Utilizing state-of-the-art brain imaging technology, I seek to understand how the brain encodes, consolidates, and retrieves information it encounters
Memory ability differs from person to person.
Even within a person, we have varying abilities across different types of memory -- memory for events in our lives (episodic), memory for facts (semantic), or memory for the layout of a route in a new city (spatial).
These differences in abilities can be reflected in the structure and function of neural substrates that support these forms of memory.
I am interested in determining what brain regions relate to these types of memory and how they do it.
In our article Adnan, Barnett, Moayedi et al (2016) we subdivided the hippocampus of healthy individuals using a k-means clustering algorithm based on the shared structural connectivity of each portion. Structural connectivity was measured with diffusion tensor imaging, and we discovered that the hippocampus could reliably be subdivided into an anterior and posterior division. Across subjects, we found that people who had stronger functional connectivity between their posterior hippocampus and the precuneus, also showed a stronger benefit to recognition memory when the original context was reinstated. Given that the precuneus has been implicated in representing contextual information, this implies that better communication between the precuneus and hippocampus may facilitate recovery of prior experiences by allowing the hippocampus to leverage the contextual representations from the precuneus. One might therefore hypothesize that the cingulum bundle or fornix -- given their role in connecting these regions -- may of critical importance in memory for source information.
The hippocampus is often the site of seizure generation in temporal lobe epilepsy (TLE)
These seizures spread throughout the brain and may have a lasting impact on how the brain communicates
Changes in communication may contribute to common memory complaints in this population
In Barnett, Man & McAndrews (2019) we used resting functional connectivity to segment the hippocampus into its anterior and posterior components using k-means clustering in people with temporal lobe epilepsy, and also with a group of healthy control participants. While segmentation was successful in both people with epilepsy and controls, people with epilepsy had reduced connectivity to the posterior cingulate/precuneus, and medial prefrontal cortex — primary hubs of the default mode network. What’s more, those who had the weakest connections to these hubs, also had poorer memory ability.
My PhD work focused largely on understanding how brain activity and structure relates to language and memory in people with temporal lobe epilepsy (TLE).
For some people with TLE, seizure activity can result in poorer than normal memory and naming ability.
Surgery to remove the seizure generating tissue is often an effective treatment when medication fails to control seizures, but can sometimes lead to further deficits.
Much of my work was directed at understanding the brain patterns related to these cognitive abilities and finding which patterns were related to greater risk following surgery.
Following surgery in the language dominant temporal lobe, a subset of people will experience a deficit in their naming ability. In Audrain, Barnett & McAndrews (2018), we found that the pattern of language network connectivity at rest preoperatively was related to naming ability following surgery. Specifically, people with abnormal patterns of network connectivity, compared to a healthy control template network, were at greatest risk of postoperative naming decline. By examining these networks in patients, clinicians may be able to provide more informative information and counselling for surgical candidates
Further reading on this topic:
Barnett, A. J., Park, M. T. M., Pipitone, J., Chakravarty, M. M., & McAndrews, M. P. (2015). Functional and structural correlates of memory in patients with mesial temporal lobe epilepsy. Frontiers in neurology, 6, 103.
McCormick, C., Protzner, A. B., Barnett, A. J., Cohn, M., Valiante, T. A., & McAndrews, M. P. (2014). Linking DMN connectivity to episodic memory capacity: what can we learn from patients with medial temporal lobe damage?. NeuroImage: Clinical, 5, 188-196.
The hippocampus is a critical brain structure for forming memories.
The cells here track where an organism is in space and are also sensitive to temporal information.
These sensitivities are thought to reflect some sort of mapping that allows for memory encoding and retrieval of context and events.
In Barnett et al. (2014), we examined whether the hippocampus was sensitive not only to temporal order information, but also temporal duration information.
We showed participants a sequence of 4 scenes, which we call events, and instructed participants to remember the sequences of these events. Each of the 4 events was displayed for a different duration of time. Following a brief delay of 3.5 seconds, we showed them the same 4 events again. This time when they saw the events, we had either changed the presentation order of the events, changed the duration of time each event was presented, changed the duration of the intervals in between the events, or left the events completely the same. The participants were asked to indicate whether the sequence had changed in ANY way or whether it was the same.
The participants were very accurate at detecting changes, though they were most accurate for detecting changes in the presentation order. When a mismatch was detected (compared to match) in the sequence of the events, the hippocampus showed an increase in activity. When a mismatch was detected for the duration information, the hippocampus showed a depression of activity.
During these duration mismatches, the hippocampus was showing increased connectivity to timing regions such as the cerebellum, caudate and pre-SMA, but also to areas thought to represent context, like the posterior cingulate cortex.
Thus, the hippocampus is sensitive to both order and duration, which may help it in its role to map contexts for memory encoding and retrieval.