Triple Dissociation in the Medial Temporal Lobes: Recollection, Familiarity, and Novelty

Memory for past events may be based on retrieval accompanied by specific contextual details (recollection) or on the feeling that an item is old (familiarity) or new (novelty) in the absence of contextual details. There are indications that recollection, familiarity, and novelty involve different medial temporal lobe subregions, but available evidence is scarce and inconclusive. Using functional magnetic resonance imaging (MRI), we isolated retrieval-related activity associated with recollection, familiarity, and novelty by distinguishing between linear and nonlinear oldness functions derived from recognition confidence levels. Within the medial temporal lobes (MTLs), we found a triple dissociation among the posterior half of the hippocampus, which was associated with recollection, the posterior parahippocampal gyrus, which was associated with familiarity, and anterior half of the hippocampus and rhinal regions, which were associated with novelty. Furthermore, multiple regression analyses based on individual trial activity showed that all three memory signals, i.e., recollection, familiarity, and novelty, make significant and independent contributions to recognition memory performance. Finally, functional dissociations among recollection, familiarity, and novelty were also found in posterior midline, left parietal cortex, and prefrontal cortex regions. This is the first study to reveal a triple dissociation within the MTL associated with distinct retrieval processes. This finding has direct implications for current memory models.

Differential parahippocampal and retrosplenial involvement in three types of visual scene recognition

Human observers can quickly and accurately interpret the meaning of complex visual scenes. The neural mechanisms underlying this ability are largely unexplored. We used functional magnetic resonance imaging to measure cortical activity while subjects identified briefly presented scenes as specific familiar locations ("Houston Hall"), general place categories ("kitchen"), or general situational categories ("party"). Scene-responsive voxels in the parahippocampal place area (PPA) and retrosplenial cortex (RSC) were highly sensitive to recognition level when identifying scenes, responding more strongly during location identification than during place category or situation identification. In contrast, the superior temporal sulcus, cingulate sulcus, and supermarginal gyrus displayed the opposite pattern, responding more strongly during place category and situation identification. Consideration of results from 4 experiments suggests that the PPA represents the visuospatial structure of individual scenes, whereas RSC supports processes that allow scenes to be localized within a larger extended environment. These results suggest that different scene identification tasks tap distinct cortical networks. In particular, we hypothesize that the PPA and RSC are critically involved in the identification of specific locations but play a less central role in other scene recognition tasks.

The Parahippocampal Cortex Mediates Spatial and Nonspatial Associations

The parahippocampal cortex (PHC) has been implicated in the processing of place-related information. It has also been implicated in episodic memory, even for items that are not related to unique places. How could the same cortical region mediate such seemingly different cognitive processes? Both processes rely on contextual associations, and we therefore propose that the PHC should be viewed not as exclusively dedicated for analyzing place-related information, or as solely processing episodic memories, but instead as more generally playing a central role in contextual associative processing. To test this proposal, we created a novel learning paradigm to form new associations among meaningless visual patterns. These new associations were created to emulate either spatial or nonspatial contexts. Both spatial and nonspatial associations activated the PHC more than noncontextual items. Moreover, items from spatial contexts activated the posterior part of the PHC, whereas items from nonspatial contexts activated the anterior PHC. Therefore, we show that the PHC plays a role of processing contextual associations in general, and that these associations are not restricted to spatial information. By modifying the existing view of the PHC function accordingly, the seemingly contradicting processes that activate it can be reconciled under one overarching framework.

Distinct roles for medial temporal lobe structures in memory for objects and their locations

The ability to learn and retain novel information depends on a system of structures in the medial temporal lobe (MTL) including the hippocampus and the surrounding entorhinal, perirhinal, and parahippocampal cortices. Damage to these structures produces profound memory deficits; however, the unique contribution to memory of each of these structures remains unclear. Here we have used functional magnetic resonance imaging (fMRI) to determine whether the perirhinal and parahippocampal cortices show differential memory-related activity. Based on the distinct patterns of cortical input to these two areas, we reasoned that these structures might show differential activity for spatial and object recognition memory. In each of 11 subjects, we found that the perirhinal cortex was active during both spatial and object memory encoding, while the anterior parahippocampal cortex was active only during spatial encoding. These data support the idea that MTL structures make distinct contributions to recognition memory performance.

A reversal of the normal pattern of parahippocampal response to neutral and fearful faces is associated with reality distortion in schizophrenia

BACKGROUND

Individuals with schizophrenia demonstrate impaired recognition of facial expressions and may misattribute emotional salience to otherwise nonsalient stimuli. The neural mechanisms underlying this deficit and the relationship with different symptoms remain poorly understood.

METHODS

We used event-related functional magnetic resonance imaging to measure neural responses to neutral, mildly fearful, and prototypically fearful facial expressions. The sample included 15 medicated individuals with chronic schizophrenia (SZ) and 11 healthy control individuals (CON), matched for gender (all male), age, and years of education.

RESULTS

A repeated measures 3 x 2 analysis of variance (ANOVA) revealed a significant interaction between expression intensity and group in right parahippocampal gyrus (p < .01). Individuals with chronic schizophrenia demonstrated a decrease, whereas CON showed an increase, in right parahippocampal gyrus response to increasingly fearful expressions. Between-group comparison revealed greater activation in SZ than CON in right parahippocampal gyrus to neutral faces. The reality distortion dimension, but not neuroleptic medication dose, was positively associated with the right parahippocampal gyral and right amygdalar response to neutral faces in SZ.

CONCLUSIONS

An abnormally increased parahippocampal response to neutral faces was positively associated with reality distortion in SZ. This may underlie the previously reported finding of a misattribution of emotional salience to nonsalient social stimuli in schizophrenia.

Time-frequency intracranial source localization of feedback-related EEG activity in hypothesis testing

The neural correlates of the response to performance feedback have been the object of numerous neuroimaging studies. However, the precise timing and functional meaning of the resulting activations are poorly understood. We studied the electroencephalographic response time locked to positive and negative performance feedback in a hypothesis testing paradigm. The signal was convoluted with a family of complex wavelets. Intracranial sources of activity at various narrow-band frequencies were estimated in the 100- to 400-ms time window following feedback onset. Positive and negative feedback were associated to 1) early parahippocampo-cingular sources of alpha oscillations, more posteriorly located and long lasting for negative feedback and to 2) late partially overlapping neural circuits comprising regions in prefrontal, cingular, and temporal cortices but operating at feedback-specific latencies and frequencies. The results were interpreted in the light of neurophysiological models of feedback and were used to discuss methodological issues in the study of high-level cognitive functions, including reasoning and decision making.

Gender differences in regional cerebral activity during the perception of emotion: A functional MRI study

Whether men activate different brain regions during various emotions compared to women or whether gender differences exist in transient emotional states has been the subject of only few studies. We used event-related functional magnetic resonance imaging (fMRI) to investigate gender differences during the perception of positive or negative emotions. The experiment comprised two emotional conditions (pleasant/unpleasant visual stimuli) during which fMRI data were acquired. Altogether, 38 healthy volunteers (19 males, 19 females) were investigated. When subtracting the activation values of men from those of women, suprathreshold positive signal changes were detected in the right posterior cingulate, the left putamen and the left cerebellum during positive mood induction, and in bilateral superior temporal gyri and cerebellar vermis during negative mood induction. The subtraction of activation values of women from those of men yielded no significant differences. Our findings suggest gender-related neural responses to emotional stimuli and could contribute to the understanding of mechanisms underlying gender-related vulnerability of the prevalence and severity of neuropsychiatric disorders.

