Position Of Recording Electrode Research Articles

Optimizing the technique for eliciting antidromic sural and superficial fibular sensory nerve action potentials.

Lower limb sensory nerve action potentials are an important component of nerve conduction studies. Most testing of the sural and superficial fibular nerves involves antidromic techniques above the ankle, which result in a falsely unobtainable response in 2%-6% of healthy people. Cadaver, surgical, and more recent ultrasound series suggest this may relate to the site of fascia penetration of the nerve, and it is hypothesized that a modified technique may be more likely to produce reliable responses and reduce false-negative errors. This article evaluates a variety of recording distances for both nerves in 100 healthy controls, including varying recording electrode positions and techniques, to provide the optimal electrodiagnostic information in healthy control subjects. Shorter stimulation distances produce higher-amplitude responses but become confounded by increasing stimulation artifact at very short distances, with the best balance found at around 10 cm. In both sural and superficial fibular nerves, amplitude increases by approximately 10%/cm compared with the standard 14 cm distance. The Daube superficial fibular technique produced a higher amplitude than the Izzo Intermediate technique (by 22.46%, p < .001). The calculated upper limit of normal for side-to-side variation in amplitude was around 50% in the sural nerve but over 70% in the superficial fibular nerve. It is proposed that the 10 cm recording distance for both nerves is optimal, with minimal false-negatives and a higher amplitude elicited than with existing techniques.

Simultaneous serotonin and dopamine monitoring across timescales by rapid pulse voltammetry with partial least squares regression

Many voltammetry methods have been developed to monitor brain extracellular dopamine levels. Fewer approaches have been successful in detecting serotonin in vivo. No voltammetric techniques are currently available to monitor both neurotransmitters simultaneously across timescales, even though they play integrated roles in modulating behavior. We provide proof-of-concept for rapid pulse voltammetry coupled with partial least squares regression (RPV-PLSR), an approach adapted from multi-electrode systems (i.e., electronic tongues) used to identify multiple components in complex environments. We exploited small differences in analyte redox profiles to select pulse steps for RPV waveforms. Using an intentionally designed pulse strategy combined with custom instrumentation and analysis software, we monitored basal and stimulated levels of dopamine and serotonin. In addition to faradaic currents, capacitive currents were important factors in analyte identification arguing against background subtraction. Compared to fast-scan cyclic voltammetry-principal components regression (FSCV-PCR), RPV-PLSR better differentiated and quantified basal and stimulated dopamine and serotonin associated with striatal recording electrode position, optical stimulation frequency, and serotonin reuptake inhibition. The RPV-PLSR approach can be generalized to other electrochemically active neurotransmitters and provides a feedback pipeline for future optimization of multi-analyte, fit-for-purpose waveforms and machine learning approaches to data analysis.Graphical abstract

Piloting the recording of electrode voltages (REVS) using surface electrodes as a test to identify cochlear implant electrode migration, extra-cochlear electrodes and basal electrodes causing discomfort

Objectives To determine if Electrode Voltage (EV) measurements are potentially suitable as a test for detecting extra-cochlear electrodes in cochlear implants (CIs). Methods EV measurements were made using surface electrodes in live mode in 17 adult cochlear implant (CI) users. Repeatability, the effects of stimulation level, CI active electrode position, (active) recording electrode position and stimulation mode (for Nucleus devices) were investigated. Results/discussion Recordings made in monopolar mode showed good repeatability when the active recording electrode was placed on the ipsilateral earlobe; voltages increased linearly with stimulation level as expected. EVs for basal electrodes differed greatly between partially inserted/migrated devices, fully inserted devices with all electrodes activated, and those with deactivated basal electrodes [χ 2(2) = 10.2, p < 0.05 for the most basal electrode]. EVs for Nucleus devices were small for electrodes on the array when compared to those for monopolar return electrodes, except for the participant with extra-cochlear electrodes. We argue that fibrosis around the electrode array facilitated current flow across the round window in this case. Conclusion The test appears to be a viable approach to detect electrode migration and extra-cochlear electrodes in adult CI users and may also be sensitive to discomfort caused by current leakage from the basal end of the cochlea.

Optimal Trapezius Electrophysiological Recording Site.

