시각적 탐색에서 표적에 의해 유발된 N2pc 성분의 특성 및 측정
The Properties and Measurements of N2pc componentEvoked by A Target in Visual Search
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본 논문은 최근 시각적 주의 연구에서 자주 사용되는 사건관련전위(Event-Related Potentials, ERPs)인 N2pc (N2 post-contralateral) 성분 측정 과정에서 반드시 고려되어야 할 유의점과 그 이유를 설명하였다. 이를 위해 시각 탐색 과제(visual search)를 사용한 주의 전환(attentional shift) 과정에 대한 연구에서 사용되는 N2pc 측정법을 개괄하고, 측정 과정에서 발생할 수 있는 문제점에 대한 해결책을 제안하였다. N2pc 성분의 유무는 외측 표적(lateralized target)을 포함한 시각 탐색 과제에서 탐색 자극 제시 후 약 200ms후에, 표적이 속한 시야의 대측(contralateral-to-target) 두피에 위치한 전극에서 측정된 전위가 동측 (ipsilateral-to-target)에 비해 음극화되는 정도에 근거한다. 일반적으로 표적으로 주의가 전환되는 시점에 대측과 동측의 전위 차이는 가장 현저해지며, 이는 N2pc의 진폭 측정뿐 아니라 발현 시점에 대한 체계적인 조사를 필요로 한다. 비교적 진폭이 작고 발현시간이 짧은 N2pc의 특성상 주의 전환 시점의 정확한 측정을 위해서는 높은 수준의 신호-대-잡음 비율(signal-to-noise ratio)이 필요한데, 이는 충분한 반복 시행의 확보와 측정 당시의 여러 잡음을 최소화시킴으로써 달성될 수 있다. 표적 방향으로의 안구 운동은 N2pc 파형에 큰 영향을 미칠 수 있는데 안구 운동 시행은 교정 보다는 제거를, 그리고 외측화된 표적으로 인한 외측 대기 전위(lateralized readiness potentials, LRPs)의 영향을 최소화하기 위해서는 한손만을 사용하되 반응에 사용되는 양손을 구획별로 바꿔주는 역균형화가 권장된다.
Abstract
This review aims to specify problems and to explain the reason why they occur when measuring event-related potentials (ERPs)--N2pc(N2 post-contralateral) component often used in recent studies of visual attention. For these purposes, an overview of N2pc measurement technique to observe an attention shift in visual search was provided, and several solutions for the problems were discussed. Presence of N2pc component is determined according to a more negative-going potential in EEG amplitude measured from the electrodes that are contralateral to the target than those ipsilateral, elicited about 200ms after the search array onset. Generally, the amplitude difference between the contra- and ipsilateral electrodes becomes maximal when attention shifts to the target location, and the moment of this attention shift can be sought by systematic tests of the amplitude and latency of N2pc component. N2pc measurement requires high signal-to-noise ratio, and this can be accomplished by minimizing any noise occurring when EEGs are initially recorded online. Eye movements to a target position can exaggerate the N2pc contra-ipsi difference and thus trials with saccades toward the target position are typically rejected rather than corrected. To minimize lateralized readiness potentials (LRPs) possibly evoked by any lateralized search target, it is recommended to use only one hand for responses while switching the hand across blocks to counterbalance handedness.
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