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IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING. VOL. 38. NO. 9, SEPTEMBER 1991
Information Contained in Sensory Nerve Recordings Made with Intrafascicular Electrodes Eleanor V. Goodall, Student Member, IEEE, Todd M . Lefurge, and Kenneth W . Horch, Member, IEEE
Abstract-Multiunit recordings were made in anesthetized cats with chronically implanted intrafascicular electrodes over a period of six months. Neural signals recorded with these electrodes consisted of activity in sensory fibers innervating a variety of cutaneous mechanoreceptors. Mechanical stimuli were used to selectively activate individual nerve fibers, and the receptive field and receptor type were identified for each unit. Over a period of six months, there was a net shift in the recorded population, but the electrodes continued to provide a representative sample of the activity in the fascicle as a whole. The total number of units from which activity could be recorded remained roughly constant with time, and individual units persisted in the recordings for up to six months. These results indicate that intrafascicular electrodes could be used to sample information carried by individual somatosensory fibers on a long term basis.
INTRODUCTION
F
UNCTIONAL electrical stimulation (FES) is a technique which offers hope of restoring mobility and sensation to stroke and spinal cord injury patients. However, the success of FES systems currently in use is limited due to a lack of adequate closed-loop control. In order to implement a closed-loop control system for FES, it is necessary to obtain information on muscle force, limb position, and skin contact or pressure. In many cases, paralyzed patients possess intact and functional peripheral nervous systems. Signals from receptors innervated by peripheral nerves could be used to provide force, position, and touch feedback control for FES. One method of doing this bases estimates of stimulus strength on the level of activity recorded from entire nerve bundles [3]. However, we feel that individual firing patterns in ensembles of single units contain more useful information than patterns of mass activity. We have developed implantable intrafascicular electrodes which can be used to chronically record from several axons at once in peripheral nerves [6]-[8]. To estimate the potential usefulness of these electrodes for providing feedback for FES, one needs information about the stability of the recordings and the amount and type of information available from them. We report here on exManuscript received January 2, 1990; revised November I , 1990. This work was supported by grants from the NIDRR (Department of Education) and the NINDS (NIH). The authors are with the Department of Bioengineering, University of Utah, Salt Lake City, UT 841 12. IEEE Log Number 9101994.
periments performed with intrafascicular electrodes implanted in the forelimb nerves of cats over a six month period. From the results we ascertain the number and type of units from which activity could be recorded, show that individual units could be recorded from over a six-month period, and delineate the sort of sensory information carried by the fibers from which activity was recorded.
METHODS The data presented here are based on recordings from ten electrodes implanted in five cats for a period of six months. Activity was be recorded with each electrode for the six-month duration of the experiment. Electrode Fabrication and Implantation An intrafascicular electrode consisted of two Tefloninsulated 90% platinum-10% iridium wires, with diameters of 25 and 50 pm, respectively. Approximately 1 mm of insulation was removed about 20 mm from the distal end of each wire and the uninsulated region was coated with platinum black to form a recording surface. A sharpened 50 pm diameter tungsten wire was bonded to the end of the 25 pm electrode with cyanoacrylate. The recording surfaces of the two wires were aligned and the proximal ends of the wires were threaded into a 600 pm (0.d.) silicone rubber tube along with a length of 6-0 silk thread. A small loop of the thread extended from the distal end of the tube. Cats were anaesthetized with Ketamine and maintained on Halothane during the electrode implantation surgery and during subsequent recording sessions. To implant the electrode, the radial nerve, which innervates cutaneous mechanoreceptors on the top of the forearm and paw, was exposed under aseptic conditions. A single fascicle of the nerve was exposed by teasing away the epineurium. The tungsten needle was used to thread the 25 pm electrode longitudinally into the fascicle for a distance of about 1 cm, centering the recording zone in this region. The 50 pm wire was placed outside the fascicle and parallel to the intrafascicular wire. A 9-0 suture passed through the loop of thread at the distal end of the silicone rubber tubing was used to secure the electrode to the epineurium. The two wires were cut just distally to where the 25 pm wire exited the fascicle and were sutured to the epineurium at that point. Although the cut ends of the electrode
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GOODALL et al.: INFORMATION IN SENSORY NERVE RECORDINGS
wires were left uninsulated, previous work in our lab has shown that current flow through the ends of the electrode wires is insignificant [7]. The electrode leads, protected by the silicone rubber tube, were threaded under the skin and brought outside the body at a point proximal to the implant site via a small incision. The leads were placed under the skin and the incision was closed and allowed to heal between recording sessions.
