A study of parallel implicit and explicit information processing in patients with obsessive-compulsive disorder

OBJECTIVE: This study examined implicit sequence learning in patients with obsessive-compulsive disorder (OCD) under dual-task conditions. Frontal-striatal networks support implicit learning and are implicated in the pathophysiology of OCD. Neuroimaging data suggest that during implicit learning, OCD patients use neural systems normally active during explicit learning to compensate for striatal dysfunction. METHOD: The authors examined implicit sequence learning in 25 OCD patients and 25 healthy comparison subjects using a dual-task paradigm, with subjects simultaneously engaged in an explicit memory task and an implicit learning task. They predicted that implicit learning in OCD subjects would be disrupted because concurrent explicit information-processing demands would prevent use of compensatory processes. RESULTS: OCD patients failed to show evidence of implicit learning and exhibited a significant deficit in comparison with healthy subjects. CONCLUSIONS: These results are consistent with the hypothesis that concurrent explicit and implicit information-processing demands interfere with implicit learning in OCD patients.     

Reduced implicit and explicit sequence learning in first-episode schizophrenia

A high prevalence of deficits in explicit learning has been reported for schizophrenic patients, but it is less clear whether these patients are impaired in implicit learning. Deficits in implicit learning indicative of a fronto-striatal dysfunction have been reported using a serial reaction-time task (SRT), but the impact of typical neuroleptic medication and chronicity remains controversial. The present study compared 37 patients with first-episode schizophrenia treated with atypical neuroleptics and 37 healthy matched control participants on two sequence learning tasks: a modified SRT for implicit sequence learning and a serial generation task (SGT) for explicit sequence learning. The two tasks were designed to be procedurally equivalent, in order to provide better comparability between implicit and explicit performance. Although unaffected in global cognitive functioning, schizophrenic patients were significantly impaired in implicit and explicit sequence learning. Deficient sequence learning in schizophrenic patients was neither related to psychopathology nor to chlorpromazine equivalent daily dosage. As performance was impaired even though patients were exclusively treated with atypical neuroleptics, the present findings concur with converging evidence of a sequence learning deficit inherent in schizophrenia. This deficit would be consistent with a fronto-striatal dysfunction and might constitute a crucial factor for the acquisition of new information.     

Evidence of developmental differences in implicit sequence learning: an fMRI study of children and adults

Prevailing theories of implicit or unaware learning propose a developmental invariance model, with implicit function maturing early in infancy or childhood despite prolonged improvements in explicit or intentional learning and memory systems across childhood. Neuroimaging studies of adult visuomotor sequence learning have associated fronto-striatal brain regions with implicit learning of spatial sequences. Given evidence of continued development in these brain regions during childhood, we compare implicit sequence learning in adults and 7- to 11-year-old children to examine potential developmental differences in the recruitment of fronto-striatal circuitry during implicit learning. Participants performed a standard serial reaction time task. Stimuli alternately followed a fixed 10-step sequence of locations or were presented in a pseudorandom order of locations. Adults outperformed children, achieving a significantly larger sequence learning effect and showing learning more quickly than children. Age-related differences in activity were observed in the premotor cortex, putamen, hippocampus, inferotemporal cortex, and parietal cortex. We observed differential recruitment of cortical and subcortical motor systems between groups, presumably reflecting age differences in motor response execution. Adults showed greater hippocampal activity for sequence trials, whereas children demonstrated greater signal during random trials. Activity in the right caudate correlated significantly with behavioral measures of implicit learning for both age groups, although adults showed greater signal change than children overall, as would be expected given developmental differences in sequence learning magnitude. These results challenge the idea of developmental invariance in implicit learning and instead support a view of parallel developments in implicit and explicit learning systems.     

Implicit and explicit aspects of sequence learning in pre-symptomatic Huntington's disease

Learning deficits may be part of the early symptoms of Huntington's disease (HD). Here we characterized implicit and explicit aspects of sequence learning in 11 pre-symptomatic HD gene carriers (pHD) and 11 normal controls. Subjects moved a cursor on a digitizing tablet and performed the following tasks: SEQ: learning to anticipate the appearance of a target sequence in two blocks; VSEQ: learning a sequence by attending to the display without moving for one block, and by moving to the sequence in a successive block (VSEQ test). Explicit learning was measured with declarative scores and number of anticipatory movements. Implicit learning was measured as a strategy change reflected in movement time. By the end of SEQ, pHD had a significantly lower number of correct anticipatory movements and lower declarative scores than controls, while in VSEQ and VSEQ test these indices improved. During all three tasks, movement time changed in controls, but not in pHD. These results suggest that both explicit and implicit aspects of sequence learning may be impaired before the onset of motor symptoms. However, when attentional demands decrease, explicit, but not implicit, learning may improve.     

