Neural sources involved in auditory target detection and novelty processing: an event-related fMRI study.

We used event-related functional magnetic resonance imaging (erfMRI) techniques to examine the cerebral sites involved with target detection and novelty processing of auditory stimuli. Consistent with the results from a recent erfMRI study in the visual modality, target processing was associated with activation bilaterally in the anterior superior temporal gyrus, inferior and middle frontal gyrus, inferior and superior parietal lobules, anterior and posterior cingulate, thalamus, caudate, and the amygdala/hippocampal complex. Analyses of the novel stimuli revealed activation bilaterally in the inferior frontal gyrus, insula, inferior parietal lobule, and in the inferior, middle, and superior temporal gyri. These data suggest that the scalp recorded event-related potentials (e.g., N2 and P3) elicited during similar tasks reflect an ensemble of neural generators located in spatially remote cortical areas.

Functional magnetic resonance imaging: the basics of blood-oxygen-level dependent (BOLD) imaging.

There are 2 principal techniques of functional MRI (fMRI): the blood-oxygen-level dependent (BOLD) technique, which is the favoured method because no intravenous contrast medium is required, and the dynamic or exogenous technique. The BOLD technique takes advantage of the fact that the change from diamagnetic oxyhemoglobin to paramagnetic deoxyhemoglobin that takes place with brain activation results in decreased signal intensity on MRI. Commercially available scanners can be used to conduct single-slice BOLD fMRI experiments, but echo-planar hardware is needed for multislice wholebrain experiments. Sequence choices in BOLD fMRI include spin-echo and gradient-echo sequences, to which rapid acquisition with relaxation enhancement and echoplanar techniques may be applied. Optimal imaging parameters (echo time, slice thickness, field of view and flip angle) are important in maximizing signal-to-noise ratios. Various statistical techniques and software programs have been developed to interpret the large amounts of data gathered from BOLD fMRI experiments, which presents one of the biggest challenges in performing this technique with clinical MR units. Controversy exists regarding the effects of draining veins on cortical mapping, of inflow of blood into the imaging slice or volume, and of motion artifact. BOLD fMRI has demonstrated good correlation with positron emission tomography, magneto-encephalography and electrocorticographic recordings of motor responses. It has been used to study cortical activity of visual, motor, auditory and speech tasks as well as brain centres for smell, motor imagery, complex motion and memory. As such, it holds promise for the study of brain function, but must be subjected to larger studies comparing it with the gold standard of electrocorticographic mapping.