In addition to the spatial scale, differences also exist between the populations being studied with intracranial (patients with epilepsy) vs. scalp EEG/MEG. Although differences in study population could theoretically contribute to observed differences in the neural correlates of memory performance, this appears unlikely for several reasons: First, the memory contrasts reveal within-subject differences that should control for differences between subjects that are independent of memory processes. Second, the neural correlates of memory in these subjects appear in tissue far from the epileptic focus and absent any evidence of interictal spikes or sharp waves [41]. Nevertheless, more studies are needed that provide direct comparisons between epileptic and non-epileptic patients [32,35] or that record intracranial and scalp EEG in the same set of patients.
