Functional role of the prefrontal cortex in retrieval of memories: a PET study

Right prefrontal cortex and episodic memory retrieval: a functional MRI test of the monitoring hypothesis

Though the right prefrontal cortex is often activated in neuroimaging studies of episodic memory retrieval, the functional significance of this activation remains unresolved. In this functional MRI study of 12 healthy volunteers, we tested the hypothesis that one role of the right prefrontal cortex is to monitor the information retrieved from episodic memory in order to make an appropriate response. The critical comparison was between two word recognition tasks that differed only in whether correct responses did or did not require reference to the spatiotemporal context of words presented during a previous study episode. Activation in a dorsal midlateral region of the right prefrontal cortex was associated with increased contextual monitoring demands, whereas a more ventral region of the right prefrontal cortex showed retrieval-related activation that was independent of task instructions. This functional dissociation of dorsal and ventral right prefrontal regions is discussed in relation to a theoretical framework for the control of episodic memory retrieval.

Recollection and familiarity in recognition memory: an event-related functional magnetic resonance imaging study

The question of whether recognition memory judgments with and without recollection reflect dissociable patterns of brain activity is unresolved. We used event-related, functional magnetic resonance imaging (fMRI) of 12 healthy volunteers to measure hemodynamic responses associated with both studying and recognizing words. Volunteers made one of three judgments to each word during recognition: whether they recollected seeing it during study (R judgments), whether they experienced a feeling of familiarity in the absence of recollection (K judgments), or whether they did not remember seeing it during study (N judgments). Both R and K judgments for studied words were associated with enhanced responses in left prefrontal and left parietal cortices relative to N judgments for unstudied words. The opposite pattern was observed in bilateral temporoccipital regions and amygdalae. R judgments for studied words were associated with enhanced responses in anterior left prefrontal, left parietal, and posterior cingulate regions relative to K judgments. At study, a posterior left prefrontal region exhibited an enhanced response to words subsequently given R versus K judgments, but the response of this region during recognition did not differentiate R and K judgments. K judgments for studied words were associated with enhanced responses in right lateral and medial prefrontal cortex relative to both R judgments for studied words and N judgments for unstudied words, a difference we attribute to greater monitoring demands when memory judgments are less certain. These results suggest that the responses of different brain regions do dissociate according to the phenomenology associated with memory retrieval.

Brain activation in PTSD in response to trauma-related stimuli

BACKGROUND

Repetitive recall of traumatic memories and chronic intermittent hyperarousal are characteristic of posttraumatic stress disorder (PTSD). Hyperarousal and memory dysfunction implicates "limbic" brain regions, including the amygdaloid complex, hippocampal formation, and limbic cortex, such as the orbitofrontal and anterior cingulate areas. To investigate the neurobiologic role of these brain regions in PTSD, we measured regional cerebral blood flow in PTSD with single photon emission computerized tomography (SPECT) during a symptom provocation paradigm.

METHODS

Fourteen Vietnam veterans with PTSD, 11 combat control subjects, and 14 normal control subjects were studied with [99mTc]HMPAO in two sessions 48 hours apart: one session after exposure to white noise and the other following exposure to combat sounds. Skin conductance, heart rate, and subjective experience were recorded at the time of the studies.

RESULTS

Activation for all three groups occurred in the anterior cingulate/middle prefrontal gyrus. Activation in the region of the left amygdala/nucleus accumbens was found in PTSD patients only. Deactivation was found in all three groups in the left retrosplenial region.

CONCLUSIONS

These findings implicate regions of the "limbic" brain, which may mediate the response to aversive stimuli in healthy individuals and in patients suffering from PTSD.

Separating retrieval strategies from retrieval success: an event-related potential study of source memory

Event-related potentials (ERPs) were recorded while subjects performed two different source memory retrieval tasks. Each task was preceded by a study phase in which subjects heard an equal number of words spoken in a male or a female voice. A cue preceding each word indicated whether the subjects should make an active/passive (action) or a pleasant/unpleasant (liking) judgment to the word. In one retrieval task (the voice condition), subjects made a three-way distinction between new (unstudied) words, and words that had been spoken by the male or the female voice at study. In the second retrieval task (the task condition), subjects distinguished between new words, and words to which they had made an action or a liking judgment. All test words were presented visually. In keeping with previous findings, the differences between the ERPs to correct memory judgments for old and new items were characterised by two temporally and topographically dissociable modulations, with right-frontal and left-parietal maxima respectively. These 'old/new effects' displayed different sensitivities to successful retrieval of either voice or task information, providing further evidence that they index functionally dissociable processes. The direct comparison of the ERPs to correct rejections in the voice and task retrieval conditions revealed reliable differences over frontal scalp, suggesting that, irrespective of whether retrieval is successful, neural processing differs according to the source retrieval demands of the task.

Positron emission tomography correlations in and beyond medial temporal lobes

This article discusses the potential usefulness of brain/ behavior correlational analyses in functional neuroimaging studies of memory, and how such analyses can illuminate the role of medial temporal lobes (MTL) and the hippocampus in episodic and declarative memory processes such as encoding and retrieval. Reanalysis of the results of four previously reported positron emission tomography (PET) studies yielded evidence of both positive and negative between-subjects correlations between recognition-memory accuracy and regional blood flow. The sites of these correlations were in MTL regions as well as in other cortical and subcortical areas, including frontal lobes (Brodmann areas 6, 9, 10, 11, and 47), temporal lobes (BAs 21, 22, and 38), insula, fusiform gyrus, and cuneus/precuneus. These findings were discussed with respect to issues such as localization of the correlation sites, the distinction between brain sites revealed by brain/cognition correlational analyses ("how" sites) and those yielded by cognitive subtraction methods ("what" sites), the tendency of the "how" sites in MTL to occur in the left hemisphere, the tendency of other "how" sites to occur in one or the other hemisphere, rather than bilaterally, and the meaning and "reality" of both brain/behavior correlations and task-related activations. Because of the known incidence of false-positives, all neuroimaging data, including those involving the localization of "what" and "how" memory sites in MTL and other brain regions, need to be interpreted cautiously, and findings of individual studies should not be overinterpreted.