Ultrastructural organization of medial prefrontal inputs to the rhinal cortices

Accumulating evidence suggests that the medial prefrontal cortex (mPFC) plays a critical role in the formation, retrieval and long-term storage of hippocampal-dependent memories. Consistent with this, there are direct hippocampal projections to the mPFC. Moreover, the mPFC sends robust projections to the perirhinal and entorhinal cortices, two interconnected cortical fields that funnel information into and out of the hippocampus. However, the significance of the latter projection remains unclear because no data are available regarding the rhinal targets of mPFC axons. This question was examined in the present study using a combination of anterograde tracing with Phaseolus vulgaris leucoagglutinin and pre-embedding gamma-aminobutyric acid (GABA) immunocytochemistry in guinea pigs. Following Phaseolus vulgaris leucoagglutinin injections in the mPFC, anterogradely labeled axons were seen in the perirhinal (mainly superficial layers) and lateral entorhinal (mainly deep layers) cortices. In the electron microscope, the synaptic articulation of anterogradely labeled mPFC axon terminals with perirhinal and entorhinal neurons was found to be nearly identical. In these two rhinal fields, mPFC axon terminals only formed asymmetric synapses, typically with GABA-immunonegative spines ( approximately 70%) but occasionally with dendritic profiles ( approximately 30%), half of which were GABA immunopositive. In the light of earlier observations, these findings indicate that mPFC inputs exert mainly excitatory effects in the rhinal cortices, prevalently on principal neurons. Thus, these results suggest that the mPFC may affect hippocampal-dependent memories by enhancing impulse traffic into and out of the hippocampus at the level of the rhinal cortices.

Cytoarchitectonic organization of the entorhinal cortex of the canine brain

The present study describes the cytoarchitectonic and chemoarchitectonic organization of the canine entorhinal cortex (EC). We distinguished medial, laterodorsal, and latero-intermediate subdivisions based on the organization of cortical layers using Nissl and Timm staining and AChE histochemistry. The medial subdivision is located at the border of the parasubiculum and is characterized by a narrow cortex, wide layer II, and densely packed cells in layer V. At its caudal extent, distinct spherical groups of small cells are situated at the border of layer I/II. The laterodorsal subdivision is located along the rhinal sulcus and borders area 35 of the perirhinal cortex. Its cortex is wide and layers tend to merge. Layer II of the laterodorsal subdivision contains scattered "stellate" cells, which are not organized into islands. The latero-intermediate subdivision displays a complex layer organization. The most easily distinguished is layer II, which is comprised of two main cell populations; "stellate" neurons arranged into "islands" and small, round cells distributed within and below the stellate cells. Layer III contains sparse cells that are arranged into vertical clusters, whereas layer IV (lamina dissecans) is especially wide. Nine fields, named according to their rostral to caudal position, were distinguished based on further analyses of layer differentiation. The main features of the rostrocaudal differentiation are a gradual disappearance of "island" organization in layer II, increasing cortical thickness, and wider layers containing small and more densely packed cells. Cytoarchitectonic differentiation was determined by observation of specific histochemical patterns of AChE- and Timm-stained sections.

Feedforward inhibition regulates perirhinal transmission of neocortical inputs to the entorhinal cortex: ultrastructural study in guinea pigs

The rhinal cortices constitute the main route for impulse traffic to and from the hippocampus. Tracing studies have revealed that the perirhinal cortex forms strong reciprocal connections with the neo- and entorhinal cortex (EC). However, physiological investigations indicate that perirhinal transmission of neocortical and EC inputs occurs with a low probability. In search of an explanation for these contradictory findings, we have analyzed synaptic connections in this network by combining injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHAL) into the neocortex, area 36, or area 35 with gamma-aminobutyric acid (GABA) immunocytochemistry and electron microscopic observations. Within area 36, neocortical axon terminals formed only asymmetric synapses, usually with GABA-negative spines (87%), and less frequently with GABA-immunopositive (GABA+) dendrites (13%). A similar synaptic distribution was observed within area 35 except that asymmetric synapses onto GABA+ dendrites were more frequent (23% of synapses). Examination of the projections from area 36 to area 35 and from both regions to the EC revealed an even higher incidence of asymmetric synapses onto GABA+ dendrites (35 and 32%, respectively) than what was observed in the neocortical projection to areas 36 and 35. Furthermore, some of the neocortical and perirhinal terminals containing PHAL and GABA immunolabeling formed symmetric synapses onto GABA-negative dendrites in their projection sites (neocortex to area 35, 16%; area 36 to 35, 7%; areas 36-35 to EC, 12%). Taken together, these findings suggest that impulse transmission through the rhinal circuit is subjected to strong inhibitory influences, reconciling anatomical and physiological data about this network.

Neural Representation of Navigational Relevance Is Rapidly Induced and Long Lasting

Successful navigation is facilitated by the presence of landmarks. Previous functional magnetic resonance imaging (fMRI) evidence indicated that the human parahippocampal gyrus automatically distinguishes between landmarks placed at navigationally relevant (decision points) and irrelevant locations (nondecision points). This storage of navigational relevance can provide a neural mechanism underlying successful navigation. However, an efficient wayfinding mechanism requires that important spatial information is learned quickly and maintained over time. The present study investigates whether the representation of navigational relevance is modulated by time and practice. Participants learned 2 film sequences through virtual mazes containing objects at decision and at nondecision points. One maze was shown one time, and the other maze was shown 3 times. Twenty-four hours after study, event-related fMRI data were acquired during recognition of the objects. The results showed that activity in the parahippocampal gyrus was increased for objects previously placed at decision points as compared with objects placed at nondecision points. The decision point effect was not modulated by the number of exposures to the mazes and independent of explicit memory functions. These findings suggest a persistent representation of navigationally relevant information, which is stable after only one exposure to an environment. These rapidly induced and long-lasting changes in object representation provide a basis for successful wayfinding.

Altered Inhibition in Lateral Amygdala Networks in a Rat Model of Temporal Lobe Epilepsy

Clinical and experimental evidence indicates that the amygdala is involved in limbic seizures observed in patients with temporal lobe epilepsy. Here, we used simultaneous field and intracellular recordings from horizontal brain slices obtained from pilocarpine-treated rats and age-matched nonepileptic controls (NECs) to shed light on the electrophysiological changes that occur within the lateral nucleus (LA) of the amygdala. No significant differences in LA neuronal intrinsic properties were observed between pilocarpine-treated and NEC tissue. However, spontaneous field activity could be recorded in the LA of 21% of pilocarpine-treated slices but never from NECs. At the intracellular level, this network activity was characterized by robust neuronal firing and was abolished by glutamatergic antagonists. In addition, we could identify in all pilocarpine-treated LA neurons: 1) large amplitude depolarizing postsynaptic potentials (PSPs) and 2) a lower incidence of spontaneous hyperpolarizing PSPs as compared with NECs. Single-shock stimulation of LA networks in the presence of glutamatergic antagonists revealed a biphasic inhibitory PSP (IPSP) in both NECs and pilocarpine-treated tissue. The reversal potential of the early GABAA receptor–mediated component, but not of the late GABAB receptor–mediated component, was significantly more depolarized in pilocarpine-treated slices. Furthermore, the peak conductance of both fast and late IPSP components had significantly lower values in pilocarpine-treated LA cells. Finally, paired-pulse stimulation protocols in the presence of glutamatergic antagonists revealed a less pronounced depression of the second IPSP in pilocarpine-treated slices compared with NECs. Altogether, these findings suggest that alterations in both pre- and postsynaptic inhibitory mechanisms contribute to synaptic hyperexcitability of LA networks in epileptic rats.

