A familiar progression of chords blares out of your speakers as the red lights of the surrounding traffic fade into the memory of a dark stage illuminated by pulsing neon lights. You replace your current discomfort (horrendous traffic!) with the memory of the last concert you attended – reliving the percussive sensory experience and feeling the intensity of the vibrating sound waves. As you bust out the occasional air guitar move and tap out the beat on your dashboard, you are successfully retrieving a memory and reinstating a specific pattern of neural activity. This mental time travel enables you to escape the confinement of your surrounding environment and plunge into the memory of enjoyable past experiences.
To successfully retrieve an episodic memory (autobiographical memory for events and details that can be consciously recalled) an individual mentally jumps back in time to re-experience the event of interest. From our own experiences and behavioral data we know that human beings are quite adept at this form of memory retrieval. However, the neural correlates underlying this behavior have been more challenging to identify. Last week’s article featured a description of EEG (electroencephalogram) technology and its capacity to record neural responses to specific stimuli. In this post, we explore a team of researchers who used intracranial EEG (iEEG) to record the neural mechanisms employed during memory retrieval.
Researchers at Johns Hopkins were interested in using intracranial EEG to explore the timing of neural activity during retrieval of episodic memory. They set out to determine whether the retrieval process reactivates cortical representations faster than during the initial encoding phase. To answer this question, an experiment was designed using 18 males with medication-resistant epilepsy who had undergone surgery to place intracranial electrodes in their brains for monitoring seizures. The participants were evaluated on a verbal-paired association task. Participants studied approximately 223 word pairs and were asked to recall the word. The patients were able to perform this task with approximately 41% accuracy. During both the encoding and retrieval of information, the research team recorded the oscillatory power from five different frequency bands from each of the electrode locations.
The researchers were thus able to build a map of neural activity comparing the encoding and retrieval periods in order to compare the extent of reinstatement. When the words were correctly remembered, it was found that reinstatement of neural activity was greater than during incorrect trials. The team hypothesized that reinstatement during correct trials was due to neural activity “jumping back” in time to recreate the context when the information was encoded. Specifically, as demonstrated in Figure 2, it was determined that greater reinstatement for correct trials occurred in the inferior temporal lobes and the ventrolateral prefrontal cortex in both theta and high gamma frequency bands.
The importance of this research is that it demonstrates what happens during successful memory recall. During correct memory retrieval, the neural signal is recovered providing for the first time exciting evidence for the mental time travel that underlies episodic memory. The next time you slip back in time to a weekend adventure during a boring class you can thank high gamma activity and theta oscillatory activity in the temporal lobe.
Yaffe R.B., Srikanth Damera, Sridevi V. Sarma, Sara K. Inati & Kareem A. Zaghloul (2014). Reinstatement of distributed cortical oscillations occurs with precise spatiotemporal dynamics during successful memory retrieval, Proceedings of the National Academy of Sciences, 111 (52) 18727-18732. DOI: http://dx.doi.org/10.1073/pnas.1417017112