Dude, what’s that smell?

U of T study explains link between smell and memory

Dude, what’s that smell?

I am sitting at my maternal grandmother’s house in New Delhi, India. Masi, my aunt, has prepared a dish for me that she promises I will love. I don’t particularly like surprises, but I wait outside the kitchen.

I catch a whiff of something sweet. I can’t place it but it’s familiar. I close my eyes and I know it’s a smell from my childhood. Then it hits me. My Masi is making an Indian confection called almond halwa using my grandmother’s recipe.

This connection that I made — that we all make — between odour and memories, is explained in a study published in Nature Communications. The study, led by Afif J. Aqrabawi, a PhD candidate in the Department of Cell & Systems Biology at U of T, sheds light on this connection and how it could help develop new diagnostic tests for Alzheimer’s disease.

The hippocampus (HPC) is essential to episodic memory. It organizes memories of sensory events, including smell, in terms of space and time. The HPC stores the condition of the brain when said events take place, and then retrieves and recreates cerebral cortex activity of the original memory’s context when we encounter the sensation again.

The anterior olfactory nucleus (AON) is the largest source of feedback projections in the olfactory cortex, and the anatomical junction where the connection between olfactory and contextual information is made. HPC projections into the AON can alter the way smells are perceived and what behaviours are associated with specific odours.

Aqrabawi and Department of Psychology Professor Jun Chul Kim had determined that inputs from the HPC to the AON are necessary for the retrieval of odour memory based on spatial and temporal contexts. They knew the AON played a role in connecting spatial and olfactory events, but they did not know the exact function of the AON-HPC junction.

Thereafter, Aqrabawi and Kim found a neural pathway between the HPC and AON and they were able to define its role in memory retrieval. This pathway is responsible for contextual retrieval of odours and is affected in patients with Alzheimer’s.

In the study, mice whose AON-HPC junction was blocked kept returning to investigate the same scent even after being exposed to it several times prior. This was an indication that the AON plays a significant role in memory retrieval.

On the other hand, mice whose junctions were left to function normally spent less time smelling familiar odours because of the episodic memories associated with them. Inhibition of the HPC-AON pathway results in a loss of the odour memory linked to a given context in space and time.

This is the first study that demonstrates that inputs from the HPC to the olfactory cortex are necessary for forming and retrieving episodic odour memories. Findings from the study also show that the anatomical location of AON behind the olfactory bulb is an ideal bridge between olfactory and contextual information.

Multiple studies have reported a loss of olfactory function in Alzheimer’s patients. In fact, diagnostic smell tests are currently used to detect the earliest symptoms of the disease. This olfactory dysfunction is due to the neurodegeneration of the AON, which stores episodic odour engrams, during the early stages of Alzheimer’s disease.

Future research involving these findings will likely aim to better understand the connection between smell and memory, and particularly the neural circuits involved in this association.

Study finds ‘lost’ memories in mice can be recovered

SickKids researchers use light to control neurons and aid memory recovery

Study finds ‘lost’ memories in mice can be recovered

Why is it that we can’t completely remember events from our childhood?

Previous studies have shown that infants are unlikely to remember event-based memories. As infants, we lack the cognitive abilities to consolidate and store autobiographical memories. As we grow older and our brains develop, new neural pathways are made in the place of old ones, leading to a near-total loss of memories from the first few years of life.

A unique form of this circuit recalibration and the most impactful on childhood forgetting is hippocampal neurogenesis, or the generation of new neurons in the hippocampus, the region of the brain primarily responsible for memory consolidation.

A study published in Current Biology outlines how memory loss in infants occurs, and how scientists induced their recovery using optical stimulation a technique that uses light to trigger neurons in mice.

Researchers in the Frankland Lab at the Hospital for Sick Children have been studying patterns of neural activity during autobiographical memory formation. Once researchers mapped out patterns of neural ensembles during encoding, they later reactivated specific neurons in the same pattern to test whether the subject could remember the encoded memory or not.

“Successful memory retrieval occurs when some specific spatial-temporal pattern of neural firing is engaged,” wrote Axel Guskjolen, a PhD candidate in the Frankland Lab and lead author of the study, in an email to The Varsity. “If that specific pattern of neural firing fails to occur, then the animal fails to retrieve the memory, resulting in forgetting.”

