Far-field Potentials Research Articles

Subcortical, thalamic and cortical somatosensory evoked potentials to median nerve stimulation

Subcortical and cortical somatosensory evoked potentials (SEPs) to median nerve stimulation were studied in 16 normal controls, 3 patients with Parkinson's disease, and 2 patients with thalamic lesions. Multiple electrodes were placed over the scalp and vervical spines and connected with a hand electrode or Fz in grid II. Thalamic SEPs were recorded directly from the Vim nucleus in 3 patients with Parkinson's disease during the stereotaxic operation. SEPs recorded from the scalp-hand derivations were composed of 4 negative (N9, N11, N16, N18) and 3 positive potentials (P8, P10, P12), whereas, at the scalp-Fz electrode, only one negative peak was identified ( N20 ). N18 is of higher amplitude on the parieto-occipital areas and corresponds to N20 . On the other hand, N16 can be identified more clearly on the fronto-central areas at scalp-hand derivations because of intermixture with N18 on the posterior head areas and disappears at scalp-scalp derivation. This suggests that N16 represents a subcortical component that is picked up as a far-field potential at the scalp electrodes. Components preceding N16 at scalp-hand derivations are also interpreted as far-field potentials because of their short latency and wide distribution over the scalp. The peak latency of N16 is not significantly different from the major negativity recorded directly from the thalamus, whereas N18 ( N20 ) occurs significantly later. From this we conclude that N16 is most probably generated in the thalamus with the potentials of shorter latency originating caudal to the thalamus and N18 rostral to the thalamus.

Stationary peaks from a moving source in far-field recording

In 20 radial nerves, referential recording of antidromic sensory potentials from the digits consisted of two stationary far-field peaks, P I-N I and P II-N II. When compared with a bipolar recording of a moving source, the onset of P I and P II was coincident with the sensory potential approaching the wrist and the base of the digit, respectively. Of the two stationary positive peaks, P I was identical in latency irrespective of the digits tested. In contrast, the latency of P II was smaller for the first digit than the others, reflecting different distances from the stimulus point to the base of respective digit. The amplitude of P II was proportional to that of the propagating sensory nerve action potential recorded at the volume conductor junction. A bipolar recording registers a near-field potential over the sensory fibers along the length of the nerve. In contrast, a referential recording represents a mixture of the near- and far-field potentials with the latter frequently producing major alterations of the classical triphasic wave. The standing far-field peaks show a temporal relationship to the traveling volleys approaching the main borders of the volume conductor. At the moment the sensory impulse reaches the boundary, current density changes suddenly in the two adjacent conducting media, giving rise to an apparent standing potential. The same mechanism probably plays an important role for the generation of some of the short latency peaks in the scalp recorded SEPs.

Questions regarding the sequential neural generator theory of the somatosensory evoked potential raised by digital filtering

Digital bandpass filtering (300–2500 Hz) designed for zero phase shift was applied to somatosensory evoked potentials recorded with cephalic bipolar montages. Four consistent negative and corresponding positive peaks with latencies of about 16, 18, 19, and 20 msec were elicited with median nerve stimulation. Peroneal nerve stimulation also elicited 4 reproducible negative-positive peaks having latencies of about 24, 26, 28, and 30 msec. Interpeak latencies measured 1.3 ± 0.2 msec and 1.8 ± 0.25 msec for median and peroneal elicited SEPs respectively. Becaise cephalic bipolar recordings cancel most far-field potentials, multiple generators cannot account for all the additional components seen. It is hypothesized that some of the high frequency components recorded are due to activity in recurrent intrathalamic neuronal networks.

Origins of tooth pulp-evoked far-field and early near-field potentials in the cat.

