In order to understand how the loss of memory works, it's good to know how we store memories in the first place. Our brain gives us the power to think, plan, speak and imagine. It also gives us the ability to make and store memories. Physiologically speaking, a memory is the result of chemical or even structural changes in synaptic transmissions between neurons. As these changes occur, a pathway is created. This pathway is called a memory trace. Signals can travel along these memory traces through the brain.
Making and storing memories is a complex process involving many regions of the brain, including the frontal, temporal and parietal lobes. Damage or disease in these areas can result in varying degrees of memory loss. Here's a good example of how this works:
In order for short-term memory to become long-term memory, it must go through a process known as consolidation. During consolidation, short-term memory is repeatedly activated -- so much so that certain chemical and physical changes occur in the brain, permanently "embedding" the memory for long-term access. If, during this repeated activation, something interrupts the process -- let's say a concussion or other brain trauma -- then short-term memory cannot be consolidated. Memories can't be "stored" for long-term access. This may be what's going on in anterograde amnesia.
It is believed that consolidation takes place in the hippocampi, located in the temporal-lobe regions of the brain. Medical research indicates that it is the frontal and temporal lobes that are most often damaged during head injury. This is why many people who suffer severe head trauma or brain injury experience anterograde amnesia. If the hippocampi are damaged, the amnesiac will be able to recall older memories, but won't be able to make any new ones.
For more information about human memory and the brain, see the links on the next page.