Frontal brain potentials during recognition are modulated by requirements to retrieve perceptual detail

To assess the role of prefrontal cortex in retrieval and address the controversy about whether prefrontal retrieval operations are engaged only following successful retrieval, we recorded event-related brain potentials during two recognition tests with differing demands on retrieval effort. Both tests included object drawings that were (1) identical to those studied, (2) the same but with altered aspect ratios, and (3) previously unseen. Instructions were to respond "old" only if drawings were not modified (specific test) or regardless of modifications (general test). Frontal potentials were enhanced during the specific relative to the general test for all three types of drawings. We conclude that these potentials reflected differential engagement of strategic retrieval, that this function relied on left prefrontal cortex, and that it was not contingent on successful retrieval.

Brain activation related to the change between bimanual motor programs

By using positron emission tomography, we aimed to identify cerebral foci of neuronal activation associated with the initiation of a specific motor program. To that end, a state of repeatedly alternating in- and antiphase of bimanual flexion and extension movements was compared with similar movement responses except phase changing. This comparison provided the opportunity to eliminate confounding effects of attention and simple movements. Change between the two bimanual motor programs was related with activation at the posterior border of the left angular gyrus, the right precuneus, and the right premotor and right medial prefrontal cortex. In a subsequent experiment, with attention and random movements as additional variables, activation at the posterior border of the left angular gyrus was found at the same significance level. This posterior parietal activation may indicate an equivalence with the coding of intention in monkey posterior parietal cortex. Lesion of the left posterior parietal cortex in human gives rise to left-right disorientation and ideomotor apraxia. Our results may support the view that these symptoms reflect the inability to transpose a motor plan to the representation of a personal body scheme. Activation of the right premotor and right medial prefrontal cortex was related both to the change between motor programs and to the condition with strictly regular movement in which no additional responses were made to randomly presented signals. This is consistent with the concept that motor preparation is associated with both the selection of internally instructed movements and the suppression of irrelevant environmental stimuli.

Task-related and item-related brain processes of memory retrieval

In all cognitive tasks, general task-related processes operate throughout a given task on all items, whereas specific item-related processes operate differentially on individual items. In typical functional neuroimaging experiments, these two sets of processes have usually been confounded. Herein we report a combined positron emission tomography and event-related potential (ERP) experiment that was designed to distinguish between neural correlates of task-related and item-related processes of memory retrieval. Two retrieval tasks, episodic and semantic, were crossed with episodic (old/new) and semantic (living/nonliving) properties of individual items to yield evidence of regional brain activity associated with task-related processes, item-related processes, and their interaction. The results showed that episodic retrieval task was associated with increased blood flow in right prefrontal and posterior cingulate cortex, as well as with a sustained right-frontopolar-positive ERP, but that the semantic retrieval task was associated with left frontal and temporal lobe activity. Retrieval of old items was associated with increased blood flow in the left medial temporal lobe and with a brief late positive ERP component. The results provide converging hemodynamic and electrophysiological evidence for the distinction of task- and item-related processes, show that they map onto spatially and temporally distinct patterns of brain activity, and clarify the hemispheric encoding/retrieval asymmetry (HERA) model of prefrontal encoding and retrieval asymmetry.

The effects of divided attention during encoding and retrieval on amnesic patients' memory performance

This study examined the effect of divided attention during encoding or retrieval on primary (recall) or secondary (button-pressing) task performance by amnesic patients and control subjects. Experiment 1 demonstrated that control subjects' recall was affected by divided attention during encoding but not during retrieval, while the amnesic patients' recall remained at the same low level for all conditions. Both groups showed a reduction in rate of button-pressing during encoding relative to their baseline levels and a further reduction during the retrieval interval. In Experiment 2, five learning trials, instead of just one, were presented. This acted to increase all subjects' recall and also had the effect of producing superior recall following free encoding relative to encoding during divided attention for the amnesic patients as well as for the control subjects. Button-pressing performance during the encoding interval increased to baseline for both groups during the five presentation trials, but there was no increase in the rate of button-pressing during the retrieval interval. Amnesic patients' normal attention to the primary task during encoding and retrieval was felt to have several implications for future neuroimaging tasks with these patients.

Episodic retrieval activates the precuneus irrespective of the imagery content of word pair associates. A PET study

The aim of this study was to evaluate further the role of the precuneus in episodic memory retrieval. The specific hypothesis addressed was that the precuneus is involved in episodic memory retrieval irrespective of the imagery content. Two groups of six right-handed normal male volunteers took part in the study. Each subject underwent six [15O]butanol-PET scans. In each of the six trials, the memory task began with the injection of a bolus of 1500 MBq of [15O]butanol. For Group 1, 12 word pair associates were presented visually, for Group 2 auditorily. The subjects of each group had to learn and retrieve two sets of 12 word pairs each. One set consisted of highly imaginable words and another one of abstract words. Words of both sets were not related semantically, representing 'hard' associations. The presentations of nonsense words served as reference conditions. We demonstrate that the precuneus shows consistent activation during episodic memory retrieval. Precuneus activation occurred in visual and auditory presentation modalities and for both highly imaginable and abstract words. The present study therefore provides further evidence that the precuneus has a specific function in episodic memory retrieval as a multimodal association area.

Cognitive and motivational operations in primate prefrontal neurons

To investigate neuronal mechanisms of higher cognitive activity, single neuronal activity has been recorded from the prefrontal cortex of monkeys trained in cognitive tasks. It has been well documented that there are prefrontal neurons which are involved in cognitive operations such as coding the meaning of a stimulus or retaining information in working memory. On the other hand, there are also prefrontal neurons which show reward-related activity changes. Two kinds of reward-related activities are found in the primate prefrontal cortex; one kind is concerned with processing post-trial events such as coding the reinforcement and/or error, and the other is concerned with the expectancy of the specific reward. These reward-related activities, which appear to be involved in motivational operations, are related also to cognitive operations such as coding the context in which the reward is given or omitted, or monitoring the context concerning the task situation with which the animal is faced. It seems that the prefrontal cortex is involved in the integration of cognitive and motivational information for goal-directed behavior. While the function of the prefrontal cortex is often associated with working memory, I introduce the idea that the prefrontal cortex plays important roles for coding, monitoring and providing the context for goal-directed behavior.