Visual Quality Determines the Direction of Neural Repetition Effects

One ubiquitous finding in functional magnetic resonance imaging studies is that repeated stimuli elicit lower responses than novel stimuli. In apparent contradiction, some studies have reported the exact opposite effect--greater responses to repeated than novel stimuli--in many of the same brain regions. Interestingly, these latter enhancement effects are typically obtained when stimuli have been degraded. To explore this observation, the present study examines the degree to which visual quality mediates repetition effects in a stimulus-selective ventral visual area. Subjects were presented with grayscale photographs of scenes that were either near or substantially above visual threshold, as determined by calibrating image contrast to behavioral performance. The presentation of 2 identical high-contrast scenes elicited lower blood oxygen level-dependent (BOLD) responses than the presentation of 2 different high-contrast scenes (repetition attenuation). Conversely, the presentation of 2 identical low-contrast scenes elicited greater BOLD responses than the presentation of 2 different low-contrast scenes (repetition enhancement). Neurophysiological studies suggest that repetition attenuation in ventral visual areas may reflect the reactivation of perceptual representations that have become sparse and selective as a result of prior experience, whereas repetition enhancement may reflect spared access to existing representations by severely degraded input.

Brain activity associated with expectancy-enhanced placebo analgesia as measured by functional magnetic resonance imaging

In this study, a well established expectancy manipulation model was combined with a novel placebo intervention, a validated sham acupuncture needle, to investigate the brain network involved in placebo analgesia. Sixteen subjects completed the experiment. We found that after placebo acupuncture treatment, subjective pain rating reduction (pre minus post) on the placebo-treated side was significantly greater than on the control side. When we calculated the contrast that subtracts the functional magnetic resonance imaging (fMRI) signal difference between post-treatment and pretreatment during pain application on placebo side from the same difference on control side [e.g., placebo (post - pre) - control (post - pre)], significant differences were observed in the bilateral rostral anterior cingulate cortex (rACC), lateral prefrontal cortex, right anterior insula, supramarginal gyrus, and left inferior parietal lobule. The simple regression (correlation) analysis between each subject's fMRI signal difference of post-treatment and pretreatment difference on placebo and control side and the corresponding subjective pain rating difference showed that significant negative correlation was observed in the bilateral lateral/orbital prefrontal cortex, rACC, cerebellum, right fusiform, parahippocampus, and pons. These results are different from a previous study that found decreased activity in pain-sensitive regions such as the thalamus, insula, and ACC when comparing the response to noxious stimuli applied to control and placebo cream-treated areas of the skin. Our results suggest that placebo analgesia may be configured through multiple brain pathways and mechanisms.

Attentional modulation of repetition attenuation is anatomically dissociable for scenes and faces

Repeating a stimulus generally leads to a decreased response in neural activity compared to that for novel items. This neural attenuation provides a marker for stimulus-specific perceptual encoding and memory that can be detected using functional magnetic resonance imaging (fMRI). Although previously assumed to occur automatically whenever a stimulus is repeated, recent studies have begun to show that the repetition attenuation effect is task-specific and modulated by attention. Here, we demonstrate that attention is crucial for obtaining neural attenuation even after extensive stimulus repetitions. Furthermore, the effect of attention on attenuation is anatomically dissociable for stimuli that have relatively segregated neural representations in high-level perceptual cortex. To manipulate attention, we used overlapping scene and face images, and asked subjects to attend to either category. In a scene-sensitive cortical region known as the parahippocampal place area (PPA), significant attenuation in the fMRI BOLD signal was observed for the attended repeated scenes (relative to attended novel scenes), while no attenuation was observed for ignored repeated scenes or attended repeated faces against their respective novel image baselines. Conversely, in the face-sensitive region known as the fusiform face area (FFA), significant attenuation was observed for attended repeated faces, but not for ignored repeated faces or attended repeated scenes. An additional control experiment ruled out alternative explanations based on global signal level reductions due to inattention. Thus, attention actively governed when neuronal activity was attenuated to repeated perceptual input, and such attenuation was specific to the cortical regions that actively represent the attended category of stimuli.

Attentional modulation of learning-related repetition attenuation effects in human parahippocampal cortex

Two of the most fundamental processes in biological vision are attention and learning. Attention actively selects and enhances visual information that is most relevant to behavior. Learning enables the visual system to benefit from perceptual experience. The amount of visual information to learn is infinite; however, top-down control mechanisms must somehow regulate learning to achieve an adaptive balance between plasticity and stability in neural circuitry. Functional magnetic resonance imaging (fMRI) can measure learning-related changes in neural activity to previously viewed perceptual stimuli. Described variably as the repetition suppression or adaptation effect, the attenuation in neural activity to repeated stimuli versus novel stimuli provides a marker for stimuli-specific perceptual processing and memory. One important issue concerns whether repetition attenuation is automatic or not, and recent work has begun to show that it is sensitive to task demands. Accordingly, the present study further examined how attention controls the attenuated response to repeated stimuli, specifically testing whether attention is important for initial encoding, for the expression of memory traces, or for both encoding and expression. To manipulate attention, we used overlapping scene and face images and asked subjects to attend to either category. fMRI revealed significant attenuation in the parahippocampal place area for only the repeated scenes that were attended both during the initial presentation and during repetition. Thus, attention actively governs when neuronal activity is attenuated to repeated perceptual input, and such attention is important during both initial encoding and subsequent expression of the learned information.

Functionally segregated neural substrates for arbitrary audiovisual paired-association learning

To clarify the neural substrates and their dynamics during crossmodal association learning, we conducted functional magnetic resonance imaging (MRI) during audiovisual paired-association learning of delayed matching-to-sample tasks. Thirty subjects were involved in the study; 15 performed an audiovisual paired-association learning task, and the remainder completed a control visuo-visual task. Each trial consisted of the successive presentation of a pair of stimuli. Subjects were asked to identify predefined audiovisual or visuo-visual pairs by trial and error. Feedback for each trial was given regardless of whether the response was correct or incorrect. During the delay period, several areas showed an increase in the MRI signal as learning proceeded: crossmodal activity increased in unimodal areas corresponding to visual or auditory areas, and polymodal responses increased in the occipitotemporal junction and parahippocampal gyrus. This pattern was not observed in the visuo-visual intramodal paired-association learning task, suggesting that crossmodal associations might be formed by binding unimodal sensory areas via polymodal regions. In both the audiovisual and visuo-visual tasks, the MRI signal in the superior temporal sulcus (STS) in response to the second stimulus and feedback peaked during the early phase of learning and then decreased, indicating that the STS might be key to the creation of paired associations, regardless of stimulus type. In contrast to the activity changes in the regions discussed above, there was constant activity in the frontoparietal circuit during the delay period in both tasks, implying that the neural substrates for the formation and storage of paired associates are distinct from working memory circuits.

Influence of serotonin receptor 2A His452Tyr polymorphism on brain temporal structures: a volumetric MR study

Serotonin (5-HT) receptors 2A are expressed in brain regions involved in memory and learning processes. Recently, a functional single nucleotide polymorphism in the 5-HT2A receptor gene leading to an amino-acid substitution at residue 452 (His452Tyr) has been involved in memory performance, persons with the rare 452Tyr allele showing poorer memory performance compared to His452His subjects. To investigate a putative structural effect of this polymorphism on temporal areas typically involved in memory processes, we performed voxel-based morphometry (VBM) and region-of-interest (ROI) volumetric analysis on high-resolution magnetic resonance images in 15 carriers and 61 noncarriers of the 452Tyr allele. ROI volumetric analysis showed a significant reduction of the fractional volume of the temporal white matter in 452Tyr carriers (0.67+/-0.07 vs 0.73+/-0.08; P=0.007). VBM confirmed this finding and in addition showed reduced grey matter in the left hippocampus, left inferior temporal gyrus, and bilaterally in the middle and superior temporal gyrus. A possible effect on synaptic plasticity or neurodevelopment might explain the influence of the His452Tyr polymorphism on temporal brain structures, and this might be the basis for poorer memory performance in 452Tyr carriers. These findings should be considered preliminary and future replication is needed.

