“Any kind of stimulus and sensation received from the inner and outer world becomes information; all of this information is called experience. Memory is the mind’s function of retaining experiences.”
Y. Özakpınar
H.M. suffered from epileptic seizures that began in childhood, gradually intensified over time, and were resistant to pharmacological treatment. He was examined by neurosurgeon Dr. William Beecher Scoville, and in 1953 a bilateral medial temporal lobe resection was performed to reduce seizure burden. After the operation, the frequency and severity of the seizures reportedly decreased; however, a clinical picture of anterograde amnesia emerged. Anterograde amnesia is defined as the inability to encode new information in a way that allows it to form stable, long-term memory representations following brain injury or a traumatic event. Although H.M. could recall many memories from the preoperative period, he was unable to transfer episodic and semantic information into long-term memory after surgery.
The medial (inner) portions of the temporal lobe are closely associated with episodic and semantic forms of memory. They are regarded as key brain regions involved in the formation of new declarative memory and in memory consolidation. This system includes parahippocampal areas and structures of the limbic system. The amygdala and hippocampus are limbic structures located within the medial temporal lobe. The hippocampus is a critical region for encoding experiences and transforming them into long-term representations. Declarative (explicit) memory refers to information that can be consciously recalled and verbally reported, and it includes two types of knowledge: episodic and semantic. Episodic memory consists of personally experienced events embedded in a temporal and spatial context, whereas semantic memory comprises general knowledge and concepts.
In this context, H.M.’s hippocampal removal is associated with his inability to transform new information into long-term representations. The literature indicates that episodic and semantic information and events could not be transferred into long-term memory following the operation. Medial temporal lobe structures, including the hippocampus and related cortices (entorhinal cortex, perirhinal cortex, and parahippocampal regions), are associated with recognition processes, memory consolidation, and the transfer of information into long-term memory. In other words, because the hippocampus is thought to support consolidation processes that help memory traces become long-term representations within neocortical networks—rather than serving as the primary storage site for memories—H.M. was able to recall many preoperative memories. This interpretation is consistent with reports of retrograde amnesia in H.M., particularly his difficulty recalling memories from the period shortly before the surgery.
In non-declarative (implicit) memory, learned information is expressed in behavior and performance. Its main forms include priming, conditioning, and procedural memory. Procedural memory is a type of non-declarative memory in which learning becomes automatized through repetition and practice and is demonstrated through performance. Procedural memory is associated with the basal ganglia, the motor cortex, and the cerebellum. Because procedural learning and motor skills rely on subcortical systems, the relative preservation of skill learning and the dissociation between memory systems can be explained within this framework.
Working memory refers to an attention-related cognitive system that enables individuals to maintain information temporarily and manipulate it actively. Working memory is commonly linked to the dorsolateral prefrontal cortex and fronto-parietal pathways. It has been reported that H.M.’s working memory and non-declarative (implicit) memory were largely preserved. This finding indicates a distinction between temporarily holding information online and encoding it into long-term declarative memory. Thus, the case clearly demonstrates a functional separation between short-term processing systems and long-term storage mechanisms within the broader architecture of memory systems.
H.M. is a classic case characterized by anterograde amnesia resulting from the bilateral removal of medial temporal lobe structures, particularly those associated with the hippocampal formation. Postoperative findings show that, even though he could learn new information in the moment, he was unable to store episodic and semantic information in long-term memory in a lasting manner. Based on these clinical observations, it is understood within a theoretical framework that the medial temporal lobe, including the hippocampus, is closely related to declarative memory.
In addition, H.M. provides a neurological example demonstrating the distinction between short-term and long-term memory. The case also illustrates that different types of memory are supported by distinct neural pathways and anatomical regions. H.M. occupies an important place in the history of cognitive psychology and served as a foundational case in the development of modern neuropsychology.
References
Scoville, W. B., & Milner, B. (1957). Loss of recent memory after bilateral hippocampal lesions. Journal of Neurology, Neurosurgery & Psychiatry, 20(1), 11–21.
Squire, L. R. (2009). The legacy of patient H.M. for neuroscience. Neuron, 61(1), 6–9. https://doi.org/10.1016/j.neuron.2008.12.023