Biomedical subjects

K M Thomas

Publications and source records attributed to K M Thomas.

At least 19 recordsLinked to original sources

PubMed2005

Hydrogen adsorption on functionalized nanoporous activated carbons.

There is considerable interest in hydrogen adsorption on carbon nanotubes and porous carbons as a method of storage for transport and related energy applications. This investigation has involved a systematic investigation of the role of functional groups and porous structure characteristics in determining the hydrogen adsorption characteristics of porous carbons. Suites of carbons were prepared with a wide range of nitrogen and oxygen contents and types of functional groups to investigate their effect on hydrogen adsorption. The porous structures of the carbons were characterized by nitrogen (77 K) and carbon dioxide (273 K) adsorption methods. Hydrogen adsorption isotherms were studied at 77 K and pressure up to 100 kPa. All the isotherms were Type I in the IUPAC classification scheme. Hydrogen isobars indicated that the adsorption of hydrogen is very temperature dependent with little or no hydrogen adsorption above 195 K. The isosteric enthalpies of adsorption at zero surface coverage were obtained using a virial equation, while the values at various surface coverages were obtained from the van't Hoff isochore. The values were in the range 3.9-5.2 kJ mol(-1) for the carbons studied. The thermodynamics of the adsorption process are discussed in relation to temperature limitations for hydrogen storage applications. The maximum amounts of hydrogen adsorbed correlated with the micropore volume obtained from extrapolation of the Dubinin-Radushkevich equation for carbon dioxide adsorption. Functional groups have a small detrimental effect on hydrogen adsorption, and this is related to decreased adsorbate-adsorbent and increased adsorbate-adsorbate interactions.

Journal Article↗

PubMed2005

Adsorption of aqueous metal ions on oxygen and nitrogen functionalized nanoporous activated carbons.

In this study, the adsorption characteristics of two series of oxygen and nitrogen functionalized activated carbons were investigated. These series were a low nitrogen content (approximately 1 wt % daf) carbon series derived from coconut shell and a high nitrogen content (approximately 8 wt % daf) carbon series derived from polyacrylonitrile. In both series, the oxygen contents were varied over the range approximately 2-22 wt % daf. The porous structures of the functionalized activated carbons were characterized using N(2) (77 K) and CO(2) (273 K) adsorption. Only minor changes in the porous structure were observed in both series. This allowed the effect of changes in functional group concentrations on metal ion adsorption to be studied without major influences due to differences in porous structure characteristics. The surface group characteristics were examined by Fourier transform infrared (FTIR) spectroscopy, acid/base titrations, and measurement of the point of zero charge (pH(PZC)). The adsorption of aqueous metal ion species, M(2+)(aq), on acidic oxygen functional group sites mainly involves an ion exchange mechanism. The ratios of protons displaced to the amount of M(2+)(aq) metal species adsorbed have a linear relationship for the carbons with pH(PZC) < or = 4.15. Hydrolysis of metal species in solution may affect the adsorption of metal ion species and displacement of protons. In the case of basic carbons, both protons and metal ions are adsorbed on the carbons. The complex nature of competitive adsorption between the proton and metal ion species and the amphoteric character of carbon surfaces are discussed in relation to the mechanism of adsorption.

Journal Article↗

PubMed2005

Reactions of nitrogen and oxygen surface groups in nanoporous carbons under inert and reducing atmospheres.

The reactions of surface functional groups have an important role in controlling conversion of char nitrogen to NOx during coal combustion. This study involved an investigation of the thermal stability and reactions of nitrogen surface functional groups in nanoporous carbons. Four suites of carbons, which were used as models for coal chars, were prepared with a wide range of nitrogen and oxygen contents and types of functional groups. The porous structures of the carbons were characterized by gas adsorption methods while chemical analysis, X-ray photoelectron spectroscopy, and X-ray near edge structure spectroscopy were used to characterize the surface functional groups. Temperature programmed desorption and temperature programmed reduction methods were used to study the reactivity of the surface functional groups during heat treatment under inert and reducing conditions. Heat treatment studies show that the order of stability of the functional groups is quaternary nitrogen > pyridinic > pyrrolic > pyridine N-oxide. Pyridine N-oxide surface groups desorb NO and form N2 via surface reactions at low temperature. Pyrrolic and pyridinic functional groups decompose and react with surface species to give NH3, HCN, and N2 as desorption products, but most pyrrolic groups are preferentially converted to pyridinic and quaternary nitrogen. The main desorption product is N2. Approximately 15-40 wt % of the original nitrogen was retained in the carbons mainly as quaternary nitrogen after heat treatment to 1673 K. The results are discussed in terms of decomposition ranges for surface functional groups and reaction mechanisms of surface species.

