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Decoding motor intent from recorded neural signals is essential for the development of effective neural-controlled prostheses. To facilitate the development of online decoding algorithms we have developed a software platform to simulate neural motor signals recorded with peripheral nerve electrodes, such as longitudinal intrafascicular electrodes (LIFEs). The simulator uses stored motor intent signals to drive a pool of simulated motoneurons with various spike shapes, recruitment characteristics, and firing frequencies. Each electrode records a weighted sum of a subset of simulated motoneuron activity patterns. As designed, the simulator facilitates development of a suite of test scenarios that would not be possible with actual data sets because, unlike with actual recordings, in the simulator the individual contributions to the simulated composite recordings are known and can be methodically varied across a set of simulation runs. In this manner, the simulation tool is suitable for iterative development of real-time decoding algorithms prior to definitive evaluation in amputee subjects with implanted electrodes. The simulation tool was used to produce data sets that demonstrate its ability to capture some features of neural recordings that pose challenges for decoding algorithms.

The goal of the study was to provide specifications for a stimulating electrode array to be implanted in the lumbosacral spinal cord as part of a functional neuromuscular stimulation (FNS) system for control of lower extremity muscles in paralyzed individuals. Dual channel stimulation of the quadriceps activation pool in the feline ventral lumbo-sacral spinal cord was performed to measure electrode interactions and to explore the effect of various stimulation paradigms on muscle fatigue. There was no measurable overlap in the populations of motor neurons activated from 2 different electrodes for spacings /spl ges/1 mm with currents below 100 /spl mu/A. However, a statistically significant increase in the population of activated fibers due to current summation was observed when stimuli /spl ges/70 /spl mu/A. were simultaneously presented through pairs of electrodes within 3 mm of each other. Fatigue effects were studied with 3 paradigms: (1) stimuli were delivered through a single electrode, (2) stimuli were delivered through 2 electrodes with the stimulus to the second electrode presented during the refractory period of fibers stimulated by the first electrode, and (3) stimuli were interleaved between the 2 electrodes such that the stimulus to 1 electrode was presented midway between stimuli to the other electrode, and the rate of stimulation through a single electrode was half that used in the first 2 paradigms. During channel refractory and single channel stimulation did not differ from each other in the rate at which the muscle fatigued, in both cases the force decayed to 30% of its initial level within 2 min of the initiation of the stimulation regime, whereas the force with interleaved stimulation was still above the initial force at this time due to strong potentiation. Based on these results and on and activation pool dimensions obtained in an earlier study, preliminary specifications are presented for an electrode array to be implanted in the human spinal cord for functional neuromuscular stimulation.

The limits of present electrode technology are being reached in current motor prostheses for restoring functional movement in paralyzed people. Improved devices require electrodes and stimulation methods that will activate muscles selectively and independently with less implanted hardware. A practical functional neuromuscular stimulation (FNS) system may need to employ extraneural, intraneural, epimysial, or intramuscular electrodes or a combination of these types. The limitations of current muscle electrodes and the anatomy of peripheral nerve innervation of muscle have pointed to stimulation of peripheral nerve trunks as a promising area for investigation. Attempts to use conventional (extraneural) peripheral nerve electrodes for selective activation of muscles in chronic applications have met with only limited success. Intraneural (intrafascicular) electrodes offer the advantages of greater selectivity and lower power requirements, but these may be offset by the difficulty of inserting delicate electrodes through the collagenous epineurium and perineurium while avoiding unacceptable amounts of trauma. Cuff electrodes require more power than intrafascicular ones but may provide more stable recruitment patterns over time, and the opportunity for retrieval and replacement. >

Using recordings of peripheral nerve activity made with carbon fiber intrafascicular electrodes, the authors compared the performance of three different recording techniques (single channel, differential, and dual channel) and four different unit classification methods (linear discriminant analysis, template matching, a novel time amplitude windowing technique, and neural networks) in terms of errors in waveform classification and artifact rejection. Dual channel recording provided uniformly superior unit separability, neural networks gave the lowest classification error rates, and template matching had the best artifact rejection performance. >

