Biomedical subjects

James M Hyman

Publications and source records attributed to James M Hyman.

7 recordsLinked to original sources

PubMed2006

Identification of case clusters and counties with high infective connectivity in the 2001 epidemic of foot-and-mouth disease in Uruguay.

OBJECTIVE: To evaluate the influence of individual spatial units (ie, counties) on the epidemic spread of foot-and-mouth disease (FMD) virus. SAMPLE POPULATION: 163 counties in Uruguay where there was an outbreak of FMD between April 23 and July 11, 2001. PROCEDURE: A geographically referenced database was created, and the distance between counties (13,203 county pairs), road density of counties (163 counties), and time when cases were reported in those counties (11 weeks of the epidemic) were considered to assess global spatial and spatial-temporal autocorrelation, determine the contribution of links connecting pairs of counties with infected animals, and allow us to hypothesize the influence for spread during the epidemic for counties with greater than the mean infective link contributions. RESULTS: Case clusters were indicated by the Moran Iand Mantel tests during the first 6 weeks of the epidemic. Spatial lags between pairs of counties with infected animals revealed case clustering before and after vaccination was implemented. Temporal lags predicted autocorrelation for up to 3 weeks. Link indices identified counties expected to facilitate epidemic spread. If control measures had been implemented in counties with a high index link (identifiable as early as week 1 of the epidemic), they could have prevented (by week 11 of the epidemic) at least 2.5 times as many cases per square kilometer than the same measures implemented in counties with average link indices. CONCLUSIONS AND CLINICAL RELEVANCE: Analysis of spatial autocorrelation and infective link indices may identify network conditions that facilitate (or prevent) disease spread.

Animals↗

PubMed2005

Medial prefrontal cortex cells show dynamic modulation with the hippocampal theta rhythm dependent on behavior.

Both the hippocampus and the medial prefrontal cortex are essential for successful performance in learning- and memory-related tasks. Within the hippocampus the theta rhythm plays an integral role in the timing of action potentials of hippocampal neurons responding to elements of any given task. Medial prefrontal cortex (mPFC) neurons display firing rate changes to specific facets of behavioral tasks (Jung et al., 1998. Cereb Cortex 8:437--450). We recorded units in the mPFC and field potentials in the hippocampus to determine whether behaviorally correlated mPFC cells fired with phase relationships to the hippocampal theta rhythm. In two different behavioral tasks (running a linear track and foraging in two distinct environments) we found mPFC cells that alternated between theta entrained firing and nonphasic firing depending on the ongoing behavior, while other cells were modulated during all conditions in both tasks. The majority of the mPFC cells with a significant correlation of firing rate changes with behavior were entrained to hippocampal theta. Cells that fired to specific events during only one direction of running were predisposed to theta modulation only in that direction. mPFC neurons have the capability to respond to behaviorally relevant elements by dynamically alternating between hippocampal theta entrained and nonphasic firing.

Action Potentials↗

PubMed2004

Differential susceptibility epidemic models.

We formulate compartmental differential susceptibility (DS) susceptible-infective-removed (SIR) models by dividing the susceptible population into multiple subgroups according to the susceptibility of individuals in each group. We analyze the impact of disease-induced mortality in the situations where the number of contacts per individual is either constant or proportional to the total population. We derive an explicit formula for the reproductive number of infection for each model by investigating the local stability of the infection-free equilibrium. We further prove that the infection-free equilibrium of each model is globally asymptotically stable by qualitative analysis of the dynamics of the model system and by utilizing an appropriately chosen Liapunov function. We show that if the reproductive number is greater than one, then there exists a unique endemic equilibrium for all of the DS models studied in this paper. We prove that the endemic equilibrium is locally asymptotically stable for the models with no disease-induced mortality and the models with contact numbers proportional to the total population. We also provide sufficient conditions for the stability of the endemic equilibrium for other situations. We briefly discuss applications of the DS models to optimal vaccine strategies and the connections between the DS models and predator-prey models with multiple prey populations or host-parasitic interaction models with multiple hosts are also given.

Communicable Diseases↗

PubMed2004

Analysis of theta power in hippocampal EEG during bar pressing and running behavior in rats during distinct behavioral contexts.

