John P. Hilton

Exploring the Neural Basis of Cognitive Reserve

There is epidemiologic and imaging evidence for the presence of cognitive reserve, but the neurophysiologic substrate of CR has not been established. In order to test the hypothesis that CR is related to aspects of neural processing, we used fMRI to image 19 healthy young adults while they performed a nonverbal recognition test. There were two task conditions. A low demand condition required encoding and recognition of single items and a titrated demand condition required the subject to encode and then recognize a larger list of items, with the study list size for each subject adjusted prior to scanning such that recognition accuracy was 75%. We hypothesized that individual differences in cognitive reserve are related to changes in neural activity as subjects moved from the low to the titrated demand task. To test this, we examined the correlation between subjects’ fMRI activation and NART scores. This analysis was implemented voxel-wise in a whole brain fMRI dataset. During both the study and test phases of the recognition memory task we noted areas where, across subjects, there were significant positive and negative correlations between change in activation from low to titrated demand and the NART score. These correlations support our hypothesis that neural processing differs across individuals as a function of CR. This differential processing may help explain individual differences in capacity, and may underlie reserve against age-related or other pathologic changes.

Covariance PET patterns in early Alzheimer's disease and subjects with cognitive impairment but no dementia: utility in group discrimination and correlations with functional performance.

Although multivariate analytic techniques might identify diagnostic patterns that are not captured by univariate methods, they have rarely been used to study the neural correlates of Alzheimers disease (AD) or cognitive impairment. Nonquantitative H2(15)O PET scans were acquired during rest in 17 probable AD subjects selected for mild severity [mean-modified Mini Mental Status Examination (mMMS) 46/57; SD 5.1], 16 control subjects (mMMS 54; SD 2.5) and 23 subjects with minimal to mild cognitive impairment but no dementia (mMMS 53; SD 2.8). Expert clinical reading had low success in discriminating AD and controls. There were no significant mean flow differences among groups in traditional univariate SPM Noxel-wise analyses or region of interest (ROI) analyses. A covariance pattern was identified whose mean expression was significantly higher in the AD as compared to controls (P = 0.03; sensitivity 76-94%; specificity 63-81%). Sites of increased concomitant flow included insula, cuneus, pulvinar, lingual, fusiform, superior occipital and parahippocampal gyri, whereas decreased concomitant flow was found in cingulate, inferior parietal lobule, middle and inferior frontal, supramarginal and precentral gyri. The covariance analysis-derived pattern was then prospectively applied to the cognitively impaired subjects: as compared to subjects with Clinical Dementia Rating (CDR) = 0, subjects with CDR = 0.5 had significantly higher mean covariance pattern expression (P = 0.009). Expression of this pattern correlated inversely with Selective Reminding Test total recall (r = -0.401, P = 0.002), delayed recall (r = -0.351, P = 0.008) and mMMS scores (r = -0.401, P = 0.002) in all three groups combined. We conclude that patients with AD may differentially express resting cerebral blood flow covariance patterns even at very early disease stages. Significant alterations in expression of resting flow covariance patterns occur even for subjects with cognitive impairment. Expression of covariance patterns correlates with cognitive and functional performance measures, holding promise for meaningful associations with underlying biopathological processes.

Relation of cognitive reserve and task performance to expression of regional covariance networks in an event-related fMRI study of nonverbal memory

Cognitive reserve (CR) has been established as a mechanism that can explain individual differences in the clinical manifestation of neural changes associated with aging or neurodegenerative diseases. CR may represent individual differences in how tasks are processed (i.e., differences in the component processes), or in the underlying neural circuitry (of the component processes). CR may be a function of innate differences or differential life experiences. To investigate to what extent CR can account for individual differences in brain activation and task performance, we used fMRI to image healthy young individuals while performing a nonverbal memory task. We used IQ estimates as a proxy for CR. During both study and test phase of the task, we identified regional covariance patterns whose change in subject expression across two task conditions correlated with performance and CR. Common brain regions in both activation patterns were suggestive of a brain network previously found to underlie overt and covert shifts of spatial attention. After partialing out the influence of task performance variables, this network still showed an association with the CR, i.e., there were reserve-related physiological differences that presumably would persist were there no subject differences in task performance. This suggests that this network may represent a neural correlate of CR.

Effects of vertex truncation of polyhedral nanostructures on localized surface plasmon resonance

Polyhedral nanostructures are widely used to enable localized surface plasmon resonance (LSPR). In practice, vertices of such structures are almost always truncated due to limitations of nanofabrication processes. This paper studies the effects of vertex truncation of polyhedral nanostructures on the characteristics of LSPR sensing. The optical properties and sensing performance of triangular nanoplates with truncated vertices are investigated using electrodynamics analysis and verified by experiment. The experimental results correlated with simulation analysis demonstrate that the fabricated triangular nanoplate array has a truncation ratio, defined as the length of truncation along an edge of the triangle over the edge length, of approximately 12.8%. This significantly influences optical properties of the nanostructures, resulting in poorer sensing performance. These insights can be used to guide the design and fabrication of nanostructures for high performance LSPR sensors.