The role of the basal ganglia and its cortical connections in sequence learning: Evidence from implicit and explicit sequence learning in Parkinson's disease

Implicit (unconscious/incidental) and explicit (conscious/intentional) learning are considered to have distinct neural substrates. It is proposed that implicit learning is mediated by the basal ganglia (BG), while explicit learning has been linked to the medial temporal lobes (MTL). To test such a dissociation we investigated implicit and explicit sequence learning in Parkinson's disease (PD), a disorder characterized by striatal dysfunction. We studied both implicit and explicit learning of a 12-item sequence of target locations in 13 PD patients and 15 age-matched controls. In the implicit sequence learning task all participants completed 10 blocks of a probabilistic serial reaction time (SRT) task in which they were exposed to the sequence without explicit knowledge of it. Participants also completed between 1 and 10 blocks of an explicit sequence learning task in which the sequence was learned deliberately by trial-and-error. Both implicit and explicit sequence learning were significantly impaired in PD patients compared to controls. The results indicate that, in addition to playing a role in implicit sequence learning, the BG and its frontal projections are also involved in explicit sequence learning.     

Reversal learning as a neuropsychological indicator for the neuropathology of obsessive compulsive disorder? A behavioral study

A dysfunction of the fronto-striatal loop has been associated with obsessive-compulsive disorder (OCD). Functional imaging studies suggest that reversal learning is affected by deficits in fronto-striatal brain areas and thus should be impaired in patients with OCD. The authors compared patients with OCD and healthy comparison subjects on a reversal learning task. Correlation analyses and group comparisons showing prolonged reaction times of different response parameters are associated with increasing severity of compulsions. The reversal learning task has been shown to be associated with ventral fronto-striatal brain activation by functional magnetic resonance imaging (fMRI) in healthy comparison subjects. The purpose of this article is to suggest that the reversal learning task can be used as a neuropsychiatric measurement of the ventral fronto-striatal dysfunction in OCD.     

Impaired procedural learning in obsessive-compulsive disorder and Parkinson's disease, but not in major depressive disorder

The striatum has been consistently implicated in the pathophysiology of obsessive-compulsive disorder (OCD), yet, studies assessing the performance of OCD patients in procedural learning tasks, assumed to rely on the intact functioning of the striatum, have yielded inconsistent results. Recently, Rauch et al. [Rauch SL, Savage CR, Alpert NM, Dougherty D, Kendrick A, Curran T, et al. Probing striatal function in obsessive-compulsive disorder: a PET study of implicit sequence learning. J Neuropsychiatry Clin Neurosci 1997;9:568-73] have obtained evidence suggesting that seemingly intact performance of OCD patients in such tasks may be achieved by recruiting systems which in normal subjects are reserved for explicit or declarative, rather than implicit or procedural, processing. The present study assessed procedural learning in OCD patients using a card betting task in which explicit processing impairs, rather than assists, acquisition. In addition, we tested a group of Parkinson's disease (PD) patients, in order to better establish the dependence of the task on procedural learning, and a group of major depressive disorder (MDD) patients, in order to test the possibility that impaired learning in the card betting task may be a result of concurrent depression. The majority of OCD (15/18) and PD patients (14/16) did not acquire the task, whereas MDD patients acquired the task similarly to normal control subjects. These results demonstrate that OCD patients are impaired on a procedural learning task in which explicit processing impairs acquisition. Two different interpretations are suggested: that the striatal system is dysfunctional in OCD, or that inappropriate explicit processing in OCD interferes with the functioning of the striatal system.     