Journal Article↗

PubMed2004

Competitive adsorption of aqueous metal ions on an oxidized nanoporous activated carbon.

Competitive adsorption is the usual situation in real applications, and it is of critical importance in determining the overall performance of an adsorbent. In this study, the competitive adsorption characteristics of all the combinations of binary mixtures of aqueous metal ion species Ca2+(aq), Cd2+(aq), Pb2+(aq), and Hg2+(aq) on a functionalized activated carbon were investigated. The porous structure of the functionalized active carbon was characterized using N2 (77 K) and CO2 (273 K) adsorption. The surface group characteristics were examined by temperature-programmed desorption, Fourier transform infrared spectroscopy, Raman spectroscopy, acid/base titrations, and measurement of the point of zero charge (pHpzc). The adsorption of aqueous metal ion species, M2+(aq), on acidic oxygen functional group sites mainly involves an ion exchange mechanism. The ratios of protons displaced to the amount of M2+(aq) metal species adsorbed have a linear relationship for both single-ion and binary mixtures of these species. Hydrolysis of metal species in solution may affect the adsorption, and this is the case for adsorption of Hg2+(aq) and Pb2+(aq). Competitive adsorption decreases the amounts of individual metal ions adsorbed, but the maximum amounts adsorbed still follow the order Hg2+(aq) > Pb2+(aq) > Cd2+(aq) > Ca2+(aq) obtained for single metal ion adsorption. The adsorption isotherms for single metal ion species were used to develop a model for competitive adsorption in binary mixtures, involving exchange of ions in solution with surface proton sites and adsorbed metal ions, with the species having different accessibilities to the porous structure. The model was validated against the experimental data.

Journal Article↗

PubMed2003

Imaging the developing brain with fMRI.

Advancements in magnetic imaging techniques have revolutionized our ability to study the developing human brain in vivo. The ability to noninvasively image both anatomy and function in healthy volunteers, including young children, has already enhanced our understanding of brain and behavior relations. The application of these techniques to developmental research offers the opportunity to further explore these relationships and allows us to ask questions about where, when and how cognitive abilities develop in relation to changes in underlying brain systems. It is also possible to explore the contributions of maturation versus learning in the development of these abilities through cross-sectional and longitudinal research involving training and intervention procedures. Current imaging methodologies, in conjunction with new and rapidly evolving techniques, hold the promise of even greater insights into developmental issues in the near future. These methodologies and their application to development and learning are discussed in the current paper.

Adult↗

PubMed2003

Parametric manipulation of conflict and response competition using rapid mixed-trial event-related fMRI.

In the current study we examined the influence of preceding context on attentional conflict and response competition using a flanker paradigm. Nine healthy right-handed adults participated in a rapid mixed trial event-related functional magnetic resonance imaging (fMRI) study, in which increasing numbers of either compatible or incompatible trials preceded an incompatible trial. Behaviorally, reaction times on incompatible trials increased as a function of the number of preceding compatible trials. Several brain regions showed monotonic changes to the preceding context manipulation. The most common pattern was observed in anterior cingulate, dorsolateral prefrontal, and superior parietal regions. These areas showed an increase in activity for incompatible trials as the number of preceding compatible trials increased and a decrease in activity for incompatible trials as the number of preceding incompatible trials increased. Post hoc analysis showed that while the MR signal in the anterior cingulate and dorsolateral prefrontal regions peaked before the superior parietal region, the dorsolateral prefrontal MR signal peaked early and remained at this level. These findings are consistent with the conflict monitoring theory that postulates that the anterior cingulate cortex detects or monitors conflict, while PFC is involved in control adjustments that may then lead to modulation of superior parietal cortex in top-down biasing of attention.

Adult↗

PubMed2002

The effect of preceding context on inhibition: an event-related fMRI study.

