Effects of inter-item lag on recognition memory in seizure patients preceding temporal lobe resection: evidence from event-related potentials.

The electrophysiological correlates of word recognition are well characterized. Repeated 'old' words evoke a more positive-going waveform starting at approximately 300 ms compared with first-presented, 'new' words. The old/new effect is thought to be generated, in part, by structures within the medial temporal lobe. In the present study, event-related potentials were recorded during a continuous verbal recognition memory task in unoperated patients with either left (L) or right (R) unilateral temporal lobe epilepsy (Epil) and neurologically intact controls. To manipulate the difficulty of the memory task, the lag between the initial and subsequent presentation of the repeated words was varied from one, four to 16 items. In the controls, ERPs to old words were more positive going than new words from approximately 350-650 ms. The old/new effect diminished as the inter-item lag increased. Patient old/new effects showed a later onset (450 ms) and resolution (750 ms) compared with the controls. Furthermore, the late component of the old/new effect was significantly reduced in the L Epil. Although patient behavioral performance did not differ significantly from that of the controls, neuropsychological testing revealed impaired verbal memory function in the L Epil patients. It is concluded that the reduced old/new effect in the L Epil patients provides evidence that medial temporal lobe structures contribute to the scalp-recorded old/new effect.