Recording and Unit Identijication Recordings were made immediately following electrode implantation, and 0.5, 1, 2 , 4 and 6 months following implantation. The proximal ends of the electrode wires were exposed by reopening the incision and connected to the inputs of a differential amplifier with a gain of 1.5 x lo4 and half-power points at 450 and 2500 Hz. This bandwidth was chosen to maximize the signal-to-noise ratio of the recordings [7]. Additional amplification (10-20) was provided to bring the signal to the range of rfr5 V before it was recorded using a Vetter model 3000A pulse code modulator attached to a Magnavox Video Cassette Recorder. The feline superficial radial nerve is a pure cutaneous sensory nerve, so all the large myelinated fibers innervate cutaneous mechanoreceptors. The hair on the innervated area of the paw and forearm was clipped to a length of about 1-2 mm to facilitate identification of single sensory units. The physiological basis for unit identification is as follows. A single peripheral sensory nerve fiber innervates one or more sensory receptors, but a given fiber innervates only one type of receptor [4], [lo]. For cutaneous innervation, the area of skin containing the mechanoreceptor(s) innervated by a single nerve fiber is called the “receptive field” for that neuron. The neuron and the mechanoreceptor(s) which it innervates are commonly referred to as a “unit.” Cutaneous mechanoreceptors can be grouped into several types or classes, each of which exhibits distinctive response properties [l], [4]. For example, phasic mechanoreceptors respond preferentially to transient stimuli such as brisk brushing of the hair or skin, while tonic receptors are well activated by slow, steady pressure applied to the skin. Some receptors are associated with hair follicles, and respond to displacement of a single hair, while others are located in the skin between hairs, and do not respond to movement of single hairs. One can identify the type of receptor innervated by a nerve fiber, independent of the size or shape of its action potentials, by applying controlled mechanical stimuli and noting which types of stimuli are effective in evoking a response [l], [4]. One can also determine the receptive field of a nerve fiber by noting where the unit is most sensitive to mechanical stimulation. Sinusoidal mechanical stimuli are useful for isolating and classifying cutaneous mechanoreceptors because the receptors exhibit phase-locked firing when a sinusoidal
847
mechanical stimulus is applied, i.e., action potentials occur at a fixed time in the stimulus cycle [9]. In addition, different receptor types have different “tuning curves,” that is, the threshold stimulus intensity is lowest at a particular frequency for each type of receptor [4], [9]. By varying stimulus frequency and intensity and selecting the location where the stimulus is applied, single units can be activated in isolation. The area of skin innervated by the implanted fascicle was determined by brushing the paw and forearm with a small paintbrush, a “broad-band” stimulus which activated many units. The presence of tonic units could be detected by squeezing or pressing on the paw, or identing with a blunt probe. Localized sinusoidal stimuli in the range of 20-300 Hz were delivered to the skin by a blunt probe mounted on a small speaker cone which was driven by a sine wave generator. The stimulator was held in a micromanipulator for stability and ease of positioning. The magnitude of the stimulus was controlled by adjusting the amplitude of the sine wave generator output. A synchronizing pulse from the sine wave generator was used to trigger the oscilloscope when viewing the recorded activity. Single units were selectively activated with sinusoidal stimuli, and the receptive field location and receptor type for each unit was determined by its response to various mechanical stimuli according to the procedure described by Horch et al. [4]. The size or shape of the action potentials produced by a given unit were not used in making this determination. The procedure was performed at each recording session for all units with a signal-to-noise level greater than 1.4 [6]. Receptors were classified as F1, F-intermediate, or F2 field; G1, G-intermediate, or G2 hair; or Type I or Type I1 tonic [4]. If a unit could not be unambiguously assigned to one of these categories, it was classified as either phasic or tonic, or, in a few cases, simply “unidentified.”
Data Analysis In order to determine the persistence of individual units over time, the location and types of receptors were compared at different recording times. If the location of the receptive field and the receptor type (as determined by the response to different mechanical stimuli) were the same at two different times, it was assumed that the same unit was being observed each time. The rationale for this approach is based on the observation that the size of individual unit fields, the extent to which receptive fields of different units overlap, and the limited number of units present in a given recording make it unlikely that two recorded units with similar response properties would have similar receptive field locations [2], [ 101. At each recording session, each unit was identified as described above. However, to provide large enough sample sizes for purposes of statistical analysis of the population of units as a whole, they were then grouped into three classes: phasic, tonic, and unidentified. Differences
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IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING. VOL. 38, NO. 9. SEPTEMBER 1991
(b) Fig. 1. Maps of receptive field locations of cutaneous mechanoreceptor units recorded with an intrafascicular electrode implanted in the radial nerve of the cat. Shown are an original map (a) and one made one month later (b). Each receptive field was innervated by a single unit in the sample. Examples of action potentials are shown for two of the units. Both units were type GI guard hair units. Action potentials were elicited by sinusoidal mechanical stimuli with frequencies o f (a) left-104 H z , (a) right-91 Hz, (b) left-60 Hz, and (b) right-63 H z . Calibration: 5 p V x 0 . 5 s.
in the distribution of these grouped receptor classes and the locations of receptive fields over time or between electrodes were tested by casting the data in a contingency table and applying the X 2 test.
