Impulse Patterns Research Articles

Neurons in the inferior colliculus, auditory cortex and pontine nuclei of the FM bat, Eptesicus fuscus respond to pulse repetition rate differently

Single-neuron responses to pulse repetition rate in the inferior colliculus, auditory cortex and pontine nuclei of the FM bat, Eptesicus fuscus were studied under free-field stimulation conditions. The best frequency (BF) and minimum threshold (MT) of each neuron were first determined with a 4 ms pulse broadcast from a specific point (response center) of the bat's frontal auditory space at which the neuron had maximal spatial sensitivity. The neuron's intensity-rate function was then studied with a 4 ms BF pulse delivered at 10 dB increments above its MT in order to determine the best intensity to which the neuron discharged maximally. The neuron's discharge pattern and number of impulses to 32 trials of 300 ms train stimuli, which consisted of different number of 4 ms BF and best intensity pulses (1, 2, 3, 8, 12, 19, 24, 29 pulses/train) and delivered at an interpulse interval of 1000, 250, 170, 100, 40, 25, 15, 12 and 10 ms (i.e. at a pulse repetition rate of 1, 4, 6, 10, 25, 40, 67, 83, 100 pulses/s), were sequentially recorded. All neurons recorded from the inferior colliculus, auditory cortex and pontine nuclei discharged phasically (1–3 impulses) but they responded to the pulse repetition rate in different manners. More than 63% of 38 inferior collicular and 65 pontine neurons studied discharged impulses to each pulse within a train stimulus when the pulse repetition rate was up to 40 pulses/s. Although the number of neurons showing such a one-to-one following response decreased with increasing pulse repetition rate, at least 20% of neurons still maintained such a response pattern even at a pulse repetition rate of 100 pulses/s. In contrast, responses of 60% of 36 cortical neurons followed the pulse repetition rate only up to 10 pps beyond which all but one discharged impulses to the first pulse of a train stimulus. We suggest that the different discharge pattern to pulse repetition rate among individual neurons from these three brain centers is primarily due to their different recovery cycles.

Impulse patterns of atmospherics over the North Atlantic

Impulse patterns of atmospherics over the North Atlantic

Audibility of partials in inharmonic complex tones

These experiments examined the ability of musically trained subjects to hear out individual partials in complex tones with partials uniformly spaced on a scale related to the equivalent rectangular bandwidth (ERB) of the auditory filter. ERB spacings of 0.75, 1.0, 1.25, 1.5, and 2 were used, and the central component always had a frequency of 1000 Hz. All components had a level of 65 dB SPL. On each trial, subjects heard a pure tone (the "probe") followed by a complex tone. The probe was close in frequency to one of the partials in the complex, but was mistuned downward by 4.5% on half the trials (at random) and mistuned upward by 4.5% on the other half. The task of the subject was to indicate whether the probe was higher or lower in frequency than the nearest partial in the complex. The partial that was "probed" varied randomly from trial to trial. Scores for the highest and lowest components in the complexes were generally high (> 90%) for component spacings greater than 1 ERB, but worsened somewhat for ERB spacings of 0.75 and 1.0. Scores for the inner components were close to chance level at 0.75-ERB spacing, and improved progressively as the ERB spacing was increased from 1 to 2 ERBs. For ERB spacings of 1.25 or less, the scores did not change smoothly with component frequency; marked irregularities were observed, as well as systematic errors. An explanation for these is suggested in terms of irregularities in the middle ear transfer function. Performance for the inner components tended to be worse for component frequencies above 1000 Hz than below 1000 Hz. It is suggested that this happens because the pitches of partials are partly coded in the time patterns of neural impulses (phase locking), and the precision of phase locking deteriorates progressively with increasing frequency above 1000 Hz. The auditory filter shapes of the subjects were measured for a center frequency of 1000 Hz, using the notched noise method. One subject had a broader auditory filter than the other three subjects, and this same subject generally had more difficulty in hearing out partials from complex tones than the other subjects.

