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R M Chapman

Publications and source records attributed to R M Chapman.

At least 19 recordsLinked to original sources
PubMed1995

EP component identification and measurement by principal components analysis.

Between the acquisition of Evoked Potential (EP) data and their interpretation lies a major problem: What to measure? An approach to this kind of problem is outlined here in terms of Principal Components Analysis (PCA). An important second theme is that experimental manipulation is important to functional interpretation. It would be desirable to have a system of EP measurement with the following characteristics: (1) represent the data in a concise, parsimonous way; (2) determine EP components from the data without assuming in advance any particular waveforms for the components; (3) extract components which are independent of each other; (4) measure the amounts (contributions) of various components in observed EPs; (5) use measures that have greater reliability than measures at any single time point or peak; and (6) identify and measure components that overlap in time. PCA has these desirable characteristics. Simulations are illustrated. PCA's beauty also has some warts that are discussed. In addition to discussing the usual two-mode model of PCA, an extension of PCA to a three-mode model is described that provides separate parameters for (1) waveforms over time, (2) coefficients for spatial distribution, and (3) scores telling the amount of each component in each EP. PCA is compared with more traditional approaches. Some biophysical considerations are briefly discussed. Choices to be made in applying PCA are considered. Other issues include misallocation of variance, overlapping components, validation, and latency changes.

Brain↗
PubMed1994

Frequent homozygous deletions of the D13S25 locus in chromosome region 13q14 defines the location of a gene critical in leukaemogenesis in chronic B-cell lymphocytic leukaemia.

Cytogenetic studies of B-cell chronic lymphocytic leukaemia show structural abnormalities involving the 13q14 chromosome region as the only karyotypic change in a significant proportion of tumours. This observation suggests the location of a gene important in leukaemogenesis. A series of 68 BCLL tumours have been analysed for allele loss using a series of probes from 13q14. Using intragenic polymorphic markers from the retinoblastoma predisposition gene LOH was observed in 25% of tumours including 3/6 showing cytogenetically obvious deletions of the 13q14 region and 3/6 showing translocations involving 13q14. However, three deletions with proximal breakpoints in 13q14 did not show allele loss, demonstrating that the breakpoint lay distal to RB1. Using the D13S25 locus, which lies 1.6 cM distal to RB1, allele loss was seen in 90% of tumours with structural rearrangements of 13q14 and 75% of tumours with an apparently normal karyotype. 50% of these tumours showed homozygous loss of D13S25, suggesting that a 'tumour suppressor gene' lies in this region. The more distal D13S31 locus, 1 cM distal to D13S25, was infrequently involved in allele loss demonstrating that the minimum region of overlap for homozygous deletions is approximately 1 Mbp around the D13S25 locus.

Alleles↗
PubMed1993

Word meaningfulness and event-related potentials during phrase processing.

Evoked Potentials (EP) were recorded to visual presentation of words in phrases. Three EP components, P250, N400, P560, and their dependence on the role that each word plays in the phrase were studied. Subjects were requested to perform a congruity/incongruity task after the presentation of short phrases of two-types that differed in the position of meaningful keywords. We show how these keywords contribute substantially to N400 and possibly to P560 but do not reliably affect P250.

Adolescent↗
PubMed1990

Neuromagnetic topography of photoconvulsive response in man.

The neuromagnetic method was applied to the study of photoconvulsive responses. The identification of specific magnetic field distributions over the scalp was achieved by; (a) a stimulation paradigm consisting of series of trains of flicker stimuli randomly presented to the epileptic patient, after eye closure, to get epileptic responses while avoiding seizures; (b) a novel procedure for data analysis, to select consistent responses. These patterns, when sufficiently stable in time and dipolar in shape, were used for source localization in the usual biomagnetic framework of the equivalent current dipole source representation. The results of this approach suggest that different specific cortical areas are repeatedly and randomly activated, involving mainly the frontal, occipital and temporal areas, often with a hemispheric prevalence.

Brain↗
PubMed1989

Evoked potentials and the study of sentence comprehension.

Evoked brain potentials were used to monitor moment-by-moment decisions during language comprehension. Subjects read sentences containing temporary syntactic ambiguities for which one of the possible interpretations was semantically implausible. The N400 component of the evoked potential, which is sensitive to implausibility, was used to discover when during a sentence subjects made a decision about the ambiguity. The results demonstrate that readers try to interpret a syntactic ambiguity early in a sentence rather than waiting for disambiguating information. This introduces a new way to use brain activity to study sentence comprehension processes.

Decision Making↗
PubMed1989

A simple method for 3-dimensional localization of epileptic activity recorded by simultaneous EEG and MEG.