50
b
30
RESULTS Fig. 1 shows a receptive field maps for all the units with signal-to-noise ratios greater than 1.4 recorded by one of the electrodes at two different times. Also shown are action potential waveforms for two of the units. Each unit had a distinct, nonoverlapping receptive field. This is representative of our data and illustrates several features of our results. The receptive fields are distributed over a fairly large area of the paw, and this area does not change with time. Although not indicated in Fig. 1, each of the receptive fields was also identified with a particular receptor type, e.g., field receptor, hair receptor, Type I receptor. It was possible to follow the presence of units over time by matching those units which have the same receptive field locations and the same receptor types. On average, each electrode could record activity from about ten units, but the population of afferent fibers recorded from tended to change with time. Fig. 2 shows the rate at which new units appeared and old units dropped out. The two curves parallel each other, their difference giving the number of units recorded at any given time. The slope of the curves indicate a shift in the recorded population of three to four units per month. The changes seen during the first month presumably reflect tissue reaction to the implantation procedure; subsequent changes may indicate movement of the electrode within the fascicle. In order to determine how long an individual unit would persist in a recording, the units present in each recording at one month post-implantation were identified, and the number of these units remaining at two, four, and six
I Il l l lIl I I 111 1I
___--___---
Months + Cumulative Additions
--A--
Cumulative Drops
Fig. 2 . Stability of the recorded population. The upper curve is the cumulative number of units added to intrafascicular electrode recordings at the time of implantation and 0.5, 1, 2, 4, and 6 months later. The lower curve is the cumulative number of units dropped from the recordings. The difference between the two lines represents the number of units in the recording at any given time. Shown are mean values from ten electrodes, with error bars indicating standard deviation.
months was then determined. These values were expressed relative to the number of units present at one month, and are plotted in Fig. 3 . The data indicate that about 30% of the units present at one month were still present five months later. To assess how representative our recorded sample was of the total population of fibers in the fascicle we analyzed the types and receptive field locations of units present in each recording session. The receptor types were grouped into three classes-phasic, tonic, and unidentified-and the area of skin innervated by the radial nerve was divided into three regions. Counts were made of each receptor class in each region at each recording time. The data are given in Table I. All receptor classes were present in each area of the paw.
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GOODALL er al. : INFORMATION I N SENSORY NERVE RECORDINGS
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Unit Persistence 100
I
to differ for different electrodes ( X 2 test, p < 0.001). This result was expected because the same fascicle of the radial nerve was not used for each implant.
DISCUSSION
1'
I
0 ' 1
3
2
4
6
5
Months
Fig. 3. Persistence of individual units in intrafascicular recordings. Individual units present at one month were taken as a starting point, and the number of these persisting at later times is plotted as a percent of the original population. Shown are mean S . E . from ten electrodes.
TABLE I Months Post-Implant Receptor Type
0
0.5
1
2
4
6
Row Totals
Area I
Phasic Tonic Unidentified
19 3 3
23 3 0
33 3 0
33 1 3
35 5 1
45 5 0
I88 20 7
Area 2
Phasic Tonic Unidentified
20 1 2
14 3 2
38 2 0
43 0 1
42 1 4
37 2 0
194 9 9
Area 3
Phasic Tonic Unidentified
20 1 6
15 0 3
21 1 7
27 0 4
20 0
25 0
128 2
75
63
105
112
109
Totals
1
0
114
2
1
578
Distribution of cutaneous mechanoreceptors on the paw over time. Receptors were grouped into three classes. as described in the text, and the paw was divided into three areas. Area I included the top of the forearm, the wrist, and the top and medial side of the paw. Area 2 included the knuckles and the tops and sides of the toes. Area 3 included the distal portion of the toes.
To determine whether the area of skin innervated by the units from which activity could be recorded changed with time, data from the different electrodes were combined, and a 3 X 6 contingency table showing receptive field location versus time for each class of units was constructed. There was no statistically significant difference in receptor distribution at different times ( X 2 test, p > 0.31). To determine whether the area of skin from which activity could be recorded differed from one electrode to another, data from different times for each electrode were combined, and a 3 X 10 contingency table showing the distribution of receptive field locations versus electrode was constructed. The receptive field locations were found
In a typical recording, activity could be evoked in about ten identifiable units. The response properties of the units indicated that the implant provided a representative sample of the receptor types present in the skin, and the receptive fields of these units were distributed throughout the region innervated by the fascicle. There was a gradual shift in the population of units from which activity could be recorded over time. However, as units dropped out, new units appeared in the recordings, so the overall number of units remained roughly constant. The appearance of new units in the recordings could have been due to the recovery of fibers which were damaged by the implantation procedure, and the loss of units due to degeneration of units injured by this procedure. However, it is unlikely that these two events would balance each other well enough to keep the total population constant. Moreover, the fact that the shift continued after the first month post-implantation, by which time most of the response to implant trauma would be expected to have taken place [5], suggests that the shift in population cannot be accounted for by cell recovery and degeneration in the vicinity of the electrode. This suggests that changes in the recorded population were due to small movements of the electrode within the fascicle, rather than loss of units due to damage caused by the electrode. The intrafascicular electrode recordings included fibers innervating several kinds of mechanoreceptors, which respond to various types of stimuli. For purposes of unit identification, we used controlled mechanical stimuli delivered to restricted regions of skin. However, if individual units could be identified on the basis of action potential size or shape, it would be possible to obtain both locus and modality specific information about any externally applied stimulus. Since the electrodes record from a restricted number of units, which limits the number of units to be sorted and minimizes superpositions, it may be feasible to use an on-line action potential sorting method to track single unit activity. Work in our lab is currently pursuing this possibility. The distribution of receptors and the receptor types from which activity could be recorded was representative of the activity in the whole fascicle. Hence, these recordings could provide both specific information about stimuli and a measure of the full range of information carried in the fascicle. Since individual units persisted in the intrafascicular electrode recordings for several months, these recordings could be used as a source of long-term information on single-unit activity. Additional modifications to the electrode design, currently under investigation in our lab, might improve the lifetime of single units in the recordings; but, even if drift in the population cannot be entirely
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eliminated, it is sufficiently slow that periodic “updating,” i.e., remapping of the unit population would be adequate if the electrodes were to be used for feedback control in FES. Mapping of receptive fields and receptor classification can be performed relatively quickly, and in human subjects there would be no need for anesthesia during the procedure. The present experiments were performed in anesthetized animals and were designed to determine what kind of information was present in the recordings. Application of this technique would require recordings to be made in awake, behaving animals or people. This would necessitate identifying units on the basis of action potential configuration in the presence of movement and EMG artifacts. These artifacts can be minimized in a number of ways. First, our bipolar electrode design provides for common mode noise rejection. Second, the use of a restricted bandwidth amplifier tends to eliminate low frequency movement artifacts. Finally, if necessary the implant zone of the nerve could be surrounded by a nonconductive cuff, providing EMG rejection capabilities similar to that of cuff electrodes. These issues remain to be pursued, after it has been demonstrated that unit identification on the basis of action potential size and shape can be performed and once a new generation of highly flexible, implantable electrodes has been developed.