Electrical stimulation of the mandibular canine tooth pulp in barbiturate-anesthetized cats activated myelinated A-beta-fibers in the inferior alveolar (dental) nerve and evoked a complex pattern of short-latency potentials that were recorded epidurally from the cortex. Three far-field components ( FFPs designated I, IIa,b, and IIIa,b) and two early near-field components ( ENFPs designated P1 and N1) were identified from computer-averaged potentials recorded over the contralateral lateral sigmoid gyrus or ipsilateral presylvian /anterior coronal gyri. Several corroborative lines of evidence indicated that tooth pulp-evoked FFPs and ENFPs originate from the following generator sources: I, inferior alveolar (dental) nerve/semilunar (gasserian) ganglion; IIa,b, trigeminal lemniscal fibers from the principal (main) sensory nucleus; IIIa,b, thalamocortical fibers from the thalamic ventral posteromedial nucleus; P1, N1, cytoarchitectural area 3 on the banks of the coronal or orbital sulcus. The latencies and waveforms of volume-conducted, surface-recorded potentials were not significantly different from those recorded at putative generator sites along the trigeminal lemniscal projection system. The short latencies and distribution of surface- and depth-recorded tooth pulp-evoked potentials suggested bilateral, trisynaptic paths from peripheral nerve to cortical neurons. Amplitude measurements of FFPs and ENFPs recorded at the cortical, epidural surface were compared to calculated values based on a mathematical model of two concentric spheres of inhomogeneous media in which a single equivalent dipole is radially oriented at some distance from the center. The morphology and distribution of FFPs and ENFPs are discussed in relationship to source geometry. Surface- and depth-recorded tooth pulp-evoked potentials had recovery functions that were covariant with rates of stimulation. In addition, stimulation of putative generator sites along the trigeminal lemniscal system reproduced portions of the tooth pulp-evoked surface-recorded potentials. Selective ablation of trigeminal cortical area 3 abolished ENFPs and lesions of lemniscal brain stem sites abolished FFPs . Disruption of extralemniscal brain stem sites had no effect on the surface-recorded evoked potentials examined. These findings and other supporting evidence from previous studies imply that the projection of fast-conducting tooth pulp input to the cortex may subserve a nonnociceptive modality and participate in sensorimotor integration.

The cervical somatosensory evoked potential in man: Far-field, conducted and segmental components

Somatosensory evoked potentials (SEPs) to median nerve stimulation were recorded from neck and scalp electrodes in 23 normal adults using cephalic and non-cephalic (knee) references simultaneously. With a cephalic reference, the neck SEP consisted of several ‘negative’ potentials that had the same latency at all recording locations. Simultaneous recordings from neck-scalp, neck-knee and scalp-knee derivations demonstrated that scalp far-field potentials significantly contributed to neck-to-scalp recordings and obscured the cervical SEPs. With a non-cephalic reference, the neck SEP consisted of a prominent positive wave (P9) followed by a large negative component (N13). A small positive potential, P10, seen in the lower neck, gradually increased in latency and amplitude from lower to upper neck and appeared as a P11 potential at upper cervical levels. In lower neck recordings, a negative wave, N11, was also present and in some subjects exhibited a latency shift from lower to upper neck. P9, P11 and N11 had a short refractory period suggesting a presynaptic origin whereas N13 had a longer refractory period indicating a postsynaptic generator. The consensus that P9 originates in the peripheral nervous system is consistent with its rapid recovery cycle. The bipolar characteristics of N11 and P11 as well as their latency shift and their short recovery cycle suggest that they reflect activity in the cervical dorsal columns. N13, that displayed no latency shift and had a longer recovery cycle, may originate in spinal cord dorsal horn interneurons.

Scalp topography of the short latency somatosensory evoked potentials following posterior tibial nerve stimulation in man

The scalp topography of the short latency somatosensory evoked potentials (SEPs) to unilateral posterior tibial nerve stimulation at the ankle was studied by using a non-cephalic reference in 22 normal young adults. At least 3 components (P28, N31 and N32) were identified preceding the major positive peak (P36). The first 2 components had similar peak latency at all scalp electrodes, and were considered to be generated in deep structures. However, N32 was localized to the hemisphere contralateral to the side of stimulation. P36 was maximal at the midline foot sensory area, or at the contralateral parasagittal area, and its amplitude decreased more steeply anteriorly than posteriorly. The peak latency of P36 progressively increased from ipsilateral to the side of stimulation in the coronal plane. P36 occurred earlier in the somatosensory area, and increased in peak latency anteriorly. Generator source of scalp-recorded far-field potentials (P28 and N31) remains to be elucidated. N32 might reflect activities of the thalamo-cortical pathway or an initial cortical response. P36 appeared to be generated in the somatosensory foot area.

