Final answer:
The amnesia patient HM showed memory loss for events before his operation, known as retrograde amnesia. HM also had anterograde amnesia, unable to form new episodic and semantic memories, which is linked to damage in the medial temporal lobes and hippocampus.
Step-by-step explanation:
The amnesia patient HM experienced memory loss for events in his life that occurred one to two years before his operation that accidentally caused his memory damage. This type of memory loss is an example of retrograde amnesia, which is a loss of memory for events that occurred before the trauma.
HM also suffered from anterograde amnesia after his bilateral lobectomy, which removed his hippocampus and amygdala to alleviate seizures. This condition prevented him from forming new memories, specifically new episodic and semantic memories while retaining the ability to form new procedural memories. For example, although HM could not remember reading a magazine or meeting new people, he could become faster at solving puzzles through repetition and relearning, indicating procedural learning was still functioning.
Retrograde and anterograde amnesias are both related to damage in the medial temporal lobes and the hippocampus, which play crucial roles in the consolidation of new learning into explicit memory.