Various active recording electrode positions for the trapezius motor nerve conduction study have been suggested. To determine the optimal recording site of the trapezius muscles in a spinal accessory nerve conduction study. Retrospective descriptive study. Department of physical medicine and rehabilitation in a tertiary clinic center. Thirty-four patients between 33 and 82 years of age with preoperative head and neck cancer (68 upper and middle trapezius muscles). Analysis of compound muscle action potentials (CMAPs) of upper and middle trapezius. CMAP latency and amplitudes were obtained at five and four recording points with constant intervals on the horizontal line of the upper and middle trapezius, respectively. The maximum CMAP amplitude of the upper trapezius was (mean ± SD) 8.5 ± 1.4 mV with the recording at the midpoint between the C7 spinous process and the lateral margin of the acromion. The maximum CMAP amplitude of the middle trapezius was 4.2 ± 1.4 mV, recorded on the horizontal line at the junction of the middle and lateral thirds between the root of the scapular spine and the vertebral spinous process. The optimal recording site in motor nerve conduction study of the trapezius is the midpoint between the C7 spinous process and the acromion for the upper trapezius, and the junction of middle and lateral thirds of the line between the root of scapular spine and the vertebral spine for the middle trapezius.

Impact of stimulus frequency and recording electrode on electrocochleography in Hybrid cochlear implant users

This report explores the impact of recording electrode position and stimulus frequency on intracochlear electrocochleography (ECoG) responses recorded from six Nucleus L24 Hybrid CI users. Acoustic tone bursts (250 Hz, 500 Hz, 750 Hz, and 1000 Hz) were presented to the implanted ear via an insert earphone. Recordings were obtained from intracochlear electrodes 6 (most basal), 8, 10, 12, 14, 16, 18, 20, and 22 (most apical). Responses to condensation and rarefaction stimuli were subtracted from one another to emphasize hair cell responses (CM/DIF) and added to one another to emphasize neural responses (ANN/SUM). For a fixed stimulus frequency, the CM/DIF and ANN/SUM magnitudes increased as the recording electrode moved apically. For a fixed recording electrode, as the stimulus frequency was lowered, response magnitudes increased. The CM/DIF and ANN/SUM response phase were generally stable across recording electrodes, although substantial phase shifts were noted for a few conditions. Given the recent interest in ECoG for assessing peripheral auditory function in CI users, the impact of stimulus frequency and recording electrode position on response magnitude should be considered. Results suggest optimal ECoG responses are obtained using the most apical recording electrode and a low frequency acoustic stimulus (250 Hz or 500 Hz).

Auditory evoked potentials of the plainfin midshipman fish (Porichthys notatus): implications for directional hearing.

The plainfin midshipman (Porichthys notatus) is an acoustically communicative teleost fish. Here, we evaluated auditory evoked potentials (AEPs) in reproductive female midshipman exposed to tones at or near dominant frequencies of the male midshipman advertisement call. An initial series of experiments characterized AEPs at behaviorally relevant suprathreshold sound levels (130-140 dB SPL re. 1 µPa). AEPs decreased in magnitude with increasing stimulus frequency and featured a stereotyped component at twice the stimulus frequency. Recording electrode position was varied systematically and found to affect AEP magnitude and phase characteristics. Later experiments employed stimuli of a single frequency to evaluate contributions of the saccule to the AEP, with particular attention to the effects of sound source azimuth on AEP amplitude. Unilateral excision of saccular otoliths (sagittae) decreased AEP amplitude; unexpectedly, decreases differed for right versus left otolith excision. A final set of experiments manipulated the sound pressure-responsive swim bladder. Swim bladder excision further reduced the magnitude of AEP responses, effectively eliminating responses at the standard test intensity (130 dB SPL) in some animals. Higher-intensity stimulation yielded response minima at forward azimuths ipsilateral to the excised sagitta, but average cross-azimuth modulation generally remained slight. Collectively, the data underscore that electrode position is an essential variable to control in fish AEP studies and suggest that in female midshipman: (1) the saccule contributes to the AEP, but its directionality as indexed by the AEP is limited, (2) a left-right auditory asymmetry may exist and (3) the swim bladder provides gain in auditory sensitivity that may be important for advertisement call detection and phonotaxis.

Click reception in the harbor porpoise (Phocoena phocoena): Effects of electrode and contact transducer location on the auditory brainstem response.