The purpose of this study was to define the testing parameters that are most sensitive to sensory loss in carpal tunnel syndrome and then to track recovery of these sensations postoperatively. Two dozen patients underwent standard nerve decompression and were subsequently re-evaluated at six weeks, three months, and six months. The test battery included provocative maneuvers, light-touch threshold determined by manually applied monofilaments and skin indentation with the Automated Tactile Tester (ATT), manual two-point discrimination, manual high-frequency vibration and ATT low-frequency vibration, and ATT warmth detection. The most sensitive indicators of sensory abnormality were the ATT low-frequency vibration and skin indentation tests. Responses to all but these two tests returned to normal within two months postoperatively. The ATT indentation and vibration tests showed continual improvement over the study period, returning to nearly normal values by six months. Recommendations concerning the use of automated methods for testing sensory function are made in light of these findings.

The Automated Tacticle Tester (ATT) is a computer-controlled device designed to measure patients' cutaneous perception of touch, vibration, temperature, and pain. The ATT provides repeatable and precise control of the amplitude, rate of application, and duration of stimuli. Threshold values for skin indentation (touch), high- and low-frequency vibration, pinprick (sharpness), warmth, and two-point discrimination were obtained with the ATT from the fingers of 62 normal subjects. Manual monofilament and two-point discrimination tests were also performed on the same subjects. All the tests with the ATT, except pinprick, showed a statistically significant increase in threshold with age. There were no significant differences attributable to the hand or digit tested or the sex of the subject. These data were used to derive age-adjusted criteria for normal sensory function in the glabrous skin of the fingers. Thresholds were found to remain within normal limits when these subjects were retested at various time intervals. We conclude that the ATT provides repeatable and reliable measurements of sensory function in the skin and has potential application in the diagnosis and evaluation of compression and other peripheral neuropathies.

A visual prosthesis for the blind using electrical stimulation of the visual cortex will require the development of an array of electrodes. Passage of current through these electrodes is expected to create a visual image made up of a matrix of discrete phosphenes. The quality of the visual sense thus provided will be a function of many parameters, particularly the number of electrodes and their spacing. We are conducting a series of psychophysical experiments with a portable "phosphene" simulator to obtain estimates of suitable values for electrode number and spacing. The simulator consists of a small video camera and monitor worn by a normally sighted human subject. To simulate a discrete phosphene field, the monitor is masked by an opaque perforated film. The visual angle subtended by images from the masked monitor is 1.7 degrees or less, depending on the mask, and falls within the fovea of the subject. In the study presented here, we measured visual acuity as a function of the number of pixels and their spacing in the mask. Visual acuity was inversely proportional to pixel density, and trained subjects could achieve about 20/26 visual acuity with a 1024 pixel image. We conclude that 625 electrodes implanted in a 1 cm by 1 cm area near the foveal representation of the visual cortex should produce a phosphene image with a visual acuity of approximately 20/30. Such an acuity could provide useful restoration of functional vision for the profoundly blind.

Psychophysical and electrophysiological techniques were used to study the encoding and processing of information about the frequency content of vibrational stimuli applied to glabrous skin in humans and cats. Trained human subjects were asked to discriminate changes in stimulus frequency and harmonic content for pairs of mono- and diharmonic sinusoidal vibrations applied to the fingertips. These psychophysical tests supplied data on what information is available to the central nervous system about the frequency components of vibratory stimuli. Electrophysiological recordings from nerves innervating the glabrous skin of the paw in cats during presentation of the same stimuli used in the psychophysical study provided data on how the peripheral nervous system encodes information about the physical parameters of cutaneous vibratory stimuli. The two sets of data indicated that the subjects derived information about the frequency of vibrotactile stimuli from the mean interval between action potentials in afferent nerve fibers activated by the stimulus.

Nerve stimulation typically employs charge-balanced current injection with a delay between the cathodal and anodal phases. Typically these waveforms are produced using a microprocessor. However, once appropriate stimulus parameters are chosen, they tend to remain fixed within an application, making computational power unnecessary. In such cases, it would be advantageous to replace the microprocessor with integrated circuitry and hardware controls for maintaining fixed pulse parameters. We describe here an architecture that generates controllable charge-balanced pulses but requires no computer processing components. The circuitry has been engineered such that minimum size and power consumption can be achieved when fabricated into an IC chip, making it ideal for many long term, portable nerve stimulation devices and applications.

