Increasing evidence suggests that the neural processes associated with identifying everyday stimuli include the classification of those stimuli into a limited number of semantic categories. How the neural representations of these stimuli are organized in the temporal lobe remains under debate. Here we used functional magnetic resonance imaging (fMRI) to identify correlates for three current hypotheses concerning object representations in the inferior temporal (IT) cortex of monkeys and humans: representations based on animacy, semantic categories, or visual features.
Selective visual attention can strongly influence perceptual processing, even for apparently low-level visual stimuli. Although it is largely accepted that attention modulates neural activity in extrastriate visual cortex, the extent to which attention operates in the first cortical stage, striate visual cortex (area V1), remains controversial. Here, functional MRI was used at high field strength (3 T) to study humans during attentionally demanding visual discriminations. Similar, robust attentional modulations were observed in both striate and extrastriate cortical areas.
Retinotopic organization is a ubiquitous property of lower-tier visual cortical areas in human and nonhuman primates. In macaque visual cortex, the retinotopic maps extend to higher-order areas in the ventral visual pathway, including area TEO in the inferior temporal (IT) cortex. Distinct regions within IT cortex are also selective to specific object categories such as faces. Here we tested the topographic relationship between retinotopic maps and face-selective patches in macaque visual cortex using high-resolution fMRI and retinotopic face stimuli.
Edges are important in the interpretation of the retinal image. Although luminance edges have been studied extensively, much less is known about how or where the primate visual system detects boundaries defined by differences in surface properties such as texture, motion or binocular disparity. Here we use functional magnetic resonance imaging (fMRI) to localize human visual cortical activity related to the processing of one such higher-order edge type: motion boundaries. We describe a robust fMRI signal that is selective for motion segmentation.
Magnetic resonance imaging (MRI) reconstruction techniques are often validated with signal-to-noise ratio (SNR), contrast-to-noise ratio, and mean-to-standard-deviation ratio measured on example images. We present human and model observers as a novel approach to evaluating reconstructions for low-SNR magnetic resonance (MR) images. We measured human and channelized Hotelling observers in a two-alternative forced-choice signal-known-exactly detection task on synthetic MR images. We compared three reconstructions: magnitude, wavelet-based denoising, and phase-corrected real.
Images of the MRI signal intensity are normally constructed by taking the magnitude of the complex-valued data. This results in a biased estimate of the true signal intensity. We consider this as a problem of parameter estimation with a nuisance parameter. Using several standard techniques for this type of problem, we derive a variety of estimators for the MRI signal, some previously published and some novel. Using Monte Carlo experiments we compare the estimators we derive with others previously published.
Prospective motion correction methods using an optical system, diffusion-weighted prospective acquisition correction, or a free induction decay navigator have recently been applied to correct for motion in diffusion tensor imaging. These methods have some limitations and drawbacks.
OBJECTIVE: To report the ictal magnetoencephalography (MEG) in a patient with ring chromosome 20 mosaicism, a rare chromosomal anomaly associated with intractable epilepsy.
METHODS: MEG and simultaneous EEG were recorded with a 204 channel whole head MEG system. Ten habitual seizures occurred during the acquisition, which was done twice. The equivalent current dipoles (ECDs) for ictal discharges on MEG were calculated using a single dipole model. The ECDs were superimposed on a magnetic resonance image.
A 17-year-old woman developed left hemiparesis at the age 6 months. She had suffered from focal motor seizures associated with tonic extension of her left extremities since the age of 10 years. The interictal scalp EEG demonstrated frequent spike-and-slow-wave complexes dominantly in the right frontal area. MRI showed an old cerebral infarction in the right frontal lobe. Simultaneous recordings of magnetoencephalography (MEG) and EEG were obtained by using a 204-channel whole-head MEG system.