Neural correlates of semantic and episodic memory retrieval

To investigate the functional neuroanatomy associated with retrieving semantic and episodic memories, we measured changes in regional cerebral blood flow (rCBF) with positron emission tomography (PET) while subjects generated single word responses to achromatic line drawings of objects. During separate scans, subjects either named each object, retrieved a commonly associated color of each object (semantic condition), or recalled a previously studied uncommon color of each object (episodic condition). Subjects were also scanned while staring at visual noise patterns to provide a low level perceptual baseline. Relative to the low level baseline, all three conditions revealed bilateral activations of posterior regions of the temporal lobes, cerebellum, and left lateralized activations in frontal regions. Retrieving semantic information, as compared to object naming, activated left inferior temporal, left superior parietal, and left frontal cortices. In addition, small regions of right frontal cortex were activated. Retrieving episodic information, as compared to object naming, activated bilateral medial parietal cortex, bilateral retrosplenial cortex, right frontal cortex, thalamus, and cerebellum. Direct comparison of the semantic and episodic conditions revealed bilateral activation in temporal and frontal lobes in the semantic task (left greater than right), and activation in medial parietal cortex, retrosplenial cortex, thalamus, and cerebellum (but not right frontal regions) in the episodic task. These results support the assertion that distinct neural structures mediate semantic and episodic memory retrieval. However, they also raise questions regarding the specific roles of left temporal and right frontal cortices during episodic memory retrieval, in particular.

Analysis of brain activation patterns using a 3-D scale-space primal sketch

A fundamental problem in brain imaging concerns how to define functional areas consisting of neurons that are activated together as populations. We propose that this issue can be ideally addressed by a computer vision tool referred to as the scale-space primal sketch. This concept has the attractive properties that it allows for automatic and simultaneous extraction of the spatial extent and the significance of regions with locally high activity. In addition, a hierarchical nested tree structure of activated regions and subregions is obtained. The subject in this article is to show how the scale-space primal sketch can be used for automatic determination of the spatial extent and the significance of rCBF changes. Experiments show the result of applying this approach to functional PET data, including a preliminary comparison with two more traditional clustering techniques. Compared to previous approaches, the method overcomes the limitations of performing the analysis at a single scale or assuming specific models of the data.

Memory, consciousness and neuroimaging

Neuroimaging techniques that allow the assessment of memory performance in healthy human volunteers while simultaneously obtaining measurements of brain activity in vivo may offer new information on the neural correlates of particular forms of memory retrieval and their association with consciousness and intention. We consider evidence from studies with positron emission tomography and functional magnetic resonance imaging indicating that priming, a form of implicit retrieval, is associated with decreased activity in various cortical regions. We also consider evidence concerning the question of whether two components of explicit retrieval--intentional or effortful search and successful conscious recollection--are preferentially associated with increased activity in prefrontal and medial temporal regions, respectively. Last, we consider recent efforts to probe the relation between the phenomenological character of remembering and neural activity. In this instance we broaden our scope to include studies employing event-related potentials and consider evidence concerning the neural correlates of qualitatively different forms of memory, including memory that is specifically associated with a sense of self, and the recollection of particular temporal or perceptual features that might contribute to a rich and vivid experience of the past.

A spatio-temporal comparison of semantic and episodic cued recall and recognition using event-related brain potentials

The event-related brain potential (ERP) was used to spatially and temporally map the brain areas active as a function of type of recall (semantic vs. episodic) and episodic retrieval mode (recall vs. recognition) while difficulty of episodic recall was manipulated. ERPs were recorded from 32 scalp sites in 12 subjects, along with behavioral accuracy and recall speed. The results revealed that different but overlapping patterns of ERP activity were elicited during semantic and episodic recall. Recall of both types of information was characterized by ERP activity over left inferior frontal, central, bilateral temporal and posterior inferior brain areas. Compared to semantic recall, episodic recall elicited more activity over the frontal poles and right frontal scalp. Different but overlapping patterns of ERP activity were also found as a function of episodic retrieval mode. While episodic recall and recognition showed similar activity over the frontal poles and central scalp, there was no left inferior frontal activity elicited during recognition and no large, topographically widespread, late positive component (LPC) elicited when the same words were recalled. Manipulation of episodic recall difficulty and analysis of trials when recall failed indicated that these task (i.e., episodic vs. semantic) and retrieval mode (recall vs. recognition) differences in ERP activity were not likely to be due to differences in task difficulty. The results are discussed in terms of processes that the ERP activity may reflect and their similarity to results of PET studies of semantic and episodic retrieval.

Neural correlates of memory retrieval during recognition memory and cued recall

Regional brain activity, measured by H215O PET, was investigated during recognition memory and word-stem cued recall of words in order to compare the neural correlates of two components of memory retrieval-effort and success-as a function of task. For each task there was a baseline and two retrieval conditions. In one retrieval condition (zero density), none of the test items corresponded to words encoded in a preceding study phase. Differences in activity between this condition and the baseline were employed to characterize the neural correlates of retrieval effort in each task. In the other retrieval condition (high density), 80% of the test items had been studied previously. Differences in brain activity between this condition and the zero-density condition were taken to represent the neural correlates of successful retrieval. The principal findings concern the right anterior prefrontal cortex, a region demonstrated previously to be active during episodic retrieval. Relative to baseline, this region showed no evidence of activation in the zero-density condition of the recognition task, but did show enhanced activity in the equivalent condition of the cued-recall task. In contrast, relative to the zero-density condition, the high-density condition was associated with increased right prefrontal activity during recognition, but reduced activity during cued recall. It is proposed that the right prefrontal cortex supports cognitive processes that operate on information retrieved in response to a test item and that these processes contribute to the evaluation of whether the information represents an appropriate prior episode.

Cortical networks implicated in semantic and episodic memory: common or unique?

While previous functional neuroimaging studies have shown that semantic and episodic memory tasks activate different cortical regions, they never compared regional cerebral blood flow (rCBF) patterns associated with semantic and episodic memory within the same experimental design. In this study, we used H2(15)O PET to study subjects in the course of semantic and episodic memory tasks. rCBF was measured in 9 normal volunteers during a resting baseline condition and two cognitive tasks. In the semantic categorisation task subjects heard a list of concrete words and had to respond to words belonging to the "animals" or "food" category. In the episodic recognition task subjects heard a list of concrete words, half "old", i.e. belonging to the list of the semantic categorisation task, and half "new", i.e. presented for the first time. Subjects had to respond to the "old" words. Both tasks were compared to a resting condition. Statistical analysis was performed with Statistical Parametric Mapping (SPM). Compared to the resting condition, the semantic tasks, activated the superior temporal gyri bilaterally, the left frontal cortex, and right premotor cortex. The episodic tasks activated the left superior temporal gyrus, the frontal cortex bilaterally, and the right inferior parietal cortex. Compared to the episodic memory tasks, the semantic memory tasks activated the superior temporal/insular cortex bilaterally and the right premotor cortex. Compared to the semantic memory tasks, the episodic memory tasks activated the right frontal cortex. These results suggest that cortical networks implicated in semantic and episodic memory show both common and unique regions, with the right prefrontal cortex being the neural correlate specific of episodic remembering.