REFERENCES [ l ] P. R. Burgess, D. Petit, and R. M. Warren, “Receptor types in cat hairy skin supplied by myelinated fibers,” J . Neurophysiol., vol. 31, pp. 833-848, 1968. [2] P. R. Burgess, J . F. Howe, M. J . Lessler, and D. Whitehorn, “Cutaneous receptors supplied by myelinated fibers in the cat. 11. Number of mechanoreceptors excited by a local stimulus, J . Neurophysiol. , vol. 37, pp. 1373-1386, 1974. 131 J. A. Hoffer, M. Haugland, and T. Li, “Obtaining skin contact force information from implanted nerve cuff recording electrodes,” Proc. IEEEIEMBS Internat. Conj., vol. 11, pp. 928-929, 1989. [4] K . W. Horch, R. P. Tuckett, and P. R. Burgess, “A key to the classification of cutaneous mechanoreceptors,” J . Invesf. Dermarol. , vol. 69, no. 1, pp. 75-82, 1977. [ 5 ] K . W. Horch and S . J. W. Lisney, “On the number and nature of regenerating myelinated axons after lesions of cutaneous nerves in the cat,” J . Physiol., vol. 313, pp. 275-286, 1981. 161 T. M. Lefurge, E. V. Goodall, K . W. Horch, L. Stensaas. and A. A. Schoenberg, “Chronically implanted intrafascicular recording electrodes,” Ann. Eiomed. Eng., vol. 19. pp. 197-207. 1991.
M. S . Malagodi, K . W. Horch. and A. A. Schoenberg, “An intrafascicular electrode for recording of action potentials in peripheral nerves,” Ann. Biomed. Eng., vol. 17, pp. 397-410, 1989. A. A. Schoenberg, M. Malagodi, and K . Horch, “Extraction of somatosensory information from peripheral nerves for FNS applications,” Advances in External Conrr. Human Extremities IX,pp. 363373, 1987. W. H. Talbot, I. Darian-Smith, H . H. Kornhuber, and V . B. Mountcastle, “The sense of flutter-vibration: Comparison of the human capacity with response patterns of mechanoreceptive afferents from the monkey hand,” J . Neurophys., vol. 31, pp. 301-334, 1968. A. B. Vallbo and R. S . Johansson, “Properties of cutaneous mechanoreceptors in the human hand related to touch sensation,” Human Neurobiol., vol. 3 , pp. 3-14, 1984.
Eleanor V. Goodall (S’91) was born in Redwood City, CA, in 1963. She received the B.A. degree in biology from Reed College, Portland, OR, in 1985 and the M.E. degree in bioengineering from the University of Utah, Salt Lake City, in 1988. She is currently a doctoral candidate in bioengineering at the University of Utah. Her interests include the acquisition and processing of biological signals, modeling of neuromuscular systems, and neural networks.
Todd M. Lefurge was born in New Jersey in 1965. He received the B.S. degree in biomedical engineering from Tulane University, New Orleans, LA, in 1987 and the M.S. degree in bioengineering from the University of Utah, Salt Lake City, in 1990. His research interests include neural prosthetics and instrumentation.
”
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Kenneth W. Horch (M’88) was born in Cleveland, OH, in 1942. He received the B.S. degree from Lehigh University, Bethlehem, PA, and the Ph.D. degree from Yale University, New Haven, CT. He is an Associate Professor of Physiology and Bioengineering at the University of Utah, Salt Lake City, and Director of Neurological Testing at Topical Testing, Inc. His research activities are in the areas of neuroprosthetics, peripheral nerve regeneration and repair, sensory evaluation, and somatosensory physiology.
