APOE ε4 non-demented older adults showed greater BOLD response in multiple right hemisphere brain regions (anterior cingulate, lingual gyrus, middle temporal gyrus, middle frontal gyrus, posterior cingulate, precuneus, and cerebellar tonsil) during a verbal paired-associate learning task than their demographically similar non-ε4 counterparts. These differences were most salient in the OLD–FIX condition likely due to more successful encoding of familiar word pairs versus novel word pairs, as confirmed by the relatively poor post-scanning cued recall rates for new word pairs among all subjects. Results with the OLD–FIX contrast also suggest greater effort with consolidation processes among the APOE ε4 group. Furthermore, these differences in BOLD activation were seen in the absence of differences on multiple neuropsychological measures of learning and memory (DRS, WMS-R, CVLT, post-scanning cued recall memory) and despite comparable values for structural segmentation volumes of gray matter, white matter, CSF, and hippocampus. ROI analyses of activation in native space renderings of the hippocampi to the NEW–FIX and OLD–FIX conditions revealed similar patterns of relatively less activation in response to previously learned word pairs for the left hippocampus for both groups. However, they revealed discrepant response patterns for the right hippocampus by APOE genotype. Specifically, there was an enhanced response to OLD words among the APOE ε4 participants.
The current findings are largely congruent with previous functional neuroimaging studies investigating the role of APOE genotype on episodic memory encoding in older non-demented populations. Bookheimer et al. [6
] were the first to identify increased BOLD signal in APOE ε4 participants during learning and recall, although their APOE ε4 group also showed lower performances in delayed verbal recall relative to their non-ε4 counterparts, increased variability with respect to age range, and no formal assessment for differential atrophy. The present study carefully controlled for group demographic characteristics and neuropsychological status, and special consideration was given to the possibility of structural differences in the form of gray matter, white matter, CSF, and hippocampal components. Importantly, these latter results support that our observed BOLD response differences within each of the hippocampal regions were not due to differential atrophy or partial volume effects between APOE genotype groups. We also undertook an additional learning-to-criterion procedure prior to scanning to maximize the contrast of OLD to NEW items and to focus our efforts on possible neurobiological distinctions between encoding and consolidation. This is in contrast to Bookheimer et al. [6
] who examined signal change from combined learning and recall periods. Previous studies from our laboratory [4
] also have consistently reported increased MTL signal associated with the APOE ε4 allele, and ROI analyses involving the hippocampus in these and other former studies have typically been based on a standardized anatomic atlas [12
], thus not allowing for consideration of individual variation with respect to hippocampal size and consequent individual hippocampal BOLD activity. In fact, most fMRI studies average groups of different subjects into standard coordinate space, and anatomic differences in brain structure create additional variance and loss of registration accuracy [46
]. We utilized native space manual outlining of individual hippocampi as structural masks for functional overlays, thereby allowing for better anatomic accuracy of hippocampal activity (see Vandenbroucke et al. [46
] for discussion).
In addition, in the present study both new and old word pair conditions were contrasted against the same low level baseline condition (i.e. visual fixation) rather than with one another, thus obviating some of the difficulties seen in prior studies in interpretability of direction of BOLD signal changes from novel to familiar items. For example, prior studies – including our own – have often examined the difference in BOLD signal change between two ‘higher level’ contrasts (novel items versus familiar items), rather than contrasting either higher level condition with a lower level baseline condition such as visual fixation. It is conceivable that both higher level conditions have a good deal of neural activity associated with them and that encoding is not solely occurring during presentation of novel items. Buckner et al. [8
], for example, have demonstrated encoding processes to be active during retrieval tasks as well during learning trials. Post-hoc between-group contrasts of the NEW versus OLD conditions in the present study revealed no significant differences, supporting this notion. Thus, when the method involves the higher level subtraction of novel from familiar items, either isolation of encoding processes to the novel condition or directionality of BOLD activity by APOE genotype cannot easily be inferred. The latter effect could be reflective of greater hippocampal signal change to novel items among ε4 subjects or greater signal change to familiar items among ε3 subjects. Our finding that hippocampal right hemisphere overactivation is confined to OLD word pairs among ε4 subjects is illustrative of this point.
