There is a range of literature that explores oestrogen levels in the brain and its influence on cognitive functions, specifically memory. This link is explored in a variety of ways: some studies look at the memory problems and cognitive function deficits that occur when females go through the menopause; some examine the role of oestrogen during the oestrus cycle in female rats. There is less research, however, into how fluctuating oestrogen levels have an effect on memory over the course of the human menstrual cycle.
Many studies have shown that oestrogen receptors are numerous in the brain, and occur in areas important for memory - the hippocampus, amygdala and cerebral cortex (Sutcliffe, Marshall & Neill, 2007; Shilling, Jenkins, Fallowfield & Howell, 2001; Compton, van Amelsvoort & Murphy, 2001). Importantly, high levels of oestrogen have been found to increase gray matter in the hippocampus during the postmenstrual phase of the menstrual cycle (Protopopescu et al., 2008). Further studies have highlighted the increased density and number of dendritic spines and synapses in the CA1 region in rats when high levels of oestrogen are present (e.g. Luine, 2008); Craig and Murphy (2006) suggest that oestrogen aids filopodial outgrowth in hippocampal neurones. Craig and Murphy (2009) also discuss how neurogenesis increases in the hippocampus of rats as a result of oestrogen therapy (OT). This can lead to a substantially increased level of long-term potentiation during the pro-oestrus stage of the menstrual cycle (Warren et al., 1995, as cited in Good, Day & Muir, 1999), possibly as a result of oestrogen's role in modulating different neurotransmitter systems that have previously seen to be important in memory - cholinergic, serotonergic and dopaminergic systems (Craig & Murphy, 2006). This suggests that memory should be improved by higher oestrogen levels.
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An area of interest when studying the effect of oestrogen on memory is in postmenopausal women. After the menopause, oestrogen levels drop significantly, to around one-fifth of the premenopausal level (Sherwin, 1983, as cited in O'Neal, Means, Poole & Hamm, 1996), leading to decline in some areas of cognitive function. Therefore, OT and hormone replacement therapy (HRT) are important to study to investigate if increasing oestrogen levels lead to a reversal of this decline. Studies into Alzheimer's Disease (AD) suggest there is a significantly greater brain mass in women who receive OT than those who do not (Craig & Murphy, 2009) and that women receiving oestrogen for six weeks demonstrate cognitive improvements (Luine, 2008). However, Luine also concludes that, in most cases, positive effects of oestrogen are small and inconsistent in models of humans and animals.
Research into HRT in younger mid-life women suggests that it is associated with improved memory (Stephens, Pachana & Bristow, 2006), which confirms Sherwin and Tulandi's (1996) work that found OT could be important in maintaining memory function in women who have lower than usual levels of oestrogen (as cited in Almeida, 1999). Additionally, Resnick et al. (1997, as cited in Almeida) found that women undertaking OT performed significantly better on a memory test than those women who do not take it post-menopause. Despite this and other evidence supporting oestrogen's positive effect on memory, however, O'Neal, Means, Poole and Hamm (1996) conclude there are still different viewpoints as to its effects (some state positive results, others none at all) and suggest this may be because of the different methods employed by different studies.
It seems high levels of oestrogen present in the brain during certain phases of the menstrual or oestrous cycles appear to facilitate memory. Sutcliffe, Marshall and Neill (2007) emphasise that oestrogen levels peak twice during the menstrual cycle - firstly, just prior to ovulation (days 6-14) and again during the mid-luteal phase (days 21-22). Research that supports an increase in memory performance during these stages is numerous - when working memory is assessed in spatial tasks, oestrogen enhancements are seen (Luine, 2008); Rosenberg and Park (2002, as cited in Konishi, Kumashiro, Izumi & Higuchi, 2008) reported an improved performance in a language working memory task on days seven and 14 of the menstrual cycle; and Konishi et al.'s own study found a high performance of language working memory was most likely associated with highly concentrated oestrogen levels, to name a few. However, as with research into postmenstrual memory decline, there are other studies that differ, (e.g. Konishi, Kumashito, Izumi, Higuchi & Awa, 2009) showing that working memory performance can worsen in high oestrogen phases. It is therefore clear that more research needs to be conducted into how exactly fluctuating oestrogen levels influence memory.
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Research that has been conducted in this area previously provides evidence for oestrogen's effect on a variety of cognitive functions. As oestrogen levels fluctuate over the menstrual cycle depending on which phase of the cycle a person is currently in, it is predicted that cognitive abilities will differ over the menstrual cycle depending on the levels of oestrogen present in the body at that time. More specifically, it is predicted that working memory performance will be improved when participants are in higher oestrogen phases of their menstrual cycle, and will deteriorate as levels of oestrogen are lower in the cycle.
Participants will be female university undergraduates, recruited via the EPR system - they will take part in this experiment in return for required course credits. Thirty participants will be tested in the experiment, and as this study requires natural menstrual cycles, participants who are selected will not be on any form of hormonal contraception or will not have been in the previous six months (to allow the natural cycle to return to usual).
To test working memory, the Corsi Block-Tapping (CBT) test will be employed. This involves a 10" x 8" board, with nine raised 3cm x 3cm squares (see Appendix A for an example of the apparatus, as cited in Berch, Krikorian & Huha, 1998). Standardized administration of the test (tapping rate, measurements obtained, etc.) has been determined using the work of Kessels, van Zandvoort, Postma, Kappelle and de Haan (2000); block tapping sequences were obtained from Pagulayan, Busch, Medina, Bartok and Krikorian (2006); Capitani, Laiacona and Ciceri (1991); and Kessels et al.
The experiment will manipulate one independent variable, oestrogen level at time of test, which is a within subjects factor. This variable has two levels - higher and lower, depending on the phase of the menstrual cycle the participant is in. In the first testing session, the oestrogen level (higher/lower) of the participant will be randomised; therefore some participants taking the experiment for the first time will be in a higher phase, and other participants in a lower phase (natural counterbalancing). The dependent variable will be score on the CBT test, as measured by 'total score' which measures the product of the number of correct trials in the test and the 'block span' (which is the length of the last correctly repeated sequence by the participant). Participants will be given two trials per each sequence length, up to a sequence of nine numbers (therefore the minimum and maximum trials a participant will undertake are 2 or 18 respectively in each testing session).
Participants will be informed of their right to withdraw and that they will be required to complete a small questionnaire about their menstrual cycle (see Appendices B and C). Then participants will be informed of what the test requires, briefly showing them an example of how the test will be performed and how they should respond. The test will then begin with trials of sequences two items long, up to a possible sequence of nine items long. The experimenter will point at each block in the sequence at a rate of one per second, after which the participant will repeat the sequence back. If at any point the participant does not correctly replicate the sequence, the test will stop and their scores will be noted down. Exactly 14 days after this first testing session, another identical session will be administered to the same participants. Testing sessions will take a maximum of approximately 10 minutes each time. Participants will be debriefed at the end of the second testing session (see Appendix D for a copy).
Plans for Data Analysis
The paired-samples t-test is the most appropriate statistical test as the experiment will have one dependent variable and one independent variable with two levels. Because the sets of scores are correlated as they will come from the same individual, and as there will only be two sets of scores, this test is the most appropriate to perform the analysis of the data.