A PET investigation of implicit and explicit sequence learning

The purpose of this study was to determine the mediating neuroanatomy of implicit and explicit sequence learning using a modified version of the serial reaction time (SRT) paradigm. Subjects were seven healthy, right-handed adults (three male, four female, mean age 26.7, range 18–43 yr). PET data were acquired via the oxygen-15-labeled-carbon dioxide inhalation method while subjects performed the SRT. Subjects were scanned during two blocks each of (1) no sequence (Random), (2) single-blind, 12-item sequence (Implicit), and (3) unblinded, same sequence (Explicit). Whole-brain-normalized images reflecting relative regional cerebral blood flow (rCBF) were transformed to Talairach space, and statistical parametric maps (SPMs) of z-scores were generated for comparisons of interest. The threshold for significant activation was defined as z-score ≥ 3.00. Behavioral data demonstrated significant learning (P < .05) for Implicit and Explicit conditions. Tests of explicit knowledge reflected non-significant explicit contamination during the Implicit condition. Foci of significant activation in the Implicit condition were found in right ventral premotor cortex, right ventral caudate/nucleus accumbens, right thalamus, and bilateral area 19; activation in the Explicit condition included primary visual cortex, peri-sylvian cortex, and cerebellar vermis. Activations in visual and language areas during the Explicit condition may reflect conscious learning strategies including covert verbal rehearsal and visual imagery. Right-sided premotor, striatal, and thalamic activations support the notion that implicit sequence learning is mediated by cortico-striatal pathways, preferentially within the right hemisphere. © 1996 Wiley-Liss, Inc.     

Towards an explicit account of implicit learning

PURPOSE OF REVIEW: The human brain supports acquisition mechanisms that can extract structural regularities implicitly from experience without the induction of an explicit model. Reber defined the process by which an individual comes to respond appropriately to the statistical structure of the input ensemble as implicit learning. He argued that the capacity to generalize to new input is based on the acquisition of abstract representations that reflect underlying structural regularities in the acquisition input. We focus this review of the implicit learning literature on studies published during 2004 and 2005. We will not review studies of repetition priming ('implicit memory'). Instead we focus on two commonly used experimental paradigms: the serial reaction time task and artificial grammar learning. Previous comprehensive reviews can be found in Seger's 1994 article and the Handbook of Implicit Learning. RECENT FINDINGS: Emerging themes include the interaction between implicit and explicit processes, the role of the medial temporal lobe, developmental aspects of implicit learning, age-dependence, the role of sleep and consolidation. SUMMARY: The attempts to characterize the interaction between implicit and explicit learning are promising although not well understood. The same can be said about the role of sleep and consolidation. Despite the fact that lesion studies have relatively consistently suggested that the medial temporal lobe memory system is not necessary for implicit learning, a number of functional magnetic resonance studies have reported medial temporal lobe activation in implicit learning. This issue merits further research. Finally, the clinical relevance of implicit learning remains to be determined.     

The striatum and probabilistic implicit sequence learning

The distinction between implicit (unconscious) and explicit (conscious) learning is controversial. Some argue that explicit learning is dependent on the medial temporal lobes, whereas implicit learning is mediated by the basal ganglia and others propose that all learning is explicit. The purpose of the present study was to investigate the involvement of the basal ganglia in implicit learning by examining learning of a probabilistic sequence of targets, in patients with Parkinson's disease (PD) and controls. Following learning, we assessed participants' awareness of the sequence structure by asking them to generate or withhold sequence consistent responses (process dissociation procedure) and to perform a recognition test in which measures of priming and recognition were obtained concurrently. Although the PD group demonstrated evidence of probabilistic sequence learning in this study, learning was significantly attenuated compared to controls. Residual learning in the PD group was genuinely implicit in nature because (a) PD patients were not able to control the expression of their acquired knowledge, and (b) their knowledge supported subsequent priming of sequence-consistent responses but recognition ratings were at chance. In contrast, following learning controls were capable of above chance recognition indicating that their sequential knowledge was acquired in a more explicit way. The results support the view that (i) the basal ganglia contribute to probabilistic implicit sequence learning (ii) that such learning can occur implicitly without explicit knowledge in PD patients.     

Primary motor and premotor cortex in implicit sequence learning - evidence for competition between implicit and explicit human motor memory systems

