Spatial selectivity in a rotationally symmetric model of the electrically stimulated cochlea.

A rotationally symmetric model of electrical stimulation of the guinea pig cochlea with active neural elements is used to study the influence of temporal stimulus parameters and electrode configurations on the spatial selectivity of electrical stimulation by cochlear implants. The width of the excitation patterns is determined with respect to the position of the stimulating electrode pairs in the cochlea. Computed O10 AB values are compared against single fibre data from the cat cochlear nerve as measured by Van den Honert and Stypulkowsky (1987). It turns out that the use of charge-balanced asymmetric rather than symmetric biphasic pulses approximately doubles the number of independent channels that can be applied in a cochlear implant with longitudinal bipolar electrodes, like a configuration with radial electrode pairs using symmetric biphasic pulse stimulation will also do. Finally, the influence on Selectivity of the physiological variation in diameter of the cochlear nerve fibres and of a possible loss of their peripheral processes is studied.

A model of myelinated nerve fibres for electrical prosthesis design.

Starting with the spatially extended non-linear node model (Reilly et al., 1985), which incorporates Frankenhaeuser-Huxley non-linearities at each of several nodes in a row, a model is developed to describe many aspects of the behaviour of mammalian nerve fibres in a quantitative way. By taking into account the effects of temperature and by introducing a realistic nerve morphology, a good fit is obtained between the shape, duration and conduction velocity of simulated and in vivo action potentials in mammals. The resulting model correctly predicts the influence of physiological variations of body temperature on various aspects of nerve behaviour. It is shown that the absolute refractory period predicted by the model is within physiological ranges. Both in vivo and in the model, the spike amplitude and the spike conduction velocity are reduced in the relative refractory period. It is concluded that single-node models (although widely used) cannot replace this multiple nonlinear node model, as the stimulus repetition rates that can be followed by the simulated nerve fibre are limited by impulse conduction properties, rather than by the frequency following behaviour of a single node.

A quantitative approach to modeling mammalian myelinated nerve fibers for electrical prosthesis design.

This paper presents an upgraded cable model of mammalian myelinated nerve fibers in an extracellularly applied field. The kinetics of the nodes is based upon voltage clamp data in rat motor fibers at 37 degrees C, while the resting membrane potential is computed with the Goldman equation. The resulting spike shape, conduction velocity, strength/duration behavior, and absolute and relative refractory period are in good quantitative agreement with published experimental data in mammals at normal body temperature and at 20 degrees C. Results at intermediate temperatures however, suggest that the widely used concept of a constant Q10 for the rate constants is invalid. In addition, the model generates realistic abortive spikes towards the end of the absolute refractory period and it can describe the consequences of repetitive firing. The results stress the advantages of a multiple nonlinear node model even if only time aspects of nerve behavior are under study. It turned out, that the model presented here describes in vivo neural properties relevant for electrical prosthesis design better than previous models in literature.

Synchrony-dependent autocorrelation in eighth-nerve-fiber response to rippled noise.

Autocorrelograms and interval and delay histograms of the eighth-nerve-fiber response in cats to cosine noise were recorded. A good detectability level for the delay oscillations in the autocorrelograms was always found, when the fiber had a high value of the synchronization index at characteristic frequency (CF). In order to study their relationship, autocorrelograms for cosine noise stimulation were computed by the substitution of synchronization indices in the Poisson distribution formulas for higher-order intervals. The approach was made possible through a further exploration of Johnson's synchronization theory [D. H. Johnson, "The response of single auditory-nerve fibers in the cat to single tones: synchrony and average discharge rate," Ph. D. thesis, MIT, Cambridge, MA (1974)]. It was shown that computed autocorrelograms somewhat resembled experimental ones. The differences between them concerned the envelopes of the "onset" and "delay" oscillations, viz., in their constrictions and amplitudes. These constrictions were shown to be significant only in recordings at delays greater than or equal to 4/CF, corresponding to the dominant region for human repetition pitch.

The design of a saccade-size predictor for eye communication.