Cortical efferents of the entorhinal cortex and the adjacent parahippocampal region in the monkey (Macaca fascicularis)

Entorhinal cortex (EC) relays information from the hippocampus to the cerebral cortex. The origin of this entorhino-cortical pathway was studied semiquantitatively and topographically with the use of 23 retrograde tracer injections in cortical areas of the frontal, temporal, and parietal lobes of the monkey. To assess possible alternative, parallel pathways, the parahippocampal region, comprised of temporal pole (TP), perirhinal (PRC), and posterior parahippocampal cortices (PPH), was included in the study. The majority of the cortical areas receive convergent projections from EC and the parahippocampal region. Strong EC layer V output is directed to temporal pole, medial frontal and orbitofrontal cortices, and the rostral part of the polysensory area of the superior temporal sulcus (sts). Moderate EC output is directed to the caudal superior temporal gyrus, area TE, and parietal cortex, and little to none to the lateral frontal cortex. With the exception of the projection to the medial frontal cortex, output from TP, PRC, and PPH surpassed that from EC, although with regional differences. TP layers II-III, V-VI project strongly to all areas injected except parietal cortex and caudal superior temporal gyrus, while PRC layers III/V-VI send strong projections to rostral parts of area TE and sts. PPH layers III/V-VI project heavily to parietal cortex and caudal superior temporal gyrus. These results suggest that the medial temporal output is primarily organized hierarchically, but at the same time, it has multiple exits of information. These parallel, alternative routes may influence local circuitry in the cerebral cortex and participate in the consolidation of declarative memory.

Effects of baseline task position on apparent activation in functional imaging of memory

Brain activation dissociates during the repeated performance of memory tasks with decreasing medial temporal and increasing orbitofrontal activation. The impact of such adaptations on a baseline task is unknown. In this study, we used H2(15)O positron emission tomography (PET) in two groups of subjects performing a continuous recognition task and a baseline task. The group performing the baseline task after the main task showed significant medial temporal activation in the subtraction (recognition task-baseline). The group performing the baseline task at the beginning showed right orbitofrontal activation. These differences appeared to result primarily from different activations during the baseline task. It thus appears that the temporal context of a baseline task may fundamentally alter cognitive requirements and substantially influence apparent brain activation during a memory task. We suggest that the automatic filtration of memories according to their relevance for ongoing reality, a capacity mediated by the orbitofrontal cortex, is one such influence on apparent activation.

No effect of hippocampal lesions on perirhinal cortex-dependent feature-ambiguous visual discriminations

Previous studies have shown that perirhinal cortex lesions in monkeys impair visual discriminations with a high degree of "feature ambiguity," a property of visual discriminations that can emerge when features are a part of both rewarded and unrewarded stimuli. The effects of damage to the hippocampus on these perirhinal-dependent feature-ambiguous tasks are, however, unknown. Prominent theories of medial temporal lobe function predict similar effects of perirhinal cortex and hippocampal lesions on cognitive tasks. In contrast, our hypothesis is that perirhinal cortex, and not the hippocampus, is important for nonspatial complex feature-ambiguous discriminations. We sought to distinguish between these competing theories in a straightforward way, by testing rhesus monkeys with hippocampal lesions on the same feature-ambiguous tasks shown previously to depend on perirhinal cortex. It was found that hippocampal lesions had no effects on any of these tasks. The findings support the perceptual-mnemonic/feature conjunction model of perirhinal cortex function, and provide further evidence for heterogeneity of function within the putative medial temporal lobe memory system.

Perirhinal cortex and its neighbours in the medial temporal lobe: contributions to memory and perception

As promised in the Introduction, this Special Issue presents several recurring themes concerning the perirhinal cortex and its neighbours within the medial temporal lobe (MTL). First, although orthodoxy insists that the diverse constituents of the MTL operate as a single functional entity, several papers presented here challenge that idea, although some defend it. Second, although many experts hold that the MTL subserves memory but not perception, several papers presented here point to a role for certain MTL structures in both. Third, although some researchers have invoked “species differences” to account for discrepant findings, several papers presented here document a striking convergence of findings in humans, nonhuman primates, and rodents. We close this Special Issue by high-lighting these recurring themes, acknowledging discrepant findings and pointing to future research that might resolve some current controversies.

Differential contributions of hippocampus, amygdala and perirhinal cortex to recognition of novel objects, contextual stimuli and stimulus relationships

This study examined contributions of the hippocampus, amygdala and perirhinal cortex to memory. Rats performed a cover task, and changes to stimulus identity or relationships were used to test incidental memory. Rats with hippocampal damage showed deficient responses to relationship changes, but demonstrated knowledge of the position and identity of the target object. They over-focused on the most predictive stimuli, and failed to acquire associations including surrounding cues. Rats with amygdala damage responded to changes involving distal stimuli, and showed deficient responses to novel objects and object relationships. These rats may be highly reliant on relational representations, resulting in a reduced salience for individual novel stimuli. Rats with perirhinal damaged responded to novel stimulus relationships and distal cues, but showed deficient responses to novel objects, suggesting that changes in identity had reduced salience. Implications for declarative and conjunctive hippocampal theories are discussed.

Cortical correlates of face and scene inversion: a comparison

Face recognition is more strongly impaired by stimulus inversion than nonface object recognition. This phenomenon, known as the face inversion effect (FIE), suggests that the visual system contains specialized processing mechanisms that are more engaged by upright faces than by inverted faces or nonface objects. Neuroimaging and neuropsychological studies indicate that environmental scenes may also recruit specialized-purpose processing machinery but a comparable inversion effect for scenes has not been established. Here we demonstrate that both face and scene inversion lead to behavioral penalties during performance of a continuous visual matching task; however, the scene inversion effect was less robust and declined in magnitude over the course of the experiment. Scene inversion led to greater neural response in the functionally defined lateral occipital (LO) object area for inverted versus upright scenes and reduced response in the parahippocampal place area (PPA), while face inversion lead to greater response in LO and the right middle fusiform (MF) object area for inverted versus upright faces but no change in the fusiform face area (FFA). A whole-brain analysis revealed several regions that responded more strongly to either upright versus inverted faces or upright versus inverted scenes, some of which may be involved in post-recognition processing. These results demonstrate that both face and scene inversion cause a shift from specialized processing streams towards generic object-processing mechanisms; however, this shift only leads to a reliable behavioral penalty in the case of face inversion.

Ecphory of autobiographical memories: an fMRI study of recent and remote memory retrieval

Ecphory occurs when one recollects a past event cued by a trigger, such as a picture, odor, or name. It is a central component of autobiographical memory, which allows us to "travel mentally back in time" and re-experience specific events from our personal past. Using fMRI and focusing on the role of medial temporal lobe (MTL) structures, we investigated the brain bases of autobiographical memory and whether they change with the age of memories. Importantly, we used an ecphory task in which the remote character of the memories was ensured. The results showed that a large bilateral network supports autobiographical memory: temporal lobe, temporo-parieto-occipital junction, dorsal prefrontal cortex, medial frontal cortex, retrosplenial cortex and surrounding areas, and MTL structures. This network, including MTL structures, changed little with the age of the memories.

