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D P Russell

Publications and source records attributed to D P Russell.

6 recordsLinked to original sources
PubMed1999

Increased synchronization of neuromagnetic responses during conscious perception.

In binocular rivalry, the observer views two incongruent images, one through each eye, but is conscious of only one image at a time. The image that is perceptually dominant alternates every few seconds. We used this phenomenon to investigate neural correlates of conscious perception. We presented a red vertical grating to one eye and a blue horizontal grating to the other eye, with each grating continuously flickering at a distinct frequency (the frequency tag for that stimulus). Steady-state magnetic fields were recorded with a 148 sensor whole-head magnetometer while the subjects reported which grating was perceived. The power of the steady-state magnetic field at the frequency associated with a grating typically increased at multiple sensors when the grating was perceived. Changes in power related to perceptual dominance, presumably reflecting local neural synchronization, reached statistical significance at several sensors, including some positioned over occipital, temporal, and frontal cortices. To identify changes in synchronization between distinct brain areas that were related to perceptual dominance, we analyzed coherence between pairs of widely separated sensors. The results showed that when the stimulus was perceived there was a marked increase in both interhemispheric and intrahemispheric coherence at the stimulus frequency. This study demonstrates a direct correlation between the conscious perception of a visual stimulus and the synchronous activity of large populations of neocortical neurons as reflected by stimulus-evoked steady-state neuromagnetic fields.

Adult↗
PubMed1998

Investigating neural correlates of conscious perception by frequency-tagged neuromagnetic responses.

In binocular rivalry, a subject views two incongruent stimuli through each eye but consciously perceives only one stimulus at a time, with a switch in perceptual dominance every few seconds. To investigate the neural correlates of perceptual dominance in humans, seven subjects were recorded with a 148-channel magnetoencephalography array while experiencing binocular rivalry. A red vertical grating flickering at one frequency was presented to one eye through a red filter and a blue horizontal grating flickering at a different frequency was presented to the other eye through a blue filter. Steady-state neuromagnetic responses at the two frequencies were used as tags for the two stimuli and analyzed with high-resolution power spectra. It was found that a large number of channels showed peaks at both frequencies, arranged in a horseshoe pattern from posterior to anterior regions, whether or not the subject was consciously perceiving the corresponding stimulus. However, the amount of power at the stimulus frequency was modulated in relation to perceptual dominance, being lower in many channels by 50-85% when the subject was not conscious of that stimulus. Such modulation by perceptual dominance, although not global, was distributed to a large subset of regions showing stimulus-related responses, including regions outside visual cortex. The results demonstrate a correlation between the conscious perception of a visual stimulus and the synchronous activity of large populations of neurons as reflected by steady-state neuromagnetic responses.

Adult↗
PubMed1998

Functional clustering: identifying strongly interactive brain regions in neuroimaging data.

Brain imaging data are generally used to determine which brain regions are most active in an experimental paradigm or in a group of subjects. Theoretical considerations suggest that it would also be of interest to know which set of brain regions are most interactive in a given task or group of subjects. A subset of regions that are much more strongly interactive among themselves than with the rest of the brain is called here a functional cluster. Functional clustering can be assessed by calculating for each subset of brain regions a measure, the cluster index, obtained by dividing the statistical dependence within the subset by that between the subset and rest of the brain. A cluster index value near 1 indicates a homogeneous system, while a high cluster index indicates that a subset of brain regions forms a distinct functional cluster. Within a functional cluster, individual brain regions are ranked at the center or at the periphery according to their statistical dependence with the rest of that cluster. The applicability of this approach has been tested on PET data obtained from normal and schizophrenic subjects performing a set of cognitive tasks. Analysis of the data reveals evidence of functional clustering. A comparative evaluation of which regions are more peripheral or more central suggests distinct differences between the two groups of subjects. We consider the applicability of this analysis to data obtained with imaging modalities offering higher temporal resolution than PET.

Adult↗
PubMed1989

Urologic aspects of tethered cord.

Tethered cord syndrome, a form of spinal dysrhaphism, may involve vesical neurologic dysfunction. We present herein 60 cases of tethered cord syndrome, including 24 patients who underwent preoperative urodynamics studies. Preoperative cystometrography revealed areflexic bladders in 71 percent of cases and hyperreflexic bladders in 29 percent. Eight patients had serial pre- and postoperative urodynamics testing. Slight improvement was noted in postoperative urodynamics studies performed on 4 of 6 areflexic bladders and in the only hyperreflexic bladder studied. In addition, of the 3 patients in the serial urodynamics groups who had urinary symptoms preoperatively, 2 noted clinical improvement postoperatively. The urologist's role is important in the early detection, evaluation, and treatment of tethered cord syndrome and the neuropathic bladders that may result.

Adolescent↗
PubMedDate not supplied

Treatment of baseline drifts in fMRI time series analysis.

PURPOSE: Gradual drifting of baseline signal intensity is common in functional MRI (fMRI) time course data. Methods for dealing with this effect are studied. METHOD: Simulations and fMRI data are used to study three statistical models that account for baseline drift. A method is proposed in which the time course data are linear least-squares fit to a reference function that includes the slope of the baseline drift as a free parameter. RESULTS: It is shown that the least-squares method is equivalent to cross-correlation with Gram-Schmidt orthogonalization. Additionally, it is shown that certain paradigm designs improve the sensitivity of statistical tests when using any of the drift correction methods commonly employed. The least-squares method results in a variety of useful parameters such as activation amplitude, with a well characterized error. CONCLUSION: Very simple techniques can effectively account for observed drifts. It is important to design paradigms that are symmetric about the midpoint of the time series. In calculating confidence levels, a proper statistical model that accounts for baseline drifts is necessary to ensure accurate confidence level assessment.

Humans↗