The Role of Neural Oscillations in Episodic Memory Formation

Post by Elisa Guma

What's the science?

Episodic memory is a category of long-term memory that involves the recollection of specific events, situations, and experiences, typically anchored to a specific time and space. Two distinct cognitive processes, with two distinct types of neural activity, are associated with this type of memory. First one must process a vast amount of sensory information about an event; next, these representations must be bound together to form a unique memory trace. It has been hypothesized that brain activity from the neocortex (in the alpha/beta oscillations) supports the prior (processing sensory information), while activity in the hippocampus (theta/gamma oscillations) supports the latter (mnemonic binding). This week in NeuroImage, Griffiths and colleagues set out to test this hypothesis by recording brain activity during an episodic memory task in healthy young adults.

How did they do it?

Seventeen participants performed a visual association memory task while fluctuations in brain activity were recorded using magnetoencephalography (MEG). During the encoding phase, participants were presented with a line drawing of an object (ex: a giraffe), a pattern (ex: blue background with orange dots), and a scene (ex: a train). Following a short interval, participants were given a short interval to create a mental image fusing all three stimuli (i.e., mnemonic binding) to help them recall this for a later memory test (ex: a blue and orange giraffe on a train). After associating 48 triads, and performing a distractor task, participants performed the retrieval task. Here, participants were presented either a line drawing or a scene and asked to recall the mental image they had made during encoding and asked to identify the pattern associated with the line drawing. Further, participants had to rate how confident they were in their choice (‘guess’, ‘unsure’, ‘certain’). For each trial, memory performance was coded as either ‘complete’ (remembered both the scene and pattern), ‘partial’ (remembered only one of the associations), or ‘forgotten’ (remembered neither scene or pattern).

The MEG data, which provides excellent temporal specificity compared to other brain imaging modalities, was preprocessed and corrected for potential motion (e.g. head movement) artifacts. Brain oscillations occurring at different wavelengths (alpha, beta, gamma, theta) were extracted from the neocortex and the hippocampus in order to test whether brain activity was associated with the following aspects of the task: (1) number of items recalled, (2) whether the scene was recalled, (3) whether the pattern was recalled, (4) the change in head position. Most importantly, the authors investigated whether alpha/beta power decreased with stimuli presentation and whether theta/gamma coupling (i.e. do the peaks and troughs of these two power spectra align) increased during mnemonic binding. The authors estimated the relationship between power (i.e. alpha, beta, gamma, theta) and the number of items recalled by participants (tested for significance using cluster-based permutation testing).

What did they find?

On average, participants correctly recalled both the associated pattern and associated scene on 38.3% of trials, recalled only one associated stimulus on 34.4% of trials, and failed to recall an associated stimulus on 27.3% of trials.

Next, the authors found that decreased alpha/beta power was associated with better memory performance noted by an increased number of items recalled. In the brain, this was localized to the bilateral occipital regions. Furthermore, no such relationship was observed with the mnemonic binding phase of the task, or with gamma/theta power, which indicates that this relationship is specific to the neocortex alpha/beta power, and sensory integration. The authors also found that hippocampal theta/gamma phase-amplitude coupling was indeed related to mnemonic binding; the number of items recalled scaled with the degree of coupling of theta/gamma power. This relationship was not observed for the sequence perception phase of the task, nor for other brain regions. 

What's the impact?

These data presented here suggest that memory-related decreases in neocortical alpha/beta power and memory-related increases in hippocampal theta/gamma phase-amplitude coupling arise at distinct stages of memory formation, with the former supporting information representation and the latter supporting mnemonic binding. This does not suggest that these two processes can occur completely independently, as mnemonic binding cannot occur without relevant perceived information. However, it does provide a deeper understanding of how two distinct cognitive processes are associated with distinct neural phenomena to create an episodic memory. Future work may investigate the integrity of these neural processes in the brains of individuals suffering from memory dysfunction. 

Griffiths BJ et al. Disentangling neocortical alpha/beta and hippocampal/theta/gamma oscillations in human episodic memory function. Neuroimage (2021). Access the original scientific publication here.