A study suggests that aphantasia – the inability to create images in the ‘mind’s eye’ – is underpinned by the functional alteration of two key brain regions: the hippocampus and the occipital lobe.
Image credit: Joel Staveley on Unsplash
The research, published today as an eLife Reviewed Preprint, is described by the editors as an important study that substantially advances our understanding of autobiographical episodic memory in individuals with aphantasia. The authors present what the editors say is convincing evidence that alterations to the hippocampus and visual-perceptual cortex in the occipital lobe, and the interactions between them, underpin the impaired autobiographical memory retrieval associated with aphantasia.
Personal experiences and events from one’s life are stored in the brain as autobiographical memories, providing stability and continuity in our sense of self. Most people are able to recall autobiographical memories that are decades old with vivid mental images. However, it remains uncertain to what extent this retrieval of autobiographical memories relies on the ability to generate mental imagery.
One way to identify the consequences of a lack of mental imagery on long-term memory is to study people with aphantasia – a neuropsychological variant characterised by the inability to visualise objects, scenes or memories.
“A region of the brain called the hippocampus is well known to play an important role in retrieving vivid, detail-rich autobiographical memories,” says co-lead author Pitshaporn Leelaarporn, a PhD student of the Clinic for Neurodegenerative Diseases and Geriatric Psychiatry, at the University Hospital Bonn, Germany. “An important question to answer is whether the hippocampus, especially its connectivity to other regions of the brain, is altered in people with aphantasia.”
Leelaarporn, along with co-lead author Merlin Monzel, a PhD student in the Institute of Psychology, University of Bonn, and their colleagues set out to examine the brain activities and structures associated with autobiographical memory deficits in aphantasia. They recruited 14 people with aphantasia and 16 controls without aphantasia from the database of the Aphantasia Research Project Bonn. Levels of aphantasia were assessed using the standard Vividness of Visual Imagery Questionnaire and a binocular rivalry task. Autobiographical memory was assessed using a detailed autobiographical interview in which participants were asked to recall five autobiographical memories from different periods of their life.
On average, the team found that those with aphantasia had more difficulty in recalling memories, were less confident about them, and described less episodic detail than those without aphantasia. “Not only do people with aphantasia report fewer details during autobiographical memory retrieval, they also show decreased confidence and emotionality associated with these memories,” adds Monzel.
Next, the team investigated how the hippocampus and the visual-perceptive cortex interact with each other during autobiographical memory retrieval in the aphantasia group compared to the control group. The participants were asked to perform an autobiographical memory retrieval task and a simple maths task, whilst undergoing functional magnetic resonance imaging (fMRI). The team measured brain activity during the task by detecting changes in blood flow. The outputs from the fMRI revealed stark differences in hippocampal activation between the two groups. Participants with aphantasia showed a reduced activation of the hippocampus on both sides of the brain, compared to the control group. This indicates that the autobiographical memory deficit associated with aphantasia is reflected by reduced activation of the hippocampus.
They then examined activation of the whole brain during autobiographical memory retrieval. Participants with aphantasia displayed a greater activation of their visual-perceptual regions in the occipital lobe than those without aphantasia. In addition, those with higher visual-perceptual cortex activation had less hippocampal activation, indicating a tradeoff between the two brain regions.
This finding strengthened the team’s hypothesis that a core difference in people with aphantasia lies in the interplay between the visual-perceptual cortex and the hippocampus. So, to test this, they examined the functional connectivity between the two regions during autobiographical memory retrieval. Whilst those with aphantasia showed almost no functional connectivity between the hippocampus and visual-perceptual cortex, those with aphantasia displayed a strong negative connectivity between the two regions.
The team proposes that the heightened activation of the visual-perceptual cortex prohibits aphantasics to experience visual imagery in their mind’s eye and without these visual details, autobiographical memory and hippocampal activation are reduced. These results suggest that the hippocampus collaborates during memory retrieval with primary sensory brain regions, such as the occipital lobe, in order to recall visual and perceptual information associated with the specific memory. In aphantasia, this process is disrupted, and the hippocampus can no longer incorporate visual-perceptual details in one coherent mental event.
The authors caution that more research is required to further explore the cognitive landscape associated with aphantasia, such as spatial cognition and scene construction – a notion shared by the eLife editors.
“We highlight the central role of the functional connectivity between the hippocampus and occipital lobe in assembling visual-perceptual details into one coherent memory,” concludes senior author Cornelia McCormick, Group Leader “Memory and Imagination” and Psychiatrist at the Department of Neurodegenerative Diseases and Geriatric Psychiatry of the University Hospital Bonn, and who also conducts research at the German Center for Neurodegenerative Diseases (DZNE) and the University of Bonn. “We are excited to see further research in the area, examining hippocampal-dependent spatial cognition in aphantasia or investigating whether neuroscientific interventions could be used to enhance autobiographical memory retrieval by enhancing visual imagery.”
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