Comparison of implicit memory encoding paradigms for the activation of mediotemporal structures

The medial temporal lobes (MTLs) are essential for both encoding and retrieval processes in declarative memory. In addition, they are a frequent seizure focus for medically refractory epilepsy. One of the major side effects of MTL resection is a decline in memory functions. Most functional imaging paradigms have been developed to find preoperative measures that, to obtain a prognosis of postoperative memory performance, employ explicit memory encoding strategies to elicit MTL activation, and require a great amount of cognitive effort. We applied three different implicit encoding tasks, which require less effort and time, to a group of healthy subjects. We found left-lateralized activation for verbal stimuli, bilateral activation for pictures, and right-lateralized activation for faces. The present study shows that even with an implicit memory-encoding paradigm, a lateralized activation of MTL structures can be achieved. This may lead to paradigms for routine clinical application that require less cognitive effort and time on the part of patients.