Implicit and explicit memory systems for motor skills compete with each other during and after motor practice. Primary motor cortex (M1) is known to be engaged during implicit motor learning, while dorsal premotor cortex (PMd) is critical for explicit learning. To elucidate the neural substrates underlying the interaction between implicit and explicit memory systems, adults underwent a randomized crossover experiment of anodal transcranial direct current stimulation (AtDCS) applied over M1, PMd or sham stimulation during implicit motor sequence (serial reaction time task, SRTT) practice. We hypothesized that M1‐AtDCS during practice will enhance online performance and offline learning of the implicit motor sequence. In contrast, we also hypothesized that PMd‐AtDCS will attenuate performance and retention of the implicit motor sequence. Implicit sequence performance was assessed at baseline, at the end of acquisition (EoA), and 24 h after practice (retention test, RET). M1‐AtDCS during practice significantly improved practice performance and supported offline stabilization compared with Sham tDCS. Performance change from EoA to RET revealed that PMd‐AtDCS during practice attenuated offline stabilization compared with M1‐AtDCS and sham stimulation. The results support the role of M1 in implementing online performance gains and offline stabilization for implicit motor sequence learning. In contrast, enhancing the activity within explicit motor memory network nodes such as the PMd during practice may be detrimental to offline stabilization of the learned implicit motor sequence. These results support the notion of competition between implicit and explicit motor memory systems and identify underlying neural substrates that are engaged in this competition.     

Anterior limb lesions in bilateral internal capsules and memory function in patients with refractory obsessive-compulsive disorder

BACKGROUND: Previous studies have shown that lesions in the anterior limb of the internal capsule contribute to obsessive-compulsive symptoms in patients with refractory obsessive-compulsive disorder (OCD). However, few reports have addressed the effects of lesions in the anterior limb of the internal capsule on cognition, learning, and memory functions in patients with refractory OCD.OBJECTIVE: To investigate the degree of damage to memory tasks in refractory OCD patients following lesions to the anterior limb of the internal capsule. DESIGN, TIME AND SETTING: A case-controlled, observational study was performed at the Department of Functional Neurosurgery, Ruijin Hospital, Shanghai Jiao-Tong University, China from May 2007 to March 2008. PARTICIPANTS: A total of 10 refractory OCD patients were admitted to the Department of Functional Neurosurgery, Ruijin Hospital, Shanghai Jiao-Tong University, China from May 2007 to March 2008 and were recruited for this study. The OCD patients were of equal gender, with an average age of (25.1 ± 9.6) years. An additional 10 healthy volunteers were enrolled from a community of Shanghai City as controls; they were of equal gender and aged (25.1 ± 8.6) years. METHODS: A total of 10 refractory OCD patients were subjected to lesions in the anterior limbs of the bilateral internal capsules. Wechsler Memory Scale-Chinese Revision (WMS-CR, as a task of explicit memory) and the Nissen Version (serial reaction time task) software (SRTT, as a task of implicit memory) were applied to determine memory functions and learning performance in pre- and post-operative OCD patients and controls. MAIN OUTCOME MEASURES: WMS scores, reaction time in SRTT, and Yale-Brown obsessive compulsive scale scores were measured in pre- and post-operative OCD patients and controls. RESULTS: Compared to controls, the pre-operative OCD patients exhibited reduced memory task scores (P = 0.005), whereas scores for reciting numbers of backwards digits were greater (P = 0.000). Figure recall and associative memory were less in OCD patients at 1 week following surgery than in the pre-operative OCD patients (P = 0.042, P = 0.002, respectively). Reaction time in implicit SRTT was significantly longer in pre-operative OCD patients compared with controls and post-operative OCD patients (P = 0.01, P = 0.03, respectively). These results suggested ameliorated SRTT following neurosurgery. Yale-Brown Obsessive Compulsive Scale results revealed significantly improved OCD following lesions in the internal capsule (P = 0.04). Some post-operative OCD patients suffered from deficits in short-term memory and implicit memory. CONCLUSION: Lesions in anterior limbs of bilateral internal capsules improve obsessive- compulsive symptoms and implicit memory in OCD patients, but result in aggravated short-term memory deficits.     

Severity of explicit memory impairment due to Alzheimer’s disease improves effectiveness of implicit learning

Background Consistent evidence from human and experimental animals studies indicates that memory is organized into two relatively independent systems with different functions and brain mechanisms. The explicit memory system, dependent on the hippocampus and adjacent medial temporal lobe structures, refers to conscious knowledge acquisition and intentional recollection of previous experiences. The implicit memory system, dependent on the striatum, refers to learning of complex information without awareness or intention. The functioning of implicit memory can be observed in progressive, gradual improvement across many trials in performance on implicit learning tasks. The influence of explicit memory on implicit memory has not been precisely identified yet. According to data from some studies, explicit memory seems to exhibit no influence on implicit memory,whereas the other studies indicate that explicit memory may inhibit or facilitate implicit memory. Objectives The analysis of performance on implicit learning tasks in patients with different severity of explicit memory impairment due to Alzheimer’s disease allows one to identify the potential influence of the explicit memory system on the implicit memory system. Patients and methods 51 patients with explicit memory impairment due to Alzheimer’s disease (AD) and 36 healthy controls were tested. Explicit memory was examined by means of a battery of neuropsychological tests. Implicit habit learning was examined on probabilistic classification task (weather prediction task). Results Patients with moderate explicit memory impairment performed the implicit task significantly better than those with mild AD and controls. Conclusion Results of our study support the hypothesis of competition between the implicit and explicit memory systems in humans.     

