Publication details
High frequency ripple oscillations in human memory encoding and recall
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| Year of publication | 2022 |
| Type | Conference abstract |
| Citation | |
| Description | High frequency ripple oscillations (HFO, 80 - 200 Hz) are induced by cognitive processing and likely represent underlying neuronal activity. These events can be directly recorded from the human brain of epileptic patients who undergo monitoring with implanted intracranial or subdural electrodes as part of their epilepsy diagnosis. In this study, we analyzed the occurrence of HFO across brain structures during word presentation and memory encoding tasks. We automatically detected HFO in intracranial EEG data sampled at 5 kHz from 7 patients who performed two cognitive tasks. In the word screening (WS) task 180 distinct nouns were presented in 5 pseudorandomly ordered trials (i.e. each word 5 times). During the free recall (FR) task the patients were asked to remember 12 words presented one at a time, and then to recall them in any order after a short distractor task with simple algebraic equations. 15 trials were performed by each patient (180 words in total matched with the WS task).To determine HFO active channels in performed tasks we binned the detected HFO into 100ms windows from the word presentation to one second after presentation and identified the channels where the number of detections was higher than mean+3*std of all channels. Channels where at least one time bin was identified were marked as active. To investigate whether the same brain sites are activated during WS task and the encoding phase of the FR task we compared the number of active channels in each anatomical structure between the two tasks. To further determine whether there are specific word responses in the individual channels and structures we compared individual time bins to the mean+3*std of the average HFO rate across all words. We then calculated the percentage of the presented words that showed HFO activation. To assess whether any brain structures are connected to memory encoding we calculated the proportion of recalled words during FR task that showed HFO response during the FR encoding phase. The most active channels during WS were found in the lingual gyrus (N=10), precuneus (N=9) and middle temporal gyrus (N=8). The same structures were active during FR encoding with additional increased activation in orbital gyrus (N=5). The highest proportion of HFO word responses during WS task was observed in supramarginal gyrus (61% of the presented words) and superior frontal gyrus (54% of the presented words). The same structures showed a high percentage of word HFO activation of words that were subsequently recalled during the FR task. Word presentation and encoding show ripple activation in brain areas responsible for visual and word processing, language and memory. |