Novelty responses to relational and non-relational information in the hippocampus and the parahippocampal region: a comparison based on event-related fMRI

We conducted two functional magnetic resonance imaging (fMRI) experiments that examined novelty responses in the human medial temporal lobe (MTL) to determine whether the hippocampus makes contributions to memory processing that differ from those of structures in the adjacent parahippocampal region. In light of proposals that such differential contributions may pertain to relational processing demands, we assessed event-related fMRI responses in the MTL for novel single objects and for novel spatial and non-spatial object relationships; subjects were asked to detect these different types of novelties among previously studied items, and they successfully performed this task during scanning. A double dissociation that emerged from the response pattern of regions in the hippocampus and perirhinal cortex provided the strongest support for functional specialization in the MTL. A region in the right middle hippocampus responded to the novelty of spatial and non-spatial relationships but not to the novelty of individual objects. By contrast, a region in right perirhinal cortex, situated in the anterior collateral sulcus, responded to the novelty of individual objects but not to that of either type of relationship. Other MTL regions that responded to novelty in the present study showed no reliable difference in their response to the various novelty types; these regions included anterior parts of the hippocampus and posterior aspects of parahippocampal cortex. Together, our findings indicate that relational processing demands are a critical determinant of functional specialization in the human MTL. They also suggest, however, that a neuroanatomical framework that only distinguishes between the hippocampus and the parahippocampal region is not sufficiently refined to account for all functional differences and similarities observed with respect to relational processes in the human MTL.

Lag-sensitive repetition suppression effects in the anterior parahippocampal gyrus

Single-unit recording studies of monkeys have shown that neurons in perirhinal and entorhinal cortex exhibit activity reductions following stimulus repetition, and some have suggested that these "repetition suppression" effects may represent neural signals that support recognition memory. Critically, repetition suppression effects are most pronounced at short intervals between stimulus repetitions. Here, we used event-related functional magnetic resonance imaging (fMRI) to identify repetition suppression effects in the human medial temporal lobe and determine whether these effects are sensitive to the length of the interval between repetitions. Twenty-one participants were scanned while performing a continuous recognition memory task in which the interval between item repetitions was parametrically varied from 2 to 32 intervening items. We found evidence of repetition suppression in the anterior parahippocampal gyrus, but only when the repetition interval was relatively short. Moreover, bilateral hippocampal regions showed lag-sensitive repetition effects. Our results demonstrate that activity in the human medial temporal cortex, like that of monkeys, exhibits repetition suppression effects that are sensitive to the length of the interval between repetitions.

Language processing within the human medial temporal lobe

Although the hippocampal formation is essential for verbal memory, it is not fully understood how it contributes to language comprehension. We recorded event-related potentials (ERPs) directly from two substructures of the medial temporal lobe (MTL), the rhinal cortex and the hippocampus proper, while epilepsy patients listened to sentences that either were correct or contained semantic or syntactic violations. Semantic violations elicited a large negative ERP response peaking at approximately 400 ms in the rhinal cortex. In contrast, syntactically incorrect sentences elicited a negative deflection of 500-800 ms in the hippocampus proper. The results suggest that functionally distinct aspects of integration in language comprehension are supported by different MTL structures: the rhinal cortex is involved in semantic integration, whereas the hippocampus proper subserves processes of syntactic integration. An analysis of phase synchronization within the gamma band between rhinal and hippocampal recording sites showed that both of the above-mentioned ERP components were preceded by an increase of phase synchronization. In contrast to these short phasic increases of phase synchronization in both violation conditions, correct sentences were associated with a long-lasting synchronization in a late time window, possibly reflecting the integration of semantic and syntactic information as required for normal comprehension.

Dissociable brain activations during the retrieval of different kinds of spatial context memory

Although memory for spatial information has often been regarded as unitary, it may be divided into two distinct types: memory for the place where an individual experienced an event and memory for the location of an experienced event within a specific reference object. We used functional magnetic resonance imaging (fMRI) to elucidate the distinctions between the retrieval of these two types of spatial context memory. During scanning, subjects judged the room (Place task) in which a photograph had been presented or the location of the photograph on the computer display (R-L task) during the encoding phase. In a control task, subjects were asked to judge whether the photograph had been presented or not. The left middle frontal gyrus, lateral parietal and occipital regions, and bilateral precunei were found to be active during both the Place task and the R-L task compared with the control task. Critically, the place task, compared with the R-L task, was associated with activations in the right lateral prefrontal gyri, the posterior part of the left parahippocampal gyrus, bilateral retrosplenial and lateral parieto-occipital areas, whereas the R-L task, relative to the place task, with activation only in the right lateral parietal cortex. These findings indicate that the retrieval processes of spatial context memory are not associated with a single network, but may vary and recruit different neural networks depending on the type of spatial information to be retrieved.

The reliability of fMRI activations in the medial temporal lobes in a verbal episodic memory task

The test-retest reliability of activation patterns elicited by encoding and recognition of word-pair associates within the whole brain and a predefined medial temporal region of interest (ROI) was investigated. Twenty healthy right-handed subjects were studied within two sessions, either on the same day or 210-308 days later. Three quantitative measures of reliability were calculated for the contrasts encoding and recognition versus a control condition within the ROI and also for the whole brain: A group correlational analysis between the lateralization indices of the first and second session, correlations of the individual SPM(t) maps of the first and the second run, and overlap ratios between both sessions. For the ROI, correlational analysis of lateralization indices during both encoding trials was significant. Eighty percent of the individual positive correlation coefficients of SPM(t) maps during encoding, and 75% during recognition reached significance. The mean percentage of overlapping voxels was 18% during encoding and 19% during recognition. The reproducibility measures evaluated for the whole brain demonstrated significantly higher values compared to the ROI. For the group that stayed inside the scanner, better whole brain test-retest reliability was observed, and no influence of the memory process (encoding or recognition) on reproducibility was found.

Episodic and semantic memory tasks activate different brain regions in Alzheimer disease

OBJECTIVE

To compare brain activity identified by fMRI in subjects with Alzheimer disease (AD) and older healthy controls (HCs) performing an episodic/working memory (EWM) and semantic memory (SM) task.

METHODS

Nine AD (mean age 73.6) and 10 HC (mean age 71.8) subjects underwent an fMRI memory paradigm. Tasks comprised 1) baseline (recognizing a single digit presented for 1 second), 2) SM (addition of two single digits, always producing a single digit answer), and 3) EWM (recall of the previous single digit on the stimulus of the next digit). Each condition was presented in 2-minute blocks with a shorter and longer time interval for the first and second minute within blocks.

RESULTS

Comparing AD and HC subjects, there were no activated brain regions in common for EWM > SM, but left anterior cingulate (Brodmann area [BA] 24, 0, 31, 4) and left medial frontal lobe gyrus (BA 25, -6, 23, -15) were activated by both groups for SM > EWM. Key differences were that for EWM > SM, HC subjects activated the right parahippocampal gyrus, whereas subjects with AD activated the right superior frontal gyrus and left uncus.

CONCLUSIONS

Subjects with Alzheimer disease (AD) recruited brain regions for easier episodic/working memory (EWM) tasks used by healthy controls (HCs) for more difficult EWM tasks. AD subjects recruited brain regions for semantic memory tasks used by HCs for more difficult EWM tasks. The authors propose a functional "memory reserve" model of compensatory recruitment according to task difficulty and underlying neuropathology.

Functional magnetic resonance imaging study of human olfaction and normal aging

BACKGROUND

The function of human olfaction declines with advancing age. An important question centers on whether functional alterations to olfactory brain structures accompany age-related behavioral changes. In the present study, we tested the hypothesis that aged adults have intact though reduced activity in the central olfactory system using functional magnetic resonance imaging (fMRI).