A Nonlinear Method for Predicting Unsteady Sheet Cavitation on Marine Propellers

A method is presented which provides a basis for predicting the nonlinear dynamic behavior of unsteady propeller sheet cavitation. The method separates the fluid velocity potential boundary-value problem into two parts, static and dynamic, which are solved sequentially in a forward time stepping procedure. The static potential problem is for the cavity fixed instantaneously relative to the propeller and the propeller translating through the nonuniform wake field. This problem can be solved by standard methods. The dynamic potential represents the instantaneous reaction of the cavity to the static potential field and thus predicts the cavity's deformation and motion relative to the blade. A solution is obtained for the dynamic potential by using the concepts of slender-body theory to define near-and far-field potentials which are matched to form the complete solution. In the far field, the cavity is represented by a three-dimensional spanwise line distribution of sources. In the near field, the cavity is approximated at each cross section as a semi-ellipse with unknown axes a(t), b(t), and position l(t) along the chord of the foil section. Conditions are derived that determine (a, b, l) by minimizing the square error in satisfying the dynamic boundary condition. These conditions yield the equations of motion of the cavity in the form of three coupled nonlinear second-order ordinary differential equations with time as the independent variable. The theory is presented for the general foil and not specifically for propellers. However, the method incorporates features in its formulation which facilitate its application to marine propellers. The method is demonstrated by using the steady noncavitating potential for the two-dimensional half-body as an approximation to the static potential. Both fixed and unsteady cavities are calculated. The unsteady cavities are calculated by varying the hydrostatic pressure in the half-body pressure field sinusoidally.

Far-field potentials as a method of monitoring acute and chronic spinal cord injury.

Far-field potentials (FFPs) were studied in monkeys to determine the utility of such responses in evaluating acute and chronic spinal cord injury. Monkeys (Macaca fascicularis) were anesthetized with 70% N2O 30% O2, immobilized with pancuronium bromide, and maintained on a respirator. Spinal cord transections were made at level T3-T4 and included bilateral dorsal columns or anterolateral columns, right or left hemisection, or central cord lesions. Percutaneous stimulation of both posterior tibial nerves was performed at a frequency of 3 Hz using 3-4 mA for 0.3 msec duration. Potentials were recorded from chronically implanted epidural electrodes (right ear reference) through a 300-3,000 Hz band pass filter. Each far-field potential represents the average of 256 individual responses recorded for a duration of 25 msec before and after cord lesion for up to 8 weeks. Seven reproducible components in the far-field potential could be identified. Following dorsal column transection all components were reduced in amplitude. After anterolateral column transection only latencies were altered. Right or left hemisections caused both attenuation of component amplitudes and latency alteration. Central cord lesions resulted in no detectable amplitude or latency disturbances. Control records likewise showed no changes. No major alterations, following day 1 posttransection, were observed, attesting to the relative stability of far-field potentials over long periods of time. Far-field potentials therefore may be useful in monitoring spinal cord injury.

The low-frequency limit for wave-power systems

Relations are derived for the maximum power absorbed by a system of devices in the low-frequency or long-wavelength limit. A Fourier series is used to represent the far-field radiation potential, and the number of terms in the Fourier series determines the maximum output of the system subject to the restriction of optimum motions. For an array with N devices, or a single device with N separate degrees of freedom, the maximum output is a function of N and the angle of the incident waves.

Short-latency somatosensory evoked potentials to median nerve stimulation: components N 13--P 13, N 14--P 14, P 15, P 16 and P 18 with different recording methods.

Comparative recordings of short-latency somatosensory evoked potentials with midfrontal (Fz), ear and non-cephalic reference electrodes were obtained in 12 normal subjects to define the site of the generators of the different components and the spatial distribution and interference of the different components recorded at the neck and scalp. Such investigations provide the basis for identical investigations in patients with focal lesions of the central nervous system. The N 13 recorded from the neck (C2-Fz) and the inconsistently recorded component N 14 were quite similar in latency to the far-field potentials P 13 and P 14 recorded at the scalp (C3', C4'--ear or hand). The component P 15 was clearly visible only in scalp-Fz leads and occurred 0.2--0.9 ms (mean 0.5 ms) later than P 14 in ear or hand reference recordings. This is evidence against the identity of these two components proposed by others. In a larger group of 48 subjects, using an exclusive Fz-reference lead, normal values were established of the components P 15, P 16 and P 18, as well as their scalp distribution.

Far-field somatosensory evoked potentials after stimulation of the tibial nerve.

In 21 normal subjects, far-field somatosensory potentials were recorded from the scalp after stimulation of the tibial nerve at the ankle (tibial SEP). With the use of a knee reference contralateral to the side of stimulation, the tibial SEP consisted of three major positive peaks, P17, P24, and P31, and three additional but inconsistent components, P11, P21, and P27. Presumable generator sources of the tibial SEP are the popliteal fossa for P11, entry to the sacral plexus for P17, the cauda equina for P21, entry to the conus medullaris for P24, the rostral spinal cord for P27, and the brainstem for P31.

The somatosensory evoked potential.

