Abstract
The prefrontal cortex and hippocampus function in tight coordination during multiple cognitive processes. During spatial navigation, prefrontal neurons are linked to hippocampal theta oscillations, presumably in order to enhance communication. Hippocampal ripples have been suggested to reflect spatial memory processes. Whether prefrontal-hippocampal-interaction also takes place within the ripple band is unknown. This intracranial EEG study aimed to investigate whether ripple band coherences can also be used to show this communication. Twelve patients with epilepsy and intracranial EEG evaluation completed a virtual spatial navigation task. We calculated ordinary coherence between prefrontal and temporal electrodes during retrieval, re-encoding, and pre-task rest. Coherences were compared between the conditions via permutation testing. Additionally, ripples events were automatically detected and changes in occurrence rates were investigated excluding ripples on epileptic spikes. Ripple-band coherences yielded no general effect of the task on coherences across all patients. Furthermore, we did not find significant effects of task conditions on ripple rates. Subsequent analyses pointed to rather short periods of synchrony as opposed to general task-related changes in ripple-band coherence. Specifically designed tasks and adopted measures might be necessary in order to map these interactions in future studies.
Highlights
Throughout the last decades, EEG research has increasingly focused on frequencies above the gamma band range
While being most extensively researched in the context of epilepsy, where high frequency oscillations (HFOs) have been proposed as a possible surrogate marker for epileptogenesis (e.g., References [2,4,8]), in some brain regions HFOs have been observed in connection with cognitive processing, such as during sensorimotor [9,10,11], visual [12,13], or memory processes [14,15]
We neither found a significant increase in ripple occurrence rates from resting to the experimental task conditions, nor did we observe an overall effect of increased coherences within the suspected frequency bands
Summary
Throughout the last decades, EEG research has increasingly focused on frequencies above the gamma band range. The so-called high-frequency EEG components have predominantly gained interest as a surrogate marker for epileptogeneity (e.g., References [1,2,3]). In this context these oscillations have been studied extensively as distinctive events within the EEG signal, so-called high frequency oscillations (HFOs), defined as spontaneous oscillatory events of at least four periods in a frequency range from 80 to 500 Hz that distinctively stand out from the background signal [4]. Kunii et al [15]
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