Evidence indicates that user acceptance of modern artificial limbs by amputees would be significantly enhanced by a system that provides appropriate, graded, distally referred sensations of touch and joint movement, and that the functionality of limb prostheses would be improved by a more natural control mechanism. We have recently demonstrated that it is possible to implant electrodes within individual fascicles of peripheral nerve stumps in amputees, that stimulation through these electrodes can produce graded, discrete sensations of touch or movement referred to the amputee's phantom hand, and that recordings of motor neuron activity associated with attempted movements of the phantom limb through these electrodes can be used as graded control signals. We report here that this approach allows amputees to both judge and set grip force and joint position in an artificial arm, in the absence of visual input, thus providing a substrate for better integration of the artificial limb into the amputee's body image. We believe this to be the first demonstration of direct neural feedback from and direct neural control of an artificial arm in amputees.

We are proposing to build a visual prosthesis based upon electrical stimulation of the visual cortex via an array of penetrating electrodes. To establish design specifications for the prosthesis we have conducted psychophysical experiments with a portable phosphene simulator. The simulator consists of a small video camera and monitor masked with an array of 63.5 pm diameter pinholes. The visual fizld of the monitor covers 1.7 degrees and falls within the fovea of the subject. The total number of phosphenes, their spacing, and the field magnification ratio can be varied. Subjects can achieve about 20130 visual acuity with a 1024 pixel image. Visual acuity is approximately an inverse linear function of pixel density.

This paper reviews behavioral, physiological, anatomical, and ecological aspects of sound and vibration detection by decapod crustaceans. Our intent is to demonstrate that despite very limited work in this area in the past 20 years, evidence suggests that at least some decapod crustaceans are able to detect and use sounds in ways that parallel detection and processing mechanisms in aquatic and terrestrial vertebrates. Some aquatic decapod crustaceans produce sounds, and many are able to detect substrate vibration at sensitivities sufficient to tell of the proximity of mates, competitors, or predators. Some semi-terrestrial crabs produce and use sounds for communication. These species detect acoustic stimuli as either air- or substrate-borne energies, socially interact in acoustic "choruses," and probably use "calls" to attract mates.

Purpose It is not known whether motor and sensory pathways associated with a missing or denervated limb remain functionally intact over periods of many months or years after amputation or chronic peripheral nerve transection injury. We examined the extent to which activity on chronically severed motor nerve fibers could be controlled by human amputees and whether distally referred tactile and proprioceptive sensations could be induced by stimulation of sensory axons in the nerve stumps. Methods Amputees undergoing elective stump procedures were invited to participate in this study. Longitudinal intrafascicular electrodes were threaded percutaneously and implanted in severed nerves of human amputees. The electrodes were interfaced to an amplifier and stimulator system controlled by a laptop computer. Electrophysiologic tests were conducted for 2 consecutive days after recovery from the surgery. Results It was possible to record volitional motor nerve activity uniquely associated with missing limb movements. Electrical stimulation through the implanted electrodes elicited discrete, unitary, graded sensations of touch, joint movement, and position, referring to the missing limb. Conclusions These findings indicate that both central and peripheral motor and somatosensory pathways retain significant residual connectivity and function for many years after limb amputation. This implies that peripheral nerve interfaces could be used to provide amputees with prosthetic limbs that have more natural feel and control than is possible with current myoelectric and body-powered control systems.

In response to the need for providing advanced engineering education in a shorter time period than is currently possible by traditional curricula, we have created an Accelerated Dual Degree program in biomedical engineering. The purpose of this pilot program is to attract the brightest students, get them involved in a research project at an early stage, and provide them with an intensive four-year educational program built around a cohort environment. Upon completion of the program, the students will receive a BS degree in biomedical engineering and an MS degree in bioengineering.