Three decades have elapsed since Dawson (1947) recorded the first somatosensory evoked potential (SEP). Simple superimposition of individual responses was possible because the patient had progressive myoclonic epilepsy. In this disease the SEP amplitude is much enhanced (Shibasaki et al, 1978; Kelly et al, 1981). Subsequently Dawson (1951, 1954) presented his averager to the Physiological Society, thereby initiating the present-day explosive growth of evoked potentials.SEPs are made up of components with varying latencies. The components are best identified by latency and polarity as recorded at the scalp (P = positive and N = negative). Nevertheless, the nomenclature of somatosensory evoked potentials can be extremely confusing, mainly because the same component can have a different polarity depending on the electrode montage used. Generally speaking (but this is not a firm rule), far-field (subcortical) potentials are positive in polarity when a non-cephalic reference is used, whereas these same components have a negative polarity when the reference is on the scalp. It is therefore useful to always indicate the recording montage being employed. In addition, use of absolute latencies in the terminology can cause confusion because they are dependent upon length and body height. For example, the brachial plexus component usually occurs at about 9 msec, but may extend to as long as 11 or more msec in a very tall individual. Subsequent components then become difficult to identify in relation to normal means.

Origins of cat somatosensory far-ffield and early near-field evoked potentials

A complex pattern of potentials evoked by forelimb cutaneous nerve stimulation was recorded from the skull surface of barbiturate anesthetized cats and was resolved by computer averaging. Seven far-field components (designated I–VII) and several larger early near-field components (designated P 1, N 1, P 2, N 2 and P 3) have been identified from averaged potentials. All far- and early nearfield components were elicited by activation of myelinated nerve fibers. Their latency and amplitude depended entirely on the number of large myelinated nerve fibers recruited into the nerve volley. Spinal lesions showed that peripherally evoked far- and early near-field potentials were generated from both lemniscal and extralemniscal sources. Far-field and early near-field potentials were evoked either from the lemniscal system by dorsal column stimulation or from the extralemniscal system by cutaneous nerve stimulation after dorsal column lesions. These potentials were similar in configuration to those evoked by cutaneous nerve stimulation in the intact cat. Sequential rostral to caudal ablations and lesions within the lemniscal or extralemniscal system eliminated the potentials selectively. The following sites of origin were proposed: I — peripheral nerve; II (IIa) — dorsal columns; IIb — spinothalamic tract; III — dorsal column nuclei or medial lemniscus; III, IV — spinocerebellar and spinoreticular tracts; V — lateral reticular nucleus or reticulocerebellar tract; VI, VII — cerebellum; P 1— lateral and medial thalamic nuclei or thalamocortical projection; N 1, P 2 — sensorimotor cortical areas; N 2, P 3 — association cortical areas. Lemniscal and extralemniscal far- and early near-field potentials were generated from common as well as separate sites. The latencies of somatosensory far- and early near-field potentials recorded extracranially in this study closely correlated with the latencies of local potentials recorded by others at the proposed lemniscal and extra-lemniscal sites of origin. The contribution of extralemniscal sources to far- and early near-field potentials and their importance to clinical measurement of somatosensory evoked potentials and diagnosis of neurological disorders are discussed.

Brain-stem auditory-evoked potentials recorded directly from human brain-stem and thalamus.

Brain-stem auditory-evoked potentials were recorded in neurosurgical patients from surface electrodes applied to the VIIIth nerve, medulla, pons, midbrain and cortex; from depth electrodes in the thalamus; and from a movable electrode in the IVth, IIIrd, and lateral ventricles. The potentials recordable over the scalp within 10 ms after click stimulation are characterized by a slow positive wave (peaking at 5 to 6 ms) and a negative wave (8 to 10 ms) with 7 small positive wavelets superimposed upon them. The sources of these components have been identified by observing their increased amplitude in depth recordings, and by tracing the potentials from their intracranial maxima to the scalp. Wave I is generated within the most distal portion of the VIII nerve; Wave V in the midbrain (inferior colliculus); and Wave VI the medial geniculate body. Both low positive and negative components have their origins in the inferior colliculus. Intracranially-recorded brain-stem auditory-evoked potential showed very rapid changes in amplitude within the brain-stem but only slight changes in the more rostral regions, although their amplitude gradients varied in the different components. They also demonstrated minor but systematic shifts in latency with distance from the potential sources, reflecting a significant overlap of separate potentials. This effect must be taken into account in the interpretation of a 'concurrent' intracranial potential as the source of a far-field surface-recorded potential.