A simple method is presented to average target patterns in the magnetoencephalographic trace of epileptic activity by finding markers in an EEG channel. This method proves to be useful even in cases of low magnetic signal-to-noise ratio. After simultaneously recording electric and magnetic activity the user chooses in the electric trace a template of interest and determines within the same trace all the time points at which the EEG pattern is highly correlated with the chosen template. The magnetic trace portions recorded at those times are averaged together, extracting in this way the magnetic signal from the noise. The MEG results are applied to localize in the brain the active sources that contribute to the recorded signals.

Electroencephalography↗
PubMed1989

Tobacco or health?

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Health Promotion↗
PubMed1984

Selective localization of alpha brain activity with neuromagnetic measurements.

A method of localizing brain activity by a new combination of magnetic and electrical recording, relative covariance, is described. The successful application of this method to alpha EEG is reported. Spontaneous human brain activity was recorded simultaneously with fixed scalp electrodes and a movable magnetometer. The analysis was restricted to the alpha rhythm, which was selected by a narrow bandpass filter centered at the observed alpha frequency. For each magnetometer location, the ratio of the covariance of the magnetic and electric signals to the electric variance was calculated, producing a map reflecting the magnetic field pattern. Clear maxima of opposite polarity over the left and right parietotemporal areas indicate bilateral current source areas near the midline, in the vicinity of the calcarine fissure, at a depth of 4-6 cm from the scalp. This relative covariance method may prove generally useful in localizing bioelectrical sources such as spontaneous brain rhythms.

Adult↗
PubMed1983

Gonadal injury resulting from chemotherapy.

Radiation, alkylating agents and the vinca alkaloids produce variable degrees of gonadal dysfunction in humans; additional drugs have been linked to gonadal damage in males only. Age at time of exposure and total dose of drug administered are associated with the degree of gonadal injury. Irreversible sterility develops at lower doses in men than in women. The infertile men usually retain sexual function, whereas the infertile women experience premature menopause and sexual dysfunction. Subfertility with normal sexual function is the sequela of lesser degrees of gonadal injury in men and women. For a given drug dose, children apparently sustain less gonadal damage than their adult counterparts. Rigorous reporting of basic essential data will enhance our understanding of drug-induced gonadal damage; incomplete reports only cloud the issue.

Adolescent↗
PubMed1982

Testicular dysfunction in untreated Hodgkin's disease.

Gonadal function was examined in 19 young men with Hodgkin's disease before therapy and compared with that of 11 men with other malignancies, 13 men with primary testicular failure, and 19 normal men of similar age. Total (p less than 0.01) and free (p less than 0.05) testosterone levels were decreased in Hodgkin's disease. In those with advanced (stage III + IV) and symptomatic (B), Hodgkin's disease serum testosterone levels were indistinguishable from those in primary testicular failure, yet serum levels of luteinizing hormone were normal. Moreover, the acute response of serum testosterone to exogenous human chorionic gonadotropin (HCG) was significantly greater in Hodgkin's disease than in primary testicular failure (p less than 0.03). These data and the finding that basal serum follicle-stimulating hormone levels are significantly lower than normal in Hodgkin's disease (p less than 0.05) suggest that the cause of pretreatment hypogonadism in Hodgkin's disease is not simple primary testicular failure. Total sperm count was decreased in 40 percent of men with Hodgkin's disease but in none of the men with other malignancies (p less than 0.05), suggesting specific seminiferous tubular dysfunction in Hodgkin's disease. However, motility was abnormal in 69 percent of men with Hodgkin's disease and 60 percent of those with other malignancies, suggesting that this is a nonspecific effect of cancer. Serum prolactin levels were significantly higher than normal in Hodgkin's disease (p less than 0.05) but not in other malignancies. Our findings suggests that the cause of testicular dysfunction that is present before treatment of Hodgkin's disease is complex, perhaps involving both pituitary and gonadal abnormalities.

Adult↗
PubMed1982

Effect of cytotoxic therapy on sexuality and gonadal function.

Many chemotherapeutic agents have been shown to cause variable degrees of gonadal dysfunction in both sexes and in all age groups. The severity of the dysfunction depends on the total drug dose and the age at time of therapy. In general, cytotoxic agents produce gonadal dysfunction in men while they produce premature gonadal failure in women. Men develop azoospermia and compensated Leydig-cell function; women sustain ovarian damage causing impaired fertility in the short term and early ovarian failure later. This dysfunction is associated with sexual and emotional difficulties in many patients. In order to discover these problems the physician must sympathetically ask patients and families about their sexual and emotional health. Endocrine and psychologic evaluation help the physician identify the problem. Appropriate counseling and hormone replacement therapy may ameliorate most symptoms and help the patient emotionally adjust to illness and infertility. Prevention of gonadal damage during cytotoxic therapy may be possible in the future. For those young people who retain fertility after cytotoxic therapy, prognosis should be taken into account when counseling about parenthood is given. There is no evidence of genetic abnormalities in the offspring of people previously treated with chemotherapy or irradiation.

Adolescent↗