The temporal lobe is temporarily needed when consolidating new information; as the learning becomes stronger, the neocortex becomes more independent of the temporal lobe.Studies on specific cases demonstrate how particular impaired areas of the hippocampus are associated with the severity of RA. Damage can be limited to the CA1 field of the hippocampus, causing very limited RA for about one to two years. More extensive damage limited to the hippocampus causes temporally graded amnesia for 15 to 25 years. Another study suggests that large medial temporal lobe lesions, that extend laterally to include other regions, produce more extensive RA, covering 40 to 50 years. These findings suggest that density of RA becomes more severe and long-term as the damage extends beyond the hippocampus to surrounding structures. The common studied causes of RA do not always lead to the onset of RA. It may even be that in some cases both conscious feigning and unconscious processes are at play. Traumatic brain injury (TBI) or post-traumatic amnesia
Traumatic brain injury (TBI), also known as post-traumatic amnesia, occurs from an external force that causes structural damage to the brain, such as a sharp blow to the head, a diffuse axonal injury, or childhood brain damage (e.g., shaken baby syndrome). In cases of sudden rapid acceleration, the brain continues moving around in the skull, harming brain tissue as it hits internal protrusions.TBI varies according to impact of external forces, location of structural damage, and severity of damage ranging from mild to severe.
Thus, the brain can learn to be independent of the impaired hippocampus, but only to a certain extent. For example, older memories are consolidated over time and in various structures of the brain, including Wernickes area and the neocortex, making retrieval through alternate pathways possible. Diagnosis
Testing for retrograde amnesia
As previously mentioned, RA can affect peoples memories in different degrees, but testing is required to help determine if someone is experiencing RA. Several tests exist, for example, testing for factual knowledge such as known public events. A problem with this form of testing is that people generally differ in their knowledge of such subjects. The tests also have to be changed regularly to adapt to the period of time patients have been alive, and include questions about autobiographical events that range in amount of detail. Due to the nature of the information being tested, it is often difficult to verify the accuracy of the memories being recalled, especially if they are from a distant past. One way to confirm information is by consulting close family members or caregivers. Another problem that arises is that this type of testing can result in a significant amount of data that needs to be processed and confirmed. Other ways to test someone is via autobiographical knowledge using the Autobiographical Memory Interview (AMI). A test that comprises names of relatives, personal information, and job history. The AMI was created by Kopelman, Wilson, and Baddeley (1990). This information could help  determine if someone is experiencing RA and the degree of memory affected.