Non-cephalic reference recording of early somatosensory potentials to finger stimulation in adult or aging normal: differentiation of widespread N18 and contralateral N20 from the prerolandic p22 and N30 components

Prerolandic and parietal SEPs to electrical stimulation of fingers or median nerve were studied with non-cephalic reference in 40 normal young adults and in 35 healthy octogenarians. Limb temperatures were 36–37°C. Intersubject variations of SEP components were analysed. A new widespread components N18 was identified and shown to be generated below the cortex. This N18 is about the only early component recorded at the parietal ipsilateral region after the positive far-field potentials P9, P11 and P13–P14. Transit times along the central somatosensory pathway were replicated and discussed as well as other evidence about the sequential activation of the various neural structures involved. The N20 potential representing the earliest cortical response is recorded from the contralateral parietal region, but is absent ipsilaterally. The prerolandic potential is related to distinct generators and is elicited by a separate thalamocortical pathway rather than by corticocortical connections from areas 2 and 5 in parietal cortex. The changes associated with normal aging have been confirmed and extended.

Origin of far-field subcortical potentials evoked by stimulation of the posterior tibial nerve

Evoked potentials to stimulation of posterior tibial nerves were recorded from cervical-scalp derivation (cervical electrode: surface electrode on spinal process of the fifth cervical vertebrae; scalp: vertex). Four subcortical potentials labeled as N24, P27, N30 and P32 were identified. Evidence indicating that these components are originated in the following structures is presented: N24, posterior columns at the level of C5; P27, high cervical posterior columns or brain stem; N30, medial lemniscus or thalamus; P32, thalamo-cortical radiations.

Acute effects of electroconvulsive therapy on brain stem auditory-evoked potentials

The acute effects of electroconvulsive therapy on brain stem auditory evoked potentials were investigated in 6 psychiatric inpatients. Even during the induced seizure, no significant effects were noted, a finding consistent with the stability of these far-field potentials.

Somatosensory evoked potentials elicited by mechanical and electrical stimulation of each single pain or tactile spot of the skin.

Single pain and tactile spots on the dorsum of the right hand in man were stimulated by electric pulses and mechanical taps using a needle to a pain spot and a horse tail bristle to a tactile one. Somatosensory evoked potentials (SEPs) and far field potentials (FFPs) were observed in three volunteers by averaging of 200 or 400 samples of responses recorded from scalp points corresponding to the left and right somatosensory areas (LSA and RSA). For SEP measurements, the difference between responses at LSA and RSA was obtained in order to specify the optimum response at LSA. The patterns of SEPs elicited by mechanical or electrical stimulation to a pain spot were similar to those to a tactile one. The typical SEP to mechanical stimulation consisted of N23, P31, N40, P49, N64, P87, N114 and P147, while that to electrical stimulation did of N24, P33, N42, P52, N66, P95, N122 and P156. Consequently, mean peak latencies of the later components in SEPs elicited by mechanical stimulation were earlier than those to electrical one. The SEP amplitudes for mechanical stimulation were 1-3 microV and were larger than those to electrical one. By mechanical stimulation FFPs could not be obtained, while by electrical stimulation FFPs in peak latency of approximately 14 msec (P14) were seen. No specific components to sensory modality of pain or tactile sensation were observed either in SEPs or in FFPs.

Erprobung einer K�rperschallsonde zur Evozierung von Hirnstammpotentialen im Tierexperiment

A new procedure of evoking brainstem potentials shall be introduced. Contrary to evoking brainstem potentials by acoustic stimulation via earphone or loudspeaker we touch the middle ear apparatus at various locations using a special mechanical vibrating system thus evoking movements within the sound-conducting structures. As a reaction to this kind of stimulation far-field brainstem potentials can be recorded; the intraoperative control of sound transmission to the inner ear is ascertained by these potentials' latency and amplitude. In order to apply the above procedure in tympanoplasty we have made a number of recordings after having simulated various kinds of middle ear pathology in the guinea pig.

Auditory-evoked far-field potentials in myelin deficient mutant quaking mice

Auditory evoked far-field potentials were recorded from mature quaking mice and their littermate controls. There was a significant prolongation in the latencies of all FFP peaks in the quaking mutants. Examination of the interaction between group (normal, quaking) and for selected pair-wise peak comparisons showed that the degree of retardation was not uniform for each FFP peak. In view of the reported histological and biochemical findings that the CNS of the quaking mutant is specifically deficient in myelin, we conclude that slowed conduction in the quaking mouse brain is due to myelin deficiency and that non-uniform retardation of each FFP peak reflects regional variation in the extent of myelin deficiency. These data suggest that the changes in interpeak latencies of far-field potentials may be useful in identifying the demyelinated region(s) of the diseased central nervous system.