Responses of slowly adapting type II afferent fibres in cat hairy skin to vibrotactile stimuli.

1. Slowly adapting type II (SAII) afferent fibres that supply the forelimb were isolated from the medial cutaneous nerve of anaesthetized cats and examined for their capacity to signal information about vibrotactile events in the hairy skin. 2. The SAII fibres had a single spot-like receptive field focus where they were highly sensitive to steady indentation and vibration applied with probes normal to the skin surface. However, their sensitivity was affected profoundly by the size of the stimulus probe, its position in relation to the receptive field focus and, to a lesser extent, the magnitude of any pre-indentation on which vibration was superimposed. Small stimulus probes (e.g. 250 microns diameter) were much more effective than larger (> or = 1-2 mm) ones, and small shifts in the position of the perpendicularly applied probe away from the receptive field focus led to a marked decline in responsiveness. 3. With appropriate choice of stimulus parameters for vibratory stimuli applied at the receptive field focus, the SAII fibres could respond at low threshold (< 100 microns), with a tightly phase-locked, regular 1:1 impulse pattern (one impulse per vibration cycle) that accurately signalled the vibration frequency over a bandwidth that extended to 600 Hz. Furthermore, their responses remained phase-locked up to 1000 Hz. Phase-locking in SAII fibres was marginally tighter than that in SAI fibres and comparable to that of Pacinian corpuscle fibres. 4. The sensitivity of forelimb SAII fibres to tangential skin stretch was directionally selective; stretch across the forelimb was much more effective than along its long axis. Vibration associated with tangential skin stretch led to a marked spatial expansion of the field of vibration sensitivity. SAII fibres could therefore signal information about natural stimuli that contain elements of skin stretch and vibration, as may be encountered when the forelimb brushes against textured surfaces. Should the SAII fibres fail to contribute to the sensory experience of vibrotactile stimuli, the explanation may be related to limitations imposed centrally on the processing of their signals. Nevertheless, the present results demonstrate that, with appropriate stimulus conditions, the SAII afferent fibres have much greater vibrotactile sensitivity than has been suggested by past studies.

Intra-neural electrical stimulation of cutaneous nociceptive fibres in humans: effects of different pulse patterns on magnitude of pain.

A study was performed to elucidate how different impulse frequencies and impulse patterns in cutaneous nociceptive fibres influence the subjective magnitude of pain. Groups of nociceptive A delta and C fibres were co-activated by electrical intraneural stimulation at constant intensity in cutaneous fascicles of the peroneal nerve in healthy human subjects. The resulting pain sensations were rated on a modified visual analogue scale. Five-second trains were administered randomly at irregular intervals of at least 30 s. Five of the stimulus patterns had regular interpulse intervals, corresponding to frequencies of 1, 2, 4, 10 and 15 Hz, and three other patterns were constructed to mimic to some extent the initially phasic and subsequently slowly adapting discharge patterns which may be encountered in recordings from human nociceptors. The results from these experiments using stimulation frequencies within physiological discharge ranges for human nociceptors indicate that the subjective magnitude of pain increases monotonously as a function of stimulus frequency, and that patterns mimicking nociceptor discharges in response to natural stimuli give rise to greater peak magnitudes of pain than artificial regular patterns with a corresponding number of impulses.

Vibrotactile sensitivity of slowly adapting type I sensory fibres associated with touch domes in cat hairy skin.

