Abstracts

Rationale: Slow oscillatory activity in the 3-10 Hz range in the hippocampus and the neocortex are known to play a role in the encoding of new memories and long-term recall. Intracranial electrode field potential recordings provide the temporal and spatial resolution for studying these oscillations in humans during cognitive function. In humans, theta (3-12 Hz) oscillations have been clearly observed in many structures, including the hippocampus during memory encoding [1] and virtual maze navigation [2]. We characterized the encoding of memory signals at multiple temporal scales and different locations in patients with epilepsy and show selective modulation of theta activity for successfully recalled memories. This effort lays the groundwork for diagnostic memory testing in epilepsy patients based on quantifying a physiologic brain rhythm. Methods: Epileptic patients implanted with subdural electrodes for seizure localization were tested on a multi-item short-term memory task. The subjects were presented with a series of four images on a computer screen. After a 1-second delay, the patients were instructed to arrange images in the order in which they appeared using the buttons on a key-pad. Results: Medial temporal electrodes showed transient increases in power in the 3-10 Hz band during the task, and these modulations were spatially localized to the temporal and prefrontal cortices. Furthermore, this increase in power in temporal lobe electrodes was significant during the correctly recalled trials (p<0.01, Wilcoxon's rank sum test, with respect to pre-task baseline, Figure 1A), while in the incorrect trials power dropped to levels pre-trial (Figure 1B). Conclusions: These results suggest a role of slow oscillations in the encoding of successfully recalled memories. This spatially and temporally selective increase of theta power in the temporal structures during successful encoding of new information may be related to hippocampal theta oscillations implicated in memory formation [3]. This physiologically based quantitative assay of memory function will now be probed for utility in predicting postoperative memory outcomes in patients undergoing seizure surgery. References 1. U. Rutishauser, I.B. Ross, A.N. Mamelak, E.M. Schuman (2010), Human memory strength is predicted by theta-frequency phase-locking of single neurons. Nature (464), 903-907. 2. M.J. Kahana, R. Sekuler, J.B. Caplan, M.Kirschen, J.R. Madsen (1999), Human theta oscillations exhibit task dependence during virtual maze navigation. Nature (399), 781-784. 3. G. Buzsaki (2002), Theta oscillations in the hippocampus. Neuron, 33(3), 325-340. Figure 1: Temporal lobe theta activity (3-10Hz) is related to task outcome. Data from an electrode on the temporal pole. A. Temporal evolution of theta for correct trials. Theta power during encoding and delay epochs was significantly higher than pre-trial baseline for successful trials (p<0.01, rank sum test). B. Temporal evolution of theta for incorrect trials. Theta power during encoding and delay epochs was comparable to baseline (p>0.01).