In this study we combined event-related fMRI with a parametric manipulation of the go nogo paradigm to examine the effect of preceding context on inhibitory processes. Nogo trials were preceded by either 1, 3, or 5 go trials and then compared to one another. Two distinct patterns of activation were associated with behavioral inhibition: First, the ventral prefrontal cortex, cingulate gyrus, and superior parietal regions showed a context effect with an increase in MR signal to nogo trials with increasing number of preceding go trials. Second, anterior regions in the supplementary and premotor cortex showed an increase in MR signal on the nogo condition after 5 preceding go trials, but not after only 1 or 3. A model using the BOLD response in our data was used to verify that the effect of context was not an artifact of the randomization scheme used in the design.

Adult↗

PubMed2001

Adsorption dynamics of gases and vapors on the nanoporous metal organic framework material Ni2(4,4'-bipyridine)3(NO3)4: guest modification of host sorption behavior.

This study combines measurements of the thermodynamics and kinetics of guest sorption with powder X-ray diffraction measurements of the nanoporous metal organic framework adsorbent (host) at different adsorptive (guest) loadings. The adsorption characteristics of nitrogen, argon, carbon dioxide, nitrous oxide and ethanol and methanol vapors on Ni2(4,4'-bipyridine)3(NO3)4 were studied over a range of temperatures as a function of pressure. Isotherm steps were observed for both carbon dioxide and nitrous oxide adsorption at approximately 10-20% of the total pore volume and at approximately 70% of total pore volume for methanol adsorption. The adsorption kinetics obey a linear driving force (LDF) mass transfer model for adsorption at low surface coverage. At high surface coverage, both methanol and ethanol adsorption follow a combined barrier resistance/diffusion model. The rates of adsorption in the region of both the carbon dioxide and methanol isotherm steps were significantly slower than those observed either before or after the step. X-ray diffraction studies at various methanol loadings showed that the host structure disordered initially but underwent a structural change in the region of the isotherm step. These isotherm steps are ascribed to discrete structural changes in the host adsorbent that are induced by adsorption on different sites. Isotherm steps were not observed for ethanol adsorption, which followed a Langmuir isotherm. Previous X-ray crystallography studies have shown that all the sites are equivalent for ethanol adsorption on Ni2(4,4'-bipyridine)3(NO3)4, with the host structure undergoing a scissoring motion and the space group remaining unchanged during adsorption. The activation energies and preexponential factors for methanol and ethanol adsorption were calculated for each pressure increment at which the linear driving force model was obeyed. There was a good correlation between activation energy and ln(preexponential factor), indicating a compensation effect. The results are discussed in terms of reversible adsorbate/adsorbent (guest/host) structural changes and interactions and the adsorption mechanism. The paper contains the first evidence of specific interactions between guests and functional groups leading to structural change in flexible porous coordination polymer frameworks.

Journal Article↗

PubMed2001

Amygdala response to facial expressions in children and adults.

BACKGROUND: The amygdala plays a central role in the human response to affective or emotionally charged stimuli, particularly fear-producing stimuli. We examined the specificity of the amygdala response to facial expressions in adults and children. METHODS: Six adults and 12 children were scanned in a 1.5-T scanner during passive viewing of fearful and neutral faces using an EPI BOLD sequence. All scans were registered to a reference brain, and analyses of variance were conducted on the pooled data to examine interactions with age and gender. RESULTS: Overall, we observed predominantly left amygdala and substantia innominata activity during the presentation of nonmasked fearful faces relative to fixation, and a decrease in activation in these regions with repeated exposure to the faces. Adults showed increased left amygdala activity for fearful faces relative to neutral faces. This pattern was not observed in the children who showed greater amygdala activity with neutral faces than with fearful faces. For the children, there was an interaction of gender and condition whereby boys but not girls showed less activity with repeated exposure to the fearful faces. CONCLUSIONS: This is the first study to examine developmental differences in the amygdala response to facial expressions using functional magnetic resonance imaging.

Adult↗

PubMed2001

Amygdala response to fearful faces in anxious and depressed children.

BACKGROUND: Alterations in amygdala function have been implicated in the pathophysiological characteristics of adult anxiety and depressive disorders. Studies with healthy adults and children, as well as with adults who have amygdala lesions, have found facial expressions of emotion to be useful probes of amygdala activity. Our study examined the amygdala response to fearful and neutral facial expressions in healthy, anxious, and depressed children. We hypothesized that children with anxiety and depression may show atypical amygdala responses to emotional stimuli. METHODS: Twelve children (8-16 years of age) with generalized anxiety or panic disorder and 12 healthy comparison children underwent noninvasive functional magnetic resonance imaging while viewing photographs of fearful and neutral facial expressions. In a second comparison, 5 girls with major depressive disorder were compared with 5 anxious and 5 healthy girls from the previous sample. RESULTS: Children with anxiety disorders showed an exaggerated amygdala response to fearful faces compared with healthy children, whereas depressed children showed a blunted amygdala response to these faces. In addition, the magnitude of the amygdala's signal change between fearful and neutral faces was positively correlated with the severity of everyday anxiety symptoms. CONCLUSIONS: Our results suggest that amygdala function is affected in both anxiety and depression during childhood and adolescence. Moreover, this disruption appears to be specific to the child's own rating of everyday anxiety.

