Biomedical subjects
Publications and source records attributed to J D Cowan.
A model for the interactions of cortical neurons is derived and analyzed. It is shown that small amplitude spatially inhomogeneous standing oscillations can bifurcate from the rest state. In a periodic domain, traveling wave trains exist. Stability of these patterns is discussed in terms of biological parameters. Homoclinic and heteroclinic orbits are demonstrated for the space-clamped system.
Neuronal activity in a two-dimensional net is analyzed in the neighborhood of an instability. Bifurcation theory and group theory are used to demonstrate the existence of a variety of doubly-periodic patterns, hexagons, rolls, etc., as solutions to the field equations for the net activity. It is suggested that these simple geometric patterns are the cortical concomitants of the "form constants" seen during visual hallucinosis.
It is now generally accepted that several mechanisms mediate contrast detection at threshold. Many attempts to measure the bandwidths of these mechanisms using subthreshold summation have yielded estimates suggesting very narrow frequency tuning. We have measured thresholds for three classes of stimuli: gratings containing two sinusoidal components, sums of "difference of Gaussian" (DOG) patterns, and square waves. The results of the subthreshold summation experiments with gratings are consistent with those of other studies. All of these thresholds can be predicted by a model requiring only four center-surround-type mechanisms at each retinal locus. These mechanisms have a full bandwidth at a half-height of about 1.75 octaves. Quantitative prediction of sensitivities depends critically upon spatial probability summation. It is this spatial nonlinearity that yields these somewhat conterintuitive results. The much narrower bandwidths inferred by others are due to the assumption of linearity used in their analyses.
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SUMMARY: A review of osteomyelitis in 54 patients treated at the Dr. Charles A. Janeway Child Health Centre over a 4-year period revealed equal frequencies of secondary and hematogenous osteomyelitis. Although the clinical picture in patients with hematogenous osteomyelitis was classic, patients with secondary osteomyelitis presented with an altered clinical response. Patients with secondary osteomyelitis have a history of an antecedent puncture wound or an inadequately treated contiguous focus of infection; antistaphylococcal antimicrobial therapy was ineffective for most because gram-negative bacilli were isolated in this group of patients. In contrast to patients with hematogenous osteomyelitis, who frequently respond to intensive antimicrobial therapy, those with secondary osteomyelitis will frequently require surgical intervention to eradicate the infection.
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Serious complications occurred in 29 of 887 children with puncture wounds of the feet treated over a four-year period at the Dr. Charles A. Janeway Child Health Centre. Osteomyelitis in one of the small bones of the foot was the commonest complication and occurred when a cartilaginous surface (physeal plate or articular cartilage) had been violated. Although systemic signs of osteomyelitis frequently are absent, this infectious process is refractory to medical management. The combined surgical and medical management of this complication is outlined. Management of puncture wounds in the emergency room should include a thorough history concisely recorded, tetanus prophylaxis, and cleansing, debridement, and probing of the wound. Antimicrobial agents are not routinely required but should be reserved for patients presenting late with cellulitis or an established infection. A semisynthetic penicillinase-penicillin appears to be the agent of choice until the results of microbiologic studies are available.
A simple graphical calculus is developed that generates analytic solutions for membrane potential transforms at any point on the dendritic tree of neurons with arbitrary dendritic geometries, in response to synaptic "current" inputs. Such solutions permit the computation of transients in neurons with arbitrary geometry and may facilitate analysis of the role of dendrites in such cells.
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Coupled nonlinear differential equations are derived for the dynamics of spatially localized populations containing both excitatory and inhibitory model neurons. Phase plane methods and numerical solutions are then used to investigate population responses to various types of stimuli. The results obtained show simple and multiple hysteresis phenomena and limit cycle activity. The latter is particularly interesting since the frequency of the limit cycle oscillation is found to be a monotonic function of stimulus intensity. Finally, it is proved that the existence of limit cycle dynamics in response to one class of stimuli implies the existence of multiple stable states and hysteresis in response to a different class of stimuli. The relation between these findings and a number of experiments is discussed.
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Because of the considerable differences in the reported values for serum folate in normal subjects using Lactobacillus casei as assay organism the effect of daylight when preparing media for this assay was studied. It was found that the growth properties of the medium were destroyed by direct sunlight and impaired by bright reflected sunlight, therefore reducing the sensitivity of the assay. Adequate precautions regarding exposure to light must be taken.
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