[New protocol for memory-related potential]

A new P300 testing protocol for evaluating memory function is proposed. Four stimuli (S1, S2, S3 and S4) are presented to the subject at 1.5 sec intervals and the subject is instructed to judge whether S4 was the same (80%) or not (20%) as S1, as well as whether S3 was the same (80%) or not (20%) as S2. The S2-S3 comparison acts as an attention shifting mechanism, and by inserting this comparison between S1 and S4, the subject must memorized S1 at the memory site in the brain until S4 is presented. This protocol was performed using both auditory and visual stimulations with 24 college students (age range: 19-28 years old) and a patient (23 years old) with bilateral medial temporal lobe lesions caused by limbic encephalitis. The P300 brain potentials after S3 and S4 were compared. The Wechsler memory scale-revised (WMS-R) test was also assessed in all subjects. The latency of P300 after S4 had a significantly (p < 0.01) longer than that after S3. The latency of P300 after S4 in the patient, who showed the severe auditory as well as the mild visual memory impairment, was longer than the mean + 2 SD in the normal subjects for auditory stimuli, but not for visual stimuli, whereas the latency of P300 after S3 in the patient was longer than the mean + 2 SD in the normal subjects only for visual stimuli. The brain potential after S4 might be more reflective of a degree of memory disorder as compared with that after S3. Furthermore, the latency of P300 after S4 showed a more significant correlation with some WMS-R scores (verbal memory, general memory and verbal delay) than the latency of P300 after S3. These results suggest that measuring the P300 latency after attention shifting in the new protocol is useful for the evaluation of memory functions.