Unlike terrestrial mammals that have unambiguous aerial sound transmission pathways via the outer ear and tympanum, sound reception pathways in most odontocetes are not well understood. Recent studies have used auditory brainstem response (ABR) measurements to examine sound reception pathways. This study sought to determine how sound source placements, recording electrode arrangements, and ABR peak analyses affect interpretations of sound reception in the harbor porpoise (Phocoena phocoena). Click stimuli were delivered in air from a contact transducer ("jawphone"). Early ABR peaks (representing auditory nerve responses), and later peaks reflecting higher brainstem activity, were analyzed across jawphone and recording electrode positions. Auditory nerve responses were similar for jawphone placements from the ipsilateral posterior mandible to the tip of the rostrum. Later peaks, however, suggested a possible region of highest sensitivity midway between the posterior mandible and the rostrum tip. These findings are generally similar to previous data for porpoises. In contrast to auditory nerve responses that were largest when recorded near the ipsilateral meatus, later ABR peaks were largest when recorded with a contralateral (opposing) electrode. These results provide information on the processes underlying peaks of the ABR, and inform stimulus delivery and ABR recording parameters in odontocete sound reception studies.

Nerve Conduction Studies as a Measure of Disease Progression: Objectivity or Illusion?

Clinical nerve conduction studies (NCS) are often used as a secondary outcome measure in therapeutic trials, but show a high degree of inter-trial variability even when technical factors known to affect the recorded responses are minimised. This raises the intriguing possibility that some of the observed variability may reflect true changes in nerve activity. Our aim was determine how much variability these factors might produce, and how this might affect the results of commonly used neuropathy rating scales. A standardised protocol was repeated over forty consecutive trials by the same operators in two healthy subjects. The protocol included recordings that shared either a stimulating or a recording electrode position, such that changes due to electrode position could be excluded, and hand temperature was closely controlled. Despite controlling for inter-operator differences, electrode position, and hand temperature, the variability in sensory nerve action potential (SNAP) amplitude was extremely high (Range 23 μV, CoV = 10.7-18.8). This variability was greater than the change in amplitude needed to move a subject from point 0 to point 4 on the CMT neuropathy rating scale. Neither temperature or electrode position accounted for all of this variability, suggesting that additional as yet unidentified factors are responsible. Even under closely controlled conditions and sophisticated laboratory methods, test-to-test variability can be significant. The factors responsible for this variability may be difficult to control, limiting the utility of single nerve recordings as a trial outcome measure.

CVEMP morphology changes with recording electrode position, but single motor unit activity remains constant.

Cervical vestibular evoked myogenic potentials (cVEMPs) recorded over the lower quarter of the sternocleidomastoid (SCM) muscle in normal subjects may have opposite polarity to those recorded over the midpoint. It has thus been suggested that vestibular projections to the lower part of SCM might be excitatory rather than inhibitory. We tested the hypothesis that the SCM muscle receives both inhibitory and excitatory vestibular inputs. We recorded cVEMPs in 10 normal subjects with surface electrodes placed at multiple sites along the anterior (sternal) component of the SCM muscle. We compared several reference sites: sternum, ipsilateral and contralateral earlobes, and contralateral wrist. In five subjects, single motor unit responses were recorded at the upper, middle, and lower parts of the SCM muscle using concentric needle electrodes. The surface cVEMP had the typical positive-negative polarity at the midpoint of the SCM muscle. In all subjects, as the recording electrode was moved toward each insertion point, p13 amplitude became smaller and p13 latency increased, then the polarity inverted to a negative-positive waveform (n1-p1). Changing the reference site did not affect reflex polarity. There was a significant short-latency change in activity in 61/63 single motor units, and in each case this was a decrease or gap in firing, indicating an inhibitory reflex. Single motor unit recordings showed that the reflex was inhibitory along the entire SCM muscle. The cVEMP surface waveform inversion near the mastoid and sternal insertion points likely reflects volume conduction of the potential occurring with increasing distance from the motor point.

Superficial radial sensory neuropathy: Medial and lateral branch injury.

Superficial radial sensory nerve (SRN) injury may involve the main nerve or its distal medial or lateral branch. We investigated the utility of SRN conduction studies on the medial and lateral branches in patients with suspected SRN injury. Fifteen patients with symptoms of SRN neuropathy were studied. Their clinical and electrophysiological findings were assessed according to sensory symptom areas. Three active recording electrode positions were used: snuff box (position A); medial branch (position B); and lateral branch (position C). In 7 patients with medial area symptoms, abnormal findings were seen in position B (100%) and position A (71%). In 3 patients with medial and lateral area symptoms, abnormal findings were seen in all positions. In 5 patients with lateral symptoms, abnormal findings were seen in position C only. Nerve conduction studies for each branch of the SRN are useful in patients with suspected SRN neuropathy.