As in previous investigations from our laboratory, results appear to be consistent with a compensatory hypothesis wherein APOE ε4 participants may require additional neurocognitive effort, as manifested by an increased BOLD response, to maintain an equivalent level of performance with non-ε4 counterparts. At-risk older persons may employ other brain regions to effectively compensate for episodic memory performance, and in so doing, actively prevent memory decline for a period of time. Our data suggest that additional frontal and temporal cortical resources are invoked for the ε4 group during episodic memory encoding, and thus implicate additional executive functions and semantic memory processes, respectively, in order to maintain comparable behavioral performances (see also Lange et al. [27
]). These data are generally consistent with the HAROLD model [9
], which posits a reduction in hemispheric asymmetry due to the aging process. Although left hemisphere differences were negligible, those participants at increased genetic risk for developing AD displayed greater areas of activation in the right hemisphere for previously learned stimuli. The correspondence of relatively greater activation in multiple right hemisphere regions with equivalent behavioral memory performances supports the notion of compensation through the employment of more bilateral network regions (see network
view in Cabeza [9
]). In all, support for the compensatory hypothesis has been demonstrated across a whole host of neuropsychological [2
] and neuroimaging [3
] investigations, as well as with neurochemical [13
], neurotrophic [18
], and mitochondrial DNA alterations [33
Although the current study may be considered tentative support for the compensation hypothesis given the presence of APOE ε4 affiliated increases in activity in brain regions subsuming learning and memory, a necessary relationship must be established between hippocampal BOLD response and reported areas of increased activity in whole brain analyses. Hippocampal activity in the present study did show an expected adaptation effect with greater activation for NEW–FIX versus OLD–FIX in the left hippocampus, consistent with a growing body of evidence describing decreased activation in response to repeated stimuli and repetition priming paradigms in multiple cortical areas [7
] as well as in the hippocampus [25
]. However, the right hippocampus showed an APOE-dependent dissociation such that non-ε4 hippocampal activation remained stable and undifferentiated between the conditions whereas ε4 hippocampal activation showed a visible overactivation effect, with greater response in the OLD–FIX than the NEW–FIX contrast. Although newly developing functional connectivity analyses may further clarify the relationship between hippocampal activation and activity in other functionally related cortical regions, the consistency with regard to hemisphericity of genotypic cortical differences and hippocampal activation differences (all occurring in the right hemisphere) lends further support for the notion of a network of widely distributed, yet interconnected, cortical regions implicated in a circuit that may be over-activated to compensate for possible preclinical changes in episodic memory encoding.
Our present findings, combined with those of our prior studies [4
] as well as those of Dickerson et al. [15
] and Johnson et al. [25
] provide converging support for alterations of the right MTL, irrespective of the modality of the encoding stimuli (i.e. verbal versus pictorial versus face-name combinations). Johnson et al. [26
] have implicated right hemisphere regions as possibly facilitating more successful memory processing of verbal material in an fMRI study of CVLT performance. They extended these findings to patients with mild cognitive impairment (MCI) and showed a lack of adaptation among MCI patients to repeated stimuli in face learning [25
]. In a pair of studies, Dickerson et al. [15
] have also demonstrated specific activation patterns implicating the right medial temporal lobe in older adults. In the first of these studies, Dickerson et al. [16
] showed that MCI patients recruited a larger extent of right hippocampal gyrus during picture encoding, and the greater extent of activation in this region appeared to herald subsequent decline over 2 years later. In the second of their studies, Dickerson et al. [15
] found that memory performance on a face-name associative learning task was best predicted by right entorhinal cortex and hippocampal activation, and right hippocampal volume, in normally aging, MCI, and AD groups. Greater extent of entorhinal cortex activity was also shown for APOE ε4 carriers. Our prior work [4
] with picture learning also demonstrated dissociations between left and right MTL activity between APOE genotype groups. Given our findings of greater activation in multiple right hemisphere regions among ε4 participants, we suggest that ε4 subjects may be more heavily recruiting such right medial temporal and association areas as a compensation strategy to maintain an equivalent level of performance.
One limitation of the present study is that cerebral blood flow was not measured, leaving open the possibility that other hemodynamic considerations might explain the current BOLD contrasts. Another limitation is the need for longitudinal follow-up of the cognitive and neurophysiologic status of the subjects to confirm whether or not present findings are truly indicative of a compensatory mechanism and predictive of subsequent cognitive decline or conversion to AD. Plans to monitor cognitive decline and possible conversion to AD in the present group are currently underway. Although group differences were significant in the right and not the left hemisphere, a third limitation is that these results may be construed as indirect evidence for a laterality effect, as this would be more stringently supported by direct comparisons of contra-lateral activation arising from ROIs of active right hemisphere regions from the current whole brain between-group analyses and their left hemisphere manually outlined native space correlates. A fourth consideration is that our results of greater activation in the ε4 subjects may also be construed as a progressive disinhibition of brain response, and therefore a precursor to a possible clinical disconnection syndrome [28
]. Although a number of neuropsychological and functional neuroimaging studies of aging both within our laboratory and elsewhere provide support for viewing this greater activation as a compensatory mechanism, correlates of the present data set with longitudinal neurocognitive functioning will help elucidate which process, compensation or disinhibition, is most characteristic of the present findings. A fifth consideration is that gender may be partially influencing our results. Although there was not a significant gender difference between our two APOE groups, post-hoc analyses were conducted to investigate the effect of gender on the functional neuroimaging results. These analyses revealed no difference in hippocampal ROI activation according to gender. Small between-group gender differences were observed for whole brain analyses such that men appeared to show greater activation in right superior temporal regions and women appeared to show greater activation in left frontal regions; however, none of these regions overlapped with the reported differences in activation according to APOE genotype. Finally, the present study did not include post-hoc analyses to determine if the ε4 and non-ε4 groups differ in BOLD responses to the baseline condition. It should be noted, however, that no group baseline condition differences were observed in two previous studies from our laboratory [4
To summarize, fMRI study of episodic encoding combined with genetic assessment presents a promising approach for early identification and possible treatment of preclinical AD. The present findings extend the growing literature on APOE ε4 genotypic differences in episodic memory among non-demented adults and support a compensatory hypothesis possibly targeting an interconnected network of broadly distributed right hemisphere regions. Further research is needed to clarify the issue of blood perfusion as related to APOE genotype as well as to concretely correlate the present findings with longitudinal cognitive and neurophysiologic changes.