Somatosensory evoked potentials and magnitude of perception

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Exp. Brain Res. 22, 331--334 (1975) 9 by Splqnger-Verlag 1975 Somatosensory Evoked Potentials and Magnitude of Perception D. JOHNSON, R. JORGENS, (]-. KO~qGEHL and H. It. KOI~NHU~E~ University of Ulm, Ulm (FRG) Received December 15, 1974 Summary. With step indentations of the index finger tip in randomized order, via a mechanostimu]ator, the tactile receptors of human skin were adequately stimulated. Recording the EEG over the contralateral and ipsilateral cortex, the evoked potentials and their 95% confidence limits were analysed. Simultaneously the psychophysical magnitude estimations were analysed. 1. The perceptual estimations were linearly related to step amplitude. 2. The early components of the E.P. show no obvious correlation to stimulus amplitude. 3. The later components (with peak latencies of 120 msec or more) show a monotone, non-linear rising function with respect to stimulus amplitude. 4. The early waves of the evoked potentials up to about 120 msec are well localized over the contralateral postcentral hand area while the late components resemble the alpha rhythm in wave length and distribution over both hemispheres. The possible role of alpha-synchronisation in the later components is discussed. Key words: Somatosensory - - Evoked potentials - - Psychophysics - - Merkel discs - - Man During the last decade, the late components of cortical evoked potentials (E.P.) with peak latencies of 120 ms or more have been considered to reflect the neural processes underlying perception. Power function relationships have been found between response and stimulus intensity with the same exponent for the late components of the E.P. and the psychophysical estimations (Keidel and Spreng, 1965; FranzSn and Offenloch, 1969). We have investigated the somatosensory system, in which the physiological bases are well understood from the peripheral receptor up to the cortical neurons. Based on their different frequency response characteristics, we can nearly selecti- vely stimulate three receptor sets in the glabrous skin of the human finger tip. The only receptor type with large myelinated fibers able to discriminate steps of different indentation magnitudes is the slowly adapting Merkcl cell (Mountcastle, 1968; Kornhuber, 1972). These receptors are known to have a strictly linear stimulus-response relation ()/fountcastle et al., 1966). The cortical neurons in the hand area of Rhesus postcentral gyrus with Merkel afferents also show the same linear response (Mountcastle, (1968). We have tried to establish the quantitative relationship between the evoked potentials and the magnitude of perception for the step indentation stimulus. 332 D. Johnson et al. Methods With 9 normal, awake human subjects, a step indentation of the finger tip was used as the adequate mechanical stimulus for the S.A. Merkel receptors. Randomized stimulus inten- sities were applied via a mechanostimulator (Burchard et al., 1967) which drove a plexiglas probe (1 mm contact diameter). Four different magnitude categories were given per experi- ment, with 128 randomized repetitions. The indentation was automatically controlled and the course of the applied force was simultaneously recorded. The start position in relation to the skin was also automatically reset prior to the stimuli. Auditory artefacts were masked in all cases. Besides the Merkel discs, the other receptors will also respond to the stimulus but only with one or two action potentials and thus could only play a minor role in perception of magni- tude (Talbot et al., 1968). The stimulus was maintained throughout the response to avoid overlap of "off-effects". The rise time of the step with the finger on the probe was 5 msec for the smallest (10O/2m) and 12 msee for the largest (1.000/zm) stimulus. The cerebral evoked potentials were recorded monopolarly over the hand area of the post- central gyrus of both hemispheres (reference electrode at joined ears) from the scalp with standard EEG electrodes and off-line computer averaging. Via reverse tape analysis and after elimination of artefacts the E.P.'s were averaged and the 95O/o confidence limits were calculat- ed for each recording (Decus program 12--98). The baseline prior to the stimulus, as an amplitude reference level, was rejected and only peak-to-peak measurements were used. Simultaneously, the psychophysical estimations from the same experiments were analysed. (Method of absolute Judgement). Results As can be readily seen from Fig. 1, the psychophysical stimulus-response relation is linear under these experimental conditions. In contrast to this and to the peripheral receptors, to the cortical neurons and to perception, the