Biomedical subjects
Publications and source records attributed to H Stowell.
In 1969 two authors proposed that human visual evoked potentials could be predictors of psychometrically scored "intelligence." This interpretation of their data was subsequently invalidated. In 1982 two other authors made a similar claim for auditory evoked potentials; their published methodology and general theoretical background in sensory neurophysiology suggest even less likelihood of predictive validation.
An evoked potential report by four authors has revived the question of the somatosensory specificity of the vertex potential of somatosensory evoked potentials, when these are derived by scalp conductance in response to subjectively painful skin stimulation. By a peripheral nerve-block maneuver combined with subjective sensoriperceptual reports, the authors show a late, slow positive wave in the somatosensory evoked potential whose appearance and disappearance are claimed to reflect the brain's response to a uniquely C-fiber input and to a combination of A delta and C-fiber input respectively. But their idiosyncratic interpretation of their data has merely revived an unresolved question: How much, if anything, of these familiar "ultralate," slow positivities can be ascribed to any form of neural encoding of nociception, as distinct from a nonspecific brain response to a behaviorally important stimulus?
The so-called Vertex Potential (VP) of human scalp-conducted and event related brain potential (ERBP), which occur as a slow and often large, biphasic sinusoid within the 100-400 msec time segment after transient stimulation in the three main sensory modalities, are the longest researched of all human evoked potential (EP) phenomena. Its variable amplitude has been directly correlated, in experiments expressly tailored for the purpose, with input/output variables such as the rate of acceleration of given stimulus parameters from a state of relative rest (RM function), interstimulus interval (ISI), stimulus intensity, skin potential and resistance changes (SPR and SRR), the peripheral electroneurogram (ENG), and experimentally isolated C-fiber afference; and with neuropsychological variables such as attention or vigilance, visual acuity, response time, subjective stimulus probability or expectancy, acute pain of both fast and slow kinds, intelligence quotient (IQ), and psychometric personality scores (e.g., extraversion versus introversion and neuroticism versus normality). Unfortunately, the cerebral, neural origins of the VP, if any, are unknown; it is reported as usually absent from cortex-surface EP in those primates and mammals hitherto studied, and also from human extracranial event related magnetic fields of the brain (ERMFb) insofar as these reveal only superficial tangential sources; but a possible analog has been recorded from deep subcortical electrodes during human neurosurgery. In view of the increasing published range and quantity of direct correlates of VP amplitude, and of the scarcity of data about its neuroanatomy and neurophysiology, it seemed a good idea to do some rudimentary signal analysis. Preliminary results from five subjects confirm earlier data: The VP of somatosensory (SEP) and auditory (AEP) evoked potentials, as obtained by scalp-conductance and either averaged or single-epoch, can be resolved into inconsistently stimulus synchronized frequency components which are also present as relatively unsynchronized waves in the theta and alpha bands (approx. 2-13 Hz) of the unstimulated or near-threshold-stimulated electroencephalogram (EEG). In averages of numerous single trials (20 less than N less than 102), initiated at interstimulus intervals longer than 2.5 sec and deliberately sequenced so that the initiator could learn to estimate the timing of stimulus onsets, the phase coherence of the power-dominant alpha and theta waves within the 100-400 msec time segment of ERBP is obvious when the stimulus is an intense transient and psychologically not "habituated".(ABSTRACT TRUNCATED AT 400 WORDS)
Since 1960 systematic studies of the human scalp-conducted cerebral slow-wave response to painful stimulation have shown only amplitude augmentation of the vertex components of the somatosensory evoked potential (SEP) to be indicators of subjective perception of a noxious or aversive quality in the stimulus. The vertex potential (VP) of the SEP occurs relatively late after onset of either transient or maintained stimuli (100-400 ms depending on stimulus mode and site), is amplitude-focal at vertex on scalp, and may be asymmetrically distributed hemispherically for unilateral stimulation of the hands. Its functional neuroanatomy seems undeterminable by scalp macroelectrodes, and its relationship to analogous vertex potentials in the auditory and visual modalities (AEP and VEP) unknown. Studies of the nociceptive SEP (SEPn) since 1977 concur that VP amplitude is more readily correlatable with subjective pain magnitude estimate than with objective stimulus parameters. They also suggest that the VP is amplitude-sensitive to (a) interstimulus-intervals less than about 350 ms; (b) analgesics and their antagonists; and (c) subjective cognitive status with regard to both the expected aversiveness of the stimulus and the previous experience of chronic pain. These studies have included electrical stimulation of toothpulp and teeth, mechanical and transcutaneous electrical stimulation of palmar and digital glabrous skin, noxious thermal stimulation of hands and forearms, and electrical stimulation of lips, fingers, toes, and anogenital perineum of both sexes. They have been done in the context of both classical and signal-detectability (TSD) methods for concurrent reports of painful versus painless, and in the methodological contexts of conventionally signal-averaged SEP and of single-epoch SEP recovered both raw and processed. Studies designed to analyze differences in the early-intermediate (25-95 ms) and late (500-1000 ms) time-segments of SEP for painful and painless stimulation have provided no convincing evidence of cortical nociceptive signals. Therefore existing data on the SEPn imply that the only cortical slow-wave sign of nociception reflects perceptual-cognitive and endogenous, rather than sensory-discriminative and exogenous, aspects of the conscious pain experience.