Intrafascicular electrodes have been implanted in peripheral nerves of cats and have recorded neural activity from a small population of nerve fibers for as long as six months, suggesting that these electrodes can be used on a chronic basis to acquire information about limb position, muscle force and skin contact. Preliminary results suggest that the activity of a small population of nerve fibers can be reliably monitored on a long-term basis using intrafascicular electrodes. They appear to cause little or no permanent damage and so may be suitable for use in an functional neuromuscular stimulation feedback system. >

Intraocular recordings were made from the eyestalks of dark-adapted fiddler crabs (Uca thayeri) during presentation of monochromatic light flashes of different wavelengths and intensities. Two types of signals were recorded in different experiments: slow potentials (electroretinogram) and fast potentials (spikes). The latter were also recorded in the presence of a continuous green or red adapting light. The resulting visual spectral-sensitivity curves, when fitted to rhodopsin-based visual pigment absorption spectra (from Dartnall nomograms), indicated the presence of two visual pigments, one with an absorption maximum near 430 nm, and the other with a peak absorption between 500 nm and 540 nm. The data also provided evidence for some differential bleaching of the pigments in the presence of a colored adapting light, but most of the adaptation effect was probably due to changes in screening pigment and neural desensitization or inhibition. These two observations suggest that an adequate substrate for color vision may exist in this and other species of fiddler crabs. The electroretinogram and spike-recording methods produced similar visual-sensitivity data, suggesting that latter technique, a much more efficient way of collecting data that is physiologically relevant, may be the method of choice for determining spectral sensitivity in crustaceans.

The goal of this study was to determine the feasibility of producing graded muscle contraction in individual muscles or muscle groups by electrically stimulating motor neurons in the lumbo-sacral spinal cord. Recruitment curves were obtained for quadriceps, tibialis anterior and triceps surae/plantaris by stimulating their activation pools in the ventral horn of the feline spinal cord. Mean twitch times-to-peak for quadriceps, tibialis anterior and triceps surae/plantaris were 33.0, 41.0, and 36.0 ms, respectively. Twitch duration as a function of stimulus strength demonstrated a mixed motor unit recruitment order, distinctively different from the inverse recruitment order exhibited by conventional methods of electrical stimulation of peripheral nerve. The recruitment curve slopes (expressed as a percentage of maximum force per nanocurrent of delivered charge) were shallow: 7.9 for quadriceps, 2.6 for tibialis anterior and 8.5 for triceps surae/plantaris. These results show that graded control of force in individual muscles or muscle groups can be obtained through spinal cord stimulation, and suggest that spinal cord stimulation could be used for functional neuromuscular stimulation applications.

Studies have demonstrated that muscle spindle organs provide the majority of the proprioceptive information available to the nervous system about limb position. Other studies suggest that a sense of position may be lacking in the fingers, as subjects were unaware of rather large excursions of finger joints if the excursions were made slowly enough. We sought to investigate the basis for this unexpected finding with a biomechanical model of the human long finger and the forearm muscles which actuate it, in order to study potential contributions of spindle organs in the extrinsic muscles of the hand to a sense of position of the finger. The model, based on cadaver data, allowed us to determine how precisely estimates of the lengths of the extrinsic finger muscles can be transformed into estimates of: (1) the flexion/extension angles of the individual finger joints, and (2) the location of the fingertip in the flexion/extension plane. We found that, for some finger positions, length information from all three extrinsic muscles was not sufficient to precisely estimate the flexion angles of all finger joints. Precision of joint angle estimates could be as poor as +/- 18% of joint range of motion. However, length information from just two of the extrinsic muscles taken together could always provide information sufficient to estimate the location of the fingertip relative to the metacarpophalangeal joint within a reasonably small tolerance (+/- one-half thickness of the fingertip). Furthermore, it was possible to make this estimate without determining any of the finger joint angles. These results suggest that spindles in the extrinsic muscles alone can signal fingertip location, even though they may not provide sufficient information to estimate the individual joint angles that set the position of the fingertip. Thus, an absence of position sense for individual joints (the sense many studies have tried to measure) may say little about a sense of location of the tip of the finger.