METHODS

University of Pennsylvania Smell Identification Test (UPSIT) was used to test the smell function of 11 young (23.9 +/- 1.6 years) and 8 aged (66.4 +/- 4.4 years) healthy participants. Then, the participants received fMRI at 3.0 T with lavender and spearmint as stimulants. After fMRI, the participants provided ratings for the odorants' intensity and pleasantness.

RESULTS

The average UPSIT score of the aged adults was 34.1 +/- 1.5, which was significantly lower than that of the young adults (37.3 +/- 1.1) (p =.0004). Both age groups showed significant activation in major olfactory brain structures, including the primary olfactory cortex, entorhinal cortex, hippocampus and parahippocampal cortex, thalamus, hypothalamus, orbitofrontal cortex, and insular cortex and its extension into the inferior lateral frontal region. The aged adults showed less brain activity in olfactory structures (p =.022), consistent with lower ratings of odor intensity and UPSIT scores. Activation intensity in bilateral primary olfactory cortex areas and right insular cortex was also comparatively weaker (p <.019).

CONCLUSION

Results demonstrate that significant activation in aged adults can be observed in all the olfactory brain structures that are activated in young adults, but with lower activation volume and intensity. This finding provides a necessary baseline for further investigations in olfaction and aging.

Persistence of parahippocampal representation in the absence of stimulus input enhances long-term encoding: a functional magnetic resonance imaging study of subsequent memory after a delayed match-to-sample task

Recent theoretical models based on cellular processes in parahippocampal structures show that persistent neuronal spiking in the absence of stimulus input is important for encoding. The goal of this study was to examine in humans how sustained activity in the parahippocampal gyrus may underlie long-term encoding as well as active maintenance of novel information. The relationship between long-term encoding and active maintenance of novel information during brief memory delays was studied using functional magnetic resonance imaging (fMRI) in humans performing a delayed matching-to-sample (DMS) task and a post-scan subsequent recognition memory task of items encountered during DMS task performance. Multiple regression analyses revealed fMRI activity in parahippocampal structures associated with the active maintenance of trial-unique visual information during a brief memory delay. In addition to a role in active maintenance, we found that the subsequent memory for the sample stimuli as measured by the post-scan subsequent recognition memory task correlated with activity in the parahippocampal gyrus during the delay period. The results provide direct evidence that encoding mechanisms are engaged during brief memory delays when novel information is actively maintained. The relationship between active maintenance during the delay period and long-term subsequent memory is consistent with current theoretical models and experimental data that suggest that long-term encoding is enhanced by sustained parahippocampal activity.

Cortical areas involved in object, background, and object-background processing revealed with functional magnetic resonance adaptation

Previous work has suggested that object and place processing are neuroanatomically dissociated in ventral visual areas under conditions of passive viewing. It has also been shown that the hippocampus and parahippocampal gyrus mediate the integration of objects with background scenes in functional imaging studies, but only when encoding or retrieval processes have been directed toward the relevant stimuli. Using functional magnetic resonance adaptation, we demonstrated that object, background scene, and contextual integration of selectively repeated objects and background scenes could be dissociated during the passive viewing of naturalistic pictures involving object-scene pairings. Specifically, bilateral fusiform areas showed adaptation to object repetition, regardless of whether the associated scene was novel or repeated, suggesting sensitivity to object processing. Bilateral parahippocampal regions showed adaptation to background scene repetition, regardless of whether the focal object was novel or repeated, suggesting selectivity for background scene processing. Finally, bilateral parahippocampal regions distinct from those involved in scene processing and the right hippocampus showed adaptation only when the unique pairing of object with background scene was repeated, suggesting that these regions perform binding operations.

A Hierarchy of Associations in Hippocampo-Cortical Systems: Cognitive Maps and Navigation Strategies

In this letter we describe a hippocampo-cortical model of spatial processing and navigation based on a cascade of increasingly complex associative processes that are also relevant for other hippocampal functions such as episodic memory. Associative learning of different types and the related pattern encoding-recognition take place at three successive levels: (1) an object location level, which computes the landmarks from merged multimodal sensory inputs in the parahippocampal cortices; (2) a subject location level, which computes place fields by combination of local views and movement-related information in the entorhinal cortex; and (3) a spatiotemporal level, which computes place transitions from contiguous place fields in the CA3-CA1 region, which form building blocks for learning temporospatial sequences.

At the cell population level, superficial entorhinal place cells encode spatial, context-independent maps as landscapes of activity; populations of transition cells in the CA3-CA1 region encode context-dependent maps as sequences of transitions, which form graphs in prefrontal-parietal cortices. The model was tested on a robot moving in a real environment; these tests produced results that could help to interpret biological data.

Two different goal-oriented navigation strategies were displayed depend-ing on the type of map used by the system.

Thanks to its multilevel, multimodal integration and behavioral imple-mentation, the model suggests functional interpretations for largely un-accounted structural differences between hippocampo-cortical systems. Further, spatiotemporal information, a common denominator shared by several brain structures, could serve as a cognitive processing frame and a functional link, for example, during spatial navigation and episodic memory, as suggested by the applications of the model to other domains, temporal sequence learning and imitation in particular.

Reduced Parahippocampal Connectivity Produces Schizophrenia-like Memory Deficits in Simulated Neural Circuits With Reduced Parahippocampal Connectivity

CONTEXT

Episodic memory impairments are well characterized in schizophrenia, but their neural origin is unclear.

OBJECTIVE

To determine whether the episodic memory impairments in schizophrenia may originate from reduced parahippocampal connectivity.

DESIGN

Experimental in silico model.

SETTING

Department of Psychology, University of Amsterdam, Amsterdam, the Netherlands.

INTERVENTIONS

A new, in silico medial temporal lobe model that simulates normal performance on a variety of episodic memory tasks was devised. The effects of reducing parahippocampal connectivity in the model (from perirhinal and parahippocampal cortex to entorhinal cortex and from entorhinal cortex to hippocampus) were evaluated and compared with findings in schizophrenic patients. Alternative in silico neuropathologies, increased noise and loss of hippocampal neurons, were also evaluated.

RESULTS

In the model, parahippocampal processing subserves integration of different cortical inputs to the hippocampus and feature extraction during recall. Reduced connectivity in this area resulted in a pattern of deficits that closely mimicked the impairments in schizophrenia, including a mild recognition impairment and a more severe impairment in free recall. Furthermore, the schizophrenic model was not differentially sensitive to interference, also consistent with behavioral data. Notably, neither increased noise levels nor a reduction of hippocampal nodes in the model reproduced this characteristic memory profile.

CONCLUSIONS

Taken together, these findings highlight the importance of parahippocampal neuropathology in schizophrenia, demonstrating that reduced connectivity in this region may underlie episodic memory problems associated with the disorder.

A 5-month period of epilepsy impairs spatial memory, decreases anxiety, but spares object recognition in the lithium-pilocarpine model in adult rats

PURPOSE

In temporal lobe epilepsy (TLE), interictal behavioral disorders affect patients' quality of life. Therefore we studied long-term behavioral impairments in the lithium-pilocarpine (li-pilo) model of TLE.

METHODS

Eleven li-pilo adult rats exhibiting spontaneous recurrent seizures (SRSs) during 5 months were compared with 11 li-saline rats. Spatial working memory was tested in a radial arm maze (RAM), anxiety in an elevated plus-maze (EPM), and nonspatial working memory in an object-recognition paradigm. Neuronal loss was assessed on thionine brain sections after behavioral testing.