Human somatosensory evoked potentials (SEP), especially those evoked by electrical stimulation of toothpulp and teeth or by laser thermal stimulation of skin, have attracted biomedical attention since 1975 as possible indicators of the quantity of acute pain being perceived by the subject. The dental variety has been claimed as an "objective correlate of acute laboratory pain." But investigators of SEP for mechanical, electrical, and thermal stimulation of skin and mucocutaneous junctions have provided data which seriously question the meaning of that claim. Most recently dental SEP workers have rediscovered some well-known ambiguities in their own data, all of which refer only to time-domain or transient characteristics of event related brain potentials (ERBP). Perhaps we should reconsider an older approach to understanding ERBP, using concurrent analysis of their transient temporal and steady-state frequency characteristics.
Scalp-recorded somatosensory evoked potentials (SEPs) in humans and dogs suggest that it takes longer for peripheral information to reach the brain from the anogenital perineum than from the distal hind limbs, in spite of a shorter afferent pathway. The delay seems to be centrally determined, rostral to the lumbar segments, rather than by slower conduction velocities in the peripheral pudendal-sacral afferents. Thus the mammalian brain may temporally conserve the spatial sequence of segmental inputs in spite of variable afferent pathway lengths and/or conduction velocities. Human SEPs indicate the arrival timetable in cortex to be ordered: Lips, fingers, toes, genitals, anus; with a delay between lips and anogenital perineum of 30-40 msec. This delay is comparable to the interpair-interval threshold for human detection of two successive tactile events on the fingers, while ambiguous temporal defusion of two successive identical stimuli can be perceived down to objective separations of about 15 msec. It is suggested that, whereas the metric tensor model proposes central correction of asynchronous sensorimotor signals within an approximately 30 msec predictive function by cerebellar circuitry (in order to avoid spatial distortion in a spacetime tensor of sensorimotor processing), the brain may use longer time differences between proximal and distal receptive fields to develop a Cartesian, four-dimensional body map within its adaptive, behavioral world.
Somatosensory evoked potentials (SEPs) were averaged from several scalp derivations in 3 subjects (1 M, 2 F) for transcutaneous electrical stimulation of toes and perineum. These lumbosacral SEPs were amplitude-focal at vertex for 40-300 msec components. Peak latencies in the 40-120 msec time segment were significantly later for perineal stimulation of equivalent subjective magnitude, contrary to expectation from relative afferent pathway lengths. One subject (M) showed a monotonic latency retardation of an intermediate component (SP50-SN75) for equivalent stimuli at lips, fingers, toes, genitalia, and perianum. Vertex potentials (SN140-SP200) did not show this temporal sequence. Though interpretation of such data is currently ambiguous, systematic application of comparative evoked potential/magnetic field techniques could yield data useful to clinical neurologists.
Evidence is offered for the requirement of more careful comparisons between the new event related magnetic fields of the brain (ERMFb) and their conventional counterparts the event related brain potentials (ERBP), in the somato-sensory (SEP) and auditory (AEP) modalities at least, because the existing ERBP literature contains a wealth of well-documented data on these crucial issues: (1) Contralateral and ipsilateral SEP; (2) the relevance of "background" intrinsic periodicity; and (3) the relevance of stimulus parameters, receptive fields, and subjective perceptual status to variations in the waveforms. The importance of the new 2nd derivative magnetic gradiometry for better spatiotemporal resolution of sources and for functional understanding of cerebral electrophysiology cannot be overestimated. So it may be equally important to ensure that existing, historical data from conventional scalp-conductance, systematically recorded and analyzed during the past twenty years, are not ignored at the inception of essential comparisons using complementary electromagnetic techniques.
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Signal summation and minimally adapting tactile stimulation techniques permit the resolution of a critical time domain of the somatosensory evoked response, recorded from human scalp, which is sensitive to the stimulus variables represented by the sensations of tapping and pinprick respectively. The variation of this time-domain suggests the influence of myelinated mechanical nociceptors in the Group III range.
Primary somatosensory evoked responses to tapping and pinprick of contralateral central palmar skin, recorded from human scalp, yielded significant differences of peak latency but not of amplitude, suggesting afferent conduction by two populations of primary fibers, Group II and Group III respectively. Pinprick first positives were always later for all conscious subjects, and sometimes larger. A model for cortical discrimination of signal phase and frequency is related to these data.
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