The aim of this study was to investigate the influence of continuous use of HT from the perimenopausal stage on patterns of hippocampal activation during performance of a verbal memory test. To achieve this aim, we studied healthy midlife women whose timing of exposure to HT was prospectively validated. Seventeen women who initiated HT during the perimenopausal stage were compared with 17 controls who were similar in age, education, depressive symptoms, and estimated verbal intelligence, but who never used HT. Based on our previous neuroimaging studies in older long-term HT users (Maki and Resnick, 2000
; Resnick et al., 1998
), we hypothesized that HT would be associated with enhanced verbal memory performance and with differences in activation in both the parahippocampal gyrus and the hippocampus proper. Each of these hypotheses was supported.
Perimenopausal HT users compared to never users showed deactivation in the parahippocampal region. This deactivation contrasted with the typical pattern of increased activation in the parahippocampal region observed across the combined sample of perimenopausal and never users. Perimenopausal HT users also showed increased activation in the left hippocampus. This activation was an exaggeration of a typical pattern of increased activation observed in the hippocampus across the combined sample. Notably, lower activation in the left parahippocampal gyrus and higher activation in the left hippocampus were each associated with better performance on the verbal memory task, whereas activation during the match task was unrelated to verbal recognition or match performance. The association between the activation pattern during verbal recognition in perimenopausal hormone initiators and better memory performance suggests that the alterations in brain function associated with perimenopausal initiation of HT are favorable. Overall, these results support the hypothesis that early and continued HT use confers a benefit to verbal memory performance and modulates hippocampal-parahippocampal function underlying verbal memory performance.
Significant performance differences on verbal memory tests in favor of the perimenopausal users were evident on the imaging verbal recognition test (p < .05, two-tail). A trend (p < .10) was observed on both the immediate and delayed trials of the EBMT, a story recall test that was performed outside the scanner. Notably, the effect size for the immediate and delayed trials of the EBMT were .59 and 0.62 standard deviations, respectively, which according to standard classifications are substantive medium (0.50) to large (0.80) effect sizes. The lack of statistical significance for these medium to large effects stemmed from a small sample size. Our findings on the EBMT are consistent with previous placebo-controlled trials in surgically menopausal women showing that estradiol valerate (with or without testosterone enanthate) improves story recall (Sherwin, 1988
); (Phillips and Sherwin, 1992
). The importance of menopausal stage and hormone use on story recall is further supported by new findings from the Study of Women’s Health Across the Nation (SWAN) based on 2362 women who completed the EBMT annually over a 6-year period as they transitioned from premenopause to postmenopause (Greendale et al., 2009
). As in the present study, prior HT users in SWAN showed improved performance on the first administration of the story recall test compared to never users. Furthermore, perimenopausal women in SWAN did not show the same magnitude of improvement over time on the EBMT that was observed in premenopausal women (p < .10), suggesting a decrease in story recall during the perimenopausal stage. Our data indicate that intervening during the perimenopausal stage with HT is associated with improvements in story recall later in life. Some of the benefit of HT on story recall may be associated with improvement in hot flashes. In a pilot study involving 29 early postmenopausal women with moderate to severe hot flashes, we found that lower performance on story recall was associated with an increase in physiological hot flashes, as measured by ambulatory skin conductance monitors, but not with self-reported hot flashes (Maki et al., 2008
In contrast to the effect of HT on story recall, the effect of HT on memory for word lists was very small (i.e., range 0.5 to .20 standard deviations) in the present study. Results of a prior study from the Melbourne Women’s Midlife Health Project (n = 326) (Henderson et al., 2003a
) suggest that larger samples may be needed to see an effect of perimenopausal HT use on word list memory. In that prior study, there was no overall effect of HT on word list memory (story recall was not assessed), but in post-hoc analyses women who initiated HT before the final menstrual period (i.e., during the perimenopausal stage) performed better on that memory test compared to women who initiated HT after the final menstrual period (Henderson et al., 2003a
). Furthermore, word list recall was positively associated with years of HT. The Nurses Health Study found no effect of postmenopausal estrogen use within the three years following the final menstrual period on 2-year change in performance on a telephone version of the EBMT in a sample of older women (mean age = 74) (Kang et al., 2004
). Together, these findings raise the possibility that HT intervention during the perimenopausal stage rather than during the early postmenopausal stage may be key to enhancement in story recall later in life.
