Biomedical subjects

George R Mangun

Publications and source records attributed to George R Mangun.

5 recordsLinked to original sources

PubMed2007

Evaluation of PCA and ICA of simulated ERPs: Promax vs. Infomax rotations.

Independent components analysis (ICA) and principal components analysis (PCA) are methods used to analyze event-related potential (ERP) and functional imaging (fMRI) data. In the present study, ICA and PCA were directly compared by applying them to simulated ERP datasets. Specifically, PCA was used to generate a subspace of the dataset followed by the application of PCA Promax or ICA Infomax rotations. The simulated datasets were composed of real background EEG activity plus two ERP simulated components. The results suggest that Promax is most effective for temporal analysis, whereas Infomax is most effective for spatial analysis. Failed analyses were examined and used to devise potential diagnostic strategies for both rotations. Finally, the results also showed that decomposition of subject averages yield better results than of grand averages across subjects.

Algorithms↗

PubMed2006

Pre-target activity in visual cortex predicts behavioral performance on spatial and feature attention tasks.

Physiological studies in humans and monkeys have revealed that, in response to an instruction to attend, areas of sensory cortex that code the attributes of the expected stimulus exhibit increases in neural activity prior to the arrival of the stimulus. Models of selective visual attention posit that these increases in activity give attended stimuli a processing advantage over distracting stimuli. Here, we test two key predictions of this view by using functional magnetic resonance imaging to record human brain activity during a cued voluntary orienting task. First, we tested whether pre-stimulus modulations are observed during both cued spatial and cued feature attention. Secondly, we tested whether the magnitude of pre-stimulus modulations predicts behavioral performance. Our results indicate that cue-triggered expectation of targets with particular spatial or nonspatial features activates areas of the visual cortex selective for these features. Furthermore, the magnitude of the cue-triggered modulations correlated with behavioral measures, such that those subjects who exhibited relatively large pre-stimulus modulations of activity performed better on the behavioral task. These findings support the view that top-down control systems bias activity in sensory cortices to favor the processing of expected target features and that this bias is related to behavior.

Action Potentials↗

PubMed2005

Control networks and hemispheric asymmetries in parietal cortex during attentional orienting in different spatial reference frames.

Neuropsychological research has consistently demonstrated that spatial attention can be anchored in one of several coordinate systems, including those defined with respect to an observer (viewer-centered), to the gravitational vector (environment-centered), or to individual objects (object-centered). In the present study, we used hemodynamic correlates of brain function to investigate the neural systems that mediate attentional control in two competing reference frames. Healthy volunteers were cued to locations defined in either viewer-centered or object-centered space to discriminate the shape of visual targets subsequently presented at the cued locations. Brain responses to attention-directing cues were quantified using event-related functional magnetic resonance imaging. A fronto-parietal control network was activated by attention-directing cues in both reference frames. Voluntary shifts of attention produced increased neural activity bilaterally in several cortical regions including the intraparietal sulcus, anterior cingulate cortex, and the frontal eye fields. Of special interest was the observation of hemispheric asymmetries in parietal cortex; there was significantly greater activity in left parietal cortex than in the right, but this asymmetry was more pronounced for object-centered shifts of attention, relative to viewer-centered shifts of attention. Measures of behavioral performance did not differ significantly between the two reference frames. We conclude that a largely overlapping, bilateral, cortical network mediates our ability to orient spatial attention in multiple coordinate systems, and that the left intraparietal sulcus plays an additional role for orienting in object-centered space. These results provide neuroimaging support for related claims based on findings of deficits in object-based orienting in patients with left parietal lesions.

Adolescent↗

PubMed2002

Selective attention to spatial frequency: an ERP and source localization analysis.

OBJECTIVES: Physiological correlates of visual selective attention have been observed by recording ERPs to attended versus ignored target stimuli. Over many such studies, spatial attention has been observed to modulate early sensory components beginning 70 ms after stimulus onset, while effects of selection based on other stimulus features such as color and spatial frequency occur at longer latencies. Together, these findings argue for a primacy of location in early attentional selection. However, there have been some reports suggesting attention effects on short latency sensory-evoked potentials during selection of spatial frequency. The prime objective of the present study was to assess whether or not spatial frequency-dependent potentials are modulated by attention at a latency as early as 70-100 ms. METHODS: Checkerboard patterns were flashed to the subject, one being the target requiring a response. We investigated attentional effects using high-density scalp mapping and inverse dipole modeling. RESULTS: The earliest robust signs of selective attention to spatial frequencies consisted of an occipital selection negativity (OSN) and a frontal selection positivity (FSP). The OSN started at a latency of 140 ms, the FSP somewhat earlier at 120 ms. These attention effects were readily modeled by sources in cortical areas ventrally and laterally to the more primary areas generating the shorter-latency sensory components. CONCLUSIONS: This pattern of results has been found for non-spatial stimulus features in several studies, and is clearly different from the ERP correlates of spatial selection.

Adolescent↗

PubMed2002

Enhanced spatial localization of neuronal activation using simultaneous apparent-diffusion-coefficient and blood-oxygenation functional magnetic resonance imaging.

Functional MRI (fMRI) can detect blood oxygenation level dependent (BOLD) hemodynamic responses secondary to local neuronal activity. The most commonly used method for detecting fMRI signals is the gradient-echo echo-planar imaging (EPI) technique because of its sensitivity and speed. However, it is known that much of the signal obtained with this approach arises from large veins, with additional contribution from the capillaries and venules. Early experiments using diffusion-weighted gradient-echo EPI have suggested that intravoxel incoherent motion (IVIM) weighting can selectively attenuate contributions from large vessels based on the differences in the mobility of the blood within them, thereby revealing the contributions from hemodynamic changes in capillaries, which are in close spatial proximity to the activated neural tissue. Using this differential sensitivity of the various neurovascular compartments to IVIM weighting, we present a new approach for deriving functional maps of neural activity. This method is based on task-induced changes of the apparent diffusion coefficients (ADC), a signal that we demonstrate is generated in vascular compartments that only partially overlap with those generating the BOLD signal. The approach allows both the ADC-based maps and the more commonly used BOLD-based maps to be acquired simultaneously. The spatial overlap between these maps can be used to create composite maps that permit improved localization of the underlying neuronal activity patterns by identifying signals generated in those vascular components that are in closest proximity to the active neuronal populations of interest.

Adult↗