Can somatosensory system generate frequency following response?

The aim of this study was to establish whether functional characteristics of the somatosensory system structures in man comply with the frequency following response (FFR) generators. Somatosensory cerebral evoked potentials (SsCEP) were recorded by skin electrodes, and spinal somatosensory evoked potentials (SpEP) both by epidural and skin electrodes. In SpEP and SsCEP to trains of electrical or mechanical stimuli, a decrease of the amplitude to subsequent stimuli was found. SpEP were also attenuated by higher stimulation rates. It is highly improbable, therefore, that somatosensory system can contribute to the FFR-like response recorded in profoundly deaf people.

A noninvasive method of neurography in meralgia paraesthetica.

The lateral femoral cutaneous nerve (LFCN) becomes superficial 10 cm distal to the anterior superior iliac spine, where it can be located and stimulated by superficial electrodes. This is not the case in the inguinal region. In the present study the LFCN compound nerve action potential (NAP) was recorded with a pair of 8-cm-long strip electrodes placed on the anterolateral aspect of the thigh 25 cm distal to the stimulating electrodes. Normative values were obtained in 58 healthy nerves. The conduction velocity (CV) was 62.3 +/- 5.5 m/s for NAP onset and 55.3 +/- 4.1 m/s for the negative NAP peak. The CV variability was comparable to that obtained with needle recordings despite a relatively low NAP amplitude (2.0 +/- 1.0 muV). This method provided definite neurophysiological evidence of the disorder in 12 of 13 patients with meralgia paraesthetica. According to our results, a slowing of CV is a more reliable sign of the condition than a decrease of NAP amplitude alone.

Measurement of light touch perception threshold by contingent negative variation.

An objective method developed to measure the threshold of light touch perception using contingent negative variation (CNV) is described. The light touch stimulus was a slight indentation of the skin produced through a displacement controlled stimulating probe (tip diameter of 2 mm). It was applied as the conditioning (S1) stimulus of the classical CNV paradigm of S1, S2, and R. To increase the CNV amplitude, the S2 stimulus was either a red or a yellow LED. The subjects were required to respond to only one of two by pressing a button. When the light touch stimulus was perceived, the CNV was recorded in all 19 healthy adult volunteers. In 14 of them, a systematic determination of the threshold of the ball of the thumb and index finger, thenar, hypothenar, face, shoulder, anterior thigh, foot dorsum and great toe ball, have been done. The thresholds of the light touch perception as defined by objective CNV measurement are very close to the results of the subjective psychophysiological determinations in normals. The amplitude of the averaged CNV (12 responses) started to decrease when stimulus intensity was reduced to the point that the subjects were able to perceive only a fraction of the presented touch stimuli. We believe, therefore, that the objective CNV determination of the light touch perception threshold is precise and sensitive enough to be used in research as well as in clinical applications.

Epidurally recorded cervical spinal activity evoked by electrical and mechanical stimulation in pain patients.

Spinal SEPs to electrical and mechanical stimulation of the upper limb of the non-painful side in 7 pain patients were recorded from the cervical epidural space. In response to electrical stimulation of the median nerve, the longitudinal distribution of the spinal postsynaptic negativity (N13) along the cord had a distinct level of maximal amplitude at the C5 vertebral body. When recorded at increasing distances cranial or caudal to this level, the latency of N13 was successively prolonged, in agreement with a spread-out near-field generator in the dorsal horn. Similar patterns of distribution and levels of maximal amplitude were demonstrated for the N13 wave evoked by electrical stimulation of the ulnar and thumb nerves as well as by mechanical stimulation of the thumb ball. The amplitude ratios of the N13 waves evoked by electrical stimulation of the median nerve and the thumb nerves, and by mechanical stimulation of the thumb ball were 3.9 to 1.4 to 1. The slow positive wave (P18), which has been assumed to represent recurrent presynaptic activity, had a somewhat different distribution, with a lower maximal amplitude and a less marked falling off in amplitude along the cord, as compared to the N13 component. The initial presynaptic positivity (P10) appeared with an almost constant amplitude along the cord. Tactile stimuli produced responses with considerably longer latency and duration than those obtained with electrical stimulation. There seemed to be a non-linear relationship between the amplitude of the response and the depth of skin indentation. The presented data contribute a more detailed picture of epidurally recorded spinal SEPs than previous studies. They will serve as a reference for further analysis of SEPs evoked by stimulation of the affected side in pain patients, to explore whether the painful state is associated with altered SEPs before or after therapeutic intervention.