1. Recordings were made from single slowly adapting type I (SAI) afferent fibres associated with touch domes in the cat hairy skin. Controlled vibratory stimuli were used first, to characterize the precision with which these SAI afferents reflect the temporal aspects of vibrotactile stimuli, and second, to determine whether earlier disparate reports of SAI responsiveness to vibration may be attributable to highly specific stimulus requirements. 2. Eighteen SAI fibres from femoral cutaneous nerve branches were examined; each was associated with one to three touch domes. SAI responses to both steps and sinusoidal vibration (1-1.5 s in duration) were affected profoundly by both probe size and position. Punctate stimulus probes (250 microns) produced much higher response levels and steeper stimulus-response relations than those elicited with large (2 mm) probes, probably on account of focal distortion created within the dome by the smaller probes. SAI sensitivity to vibration was also affected markedly by the amplitude of any pre-indentation on which the vibration was superimposed; sensitivity was much lower when the pre-indentation exceeded 100 microns, in particular with larger stimulus probes. 3. Measures of both vibration sensitivity and the precision of impulse patterning demonstrated that, if appropriate stimulus parameters are chosen, the SAI fibres can respond to 1 s trains of vibration (amplitude < or = 100 microns) in a tightly phase-locked, 1:1 manner for frequencies up to 500 Hz. At frequencies from approximately 100-500 Hz the SAI fibres displayed broad 1:1 plateaus, where their response rate remained constant over a range of amplitudes, and phased-locking was tightest. Responses remained phase-locked up to 1000 Hz, but could not follow the vibration with a 1:1 pattern above 500 Hz. 4. The results demonstrate that with appropriate stimulus parameters, touch dome-associated SAI fibres are capable of signalling vibrotactile information over a similar bandwidth of frequencies as do Pacinian sensory fibres. The variability in past reports of SAI vibration sensitivity may relate principally to differences in stimulus conditions. However, in view of the SAI capacity for responding to vibration with temporally precise, patterned activity, it appears that their reported failure to contribute to vibrotactile sensibility must be attributed to limitations imposed in the central processing of SAI signals.

Impulse conduction in sympathetic nerve terminals in the guinea-pig vas deferens and the role of the pelvic ganglia

Focal extracellular recording techniques were used to study nerve impulse propagation and the intermittent transmitter release mechanism in sympathetic nerve terminals of the guinea-pig vas deferens in vitro. In particular, the nature of impulse propagation in postganglionic nerve fibres was characterized following pre- or postganglionic stimulation. Conventional intracellular recording techniques were also used to study directly ganglionic transmission in cell bodies in the anterior pelvic ganglia. When brief electrical stimuli were applied to the hypogastric nerve trunk close to the prostatic end of the vas deferens, the nerve terminal impulses recorded extracellularly could be evoked either directly by stimulation of the parent axon (i.e. postganglionically) or indirectly by stimulation of the preganglionic nerve fibre. In 364 separate recordings, nerve terminal impulse conduction failure was not observed during trains of stimuli at 1 Hz. However, apparent “intermittent conduction” of nerve impulses was noted on 16 occasions. In these fibres, the degree of intermittent conduction decreased as the frequency of stimulation was increased. Conduction in these intermittent fibres was reversibly interrupted by removing calcium from the Krebs' solution or by the addition of the ganglion blocker, hexamethonium (30–100 μM). Thus, the cause of intermittent conduction is failure of the transmission of excitation in the sympathetic ganglia. Impulses evoked by postganglionic stimulation never failed to propagate into the nerve terminals, and changes in the shape or amplitude of the nerve terminal impulse during trains of stimuli were not detected. One effect of stimulation was a frequency-dependent increase in the latency of the nerve terminal impulse which developed during the train of stimuli. Thus, intermittence of transmitter release from individual varicosities cannot be attributed to failure of impulse propagation in sympathetic nerve terminals. Transmission in the anterior pelvic ganglia was investigated directly by making intracellular recordings from cell bodies whose terminals projected to the vas deferens. Many cell bodies received a strong synaptic input which generated an action potential in the postganglionic cell body on a one-to-one basis. However, in some cell bodies there was a low safety factor for the generation of the action potential by the excitatory postsynaptic potential. The safety factor for generating an action potential in the postganglionic cell body was raised by increasing the frequency of stimulation. These findings suggest that peripheral ganglia are not simple one-to-one relay stations, but may well play an important role in controlling the patterns of nerve impulse traffic in postganglionic sympathetic neurons.