Age-related shift in brain region activity during successful memory performance

Coregistered positron emission tomography (PET)/magnetic resonance imaging (MRI) was used to characterize brain function in 70 volunteers, aged 20-87 years, during a verbal memory task. Frontal activity showed an age-related decline that remained significant after statistical control for sulcal atrophy. Analyses of young and old subgroups matched for memory scores revealed that young good performers activated frontal regions, whereas old good performers relied on occipital regions. Although activating different cortical regions, good performers of all ages used the same cognitive strategy semantic clustering. Age-related functional change may reflect dynamic re-allocation in a network of brain areas, not merely anatomically fixed neuronal loss or diminished capacity to perform.

The effect of emotional content on visual recognition memory: a PET activation study

The emotional content of stimuli can enhance memory for those stimuli. This process may occur via an interaction with systems responsible for perception and memory or via the addition of distinct brain regions specialized for emotion which augment mnemonic processing. We performed an 15O PET study to identify neuroanatomical systems which encode visual stimuli with strong negative emotional valence compared to stimuli with neutral valence. Subjects also performed a recognition memory task for these same images, mixed with distracters of similar emotional valence. The experimental design permitted us to independently test effects of emotional content and recognition memory on regional activity. We found activity in the left amygdaloid complex associated with the encoding of emotional stimuli, although this activation appeared early in the scanning session and was not detectable during recognition memory. Visual recognition memory recruited the right middle frontal gyrus and the superior anterior cingulate cortex for both negative and neutral stimuli. An interaction occurred between emotional content and recognition in the lingual gyrus, where greater activation occurred during recognition of negative images compared to recognition of neutral images. Instead of distinct neuroanatomical systems for emotion augmenting memory, we found that emotionally salient stimuli appeared to enhance processing of early sensory input during visual recognition.

The functional neuroanatomy of episodic memory: the role of the frontal lobes, the hippocampal formation, and other areas

Because it allows direct mapping of synaptic activity during behavior in the normal subject, functional neuroimaging with the activation paradigm, especially positron emission tomography, has recently provided insight into our understanding of the functional neuroanatomy of episodic memory over and above established knowledge from lesional neuropsychology. The most striking application relates to the ability to distinguish the structures implicated in the encoding and the retrieval of episodic information, as these processes are extremely difficult to differentiate with behavioral tasks, either in healthy subjects or in brain-damaged patients. Regarding encoding and retrieval, the results from most studies converge on the involvement of the prefrontal cortex in these processes, with a hemispheric encoding/retrieval asymmetry (HERA) such that the left side is preferentially involved in encoding, and the right in retrieval. However, there are still some questions, for instance, about bilateral activation during retrieval and a possible specialization within the prefrontal cortex. More expected from human and monkey lesional data, the hippocampal formation appears to play a role in both the encoding and the retrieval of episodic information, but the exact conditions which determine hippocampal activation and its fine-grained functional neuroanatomy have yet to be fully elucidated. Other structures are activated during episodic memory tasks, with asymmetric activation that fits the HERA model, such as preferentially left-sided activation of the association temporal and posterior cingulate areas in encoding tasks and preferentially right-sided activation of the association parietal cortex, cerebellum, and posterior cingulate in retrieval tasks. However, this hemispheric asymmetry appears to depend to some extent on the material used. These new data enhance our capacity to comprehend episodic memory deficits in neuropsychology, as well as the neural mechanisms underlying the age-related changes in episodic memory performances.

Neural response during preference and memory judgments for subliminally presented stimuli: a functional neuroimaging study

Preexposing subjects to visual stimuli is sufficient to establish a subsequent preference, even when previous exposure is subliminal, such that explicit recognition is at chance. This influence of previous exposure on preference judgments, known as the "mere exposure effect," is a form of unconscious memory. The present functional neuroimaging study examines the mechanism of this effect. Nine volunteer subjects were studied using functional imaging while making forced choice judgments about abstract stimuli on the basis of either preference or memory. Each judgment type was made under two conditions: under one condition one or the other stimulus had previously been presented subliminally, whereas under the second condition both stimuli were novel. Memory judgments were associated with activation of left frontopolar cortex and parietal areas, whereas preference judgments were associated with activation of medial prefrontal cortex and regions of occipital cortex. The modulation of preference by objective familiarity (implicit memory) was associated with right lateral frontal activation. Significant activation of hippocampal gyrus was seen in response to objective stimulus novelty, regardless of judgment type required. Our data thus demonstrate activations of a memory system independent of recollective experience. Dissociable activations within this system implicate a frontopolar involvement in explicit retrieval attempt and right lateral prefrontal cortex involvement in implicit memory expressed in preference judgments. Furthermore, the results suggest that hippocampal response to stimulus novelty can be independent of conscious reportability of familiarity.

On the relations among priming, conscious recollection, and intentional retrieval: evidence from neuroimaging research

Neurobiological distinctions among forms of memory have been investigated mainly from the perspective of lesion studies in nonhuman animals and experiments with human neurological patients. We consider recent neuroimaging studies of healthy human volunteers using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) that provide new information concerning the neural correlates of particular forms of memory retrieval. More specifically, we consider evidence indicating that priming, a form of implicit retrieval, is associated with decreased activity in various cortical regions. We also consider evidence suggesting that two components of explicit retrieval-intentional or effortful search and successful conscious recollection-are preferentially associated with increased activity in prefrontal and medial temporal regions, respectively.