Epidurally recorded cervical somatosensory evoked potential in humans.

Three slow wave components, P10, N13 and P18, can be seen in the cervical somatosensory evoked potential (CSEP) in response to median nerve stimulation recorded by an electrode in the epidural space at the dorsal aspect of the cervical spinal cord referenced to an electrode at the suprasternal notch. In the region of high CSEP amplitude, which extends over several cervical segments, the peak-to-peak amplitude is more than 10 microV, permitting observation of the CSEP slow waves in single, unaveraged records. The CSEP to finger nerve stimulation had a similar wave form and the same latencies (referred to the Erb's potential) as the CSEP to median nerve stimulation. The P10 activity is of presynaptic origin; it is generated in the brachial plexus, spinal roots and terminal branches of the primary sensory fibers. The N13 slow wave is of postsynaptic origin; however, the small wave on the ascending phase of this main postsynaptic component represents superimposed presynaptic activity. In bipolar epidural recordings, 3-5 fast waves are superimposed on the slow CSEP waves, which are of lower amplitude than the slow waves in unipolar recordings. The fast waves show a slight but progressive delay at the more rostral recording sites and are present even with high frequency stimulation, presumably reflecting activity in long ascending tracts. The surface recorded CSEP to median nerve stimulation is 4-7 times lower in amplitude than the CSEP in unipolar epidural recordings. The small wave on the ascending phase of N13 and the N13 peak of the unipolar epidural recordings had the same latencies as the surface N11 and N13 peaks.

Maturation of cortical potentials evoked by tibial-nerve stimulation in newborns, infants and children aged four and eight years.

This study traced changes in cortical activity (SsCEP) evoked by electrical stimulation of leg nerves during the period of fast morphological and functional development of the nervous system from birth to eight years of age. The study revealed complex waveform changes in the SsCEP during this period. At birth low-amplitude potentials with well-defined, simple, three-phasic waveform (P1, N2P2N3, P3) were present in only eight of 26 newborns: no SsCEP could be detected in 13 cases. At one year the SsCEP had a higher amplitude and the simple three-phasic waveform was seen in the majority of cases (15 of 22). At four years an SsCEP with a notch in the middle negative wave was the most common waveform, observed in nine of 18 cases. At eight years adult-like SsCEPs prevailed in the records: they showed well-defined peaks P1 (corresponding to P40 of adults), N2, P2 and N3 (probably corresponding to N75 of adults). Such waveforms were observed in 16 of 24 children. In newborns P1 had a latency of 37 . 5 +/- 2 . 54ms: at one year the latency decreased to 32 +/- 3 . 81ms and increased again in older children, corresponding to increasing body-length.

Auditory potentials in profoundly deaf subjects.

In 12 healthy volunteers, and 5 prelingually and 5 postlingually profoundly deaf subjects, a 500 Hz filtered click stimulus was applied to the mastoid via the bone conductor. In all the healthy volunteers, middle latency response (MLR) was recorded. Among all profoundly deaf subjects, MLR was recorded only in 2 with prelingually deafness; these two subjects were also the only ones with normal vestibular function. The possible mechanisms of the MLR appearance in deaf subjects are discussed.

Frequency following response evoked by vibratory stimuli in profoundly deaf subjects.