Influence of impulse pattern on noradrenaline release from sympathetic nerves in cerebral and some peripheral vessels.

Earlier findings suggested that the physiological firing rate in sympathetic nerves did not exceed 10 Hz. Later recordings have revealed that this is a net value: impulses are usually not continuous but occur in bursts of high frequencies separated by quiet periods. The effects of continuous and burst-like neurogenic activation in various isolated blood vessels were compared. In the first part of the study changes in vascular tone were registered. Electrical field stimulatory parameters were chosen to give tetrodotoxin-blockable, neurogenic responses. From dose-response experiments in rat caudal artery a net frequency of 6 Hz was chosen, for bursts usually designed as 30 Hz during 0.2 s followed by a quiet period of 0.8 s. In the rabbit ear artery the neurogenic contraction was enhanced by a mean of near 50% during stimulation with bursts. Now appeared a minor phentolamine-resistant portion, which was not due to release of NPY, 5-HT, histamine or ATP. Also in the rat caudal artery and monkey pial artery a significant enhancement of contraction was seen during burst simulation, whereas in the rabbit facial vein no significant difference in dilatation through beta-receptor activation was obtained. In vessels that do not normally respond with purely neurogenic contractions/dilatations during continuous stimulation, like pial arteries from the rat and rabbit, not even bursts revealed a neurogenic response. In a second series of experiments the influence of continuous and burst-like nerve activation on the release of [14C]noradrenaline was studied in monkey, rabbit and rat pial arteries, rat caudal artery and rabbit central ear artery and facial vein.(ABSTRACT TRUNCATED AT 250 WORDS)

Parallel processing of tactile information in the cerebral cortex of the cat: effect of reversible inactivation of SI on responsiveness of SII neurons.

1. Localized cortical cooling was employed in anesthetized cats for the rapid reversible inactivation of the distal forelimb region within the primary somatosensory cortex (SI). The aim was to examine the responsiveness of individual neurons in the second somatosensory area (SII) in association with SI inactivation to evaluate the relative importance for tactile processing of the direct thalamocortical projection to SII and the indirect projection from the thalamus to SII via an intracortical path through SI. 2. Response features were examined quantitatively before, during, and after SI inactivation for 29 SII neurons, the tactile receptive fields of which were on the glabrous or hairy skin of the distal forelimb. Controlled mechanical stimuli that consisted of l-s trains of either sinusoidal vibration or rectangular pulses were delivered to the skin by means of small circular probes (4- to 8-mm diam). 3. Twenty-three of the 29 SII neurons (80%) showed no change in response level (in impulses per second) as a result of SI inactivation. These included seven neurons activated exclusively or predominantly by Pacinian corpuscle (PC) receptors, six that received hair follicle input, four activated by convergent input from hairy and glabrous skin, and six driven by dynamically sensitive but non-PC inputs from the glabrous skin. 4. Six SII neurons (20%), also made up of different functional classes, displayed a reduction in response to cutaneous stimuli when SI was inactivated. 5. Stimulus-response relations, constructed by plotting response level in impulses per second against the amplitude of the mechanical stimulus, showed that the effect of SI inactivation on individual neurons was consistent over the whole response range. 6. The reduced response level seen in 20% of SII neurons in association with SI inactivation cannot be attributed to direct spread of cooling from SI to the forelimb area of SII, as there was no evidence for a cooling-induced prolongation in SII spike waveforms, an effect that is known to precede any cooling-induced reduction in responsiveness. 7. As SI inactivation produced a fall in spontaneous activity in the affected SII neurons, we suggest that the inactivation removes a source of background facilitatory influence that arises in SI and affects a small proportion of SII neurons. 8. Phase-locking and therefore the precision of impulse patterning were unchanged in the responses of SII neurons to vibration during SI inactivation. This was the case whether response levels of neurons were reduced or unchanged by SI inactivation.(ABSTRACT TRUNCATED AT 400 WORDS)

