ISSN : 1226-9654
파블로프 공포 조건화의 소거에 영향을 미치는 여러 요인 중 학습-소거간 간격의 효과에 대한 연구가 주목을 받아왔다. 특히 학습-소거 간 간격이 짧을 경우 소거 학습에서의 결함, 즉 즉각소거 결함(immediate extinction deficit; IED)이 일어난다. 한편, IED를 조절하는 요인들 특히, 무조건 자극(unconditioned stimulus; US)의 강도에 관한 연구는 초기 단계이다. 본 연구에서는 US 강도에 따른 IED 효과를 검증하고자 소거된 반응이 다른 맥락에서 다시 나타나는 재발(renewal)현상을 중심으로 실험 결과를 얻었다. 실험 1에서, 실험동물들은 소리 조건 자극(conditioned stimulus: CS)과 강한(1mA) 발바닥 전기 충격 무조건 자극(unconditioned stimulus: US)으로 공포 조건화 학습을 받았다. 이후, 실험동물들은 소거학습 시점 및 자극 제시 여부에 따라 4집단으로 나뉘었다: 즉각소거 집단(immediate extinction; IE), 지연소거 집단(delayed extinction; DE), 즉각무소거 집단(immediate no-extinction; IE_NO), 지연무소거 집단(delayed no-extinction; DE_NO). IE 집단과 DE 집단은 공포 조건화 학습 후, 10분 혹은 24시간 후에 새로운 맥락에서 CS에 30차례 노출되었고 IE_NO 집단과 DE_NO 집단은 CS없이 맥락에만 노출되었다. 공포조건화 학습 48시간 후 CS에 대한 공포 반응을 측정하는 보유(retention test)검사를 받았다. 보유 검사를 받은 지 24시간 후, 맥락을 바꾸어 CS에 대한 공포반응을 측정하는 재발검사를 받았다. 실험 결과 오직 DE 집단만이 공포의 소거 및 재발을 보여주었고 IE 집단에서는 소거가 이루어지지 않았다. 실험 2에서는 실험 1과 동일한 실험 패러다임을 사용하되 약한 US(0.4mA)를 사용하였다. 실험1에서의 결과와 다르게, IE 집단과 DE 집단 모두 새로운 환경에서 CS에 대한 공포기억의 소거가 이루어졌다. 흥미롭게도 DE 집단은 소거된 공포의 재발을 보였으나 IE 집단은 소거된 공포의 재발이 나타나지 않았다. 즉각소거 후 재발결함(immediate extinction renewal deficit; IRD)이 나타났다. 이 결과는 IRD는 US의 강도에 따라 소거 기억이 저장되는 신경생물학적 메커니즘이 다를 수 있음을 시사한다.
Impaired extinction following fear conditioning in the immediate past, dubbed immediate extinction deficit (IED) has been repeatedly demonstrated in animal models. However, whether IED is a universal phenomenon across all different intensities of the US is unknown. In the current experiment, we tested the effect of strong vs. mild footshock US during acquisition on the subsequent extinction, retention and renewal. In Exp.1, rats were subjected to Pavlovian fear conditioning with a tone conditioned stimulus (CS, 5kHz, 80dB, ITI between 45~75sec) and footshock unconditioned stimulus (US, 1.0mA). Then they were divided into four groups: immediate extinction (IE); delayed extinction (DE); immediate no-extinction (IE_NO); delayed no-extinction (DE_NO). IE and DE received 30 CS-only trials 10 min or 24 hr after the conditioning, respectively. IE_NO and DE_NO received the same treatment except for the CS. Twenty-four hours later, they all received retention test which was composed of 10 trials of CS-only trials. On the next day, they received renewal test in a different context and received 10 trials of CS-only trials. Consistent with previous studies, Only DE showed reliable extinction and renewal. IE showed extinction deficit. In Exp.2, the same experimental protocol was employed except for the shock intensity (0.4mA). In contrast to the results from Exp.1, both IE and DE showed a reliable extinction. However, only DE showed renewal effect. Taken together, the current result suggests that extinction process (consolidation of extinction memory) following Pavlovian conditioning with weak shock might involve different neural mechanism from that with strong shock.
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