Brain activation during word identification and word recognition

Previous memory research has suggested that the effects of prior study observed in priming tasks are functionally, and neurobiologically, distinct phenomena from the kind of memory expressed in conventional (explicit) memory tests. Evidence for this position comes from observed dissociations between memory scores obtained with the two kinds of tasks. However, there is continuing controversy about the meaning of these dissociations. In recent studies, Ostergaard (1998a, Memory Cognit, 26:40-60; 1998b, J. Int. Neuropsychol. Soc., in press) showed that simply degrading visual word stimuli can dramatically alter the degree to which word priming shows a dissociation from word recognition; i.e., effects of a number of factors on priming paralleled their effects on recognition memory tests when the words were degraded at test. In the present study, cerebral blood flow changes were measured while subjects performed the word identification (reading) and recognition memory tasks used previously by Ostergaard. The results are the direct comparisons of the two tasks and the effects of stimulus degradation on blood flow patterns during the tasks. Clear differences between word identification and word recognition were observed: the latter task evoked considerably more prefrontal activity and stronger cerebellar activation. Stimulus degradation was associated with focal increases in bilateral fusiform regions within the occipital lobe. No task, degradation, or item repetition effects were demonstrated in mesial temporal regions, no repetition effects were observed in any region, and there was no evidence for different effects of stimulus degradation in the priming and recognition memory conditions. Power limitations may have contributed to the null effects.

fMRI study of face perception and memory using random stimulus sequences

A new functional magnetic resonance imaging (fMRI) method was used to investigate the functional neuroanatomy of face perception and memory. Whole-brain fMRI data were acquired while four types of stimuli were presented sequentially in an unpredictable pseudorandom order at a rate of 0.5 Hz. Stimulus types were a single repeated memorized target face, unrepeated novel faces, nonsense scrambled faces, and a blank screen. Random stimulus sequences were designed to generate a functional response to each stimulus type that was uncorrelated with responses to other stimuli. This allowed fMRI responses to each stimulus type to be examined separately using multiple regression. Signal increases were found for all stimuli in ventral posterior cortex. Responses to intact faces extended to more anterior locations of occipitotemporal cortex than did responses to scrambled faces, consistent with previous studies of face perception. Responses evoked by novel faces were in regions of ventral occipitotemporal cortex medial to regions in which significant responses were evoked by the target face. The repeated target face stimulus also evoked activity in widely distributed regions of frontal and parietal cortex. These results demonstrate that cortical hemodynamic responses to interleaved novel and repeated stimuli can be distinguished and measured using fMRI with appropriate stimulus sequences and data analysis methods. This method can now be used to examine the neural systems involved in cognitive tasks that were previously impossible to study using positron emission tomography or fMRI.

Electrophysiological evidence for dissociable processes contributing to recollection

This paper reviews a number of studies in which we have employed event-related potentials (ERPs) to investigate the cognitive processes which contribute to conscious recollection. Across a range of tasks (including recognition memory, source memory, associative recall and word-stem cued recall) there is evidence for the proposal that recollection involves processes which are both functionally and neurologically dissociable. This evidence takes the form of temporally and topographically dissociable ERP effects, which attain their maximum amplitude when elicited by items that satisfy operational definitions for having been recollected. The ERP effects are interpreted as reflecting retrieval and post-retrieval processes which, we argue, constitute two separate components of recollection as defined within the process dissociation framework of Jacoby and colleagues. The ERP findings suggest that post-retrieval processing is particularly sensitive to task variables, implying that recollection may be neither functionally nor neurologically homogeneous.

Functional-anatomic study of episodic retrieval using fMRI. I. Retrieval effort versus retrieval success

A number of recent functional imaging studies have identified brain areas activated during tasks involving episodic memory retrieval. The identification of such areas provides a foundation for targeted hypotheses regarding the more specific contributions that these areas make to episodic retrieval. As a beginning effort toward such an endeavor, whole-brain functional magnetic resonance imaging (fMRI) was used to examine 14 subjects during episodic word recognition in a block-designed fMRI experiment. Study conditions were manipulated by presenting either shallow or deep encoding tasks. This manipulation yielded two recognition conditions that differed with regard to retrieval effort and retrieval success: shallow encoding yielded low levels of recognition success with high levels of retrieval effort, and deep encoding yielded high levels of recognition success with low levels of effort. Many brain areas were activated in common by these two recognition conditions compared to a low-level fixation condition, including left and right prefrontal regions often detected during PET episodic retrieval paradigms (e.g., R. L. Buckner et al., 1996, J. Neurosci. 16, 6219-6235) thereby generalizing these findings to fMRI. Characterization of the activated regions in relation to the separate recognition conditions showed (1) bilateral anterior insular regions and a left dorsal prefrontal region were more active after shallow encoding, when retrieval demanded greatest effort, and (2) right anterior prefrontal cortex, which has been implicated in episodic retrieval, was most active during successful retrieval after deep encoding. We discuss these findings in relation to component processes involved in episodic retrieval and in the context of a companion study using event-related fMRI.

On the relationship between recognition familiarity and perceptual fluency: evidence for distinct mnemonic processes

Fluent reprocessing of perceptual aspects of recently experienced stimuli is thought to support repetition priming effects on implicit perceptual memory tests. Although behavioral and neuropsychological dissociations demonstrate that separable mnemonic processes and neural substrates mediate implicit and explicit test performance, dual-process theories of memory posit that explicit recognition memory judgments may be based on familiarity derived from the same perceptual fluency that yields perceptual priming. Here we consider the relationship between familiarity-based recognition memory and implicit perceptual memory. A select review of the literature demonstrates that the fluency supporting implicit perceptual memory is functionally and anatomically distinct from that supporting recognition memory. In contrast to perceptual fluency, recognition familiarity is more sensitive to conceptual than to perceptual processing, and does not depend on modality-specific sensory cortices. Alternative possible relationships between familiarity in explicit memory and fluency in implicit memory are discussed.

Cognitive neuroscience of human memory

Current knowledge is summarized about long-term memory systems of the human brain, with memory systems defined as specific neural networks that support specific mnemonic processes. The summary integrates convergent evidence from neuropsychological studies of patients with brain lesions and from functional neuroimaging studies using positron emission tomography (PET) or functional magnetic resonance imaging (fMRI). Evidence is reviewed about the specific roles of hippocampal and parahippocampal regions, the amygdala, the basal ganglia, and various neocortical areas in declarative memory. Evidence is also reviewed about which brain regions mediate specific kinds of procedural memory, including sensorimotor, perceptual, and cognitive skill learning; perceptual and conceptual repetition priming; and several forms of conditioning. Findings are discussed in terms of the functional neural architecture of normal memory, age-related changes in memory performance, and neurological conditions that affect memory such as amnesia. Alzheimer's disease, Parkinson's disease, and Huntington's disease.