We present a method to iteratively train an artificial neural network (ANN) or other supervised pattern classifier in order to adaptively recognize and track temporally changing patterns. This method uses recently acquired data and the existing classifier to create new training sets, from which a new classifier is then trained. The procedure is repeated periodically using the most recently trained classifier. This scheme was evaluated by applying it to simulated situations that arise in chronic recordings of multiunit neural activity from peripheral nerves. The method was able to track the changes in these simulated chronic recordings and to provide better unit recognition rates than an unsupervised clustering method suited to this problem.

Neural network pattern-recognition techniques were applied to the problem of identifying the sources of action potentials in multi-unit neural recordings made from intrafascicular electrodes implanted in cats. The network was a three-layer connectionist machine that used digitized action potentials as input. On average, the network was able to reliably separate 6 or 7 units per recording. As the number of units present in the recording increased beyond this limit, the number separable by the network remained roughly constant. The results demonstrate the utility of neural networks for classifying neural activity in multi-unit recordings. >

A system for extracting single-unit activity patterns from multi-unit neural recordings was tested using real and simulated neural data. The system provided reliable estimates of firing frequency for individual units in simulated multi-unit data and allowed reliable determinations of the responses of individual cutaneous mechanoreceptor units to ‘natural’ stimuli such as brushing or pressing on the skin. An implementation of the system, which operated online and in real time, was used to obtain estimates of multiple, single-unit responses from multi-unit intrafascicular electrode recordings. The pattern of activity across the population of units in a given recording gave a reliable indication of the type of stimulus that had evoked the activity. It was concluded that this system, used in combination with intrafascicular peripheral nerve recordings, could be used to provide online, real-time information about peripheral stimuli.

Neural network pattern recognition techniques were applied to classify action potentials in multi-unit neural recordings made from chronically and acutely implanted intrafascicular electrodes in cats. The success of unit potential classification with the neural network was compared to that with a previously described template system. The neural network reliably separated 89 of the 194 recorded units, while the template system only separated 31 of the units. The network was able to reliably separate 6 or 7 units per recording, on average. The results demoustrate the potential superiority of neural networks over template matching approaches to classification of neural activity in multi-unit recordings.

The Automated Tactile Tester (ATT) was used to measure threshold values for trapezoidal skin indentation (light touch), low- and high-frequency vibration (50 and 150 Hz), pinprick (sharpdull transition point), warming (temperature awareness), and two-point discrimination in 61 patients with symptoms of median nerve compression at the wrist. We compared these data with values obtained in the same patients with manual monofilament tests, manual two-point discrimination measurements, and electrophysiologic nerve conduction studies. The ATT detected abnormal sensation in 71% of the hands tested, nerve conduction velocity was abnormal in 44% of the cases, and the manual tests indicated abnormality in 42% of the hands. The most indicative single test among those included in the present study for detecting sensory abnormality in these patients was threshold to a 50 Hz vibration administered by the ATT. We conclude that the ATT is a sensitive tool for the diagnosis and evaluation of compressive peripheral neuropathy and may allow objective documentation in a higher percent of patients than do more traditional testing methods.

A visual prosthesis, based on electrical stimulation of the visual cortex, has been suggested as a means for partially restoring functional vision in the blind. The prosthesis would create a pixelized visual sense consisting of punctate spots of light (phosphenes). The present study investigated the feasibility of achieving visually-guided mobility with such a visual sense. Psychophysical experiments were conducted on normally sighted human subjects, who were required to walk through a maze which included a series of obstacles, while their visual input was restricted to information from a pixelized vision simulator. Walking speed and number of body contacts with obstacles and walls were measured as a function of pixel number, pixel spacing, object minification, and field of view. The results indicate that a 25 × 25 array of pixels distributed within the foveal visual area could provide useful visually guided mobility in environments not requiring a high degree of pattern recognition.

A visual prosthesis based on electrical stimulation of the visual cortex with an array of penetrating electrodes is expected to produce pixelized visual images consisting of punctate spots of light (phosphenes). We measured reading speed in subjects viewing text with optically simulated phosphene fields in order to obtain estimates of the following design parameters for such an electrode array: pixel number, pixel spacing, and visual-field size. Comparisons were made between scanning the text with eye movements and scanning the text with head movements. The results indicate that a 25 x 25 array of pixels representing four letters of text projected on a foveal visual field of 1.7 degrees is sufficient to provide reading rates near 170 words/min with scrolled text and near 100 words/min with fixed text.