U IIlllllIl
l11l l l l l l
IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING. VOL. 38. NO. 9, SEPTEMBER 1991
Information Contained in Sensory Nerve Recordings Made with Intrafascicular Electrodes Eleanor V. Goodall, Student Member, IEEE, Todd M . Lefurge, and Kenneth W . Horch, Member, IEEE
Abstract-Multiunit recordings were made in anesthetized cats with chronically implanted intrafascicular electrodes over a period of six months. Neural signals recorded with these electrodes consisted of activity in sensory fibers innervating a variety of cutaneous mechanoreceptors. Mechanical stimuli were used to selectively activate individual nerve fibers, and the receptive field and receptor type were identified for each unit. Over a period of six months, there was a net shift in the recorded population, but the electrodes continued to provide a representative sample of the activity in the fascicle as a whole. The total number of units from which activity could be recorded remained roughly constant with time, and individual units persisted in the recordings for up to six months. These results indicate that intrafascicular electrodes could be used to sample information carried by individual somatosensory fibers on a long term basis.
INTRODUCTION
F
UNCTIONAL electrical stimulation (FES) is a technique which offers hope of restoring mobility and sensation to stroke and spinal cord injury patients. However, the success of FES systems currently in use is limited due to a lack of adequate closed-loop control. In order to implement a closed-loop control system for FES, it is necessary to obtain information on muscle force, limb position, and skin contact or pressure. In many cases, paralyzed patients possess intact and functional peripheral nervous systems. Signals from receptors innervated by peripheral nerves could be used to provide force, position, and touch feedback control for FES. One method of doing this bases estimates of stimulus strength on the level of activity recorded from entire nerve bundles [3]. However, we feel that individual firing patterns in ensembles of single units contain more useful information than patterns of mass activity. We have developed implantable intrafascicular electrodes which can be used to chronically record from several axons at once in peripheral nerves [6]-[8]. To estimate the potential usefulness of these electrodes for providing feedback for FES, one needs information about the stability of the recordings and the amount and type of information available from them. We report here on exManuscript received January 2, 1990; revised November I , 1990. This work was supported by grants from the NIDRR (Department of Education) and the NINDS (NIH). The authors are with the Department of Bioengineering, University of Utah, Salt Lake City, UT 841 12. IEEE Log Number 9101994.
periments performed with intrafascicular electrodes implanted in the forelimb nerves of cats over a six month period. From the results we ascertain the number and type of units from which activity could be recorded, show that individual units could be recorded from over a six-month period, and delineate the sort of sensory information carried by the fibers from which activity was recorded.
METHODS The data presented here are based on recordings from ten electrodes implanted in five cats for a period of six months. Activity was be recorded with each electrode for the six-month duration of the experiment. Electrode Fabrication and Implantation An intrafascicular electrode consisted of two Tefloninsulated 90% platinum-10% iridium wires, with diameters of 25 and 50 pm, respectively. Approximately 1 mm of insulation was removed about 20 mm from the distal end of each wire and the uninsulated region was coated with platinum black to form a recording surface. A sharpened 50 pm diameter tungsten wire was bonded to the end of the 25 pm electrode with cyanoacrylate. The recording surfaces of the two wires were aligned and the proximal ends of the wires were threaded into a 600 pm (0.d.) silicone rubber tube along with a length of 6-0 silk thread. A small loop of the thread extended from the distal end of the tube. Cats were anaesthetized with Ketamine and maintained on Halothane during the electrode implantation surgery and during subsequent recording sessions. To implant the electrode, the radial nerve, which innervates cutaneous mechanoreceptors on the top of the forearm and paw, was exposed under aseptic conditions. A single fascicle of the nerve was exposed by teasing away the epineurium. The tungsten needle was used to thread the 25 pm electrode longitudinally into the fascicle for a distance of about 1 cm, centering the recording zone in this region. The 50 pm wire was placed outside the fascicle and parallel to the intrafascicular wire. A 9-0 suture passed through the loop of thread at the distal end of the silicone rubber tubing was used to secure the electrode to the epineurium. The two wires were cut just distally to where the 25 pm wire exited the fascicle and were sutured to the epineurium at that point. Although the cut ends of the electrode
001 8-929419 110900-0846$0 1 .OO
1ll l1Il 1l1l l lll
0 199 1 IEEE
I I Il III I I I Il
GOODALL et al.: INFORMATION IN SENSORY NERVE RECORDINGS
wires were left uninsulated, previous work in our lab has shown that current flow through the ends of the electrode wires is insignificant [7]. The electrode leads, protected by the silicone rubber tube, were threaded under the skin and brought outside the body at a point proximal to the implant site via a small incision. The leads were placed under the skin and the incision was closed and allowed to heal between recording sessions.
