letters to nature Same probe a Correspondence and requests for materials should be addressed to N J D e mail njdominy hkusua hku hk 95 85 75 65 Suppress Respond 0 1 8 16 Number of repetitions c Michael C Anderson Collin Green Department of Psychology University of Oregon Eugene Oregon 97403 1227 USA 95 85 75 65 Suppress Respond 0 1 8 16 Number of repetitions d Per cent recalled Suppressing unwanted memories by executive control Independent probe b Per cent recalled We thank D Osorio for help with colour registration E Ting P Y Cheng I C Bruce R T Corlett L Ramsden N Yamashita and A Walker for comments P Kagoro B Balyeganira and M Musana for eld assistance in Uganda J Magnay R W Wrangham and C A Chapman for logistic support in Uganda and the Ugandan National Council for Science and Technology Ugandan Wildlife Authority and Makerere University Biological Field Station for permission to work at Kibale Supported by Research Grants Council of Hong Kong National Geographic Society Sigma Xi Explorer s Club and Croucher Foundation of Hong Kong Per cent recalled Acknowledgements proposed by Freud1 To test this hypothesis we adapted the go no go paradigm used to study executive control over motor actions in primates18 and humans15 17 for use in a memory retrieval task First we trained subjects on 40 unrelated word pairs for example ordeal roach so that they could recall the right hand member of each pair when provided with the left hand member Next subjects performed a critical task requiring them to exert executive control over the retrieval process On each trial of this think no think task a cue from one of the pairs appeared on the computer screen Depending on which cue appeared subjects were told either to recall and say think about the associated response word respond pairs or not to think about the response suppression pairs For the latter pairs we emphasized that subjects should not allow the associated memory to enter consciousness at all If subjects accidentally responded to a suppression pair they heard a beep signalling an error To increase the need to recruit inhibitory control mechanisms we required subjects to xate on the cue word for the entire time 4 s that it appeared on the screen discouraging perceptual avoidance and generating a constant threat that the associated memory might intrude into consciousness Thus suppression trials required the stopping of both a prepotent motor Per cent recalled 19 Terborgh J Five New World Primates Princeton Univ Press Princeton 1983 20 Leighton M Modeling dietary selectivity by Bornean orangutans evidence for integration of multiple criteria in fruit selection Int J Primatol 14 257 313 1993 21 Gautier Hion A et al Fruit characters as a basis of fruit choice and seed dispersal in a tropical forest vertebrate community Oecologia 65 324 337 1985 22 Davies A G Oates J F in Colobine Monkeys eds Davies A G Oates J F 229 249 Cambridge Univ Press Cambridge 1994 23 Wrangham R W Conklin Brittain N L Hunt K D Dietary responses of chimpanzees and cercopithecines to seasonal variation in fruit abundance I antifeedants Int J Primatol 19 949 970 1998 24 Onishi A et al Dichromatism in macaque monkeys Nature 402 139 140 1999 25 Struhsaker T T Ecology of an African Rainforest Univ Florida Press Gainesville 1997 26 Lucas P W et al Fieldkit to characterize the physical chemical and spatial aspects of potential primate foods Folia Primatol 72 11 25 2001 27 Osorio D Vorobyev M Colour vision as an adaptation to frugivory in primates Proc R Soc Lond B 263 593 599 1996 28 Darvell B W Lee P K D Yuen T D B Lucas P W Meas Sci Technol 7 954 962 1996 29 Newton Fisher N E The diet of chimpanzees in the Budongo Forest Reserve Uganda Afr J Ecol 37 344 354 1999 30 Gartlan J S McKey D B Waterman P G Mbi C N Strusaker T T A comparative study of the phytochemistry of two African rainforests Biochem Syst Ecol 8 401 422 1980 95 85 Suppress Respond 75 65 0 1 8 16 Number of repetitions 95 85 75 65 Suppress Respond 0 1 8 16 Number of repetitions 366 f 95 85 75 65 Suppress Respond 0 1 8 16 Number of repetitions Per cent recalled e 95 85 75 65 Suppress Respond 0 1 8 16 Number of repetitions g Per cent recalled Freud proposed that unwanted memories can be forgotten by pushing them into the unconscious a process called repression1 The existence of repression has remained controversial for more than a century in part because of its strong coupling with trauma and the ethical and practical dif culties of studying such processes in controlled experiments However behavioural and neurobiological research on memory and attention shows that people have executive control processes directed at minimizing perceptual distraction2 3 overcoming interference during short and long term memory tasks3 7 and stopping strong habitual responses to stimuli8 13 Here we show that these mechanisms can be recruited to prevent unwanted declarative memories from entering awareness and that this cognitive act has enduring consequences for the rejected memories When people encounter cues that remind them of an unwanted memory and they consistently try to prevent awareness of it the later recall of the rejected memory becomes more dif cult The forgetting increases with the number of times the memory is avoided resists incentives for accurate recall and is caused by processes that suppress the memory itself These results show that executive control processes not uniquely tied to trauma may provide a viable model for repression Executive control processes studied in behavioural6 9 14 and neurobiological2 4 10 13 15 17 research on cognition may provide a mechanism for the voluntary form of repression suppression Per cent recalled Respond same Respond indep Supp same Supp indep 95 85 75 65 0 1 8 16 Number of repetitions Figure 1 Final recall for respond and suppression items as a function of the number of repetitions for the same probe SP and independent probe IP tests a b Experiment 1 c d experiment 2 e f experiment 3 g averaged across experiments Note the negative slope for recall of the suppressed item indicating increasing inhibition Inhibition 0 vs 16 suppressions was signi cant P 0 01 in all experiments and did not interact with type of test cue F 1 in all cases analysis of variance Inhibition was signi cant P 0 05 in every SP and IP test for every experiment a f 2001 Macmillan Magazines Ltd NATURE VOL 410 15 MARCH 2001 www nature com letters to nature response and the entrance of an unwanted memory into
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