Don L. Jewett

Human auditory evoked potentials: possible brain stem components detected on the scalp.

Auditory potentials recorded from the vertex of humans by a modified averaging technique have very short latencies and are probably generated by brain stem structures located at a considerable distance from the recording point. The evoked waves, which shOW considerable detail and consistency within and across subjects, may be clinically useful in evaluating subcortical function.

Volume-conducted potentials in response to auditory stimuli as detected by averaging in the cat☆

Abstract With the tongue as a reference point in the deeply anesthetized cat, recordings from the scalp or rostral brain locations obtained by repeated averaging, time-locked to an auditory click, showed four positive waves which were labeled P 1 , P 2 , P 3 and P 4 . P 1 occurs simultaneously with N 1 recorded at the round window and is probably generated by the VIIIth nerve. The other waves are likely to be composites of both slow and fast wave activity, but each shows increased amplitude or inversion of polarity or both in the vicinity of classical auditory structures: P 2 near the cochlear nucleus, P 3 near the superior olive and P 4 in and on either side of the inferior colliculus. A P 5 wave was observed in the inferior colliculus or slightly rostral to it. The medial geniculate and auditory cortex showed no evoked potentials in these animals. The results indicate that widely spaced electrodes can detect potentials, by means of averaging, at considerable distances from the generator of the potentials, so that additional evidence about the location of the generator must be provided when using this recording arrangement. The technique can be used to record a sensory system from a single electrode outside the system.

Neonatal development of auditory system potentials averaged from the scalp of rat and cat

Summary Auditory evoked potentials to ‘clicks’ were averaged from the scalps of litters of anesthetized rat pups and kittens throughout neonatal development. The potentials were shown to be from the brain stem auditory system, VIIIth nerve to inferior colliculus. At a fixed intensity and rate, peak latencies of the waves shortened with increasing age, while inter-litter variability decreased. The latencies of early waves shortened to adult values at younger ages than later waves. Increasing the intensity of stimulus at any age caused reduced latency of all of the waves; sometimes, the pattern of latencies seen at a younger age could be duplicated at an older age with a decreased intensity. Increasing the stimulus rate reduced the size of the waves, and there was return to control levels after 100 stimuli/sec in rat pups, adult rats, and cats, but not in kittens. The results are consistent with the hypothesis that significant limitations in both the immature and mature auditory system, as measured by peak latencies, occur before the VIIIth nerve. Several of the parameters studied can serve as a measure of ‘maturation’ in the auditory system, but at some ages one measure may be ‘immature’ when another is ‘mature’.

Photopic luminance does not always predict perceived room brightness

The perception of room brightness over photopic luminances ranging from 30 cd m-2 to 67 cd m-2 was judged by 12 subjects in an almost uniformly white experimental chamber. Two different illuminants were compared which had different spectral compositions, but were colour matched. Brightness judgements were often opposite to large differences in photopic luminance. These results are inconsistent with models of brightness perception that depend solely on cone receptors. At the luminance levels considered here subjective evaluation of light intensity depends upon both photopic and scotopic spectral contributions. These results imply that aspects of the visual system operate mesopically under most interior lighting conditions.

The Effect of Media Inhomogeneities Upon Intracranial Electrical Fields

A three-dimensional finite difference computer simulation has been used to model entire intracranial fields from a known dipole generator in both a cat brain and an insulated cube. The model permits pointwise variation of conductivity coincident with intracranial inhomogeneities due to gray and white matter, cerebrospinal fluid, and bone. Adequacy of the model was verified by agreement between the computed results and those obtained in the cube with both homogeneous and inhomogeneous media and in the cat brain along the dipole axis. Variation of brain model parameters showed that neither increasing nor abolishing intracranial inhomogeneities would give differences great enough to be detected experimentally. On the other hand, the position of the boundary could have significant effects on the fields. The model is general enough to be applicable to a variety of multipole generators and generator locations, as well as permitting consideration of the influence of localized inhomogeneities.

Far-field potentials due to action potentials traversing curved nerves, reaching cut nerve ends, and crossing boundaries between cylindrical volumes.