Processing of acoustic signals in a cochlear model incorporating laterally coupled suppressive elements

Development of hearing machines with brain-like performance can be approached by faithfully modeling the human auditory nervous system. As an initial step in this effort, a composite cochlear model has been built which processes acoustic signals in a manner consistent with physiological responses recorded from the mammalian auditory nerve. The model integrates a current state of knowledge about cochlear function and provides a coherent picture of the representation of acoustic signals in the pattern of neural impulses distributed across a tonotopically organized array of auditory-nerve fibers. The model explicitly represents the active electromechanical responses of outer hair cells as a possible mechanism for adaptive control of basilar membrane damping, and introduces lateral coupling of resistive elements in the cochlear mechanics based on this mechanism. The output of the model is represented as an acoustic image being transferred to the central auditory nervous system in a massively parallel fashion over the auditory nerve. The impact of the cochlear nonlinearity on this image is explored. Experiments on applying this model to speech analysis show advantages of the cochlear modeling approach over traditional linear analysis methods.

The Charles Bonnet Syndrome: ‘Phantom Visual Images’

The Charles Bonnet syndrome is a condition in which individuals experience complex visual hallucinations without demonstrable psychopathology or disturbance of normal consciousness. An analysis of the sixty-four cases described in the literature reveals that the syndrome can occur at any age though it is more common in elderly people. Reduction in vision, due to peripheral eye pathology as well as pathology within the brain, is associated with the syndrome. Individual hallucinatory episodes can last from a few seconds to most of the day. Episodes can occur for periods of time ranging from days to years, with the hallucinations changing both in frequency and in complexity during this time. The hallucinations may be triggered or stopped by a number of factors which may exert their effect through a general arousal mechanism. People, animals, buildings, and scenery are reported most often. These images may appear static, moving in the visual field, or animated. Emotional reaction to the hallucinations may be positive or negative. Several theories have been proposed to account for the hallucinations. This paper highlights the sensory deprivation framework, with particular emphasis on the activity in the visual system after sensory loss that produces patterns of nerve impulses that, in turn, give rise to visual experience.

An analysis of the oscillatory patterns in the central nervous system with the wavelet method

This paper discusses a simple application of the wavelet transformation to analyse nerve cell impulse patterns. The action potentials converted into delta, or Dirac, functions were convoluted in the time domain with a modified Gauss (the negative of the second derivative of Gauss) function, varying in width between 0.6 and 384 ms. The width of the Gauss function was varied in 640 steps. Some parts of the transformation were extended, analysed and averaged in the frequency domain to explore oscillatory components of the impulse pattern. The sequences of action potentials of retinal ganglion cells evoked by short flashes are taken as examples. The present analysis demonstrate some properties of mathematical “microscopic” application to transient responses of the central nervous system (CNS), whereby the degree of magnification (steps of transformation) was varied.

Basic Studies on Skeletal Muscle for Cardiac Assistance

It is well recognized that skeletal muscle is incapable of maintaining the continuous power output needed for significant circulatory assistance unless it is rendered fatigue resistant by electrical stimulation. Most studies using conditioning patterns related to the pumping rate of the natural heart have suggested that such patterns can produce complete transformation to a slow phenotype. Such a transformation has two undesirable accompaniments: a reduction in the contractile speed of the muscle and an extreme reduction in its power-generating capacity. We have investigated the chronic effect on rabbit muscle of intermittent high frequency contractions either alone or overlaid on a continuous background of 10 Hz. If the amount of stimulation were the overriding stimulus for transformation, then the rate and/or extent of transformation should, if anything, be greater with the additional activation. If, on the other hand, components of the transformation could be modulated independently by the pattern of impulses, then the reduction in power output might be avoided to some extent by a pattern somewhat analogous to a strength-training exercise regime. Our results suggest that high force contractions may indeed preserve power output without compromising the acquisition of fatigue resistance.