An electrodiagnostic technique for assessing palmar proper digital nerves of the hand: Normative data and clinical application.

There is no standard electrodiagnostic technique for palmar proper digital nerves (PaPDNs). In this study we investigated sensory nerve action potentials (SNAPs) to PaPDN stimulation in normal subjects and patients. SNAPs of PaPDNs were recorded in response to selective antidromic stimulation at the web space and mixed nerve stimulation at the wrist in 14 controls. The selectivity of PaPDN stimulation and the effect of recording electrode position on SNAP amplitude were studied. The technique was tested in 2 patients with PaPDN lesions. The technique yielded selective PaPDN stimulation at the web space. SNAP amplitude to PaPDN stimulation was influenced by age and was larger than SNAP amplitude to wrist stimulation. The recording electrode positions influenced SNAP amplitude. In patients, we documented PaPDN lesions, which were confirmed at surgery, whereas conventional wrist mixed nerve stimulation yielded negative findings. Selective PaPDN stimulation at the web space is feasible and may be helpful for electrodiagnosis of PaPDN lesions.

Embryonic assembly of auditory circuits: spiral ganglion and brainstem

During early development, peripheral sensory systems generate physiological activity prior to exposure to normal environmental stimuli. This activity is thought to facilitate maturation of these neurons and their connections, perhaps even promoting efficacy or modifying downstream circuitry. In the mammalian auditory system, initial connections form at embryonic ages, but the functional characteristics of these early neural connections have not been assayed. We investigated processes of embryonic auditory development using a whole-head slice preparation that preserved connectivity between peripheral and brainstem stations of the auditory pathway. Transgenic mice expressing fluorescent protein provided observation of spiral ganglion and cochlear nucleus neurons to facilitate targeted electrophysiological recording. Here we demonstrate an apparent peripheral-to-central order for circuit maturation. Spiral ganglion cells acquire action potential-generating capacity at embryonic day 14 (E14), the earliest age tested, and action potential waveforms begin to mature in advance of comparable states for neurons of the ventral cochlear nucleus (VCN) and medial nucleus of the trapezoid body (MNTB). In accordance, auditory nerve synapses in the VCN are functional at E15, prior to VCN connectivity with the MNTB, which occurs at least 1 day later. Spiral ganglion neurons exhibit spontaneous activity at least by E14 and are able to drive third-order auditory brainstem neurons by E17. This activity precedes cochlear-generated wave activity by 4 days and ear canal opening by at least 2 weeks. Together, these findings reveal a previously unknown initial developmental phase for auditory maturation, and further implicate the spiral ganglion as a potential controlling centre in this process.

Compound Motor Action Potential Interexaminer Variability in Photoguided Placement of the Recording Electrodes

Interpretation of neurographic data in follow-up studies of patients with neuromuscular disease is sometimes challenging because of the expected variability in repeated tests. In this study, we explored whether the interexaminer variability of the compound motor action potential (CMAP) amplitude is reduced if the examiner is guided by the previously taken photographs of the position of the recording electrodes. The CMAPs were recorded from 20 subjects by unilateral distal stimulation of the median, ulnar, peroneal, and tibial nerve by 4 different technicians. All the subjects were examined on 2 occasions (∼1 week apart). On the first occasion, the technicians recorded the CMAP values according to routine protocols. On the second occasion, the technicians had additional guidance from photographs displaying the recording electrode positions from the first study. The CMAP coefficient of variation (CoV) was calculated for each nerve examined by the four technicians. Differences in CoV between the two types of tests (i.e., with or without photographs) were evaluated. When the examiners were guided by the photographs during electrode application, the CMAP CoV for the tibial innervated abductor hallucis (AH) muscle was reduced (P = 0.02) from 18.5% to 13%. There were, however, no significant reductions in CoV for the abductor pollicis brevis (APB, P = 0.23, median nerve), abductor digiti minimi (P = 0.37, ulnar nerve), or extensor digitorum brevis (EDB, P = 0.15, peroneal nerve) muscles. Photographic documentation of the CMAP recording electrodes seems to have a limited overall effect on interexaminer variability in a subsequent study.