Adolescent↗

PubMed2001

Sensitivity of prefrontal cortex to changes in target probability: a functional MRI study.

Electrophysiological studies suggest sensitivity of the prefrontal cortex to changes in the probability of an event. The purpose of this study was to determine if subregions of the prefrontal cortex respond differentially to changes in target probabilities using functional magnetic resonance imaging (fMRI). Ten right-handed adults were scanned using a gradient-echo, echo planar imaging sequence during performance of an oddball paradigm. Subjects were instructed to respond to any letter but "X". The frequency of targets (i.e., any letter but X) varied across trials. The results showed that dorsal prefrontal regions were active during infrequent events and ventral prefrontal regions were active during frequent events. Further, we observed an inverse relation between the dorsal and ventral prefrontal regions such that when activity in dorsal prefrontal regions increased, activity in ventral prefrontal regions decreased, and vice versa. This finding may index competing cognitive processes or capacity limitations. Most importantly, these findings taken as a whole suggest that any simple theory of prefrontal cortex function must take into account the sensitivity of this region to changes in target probability.

Adolescent↗

PubMed2001

Serial reaction time learning in preschool- and school-age children.

Visuomotor sequence learning was assessed in 4- to 10-year-old children using a serial reaction time (SRT) task with both random and sequenced trials. One-half of the children received exposure to the sequence prior to performing the reaction time (RT) task. In Experiment 1, 7- and 10-year-old children demonstrated sequence-specific decreases in RT. As in the adult SRT literature, participants with explicit awareness of the sequence at the end of the session showed larger sequence-specific reaction time decrements than those without explicit awareness. Contrary to expectation, preexposure to the sequence did not reliably predict the level of awareness attained. Results from Experiment 2 indicate that 4-year-olds also demonstrate significant sequence learning on a variant of the SRT task. This article provides preliminary data regarding developmental changes in sequential learning and the development and use of implicit and explicit knowledge. Age-related differences emerged primarily in explicit rather than implicit knowledge.

Age Factors↗

PubMed2001

Is all sexual harassment viewed the same? Mock juror decisions in same- and cross-gender cases.

Given recent court decisions, there is a need to investigate less common forms of sexual harassment, including women harassing men and same-gender harassment. The present study was a 2 (harasser gender) x 2 (target gender) x 2 (participant gender) factorial design in which 408 mock jurors made decisions in a hostile work environment case. Women harassing men were more likely to be found guilty than were men harassing women, and harassers in same-gender cases were more likely to be found guilty and were perceived more negatively than harassers in cross-gender cases. Participant gender differences were found in cross-gender, but not same-gender, conditions. Results suggest that the gender composition of the harasser and target may be an extralegal factor influencing managerial and juror decision making.

Adult↗

PubMed2000

Dissociation of response conflict, attentional selection, and expectancy with functional magnetic resonance imaging.

Two different attentional networks have been associated with visuospatial attention and conflict resolution. In most situations either one of the two networks is active or both are increased in activity together. By using functional magnetic resonance imaging and a flanker task, we show conditions in which one network (anterior attention system) is increased in activity whereas the other (visuospatial attention system) is reduced, showing that attentional conflict and selection are separate aspects of attention. Further, we distinguish between neural systems involved in different forms of conflict. Specifically, we dissociate patterns of activity in the basal ganglia and insula cortex during simple violations in expectancies (i.e., sudden changes in the frequency of an event) from patterns of activity in the anterior attention system specifically correlated with response conflict as evidenced by longer response latencies and more errors. These data provide a systems-level approach in understanding integrated attentional networks.

Adult↗

PubMed2000

A pilot study of amygdala volumes in pediatric generalized anxiety disorder.

