Abstracts

Rationale: Prior studies found an association between disruptions of cognitive task performance and the presence of focal or generalized interictal epileptiform discharges (IEDs). Memory processes may be particularly vulnerable to IED effects. In an animal model, hippocampal spikes resulted in impaired spatial memory and object recognition compared to controls without epileptiform activity¹. In humans, IEDs disrupted verbal and non-verbal short term² and long term^3,4 memory, and correlated with accelerated rates of long term forgetting³. We propose an effect of IED timing, in that IEDs during encoding will disrupt memory formation, leading to impaired recognition. Methods: Subjects included adults diagnosed with temporal lobe epilepsy undergoing hippocampal depth, foramen ovale or subtemporal strip electrode placement for clinical purposes. During EEG recording, participants completed tests of verbal and non-verbal memory. In an encoding block, subjects were shown 50 stimuli (words or faces), presented one at a time on a computer screen. After a 5 minute delay with an intervening working memory (n-back) task, the subject s recognition of the stimulus items was tested. Recognition testing was administered in a two-choice format. IEDs were identified by manual review. Mixed model logistic regression was used to assess effects of the presence and laterality of IEDs during individual encoding trials on subsequent recognition of the stimuli. Results: Seven subjects (2 female; 6 right-handed, 1 ambidextrous) of mean age 31 years (range 23-42 years) participated. Mean duration of epilepsy was 20 years (range 4-41 years). Seizures were of left-sided onset in 4 subjects, right-sided in 1 subject and bilateral in 2 subjects. No seizures occurred within 6 hours of testing. IED's were associated with very slightly higher error rates for faces (33% vs. 32%) and lower rates of errors for words (16% vs. 27%), however these differences were not statistically significant. No significant effect was evident for laterality of IEDs. Conclusions: The present data demonstrated no significant relationship between the occurrence of IEDs during encoding of stimuli and later recognition of the stimulus items, nor any effect of IED laterality. This result may be contrasted with prior findings indicating a disruptive effect of IEDs on encoding². It is possible that certain characteristics of IEDs are necessary for disruption to occur. These necessary features may relate to discharge duration, spatial extent, timing relative to stimulus onset or presence of embedded high frequency components. It is also possible that the effect of IEDs depends upon the underlying functional and structural integrity of the hippocampus. This sample, with a long duration of epilepsy, may have impaired function of the hippocampus at baseline, such that the added disruptive effect of IEDs on encoding would be minimal. Further study of these variables in a larger sample will be necessary to determine in more detail the effects of IEDs on memory formation. References: ¹Epilepsy Behav 2006;9:549-56 ²Brain 1984;107:293-308 ³Epilepsy Behav 2006;8:278-88 ⁴Epilepsia 2008;49:136-7