16 Impact of Hippocampal Damage on Dreaming

The concept of dreaming has always fascinated humans, however even with the ongoing research around it, the question of why we dream still hasn’t been fully answered. The hippocampus is associated with both sleep and memory, although there is controversy regarding whether a key part of sleep, dreaming, necessitates its involvement. Much evidence currently suggests that sleep plays a role in memory remodelling and consolidation, and consequently in improving waking cognitive function. Memory consolidation is a process that occurs over time, from weeks to years, in which hippocampus-dependent memory becomes independent and is consolidated into the general neocortex. Some theories add onto this evidence and say that dreaming aids in the processing of prior events while sleeping meaning that it necessitates the involvement of the hippocampus. If one were to sustain an injury to their hippocampus, it could result it amnesia, which as we learned can hinder people from not only developing new memories but difficulties recalling past ones. According to the research done by Spanò et al., on dreaming with patients who have amnesia has yielded varied results; some conclude that hippocampal injury has no impact on dreaming whereas others discovered that is caused individuals to have reoccurring dreams with little detail. These differences in results could be attributed to the fact that many studies asked patients about their dreams the next morning, by which time most people, especially those with amnesia, had forgotten whether or not they had dreamed (Spanò et al., 2020). To avoid these issues, within the experiment, selected participants were all right-handed men who had a variety of bilateral hippocampal lesions and a specific episodic memory deficit, compared to a control group of ten healthy participants (also all right-handed men). They were then woken up at various times though out their sleep and asked about their dreams.

Patients participated in this study for two consecutive nights, one habituation night for familiarisation using portable polysomnography (PSG) and one experimental night for dream report collecting. The goal of the PSG recording was to guarantee that participants were woken up during both non-rapid eye movement sleep and rapid eye movement sleep, and that the patient and control groups were awakened in the same way. Following an awakening, participants were encouraged to recount all that was going through their minds prior to being awakened up at that time through a two-way intercom, and those individuals who had hippocampal damage reported fewer dreams than the control group (Spanò et al., 2020). They were asked questions such as ‘What was going through your mind before being woken up’, then based on the response were prompted to share more information on what they had experienced.   

In addition to having fewer dreams, individuals with hippocampal injury stated that their dreams featured less episodic information and sensory elements. These results support the developing theory that dreams are formed by brain networks comparable to those engaged in retrieving memories and generating imagined situations while awake but could also go beyond just dreaming, potentially impacting imagination and daydreaming. A vivid dream, like memory and imagination, necessitates the creation of precise, memory-based imagined scenarios, and this process appears to rely on the hippocampus. The findings of this study imply that a healthy hippocampus is required for both memory and dreaming, further strengthening the relationship between the two. People with hippocampus damage may not have the dream-related mechanisms for aiding memory processing, which may add to their amnesia if dreaming does, in fact, serve a useful role in consolidating knowledge in long-term memory. Hippocampal injury has been linked to a variety of disorders, including dementia. If these disorders lead people to sleep less, it may exacerbate the memory problems associated with these ailments (Spanò et al., 2020).

According to Fossa. et al., 2003, about half of all dreams contain at least one element originating from a specific experience while the subject was awake. Although these dreams are rarely a faithful replication of any one memory, there are usually pieces of different recent memories. In addition, they noted that other than the study of Spanò. et al., there has been recent work in cognitive neuroscience that has established that in addition to being involved with the formation of memories, the hippocampus is involved with using memory to construct imagined scenarios and simulate possible events (Wamsley, 2020). Therefore, patients without a normal, healthy hippocampus find it difficult to imagine scenes that are coherent, possibly because the hippocampus is responsible for combining different elements of memory into a spatially coherent whole (Wamsley, 2020). This relates back to the results from Spanò’s research since individuals with hippocampal injuries not only experience fewer dreams, but they also had much less detail. This article critically examined the study conducted by Spanò. et al., highlighting that while the results of this study should be taken with a grain of salt due to the small sample size, their work is helping to shed light on the role that the hippocampus plays in the dreaming process.