Classification of action potentials in multiunit recordings was based on the use of various types of features to uniquely characterize action potentials from different cells. The authors compared classification results obtained using three types of descriptive features: digitized data points, amplitude and duration (time domain) parameters, and fast Fourier transform (FFT) coefficients. Digitized data points used as descriptive features provided good classification success and required minimal computation. Time-domain features gave comparable results but required more computation. FFT coefficients were less effective than the other features. As the signal-to-noise ratio of the recordings increased, smaller differences in feature values could be discriminated. >

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 contained 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. >

A method has been developed for the manufacture of a "three-dimensional" electrode array geometry for chronic intracortical stimulation. This silicon based array consists of a 4.2 x 4.2 x 0.12 mm thick monocrystalline substrate, from which project 100 conductive, silicon needles sharpened to facilitate cortical penetration. Each needle is electrically isolated from the other needles, and is about 0.09 mm thick at its base and 1.5 mm long. The sharpened end of each needle is coated with platinum to facilitate charge transfer into neural tissue. The following manufacturing processes were used to create this array. 1) Thermomigration of 100 aluminum pads through an n-type silicon block. This creates trails of highly conductive p+ silicon isolated from each other by opposing pn junctions. 2) A combination of mechanical and chemical micromachining which creates individual penetrating needles of the p+ silicon trails. 3) Metal deposition to create active electrode areas and electrical contact pads. 4) Array encapsulation with polyimide. The geometrical, mechanical, and electrical properties of these arrays should make them well suited as interfaces to cortical tissue.

An automatic apparatus for testing tactile responses of a patient is disclosed. The embodiments of the invention variously include components for: applying a nonambient temperature to the patient's skin to test the patient's response to thermal stimuli; pricking the patient's skin to test the patient's response to pain; indenting the patient's skin to test the patient's response to touch; vibrating the patient's skin to test the patient's response to vibration; and for making two spaced apart contacts with the patient's skin to test the patient's two point discrimination response. A general purpose computer and dedicated control circuits function to control the operation of the system and record the responses of the patient. The embodiments of the present invention are able to repeatedly reproduce each test so that the tests carried out are reproducible and accomplished in a minimum of time.

The effect of indenting the skin at different rates on the perceived intensity of the stimulus was studied by indenting the skin of the fingertip with two triangular waveforms, given as a pair. The subjects were asked to judge which member of the pair was more intense. Perceived intensity was found to increase both with the depth and the speed of the indentation. In contrast, changes in the rate of skin indentation had little influence on perceived skin indentation depth. This suggests that intensity and depth are different attributes of tactile sensibility. Since the skin is viscous, a rapid indentation is more forceful than a slow indentation of the same depth, raising the possibility that perceived intensity is related to stimulus force. Even though intensity judgments were more closely correlated with the force of a stimulus than with the indentation it produced, a rapidly increasing force was felt as more intense than one that increased more slowly but attained the same final magnitude. When mechanoreceptors in the palmar aspect of the monkey's hand were excited with triangular stimuli like those used in the psychophysical studies, their discharge frequency increased with the rate of skin indentation. However, the receptors were distinctly more rate sensitive than the human judgments of stimulus intensity, suggesting that impulse summation in the central nervous system summates (integrates in the mathematical sense) the receptor input so as to enhance, relatively, the perceived intensity of the slower stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)

✓ The radial and sural nerves in cats were transected and either left unrepaired, repaired with 11-0 epineurial sutures, sutured and treated with topical application of Kenalog, or tubulated with hypoantigenic collagen. One year later the nerves were examined electrophysiologically, using single unit recording techniques, and histologically, by light microscopy. The results indicate that regeneration through the neuroma was comparable in all four groups, but that reinnervation of sensory structures in the skin was most successful in the tubulated nerves.

Rapid skin indentations can produce strong tap-like sensations that contain little information about skin indentation depth. Although slower stimuli produce weaker sensations, subjects can more accurately identify how the position of the skin surface changes with respect to the deeper tissues. This dissociation between intensity and depth information suggests that intensity and depth are served by different neural circuits.

