Estimating ECAP threshold from the variability of the response

Electrical compound action potentials (ECAPs) of the cochlear nerve are used clinically for quick and efficient cochlear implant parameter setting. The ECAP is the aggregate response of nerve fibres at various distances from the recording electrode, and the magnitude of the ECAP is therefore related to the number of fibres excited by a particular stimulus. Current methods, such as the masker-probe or alternating polarity methods, use the ECAP magnitude at various stimulus levels to estimate the neural threshold, from which the parameters are calculated. However, the correlation between ECAP threshold and perceptual threshold is not always good, with ECAP threshold typically being much higher than perceptual threshold. The lower correlation is partly due to the very different pulse rates used for ECAPs (below 100 Hz) and clinical programs (hundreds of Hz up to several kHz). Here we introduce a new method of estimating ECAP threshold for cochlear implants based upon the variability of the response. At neural threshold, where some but not all fibers respond, there is a different response each trial. This inter-trial variability can be detected overlaying the constant variability of the system noise. The large stimulus artefact, which requires additional trials for artefact rejection in the standard ECAP magnitude methods, is not consequential, as it has little variability. The variability method therefore consists of simply presenting a pulse and recording the ECAP, and as such is quicker than other methods. It also has the potential to be run at high rates like clinical programs, potentially improving the correlation with behavioural threshold. Preliminary data is presented that shows a detectable variability increase shortly after probe offset, at probe levels much lower than those producing a detectable ECAP magnitude. Care must be taken, however, to avoid saturation of the recording amplifier saturation; in our experiments we found a gain of 300 to be optimal.

Theoretical study of Talbot-like bands observed using a laser diode below threshold

We report on a theoretical study of an interferometric system in which half of a collimated beam from a broadband optical source is intercepted by a glass slide, the whole beam subsequently being incident on a diffraction grating and the resulting spectrum being viewed using a linear CCD array. Using Fourier theory, we derive the expression of the intensity distribution across the CCD array. This expression is then examined for non-cavity and cavity sources for different cases determined by the direction from which the slide is inserted into the beam and the source bandwidth. The theoretical model shows that the narrower the source linewidth, the higher the deviation of the Talbot bands' visibility (as it is dependent on the path imbalance) from the previously known triangular shape. When the source is a laser diode below threshold, the structure of the CCD signal spectrum is very complex. The number of components present simultaneously increases with the number of grating lines and decreases with the laser cavity length. The model also predicts the appearance of bands in situations not usually associated with Talbot bands.

Talbot-like bands for a laser diode below threshold

We report on the problems encountered when replacing a tungsten filament lamp with a laser diode in a set-up for displaying Talbot bands using a diffraction grating. It is shown that the band pattern is rather complex and strong interference signals may exist in situations where Talbot bands are not normally expected to appear. In these situations, the period of the bands increases with the optical path difference (OPD). The visibility of bands as dependence on path imbalance is obtained by suitably obstructing halfway into the arms of a Michelson interferometer using opaque screens.

Distributed temperature sensor based on Brillouin loss in an optical fibre for transient threshold monitoring

A distributed temperature sensor for transient threshold monitoring with a 22 km sensing length, based on the Brillouin loss in standard communications fibre, is demonstrated. The system can be used for real-time monitoring of a preset temperature threshold. Good S/N ratios were achieved with only 8–16 sample averages giving a response time of 2 to 4 s with a temperature uncertainty of ±1 °C.

Evolution of competing strategies in a threshold model for task allocation

A nature inspired decentralised multi-agent algorithm is proposed to solve a problem of distributed task allocation in which cities produce and store batches of different mail types. Agents must collect and process the mail batches, without global knowledge of their environment or communication between agents. The problem is constrained so that agents are penalised for switching mail types. When an agent process a mail batch of different type to the previous one, it must undergo a change-over, with repeated change-overs rendering the agent inactive. The efficiency (average amount of mail retrieved), and the flexibility (ability of the agents to react to changes in the environment) are investigated both in static and dynamic environments and with respect to sudden changes. New rules for mail selection and specialisation are proposed and are shown to exhibit improved efficiency and flexibility compared to existing ones. We employ a evolutionary algorithm which allows the various rules to evolve and compete. Apart from obtaining optimised parameters for the various rules for any environment, we also observe extinction and speciation.

