N1-P2 Evoked Response as a Measure for Short-Term Visual Memory R. Palaniappan and P.Raveendran Dept. of Electrical and Telecommunication Engineering Faculty University of Malaya 50603 Kuala Lumpur Malaysia psar/ravee@fk.um.edu.my Abstract We investigate the relationship of N1-P2 evoked response (peak-to-peak amplitude and time interval) with short-term visual memory in humans. Visual evoked responses obtained from 20 subjects (10 non- amnesic alcoholics and 10 non-alcoholics) are extracted from channel P8 referenced to channel Cz during the presentation of modified delayed matching-to-sample visual task using Snodgrass and Vanderwart picture set. Our results indicate that N1- P2 amplitudes are higher for non-matching (novel) stimuli as compared to matching stimuli for all the subjects. N1-P2 time interval is also shorter for the case of matching stimuli. This indicates that information processing is increased for the non- matching stimuli as compared to matching stimuli. These results are quite consistent with a number of related studies and we conclude that N1-P2 is related to short-term visual memory involved during object recognition. The results also indicate that N1-P2 amplitude is higher for non-alcoholics as compared to alcoholics, which indicates that some form of memory impairment exist in alcoholics. 1. Introduction The general mechanism underlying the neuronal behavior that relates vision and memory is yet to be fully understood [1]. However, low frequency component analyses have identified the occipito- temporal cortex to be involved in memory process of humans [1, 11]. These studies use Visual Evoked Potential (VEP) extracted from subjects performing a modified delayed matching-to-sample task and the results indicate that a positive component around 170 to 240 ms [11] and 220 to 260 ms [1] after stimulus onset can serve as a visual memory potential. Higher frequency analysis (>20 Hz) has shown that occipitotemporal and frontal electrodes are involved in the memory process [10]. In this paper, VEP is extracted from alcoholic and non-alcoholic subjects while performing modified delayed matching-to-sample paradigm using Snodgrass and Vanderwart [9] picture set. We investigate the possibility of using N1-P2 peak-to- peak amplitude and time interval as a measure for short-term visual memory in humans during object recognition. The evoked response is extracted from channel P8 following studies by Begleiter et. al. [1] and Zhang et. al. [11]. In our analysis, N1 is the first negative peak evoked around 180 ms (±20 ms) after stimulus onset, while P2 is the positive peak immediately following N1. N1 latency to auditory response has been shown to be around 100 ms while for the flash evoked potential, it is around 130 ms [3]. Longer response time of N1 in our case is most likely due to the different stimulus i.e. visual object in our case. Although positive component similar to P2 has been suggested as a measure of short-term memory potential [1,11], the inclusion of N1 is preferable for the following reason. N1 can be thought as the start of the stimulus response. Therefore, the time interval and amplitude difference between N1-P2 peaks will be a better representative of the short-term memory. The analysis of N1-P2 response is extended to study the short-term memory impairment in alcoholics. 2. Method Twenty subjects, 10 non-amnesic alcoholics and 10 non-alcoholics participated in the experimental study. The alcoholics tested have been abstinent for a minimum period of one month and are also off all medications for the same period of time. The non- alcoholics are not alcohol or substance abusers. The