Functional neuroimaging studies of encoding, priming, and explicit memory retrieval

Human functional neuroimaging techniques provide a powerful means of linking neural level descriptions of brain function and cognition. The exploration of the functional anatomy underlying human memory comprises a prime example. Three highly reliable findings linking memory-related cognitive processes to brain activity are discussed. First, priming is accompanied by reductions in the amount of neural activation relative to naive or unprimed task performance. These reductions can be shown to be both anatomically and functionally specific and are found for both perceptual and conceptual task components. Second, verbal encoding, allowing subsequent conscious retrieval, is associated with activation of higher order brain regions including areas within the left inferior and dorsal prefrontal cortex. These areas also are activated by working memory and effortful word generation tasks, suggesting that these tasks, often discussed as separable, might rely on interdependent processes. Finally, explicit (intentional) retrieval shares much of the same functional anatomy as the encoding and word generation tasks but is associated with the recruitment of additional brain areas, including the anterior prefrontal cortex (right > left). These findings illustrate how neuroimaging techniques can be used to study memory processes and can both complement and extend data derived through other means. More recently developed methods, such as event-related functional MRI, will continue this progress and may provide additional new directions for research.

Networks of domain-specific and general regions involved in episodic memory for spatial location and object identity

Positron emission tomography (PET) was used to investigate human episodic memory for spatial location and object identity. We measured regional cerebral bloodflow (rCBF) while subjects engaged in perceptual matching of the location or the identity of line drawings of objects. Perceptual matching also involved incidental encoding of the presented information. Subsequently, rCBF was measured when subjects retrieved the location or the identity of these objects from memory. Using the multivariate partial least squares image analysis, we identified three patterns of activity across the brain that allowed us to distinguish structures that are differentially involved in processing spatial location and object identity from structures that are differentially involved in encoding and retrieval but operate across both domains. Domain-specificity was evident by increased rCBF during the processing of spatial location in the right middle occipital gyrus, supramarginal gyrus, and superior temporal sulcus, and by increased rCBF during the processing of object identity in portions of bilateral lingual and fusiform gyri. There was a nearly complete overlap between domain-specific dorsal and ventral extrastriate cortex activations during perceptual matching and memory retrieval. Evidence of domain-specificity was also found in the prefrontal cortex and the left hippocampus, but the effect interacted with encoding and retrieval. Domain-general structures included bilateral superior temporal cortex regions, which were preferentially activated during encoding, and portions of bilateral middle and inferior frontal gyri, which were preferentially activated during retrieval. Together, our data suggest that encoding and retrieval in episodic memory depend on the interplay between domain-specific structures, most of which are involved in memory as well as perception, and domain-general structures, some of which operate more at encoding and others more at retrieval.

Mapping episodic memory

This paper presents an analysis of brain regions generally associated with a frequently used episodic memory task; visual word recognition. The results from five positron emission tomography studies of regional cerebral blood flow, involving a total of nine pairwise comparisons of brain activity related to episodic retrieval and to performance on non-episodic reference tasks, were considered. Across studies, increased activity has been observed in the right anterior and posterior prefrontal, anterior cingulate, bilateral parietal, and cerebellar regions. Decreased activity has been found in bilateral temporal and left midfrontal regions. Comparison of this activation pattern with those of other memory tasks, episodic and semantic, indicate that the right anterior and posterior prefrontal regions guide processes selectively demanded by episodic memory retrieval. There is suggestive evidence from subtraction analyses that these prefrontal regions are activated by different processing components, and analyses of functional connectivity provide further support for functional differentiation. These analyses also point to a critical role of medial-temporal brain regions in episodic retrieval. Taken together, these results show that episodic memory retrieval is mediated by an extensive set of brain regions, some of which seem to be specifically engaged by episodic remembering.

Lack of an association between delayed memory and hippocampal and temporal lobe size in patients with schizophrenia and healthy controls

The purpose of the present study was to investigate putative neural substrates of long-term (delayed) memory in schizophrenia and young healthy controls. Ten "low" and 10 "high" memory patients were selected from a large sample of DSM-III-R diagnosed schizophrenia spectrum patients, based on composite verbal and nonverbal delayed recall memory scores. Ten "low" and 9 "high" memory individuals were also selected from a larger sample of young healthy controls. Magnetic resonance imaging scans were acquired on a 1.5-T GE Signa scanner using a SPGR sequence (repetition time = 24 msec, echo time = 5 msec). Hippocampal volumes were computed from manual tracings (intraclass correlation = .96), and temporal lobe and whole brain tissue volumes were obtained using a semiautomated technique. In both the patient sample and controls, there was no significant relationship between delayed memory ability and hippocampal, temporal lobe, or whole brain volume. The integration of results from this study, and from studies on normal aging and Alzheimer's disease, supports a model suggesting that hippocampal size may be an indicator of long-term memory ability, but only when hippocampal measures reflect aging and degenerative hippocampal atrophy. If the hippocampal measures reflect individual differences in hippocampal size prior to the onset of hippocampal atrophy, then hippocampal size does not appear to predict long-term memory ability.

Source monitoring and memory distortion

Memory distortion reflects failures to identify the sources of mental experience (reality monitoring failures or source misattributions). For example, people sometimes confuse what they inferred or imagined and what actually happened, what they saw and what was suggested to them, one person's actions and another's what they heard and what they previously knew, and fiction and fact. Source confusions arise because activated information is incomplete or ambiguous and the evaluative processes responsible for attributing information to sources are imperfect. Both accurate and inaccurate source attributions result from heuristic processes and more reflectively complex processes that evaluate a mental experience for various qualities such as amount and type of perceptual, contextual, affective, semantic and cognitive detail, that retrieve additional supporting or disconfirming evidence, and that evaluate plausibility and consistency given general knowledge, schemes, biases and goals. Experimental and clinical evidence regarding cognitive mechanisms and underlying brain structures of source monitoring are discussed.

False recognition in younger and older adults: exploring the characteristics of illusory memories

Roediger and McDermott (1995) demonstrated that when subjects hear a list of associates to a "theme word" that has itself not been presented, they frequently claim to recollect having heard the nonpresented theme word on the study list. In Experiment 1, we found that asking subjects to explain their remember responses, by writing down exactly what they remembered about the item's presentation at study, did not significantly diminish the rate of remember false alarms to nonpresented theme words. We also found that older adults were relatively more susceptible than younger adults to this false-recognition effect. Subjects' explanations suggested that both veridical and illusory memories were predominantly composed of associative information as opposed to sensory and contextual detail. In Experiment 2, we obtained quantitative evidence for this conclusion, using a paradigm in which subjects were asked focused questions about the contents of their recollective experience. Lastly, we found that both younger and older adults recalled more sensory and contextual detail in conjunction with studied items than with nonpresented theme words, although these differences were less pronounced in older adults.