Psychophysical tests on human subjects showed that judgments of skin indentation depth made when the fingertip was indented at rates from 0.2 to 16 mm/sec were quite insensitive to changes in indentation velocity. Similar results were obtained on the forearm at indentation velocities of 0.4 to 16 mm/sec. Recordings were made from mechanoreceptors in the monkey's hand that were able to respond over the same range of velocities and at comparable depths to determine how skin indentation depth might be signaled (coded) at the receptor level and to examine the rate sensitivity of the possible depth codes. It was found that most of the receptors with foci under the stimulator were recruited relatively early during an indentation, especially at velocities of 1.6 mm/sec and higher, making it improbable that the full range of indentation depths is signaled by the “subsurface” recruitment of different receptors at different indentation depths. A subsurface recruitment code involving subcutaneous receptors is not likely since subjects could feel virtually none of the stimuli after skin anesthesia. Progressive recruitment with depth of receptors whose foci lie further and further away from the stimulator (“lateral” recruitment) was considered an unlikely depth code because changing the area of the stimulator had little effect on its perceived depth. Also, it was shown that subjects could sense the curvature of the indentation (the profile of the depth at right angles to the skin surface), which requires information about the depth of individual patches of skin beneath the stimulator. There is no obvious way that a lateral recruitment code can provide this information. Thus it is probable that the discharge rate of some or all of the receptors excited by the indentation is involved in indicating its depth. Both impulse frequency and receptor recruitment at any given depth increased as the velocity of the indentation increased. The demonstrated reliability of information about skin indentation depth in humans indicates that the central neural circuitry responsible for judgments of skin indentation depth is able to compensate for the rate-sensitive receptor signals.

As background for a report on our current selective adaptation experiments in decapod crustaceans, the various facts and hypotheses generally relevant to intraretinal sensitivity to polarized light in arthropods as well as cephalopods have been marshaled. On the basis of this review, the following working hypotheses have been made. 1) One ommatidium in the compound eye is the functional unit in image perception but contains in its component retinular cells subunits which can work independently in detecting other visual parameters, such as polarization. 2) Single retinular cells do respond differentially to light polarized in various planes. 3) Light sensitivity, including e-vector detection, is localized in the rhab domeres, which comprise closely packed arrays of microvilli protruding axially from retinular cells; the dichroism of the photopigment molecules, which are contained within the microvilli, provides the molecular basis of e-vector detection. 4) The visual pigment molecules have their major dichroic axis aligned predominantly parallel to the long axis of the microvillus containing them; typically all microvilli in a single rhab domere are closely parallel to one another, thus comprising at the cellular level a unit dichroic analyzer with maximum optical density to photons vibrating in the direction parallel to these microvillous protrusions. 5) In most decapod crustaceans, in cephalopods, and in some insects the microvilli in all rhabdomeres of a retinula are oriented in only two directions, perpendicular. to each other. Therefore, e-vector perception must depend at the retinular level on a two channel system consisting of a pair of dichroic analyzers with their major transmitting axes fixed at a 90 degrees angle determined by the two directions of microvillus orientation. Our new results on selective adaptation in the eye of Cardisoma provide direct experimental evidence for such a two-channel analyzer in which the pair of functional units have their maximum sensitivity to polarization in the same retinal directions as the rhab dom microvilli observed in electron micrographs. In turn, these directions correspond with the vertical and horizontal axes of the animal's normal spatial orientation. In e-vector detection the seven retinular cells of a single decapod ommatidium thus form two operational subgroups of four and three cells, respectively (39). The correspondence of the electrophysiological evidence for a dual polarization analyzer with the perpendicular directions shown by the microvilli in a single rhabdom strengthens the idea that one ommatidium is enough for detecting e-vector orientation. On this evidence we may conclude that the model developed above for a two-channel polarization analyzer effectively accounts for the relevant spectrophotometric, fine-structural, electrophysiological, and behavioral data currently available for a considerable number of arthropods and cephalopods.