A vibratory stimulus at a frequency of about 100 Hz was transmitted through a bone conductor on the mastoid in 26 profoundly deaf subjects. In all of them, the function of the vestibular apparatus was tested by means of the Fitzgerald-Hallpike caloric test. In some of the deaf subjects, evoked potentials were recorded, which following the frequency of the stimulus and which according to amplitude and latency characteristics, corresponded to the frequency following response (FFR). A close correlation was found between the normal function of the vestibular apparatus and the appearance of FFR in deaf subjects. The possible receptors of the stimulus and the afferent pathways which generate the recorded response are discussed.

Frequency-following response evoked by acoustic stimuli in normal and profoundly deaf subjects.

A low-frequency acoustic stimulus was applied to the right mastoid, the right acromion and the distal phalanx of the right index finger in 10 healthy and 10 profoundly deaf subjects. The stimulus, which had a frequency of 80-120 Hz, was delivered by means of a special vibrating system, constructed for this purpose. The frequency-following response (FFR) was recorded. In all the healthy subjects, FFR appeared with a latency of about 6 ms from the stimulus onset and with a peak-to-peak amplitude of 0.3-1.2 microV. Also, in all the deaf subjects, a change in bioelectrical activity was recorded, which reproduced the stimulus frequency at a latency of about 6 ms and with a peak-to-peak amplitude of 0.3-1.2 microV. Afferent pathways by which the stimulus could be transmitted in deaf subjects are discussed.

Contingent negative variation audiometry in children.

The aim of the present work was to establish whether contingent negative variation audiometry (CNV-A) is applicable to children. In a group of 23 children aged 5-7 years, only 10 generated clearly recognizable CNV when tested with the method successfully used in adults. When the procedure was modified by prolonging the S1-S2 interval and by introducing attractive slides to serve as the S2 stimulus and by adopting a slower repetition rate, 9 children randomly selected from the former group generated high-amplitude CNV (10.1 +/- 4 mu V). The CNV-A measurements involving a longer auditory stimulus (S1), lasting nearly to the beginning of S2, and an even slower repetition rate were equally successful in 18 children aged 3-5 years, who generated CNV with an average amplitude of about 9 mu V (range 5-15 mu V). We believe that the basic problem of successful CNV recording in children is to attract their attention to the signals of the CNV paradigm. The child's attentiveness decreases rapidly. The mean difference and the absolute mean difference between the subjective hearing threshold for white noise and the perception threshold for white noise as determined by CNV-A were as follows: 8.8 +/- 8 dB (both values) for the older group, and 3 +/- 10.4 and 8.6 +/- 6.5 dB, respectively, for the younger group. These differences are quite comparable to those obtained in adults. We therefore believe that CNV-A, used in combination with the behavioral method, provides a most reliable estimate of the child's hearing threshold in dubious situations.

Somatosensory perception and cortical evoked potentials in established paraplegia.

In 66 patients who suffered severe spinal cord injury 7 months to 28 years previously, somatosensory cortical evoked potentials were recorded to electrical stimulation of the leg nerves and compared to clinical assessment of light touch, pain, position sense and two-point discrimination. The patients were separated into 4 categories according to the degree of disintegration of the somatosensory evoked potential waveform. A clear correlation was found between the impairment of somatosensory perception and the deterioration of the somatosensory evoked potential in each group. However, it was not possible to observe any direct correlation between the sensory score or impairment of a single modality and somatosensory evoked potential changes, or among the impairment of single modalities on a case by case basis. This study indicates that the somatosensory evoked potential can be used to provide electrophysiological information independent of the clinical examination on functions of the dorsal columns in the chronic stage of spinal cord injury.

Distribution of scalp somatosensory potentials evoked by stimulation of the tibial nerve in man.