Brain regions differentially involved in remembering what and when: a PET study

Recollecting a past episode involves remembering not only what happened but also when it happened. We used positron emission tomography (PET) to directly contrast the neural correlates of item and temporalorder memory. Subjects studied a list of words and were then scanned while retrieving information about what words were in the list or when they occurred within the list. Item retrieval was related to increased neural activity in medial temporal and basal forebrain regions, whereas temporal-order retrieval was associated with activations in dorsal prefrontal, cuneus/precuneus, and right posterior parietal regions. The dissociation between temporal and frontal lobe regions confirms and extends previous lesion data. The results show that temporal-order retrieval involves a network of frontal and posterior brain regions.

Frontal and parietal networks for conditional motor learning: a positron emission tomography study

Studies on nonhuman primates show that the premotor (PM) and prefrontal (PF) areas are necessary for the arbitrary mapping of a set of stimuli onto a set of responses. However, positron emission tomography (PET) measurements of regional cerebral blood flow (rCBF) in human subjects have failed to reveal the predicted rCBF changes during such behavior. We therefore studied rCBF while subjects learned two arbitrary mapping tasks. In the conditional motor task, visual stimuli instructed which of four directions to move a joystick (with the right, dominant hand). In the evaluation task, subjects moved the joystick in a predetermined direction to report whether an arrow pointed in the direction associated with a given stimulus. For both tasks there were three rules: for the nonspatial rule, the pattern within each stimulus determined the correct direction; for the spatial rule, the location of the stimulus did so; and for the fixed-response rule, movement direction was constant regardless of the pattern or its location. For the nonspatial rule, performance of the evaluation task led to a learning-related increase in rCBF in a caudal and ventral part of the premotor cortex (PMvc, area 6), bilaterally, as well as in the putamen and a cingulate motor area (CM, area 24) of the left hemisphere. Decreases in rCBF were observed in several areas: the left ventro-orbital prefrontal cortex (PFv, area 47/12), the left lateral cerebellar hemisphere, and, in the right hemisphere, a dorsal and rostral aspect of PM (PMdr, area 6), dorsal PF (PFd, area 9), and the posterior parietal cortex (area 39/40). During performance of the conditional motor task, there was only a decrease in the parietal area. For the spatial rule, no rCBF change reached significance for the evaluation task, but in the conditional motor task, a ventral and rostral premotor region (PMvr, area 6), the dorsolateral prefrontal cortex (PFdl, area 46), and the posterior parietal cortex (area 39/40) showed decreasing rCBF during learning, all in the right hemisphere. These data confirm the predicted rCBF changes in premotor and prefrontal areas during arbitrary mapping tasks and suggest that a broad frontoparietal network may show decreased synaptic activity as arbitrary rules become more familiar.

Positron-emission tomography imaging of long-term shape recognition challenges

Long-term visual memory performance was impaired by two types of challenges: a diazepam challenge on acquisition and a sensory challenge on recognition. Using positron-emission tomography regional cerebral blood flow imaging, we studied the effect of these challenges on regional brain activation during the delayed recognition of abstract visual shapes as compared with a baseline fixation task. Both challenges induced a significant decrease in differential activation in the left fusiform gyrus, suggesting that this region is involved in the automatic or volitional comparison of incoming and stored stimuli. In contrast, thalamic differential activation increased in response to memory challenges. This increase might reflect enhanced retrieval attempts as a compensatory mechanism for restoring recognition performance.

Time course of cortical activations in implicit and explicit recall

The distinction between implicit and explicit retrieval of learned material is central to recent thinking about the neural systems underlying memory. Word stem completion is one task in which subjects can be instructed either to make a deliberate recall (explicit instruction) or to be told to complete the stem with any appropriate word (implicit instruction). Positron emission tomography (PET) studies have indicated that during implicit retrieval, there is reduced blood flow in right posterior areas, whereas some tasks of explicit retrieval involve frontal and hippocampal activation. However, there is no information about the timing of these activations or how implicit and explicit retrieval might be related. We used word stem completion tasks similar to those used in the PET studies, but used high-density electrical recording designed to allow localization of the regions involved in the tasks and to provide temporal information. We found reduced activity for primed words in right posterior cortex corresponding to previous PET results. The reduction occurred within the first 200 msec after input, suggesting early interaction with the information stored in this area. Similar reductions observed during explicit recall of the previously presented words indicate that priming is similar under implicit and explicit conditions. In addition, when priming was not an adequate basis for response, then frontal areas were active. Retrieval of unprimed words under implicit instruction elicited right frontal activation, whereas explicit retrieval activated frontal areas bilaterally. Left frontal and hippocampal activations appear to occur only when the retrieval involved use of the words from the list studied previously.

Commonality of processes underlying visual and verbal recognition memory

Memory for visual and verbal material engages widely distributed systems, to a large degree focussed in different hemispheres. It might thus be expected that these disparate neuronal populations should display significantly different characteristics in regard to mnemonic performance. Visual memory, fundamental to all human beings, and whose characteristics are largely shared with macaques, was assayed using unique non-objective, colored images lacking ready verbal description and was contrasted with memory for four-letter non-offensive English words. The effects of memory loading, stimulus duration and long-term test intervals (1-2 weeks) were studied in regard to accuracy and reaction times for recognizing initial versus re-exposure to these two types of items. No effects of memory loading were apparent despite the incrementing memory load in the 240-item, running recognition sessions. Words were better remembered than images, both in the long and short term, but the detailed characteristics of reaction times and accuracy in relation to number of intervening items, and in long-term memory were strikingly similar. Given the wide and well-established disparity in cerebral loci participating in linguistic versus image analysis, these multiple similarities in the pattern of mnemonic performance indicate that the underlying neuronal processes must be comparable for remembering either images or words. Furthermore, the strong link manifested between individual items across a varying number of intervening intervals and added items suggests that a phenomenon highly similar to the "stimulus specific adaptation" (SSA), displayed by units in macaque inferotemporal cortex, occurs for each item to be recognized. Finally, the significant augmentation in accuracy both in short- and long-term memory for images when viewing time permits saccades is explained if each saccade and fixational pause recruits additional neurons into the pool displaying SSA, or its equivalent, for the item being viewed.

