Jack Lubowsky

Magnetic field influence on central nervous system function

The effects of strong static magnetic fields on the excitability of striate cortex in adult cats was studied. The visual evoked response was used as a measure of cortical excitability. In all animals a 1200-G field was associated with a significant decrease in both amplitude and variability of the evoked response. This effect began more than 50 s after the field was turned on and persisted, even after the field was turned off, for several minutes. This phenomenon appears to be due to action of the magnetic field at the synapse rather than on axonal conduction.

Modification of spontaneous unit discharge in the lateral geniculate body by a magnetic field

The effects of a 1230-G static magnetic field on spontaneous discharge frequency and discharge pattern of principle cells in the cats lateral geniculate body (LGB) were examined. In 45% of cells studied, a decrease in frequency was seen after the field was turned on. This progressed, even after the field was turned off, with return to baseline after an average duration of 250 s. Onset typically was 75 s after the field was activated, with maximum effect occurring 135 sec thereafter. In 67% of those cells which exhibited a decrease in frequency and in 50% of those which did not, a change in discharge pattern, as reflected by the interspike interval histogram, was seen. When present, this was manifested as a decrease in short interspike intervals. The change in the interspike interval histogram usually persisted longer than the change in frequency. The gradual onset and prolonged time course of changes in LGB cell activity suggest either an alteration in the synaptic ionic environment or in neurotransmitter availability. It is hypothesized that strong magnetic fields produce a partial realignment of diamagnetically anisotropic molecules within the cell membrane, thereby distorting ion-specific channels sufficiently to alter their function.

Near-infrared absorption imaging of dense scattering media by steady-state diffusion tomography

Results of experimental studies are presented based on steady-state optical tomographic measurements performed at 720 nm on tissue-like media having cylindrical geometry and a thickness equivalent to an uncompressed female breast. Experimental data were evaluated using a linear perturbation equation, previously described, derived from the transport equation. Results obtained demonstrate that high-resolution (< 1 mm edge detection) images are recoverable of small-diameter objects buried at depths not visible from the surface.

Application of Transport Theory to Infra-Red Medical Imaging

We discuss the principles of a potential imaging scheme which makes use of reflected radiation in the near infrared region and is intended for a broad spectrum of clinical applications. The object is to detect and image regions of anomalous absorption, which give information about physiological function. We describe Monte Carlo procedures used to simulate the process and give computed results for weight functions. Computed images of simulated absorbers are also presented. We discuss possible applications.

Imaging of scattering media by diffusion tomography: an iterative perturbation approach

This paper describes an iterative perturbation approach for imaging the absorption properties of a dense scattering medium. This method iteratively adjusts a current estimate until the calculated photon fluxes for the estimated medium match the detected readings. The inverse update in each iteration is accomplished by solving a linear perturbation equation. It is similar to the compensation theory method used in electrical impedance tomography. A comparison was made between the methods of conjugate gradient descent and projection onto convex sets for the solution of the perturbation equation. The former converges more rapidly, but can yield an inaccurate solution where the problem is underdetermined. The latter can incorporate many types of a priori information to reach a correct solution, but progresses very slowly. A multi-grid, progressive reconstruction technique is proposed, which computes the fine details with the help of the coarse structure. It is quite effective in forcing the correct solution and reducing computation time. These methods have been used to reconstruct several inhomogeneous media containing simple structures, from steady-state reflectance data. Two sets of data are tested: one calculated according to the perturbation model, and the other using Monte-Carlo methods. When the difference between the absorption distributions of the test medium and the initial estimate is localized, a single step of the perturbation approach can resolve the absorption distribution reasonably well to within 5 transport mean free pathlengths from the surface. At greater depths, the reconstruction is less reliable.

Imaging of diffusing media by a progressive iterative backprojection method using time-domain data

A method for the reconstruction of 3-D images of the interior of dense scattering media, based on the analysis of time-resolved backscattered signals is described. The method evaluates a linear perturbation equation by a progressive iterative backprojection scheme. A key feature of the method is the use of weighting functions which estimate the impact that absorption of photons in the interior have on the response of detectors located at the surface. Examples of reconstructed images shown are based on the analysis of simulated data for multilayered media and simulated and experimental data for media containing finite-volume absorbers. These results contain features which indicate that images having high resolution are obtainable even in the limiting case where the view angle is restricted to only backscattered signals and the absorption contrast across an interior boundary is 1%. A general scheme, similar to a layer- stripping approach, is described for the case where signals emerging about a target are measured.

Geniculate interactions with a penicillin discharge in visual cortex.

Abstract Spontaneous activity of principal cells in the dorsal nucleus of the lateral geniculate body was studied 1 sec before and 1 sec after penicillin discharges in the striate cortex. Many cells exhibited increased activity prior to the penicillin discharge suggesting that corticopetal activity from the geniculate is capable of triggering the penicillin discharge. Many cells also exhibited increased activity for as long as 1 sec after the discharge. Some cells displayed both phenomena. Two patterns of transient depression were also observed in the postdischarge period. The latter phenomena are believed to be related to the inhibitory process associated with after-positivity which follows orthodromic or antidromic stimulation of the geniculate body.

Evaluation of steady-state, time- and frequency-domain data for the problem of optical diffusion tomography

The three techniques of CW, time-gated, and frequency modulation measurements are examined and compared for their ability to provide clues to the macroscopic optical properties of multilayer random media. For each illumination scheme, a simple algorithm is described which is capable of identifying important parameters such as the ratio of macroscopic absorption cross-sections across layer boundaries, and the depth of boundaries in two- and three-layer media. The algorithms are developed by consideration of data from Monte Carlo simulations, and in the case of the time- and frequency-domains, are tested by using them to analyze experimental data.

Determination of macroscopic optical properties of multilayer random media by remote sensing

Several measurement and analysis schemes have been explored, using simulated data, in an effort to identify new strategies for the determination of the macroscopic optical properties of multilayer random media. Several simple algorithms are described which are capable of identifying the values of important parameters such as the total cross- section, (summation)t, the ratio of the absorption to total-cross section, (summation)a/(summation)t, and depth of subsurface boundaries in two- and three- layer media. The albedo values of the media varied for 0.9 to 0.99 and the superficial layer had an absorptivity either greater or less than the subsurface layers. A novel feature of the algorithms is a comparison of the responses of coupled pairs of detectors. Their utility stems from the fact that variations in the orientation and location of collimated detectors permit the selective interrogation of subsurface regions in dense scattering media.

A Computer Controlled Method for Determining the Fields of Visual System Neurons

Determining the shape of the response field of a visual system neuron by monitoring its responses to a manually manipulated light stimulus is a very tedious procedure. Equipment and computer programs are here described which allow a computer to control the shape of a stimulus image while it monitors the response of a unit (neuron). The technique was used on simulated neurons and neurons of the lateral geniculate bodies of experimental animals (cats). Preliminary results showed that the technique repeatably located and sized the response fields of these units.