A previously published computer simulation was tested in a biological preparation by recording action potentials from frog sciatic nerves within a volume conductor filled with Ringers solution. Traveling in a straight line, nerve action potentials traversed a constricted cylinder before crossing into a larger, hemicylindrical volume. Recordings from widely spaced electrodes in the larger volume demonstrated a potential associated with the action potential crossing the boundary between the two volumes. Another potential was associated with the action potential reaching the nerves cut end. These potentials did not diminish in amplitude with increasing distance from the source. In other recordings, a potential associated with a bend in the nerve was found which was dependent upon the angle of the bend. These results indicate that the simple model of a dipole in a bounded sphere in which potentials decrease as a function of distance from the generator does not explain all potentials that can be observed under conditions that approximate human and animal recordings.

Improved method for computation of potentials in a realistic head shape model

The lead field analysis (LFA) algorithm, a new computational technique for the calculation of potentials on the surface of a realistic head shaped volume conductor model based on the boundary element method and the reciprocity theorem, is presented. The new algorithm, in comparison to the standard boundary element method, offers improved computational efficiency and lower storage requirements. It also yields more accurate surface potential results in the face of varying dipole source locations for a head shape boundary element model with a given number of nodes. Additionally, the algorithm results in quasi-analytic expressions of the derivatives of the surface potential with respect to the location of the sources, allowing the use of optimization techniques with better convergence properties. A set of simulations demonstrating the increased robustness of the LFA algorithm in the face of varying dipole source parameters is also described.<<ETX>>

Far-field potentials recorded from action potentials and from a tripole in a hemicylindrical volume

There is growing evidence in support of the hypothesis that far-field potentials are recorded when action potentials encounter discontinuities in the surrounding volume. The present study found further support for this hypothesis using two methods of experimentation. The first method recorded potentials when the action potential from an isolated bullfrog sciatic nerve in a hemicylindrical volume (i) encountered a change in the shape of the surrounding volume, (ii) crossed a boundary between 2 volumes of differing resistivities, (iii) reached a bend in the nerve, or (iv) reached the functional end of the nerve. In the second method, potentials were recorded when an electrical tripole, constructed in a way to produce the electrical equivalent of an action potential, encountered the same discontinuities as well as when it was configured to simulate a curved nerve. These results are consistent with the hypothesis that dipole components of an action potential predominant in far-field recordings.

Blood-containing aerosols generated by surgical techniques: a possible infectious hazard.

The aerosols generated in an operating room during surgery were simulated in the laboratory by using a variety of common surgical power tools. A Stryker bone saw, a Hall drill, and a Shea drill were used on bone, and a Bovie electrocautery was used in both the cutting and coagulation modes on tendon, all in the presence of a thin film of blood. A 10-stage, low-pressure cascade impactor was used to determine the particle size distribution of each aerosol, and Hemastix was used to assess the hemoglobin content of each particle size fraction. The same assessment was done for another series of blood aerosols that had previously shown the ability to infect human T-cell cultures. All of the tools tested produced blood-containing aerosol particles in the respirable size range (less than 5 microns). Because surgical masks offer little protection against such particles, personal sampling is indicated to define the risk of exposure to bloodborne pathogens by this route.

EXPOSURE TO BLOOD-CONTAINING AEROSOLS IN THE OPERATING ROOM: A PRELIMINARY STUDY

A personal sampling study was conducted to assess exposure to blood aerosols in the operating room. The breathing zones of primary and assistant surgeons were monitored using a personal cascade impactor configured with three stages corresponding to effective cut-off aerodynamic diameters of 14.8 microns, 3.5 microns, and 0.52 microns, respectively. Hemastix was used to assess the hemoglobin content of each particle size fraction. The arithmetic mean exposure concentration for primary surgeons (n = 14) was 1.4 micrograms Hb/m3 (range, none detected to 7.4 micrograms Hb/m3), while that for assistant surgeons (n = 12) was 1.8 micrograms Hb/m3 (range, 0.3 to 4.8 micrograms Hb/m3). Hemoglobin was detected in Stage 2 in 26 (90%) of the samples, in Stage 5 in 19 (66%) of the samples, and in Stage 8 in 11 (38%) of the samples. These data show that the mucous membrane lining of the upper respiratory tract and alveolar macrophages in the gas-exchange region are likely to be exposed to aerosolized blood in the operating room. Until further research determines the potential of infected blood aerosols to transmit disease, the authors recommend the proper use of respiratory protection equipment instead of surgical masks because the latter do not offer adequate protection.