These experiments examine changes in theta power as measured by wavelet analysis in five rats performing a conditional visual discrimination task and a simple running task. In the conditional task, rats were trained to press one lever to initiate a trial and then to press one of two choice levers, each corresponding to one of two cue lights. Analysis of theta power in this operant task found a large decrease in theta power during the choice bar presses, in contrast to the increase in theta power during trial initiation bar presses. This result seems to stand counter to results that propose consistent relationships between motor actions and theta power (Vanderwolf, EEG Clin Neurophys 26:407-418, 1969), as well as studies suggesting that the lack of bar-press theta is the result of habituation. However, these data can be seen as being in broad agreement with the theoretical framework of sensorimotor integration (Bland and Oddie, Behav Brain Res 127:119-136, 2001). To investigate further the power of theta observed at the termination of type 1 motor activity, a runway task was devised in which rats ran back and forth between two ends of a linear track, one of which was always rewarded and the other never rewarded. Theta power decreased sharply 240 ms before movement ended at the rewarded end, but not at the unrewarded end of the track. These data extend the current scope of theory in demonstrating that hippocampal theta activity can end abruptly 200-400 ms prior to the end of type 1 motor movement when approaching the end of a motor sequence.

Action Potentials↗

PubMed2004

Model parameters and outbreak control for SARS.

Control of the 2002-2003 severe acute respiratory syndrome (SARS) outbreak was based on rapid diagnosis coupled with effective patient isolation. We used uncertainty and sensitivity analysis of the basic reproductive number R0 to assess the role that model parameters play in outbreak control. The transmission rate and isolation effectiveness have the largest fractional effect on R0. We estimated the distribution of the reproductive number R0 under perfect isolation conditions. The distribution lies in the interquartile range 0.19-1.08, with a median of 0.49. Even though the median of R0 is <1, we found that 25% of our R0 distribution lies at R0 > 1, even with perfect isolation. This implies the need to simultaneously apply more than one method of control.

Communicable Disease Control↗

PubMed2003

Stimulation in hippocampal region CA1 in behaving rats yields long-term potentiation when delivered to the peak of theta and long-term depression when delivered to the trough.

Experimental evidence suggests that the hippocampal theta rhythm plays a critical role in learning. Previous studies have shown long-term potentiation (LTP) to be preferentially induced with stimulation on the peak of local theta rhythm in region CA1 in anesthetized rats and with stimulation of the perforant path at the peak of theta in both anesthetized and behaving animals. We set out to determine the effects of tetanic burst stimulation in stratum radiatum of region CA1 in awake behaving animals, delivered during either the peak or the trough of the theta rhythm in the EEG. Bursts delivered to the peak resulted in an increase of 17.9 +/- 0.94% in potential slope. When identical stimulation bursts were delivered to the trough of local theta waves, the potential slope decreased 12.9 +/- 1.03%. This is the first report of LTP being preferentially induced at the peak of local theta rhythm in behaving animals in region CA1 and that LTD was found in response to tetanic stimulation at the trough of the local theta wave. The results are discussed within the framework of a recent theory that proposes that the theta rhythm sets the dynamics for alternating phases of encoding and retrieval (Hasselmo et al., 20021).

Animals↗

PubMed2003

Modeling the impact of random screening and contact tracing in reducing the spread of HIV.

Mathematical models can help predict the effectiveness of control measures on the spread of HIV and other sexually transmitted diseases (STDs) by reducing the uncertainty in assessing the impact of intervention strategies such as random screening and contact tracing. Even though contact tracing is one of the most effective methods used for controlling treatable STDs, it is still a controversial strategy for controlling HIV because of cost and confidentiality issues. To help estimate the effectiveness of these control measures, we formulate two models with random screening and contact tracing based on the differential infectivity (DI) model and the staged-progression (SP) model. We derive formulas for the reproductive numbers and the endemic equilibria and compare the impact that random screening and contact tracing have in slowing the epidemic in the two models. In the DI model the infected population is divided into groups according to their infectiousness, and HIV is largely spread by a small, highly infectious, group of superspreaders. In this model contact tracing is an effective approach to identifying the superspreaders and has a large effect in slowing the epidemic. In the SP model every infected individual goes through a series of infection stages and the virus is primarily spread by individuals in an initial highly infectious stage or in the late stages of the disease. In this model random screening is more effective than for the DI model, and contact tracing is less effective. Thus the effectiveness of the intervention strategy strongly depends on the underlying etiology of the disease transmission.

Contact Tracing↗