Distinguishing Female Borderline Adolescents from Normal and Other Disturbed Female Adolescents

PSYCHOANALYTIC theory would suggest that differentiating the borderline adolescent from his or her normal peers should be a difficult task. In many respects the developmental crises of adolescence dovetail with critical conflicts of Borderline Personality Disorder--e.g., identity formation and separation-individuation. Extensive semi-structured interviews (Gunderson's Diagnostic Interview for Borderlines) of normal, borderline and other disturbed adolescents provide a data base for examining the hallmarks of borderline pathology in adolescence. This paper focuses on the qualitative differences in patterns of impulsivity, affective lability, dissociative experience, and interpersonal relationships that distinguish borderline teenagers from other seriously disturbed and normal adolescents. The paper also outlines modifications in the Gunderson interview for an adolescent population.

Electromyographic changes in the isolated rat diaphragm during the development of fatigue

To investigate whether the power spectrum of the electromyogram of a fatiguing muscle can be used to infer the degree to which the muscle is fatigued, we recorded isometric tension and two monopolar electromyograms from eight isolated rat diaphragm preparations suspended in an organ bath containing a balanced salt solution. Each preparation was excited with a fixed phrenic nerve impulse pattern made up of a 70-Hz train of impulses of supramaximal voltage delivered for 170 ms with a 500-ms recovery period. Tension fell rapidly over the first 60 s of the fatigue run and more slowly for the remaining 60 s analysed. The duration of extracellular action potentials increased and their amplitude decreased as the tension developed by the diaphragm decreased; conduction velocity along muscle fibres also decreased. The centroid frequency (fcen) of the power spectrum of the first action potential elicited by each train of stimuli decreased rapidly until tension fell to approximately 70% of the initial value; thereafter little change in fcen occurred, although tension continued to fall to 33% of its initial value. Our results demonstrated that under controlled conditions, fcen provided a sensitive index of fatigue in its early stages, but provided no information once fatigue was pronounced.

Impulse patterning and relaxation propagation in excitable media

Wavetrains of impulses in homogeneous excitable media relax during propagation toward constant-speed patterns. Here we present a study of this relaxation process. Starting with the basic reaction-diffusion or cable equations, we derive kinematics for the trajectories of widely spaced impulses in the form of ordinary differential equations for the set of times at which impulses arrive at a given point in space. Stability criteria derived from these equations allow us to determine the possible asymptotic forms of propagating trains. When the recovery after excitation is monotonic, only one stable train exists for a given propagation speed. In the case of an oscillatory recovery, however, many stable trains are possible. This essential difference between monotonic and oscillatory recoveries manifests itself in qualitatively distinct relaxational behaviors.

Rotor blade-vortex interaction impulsive noise source localization

An acoustic source localization scheme applicable to noncompact moving sources is developed and applied to the blade-vortex interaction (BVI) noise data of a 40 % scale BO-105 model rotor. A generalized rotor wake code is employed to predict possible BVI locations on the rotor disk and is found useful in interpreting the acoustic localization results. The highly varying directivity patterns of different BVI impulses generated at the same test condition is explained by both the localization results and predicted tip vortex trajectories. The effects of rotor tip-path-plane angle and advance ratio on the BVI source positions is studied. Decreasing tip-path-plane angle moves the general interaction region upwind on the rotor disk, significantly changing the interaction geometry. The region of strongest BVI sources lies between 60- and 70-deg azimuth and 80 and 90 % radius.

