brain region is impaired.
The basic sub-types of amnesia can be differentiated as follows:
1) Retrograde v anterograde
Retrograde amnesia is a condition in which a patient can not remember the events that occurred before the onset of the disease, but is still able to form new memories. On a contrary, anterograde amnesia affects a person's ability to form new memories, however they still remember events that occurred in the past. This type of amnesia was outlined due to the patient Henry Molaison known as H.M., who developed anterograde amnesia followed by progressing retrograde type after his amygdalae and hippocampus were surgically removed in order to treat his severe epilepsy.
Below you will find a video about the distinction between retrograde and anterograde amnesia presented by a professor Jens Bo Nielsen - really worth checking out!
Episodic amnesia is characterised by inability to remember specific events and experiences of one's life. Patients suffering from episodic amnesia retain semantic knowledge of the world though; for example, they will not remember they ever travelled, but will still remember information about the countries they visited. Semantic amnesia is directly opposite; for example, a patient can describe a picture of a plane as 'This thing that goes in a sky, I went to Australia in one of those'.
The double dissociation provided by patients with episodic and semantic amnesias provides grounds for arguing that episodic and semantic memory systems are independent and parallel. That is, episodic memory is NOT a necessary gateway into semantic memory, because if this was the case, patients with episodic amnesia would not be able to retain semantic memory.
3) Explicit (declarative) v implicit (procedural)
Explicit amnesia is a loss of information - either episodic or semantic. Implicit (or, procedural) amnesia refers to loss of procedural memories of body movements or usage of objects - for example, that of riding a bike, writing with a pen etc.
As organic amnesia is characterised by the loss of brain cells, there is no way to treat it, as there is no way to recover those cells once they are damaged. However, depending on the severity and cause of a condition, it is sometimes possible to stop it from further development - for example, by removing a brain tumour. In a majority of cases, certain therapies are used which help patients to cope with amnesia. For example, those with anterograde type are trained to use day-to-day planners.
Visual agnosia can be subdivided in two types: apperceptive and associative. Apperceptive agnosia is a deficit of perception: for example, a patient would not be able to draw a presented object or to group it with similar ones. On a contrary, patients with associative agnosia will perceive an object correctly, and will be able to draw it - however they will struggle to name it (i.e. to access the meaning of the object). One of the sub-types of associative agnosia is prosopagnosia: inability to recognise human faces. A person will be able to perceive the stimuli correctly: eyes, mouth, nose etc., but will not be able to link this information to a specific person he/she knows.
1) Ideational apraxia
Ideational apraxia is a loss of an ability to recall a purpose of an object and to create/remember a plan to use an object, thus causing a disturbance in movement - for example, inability to open a can with a can opener. If a plan of action is formed, a sequence of movements may be disturbed; an example of this can be striking a match to use a cooker and holding it until it burns one's fingers. The motor skills themselves are intact - but a person is not be able to use them purposefully. The condition is associated with a damage of a submarginal gyrus in the parietal lobe of the brain.
2) Ideomotor apraxia
Ideomotor apraxia is inability to transform an idea into the movement. A person does have a correct plan of action, however his/her voluntary movements are disturbed, making it impossible to carry out the existing plan. In other words, the connection between the planning areas and the motor cortex is impaired - rather than the planning area itself, as is the case with ideational apraxia.