Recording and Unit Identijication Recordings were made immediately following electrode implantation, and 0.5, 1, 2 , 4 and 6 months following implantation. The proximal ends of the electrode wires were exposed by reopening the incision and connected to the inputs of a differential amplifier with a gain of 1.5 x lo4 and half-power points at 450 and 2500 Hz. This bandwidth was chosen to maximize the signal-to-noise ratio of the recordings [7]. Additional amplification (10-20) was provided to bring the signal to the range of rfr5 V before it was recorded using a Vetter model 3000A pulse code modulator attached to a Magnavox Video Cassette Recorder. The feline superficial radial nerve is a pure cutaneous sensory nerve, so all the large myelinated fibers innervate cutaneous mechanoreceptors. The hair on the innervated area of the paw and forearm was clipped to a length of about 1-2 mm to facilitate identification of single sensory units. The physiological basis for unit identification is as follows. A single peripheral sensory nerve fiber innervates one or more sensory receptors, but a given fiber innervates only one type of receptor [4], [lo]. For cutaneous innervation, the area of skin containing the mechanoreceptor(s) innervated by a single nerve fiber is called the “receptive field” for that neuron. The neuron and the mechanoreceptor(s) which it innervates are commonly referred to as a “unit.” Cutaneous mechanoreceptors can be grouped into several types or classes, each of which exhibits distinctive response properties [l], [4]. For example, phasic mechanoreceptors respond preferentially to transient stimuli such as brisk brushing of the hair or skin, while tonic receptors are well activated by slow, steady pressure applied to the skin. Some receptors are associated with hair follicles, and respond to displacement of a single hair, while others are located in the skin between hairs, and do not respond to movement of single hairs. One can identify the type of receptor innervated by a nerve fiber, independent of the size or shape of its action potentials, by applying controlled mechanical stimuli and noting which types of stimuli are effective in evoking a response [l], [4]. One can also determine the receptive field of a nerve fiber by noting where the unit is most sensitive to mechanical stimulation. Sinusoidal mechanical stimuli are useful for isolating and classifying cutaneous mechanoreceptors because the receptors exhibit phase-locked firing when a sinusoidal
847
mechanical stimulus is applied, i.e., action potentials occur at a fixed time in the stimulus cycle [9]. In addition, different receptor types have different “tuning curves,” that is, the threshold stimulus intensity is lowest at a particular frequency for each type of receptor [4], [9]. By varying stimulus frequency and intensity and selecting the location where the stimulus is applied, single units can be activated in isolation. The area of skin innervated by the implanted fascicle was determined by brushing the paw and forearm with a small paintbrush, a “broad-band” stimulus which activated many units. The presence of tonic units could be detected by squeezing or pressing on the paw, or identing with a blunt probe. Localized sinusoidal stimuli in the range of 20-300 Hz were delivered to the skin by a blunt probe mounted on a small speaker cone which was driven by a sine wave generator. The stimulator was held in a micromanipulator for stability and ease of positioning. The magnitude of the stimulus was controlled by adjusting the amplitude of the sine wave generator output. A synchronizing pulse from the sine wave generator was used to trigger the oscilloscope when viewing the recorded activity. Single units were selectively activated with sinusoidal stimuli, and the receptive field location and receptor type for each unit was determined by its response to various mechanical stimuli according to the procedure described by Horch et al. [4]. The size or shape of the action potentials produced by a given unit were not used in making this determination. The procedure was performed at each recording session for all units with a signal-to-noise level greater than 1.4 [6]. Receptors were classified as F1, F-intermediate, or F2 field; G1, G-intermediate, or G2 hair; or Type I or Type I1 tonic [4]. If a unit could not be unambiguously assigned to one of these categories, it was classified as either phasic or tonic, or, in a few cases, simply “unidentified.”
Data Analysis In order to determine the persistence of individual units over time, the location and types of receptors were compared at different recording times. If the location of the receptive field and the receptor type (as determined by the response to different mechanical stimuli) were the same at two different times, it was assumed that the same unit was being observed each time. The rationale for this approach is based on the observation that the size of individual unit fields, the extent to which receptive fields of different units overlap, and the limited number of units present in a given recording make it unlikely that two recorded units with similar response properties would have similar receptive field locations [2], [ 101. At each recording session, each unit was identified as described above. However, to provide large enough sample sizes for purposes of statistical analysis of the population of units as a whole, they were then grouped into three classes: phasic, tonic, and unidentified. Differences
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IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING. VOL. 38, NO. 9. SEPTEMBER 1991
(b) Fig. 1. Maps of receptive field locations of cutaneous mechanoreceptor units recorded with an intrafascicular electrode implanted in the radial nerve of the cat. Shown are an original map (a) and one made one month later (b). Each receptive field was innervated by a single unit in the sample. Examples of action potentials are shown for two of the units. Both units were type GI guard hair units. Action potentials were elicited by sinusoidal mechanical stimuli with frequencies o f (a) left-104 H z , (a) right-91 Hz, (b) left-60 Hz, and (b) right-63 H z . Calibration: 5 p V x 0 . 5 s.
in the distribution of these grouped receptor classes and the locations of receptive fields over time or between electrodes were tested by casting the data in a contingency table and applying the X 2 test.
