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Biomedical subjects

T Allison

Publications and source records attributed to T Allison.

At least 55 records · Page 3Linked to original sources
PubMed1988

Localization of human sensorimotor cortex during surgery by cortical surface recording of somatosensory evoked potentials.

The traditional means of localizing sensorimotor cortex during surgery is Penfield's procedure of mapping sensory and motor responses elicited by electrical stimulation of the cortical surface. This procedure can accurately localize sensorimotor cortex but is time-consuming and best carried out in awake, cooperative patients. An alternative localization procedure is presented that involves cortical surface recordings of somatosensory evoked potentials (SEP's), providing accurate and rapid localization in patients under either local or general anesthesia. The morphology and amplitude of median nerve SEP's recorded from the cortical surface varied systematically as a function of spatial location relative to the sensorimotor hand representation area. These results were validated in 18 patients operated on under local anesthesia in whom the sensorimotor cortex was independently localized by electrical stimulation mapping; the two procedures were in agreement in all cases. Similar SEP results were demonstrated in an additional 27 patients operated on under general anesthesia without electrical stimulation mapping. The following three spatial relationships between SEP's and the anatomy of the sensorimotor cortex permit rapid and accurate localization of the sensorimotor hand area: 1) SEP's with approximately mirror-image waveforms are recorded at electrode sites in the hand area on opposite sides of the central sulcus (P20-N30 precentrally and N20-P30 postcentrally); 2) the P25-N35 is recorded from the postcentral gyrus as well as a small region of the precentral gyrus in the immediate vicinity of the central sulcus: this waveform is largest on the postcentral gyrus about 1 cm medial to the focus of the 20- and 30-msec potentials; and 3) regardless of component identification, maximum SEP amplitudes are recorded from the hand representation area on the precentral and postcentral gyri.

Anesthesia, General↗
PubMed1985

Electrical sources in human somatosensory cortex: identification by combined magnetic and potential recordings.

Magnetic fields and electrical potentials produced by neuronal activity have different properties that can be used for the identification of electrical sources in the human brain. Fields and potentials occurring 20 to 30 milliseconds after median nerve stimulation in human subjects were compared in order to investigate the sources of evoked potential components that have been attributed by different investigators to the thalamus or thalamocortical afferents, to separate radial sources in somatosensory cortex and motor cortex, or to a tangential source in somatosensory cortex. The magnetic and potential wave forms were highly similar in morphology, and their spatial distributions were centered over sensorimotor cortex, were dipolar in shape, and differed in orientation by approximately 90 degrees; distances between the minimum and maximum of the magnetic distributions were about 60 percent of those of the potential distributions. These results cannot be accounted for by thalamic sources or radial cortical sources alone, but are consistent with a tangential source in somatosensory cortex, with an additional smaller contribution from radial sources.

Electrophysiology↗
PubMed1984

Developmental and aging changes in somatosensory, auditory and visual evoked potentials.

To assess developmental and aging changes in human sensory systems, components of short-latency somatosensory, brain-stem auditory and pattern-reversal visual evoked potentials, thought to originate in specific structures of these systems, were recorded in 286 normal subjects ranging in age from 4 to 95 years. Analysis was primarily restricted to peak and interpeak latencies; visual evoked potential amplitudes were also analyzed. Major results and conclusions are: (1) 'Developmental' changes (that is, decreases in latency attributable to decreased conduction time in younger subjects) were not seen in the median nerve, in brain-stem auditory pathways, or in some portions of visual cortex. Small developmental changes were seen in the somatosensory afferent pathway from the cervical spinal cord to thalamus, and large changes were seen in somatosensory and visual cortex. Cortical developmental changes appeared not to be complete until 17 years of age or later. (2) 'Aging' changes (that is, increases in latency attributable to increased conduction time in older subjects) were observed in the median nerve, cervical spinal cord, brain-stem auditory pathways, and somatosensory and visual cortex. (3) Visual evoked potential amplitudes tended to decrease with age, particularly during development; amplitude and latency effects were dissimilar for most components. (4) Males tended to show larger aging effects than females. (5) The results suggest that age-related changes in human sensory systems are not uniform, but rather are different in specific portions of these systems, different at particular epochs of the life span, and stronger in males than in females.

Adolescent↗
PubMed1983

Brain stem auditory, pattern-reversal visual, and short-latency somatosensory evoked potentials: latencies in relation to age, sex, and brain and body size.