Brain activation during implicit sequence learning in individuals with trichotillomania

Trichotillomania (TTM) may be related to obsessive-compulsive disorder (OCD) and other neuropsychiatric conditions characterized by cortico-striatal dysfunction. Functional imaging studies of OCD using an implicit learning task have found abnormalities in striatal and hippocampal activation. The current study investigated whether similar abnormalities occur in TTM. Functional MRI and the serial reaction time (SRT) task were used to assess striatal and hippocampal activation during implicit sequence learning in TTM and healthy control (HC) subjects. The results for 20 age- and education-matched participants (10 TTM, 10 HC) are reported. In comparison with HC participants, those with TTM exhibited no significant differences in implicit learning, or in activation within the striatum, hippocampus, or other brain regions. The current findings do not provide evidence for cortico-striatal dysfunction in TTM. Future studies directly comparing OCD and TTM subjects are warranted to confirm the specificity of abnormal striatal and hippocampal findings during implicit sequence learning in OCD.     

Intact memory for implicit contextual information in Korsakoff's amnesia

Implicit contextual learning is the ability to acquire contextual information from our surroundings without conscious awareness. Such contextual information facilitates the localization of objects in space. In a typical implicit contextual learning paradigm, subjects need to find a target among a number of distractors during visual search. Some of the configurations of stimuli are repeated during the experiment resulting in faster responses than for novel configurations, without subjects being aware of their repetition. Patients with Korsakoff's syndrome (KS) have been found to show devastating explicit spatial amnesia. Less is know about their implicit spatial memory abilities. The aim of the present research was to examine whether implicit contextual learning is intact in KS. Therefore, eighteen KS patients and twenty-two age-IQ- and education-matched controls performed the Implicit Contextual Learning task and a paradigm intended to assess explicit, spatial working memory, i.e. the Box task. Intact implicit contextual learning was observed in both the control group and the KS patients. In turn KS patients did have markedly lower explicit spatial working memory scores. The implicit learning effect was not related to the spatial working memory scores. Together these results clearly suggest that implicit and explicit spatial memory have a different neurocognitive basis.     

Intact implicit learning in schizophrenia

OBJECTIVE: Schizophrenia impairs performance on explicit, but not implicit, memory tasks, indicating that conscious awareness at retrieval is a critical determinant of impaired memory. The authors investigated implicit learning, i.e., knowledge acquisition in the absence of conscious awareness, in patients with schizophrenia. METHOD: An artificial grammar learning task was used to assess implicit learning in 48 patients with schizophrenia and 24 healthy comparison subjects. The subjects were first presented with letter strings that were generated according to the rules of a finite-state grammar paradigm. They were then required to indicate whether new letter strings were "grammatical," depending on whether or not the strings corresponded to the rules. IQ, working memory, explicit memory, verbal fluency, and speed of processing were also assessed. RESULTS: Patients performed significantly worse than the comparison subjects on cognitive tasks that assessed episodic memory, verbal fluency, working memory, and speed of processing. In contrast, patients classified as being correct more grammatical than nongrammatical letter strings, and the magnitude of the difference was similar to that observed in healthy comparison subjects. CONCLUSIONS: Implicit learning, as assessed with an artificial grammar learning task, is intact in patients with schizophrenia. Conscious awareness might be a critical determinant of memory impairment both at encoding and at retrieval.     

Functional magnetic resonance imaging study of regional brain activation during implicit sequence learning in obsessive-compulsive disorder.