The scalp distribution of the response to stimulation of the tibial nerve at the medial malleolus was systematically analysed. The somatosensory evoked potential (SEP) was recorded with electrodes placed in a transversal line over the ipsilateral and contralateral postcentral gyri and in a sagittal line over the longitudinal brain fissure. The SEPs recorded over the ipsilateral hemisphere and along the sagittal line were similar to the F response (the response over the foot primary somatosensory region). Over the contralateral hemisphere the waveform of the responses changed obviously from point F to the point C (contralateral hand primary somatosensory region). The C response started with N37, P40 had a longer latency, N50 was not present and the subsequent waves were also considerably different. Mathematical simulation of the responses recorded from the electrodes between points F and C has shown that they represent an electrical algebraic summation of the activity over points F and C. Although the F and C responses may be 2 potentials arising from the opposite sides of a single dipole generator which is located in the medial fissure, it is more probable that the somatosensory evoked potential on tibial nerve stimulation reflects the activity of 2 separate generators.

Dependence of visual evoked potentials on change of stimulated retinal area associated with different pattern displacements.

An attempt was made to distinguish between the effects of the moving edges and the change in the area of stimulated retina on pattern shift visual potentials, elicited by checkerboard pattern displacements varying through 0.25, 0.50, 0.75 and full pattern reversal. Twelve healthy subjects were stimulated binocularly with a horizontally orientated pattern (whole field 17 degrees, check size 28' of visual angle). Four of them were additionally presented with a diagonally orientated pattern. Two sets of 32 averages were taken for each stimulus condition and peak-to-peak amplitudes P50-N70 and P100-N140 were measured. The change of the stimulated retinal area is a quadratic function of the diagonally orientated pattern. The change in amplitude of P50-N70 fits well with the curve of the area change, but that in the amplitude of P100-N140 does not. Each of the amplitudes also seems to have a different orientation of hypothetical dipole vectors.

The early negative potential evoked by stimulation of the tibial nerve in man.

The scalp response to stimulation of the tibial nerve at the level of the medial malleolus was systematically analysed. It was recorded 2 cm posterior to the vertex and at the sites corresponding to cortical representation of the hand. The existence of an early negative wave with a peak latency of 37.2 2.29 ms and amplitude of -0.69 0.40 micro V was established (being half the amplitude of the first positive wave (P40) over the vertex). This wave was named N37 in respect of the peak latency and polarity. N37 was the first event recorded after stimulation of the tibial nerve at this level as the onset latency was 32.2 1.75 ms and that of P40 over the vertex 33.8 2.28 ms. It was recorded with the highest amplitude over the hand primary somatosensory area after stimulation of the opposite foot. N37 evoked by stimulation of the tibial nerve at the ankle and N20 evoked by stimulation of the arm nerve are both the primary negativities of the evoked potential. However, N37 is not recorded with maximum amplitude over the leg primary somatosensory area and it is rounded and longer lasting than N20. In spite of these differences the two initial negative electrical phenomena are not necessarily generated by different functional structures. The possible generators of N37 are discussed.

Improved contingent negative variation audiometry.

In earlier reports we described contingent negative variation audiometry (CNV-A) and explored the accuracy of the method used in clinical conditions. Amplitude was found to be the factor which determines the accuracy of CNV-A. With this study we aimed to increase the amplitude of the CNV so as to improve the accuracy of CNV-A. The amplitude of the CNV is dependent on the content of the information of the S2 stimulus in the CNV paradigm. For this purpose we used a light stimulus (red or green) for the S2. The patient had to recognize the stimulus and act accordingly. The tests were performed on 12 volunteers in whom we simultaneously registered the CNV and the slow vertex response. The amplitude of the CNV was on average higher than with the previous CNV-A methods. The mean value of the absolute difference of the results between CNV-A and subjective tonal audiometry (ADG) was 6.9 +/- 5.5 dB. The mean difference between the results of both methods, however, was only 4.3 +/- 7.8 dB. The new CNV-A method is more accurate than the previous one. Nevertheless, we believe that not all possibilities for the improvement of the method have been explored. By taking into account the psychological factors which influence the amplitude of the CNV, and with a better computer analysis of the recordings, it will probably be possible to obtain even more accurate objective data on the threshold of perception of auditory stimuli with the aid of CNV-A.