50
b
30
RESULTS Fig. 1 shows a receptive field maps for all the units with signal-to-noise ratios greater than 1.4 recorded by one of the electrodes at two different times. Also shown are action potential waveforms for two of the units. Each unit had a distinct, nonoverlapping receptive field. This is representative of our data and illustrates several features of our results. The receptive fields are distributed over a fairly large area of the paw, and this area does not change with time. Although not indicated in Fig. 1, each of the receptive fields was also identified with a particular receptor type, e.g., field receptor, hair receptor, Type I receptor. It was possible to follow the presence of units over time by matching those units which have the same receptive field locations and the same receptor types. On average, each electrode could record activity from about ten units, but the population of afferent fibers recorded from tended to change with time. Fig. 2 shows the rate at which new units appeared and old units dropped out. The two curves parallel each other, their difference giving the number of units recorded at any given time. The slope of the curves indicate a shift in the recorded population of three to four units per month. The changes seen during the first month presumably reflect tissue reaction to the implantation procedure; subsequent changes may indicate movement of the electrode within the fascicle. In order to determine how long an individual unit would persist in a recording, the units present in each recording at one month post-implantation were identified, and the number of these units remaining at two, four, and six
I Il l l lIl I I 111 1I
___--___---
Months + Cumulative Additions
--A--
Cumulative Drops
Fig. 2 . Stability of the recorded population. The upper curve is the cumulative number of units added to intrafascicular electrode recordings at the time of implantation and 0.5, 1, 2, 4, and 6 months later. The lower curve is the cumulative number of units dropped from the recordings. The difference between the two lines represents the number of units in the recording at any given time. Shown are mean values from ten electrodes, with error bars indicating standard deviation.
months was then determined. These values were expressed relative to the number of units present at one month, and are plotted in Fig. 3 . The data indicate that about 30% of the units present at one month were still present five months later. To assess how representative our recorded sample was of the total population of fibers in the fascicle we analyzed the types and receptive field locations of units present in each recording session. The receptor types were grouped into three classes-phasic, tonic, and unidentified-and the area of skin innervated by the radial nerve was divided into three regions. Counts were made of each receptor class in each region at each recording time. The data are given in Table I. All receptor classes were present in each area of the paw.
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GOODALL er al. : INFORMATION I N SENSORY NERVE RECORDINGS
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Unit Persistence 100
I
to differ for different electrodes ( X 2 test, p < 0.001). This result was expected because the same fascicle of the radial nerve was not used for each implant.
DISCUSSION
1'
I
0 ' 1
3
2
4
6
5
Months
Fig. 3. Persistence of individual units in intrafascicular recordings. Individual units present at one month were taken as a starting point, and the number of these persisting at later times is plotted as a percent of the original population. Shown are mean S . E . from ten electrodes.
TABLE I Months Post-Implant Receptor Type
0
0.5
1
2
4
6
Row Totals
Area I
Phasic Tonic Unidentified
19 3 3
23 3 0
33 3 0
33 1 3
35 5 1
45 5 0
I88 20 7
Area 2
Phasic Tonic Unidentified
20 1 2
14 3 2
38 2 0
43 0 1
42 1 4
37 2 0
194 9 9
Area 3
Phasic Tonic Unidentified
20 1 6
15 0 3
21 1 7
27 0 4
20 0
25 0
128 2
75
63
105
112
109
Totals
1
0
114
2
1
578
Distribution of cutaneous mechanoreceptors on the paw over time. Receptors were grouped into three classes. as described in the text, and the paw was divided into three areas. Area I included the top of the forearm, the wrist, and the top and medial side of the paw. Area 2 included the knuckles and the tops and sides of the toes. Area 3 included the distal portion of the toes.
To determine whether the area of skin innervated by the units from which activity could be recorded changed with time, data from the different electrodes were combined, and a 3 X 6 contingency table showing receptive field location versus time for each class of units was constructed. There was no statistically significant difference in receptor distribution at different times ( X 2 test, p > 0.31). To determine whether the area of skin from which activity could be recorded differed from one electrode to another, data from different times for each electrode were combined, and a 3 X 10 contingency table showing the distribution of receptive field locations versus electrode was constructed. The receptive field locations were found
In a typical recording, activity could be evoked in about ten identifiable units. The response properties of the units indicated that the implant provided a representative sample of the receptor types present in the skin, and the receptive fields of these units were distributed throughout the region innervated by the fascicle. There was a gradual shift in the population of units from which activity could be recorded over time. However, as units dropped out, new units appeared in the recordings, so the overall number of units remained roughly constant. The appearance of new units in the recordings could have been due to the recovery of fibers which were damaged by the implantation procedure, and the loss of units due to degeneration of units injured by this procedure. However, it is unlikely that these two events would balance each other well enough to keep the total population constant. Moreover, the fact that the shift continued after the first month post-implantation, by which time most of the response to implant trauma would be expected to have taken place [5], suggests that the shift in population cannot be accounted for by cell recovery and degeneration in the vicinity of the electrode. This suggests that changes in the recorded population were due to small movements