Our findings support the hypothesis that CA1-SRLM thickness correlates with delayed recall memory performance among patients with mild AD. We additionally found that the thicknesses of CA1-SP and ERC similarly correlated with delayed recall. These data establish that in addition to a role in discriminating patients with AD from healthy controls (Kerchner et al., 2010
), hippocampal microstructural metrics also track the severity of a core neuropsychological feature of the disease.
CA1-SRLM atrophy and memory dysfunction
CA1-SRLM atrophy could play a mechanistic role in the memory loss associated with AD. It is an area rich in synapses from CA3 to CA1 and from perforant pathway axons from the ERC to CA1. Synapses onto CA1 neurons, especially in the stratum radiatum, are highly plastic and have been hypothesized to play a critical role in learning and memory (Kerchner and Nicoll, 2008
). Loss of these synapses would be expected to cause a fundamental disruption of hippocampal circuitry (Amaral and Lavenex, 2007
; Insausti and Amaral, 2012
). In fact, one postmortem electron microscopic study revealed a gradient of synaptic loss in CA1-SRLM from normal controls to MCI to mild AD, and that synaptic density correlated with memory performance (Scheff et al., 2007
). It is thus likely that focal damage to CA1-SRLM plays a direct role in memory dysfunction in AD.
Other lines of evidence support the general notion that synaptic dysfunction is more pronounced than neuronal loss in early AD (Selkoe, 2002
). In a mouse model of the disease, CA1 synaptic loss precedes both neuronal loss and the appearance of pathology (Hsia et al., 1999
). In humans with early AD, pathological burden is greater in CA1-SRLM than in CA1-SP (Braak and Braak, 1997
); supporting that finding, 7 T imaging revealed no difference in CA1-SP thickness between patients with mild AD and normal controls (Kerchner et al., 2010
). In this study, delayed recall correlated significantly with both CA1-SRLM and CA1-SP thicknesses. Given our small sample size, it was not possible to detect whether one played a bigger role than the other in accounting for differences in episodic memory performance; this will be a focus of future work, which should involve the study of individuals at earlier, preclinical stages of Alzheimer's disease when discrepant contributions of CA1-SRLM and CA1-SP may be more pronounced.
In our patients, ERC thickness also correlated with both delayed free recall and delayed recognition subtests. This is not surprising, given the important reciprocal synaptic connections between CA1 and ERC (Amaral and Lavenex, 2007
; Insausti and Amaral, 2012
), and the early pathological involvement of the ERC in AD (Braak et al., 2006
). Another group found that ERC thinning correlated with apolipoprotein-E ε4 allele carrier status, a risk factor for AD (Burggren et al., 2008
), and with risk for subsequent memory decline among patients with MCI (Burggren et al., 2011
). In this study, ERC width correlated significantly with CA1-SP width, emphasizing the important association of these loci.
Immediate recall did not appear to correlate significantly with any medial temporal metric. Although our small sample size makes it hard to be sure that no relationship exists, it is notable that immediate memory is thought to localize to non-hippocampal working memory pathways in the brain. Digit span is another metric of working memory, and its negative correlation with CA1-SRLM thickness () is of uncertain significance; for immediate recall, the regression lines with the various medial temporal metrics, while not significant, all had a positive slope (), suggesting that there was no unifying pattern between these two methods of assessing working memory. Similarly, while there was a significant correlation between performance on one visuospatial task (figure copy) and the manually-derived CA1-SRLM data (), this finding was not reproduced on either of the other two visuospatial tasks (Block Design and Matrix Reasoning). Finally, DG/CA3 area correlated significantly with full scale IQ, a global metric of intelligence, but not with any measure of memory, any other specific cognitive domain, or the size of any other subfield. By contrast to these isolated findings, the relationship between hippocampal microstructure and delayed memory performance was consistent across tests and methods of measurement ( and ).
Do CA1-SRLM, CA1-SP, or ERC play specific roles in delayed recall? In theory, disruption of any node along the hippocampal trisynaptic loop (Amaral and Lavenex, 2007
; Insausti and Amaral, 2012
) could lead to medial temporal circuit dysfunction and memory failure. In this case, memory performance might be expected to degrade in direct proportion to the overall burden of pathology, wherever it first presents; in other words, the relationship between atrophy of these structures and memory performance that we observed may be nonspecific — reflecting only their particular vulnerability to early tau pathology but not any specific role in delayed recall capacity. Favoring instead a more specific role for CA1 is one lab's observation that among patients with temporal lobe epilepsy and mesial temporal sclerosis, a correlation between CA1 size and delayed recall performance emerged (Mueller et al., 2011b
) just as it did in AD (Mueller et al., 2011a
), despite somewhat different patterns of subfield atrophy in the two conditions (Mueller et al., 2009
). It would be of interest to study other patients with different sorts of focal hippocampal lesions, in order to determine whether structural perturbations of different subfields or strata of the hippocampus lead to discrete neuropsychological outcomes. Given a special role of the hippocampus – and of the CA1 subfield in particular – for place memory (Bartsch et al., 2010
), future work should incorporate navigational testing into the neuropsychological assessment.
