How has the study of patient HM helped our understanding of the role of the hippocampus in
memory?
Henry Molaison (HM) suffered from severe recurrent seizures and underwent Working memory- consolidation
a bilaterally symmetrical medial temporal lobe removal to treat his condition.
Procedural memory
His surgery was successful, and his seizures could now be treated with
Declarative memory- hippocampal
anticonvulsant drugs, however he experienced significant deficits in memory.
formation linking memories
His subsequent amnesia has helped our understanding of the role of the
Animal models:
hippocampus in memory.
PRC- object memory (recognition)
HM suffered from severe anterograde amnesia, amnesia for memories that
Hippocampus- visuospatial memory
occurred after the surgery. He also suffered from temporally graded
retrograde amnesia, amnesia for events that occurred before the surgery. He Taxi drivers- spatial memory
couldn’t remember events that happened just prior to the operation, however
his memory for earlier years was intact. His working memory was also intact,
the ability to hold items in short term memory and manipulate them. He had a normal digit span,
but only with constant rehearsal. If he was interrupted, his short-term memory was impaired. This
suggests that memory was not being consolidated from working memory to long term memory. It
has therefore been proposed that the role of the hippocampus in memory is consolidation. Although
it is debatable whether his short-term memory really was intact, as he had difficulty with
remembering visual images, suggesting a restricted working memory.
HM could learn new motor tasks, implying that his procedural memory was intact. He was able to
improve on the mirror tracing task despite not being explicitly familiar with the testing equipment.
Even though he couldn’t remember previously doing the task, he was able to improve in the long
term. This suggests a distinction between implicit and explicit memory. Explicit, or declarative
memory, was impaired in HM, which includes semantic and episodic memory. Episodic memory,
memory for personal events, was especially impaired in HM, however he had some preserved
semantic memory, memory for facts about the world that he learnt before the surgery. It has
therefore been suggested that the hippocampal formation plays a role in the process of forming
declarative memories; through connections with subcortical regions, it links memories together to
permit us to remember the relationships among the elements of the memories, for example the
order of events and the context. Memories stored in the hippocampus are then gradually
transferred to the frontal cortex, which could explain the graded nature of HM’s retrograde amnesia.
However, the role of the hippocampus in memory is further complicated by the findings of animal
models of human amnesia. In one study, animals with a restricted lesion to the hippocampus
performed as well on a memory task as controls, suggesting that the hippocampus is not as critical in
memory deficits as was once thought. Animals with a lesion to the entire temporal lobe, like HM,
performed the most poorly on the task, and so the role of another area was highlighted, the
perirhinal cortex (PRC). On a recognition memory test, animals with hippocampal lesions perform
well and could recognise novel items, whereas those with PRC lesions show a profound deficit.
Therefore, it has been suggested that the role of the PRC in memory is for object-based memory
important in recognition, whereas the hippocampus in implicated in complex visuospatial scene
memory. This has been demonstrated in evolutionary terms, as food storing birds have a larger
hippocampus to remember the location of food. Furthermore, London taxi drivers show experience
related changes in the size of the hippocampus from topographical learning, the posterior region is
larger whereas the anterior region is smaller. This correlates with their time as a taxi driver and
implies that a change in structure may result in a change in function. To demonstrate this, taxi
