To the extent that hippocampal atrophy predicts risk for future decline to AD, this raises the possibility that such behavioral tasks may be sensitive indicators of AD risk, before deficits begin to show up in “standard” neuropsychological tests of memory or in daily activities. We are currently pursuing longitudinal studies to verify this idea, and to determine whether the same individuals who perform poorly on our transfer tasks are indeed more likely to decline to AD within a few years, compared with individuals who transferred well. Preliminary data from these longitudinal studies is encouraging and a full report will follow in the near future.
Two other directions for current research are, first, the use of functional imaging, and second, the development of mouse versions of these task to facilitate translational research between animal and human studies and the assessment of novel therapeutics in transgenic mouse models of Alzheimer’s disease. These are briefly summarized below.
Functional Imaging in the Mild Cognitively Impaired
The above interpretations of our behavioral tasks assume that HA individuals’ failure to transfer reflect a qualitative difference in initial learning. Specifically, in the presence of hippocampal-region mediation, learning normally includes extra information about context and stimulus regularities, and this extra information supports subsequent transfer. HA impairs this extra learning; HA individuals can still learn the stimulus-response associations needed to master the initial discriminations, but cannot apply this learning flexibly when challenged with a transfer test involving familiar stimuli presented in new ways or new combinations.
One way to test this assumption is by functional neuroimaging (fMRI) of individuals learning these tasks. FMRI is a non-invasive imaging method useful for detecting hemodynamically coupled neurocognitive brain activity during specific cognitive tasks. FMRI offers a method of examining memory-associated brain regions while those regions are functionally engaged in memory tasks. On our tasks, we would expect that individuals who show hippocampal-region activation during initial learning would be more likely to transfer well later, while those individuals who show little hippocampal-region activation during learning might show impaired transfer later.
In initial studies, conducted at the University of Wisconsin Medical School under the direction of Sterling Johnson, we have adapted the concurrent discrimination task shown in () for event-related fMRI. During the learning phase, subjects were presented with each pair six times over the course of an 8-minute scan. In addition to the object pairs, a control condition was presented in which subjects saw two gray squares side by side; one of the squares was clearly marked as the correct choice, and the subject’s task was merely to choose that square. Feedback was provided for both the object and control trials. Subjects were provided with instructions and practice prior to scanning.
We modeled the cerebral response to the task in two ways. First we examined the main effect of response selection. This produces a robust dorsolateral frontal lobe response in connection with choosing a response among the two response alternatives (irrespective of accuracy or number of repetitions). Next we examined each subject’s time series of images for change in signal associated with increasing response accuracy. In some subjects, but not all, we have seen that the signal in the hippocampus attenuates with increasing response accuracy indicating that the hippocampus is recruited initially during learning, but as the correct response is acquired, the hippocampus becomes progressively less active. A single healthy elderly subject demonstrating this pattern is shown in . These results are consistent with our assumption that, in healthy individuals, the hippocampal region is recruited during the learning phase of this task.
Fig. (6) A montage across three slices of the SPM atlas depicting activation from a single 75 year old healthy female subject. The subject exhibited a learning-related adaptation response in the hippocampus. A region of interest analysis was used that focused (more ...)
The next question is whether this pattern of hippocampal-region involvement differs in individuals at risk for Alzheimer’s disease. To investigate this question, we are conducting fMRI studies with our concurrent discrimination task in elderly individuals with amnestic mild cognitive impairment (MCI). Amnestic MCI is a condition involving memory impairment beyond typical age-related declines, and unaccounted for by other medical conditions, and it is a major risk factor for development of AD [34
]. F-18 fluoro-deoxy-glucose (FDG) positron emission tomography (PET) studies show reduced cerebral metabolic rate of glucose (CMRgl) in many of the brain areas affected by AD [35
], and hippocampal metabolism has been found to correlate with the performance on an encoding task [36
While CMRgl and structural MRI studies have shown some sensitivity to MCI, the use of fMRI has not been widely applied to this disorder. A small body of functional imaging studies suggest that the hippocampus is responsive to new information in controls more than MCI or early AD [37
]. Not all studies agree [41
], but this discrepancy may be due to the fact that MCI subjects in those studies were not required to have deficits in memory function on neuropsychological tests, only memory complaints. Some fMRI studies of mild AD and persons at genetic risk for AD have found greater activation associated with disease presence or risk, perhaps reflecting a compensatory response [43
]. Further study is needed to resolve the discordant findings.