The authors are developing a computer-based method for identifying individual action potentials in multiunit peripheral nerve recordings. Threshold-level, sinusoidal stimuli are used to elicit phase-locked activity from single units. These action potentials are digitized, and a template is constructed using various waveform parameters. Once templates have been generated for all the units, the program is used in a sorting mode in which action potentials are detected and digitized, and parameter values are measured and compared to the template values. The output of the program is an indication of the instantaneous firing rate of each identified unit as a function of time. In principle, the sorting could be done online in real time. >

The cutaneous mechanoreceptors of the cat can be assigned to one of 11 groups by using physiologic tests performed with hand-held stimulators. The method of classification is described, and the validity and utility of this classification scheme are discussed.

The effect of proximal nerve crush on regeneration after unrepaired nerve transection was studied in feline cutaneous nerves. Delays between the initial transection and the subsequent nerve crush varied between 0 (immediate crush) and 13 months. A survival time of 6 to 12 months after the crush was allowed for regeneration to complete. Data from these animals was compared with data from animals in which the nerve was simply transected. Four measures of regeneration success were used to assess the degree of functional recovery: (i) the number of fibers crossing the neuroma and regenerating down the distal stump; (ii) the conduction velocity and diameter of individual fibers in the distal stump relative to their conduction velocity and diameter in the proximal stump; (iii) the number of fibers reinnervating cutaneous mechanoreceptors; and (iv) the number of cutaneous type I mechanoreceptors present after regeneration. As a whole, the data showed a statistically significant increase in the number of fibers entering the distal stump and in the number of fibers reinnervating the skin when the nerve was crushed 6 months after transection compared with nerves that were only transected. There was also an increase in the size and conduction velocity of fibers in the distal stump of nerves that had been crushed 4 to 6 months after transection.

1. Electrophysiological and anatomical techniques were used to investigate normal and regenerating sural and posterior femoral cutaneous nerve fibres in the cat. 2. One and a half years after transection of these nerves it was found that the regenerating neurones supported multiple sprouts in the distal stump of the nerve. The branching occurred at or beyond the level of the neuroma and some of the branched fibres innervated split receptive fields on the skin. 3. Counts of the number of axons in the proximal stumps of transected nerves showed that the whole original population of myelinated fibres persisted for at least 18 months. About 75% of these fibres successfully crossed the unrepaired transection site and regenerated into the distal stump of the nerve to re-form functional connexions in the skin. 4. After nerve crush all the myelinated axons regenerated. None showed signs of abnormal branching. 5. After crush the conduction velocities of the regenerated axons in the distal stump of the nerve reached nearly normal values by 6 months. After nerve transection the distal conduction velocities were reduced to 50% of normal even 18 months after the injury. 6. The implications of these findings for the recovery of function after nerve injury in man are discussed.

1. Micro-electrode recordings were made from normal and regenerating sural nerve fibres in cats. Increases in the excitability of the central terminals of these fibres after conditioning stimulation of other sural nerve fibres were taken as evidence for primary afferent depolarization. 2. At all recovery times studied the excitability changes seen were significantly less than those seen in control animals. Two factors contributed to the changes in primary afferent depolarization. First, the proportion of fibres that showed no evidence of primary afferent depolarization increased significantly. This proportion became smaller as recovery progressed. Secondly, where primary afferent depolarization was present, the magnitudes of the effects were slightly but significantly decreased compared with control values. 3. Excitability changes of the central terminals of sural nerve fibres were also measured after conditioning stimulation of the ipsilateral, unlesioned accessory sural nerve. One month after sural nerve transection there was a significant increase in the proportion of fibres showing no evidence of excitability changes following accessory sural nerve conditioning stimulation compared with control animals. Thus, the loss of primary afferent depolarization of regenerating sural nerve fibres was neither simply a consequence of desynchronization of the volley of impulses entering the spinal cord after conditioning stimulation of other regenerating sural fibres, nor due to fewer fibres being activated during conditioning stimulation of the lesioned nerves. 4. A possible explanation of these results is that after peripheral nerve crush or transection the central terminals of the damaged fibres retract or atrophy. Then as regeneration of the nerve proceeds, the central terminals of the fibres re-form.