Reza Habib

Hippocampal PET activations of memory encoding and retrieval: The HIPER model

A meta‐analysis of experimentally induced changes in blood flow (“activations”) in positron emission tomography (PET) studies of memory has revealed an orderly functional anatomic pattern: Activations in the hippocampal region associated with episodic memory encoding are located primarily in the rostral portions of the region, whereas activations associated with episodic memory retrieval are located primarily in the caudal portions. These findings are based on an analysis of a sample of 54 “hippocampal encoding and retrieval” activations that were culled from an overall database consisting of 52 published PET studies of memory. We refer to this general pattern of rostrocaudal gradient of encoding and retrieval PET activations as the HIPER (Hippocampal Encoding/Retrieval) model. The model suggests a division of memory‐related labor between the rostral and caudal portions of the hippocampal formation. Because functional anatomic pattern of encoding and retrieval activation that defines the HIPER model was unprecedented and unexpected, it is difficult to relate the model to what is already known or thought about functional neuroanatomy of episodic memory in the hippocampal regions. The model is interesting primarily because its exploration may yield fresh insights into the neural basis of human memory. Hippocampus 1998;8:313–322.

Hemispheric asymmetries of memory: the HERA model revisited

The hemispheric encoding/retrieval asymmetry (HERA) model is a process-specific description of experimental data provided by a large set of functional neuroimaging studies. According to HERA, left prefrontal cortex (PFC) is more involved than right PFC in episodic memory encoding, whereas right PFC is more involved than left PFC in episodic memory retrieval. Recently it has been claimed that this description does not hold for non-verbal materials. Here we propose a more precise formulation of HERA than previously, and argue that there is sufficient evidence to conclude that HERA, as reformulated, is true for both verbal and non-verbal materials.

Functional brain maps of retrieval mode and recovery of episodic information.

Positron emission tomography (PET) was used to identify brain regions associated with two component processes of episodic retrieval; those related to thinking back in subjective time (retrieval mode) and those related to actual recovery of stored information (ecphory). Healthy young subjects recognized words that had been encoded with respect to meaning or the speakers voice. Regardless of how the information had been encoded, recognition was associated with increased activation in regions in right prefrontal cortex, left anterior cingulate, and cerebellum. These activations reflect retrieval mode. Recognition following meaning encoding was specifically associated with increased activation in left temporal cortex, and recognition following voice encoding involved regions in right orbital frontal and parahippocampal cortex. These activations reflect ecphory of differentially encoded information.

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.

Large Scale Neurocognitive Networks Underlying Episodic Memory

Large-scale networks of brain regions are believed to mediate cognitive processes, including episodic memory. Analyses of regional differences in brain activity, measured by functional neuroimaging, have begun to identify putative components of these networks. To more fully characterize neurocognitive networks, however, it is necessary to use analytical methods that quantify neural network interactions. Here, we used positron emission tomography (PET) to measure brain activity during initial encoding and subsequent recognition of sentences and pictures. For each type of material, three recognition conditions were included which varied with respect to target density (0, 50, 100). Analysis of large-scale activity patterns identified a collection of foci whose activity distinguished the processing of sentences vs. pictures. A second pattern, which showed strong prefrontal cortex involvement, distinguished the type of cognitive process (encoding or retrieval). For both pictures and sentences, the manipulation of target density was associated with minor activation changes. Instead, it was found to relate to systematic changes of functional connections between material-specific regions and several other brain regions, including medial temporal, right prefrontal and parietal regions. These findings provide evidence for large-scale neural interactions between material-specific and process-specific neural substrates of episodic encoding and retrieval.

Consciousness of subjective time in the brain

“Mental time travel” refers to conscious experience of remembering the personal past and imagining the personal future. Little is known about its neural correlates. Here, using functional magnetic resonance imaging, we explored the hypothesis that mental time travel into “nonpresent” times (past and future) is enabled by a special conscious state (chronesthesia). Well-trained subjects repeatedly imagined taking one and the same short walk in a familiar environment, doing so either in the imagined past, present, or future. In an additional condition, they recollected an instance in which they actually performed the same short walk in the same familiar setting. This design allowed us to measure brain activity correlated with “pure” conscious states of different moments of subjective time. The results showed that the left lateral parietal cortex was differentially activated by nonpresent subjective times compared with the present (past and future > present). A similar pattern was observed in the left frontal cortex, cerebellum, and thalamus. There was no evidence that the hippocampal region is involved in subjective time travel. These findings provide support for theoretical ideas concerning chronesthesia and mental time travel.

A left amygdala mediated network for rapid orienting to masked fearful faces.

A rapid response to environmental threat is highly adaptive and fearful facial expressions serve as important threat cues. The biological significance of these threat cues is demonstrated by neuroimaging findings of amygdala responses to backward masked fearful faces. Additionally, behavioral dot-probe studies reveal that backward masked fearful faces modulate spatial attention. However, little is known about the behavioral impact of the amygdala sensitivity to masked fearful faces. Using a dot-probe task with event-related functional magnetic resonance imaging (fMRI), we provide the first evidence that the amygdala is involved in orienting to backward masked fearful faces. Furthermore, this spatial attention-related amygdala response was correlated with activity in the anterior cingulate, superior temporal sulcus, and lingual gyrus.

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. Hippocampus 1999;9:71–82.

Neural correlates of semantic associative encoding in episodic memory

Associations between individual items are the basic building blocks of learning and memory. Functional neuroimaging has now made it possible to study neural correlates of such associations. The present PET study examined three associative encoding conditions differing in the number of words (0, 1, or 2) semantically related to a third word representing the name of a semantic category. A recall task consisting in the presentation of the category names as cues for retrieving the other two members of the triads followed each encoding condition. As expected, retrieval performance increased as the number of semantic exemplars at encoding increased (10%, 43%, 70% items recalled, respectively). A first analysis (partial least squares, PLS) of the PET data identified task-related patterns of activity for associative encoding and cued-recall tasks. A second analysis identified brain regions whose activity was modulated by the number of semantic exemplars at encoding. Some of the task-related brain regions also showed modulated activity by semantic relatedness and consisted in the left inferior prefrontal cortex, right medial temporal lobe, fusiform gyrus and inferior temporal gyrus bilaterally. Some of these regions showed greater activity when words in a triad were unrelated, whereas others did so when the three words were semantically related. These regions have been consistently reported in previous functional neuroimaging studies of associative encoding and may constitute key structures in association formation.

Cognitive and Non-Cognitive Factors Contributing to the Longitudinal Identification of Successful Older Adults in the Betula Study

ABSTRACT Studies of successful aging have typically defined elderly who fall in the upper end of a distribution of test scores as successful. A different definition of successful aging requires that older adults fall at or above the mean level of younger adults and maintain this level over time. Here we examined this definition of successful aging in a sample of 1463 individuals between the ages of 50 of 85. Based on principal coordinate analysis of cognitive and non-cognitive variables, we identified a group of 55 (8.3%) 70–85 years olds that were high functioning. This group of elderly showed elevated performance on a range of cognitive tasks. Non-cognitive factors that characterized this group included education and subjective health. The participants were retested 5 years later and the same type of analysis was repeated. Of the remaining individuals who initially were classified as high functioning, 18 (35%) remained high functioning and thus met the definition for successful aging. Years of education was a significant predictor of who remained successful over time.