A method for predicting the end position of a saccade, based on the peak velocity/amplitude relationship, was studied for application in eye communication. An analogue delay line of the bucket brigade type was implemented for measuring the saccadic velocity on-line. The magnitude of the differentiated saccade-signal had been subdivided in discrete values, corresponding to the distances between the neighbouring LEDs at the characters in a row of the communication board. An up-down counter started one by one the successive switching-on of the LEDs at each step in the saccadic velocity staircase. A single emitting LED corresponded finally to a predicted eye position. The signal-to-noise ratio of the detecting transistors of the infrared limbus-reflection method determined the confidence limits in prediction of the saccadic sizes. Predictions turned out to be reliable over 6 characters at 3 degrees equidistant locations in a horizontal row. It is concluded in a discussion that the feasibility of a two-dimensional fast eye keyboard system can be studied by one saccade-size predictor in combination with the magnetic field scleral search coil technique.

Discrete eye-position recording for alternative communication.

A feasibility study for using Electro-Oculo-Graphy in a two-step direct-selection type of communication is carried out. The magnitude of any saccade at a certain adjustable time Ts after the start of the saccade is measured and compared to a number of previously adjusted set reference potentials by a multicomparator. Any E.O.G. signal within a certain range is classified to have the central reference-potential of a band. Once the corresponding potential band is determined, a light source will light up, serving as a visual feed-back. The automatic reset circuit will keep the potential of the E.O.G. signal at the central reference potential as long as the eye stays 'fixed' on the same light source in the column of light sources. Slow pursuit movement of the eye and/or the head are possible however below the trigger-velocity on any saccade before the selection of another light source, giving the user a certain degree of freedom. It is concluded, that the construction of a two-step E.O.G. controlled communicator by direct selection with discrete vertical eye-positions is a possible approach. Application of the saccade-discretisator to the field of rehabilitation is discussed.

Optical communication aids in rehabilitation.

Two types of low-cost optical communication aids for non-vocal paralyzed patients are described: one based on reflection via mirrors and the other based on refraction via prisms. The recognition by the partner, when the handicapped person looks straight via the mirror/prism into his eyes, is crucial in the selection of the corresponding symbol in this communication process. The configurations in which the mirrors/prisms are arranged in aids of one- or two-dimensions, respectively. Preceding the choice of an advanced eye-controlled aid, eye defects can be detected using two-dimensional aids. Both types of optical communicators have been used in rehabilitation. The prism-aids appeared to be preferable to mirror-devices and to ETRAN-window in the communication process for speech replacement or in playing simple games.

An electro-ocular switch for communication of the speechless.

The designed EOG switch was based on the principle of the detection of the saccadal velocity producing a pulse above a certain selected value of the saccadal derivative. The EOG transducer is preferable above an eye switch designed on the infrared limbus-pupil reflection method because of two reasons, namely in its use for vertical eye movements and because of its low costs when composed with operational amplifiers (741) or with IC's. AC-coupling between the differential amplifier and the main amplifier excludes the effects of the well-known DC-drift during EOG recording on switching. An optocoupler at the output of the switch serves for the safety of the non-vocal handicapped during the communication process with the Handiwriter. Experience is obtained with the EOG switch connected with the Handiwriter system by one quadriplegic CVA patient and one quadriplegic ALS patient. A possible group of patients as candidates for using the EOG switch has been indicated in the discussion.

Eye-controlled communication aids.

Two types of eye-controlled aids for two categories of nonvocal patients with paralysis are described: one based on the use of an eye switch by the partially defect eye for the selection of letters and the other based on position recording of the intact eye during its fixation on the desired letter. The eye recording device for both aids was designed to be drift-free and insensitive to ambient light and heat sources, using an infrared limbus-pupil-reflection method. Different selection procedures with and without visual feedback were analyzed. Feedback reduced the number of selection errors significantly. Maximum selection speeds obtained with the eye switch controlled aids and with the eye position controlled aid were 15, 30, and 105 letters/min, respectively. Eye switch-controlled aids have been used in rehabilitation. Possible application of the aids and plans are discussed.