Based on results from our prior PET studies in elderly long-term HT users (Maki and Resnick, 2000
; Resnick et al., 1998
) and a wealth of basic science studies showing estrogen effects on the hippocampus (Gibbs, 2000
), the present fMRI analyses focused on medial temporal lobe areas, specifically the parahippocampal gyrus and hippocampus proper. For the verbal task, group comparisons revealed significant increases in activation in perimenopausal HT users compared to controls in the left hippocampus. This increase was evident during verbal recognition and match conditions but not during verbal encoding. The left hippocampal regions subserving retrieval and matching of abstract words overlapped substantially, suggesting an overlap in cognitive processes mediated by the left hippocampus during the two conditions. The finding in HT users of increased hippocampal activation during the match task suggests that early initiators may have engaged in memory processing during the control task, particularly because the match task always followed recognition and hippocampal activation is commonly observed in verbal memory tasks (Cabeza and Nyberg, 2000
; Schacter and Wagner, 1999
). Analyses across nonusers and users combined showed that the left hippocampus is typically active during this verbal recognition memory task. In light of the perimenopausal user’s significant advantage in the recognition condition and near-significant advantage in the match condition (p < .10, effect size = .70 sd), these results suggest that this augmentation of the typical patterns of left hippocampal activation is advantageous for perimenopausal users. Regression analyses support this interpretation as well, because higher activation in the left hippocampus was associated with better memory. More generally, the pattern of results suggests that beneficial effects of estrogen on medial temporal regions subserving verbal memory are observed in regions that subserve retrieval of studied words rather than encoding of to-be-learned words. There were no group differences in activation during verbal encoding alone, and a group difference observed when verbal encoding was compared to match was due to differences in the match condition.
Group differences in patterns of brain activation were also observed in the parahippocampal gyrus during verbal tasks. Specifically, perimenopausal HT users showed less activation compared to controls in the right and left parahippocampal gyri during verbal recognition. That result replicates our previous PET findings of enhanced verbal memory and decreased parahippocampal gyrus activation in HT users compared to nonusers during verbal recognition (Resnick et al., 1998
), albeit in a more anterior and medial parahippocampal region corresponding to perirhinal cortex. The consistency of these findings across samples and imaging methods suggests that decreased parahippocampal activation may be a critical neural substrate underlying the association between use of HT and enhanced verbal memory. The typical pattern of activation in the parahippocampal gyrus is one of activation, as suggested by the analysis across the combined groups of perimenopausal users and nonusers shown in . Insights into the role of the parahippocampal gyrus during word recognition tasks can be gained from a previous fMRI study which used a mixed blocked and event-related experimental design in order to dissociate item components associated with transient recovery of word-specific information from state components associated with a sustained retrieval mode across trials to meet ongoing goals (Donaldson et al., 2001
). That study found sustained decreases in activation in bilateral regions of parahippocampal cortex, suggesting that the parahippocampal gyrus might become deactivated when participants entered into a sustained state of retrieval during recognition tasks. In the present study, regression analyses suggested that deactivation in the left parahippocampal gyrus relates to better verbal memory performance, and this effect was specific to the verbal recognition condition. Together, the findings of enhanced hippocampal activation and decreased parahippocampal activation in perimenopausal HT users suggest that perimenopausal HT enhances both state-dependent and recollective processes contributing to verbal recognition performance.