of the electrode within the fascicle, rather than loss of units due to damage caused by the electrode. The intrafascicular electrode recordings included fibers innervating several kinds of mechanoreceptors, which respond to various types of stimuli. For purposes of unit identification, we used controlled mechanical stimuli delivered to restricted regions of skin. However, if individual units could be identified on the basis of action potential size or shape, it would be possible to obtain both locus and modality specific information about any externally applied stimulus. Since the electrodes record from a restricted number of units, which limits the number of units to be sorted and minimizes superpositions, it may be feasible to use an on-line action potential sorting method to track single unit activity. Work in our lab is currently pursuing this possibility. The distribution of receptors and the receptor types from which activity could be recorded was representative of the activity in the whole fascicle. Hence, these recordings could provide both specific information about stimuli and a measure of the full range of information carried in the fascicle. Since individual units persisted in the intrafascicular electrode recordings for several months, these recordings could be used as a source of long-term information on single-unit activity. Additional modifications to the electrode design, currently under investigation in our lab, might improve the lifetime of single units in the recordings; but, even if drift in the population cannot be entirely
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IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 38. NO. 9. SEPTEMBER 1991
eliminated, it is sufficiently slow that periodic “updating,” i.e., remapping of the unit population would be adequate if the electrodes were to be used for feedback control in FES. Mapping of receptive fields and receptor classification can be performed relatively quickly, and in human subjects there would be no need for anesthesia during the procedure. The present experiments were performed in anesthetized animals and were designed to determine what kind of information was present in the recordings. Application of this technique would require recordings to be made in awake, behaving animals or people. This would necessitate identifying units on the basis of action potential configuration in the presence of movement and EMG artifacts. These artifacts can be minimized in a number of ways. First, our bipolar electrode design provides for common mode noise rejection. Second, the use of a restricted bandwidth amplifier tends to eliminate low frequency movement artifacts. Finally, if necessary the implant zone of the nerve could be surrounded by a nonconductive cuff, providing EMG rejection capabilities similar to that of cuff electrodes. These issues remain to be pursued, after it has been demonstrated that unit identification on the basis of action potential size and shape can be performed and once a new generation of highly flexible, implantable electrodes has been developed.
REFERENCES [ l ] P. R. Burgess, D. Petit, and R. M. Warren, “Receptor types in cat hairy skin supplied by myelinated fibers,” J . Neurophysiol., vol. 31, pp. 833-848, 1968. [2] P. R. Burgess, J . F. Howe, M. J . Lessler, and D. Whitehorn, “Cutaneous receptors supplied by myelinated fibers in the cat. 11. Number of mechanoreceptors excited by a local stimulus, J . Neurophysiol. , vol. 37, pp. 1373-1386, 1974. 131 J. A. Hoffer, M. Haugland, and T. Li, “Obtaining skin contact force information from implanted nerve cuff recording electrodes,” Proc. IEEEIEMBS Internat. Conj., vol. 11, pp. 928-929, 1989. [4] K . W. Horch, R. P. Tuckett, and P. R. Burgess, “A key to the classification of cutaneous mechanoreceptors,” J . Invesf. Dermarol. , vol. 69, no. 1, pp. 75-82, 1977. [ 5 ] K . W. Horch and S . J. W. Lisney, “On the number and nature of regenerating myelinated axons after lesions of cutaneous nerves in the cat,” J . Physiol., vol. 313, pp. 275-286, 1981. 161 T. M. Lefurge, E. V. Goodall, K . W. Horch, L. Stensaas. and A. A. Schoenberg, “Chronically implanted intrafascicular recording electrodes,” Ann. Eiomed. Eng., vol. 19. pp. 197-207. 1991.
M. S . Malagodi, K . W. Horch. and A. A. Schoenberg, “An intrafascicular electrode for recording of action potentials in peripheral nerves,” Ann. Biomed. Eng., vol. 17, pp. 397-410, 1989. A. A. Schoenberg, M. Malagodi, and K . Horch, “Extraction of somatosensory information from peripheral nerves for FNS applications,” Advances in External Conrr. Human Extremities IX,pp. 363373, 1987. W. H. Talbot, I. Darian-Smith, H . H. Kornhuber, and V . B. Mountcastle, “The sense of flutter-vibration: Comparison of the human capacity with response patterns of mechanoreceptive afferents from the monkey hand,” J . Neurophys., vol. 31, pp. 301-334, 1968. A. B. Vallbo and R. S . Johansson, “Properties of cutaneous mechanoreceptors in the human hand related to touch sensation,” Human Neurobiol., vol. 3 , pp. 3-14, 1984.
Eleanor V. Goodall (S’91) was born in Redwood City, CA, in 1963. She received the B.A. degree in biology from Reed College, Portland, OR, in 1985 and the M.E. degree in bioengineering from the University of Utah, Salt Lake City, in 1988. She is currently a doctoral candidate in bioengineering at the University of Utah. Her interests include the acquisition and processing of biological signals, modeling of neuromuscular systems, and neural networks.
Todd M. Lefurge was born in New Jersey in 1965. He received the B.S. degree in biomedical engineering from Tulane University, New Orleans, LA, in 1987 and the M.S. degree in bioengineering from the University of Utah, Salt Lake City, in 1990. His research interests include neural prosthetics and instrumentation.
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Kenneth W. Horch (M’88) was born in Cleveland, OH, in 1942. He received the B.S. degree from Lehigh University, Bethlehem, PA, and the Ph.D. degree from Yale University, New Haven, CT. He is an Associate Professor of Physiology and Bioengineering at the University of Utah, Salt Lake City, and Director of Neurological Testing at Topical Testing, Inc. His research activities are in the areas of neuroprosthetics, peripheral nerve regeneration and repair, sensory evaluation, and somatosensory physiology.
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