Potential importance of the left hippocampus
Among our subjects, left and right hippocampal cross-sectional areas were equal, on average, and tightly correlated. Individual subfields – CA1-SRLM, CA1-SP, and DG/CA3 – were symmetric on average, but exhibited poor left-right correlation. While increased noise associated with the measurement of tiny structures could account for these poor correlations, these data suggest that macroscopic symmetries may belie actual smaller-scale asymmetries in the contributions of individual subfields and strata to overall hippocampal size.
This phenomenon relates to our finding that although left and right CA1-SRLM width was no different on average
, the left side played a significantly greater role than the right in driving the relationship with delayed memory performance. There is substantial evidence that the left and right hippocampi contribute differentially to the encoding of verbal and visuospatial memory (Apostolova et al., 2010a
; de Toledo-Morrell et al., 2000
; Frisk and Milner, 1990
; Milner, 1965
; Nunn et al., 1998
; Petrides and Milner, 1982
; Smith and Milner, 1981
; Spiers et al., 2001
). Thus while it is reassuring to see a disproportionate role for the left CA1-SRLM (and ERC) in predicting performance on LM, a verbal contextual memory test, it is not clear why a similar trend was apparent for the visuospatial memory test (). Perhaps this reflects a peculiarity of our subjects or of the BVMT-R; for instance, it is hard to control for the possibility that subjects may employ a verbal strategy to recall shapes and positions. Importantly, all of our subjects were right-handed, and so variations in hemispheric dominance should not be an issue. Other work suggests that the relationship between visuospatial memory and the right hippocampus is weaker than between verbal memory and the left (McDermid Vaz, 2004
). Acquiring data on more AD subjects using additional visuospatial memory metrics could help to verify and expand upon this finding.
7 T hippocampal microstructural MRI: methodology
In addition to the findings outlined above, we demonstrate the utility of a high-resolution T2-weighted 7 T MRI sequence in reliably depicting hippocampal subfield and stratal microstructural anatomy. The sequence was well-tolerated among our elderly, mildly demented patients without any significant adverse effect.
One trade-off to having a high in-plane resolution (220 µm) was a low through-plane resolution (1.5 mm), which was necessary to maintain an adequate signal-to-noise ratio. Thick slices could produce partial volume artifacts, potentially blurring small structural features in-plane. We tried to compensate for this potential pitfall by carefully prescribing image acquisition perpendicular to the longitudinal axis of the hippocampus and by restricting our analysis to the hippocampal body, where the anatomy is much more dynamic in-plane than through-plane. Future sequence development could focus on strategies to reduce slice thickness, which may allow more detailed analysis of the hippocampal head, where anatomy is dynamic in all three dimensions.
We also report a semi-automated method for hippocampal stratal width determination. This technique reduces reliance on user intervention to measure small structures, with the hope of speeding analysis and enhancing reproducibility. We found that the method yielded estimates of CA1-SRLM and CA1-SP width that correlated well with manual measurements. Manual and semi-automated measurements correlated at a high level of significance but were not perfectly linked, and it is not necessarily straightforward to predict which method should be more accurate. While manual measurements are typically considered to be a gold standard, the semi-automated method, which integrates many measurements along the full length of the region of interest (compared to three manual measurements per slice), could possibly be less susceptible to noise. The semi-automated tool does still require the input of a user who is knowledgeable of hippocampal anatomy and so is not widely deployable, but it sets the stage for future development of a more fully automated analysis.
In sum, we found that among patients with mild AD, delayed recall performance correlates with CA1-SRLM, CA1-SP, and ERC widths. Left CA1-SRLM width played an especially important role in predicting delayed recall capacity. These data suggest that in addition to being a marker for the presence of AD pathology, CA1-SRLM atrophy closely tracks the magnitude of episodic memory impairment among patients with the disease. While it is not yet clear whether a high-resolution imaging biomarker of AD-associated memory loss is yet poised to assist in the early diagnosis of AD, our findings suggest that it may be useful in tracking disease severity. In future work, it will be important to follow patients longitudinally, and to examine patients with MCI or normal cognition at baseline.