In secondary analyses we examined the effects of perimenopausal HT on figural encoding and retrieval. In an earlier study we found greater parahippocampal deactivation and better figural memory among HT users compared to nonusers (Resnick et al., 1998
). In contrast, in this study perimenopausal HT users showed an increase in parahippocampal gyrus activation and no advantage over nonusers in figural memory. It is possible that our earlier findings reflected a enhanced state-dependent retrieval mode during figural tasks, though this particular role of the parahippocampal gyrus has been demonstrated only during verbal memory tasks (Donaldson et al., 2001
). A modest figural memory benefit (p < .01) with combined conjugated equine estrogen plus medroxyprogesterone acetate (CEE+MPA) but not CEE alone was observed in WHISCA, a randomized clinical trial in women over age 65 (Resnick et al., 2004
; Resnick et al., 2006
). This suggests a possible influence of MPA on figural memory in older women. Note, however, that no benefits of CEE+MPA on figural memory were observed in a clinical trial of 180 younger postmenopausal women (mean age = 52 years) (Maki et al., 2007
). These studies suggest that figural memory benefits may be most evident in samples of elderly women receiving combination HT rather than in midlife women.
Lastly, it is worth noting that the evidence of a functional enhancement in the hippocampus with perimenopausal hormone initiation contrasts with evidence of detrimental effects of HT on hippocampal structure from the Women’s Health Initiative Memory Study (WHIMS) MRI substudy (Resnick et al., 2009
). WHIMS-MRI examined brain volumes in 1,403 women aged 71–89 years who underwent brain scans on average 3.0 years post-trial for the CEE + MPA trial and 1.4 years post-trial for the CEE-alone trial. Total brain volumes, hippocampal volumes, and frontal lobe volumes were significantly lower among women randomized to receive HT compared to those randomized to receive placebo. Notably, the clinical significance of these volumetric differences was evident in the demonstration of a relationship between cognitive impairment (i.e., mild cognitive impairment or dementia) and loss of hippocampal and total brain volume among older women randomized to receive CEE-based therapies but not among older women randomized to receive placebo (Espeland et al., 2009
There were several limitations to our study. Although we were careful to take specific steps to minimize biases associated with HT, we cannot rule them out entirely. Our fMRI study of the effects of early and continued HT use on brain function later in life was observational by design and was conducted almost 10 years after menopause, on average. To ensure validity of timing and duration of HT exposure, we examined prospective daily diary and medical records for women participating in the Melbourne Women’s Midlife Health Project. We drew specifically from the pool of women who initiated HT before the final menstrual period and continued HT so that we could test the extremes of the critical window hypothesis, specifically to study whether early and continued use of HT confers benefits later in life. The users and nonusers of HT were carefully matched with respect to age, education, depression and estimated verbal intelligence. Valid ascertainment of HT exposure is critical because recall bias is common in self reports of both medication use and age at menopause (den Tonkelaar, 1997
; West et al., 1995
). In this observational study, the specific form of HT was not controlled and therefore the results reflect the most commonly used preparations, particularly estradiol (60%) and conjugated equine estrogens (35%). These results then reflect common effects of these two estrogen preparations, rather than effects particular to a certain preparation. Finally, our data do not address potential effects of perimenopausal HT on prefrontal cortex, a region shown to be functionally enhanced during an fMRI verbal memory task in randomized trials of HT (Joffe et al., 2006
; Persad et al., 2009
) and in our previous PET studies (Maki and Resnick, 2000
). Our image acquisition parameters were optimized to detect effects in medial temporal lobe structures so the number of superior slices to address frontal lobe effects was minimal. On the other hand, our functional data complement new structural neuroimaging data demonstrating larger hippocampal volumes among women who used HT at the time of menopause compared with women who had never used HT (Erickson et al.
In summary, the present study examined the effects of continuous use of HT from the perimenopausal stage to later in life (i.e., mean age = 60 years) on verbal memory and the medial temporal structures subserving verbal memory. Our design was observational by design, given the substantive logistical and financial challenges necessary to examine this issue in a randomized controlled trial. One notable strength of our design is that exposure to HT was validated through prospectively collected research records. Additionally, our two comparison groups were well matched on potential confounding variables. Our behavioral and neuroimaging findings support the critical window hypothesis and suggest that perimenopausal HT is associated with enhanced verbal memory and enhanced hippocampal and parahippocampal function. We did not find similar support for the critical window hypothesis as applied to figural memory. The present finding raises the possibility that the critical window may begin in the perimenopausal stage. Future studies should directly contrast the effects of perimenopausal versus early postmenopausal HT use on verbal memory and the functional circuitry underlying verbal memory, including the hippocampus and prefrontal cortex.