Biomedical subjects

Colette M McKay

Publications and source records attributed to Colette M McKay.

4 recordsLinked to original sources

PubMed2003

The perceptual effects of interphase gap duration in cochlear implant stimulation.

The most common current pulse shape used for cochlear implants is a biphasic rectangular pulse. The interphase gap (IPG) is the duration of the zero-current portion which lies between the two phases. It is known from single-nerve studies in animals that, as the IPG decreases, the biphasic pulse becomes less efficient in activating the nerve cell. Thus, it can be predicted that stimulation using smaller IPGs will necessitate the use of higher currents to maintain the loudness required by the cochlear implantee. The development of contemporary processing schemes commonly involves the maximization of the rate parameter, and to achieve this in sequential pulsatile stimulation, the IPG as well as the pulse phase duration must be minimized. This experiment investigated the effect of IPG on loudness in eight cochlear implantees who use the CI24 implant manufactured by Cochlear Ltd. An exponential increase in current level was required to maintain equal loudness when IPG is reduced from 100 to 45 and 8.4 micros. The effect of IPG was greater at lower levels, was greater for shorter pulse durations (26 micros compared to 52 micros), and was not significantly different for the rates (1 kHz or 4 kHz) tested.

Cochlear Implants↗

PubMed2003

A practical method of predicting the loudness of complex electrical stimuli.

The output of speech processors for multiple-electrode cochlear implants consists of current waveforms with complex temporal and spatial patterns. The majority of existing processors output sequential biphasic current pulses. This paper describes a practical method of calculating loudness estimates for such stimuli, in addition to the relative loudness contributions from different cochlear regions. The method can be used either to manipulate the loudness or levels in existing processing strategies, or to control intensity cues in novel sound processing strategies. The method is based on a loudness model described by McKay et al [J. Acoust. Soc. Am. 110, 1514-1524 (2001)] with the addition of the simplifying approximation that current pulses falling within a temporal integration window of several milliseconds' duration contribute independently to the overall loudness of the stimulus. Three experiments were carried out with six implantees who use the CI24M device manufactured by Cochlear Ltd. The first experiment validated the simplifying assumption, and allowed loudness growth functions to be calculated for use in the loudness prediction method. The following experiments confirmed the accuracy of the method using multiple-electrode stimuli with various patterns of electrode locations and current levels.

Adult↗

PubMed2002

Frequency-to-electrode allocation and speech perception with cochlear implants.

The hypothesis was investigated that selectively increasing the discrimination of low-frequency information (below 2600 Hz) by altering the frequency-to-electrode allocation would improve speech perception by cochlear implantees. Two experimental conditions were compared, both utilizing ten electrode positions selected based on maximal discrimination. A fixed frequency range (200-10513 Hz) was allocated either relatively evenly across the ten electrodes, or so that nine of the ten positions were allocated to the frequencies up to 2600 Hz. Two additional conditions utilizing all available electrode positions (15-18 electrodes) were assessed: one with each subject's usual frequency-to-electrode allocation; and the other using the same analysis filters as the other experimental conditions. Seven users of the Nucleus CI22 implant wore processors mapped with each experimental condition for 2-week periods away from the laboratory, followed by assessment of perception of words in quiet and sentences in noise. Performance with both ten-electrode maps was significantly poorer than with both full-electrode maps on at least one measure. Performance with the map allocating nine out of ten electrodes to low frequencies was equivalent to that with the full-electrode maps for vowel perception and sentences in noise, but was worse for consonant perception. Performance with the evenly allocated ten-electrode map was equivalent to that with the full-electrode maps for consonant perception, but worse for vowel perception and sentences in noise. Comparison of the two full-electrode maps showed that subjects could fully adapt to frequency shifts up to ratio changes of 1.3, given 2 weeks' experience. Future research is needed to investigate whether speech perception may be improved by the manipulation of frequency-to-electrode allocation in maps which have a full complement of electrodes in Nucleus implants.

Adult↗

PubMed2002

Benefits of syllabic input compression for users of cochlear implants.

Ten users of multielectrode cochlear implants participated in an evaluation of the perceptual effects of input-signal compression. A syllabic compressor was introduced into the microphone circuit of Spectra-22 or SPrint sound processors. The post-compression gain was adjusted to provide similar loudness for speech at an average level of 65 dBA with compression either enabled or disabled. Sentence recognition was measured at three levels. Averaged across all listeners, statistically significant score increases were obtained at each level with compression enabled (45 dBA: 19.6 percentage points, p < .0001; 55 dBA: 16.6 percentage points, p < .0001; 70 dBA: 3.1 percentage points, p = .031). A test of speech intelligibility in noise showed no significant effect of compression. Generally, participants in the trial reported improved perception of low-level sounds with compression, although a few disliked the increased loudness of some background noises. Some participants suggested that the ability to enable or disable compression with a manual switch would be helpful. Overall, the results show that input compression can improve the performance of these sound processors for users of cochlear implants, especially when listening to speech at low levels.

Adult↗