A Robotic Neural Interface for Autonomous Positioning of Extracellular Recording Electrodes

In this paper we describe a set of algorithms and a novel miniature device that together can autonomously position electrodes in neural tissue to obtain high-quality extracellular recordings. This robotic system moves each electrode to detect the signals of individual neurons, optimize the signal quality of a target neuron, and then maintain this signal over time. Such neuronal signals provide the key inputs for emerging neuroprosthetic medical devices and serve as the foundation of basic neuroscientific and medical research. Experimental results from extensive use of the robotic electrodes in macaque parietal cortex are presented to validate the method and to quantify its effectiveness.

41. The impact of the recording electrode position on the compound muscle action potential amplitude and area

41. The impact of the recording electrode position on the compound muscle action potential amplitude and area

S209 – Optimal Electrode Position for ENoG for Facial Palsy

Objectives To develop a more simple procedure for electroneurography (ENoG) of facial palsy for assessing prognosis, we conducted a preliminary study to examine an optimal electrode position for recording evoked compound muscle action potentials (CMAPs) of the perioral muscles. Methods We evaluated a new method called the submentall method, in which the electrode connected to the negative input (G1) was placed on the skin over the orbicularis oris muscle, and the other electrode connected to the positive input of the amplifier (G2) was placed on the mental protuberance. We compared the amplitudes and morphology between the CMAPs recorded with this method and those recorded using the standard electrode positions (G1 placed on the skin over the orbicularis oris muscle; G2 placed on the nasolabial fold, closely lateral to the ala nasi). Furthermore, the amplitudes and waveforms of CMAPs were recorded from 4 different G1 positions on the skin over the orbicularis oris muscle, with G2 set on the mental protuberance. The main trunk of the facial nerve was stimulated by percutaneous supramaximal electrical stimulation beneath the auricle. Results The submental method gave greater amplitudes and more obvious biphasic CMAPs than those produced with the standard method. In the submental method, the optimal G1 recording electrode position was on the philtrum. Conclusions This recording procedure has the potential to be a simpler and more reliable method for calculating ENoG values than the standard method. However, further studies to establish prognostic criteria of ENoG for facial palsy are needed.

Modulation Of Hamstring Muscle H-reflex During Human Gait

Peripheral inputs from muscle, joints and skin play a major role in the control of locomotion by the central nervous system. Assessing their influences has shown that afferent inputs are modulated with the level of motoneuron activity but also exhibit task/phase dependent modulations. Tendon taps and spinal cord electrical stimulation have shown that hamstring muscle afferent responses follow motoneuron activity. However, problems in controlling for the stimulation and recording electrode positions in such large movements impairs the interpretation of the data in regards to the assessment of the task/phase dependent modulation. PURPOSE: To test the feasibility of lumbosacral roots stimulation during human gait to elicit M and H-reflexes responses in hamstring muscles. METHODS: Participants with no know neurological disorders were tested while walking on a treadmill set to their comfortable walking speed. The lumbosacral roots were stimulated percutaneously at 12 different times within the gait cycle with a diffusing electrode placed over the L5 spinous process and the stimulating electrode placed at a 6 inch distance (45 deg downward angle, distal and lateral) away. Timing of the lumbosacral root stimulation was controlled by recording from a footswitch taped under the right heel. EMG of biceps femoris was recorded using a surface electrode and differential amplifier placed over the belly of the muscle. Stimulation intensities were applied pseudo-randomly until a stimulus-response curve was elicited for each of the 12 gait cycle bins. RESULTS: H-reflexes were successfully elicited using lumbosacral root stimulation. In all subjects (n=6) it was possible to elicit a Maximal M wave during walking (amplitude: 2.99+/-1.53 mV). In a subset of participants (n=2), the maximal H-reflex amplitude is modulated within the gait cycle (range: 20.3-81.1% of Mmax) with a moderate correlation to the background EMG (r=0.65 and-0.07). CONCLUSIONS: The results from this study show that it is feasible to use lumbosacral stimulation to record maximal M and H responses from Hamstring muscles during walking. Furthermore, it seems that the modulation of the hamstring maximal H-reflex is not simply dependent on the background EMG levels but might also be modulated in a task/phase dependent manner.