To determine standards of normality for auditory, somatosensory and visual evoked potentials commonly used in the assessment of neurological disease, 8 AEP, 1 VEP and 12 SEP components were recorded to stimulation of left and right ears, eyes, and median nerves in 286 normal subjects ranging in age from 4 to 95 years. Peak and interpeak latencies, and left-right differences in latency, were analyzed as a function of age, sex, and estimates of brain and body size. Major features of the results were: (1) Peak latencies of all components showed statistically significant increases in latency with age except that VEP P100 latency decreased significantly between 4 and 19 years and did not change between 20 and 59 years. (2) In adults the peak latencies of all components were significantly later in males than in females. For AEPs and VEPs these differences were explained by sex differences in brain size, and for adult SEPs were explained by sex differences in arm and shoulder dimensions. No significant sex differences in VEP and SEP latencies were seen in children. (3) Most interpeak latencies showed significant differences in relation to age or sex. (4) Age and sex are useful predictors of latency for nearly all peak and interpeak latencies; in addition, height is a useful predictor of SEP peak latencies. (5) Left-right latency differences showed little age-related, and no sex-related, change. The interlaboratory use of these or other normative data was discussed. It was concluded that these AEP and SEP norms can probably be used in other laboratories if stimulating and recording conditions are similar. However, VEP results are difficult to transfer due to the poorly understood effects of variation in stimulus conditions. Some issues regarding the optimal characterization of norms were also discussed.

Adolescent↗
PubMed1983

Gasser and Graham and spinal cord electrophysiology.

This year marks the 50th anniversary of the publication of the pioneering electrophysiological study of the spinal cord by H. S. Gasser and H. T. Graham (Am. J. Physiol. 103: 303-320, 1983). Gasser and Graham's paper is of historical interest, because it was the first study of the spinal cord to use a cathode ray tube. It is of scientific interest, because their careful recordings and insightful interpretations laid the groundwork for subsequent studies of spinal cord electrophysiology in anesthetized animals and, more recently, for the interpretation of noninvasive spinal cord recordings in awake, intact humans.

Animals↗
PubMed1978

On-line statistical detection of average evoked potentials: application to evoked response audiometry (ERA).

An objective method of EP detection in averaged EEG epochs is described which is based on the statistical properties of the averaged EEG in the absence of time-locked EP activity. The statistical properties of the subject's averaged background EEG are summarized by making a set of comparisons between two consecutive EEG epochs immediately preceding each stimulus presentation. After every stimulus presentation, a second set of comparisons is made between the post-stimulus EEG epoch and the immediately preceding pre-stimulus EEG epoch. These two sets of comparisons are then examined to determine whether the latter differ significantly from the former. The technique has been programmed for on-line use on a 12-bit minicomputer, validated on cooperative adult subjects and used for ERA threshold determinations in young children. Using response detection criteria yielding an empirically determined false positive error rate of between 1 and 4%, threshold estimates averaged about 10 dB higher than psychophysical thresholds. The efficiency with which such ERA thresholds may be obtained with this method is substantially greater than that of conventional ERA procedures employing subjective evaluation of averages. Further, the technique allows variation of detection criteria to suit non-audiometric needs and empirical determination of the false-positive error rate under any set of conditions.

Adult↗
PubMed1978

A method for localization of sources of human cerebral potentials evoked by sensory stimuli.

A method based on potential field theory is described for assessing the location and orientation of dipole generators of the human scalp-recorded sensory evoked potential (EP). The method assumes that the EP at a given moment is due to a single dipole source and that the head can be modeled by a homogeneous conductive sphere (brain) surrounded by inner (skull) and outer (scalp) shells of differing conductivity (three-sphere model). Solution for source location and orientation from the surface potential field is given for the case of a single homogeneous sphere (one-sphere model). It is then shown that a unique solution for the three-sphere model can be derived from the one-sphere solution. Solutions are obtained by application of an iterative procedure which minimizes the error between calculated and empirical potential fields. A test of the method is described in which the calculated location and orientation of a dipole was in good agreement with the known source of an early component of the human somatosensory EP.

Brain↗
PubMed1977

The scalp topography of human somatosensory and auditory evoked potentials.

The waveform and topography of components of the scalp recorded somatosensory evoked poal (AEP) to click stimulation of the right ear, were determined for scalp electrode locations of the 10-20 system and for locations at the eye, mastoids, and posterior neck. Twenty-one SEP and twenty-two AEP components were analyzed. Differentiation of neurogenic and myogenic components was attempted on the basis of localization and variability. Some components of extracranial origin, apparently originating in frontal musculature, were small in most experienced subjects and large in most experimentally naive subjects. These and other presumptive myogenic potentials can distort adjacent neurogenic components. These data should aid in predicting SEP and AEP characteristics and in assessing myogenic distortion of neurogenic components.

Adolescent↗