In the lab, fear was encoded into mice of varying ages, both infants and adults, by exposing them to small conditioning shocks. When returned to the training context, the older mice were able to retain the memory and freeze at the times and locations that they expected the footshock days after training. However, infant mice were a different story.

“In our experiment, infant mice successfully encode a memory but fail to retrieve it when [tested] at long retention delays (i.e. infantile amnesia). Using memory-tagging and optogenetic techniques, we were able to bring the memory back by forcing the neurons that were involved with memory encoding to become active again,” wrote Guskjolen.

When their neurons were treated with light, the infant mice were more likely to remember where to freeze. The stimulation of the neurons in the hippocampus led to artificial memory expression, even 90 days after initial training.

“This finding is a bit of an enigma because we forget the earliest experiences of our lives a phenomena known as infantile amnesia,” added Guskjolen. “The finding that the physical basis of these memories still exists in the brain in a ‘silent’ state might explain how these forgotten memories continue to influence our thoughts and behaviours as adults.”

Initially, the researchers questioned whether the loss of memory in the infant mice was due to storage failure, where there isn’t enough space in the brain to retain memories, or retrieval failure, where memories are retained but the brain isn’t able to access them. However, throughout the study, the mice who were encoded with memories and opto-stimulated were able to experience these same memories again. The memory loss was therefore a case of retrieval failure.

According to Guskjolen, the implications of these findings for human medicine are hugely significant as the many “commonalities across mammalian brains in terms of neural subtypes, structure, and function” suggest that that these results will be translatable to humans.

“Many disorders that afflict humans are at their heart disorders of forgetting. Sometimes these disorders are characterized by too much forgetting (Alzheimer’s disease) and sometimes by too little forgetting (Post Traumatic Stress Disorder),” wrote Guskjolen. “To find cures [for] these disorders, it is important that we first understand the mechanisms of forgetting under normal circumstances.”

Is there more to memory than meets the eye?

Rotman study finds that eye movements are critical for memory recall

Is there more to memory than meets the eye?

Picture the snack aisle of your local grocery store. Before you start planning your next meal, pay attention to what your eyes are doing as you bring up this mental image. You will likely notice that your eyes are moving around as you visualize different features of this scene. A new study published in Cerebral Cortex suggests that attempts to remember visual scenes benefit from the re-enactment of eye movement patterns.

Researchers from the Rotman Research Institute at Baycrest tracked the eye movements and brain activity of participants as they repeatedly viewed and later recalled complex visual scenes. They found that patterns of eye movement positively correlated with more vivid memorization and recall.

Specifically, eye movement patterns during recall represented a similar but condensed version of eye movement during original observation. To put that in grocery aisle terms: when you picture the chip section of your local snack shack, your eyes replay the critical motions that occurred when you initially scanned the shelf.

The phenomenon was first proposed in 1949 by Donald Hebb, a renowned Canadian psychologist who was influential in the field of neuropsychology.

Hebb suggested that we engage with mental images the same way we engage with the objects of our perception. We use our eyes to shift attention to different features while piecing together a coherent picture.

Before you start frantically shifting your eyes in an attempt to retrieve answers on your next exam, there are two things to note.

First, it remains unclear whether these results extend to text-based memory retrieval. “[Although] our results would be expected to extend to text-based memories if the text is memorized as part of an image of the page or screen… we have no direct evidence to support this conjecture,” wrote Michael Bone, lead author and U of T PhD student, in an email to The Varsity.

Second, it is unclear whether a deliberate eye movement can facilitate memory. “The current study and most previous studies do not investigate deliberate fixation reinstatement,” said Bone. “The participants are generally unaware that they are reinstating their fixations during imagery, and they are not instructed to do so… We have a study coming up that will directly address the causal question.”

Nevertheless, these results have great practical implications for memory assessment. Because scene-specific eye movements emerge during visualization, this motion can be used as a proxy for neural activity and memory function in some contexts. Bone said that it is possible that easy-to-use and inexpensive eye-trackers could eventually replace expensive MRI machines.

“Eye tracking technology… could detect memory decline associated with the early stages of dementia based on eye-movement irregularities detected while driving, inform the user (after they have parked), and provide the option to send the relevant data directly to their physician,” wrote Bone.

With strong potential for clinical applications, this research is certainly worth keeping an eye on.