Preterm labour detection by use of a biophysical marker: the uterine electrical activity

BackgroundThe electrical activity of the uterine muscle is representative of uterine contractility. Its characterization may be used to detect a potential risk of preterm delivery in women, even at an early gestational stage.MethodsWe have investigated the effect of the recording electrode position on the spectral content of the signal by using a mathematical model of the women's abdomen. We have then compared the simulated results to actual recordings. On signals with noise reduced with a dedicated algorithm, we have characterized the main frequency components of the signal spectrum in order to compute parameters indicative of different situations: preterm contractions resulting nonetheless in term delivery (i.e. normal contractions) and preterm contractions leading to preterm delivery (i.e. high-risk contractions). A diagnosis system permitted us to discriminate between these different categories of contractions. As the position of the placenta seems to affect the frequency content of electrical activity, we have also investigated in monkeys, with internal electrodes attached on the uterus, the effect of the placenta on the spectral content of the electrical signals.ResultsIn women, the best electrode position was the median vertical axis of the abdomen. The discrimination between high risk and normal contractions showed that it was possible to detect a risk of preterm labour as early as at the 27th week of pregnancy (Misclassification Rate range: 11–19.5%). Placental influence on electrical signals was evidenced in animal recordings, with higher energy content in high frequency bands, for signals recorded away from the placenta when compared to signals recorded above the placental insertion. However, we noticed, from pregnancy to labour, a similar evolution of the frequency content of the signal towards high frequencies, whatever the relative position of electrodes and placenta.ConclusionOn human recordings, this study has proved that it is possible to detect, by non-invasive abdominal recordings, a risk of preterm birth as early as the 27th week of pregnancy. On animal signals, we have evidenced that the placenta exerts a local influence on the characteristics of the electrical activity of the uterus. However, these differences have a small influence on premature delivery risk diagnosis when using proper diagnosis tools.

The positioner effects electrode interaction in pediatric cochlear implants

Problem: Address the effect of an implant electrode’s cochlear position on the degree of electrode interaction in pediatric patients with Clarion HiFocus arrays using objective measurements well-suited for very young implant recipients. Methods: PATIENTS: A volunteer sample of 16 patients (range 2.2 to 12.8; mean age, 6.3 years) implanted with the HiFocus electrode were tested; 9 patients with the positioner and 7 without. MAIN OUTCOME MEASURES: Electrically evoked whole nerve action potentials (EAPs) were measured using the CII’s neural response imaging. An electrode interaction function was measured by varying the masker electrode position while maintaining probe and recording electrode positions. Interaction functions were obtained at 3 probe locations (electrodes 5, 9, and 13) and 3 supra-threshold intensities. Results: While EAP thresholds did not vary along the electrode array, growth function slope increased from basal to apical location ( P < 0.01). No difference in growth function slope or EAP threshold was associated with the positioner. Interaction among apical electrodes was greater than among basal electrodes ( P < 0.01). No difference in electrode interaction could be attributed to the positioner alone ( P > 0.01). Conclusion: Both electrode interaction and growth function slope were greater toward the cochlear apex than at the cochlear base. We attribute this difference to a combination of systematic variations in electrode/neural element distances and surviving cochlear ganglion cells. Behavioral testing will determine the implication of different degrees of electrode interaction on electrical hearing. Significance: No systematic study of EAPs in children has been reported. These findings will help us understand the relationship between the auditory nerve response to electrical stimuli and the perception of hearing. Support: None reported.

The inaccuracy of Kubicek's one-cylinder model in thoracic impedance cardiography.

The validity of a one- and a two-cylinder model, underlying thoracic impedance cardiography (TIC), was investigated by studying the length dependence of the impedance parameters Z0, (dZ/dt)min, and stroke volume (SV). It can be shown that, within a one-cylinder model, all parameters are directly proportional to the length, whereas, if the volume conduction of the thorax and the neck are modeled separately, Z0 and (dZ/dt)min are expected to be linear dependent and SV will be nonlinear upon the length. The expectations were compared to results from in vivo measurements. Two electrode arrays were studied, in which the caudal recording electrode position was varied; SV was calculated using Kubicek's equation. Except for small distances, the results showed a nearly linear relation between the parameters and the length. Regression analysis of the linear part revealed statistically significant intercepts (p < 0.05). Neither the intercept nor the nonlinear part can be explained by a one-cylinder model, whereas a model consisting of two cylinders serially connected describes the experimental results accurately. Thus SV estimation based on a one-cylinder model is biased due to the invalid one-cylinder model. Corrections for the Kubicek-equation need to be developed in future research using this two-cylinder model.