Effects of evoked spike-wave discharges upon short term memory in patients with epilepsy

Memory for digits was examined during EEG examination in five female children suffering from light sensitive epilepsy. Sequences of random digits equal to the patients' digit span, span-less-one and span-less-two were presented, followed by a 2 sec. interval, after which recall of the digits was required. The 2 sec. interval could be filled in any of four possible ways by: a burst of photic stimulation which evoked a paroxysm of generalised spike-wave in the EEG; a burst of slow (3 Hz) photic stimulation having no discernible effect upon the EEG; a burst of fast (30 Hz) stimulation, again having no observable effect; or a period of silence with no photic stimulation. It was found that recall of digits series of all three lengths was impaired if a paroxysm of spike-wave had been generated between the end of presentation and the beginning of recall. There were no significant differences in correctness of recall when the slow, fast and no-stimulation photic conditions were compared. It was found that the last two digits in the digit span were more vulnerable to recall failure than were the first two. On the basis of recent work in human experimental psychology, the initial and final digit pairs appear to be retrieved from functionally different storage systems. It is tentatively suggested therefore, that since different parts of the digit span are dislocated to different extents by spike-wave activity, they may depend upon different neural substrates.