Biomedical subjects

D J Major

Publications and source records attributed to D J Major.

3 recordsLinked to original sources

PubMed1991

Age-related features of the resting pattern-reversal visual evoked response using the dipole localization method and cortical imaging technique.

Two mathematical techniques, the dipole localization method (DLM) and the cortical imaging technique (CIT), are used to analyze the resting visual response to pattern-reversal stimulation. These methods identify certain age-related features of this evoked response that are not found by using standard topographic maps. These features include the symmetry of the N1 and P1 responses. The amplitudes of the N1 and P2 responses and the latency of N2 are also significantly different between old and young groups of test subjects, findings consistent with differences seen in conventional topographical analyses. Theoretical dipole sources and simulated cortical surface maps are also constructed for the "average" normal older subject and one patient with documented progressive frontal lobe degenerative disease. Standard topographical imaging studies of this patient were unremarkable, except for the P300 auditory response. DLM and CIT analyses of the VER components were exceptional and consistent with the clinical diagnosis. These mathematical methods appear to enhance the discriminating power of traditional electrophysiological measures.

Adult↗

PubMed1991

Numerical tests of a method for simulating electrical potentials on the cortical surface.

A mathematical imaging method for simulating cortical surface potentials was introduced at recent neurosciences meetings [1a], [1b], [2] and was applied to elucidate the neural origins of evoked responses in normal volunteers and certain patient populations. This method consists of the solution of an inward harmonic continuation problem and its effect is to simulate data that has not been attenuated and smeared by the skull. This cortical imaging technique (CIT) is validated by applying it to artificially derived data. Pairs of dipolar sources with different depths and separations are introduced into a spherical conducting medium simulating the head. Scalp potential maps are constructed by interpolating the simulated data between 28 "scalp" electrode positions. Noise is added to the data to approximate the variability in measured potentials that would be observed in practice. CIT is used in each case to construct potential maps on layers concentric to and within the layer representing the scalp. In several instances when the dipole pair is deep and closely spaced, the sources cannot be separated by the scalp topographical maps but are easily separated by the "cortical" topographical maps. CIT is also applied to scalp-recorded potentials evoked by bilateral median nerve stimulation and pattern-reversal visual stimulation.

Brain Mapping↗

PubMed1983

Gibberellins and heterosis in maize : I. Endogenous gibberellin-like substances.

Under controlled environment and/or field conditions, vegetative growth (height, internode length, leaf area, shoot dry weight, grain yield) was greater in an F(1) maize hybrid than in either parental inbred. Endogenous gibberellin (GA)-like substances in apical meristem cylinders were also higher in the hybrid than in either inbred, both on a per plant and per gram dry weight basis. There were no apparent qualitative differences in GA-like substances, however. Levels of GA-like substances in all genotypes were highest prior to tassel initiation. Chromatographic comparisons of the GA-like substances and authentic standards of GA native to maize on gradient-eluted SiO(2) partition and reverse-phase C(18) high-pressure liquid chromatography columns are described. No consistent differences in abscisic acid levels of the three genotypes were observed. This correlation of heterosis for endogenous GA-like substances with heterosis for growth suggests that amounts of endogenous GA may be related to hybrid vigor in maize.

Journal Article↗