Sympathetic transmission in small mesenteric arteries from the rat: influence of impulse pattern

We have previously studied the transmitters involved in, and the calcium dependency of, the neurogenic response to continuous, regular nerve stimulation in small mesenteric arteries from the rat. We have now studied responses to irregular nerve stimulation in these respects. Small mesenteric arteries from rat were mounted into a myograph and the intramural sympathetic nerves were activated by field stimulation. Irregular nerve stimulation was patterned after recorded activity in human cutaneous nerves. The response to irregular stimulation was reduced by only approximately 50% in the presence of the alpha 1-adrenoceptor antagonist prazosin. In particular, responses to low-frequency stimulation were resistant to prazosin. When the extracellular calcium concentration was reduced from 2.5 to 1.0 mM, approximately 50% of the response to irregular nerve stimulation remained. Responses to low-frequency stimulation were particularly reduced. Thus in these arteries the neurogenic response is caused by noradrenaline and a, perhaps purinergic, co-transmitter. The co-transmitter is important for the response to low-frequency stimulation and for the initial part of the contraction caused by high-frequency stimulation. Reducing the calcium concentration affects more strongly the response to low-frequency nerve stimulation.

Random-Sequence Stimulation of the G1 Hair Afferent Unit

Impulse trains were recorded from the parent axon of the cat G1 hair afferent unit. Separate random (Poisson-like) trains of mechanical stimuli were applied to two coinnervated receptive field hairs individually or concurrently. The objective was to determine whether the parent axonal impulse train elicited by dual-hair stimulation was due to a temporal combining ("mixing"; Fukami, 1980) of the impulse trains elicited in the parent axons by the same stimulation to each hair alone. Both impulse rates and patterns were assessed. During single-hair random stimulation, impulse trains differed from stimulus trains, having lower mean rates and short-interval doublets. During dual-hair random stimulation, mean impulse frequencies were on average 36% less than those predicted for mixing. There were no correlations between stimulus amplitude and departures from mixing. As a further test of the mixing hypothesis, the two single-hair-elicited impulse trains were temporally merged (i.e., superimposed to form one impulse train). Such merged impulse trains were compared with the corresponding dual-hair-elicited impulse train. Dual-hair-elicited frequencies were typically less than those of the merged trains, despite the use of an absolute-refractory-period criterion during merging. The impulse patterns elicited by dual-hair stimulation usually differed from the merged-train patterns. Temporal coupling between stimuli and impulses was either variable or absent during single-hair random stimulation; such coupling was altered during dual-hair random stimulation. In summary, this work showed that the dual-hair responses could not be predicted from the single-hair responses. Limitations of the mixing hypothesis and possible biophysical mechanisms in the axonal arborization are discussed. The results are consistent with a general hypothesis of analog processing within the arborization of the parent axon.

Impulse Patterns of Auditory Nerve Fibres to Extra-and Intracochlear Electrical Stimulation

The impulse patterns of single auditory nerve fibres in cats with normal hearing and in deafened cats were studied during electrical stimulation of the cochlea at different locations. In the case of extracochlear round-window membrane stimulation, cats with normal hearing showed lowest thresholds (32 dB re 1 μA rms) for sinusoidal stimulation in the frequency range 50-300 Hz. These thresholds were independent of the acoustically determined characteristic frequencies of the fibres. The fibres of acutely deafened cats (by Neomycin) showed no spontaneous activity and could be activated only electrically with similar current values as in normal hearing cats. The action potentials were highly synchronized with the electrical signal up to 12.8 kHz. Intracochlear electrical stimulation was performed using a human multichannel cochlear implant electrode array. The excitation thresholds of the fibres for current stimulation showed poor place dependency when using different electrodes (monopolar) in the first turn of the cochlea with a remote indifferent electrode inside the middle ear. Better localization could be obtained when using bipolar stimulation with electrode pairs (electrode distance 0.75 mm) in the scala tympani. With parallel bipolar stimulation at two different locations (distance 1.5 mm) using two independent current sources a clear interaction could be observed in the firing pattern of single fibres. The measured gain and phase relations between current signal and neuronal response allowed the calculation of a model describing the summation of current density vectors acting on an excitable nerve membrane model. On the basis of these measurements, input networks were calculated to improve the channel separation of multichannel cochlear implants.