The medial temporal lobe (MTL) encodes and recalls memories and can be a predominant site for interictal spikes (IS) in patients with focal epilepsy. It is unclear if memory deficits are due to IS in the MTL producing a transient decline. Here, we investigated whether IS in the MTL subregions and lateral temporal cortex impact episodic memory encoding and recall.
78 participants undergoing presurgical evaluation for medically refractory focal epilepsy with depth electrodes placed in the temporal lobe participated in a verbal free recall task. IS were manually annotated during the pre-encoding, encoding and recall epochs. We examined the effect of IS on word recall using mixed-effects logistic regression.
IS in the left hippocampus (OR:0.73, CI:0.63-0.84, p < 0.001) and left middle temporal gyrus (OR:0.46, CI:0.27-0.78, p < 0.05) during word-encoding decreased subsequent recall performance. Within the left hippocampus, this effect was specific for area CA1 (OR:0.76, CI:0.66-0.88, p < 0.01) and dentate gyrus (OR:0.74, CI:0.62-0.89, p < 0.05). IS in other MTL subregions or inferior and superior temporal gyrus and IS occurring during the prestimulus window did not affect word encoding (p > 0.05). IS during retrieval in right hippocampal (OR:0.22, CI:0.08-0.63, p = 0.01) and parahippocampal regions (OR:0.24, CI:0.07-0.8, p < 0.05), reduced the probability of recalling a word.
Interictal spikes in medial and lateral temporal cortex contribute to transient memory decline during verbal episodic memory.