The influence of memory in a threshold model for distributed task assignment

A nature inspired decentralised multi-agent algorithm is proposed to solve a problem of distributed task selection in which cities produce and store batches of different mail types. Agents must collect and process the mail batches, without a priori knowledge of the available mail at the cities or inter-agent communication. In order to process a different mail type than the previous one, agents must undergo a change-over during which it remains inactive. We propose a threshold based algorithm in order to maximise the overall efficiency (the average amount of mail collected). We show that memory, i.e. the possibility for agents to develop preferences for certain cities, not only leads to emergent cooperation between agents, but also to a significant increase in efficiency (above the theoretical upper limit for any memoryless algorithm), and we systematically investigate the influence of the various model parameters. Finally, we demonstrate the flexibility of the algorithm to changes in circumstances, and its excellent scalability.

Distributed temperature sensor based on Brillouin loss in an optical fibre for transient threshold monitoring

A distributed temperature sensor for transient threshold monitoring with a 22 km sensing length, based on the Brillouin loss in standard communications fibre, is demonstrated. The system can be used for real-time monitoring of a preset temperature threshold. Good S/N ratios were achieved with only 8–16 sample averages giving a response time of 2 to 4 s with a temperature uncertainty of ±1 °C.

Theoretical study of Talbot-like bands observed using a laser diode below threshold

We report on a theoretical study of an interferometric system in which half of a collimated beam from a broadband optical source is intercepted by a glass slide, the whole beam subsequently being incident on a diffraction grating and the resulting spectrum being viewed using a linear CCD array. Using Fourier theory, we derive the expression of the intensity distribution across the CCD array. This expression is then examined for non-cavity and cavity sources for different cases determined by the direction from which the slide is inserted into the beam and the source bandwidth. The theoretical model shows that the narrower the source linewidth, the higher the deviation of the Talbot bands' visibility (as it is dependent on the path imbalance) from the previously known triangular shape. When the source is a laser diode below threshold, the structure of the CCD signal spectrum is very complex. The number of components present simultaneously increases with the number of grating lines and decreases with the laser cavity length. The model also predicts the appearance of bands in situations not usually associated with Talbot bands.

Talbot-like bands for a laser diode below threshold

We report on the problems encountered when replacing a tungsten filament lamp with a laser diode in a set-up for displaying Talbot bands using a diffraction grating. It is shown that the band pattern is rather complex and strong interference signals may exist in situations where Talbot bands are not normally expected to appear. In these situations, the period of the bands increases with the optical path difference (OPD). The visibility of bands as dependence on path imbalance is obtained by suitably obstructing halfway into the arms of a Michelson interferometer using opaque screens.

Effects of dynamic strain ageing on fatigue crack propagation and threshold behaviour in a Ni-base superalloy

In the temperature range 200-400 degree C the Ni-base superalloy, N901, develops marked dynamic strain ageing effects in its tensile behavior. These include inverse strain rate sensitivity, especially in UTS values, strongly serrated stress-strain curves and a heavily sheared failure mode at the higher test-temperatures. As for steels these effects seem to be due to interactions between the dislocations and the interstitial carbon atoms present. The results of tensile and fatigue threshold tests carried out between 20 degree C and 420 degree C are reported and the fatigue behavior is discussed in terms of the effects of surface roughness induced closure, temperature and strain aging interactions.

Residual stress effects during near-threshold fatigue crack growth

The fatigue crack propagation behaviour of a low alloy, boron-containing steel has been examined after austenitizing at 900°C or 1250°C and tempering at a range of temperatures up to 400°C. Fatigue threshold values were found to vary with austenitizing and tempering treatment in a range between 3.3 to 6 MPa √m when tested at a stress ratio (R) of 0.2. Crack propagation rates in the Paris regime were insensitive to heat treatment variations. The crack propagation path was essentially transgranular in all conditions with small regions of intergranular facets appearing at growth rates around the knee of the da/dN vs ΔK curve. The crack front shape showed marked retardation in the centre of the specimen at low tempering temperatures. Experimental determinations and computer predictions of residual stress levels in the specimens indicated that this was due to a central residual compressive stress resulting from differential cooling rates and the volume change associated with the martensite transformation. The results are discussed in terms of microstructural and residual stress effects on fatigue behaviour. © 1987.

Surface damage and near-threshold fatigue crack growth in a Ni-base superalloy in vacuum

Fatigue crack growth rate tests have been performed on Nimonic AP1, a powder formed Ni-base superalloy, in air and vacuum at room temperature. These show that threshold values are higher, and near-threshold (faceted) crack growth rates are lower, in vacuum than in air, although at high growth rates, in the “structure-insensitive” regime, R-ratio and a dilute environment have little effect. Changing the R-ratio from 0.1 to 0.5 in vacuum does not alter near-threshold crack growth rates very much, despite more extensive secondary cracking being noticeable at R= 0.5. In vacuum, rewelding occurs at contact points across the crack as ΔK falls. This leads to the production of extensive fracture surface damage and bulky fretting debris, and is thought to be a significant contributory factor to the observed increase in threshold values.