These prior reports have all more or less relied on novelty detection paradigms that echo the putative role of the hippocampus in forming new memories from previously un-encountered episodic events or stimuli. As reviewed above, models of hippocampal function that incorporate incremental associative learning may provide a fruitful approach to studying hippocampal-region dysfunction in people with MCI, with the goal of identifying those most at risk for conversion to AD.
We have conducted some preliminary fMRI studies in eight volunteers with MCI (mean age 74) and thirteen elderly controls (mean age 73, SD 9). As expected, and per criteria, the MCI subjects exhibited neuropsychological deficits on the encoding trials and delayed recall trials of declarative memory tests including the Rey Auditory Verbal Learning Test and Brief Visuospatial Memory Test-Revised. Other neuropsychological domains were relatively less impaired. The fMRI results for each group were in the direction of our hypotheses, as shown in . The controls on average exhibited attenuation in hippocampal signal over repeated trials associated with performance accuracy, while the MCI subjects did not. Both groups exhibited attenuation in the anterior cingulate over the course of the experiment.
Fig. (7) Statistical parametric maps of adaptation during the Choose task in 13 controls and 8 MCI. The activations can be interpreted as regions where the negative slope of change over repeated trials is significantly different from zero. (A) The average slope (more ...)
These data demonstrate the feasibility of adapting an associative learning task for MCI in the fMRI environment. Future experiments are planned to determine whether the fMRI of associative learning predicts subsequent conversion to AD and whether this adds additional new information beyond behavioral evaluation and existing structural imaging methods.
Translation to Mouse Paradigms
A key value to our associative learning tasks is that –unlike tests of hippocampal-dependent declarative memory (e.g., delayed paragraph recall) – they can be naturally translated into rodent paradigms. In collaboration with Michelle Nicolle at Wake Forest University, we have developed a mouse version of our learning and generalization task (based on the concurrent discrimination task of ). The long-term goal is to develop a quick, largely-automated task that can be used for inexpensive, high-throughput drug screening with transgenic mice. This project has recently begun and will proceed as follows. On the human version of the task, subjects begin by learning concurrent discriminations involving pairs of objects that vary in shape and color, with color or shape relevant, and then transfer to new pairs where the relevant features remain the same but the irrelevant features are novel. In the mouse version of this task, we present the animal with discrimination pairs consisting of digging pots that each contain two stimuli, an odor and a digging medium (). In each pair of pots, either the odor or the medium differs, but not both. Thus, for example, a pair with odor as the relevant stimulus might contain mint-scented sawdust vs. lemon-scented sawdust, while another pair with medium as the relevant stimulus might contain cinnamon-scented confetti vs. cinnamon-scented sand (). One of the pots is seeded with a chocolate reward at the bottom, and the animal’s task is to learn to dig in the correct pot to obtain the reward. To control for the odor of the food, there is chocolate pellet “dust” distributed in every pot.
(A) Mouse testing apparatus. (B) Schematic of design of rodent analog of the concurrent discrimination and transfer task.
Our preliminary data from male C57B6 wild-type mice indicates that healthy mice can learn these discriminations easily. The learning phase is followed by a transfer phase analogous to that in the human task: the irrelevant features are changed, but the relevant rules remain the same (so, mint still beats lemon regardless of medium, and confetti still beats sand regardless of odor). Just like the non-atrophied humans, healthy wild-type mice can transfer with relatively few errors, as shown in pilot data in .
Fig. (9) Pilot data from 9 wild-type mice showing enhanced transfer to phase 2 from phase 1 (fewer trials to criterion) when the relevant rule remains the same. A third phase with a novel cue pairing is included as a control for non-specific changes in performance. (more ...)
The next question is whether we will see the same pattern of spared initial learning but impaired transfer in mouse AD models as we do in humans with HA. The transgenic mouse models of AD will be particularly useful in determining relevance of specific neurobiological and/or pathological changes that may underlie hippocampal-dependent memory dysfunction in AD. If this rodent task is successful, it will be useful in evaluating the effects of new AD drugs on the prevention of AD-related cognitive impairment. New pharmacological agents can be administered to transgenic mice, and their effects on hippocampal-dependent memory assessed by performance on the transfer test. The advantage of using this transfer test over existing rodent paradigms is that it parallels the human task, making direct cross-species comparisons possible and, it would be hoped, speeding evaluation and delivery of new therapeutic agents for Alzheimer’s disease.