evoked potential components, show no linear relation between their amplitudes and stimulus magni- tude (Fig. 2). The earliest waves (N25-P59) show no relationship at all to stimulus magnitude. The differences in waveform of these early components, as compared to those of other authors, may be adequately explained by the different stimuli "ABSOLUTE" JUDGEMENT f 9 SUBJ. RI. iNDEX FINGER TIP 100 350 600 850 ,,u SKIN INDENTATION Fig. 1. Psychophysical perception curve. Estimated magnitude of stimuli, versus step skin indentation, normalized average of 9 subjects Somatosensory Evoked Potentials and Perception 333 A N158 i [IO.uV 50 msec -L:2sE P319 P 59 N=128 STEP INDENTATION I =950 Mm PEAK pV TO PEAK AMPLITUDE P 59 - N 158 15 10 N lS8 - P206 N 245- P 319 5 R 206 - N 245 ~ N 25-P 59 200 400 600 800 10()0 # m L DEPTH OF FINGER TIP iNDENTATION Fig. 2. (A) Averaged evoked potential following 128 step indentations of the right index finger tip from one typical subject. Monopolar recording, contralateral, posteentral hand area versus joined ears. Surface negativity upwards. Peak latencies of the components are given in msec SE Standard error. The time course of the stimulus is shown below. (B) Amplitude curves of the different components of the somatosensory evoked potentials for the same subject. The vertical bars give +2 x standard error, calculated from the 128 responses for each stimulus magnitude used. The later waves, however, do show a systematic relationship to the stimulus at the 95% significance level as seen in Fig. 2, but not a l inear one. All st imulus response curves had approx imate ly the same form and variance. P lott ing stimulus force as abscissa, instead of indentat ion, produces no change in form of the curves. The peak latencies of all waves decrease with increasing st imulus intensity, but only to a min imum by about 300 #m indentat ion. The late components, with onset greater than 120 reset, are found bi lateral ly over the whole scalp while the early components are str ict ly contralateral . The late components have ampl i tudes in- creasing from preccntral towards occipital. Discussion Although the magnitude perception shows the same linear relat ion to stimulus intensity as do the cortical neurons, the E. P. -eomponents do not reflect this simple relation. Actual ly, a l inear relat ion of the evoked potent ial ampl i tude, recorded 334 D. Johnson et al. from the scalp, to single unit firing frequency in the cortex should not be expected because of the neuronal and physical complexity. The late components in the somatosensory E .P . have a typical frequency of about 10 Hz (approximately alpha frequency) and are widely distr ibuted over the cortex with increasing ampl i tudes towards the occipital lobe as do those of the visual E .P . whereas the audi tory E .P . has a frequency of about 5 Hz, that is the first subharmonie of the alpha rhythm. From clinical cases, we also know that the occipital and frontal lobes as well as the whole ipsi lateral hemispheres are not required for tact i le per- ception. Due to their frequency and distr ibut ion we suspect these late components may represent a non-specific synchronisation of thalamo-cort ical alpha generators and are probably not direct ly related to sensation. A possible significance for perception of some ampl i tude components hidden in the late waves is, however, not contradicted by this hypothesis. Acknowledgement: The authors wish to express their appreciation for invaluable assistance and advice to Mr. W. Becker. References Burchard, D., Kapp, H., Kornhuber, H. H. : Ein Kraft- und Weg-kontrolliertes mechanisches Reizgerat fiir Untersuchungen der somatischen Sensiblititi~t. Pfliigers Arch. ges. Physiol. 297, 99 (1967) Franz~n, 0., Offenloch, K. : Evoked response correlates of psychophysical magnitude estimates for tactile stimulation in man. Exp. Brain Res. 8, 1--18 (1969) Keidel, W.B., Spreng, M.: Neurophysiological evidence for the Stevens power function in man. J. acoust. Soc. Amer. 38, 191--195 (1965) Kornhuber, H.H. : Tastsinn und Lagesinn. In: Gauer, Kramer, Jung (Eds.): Physiologie des Menschen, Vol. 11, S. 52--112. Miinchen: Urban und Schwarzenberg 1972 Mountcastle, V.B. (Ed.): Medical Physiology, 12th Ed., Vol. II. St. Louis: C.V. 1VIosby 1968 Mountcastle, V. B., Talbot, W. It., Kornhuber, H. H. : The neural transformation of mechanical stimuli delivered to the monkey's hand. In: De Reuck and Knight (Eds.) : CIBA Foundation Symposium on touch, he~t and pain, p. 325--345. London: Churchill 1966 Talbot, W.H., Darian-Smith, I., Kornhuber, H.H., Mountcastle, V. B. : The sense of flutter- vibration. J. Neurophysiol. 31, 301--334 (1968) D. Johnson University of Ulm Sect. Neurophysiology D - 79 Ulm Federal Republic of Germany