Functional magnetic resonance imaging study of word recognition in normal elders

Four healthy, cognitively intact elders participated. Subjects underwent fMRI scanning while performing a word recognition task with an easy condition (low demand) and a difficult condition titrated to each subjects ability (titrated demand). Relative to low, titrated demand was associated with increased activation of the left medial frontal (cluster level P <.002), right superior temporal (P <.007) and right superior parietal cortices (P<.001). Increased activation of bilateral cortical areas by elders during the more challenging titrated demand compared with low demand may indicate recruitment of additional brain regions, enabling subjects to maintain performance despite increasing difficulty. Alternatively, the bilateral activation on this word recognition task may reflect compensatory use of right hemisphere networks.

Isolation of thermally sensitive aptamers on a microchip

We present a microchip for isolation of aptamers that bind to target ligands at prespecified temperatures. The device uses integrated resistive heaters and sensors to control the temperatures of (poly)dimethylsiloxane (PDMS) microchambers for temperature-specific selection and bead-based amplification of aptamers. Aptamers are isolated from a randomized DNA library at specified temperatures, and amplified onto microbeads using bead-based polymerase chain reaction (PCR). As a proof of concept, the device was used to isolate aptamers for human immunoglobulin E (IgE), with the enriched pool of candidate aptamers exhibiting a much higher and temperature-dependent affinity for the target protein. The procedure was performed with significantly less reagent use in a much shorter time period (4 hours) than with conventional devices (up to 2 days).

A microchip for nucleic acid isolation and enrichment

This paper presents a microchip that effectively isolates and enriches target-binding single-stranded DNA (ssDNA) in a randomized DNA mixture using solid-phase extraction (SPE) and electrophoresis. The microchip consists of isolation and enrichment microchambers that are connected by a microchannel partially filled with agarose gel. Single-stranded DNA oligomers in the randomized mixture are captured via specific binding onto human immunoglobulin E (IgE) that is immobilized on microbeads in the isolation chamber. Following elution, the oligomers are separated from impurities via electrophoretic transport through the gel-filled microchannel to the enrichment chamber. Experimental results show that this approach can enhance the sensitivity of ssDNA detection methods in dilute and complex biological samples.

Integrated Microfluidic Isolation of Aptamers Using Electrophoretic Oligonucleotide Manipulation

We present a microfluidic approach to integrated isolation of DNA aptamers via systematic evolution of ligands by exponential enrichment (SELEX). The approach employs a microbead-based protocol for the processes of affinity selection and amplification of target-binding oligonucleotides, and an electrophoretic DNA manipulation scheme for the coupling of these processes, which are required to occur in different buffers. This achieves the full microfluidic integration of SELEX, thereby enabling highly efficient isolation of aptamers in drastically reduced times and with minimized consumption of biological material. The approach as such also offers broad target applicability by allowing selection of aptamers with respect to targets that are either surface-immobilized or solution-borne, potentially allowing aptamers to be developed as readily available affinity reagents for a wide range of targets. We demonstrate the utility of this approach on two different procedures, respectively for isolating aptamers against a surface-immobilized protein (immunoglobulin E) and a solution-phase small molecule (bisboronic acid in the presence of glucose). In both cases aptamer candidates were isolated in three rounds of SELEX within a total process time of approximately 10 hours.

Pathogen detection using microfluidic bead-based polymerase chain reaction

This paper presents a microfluidic device which uses bead-based polymerase chain reaction (PCR) to amplify and detect genomic DNA of Bordetella Pertussis. PCR is a biochemical amplification process in which template DNA is duplicated by repeated thermal cycling and enzymatic amplification. The device uses an integrated resistive heater and temperature sensor beneath a (poly) dimethylsiloxane microfluidic chamber to control the temperature of a solution containing the template DNA and PCR reagents, including bead-based primers. The PCR reaction results in fluorescently tagged double-stranded DNA immobilized on microbeads. The combination of chemical amplification and fluorescent signal concentration on microbeads allows sensitive detection of template DNA at a concentration of 1 pM in only 10 PCR cycles within 13 minutes.

Electrokinetically integrated microfluidic isolation and amplification of biomolecule-and cell-binding nucleic acids

This paper presents electrokinetically based microfluidic integration of isolation and amplification of target-binding nucleic acids. A microfluidic device is used that consists of two microchambers for nucleic acid isolation and amplification connected by a microchannel filled with agarose gel. In the device, target-binding DNA strands are isolated and amplified on surfaces while gel-based electrophoresis is used to transfer the strands between chambers, eliminating the need for complicated flow control components. Experimental results show that the device has the potential to rapidly isolate and amplify nucleic acids in random mixtures against a variety of biological targets, such as biomolecules and cells, with increased binding affinity.