Frontal lobes pathology and dementia. An appraisal of the contribution of Leonardo Bianchi

This paper which was inspired by Leonardo Bianchi's work, published in English a hundred years ago, summarizes the main features of frontal dementias, with particular emphasis on Pick's Disease (PiD) and on Frontal Lobe Dementia (FLD). We intend to examine on one hand whether these behavioral changes follow pathology strictly limited to the frontal lobes and on the other hand whether these changes truly constitute a dementia. Currently available data suggest the following conclusions: 1) Lesions to the frontal lobes produce a dementia which is clearly different from the typical picture of Alzheimer's disease and which can be called a behavioral dementia. 2) The pathology of the so-called frontal dementias usually extends beyond the limits of the prefrontal cortex. 3) Executive functions classically thought to be related to frontal lobe structures are in fact associated with structures outside the frontal lobes, particularly in the telencephalic and limbic cortex. These findings in no way diminish the value of the work of Leonardo Bianchi. Rather, they strengthen it. We consider that the work of this great Neuroscientist remains highly relevant and that after 100 years, it still represents the starting basis for further works and ideas.

Event-related brain potential correlates of two states of conscious awareness in memory

We report an event-related potential (ERP) experiment of human recognition memory that explored the relation between conscious awareness and electrophysiological activity of the brain. We recorded ERPs from healthy adults while they made "remember" and "know" recognition judgments about previously seen words. These two kinds of judgments reflect "autonoetic" and "noetic" awareness, respectively. The ERP effects differed between the two kinds of awareness while they were similar for "true" and "false" recognition. Noetic awareness was associated with a temporoparietal positivity in the N400 range (325-600 ms) and a late (600-1,000 ms) frontocentral negativity, whereas autonoetic awareness was associated with a widespread, late, bifrontal and left parietotemporal (600-1000 ms) positivity. In the very late (1,300-1, 900 ms) time window, a right frontal positivity was observed for both remember and know judgments of both true and false targets. These results provide physiological evidence for two types of conscious awareness in episodic memory retrieval.

The functional neuroanatomy of episodic memory

Functional neuroimaging studies have revealed that effective encoding in episodic memory is associated with enhanced activity in left prefrontal cortex, whereas retrieval is accompanied by the enhancement of predominantly right-sided prefrontal activity. The extent of the contribution of prefrontal cortex to episodic memory, and the fact that the encoding and retrieval operations it supports are differentially lateralized, were unexpected on the basis of evidence from lesion studies. Such studies have highlighted the crucial role in episodic memory played by the hippocampus and related medial temporal lobe structures. Neuroimaging studies, however, have had only limited success in elucidating the role of the hippocampus in episodic memory. Refinements in experimental design and improved spatial resolution should promote rapid future progress with respect to this issue.

Factors that influence effect size in 15O PET studies: a meta-analytic review

The PET literature is growing exponentially, creating a need and an opportunity to perform a meta-analytic review consolidating the published information. This study describes the use of effect size as an index in PET studies and discusses how this measure can be used for comparing findings across studies, laboratories, and paradigms. In comparing studies across laboratories it is essential to know how the methods employed affect the results and conclusions drawn. This study also compared effect size for two different methods of tracer delivery in 15O PET studies ([15O]H2O bolus injection versus inhalation of [15O]CO2), whether averaged versus single-scan conditions were used, and the data analytic strategy employed. The effect sizes observed across studies were consistently large with a median effect size of 8.55, indicating that the phenomena investigated in 15O PET studies are strong. The largest peak activation reported in a study was found to be affected by variability in sample size, data analytic strategy, and repeat versus single-scan conditions. However, the impact of these factors was not examined on smaller or less intense peaks. Minimal standards for reporting statistical results are discussed.

Memory beyond the hippocampus

Improved neuroanatomical knowledge, technical and methodological innovations (such as PET), and more refined conceptualizations of memory have inspired a reappraisal of theoretical beliefs regarding the role of the hippocampus in memory. In the past few years, it has become apparent that the influence of the medial temporal lobe regions extends beyond memory and that memory processes (such as encoding, consolidation and retrieval) involve not only the hippocampus and the medial temporal and diencephalic regions, but also widely distributed neocortical and perhaps even cerebellar regions.

Age-related differences in neural activity during memory encoding and retrieval: a positron emission tomography study

Positron emission tomography (PET) was used to compare regional cerebral blood flow (rCBF) in young (mean 26 years) and old (mean 70 years) subjects while they were encoding, recognizing, and recalling word pairs. A multivariate partial-least-squares (PLS) analysis of the data was used to identify age-related neural changes associated with (1) encoding versus retrieval and (2) recognition versus recall. Young subjects showed higher activation than old subjects (1) in left prefrontal and occipito-temporal regions during encoding and (2) in right prefrontal and parietal regions during retrieval. Old subjects showed relatively higher activation than young subjects in several regions, including insular regions during encoding, cuneus/precuneus regions during recognition, and left prefrontal regions during recall. Frontal activity in young subjects was left-lateralized during encoding and right-lateralized during recall [hemispheric encoding/retrieval asymmetry (HERA)], whereas old adults showed little frontal activity during encoding and a more bilateral pattern of frontal activation during retrieval. In young subjects, activation in recall was higher than that in recognition in cerebellar and cingulate regions, whereas recognition showed higher activity in right temporal and parietal regions. In old subjects, the differences in blood flow between recall and recognition were smaller in these regions, yet more pronounced in other regions. Taken together, the results indicate that advanced age is associated with neural changes in the brain systems underlying encoding, recognition, and recall. These changes take two forms: (1) age-related decreases in local regional activity, which may signal less efficient processing by the old, and (2) age-related increases in activity, which may signal functional compensation.

Event-related potentials and the recognition memory exclusion task

Subjects heard words that were presented in either a male or a female voice, and were required to perform one of two encoding tasks according to the gender of the voice. At test studied words were presented visually, along with a set of words new to the experiment. Subjects were required to respond on one key to words belonging to one of the two classes of studied word (targets), and to respond on a different key both to words belonging to the other study class (non-targets), and to words new to the experiment. In comparison to the event-related potentials (ERPs) elicited by new words, the ERPs elicited by correctly detected targets exhibited two temporally and topographically distinct positive going effects: one of these was phasic, showed a parietal maximum, and was larger over the left than the right hemisphere. The second effect was more sustained in time, frontally distributed, and was larger over the right hemisphere. The ERPs elicited by correctly classified non-targets contained the parietal effect only. These findings confirm that retrieval of contextual information in tests of recognition memory (recollection) is associated with two distinct ERP modulations. While one of these may be closely tied to process necessary for recollection, the other may reflect less obligatory processes which operate on the products of successful retrieval.