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.

Blind as a bat

Visual constraints were necessary for the evolution of echolocation in bats

Blind as a bat

 

Believe it or not, Batman was not the first winged creature to use echolocation to hunt down his prey — ancestors of the modern bat began using echolocation, the process in which objects are located by reflected sound, between 65 million and 85 million years ago.

John Ratcliffe, an Assistant Professor of Biology at UTM, and his team used molecular phylogeny techniques to decode the evolution of echolocation in bats, reconstructing our perspective on how and why these creatures came to be sonar crusaders of the night.

Their study, recently published in Nature Communications, has built on an existing phylogenetic study to reveal that bats evolved advanced biosonar capabilities through ancient tradeoffs between vision and echolocation.

Bats are the second largest order of mammals in the world, with more than 1,300 species. The bulk of these species are carnivorous and nocturnal, hunting small insects in the night using echolocation.

The researchers used phylogenetic comparative methods to understand evolutionary relationships between sensory systems, neuroanatomy, and morphology in bats. The results of the study indicate that ancestral bats possessed insufficient visual capabilities to hunt small prey at night but had the neuroanatomical potential to echolocate. This proved that there was a pre-existing opportunity for echolocation to flourish into an advanced sensory system in future descendants.

Evolutionarily, bats have continuously exhibited processes of behavioural tradeoff: species with poor echolocation abilities tend to have better visual resolution, and vice versa.

They further found that modern bats that use echolocation of the weakest frequency had the best vision out of those in all other sensory frequency groups, demonstrating that this tradeoff can function in either direction. The non-echolocating pteropodid is an example of a family with this tradeoff.

Surprisingly, the pteropodids have auditory brain regions the same size as echolocators. This vestige confirmed that bats from this family were once capable of echolocating but have since lost the trait. This is in contrast to the alternative scenario that proposes they evolved independently of echolocators — instead they are descendants of the same group.

This study not only answers questions about the evolution of a key trait of one of the most diverse mammals in the world, but it also cements the importance of phylogenetic research.

“It’s like replaying the video of evolution under different scenarios,” said Jeneni Thiagavel, the primary author of the paper, who has been working in this area of research for the past four years. “That essentially gives users [of these comparative methods] the power to reconstruct what that was like.”

Love makes scents

UTSC professor’s study finds ring-tail lemurs use ‘stink-flirting’ to attract mates

Love makes scents

If you think the smell of Axe is bad, be glad you’re not a lemur. In a study led by Amber D. Walker-Bolton, a UTSC professor in the Department of Anthropology, researchers found that male ring-tailed lemurs use their ‘stink’ to impress potential mates.

This unique behaviour allows lemurs to display their rank among other males and attract suitable mates, albeit at a cost.

Lemurs belong to the Strepsirrhine sub order and live in male groups that have a core female lineage. Each of these groups have dominant central males and periphery males, where rank is correlated with age.

‘Stink-flirting’ refers to male display of tail anointing and wafting, which is considered “a submissive display prior to a close approach.” The study found these displays are associated with male dominance and that they are reciprocated by females. Females preferred the exaggerated displays and are said to set the male apart from the rest of the population.

Additionally, the study found females showed a preference for dominant resident males as opposed to lower-ranking immigrant males. Immigrants are rarely found mating with females of the group.

Surprisingly, male lemurs are more frequently faced with aggression from both females and other males when they perform stink-flirting displays compared to other mating rituals. Only when females in estrus were receiving the olfactory display would they then mate.

Some of the females are also mate-guarded by a male. These guarded females were found to receive a higher rate of displays than non-guarded females. Although mate-guarding doesn’t completely eliminate displays from other males, it hinders approach to guarded females.

While the majority of females chose the most dominant mate, the opposite was not the case. According to the study, male ring-tailed lemurs did not “preferentially target high-ranked females for olfactory displays.”

In the future, Walker-Bolton’s team hopes to study the correlation between ‘stink-flirting’ and reproductive success.