Biomedical subjects

Eric Halgren

Publications and source records attributed to Eric Halgren.

24 records · Page 2Linked to original sources

PubMed2004

Laminar analysis of human neocortical interictal spike generation and propagation: current source density and multiunit analysis in vivo.

Multicontact microelectrodes were chronically implanted in epilepsy patients undergoing subdural grid implantation for seizure localization. Current source density and multiple unit activity of interictal spikes (IISs) were sampled every approximately 150 microm in a line traversing all layers of a cortical column. Our data suggest that interictal epileptiform events in humans are initiated by large postsynaptic depolarizations, consistent with the hypothesis that human IISs correspond to animal paroxysmal depolarization shifts. Furthermore, the cortical layer where the initial depolarization occurs may differ according to whether the IIS is locally generated or propagated from a distant location, and among the propagated IISs, whether the IIS is in the direct path of propagation or on the periphery of that path.

Action Potentials↗

PubMed2003

Spatiotemporal dynamics of modality-specific and supramodal word processing.

The ability of written and spoken words to access the same semantic meaning provides a test case for the multimodal convergence of information from sensory to associative areas. Using anatomically constrained magnetoencephalography (aMEG), the present study investigated the stages of word comprehension in real time in the auditory and visual modalities, as subjects participated in a semantic judgment task. Activity spread from the primary sensory areas along the respective ventral processing streams and converged in anterior temporal and inferior prefrontal regions, primarily on the left at around 400 ms. Comparison of response patterns during repetition priming between the two modalities suggest that they are initiated by modality-specific memory systems, but that they are eventually elaborated mainly in supramodal areas.

Adult↗

PubMed2003

Reduction in distractibility with AF102B and THA in the macaque.

Distractibility in primates may be influenced by central cholinergic systems. Two cholinomimetics, the m-1 muscarinic agonist (+/-)-cis-2-methyl-spiro(1,3-oxathiolane-5,3')quinuclidine (AF102B, civemeline) and the cholinesterase inhibitor tetrahydroaminoacridine (THA, tacrine), were compared to vehicle controls for effects on distractibility in an automated visuospatial attention task. The task required visual pursuit of a moving target amongst distractor stimuli that acted to impair performance and was executed by seven healthy adult bonnet macaque monkeys. Task accuracy and reaction time were measured 1.5 h after systemic administration of each substance. For the seven-subject group at individually titrated best doses, accuracy increased significantly relative to vehicle for both drugs. Reaction time at best dose decreased for both drugs, but not significantly. Muscarinic agonists and cholinesterase inhibitors may reduce distractibility in primates.

Animals↗

PubMed2003

Cortical activation to illusory shapes as measured with magnetoencephalography.

Spatiotemporal patterns of cortical activation during the perceptual grouping of elements to form illusory shapes were estimated using anatomically constrained magnetoencephalography. Subjects were shown an array of Kanizsa-style figures which were either aligned to form illusory squares or misaligned so that no illusory contour or shape was perceived. Differential activity is more pronounced in the right hemisphere. After a weakly significant modulation at approximately 110 ms in the occipital pole, a prominent peak appears at approximately 155 ms in the lateral occipital cortex. Modulation then appears to spread back from this location toward the occipital pole, as well as ventrally to involve ventral occipital and temporal cortices for the next 180 ms, eventually involving ventral orbitofrontal cortex at 325 ms. The prominent lateral occipital response is consistent with fMRI studies with similar stimuli which found activation in that region as well as in V3A, V4v, V7, and V8. Furthermore, the timing of this activation, after the occipital pole but before ventral temporal, is consistent with a putative role for this region in midlevel vision. The late ventral temporal response (235 ms) is centered in the lingual and fusiform areas implicated in object identification. The V1/V2 modulation at this time may reflect top-down modulation by lateral occipitotemporal and ventral temporal areas.

Adult↗

PubMed2003

Spatiotemporal maps of past-tense verb inflection.

Does the brain inflect verbs by applying rules, by associative retrieval of the inflected form, or both? We used whole-head magnetoencephalography to spatiotemporally map the brain response underlying verb past-tense inflection. Placing either regular or irregular verbs into the past tense sequentially modulates the bilateral visual, left inferotemporal, posterior superior temporal (Wernicke's area), left inferior prefrontal (Broca's area), and right prefrontal cortices. Although irregular and regular verb inflection evokes similar cortical response patterns, differences in specific frontotemporal regions are observed. At approximately 340 ms, irregular verbs evoke greater response modulation in left occipitotemporal cortex. This modulation occurs when widespread areas are simultaneously active, suggesting that it reflects associative activation necessary for generation of past-tense forms. Subsequently, regular verbs show increased response at approximately 470 ms within left inferior prefrontal regions associated with rule-based inflection. Increased right dorsolateral prefrontal response at approximately 570 ms may represent directed/effortful retrieval of irregular past-tense forms. Thus, the brain inflects verbs by dynamically modulating different functional divisions of an integrated language system.

Adult↗

PubMed2002

N400-like magnetoencephalography responses modulated by semantic context, word frequency, and lexical class in sentences.

Words have been found to elicit a negative potential at the scalp peaking at approximately 400 ms that is strongly modulated by semantic context. The current study used whole-head magnetoencephalography (MEG) as male subjects read sentences ending with semantically congruous or incongruous words. Compared with congruous words, sentence-terminal incongruous words consistently evoked a large magnetic field over the left hemisphere, peaking at approximately 450 ms. Source modeling at this latency with conventional equivalent current dipoles (ECDs) placed the N400 m generator in or near the left superior temporal sulcus. A distributed solution constrained to the cortical surface suggested a sequence of differential activation, beginning in Wernicke's area at approximately 250 ms, spreading to anterior temporal sites at approximately 270 ms, to Broca's area by approximately 300 ms, to dorsolateral prefrontal cortices by approximately 320 ms, and to anterior orbital and frontopolar cortices by approximately 370 ms. Differential activity was exclusively left-sided until >370 ms, and then involved right anterior temporal and orbital cortices. At the peak of the N400 m, activation in the left hemisphere was estimated to be widespread in the anterior temporal, perisylvian, orbital, frontopolar, and dorsolateral prefrontal cortices. In the right hemisphere, the orbital, as well as, weakly, the right anterior temporal cortices were activated. Similar but weaker field patterns were evoked by intermediate words in the sentences, especially to low-frequency words occurring in early sentence positions where there is little preceding context. The locations of the N400 m sources identified with the distributed solution correspond well with those previously demonstrated with direct intracranial recordings, and suggested by functional magnetic resonance imaging (fMRI). These results help identify a distributed cortical network that supports online semantic processing.

Adult↗