RESULTS

In the RAM, the time to complete each session and the number of errors per session decreased over a 5-day period in li-saline rats but remained constant and significantly higher in li-pilo rats. In the EPM, the number of entries in and time spent on open arms were significantly higher in li-pilo than li-saline rats. In the object-recognition task, the two groups exhibited a comparable novelty preference for the new object. Neuronal loss reached 47-90% in hilus, CA1, amygdala, and piriform and entorhinal cortex.

CONCLUSIONS

In li-pilo rats having experienced SRS for 5 months, performance in the object-recognition task is spared, which suggests that object discrimination remains relatively intact despite extensive damage. Neuronal loss in regions mediating memory and anxiety, such as hippocampus, entorhinal cortex, and amygdala, may relate to impaired spatial orientation and decreased anxiety.

Stimulus pacing affects the activation of the medial temporal lobe during a semantic classification task: an fMRI study

Our purpose was to explore the influence of stimulus pacing in blocked functional MRI studies on the activation pattern elicited by a semantic retrieval task. Twenty-two participants performed both a fixed-paced and a self-paced functional MR imaging experiment in which a semantic categorization (animal/object) task was contrasted with a perceptual (small/capital letter string) categorization task. Group and single-subject ROI analyses were computed. In both the fixed-paced and self-paced experiments, semantic categorization contrasted with perceptual categorization elicited a cerebral network generally accepted to be involved in semantic processing comprising left inferior prefrontal, left lateral temporal, paracingular and right cerebellar areas. Our main finding was that the self-paced stimulus modality additionally yielded significant activation in the medial temporal lobe (MTL) structures including the hippocampus and the parahippocampal gyrus. More elaborative single-subject ROI analyses revealed MTL activation in 86% of the subjects for the self-paced design, but only in 21% of subjects for the fixed-paced design. The discussion focuses on possible explanations for this finding. We conclude that a self-paced as opposed to a fixed-paced semantic retrieval paradigm is able to detect significant MTL activation in groups as well as in single-subjects. This offers opportunities for the application of such a design in clinical practice.

Concrete spatial language: see what I mean?

Conveying complex mental scenarios is at the heart of human language. Advances in cognitive linguistics suggest this is mediated by an ability to activate cognitive systems involved in non-linguistic processing of spatial information. In this fMRI-study, we compare sentences with a concrete spatial meaning to sentences with an abstract meaning. Using this contrast, we demonstrate that sentence meaning involving motion in a concrete topographical context, whether linked to animate or inanimate subjects nouns, yield more activation in a bilateral posterior network, including fusiform/parahippocampal, and retrosplenial regions, and the temporal-occipital-parietal junction. These areas have previously been shown to be involved in mental navigation and spatial memory tasks. Sentences with an abstract setting activate an extended largely left-lateralised network in the anterior temporal, and inferior and superior prefrontal cortices, previously found activated by comprehension of complex semantics such as narratives. These findings support a model of language, where the understanding of spatial semantic content emerges from the recruitment of brain regions involved in non-linguistic spatial processing.

Human parahippocampal activity: non-REM and REM elements in wake–sleep transition

The covert-rapid-eye-movement (REM) sleep hypothesis of dreaming suggests that elements of REM sleep emerge during sleep onset, leading to vivid hypnagogic imagery. Based on parahippocampal electrocorticography of epileptic patients we found an increase in REM-like 1.5-3.0 Hz parahippocampal activity during wake-sleep transition, which peaks after on average 30s of sleep onset, and reaches 82% of REM sleep value. The increase in 1.5-3.0 Hz parahippocampal activity followed alpha dropout, but did not relate to short-term fluctuations in alpha waves or sleep spindles. Non-REM sleep-specific slow (<1.25 Hz) activity showed a continuous increase during wake-sleep transition in both temporal scalp and parahippocampal recordings. It is suggested that REM-like parahippocampal rhythmic slow activity is an after-effect of hypothalamic wake-promoting centers' switch-off at sleep onset, leading to an inhibited hippocampal functioning and hypnagogic hallucinations.

Neural basis of spontaneous thought processes

Studies examining thought processes have focused upon the deliberate, goal-directed mental processes occurring during complex cognitive tasks. Spontaneously occurring thought processes have, on the other hand, received much less attention. Such spontaneous thought processes occur frequently when no task is present or when task demands are low. Although their existence has been recognised, their study has been difficult due to lack of direct behavioural measures. Nevertheless, a number of behavioural methods based on subjects' verbal reports have been developed. Findings derived using such behavioural methods suggest that spontaneous thought processes share common cognitive mechanisms with purposeful, task-related thought processes. Furthermore, evidence from neuroimaging observations is accumulating suggesting similar conclusions about the neural basis of spontaneous thought processes. These neuroimaging findings demonstrate an overlap in the pattern of activation between various cognitive tasks and rest, with a number of higher cortical regions activated in common, including visual areas, medial temporal lobe, and lateral cortical association areas. Many of these observations have, however, been based upon comparisons between rest and tasks posing relatively high cognitive demands. In contrast, here we report an fMRI study in which rest was compared to a simple left/right response task of minimal cognitive demands. Rest was associated with greater activation in temporopolar cortex, parahippocampus, rostrolateral prefrontal cortex, parietal and visual cortical areas. Activation of temporal lobe structures was particularly extensive and robust, suggesting that long-term memory processes may form the core of spontaneous thought. By considering such long-term memory processes as an essential part of thought mechanisms, it may be possible to gain better understanding into spontaneous thought phenomena that have remained unaccounted for until now.

Medial temporal lobe activation during encoding and retrieval of novel face-name pairs

The human medial temporal lobe (MTL) is known to be involved in declarative memory, yet the exact contributions of the various MTL structures are not well understood. In particular, the data as to whether the hippocampal region is preferentially involved in the encoding and/or retrieval of associative memory have not allowed for a consensus concerning its specific role. To investigate the role of the hippocampal region and the nearby MTL cortical areas in encoding and retrieval of associative versus non-associative memories, we used functional magnetic resonance imaging (fMRI) to measure brain activity during learning and later recognition testing of novel face-name pairs. We show that there is greater activity for successful encoding of associative information than for non-associative information in the right hippocampal region, as well as in the left amygdala and right parahippocampal cortex. Activity for retrieval of associative information was greater than for non-associative information in the right hippocampal region also, as well as in the left perirhinal cortex, right entorhinal cortex, and right parahippocampal cortex. The implications of these data for a clear functional distinction between the hippocampal region and the MTL cortical structures are discussed.

Functional neuroanatomy of perceiving surprised faces

Surprise is one of six emotions having a specific and universally recognized facial expression. Functional imaging and neuropsychologic studies have uncovered partly separable neural substrates for perceiving different facial expressions; however, the functional neuroanatomy of perceiving surprised faces has not yet been investigated. Using functional magnetic resonance imaging (fMRI), we aimed to identify the neural substrate of surprise perception from facial expressions. Based on the assumption of unexpectedness and novelty as elicitors of facial surprise reactions, we hypothesized recruitment of medial temporal lobe structures implicated in novelty detection during the perception of surprise in others. Healthy subjects were scanned while they were presented with surprised faces. As a control, they viewed faces depicting neutral or disgust expressions. Activations during the emotional conditions were contrasted with each other and with the neutral face condition. Compared to both control conditions, perception of surprised facial expressions yielded consistently increased signals in the parahippocampal region, an area associated previously with novelty detection. Our findings therefore suggest a close relation between perceiving surprise in others and the response to novel events. Additionally, we confirmed activation of the insula during perception of disgust expressions.