BACKGROUND: The neurodevelopment of childhood anxiety disorders is not well understood. Basic research has implicated the amygdala and circuits related to these nuclei as being central to several aspects of fear and fear-related behaviors in animals. METHODS: Magnetic resonance imaging was used to measure amygdala volumes and comparison brain regions in 12 child and adolescent subjects with generalized anxiety disorder and 24 comparison subjects. Groups were matched on age, sex, height, and handedness and were also similar on measures of weight, socioeconomic status, and full scale IQ. RESULTS: Right and total amygdala volumes were significantly larger in generalized anxiety disorder subjects. Intracranial, cerebral, cerebral gray and white matter, temporal lobe, hippocampal, and basal ganglia volumes and measures of the midsagittal area of the corpus callosum did not differ between groups. CONCLUSIONS: Although these data are preliminary and from a small sample, the results are consistent with a line of thinking that alterations in the structure and function of the amygdala may be associated with pediatric generalized anxiety disorder.

Adolescent↗

PubMed2000

Structural and functional brain development and its relation to cognitive development.

Despite significant gains in the fields of pediatric neuroimaging and developmental neurobiology, surprisingly little is known about the developing human brain or the neural bases of cognitive development. This paper addresses MRI studies of structural and functional changes in the developing human brain and their relation to changes in cognitive processes over the first few decades of human life. Based on post-mortem and pediatric neuroimaging studies published to date, the prefrontal cortex appears to be one of the last brain regions to mature. Given the prolonged physiological development and organization of the prefrontal cortex during childhood, tasks believed to involve this region are ideal for investigating the neural bases of cognitive development. A number of normative pediatric fMRI studies examining prefrontal cortical activity in children during memory and attention tasks are reported. These studies, while largely limited to the domain of prefrontal functioning and its development, lend support for continued development of attention and memory both behaviorally and physiologically throughout childhood and adolescence. Specifically, the magnitude of activity observed in these studies was greater and more diffuse in children relative to adults. These findings are consistent with the view that increasing cognitive capacity during childhood may coincide with a gradual loss rather than formation of new synapses and presumably a strengthening of remaining synaptic connections. It is clear that innovative methods like fMRI together with MRI-based morphometry and nonhuman primate studies will transform our current understanding of human brain development and its relation to behavioral development.

Adolescent↗

PubMed2000

Functional neuroanatomy of spatial working memory in children.

Functional magnetic resonance imaging (fMRI) was used to examine spatial working memory in 8- to 11-year-old children tested under three conditions. In the visual condition, children were asked to examine the location of a dot on a screen. In the motor condition, children were instructed to push a button that corresponded to the location of a dot presented on a screen. In the memory condition, children were asked to remember the location of a dot presented 1 or 2 trials previously. Subtracting the activation of the motor condition from the memory condition revealed activity in the dorsal aspects of the prefrontal cortex and in the posterior parietal and anterior cingulate cortex. These findings were also obtained in the analysis of the memory minus visual conditions except that motor cortex activation was also observed. These findings parallel those reported in comparable studies of adults and suggest that fMRI may be a useful means of examining function-structure relations in developmental populations.

Adult↗

PubMed2000

Neurophysiologic evaluation of auditory recognition memory in healthy newborn infants and infants of diabetic mothers.

OBJECTIVES: We evaluated the integrity of neural pathways for auditory recognition memory in normal newborn infants (n = 32) and infants of diabetic mothers (IDMs, n = 25). IDMs are at risk for fetal metabolic abnormalities that potentially damage recognition memory pathways. We hypothesized that newborn IDMs would have recognition memory deficits that would be correlated with later cognitive development. STUDY DESIGN: Recognition memory was assessed with event-related potentials (ERPs). Neonatal ERPs elicited by the maternal voice were compared with those elicited by a stranger's voice. The Bayley Scales of Infant Development were administered at 1 year of age. RESULTS: Infants in both the control and IDM groups demonstrated recognition of the maternal voice, but their ERP patterns differed. Both groups demonstrated increased amplitude and latency for the "P2" peak elicited by the maternal voice compared with the stranger's voice. In the control group the stranger's voice also elicited a negative slow wave, which was attenuated in the IDMs. The negative slow wave correlated significantly with the 1-year Mental Developmental Index. CONCLUSIONS: The presence of a specific neonatal ERP pattern indicated better 1-year cognitive development in infants in the control and IDM groups. ERPs from IDMs demonstrated subtle evidence of recognition memory impairments.

Analysis of Variance↗