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M A Tagamets

Publications and source records attributed to M A Tagamets.

10 recordsLinked to original sources
PubMed2001

Interpreting PET and fMRI measures of functional neural activity: the effects of synaptic inhibition on cortical activation in human imaging studies.

Human brain imaging methods such as postiron emission tomography and functional magnetic resonance imaging have recently achieved widespread use in the study of both normal cognitive processes and neurological disorders. While many of these studies have begun to yield important insights into human brain function, the relationship between these measurements and the underlying neuronal activity is still not well understood. One open question is how neuronal inhibition is reflected in these imaging results. In this paper, we describe how large-scale modeling can be used to address this question. Specifically, we identify three factors that may play a role in how inhibition affects imaging results: (1) local connectivity; (2) context; and (3) type of inhibitory connection. Simulation results are presented that show how the interaction among these three factors can explain seemingly contradictory experimental results. The modeling suggests that neuronal inhibition can raise brain imaging measures if there is either low local excitatory recurrence or if the region is not otherwise being driven by excitation. Conversely, with high recurrence or actively driven excitation, inhibition can lower observed values.

Animals↗
PubMed2001

Functional interactions of the inferior frontal cortex during the processing of words and word-like stimuli.

The hypothesis that ventral/anterior left inferior frontal gyrus (LIFG) subserves semantic processing and dorsal/posterior LIFG subserves phonological processing was tested by determining the pattern of functional connectivity of these regions with regions in left occipital and temporal cortex during the processing of words and word-like stimuli. In accordance with the hypothesis, we found strong functional connectivity between activity in ventral LIFG and activity in occipital and temporal cortex only for words, and strong functional connectivity between activity in dorsal LIFG and activity in occipital and temporal cortex for words, pseudowords, and letter strings, but not for false font strings. These results demonstrate a task-dependent functional fractionation of the LIFG in terms of its functional links with posterior brain areas.

Adult↗
PubMed2000

A parametric approach to orthographic processing in the brain: an fMRI study.

Brain activation studies of orthographic stimuli typically start with the premise that different types of orthographic strings (e.g., words, pseudowords) differ from each other in discrete ways, which should be reflected in separate and distinct areas of brain activation. The present study starts from a different premise: Words, pseudowords, letterstrings, and false fonts vary systematically across a continuous dimension of familiarity to English readers. Using a one-back matching task to force encoding of the stimuli, the four types of stimuli were visually presented to healthy adult subjects while fMRI activations were obtained. Data analysis focused on parametric comparisons of fMRI activation sites. We did not find any region that was exclusively activated for real words. Rather, differences among these string types were mainly expressed as graded changes in the balance of activations among the regions. Our results suggest that there is a widespread network of brain regions that form a common network for the processing of all orthographic string types.

Adult↗
PubMed1999

Predicting human functional maps with neural net modeling.

Formidable difficulties exist in interpreting positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) hemodynamic signals in terms of the underlying neural activity. These include issues of spatial and temporal resolution and problems relating neuronal activity (i.e., action potentials) measured in nonhuman studies by single unit electrodes to hemodynamic measurements reflecting synaptic activity. Also, regional hemodynamic measurements correspond to a mixture of local and afferent synaptic activity. To surmount these difficulties, we propose using large-scale neurobiologically realistic models in which data at various spatial and temporal levels can be simulated and cross-validated by multiple disciplines, including functional neuroimaging. A delayed match-to-sample visual task is used to illustrate this approach.

Brain↗
PubMed1998

Integrating electrophysiological and anatomical experimental data to create a large-scale model that simulates a delayed match-to-sample human brain imaging study.

We propose a model that draws together experimental evidence from anatomical, electrophysiological and imaging experiments in order to understand better the neural substrate of human imaging studies using positron electron tomography (PET) and functional magnetic resonance imaging (fMRI). First, we define a simple local circuit that reflects the major role that local connectivity plays in producing PET and fMRI data, which are thought to mainly reflect synaptic activity. Second, in order to account for the role of varying behaviors during the course of a typical imaging experiment, we propose a local circuit that can perform a delayed match-to-sample task. The elements of this circuit behave very much like neurons that have been found in the prefrontal cortex during similar tasks in monkeys. One subpopulation responds selectively only when stimuli are present. Two different populations show the two types of delay-period activity that have been identified, one with high activity both during the cue and the delay period, the other with a rise during the delay period only. Last, a subpopulation shows a brief response only if the second stimulus matches the first, thus mediating the decision about whether the stimuli match. We show that in addition to performing the task, the integrated summed synaptic activities of the model are similar to experimental PET data.

Behavior↗
PubMed1972

Sequence of spontaneous Epstein-Barr virus activation and selective DNA synthesis in activated cells in the presence of hydroxyurea.

The sequence of spontaneous Epstein-Barr virus activation was studied in P3HR-1 carrier cells and in P3HR-1(BrdU) cells made resistant to 5-bromodeoxyuridine. Virus activation was initiated during the normal cell cycle, and recruitment of additional virus-activated cells was prevented by the DNA inhibitors, 1-beta-D-arabinofuranosylcytosine and hydroxyurea. Virus activation was followed by synthesis of the early antigen complex in the absence of additional detectable DNA synthesis. Early antigen synthesis was followed by hydroxyurea-resistant synthesis of new DNA, which in the case of P3HR-1(BrdU) cells was characterized by the appearance of thymidine kinase. The newly synthesized DNA banded in neutral cesium chloride at peaks corresponding to normal human DNA and Epstein-Barr viral DNA. Synthesis of viral antigen was seen only in cells that had undergone hydroxyurea-resistant DNA synthesis.

Adenine↗
PubMedDate not supplied

A model of working memory: bridging the gap between electrophysiology and human brain imaging.

Human neuroimaging methods such as positron emission tomography and functional magnetic resonance imaging have made possible the study of large-scale distributed networks in the behaving human brain. Although many imaging studies support and extend knowledge gained from other experimental modalities such as animal single-cell recordings, there have also been a substantial number of experiments that appear to contradict the animal studies. Part of the reason for this is that neuroimaging is an indirect measure of neuronal firing activity, and thus interpretation is difficult. Computational modeling can help to bridge the gap by providing a substrate for making explicit the assumptions and constraints provided from other sources such as anatomy, physiology and behavior. We describe a large-scale model of working memory that we have used to examine a number of issues relating to the interpretation of imaging data. The gating mechanism that regulates engagement and retention of short-term memory is revised to better reflect hypothesized underlying neuromodulatory mechanisms. It is shown that in addition to imparting better performance for the memory circuit, this mechanism also provides a better match to imaging data from working memory studies.

Brain↗