BACKGROUND: Corticostriatal circuitry has been implicated in the pathophysiology of obsessive-compulsive disorder (OCD). The serial reaction time (SRT) task, a paradigm that tests implicit sequence learning, has been used with imaging to probe striatal function. Initial studies have indicated that OCD patients exhibit deficient striatal activation and aberrant hippocampal recruitment compared with healthy control (HC) subjects. Here, we used the SRT and functional magnetic resonance imaging (fMRI) to replicate prior results in a larger sample and to test for relationships between regional activation and OCD symptom dimensions. METHODS: Using SPM99, fMRI-SRT data from 12 OCD and 12 matched HC subjects were analyzed. Symptom dimensions followed a four-factor model scored on a 0- to 10-point scale. RESULTS: For the implicit learning versus random contrast, group by condition interactions revealed aberrant recruitment within the hippocampus as well as orbitofrontal cortex (OCD > HC) but no striatal group differences. However, an inverse correlation was found between striatal activation and specific symptom factors. CONCLUSIONS: These results replicate previous smaller studies showing aberrant hippocampal recruitment in OCD during SRT performance. Although findings of deficient striatal activation in OCD were not replicated, correlation results suggest that this inconsistency may be attributable to differences among OCD symptom dimensions.     

Selective impairments in implicit learning in Parkinson's disease

The basal ganglia are thought to participate in implicit sequence learning. However, the exact nature of this role has been difficult to determine in light of the conflicting evidence on implicit learning in subjects with Parkinson's disease (PD). We examined the performance of PD subjects using a modified form of the serial reaction time task, which ensured that learning remained implicit. Subjects with predominantly right-sided symptoms were trained on a 12-element sequence using the right hand. Although there was no evidence of sequence learning on the basis of response time savings, the subjects showed knowledge of the sequence when performance was assessed in terms of the number of errors made. This effect transferred to the left (untrained) hand as well. Thus, these data demonstrate that PD patients are not impaired at implicitly learning sequential order, but rather at the translation of sequence knowledge into rapid motor performance. Furthermore, the results suggest that the basal ganglia are not essential for implicit sequence learning in PD.     

Achieving enlightenment: what do we know about the implicit learning system and its interaction with explicit knowledge?

Two major memory and learning systems operate in the brain: one for facts and ideas (ie, the declarative or explicit system), one for habits and behaviors (ie, the procedural or implicit system). Broadly speaking these two memory systems can operate either in concert or entirely independently of one another during the performance and learning of skilled motor behaviors. This Special Issue article has two parts. In the first, we present a review of implicit motor skill learning that is largely centered on the interactions between declarative and procedural learning and memory. Because distinct neuroanatomical substrates support unique aspects of learning and memory and thus focal injury can cause impairments that are dependent on lesion location, we also broadly consider which brain regions mediate implicit and explicit learning and memory. In the second part of this article, the interactive nature of these two memory systems is illustrated by the presentation of new data that reveal that both learning implicitly and acquiring explicit knowledge through physical practice lead to motor sequence learning. In our new data, we discovered that for healthy individuals use of the implicit versus explicit memory system differently affected variability of performance during acquisition practice; variability was higher early in practice for the implicit group and later in practice for the acquired explicit group. Despite the difference in performance variability, by retention both groups demonstrated comparable change in tracking accuracy and thus, motor sequence learning. Clinicians should be aware of the potential effects of implicit and explicit interactions when designing rehabilitation interventions, particularly when delivering explicit instructions before task practice, working with individuals with focal brain damage, and/or adjusting therapeutic parameters based on acquisition performance variability.     

Implicit and explicit memory in patients with obsessive-compulsive disorder: an event-related potential study

The implicit and explicit memory in patients with obsessive-compulsive disorder (OCD) was investigated using the event-related potential (ERP). For the assessment of implicit memory, a lexical decision task was administered. Among a total of 320 words and 140 non-words, 200 words were repeated, while the remaining 120 words and the 140 non-words were not repeated. For explicit memory, a continuous recognition task was administered, in which 280 words were repeated and 100 were not repeated. On the recognition task, both the controls and OCD patients showed more positivity to the old words than to the new words during the 200-600 ms period post-stimulus. Both groups showed faster response time to the old words than to the new words. On the lexical decision task, the controls showed the old/new effect during the 200-500 ms period post-stimulus, while the OCD patients did not. However, OCD patient showed faster response time to the old words than to the new words, although OCD patients showed prolonged response times to the old words compared to the controls. These results indicate that OCD patients have preserved explicit and implicit memory. The absence of old/new effect on ERP in OCD patients was discussed in terms of dysfunction of frontostriatal system, which plays an important role in both OCD and implicit memory.