Periodic oscillatory activity in parahippocampal slices maintained in vitro

Brain slices maintained in vitro have been extensively used for studying neuronal synchronization. However, the validity of this approach may be questioned since pharmacological procedures are usually required to elicit spontaneous events similar to the EEG activity recorded in vivo. Here, we report that when superfused with control medium, rat brain slices comprising the entorhinal and perirhinal cortices along with a portion of the basolateral/lateral nuclei of the amygdala can synchronously generate periodic oscillatory activity at 5-11 Hz every 5-30 s. The periodic events: (i) correspond intracellularly to synaptic depolarizations in regularly firing neurons analyzed in the three areas; (ii) have no fixed site of onset; (iii) spread with time lags of 8-20 ms; and (iv) continue to occur asynchronously after their surgical isolation. NMDA receptor antagonism reduced the duration of the oscillatory events, while glutamatergic non-NMDA receptor antagonism abolished them. Activation of mu-opioid receptors, a procedure that hyperpolarizes interneurons thus decreasing GABA release, reversibly decreased the rate of occurrence of periodic oscillatory activity (POA). However, periodic events continued to occur during application of GABA(A) or GABA(B) receptor antagonists as well as in the presence of the cholinergic agent carbachol. We also found that POA was abolished by baclofen and irreversibly reduced by the gap junction decoupler carbenoxolone. These findings demonstrate that parahippocampal networks in a brain slice preparation can generate periodic, synchronous activity under quasi-physiological conditions. These network oscillations (i) reflect the activation of ionotropic glutamatergic and GABAergic receptors, (ii) are contributed by gap-junction interactions, and (iii) are controlled by GABA(B) receptors that are presumably located presynaptically.

Cortical mechanisms of visual self-recognition

Several lines of evidence have suggested that visual self-recognition is supported by a special brain mechanism; however, its functional anatomy is of great controversy. We performed an event-related functional magnetic resonance imaging (fMRI) study to identify brain regions selectively involved in recognition of one's own face. We presented pictures of each subject's own face (SELF) and a prelearned face of an unfamiliar person (CONT), as well as two personally familiar faces with high and low familiarity (HIGH and LOW, respectively) to test selectivity of activation to the SELF face. Compared with the CONT face, activation selective to the SELF face was observed in the right occipito-temporo-parietal junction and frontal operculum, as well as in the left fusiform gyrus. On the contrary, the temporoparietal junction in both the hemispheres and the left anterior temporal cortex, which were activated during recognition of HIGH and/or LOW faces, were not activated during recognition of the SELF face. The results confirmed the partial distinction of the brain mechanism involved in recognition of personally familiar faces and that in recognition of one's own face. The right occipito-temporo-parietal junction and frontal operculum appear to compose a network processing motion-action contingency, a role of which in visual self-recognition has been suggested in previous behavioral studies. Activation of the left fusiform gyrus selective to one's own face was consistent with the results of two previous functional imaging studies and a neuropsychological report, possibly suggesting its relationship with lexical processing.

Learning Places from Views: Variation in Scene Processing as a Function of Experience and Navigational Ability

Humans and animals use information obtained from different viewpoints to form representations of the spatial structure of the world. We used functional magnetic resonance imaging (fMRI) adaptation to investigate the neural basis of this learning process and to show how the concomitant representations vary across individuals as a function of navigational ability. In particular, we examined the effect of repeating viewpoint and/ or place information over both short (within-trial) and long (across-scan) intervals on the neural response in scene processing regions. Short-term fMRI adaptation effects in the parahippocampal cortex were initially highly viewpoint-specific but became less so over time. Long-term fMRI repetition effects included a significant viewpoint-invariant component. When individual differences in navigational ability were considered, a significant correlation between the strength of these effects and self-reported navigational competence was observed. In particular, good navigators encoded representations that differed between new and old views and new and old places, whereas bad navigators did not. These results suggest that cortical scene representations evolve over time to become more viewpoint-invariant and that the quality of these representations directly impacts navigational ability.

Mediotemporal contributions to semantic processing: fMRI evidence from ambiguity processing during semantic context verification

The medial temporal lobe (MTL) is well known to be crucial for various types of memory; however, controversy remains as to which of its substructures contribute to semantic processing and, if so, to what extent. The current study addresses the issue of MTL contributions to semantic processing during lexico-semantic ambiguity processing by using functional magnetic resonance imaging (fMRI) in combination with a context verification task (CVT). The CVT required decisions on the semantic fit of congruent and incongruent target words to the overall meaning of preceding sentential contexts with and without semantic ambiguity. In two of the four experimental conditions (congruent homographic, incongruent homographic), target decisions were critically dependent on the successful processing of prior sentence-final lexico-semantic ambiguity. Semantic context verification per se evidenced bilateral activations of the hippocampus that were part of a functional network including inferior prefrontal and superior parietal cortices. Commonalities in activation differences pertaining to the specific cognitive component of lexico-semantic ambiguity processing were found in a left temporal lobe network that comprised activation foci in the temporal pole, the parahippocampal and fusiform gyri. The present results suggest that the hippocampus may well contribute to semantic processing, namely by a mnemonic function that serves to link the target meaning representation with the meaning of a prior sentence context. Contrary to previous reports from human lesion studies, the present findings further suggest, that the specific cognitive component of lexico-semantic ambiguity processing is neither dependent on the hippocampus nor exclusively subserved by the temporal pole, but also recruits an associative semantic memory function from the parahippocampal gyrus as well as a more general (bottom-up) semantic function from the fusiform gyrus.

Representation of attitudinal knowledge: role of prefrontal cortex, amygdala and parahippocampal gyrus

It has been proposed that behavior is influenced by representations of different types of knowledge: action representations, event knowledge, attitudes and stereotypes. Attitudes (representations of a concept or object and its emotional evaluation) allow us to respond quickly to a given stimulus. In this study, we explored the representation and inhibition of attitudes. We show that right dorsolateral prefrontal cortex mediates negative attitudes whereas left ventrolateral prefrontal cortex mediates positive attitudes. Parahippocampal regions and amygdala mediate evaluative processing. Furthermore, anxiety modulates right dorsolateral prefrontal activation during negative attitude processing. Inhibition of negative attitudes activates left orbitofrontal cortex: a region that when damaged is associated with socially inappropriate behavior in patients. Inhibition of positive attitudes activates a brain system involving right inferior frontal gyrus and bilateral anterior cingulate. Thus, we show that there are dissociable networks for the representation and inhibition of attitudes.

Phase/amplitude reset and theta-gamma interaction in the human medial temporal lobe during a continuous word recognition memory task

We analyzed intracranial electroencephalographic (EEG) recordings from the medial temporal lobes of 12 epilepsy patients during a continuous word recognition paradigm, contrasting trials of correctly recognized repeated words (hits) and correctly identified new words (correct rejections). Using a wavelet-based analysis, we investigated how power changes and phase clustering in different frequency bands contribute to the averaged event-related potentials (ERPs). In addition, we analyzed the actual mean phases of the different oscillations. Our analyses yielded the following results: (1) power changes contributed significantly only to the late components of the ERPs (>400 ms) (2) earlier ERP components were produced by a stimulus-related broad-band phase and amplitude reset of ongoing oscillatory activity about 190 ms after stimulus onset that involved not only the theta band, but also covered alpha and lower beta band frequencies (3) phase and amplitude reset occurred during an epoch of increased phase entrainment over time that lasted for about two oscillation periods for all involved frequencies and was more pronounced for correct rejections than for hits. The broad-band phase and amplitude reset was observed for both hits and correct rejections, and therefore, did not appear to support a specific cognitive function, but rather to act as a general facilitating factor for the processes involved in this memory task. Further analyses of synchronization between oscillations and power changes in different frequency bands revealed a task-dependent modulation of gamma activity by the entrained theta cycle, a mechanism potentially related to memory encoding and retrieval in the rhinal cortex and hippocampus, respectively.