A psychophysical method for measuring spatial resolution in cochlear implants

Abstract

A novel psychophysical method was developed for assessing spatial resolution in cochlear implants. Spectrally flat and spectrally peaked pulse train stimuli were generated by interleaving pulses on 11 electrodes. Spectrally flat stimuli used loudness-balanced currents and the spectrally peaked stimuli had a single spatial ripple with the current of the middle electrode raised to create a peak while the currents on two electrodes equally spaced at variable distance from the peak electrode were reduced to create valleys. The currents on peak and valley electrodes were adjusted to balance the overall loudness with the spectrally flat stimulus, while keeping the currents on flanking electrodes fixed. The psychometric functions obtained from percent correct discrimination of peaked and flat stimuli versus the distance between peak and valley electrodes were used to quantify spatial resolution for each of the eight subjects. The ability to resolve the spatial ripple correlated strongly with current level difference limens measured on the peak electrode. The results were consistent with a hypothesis that a factor other than spread of excitation (such as neural response variance) might underlie much of the variability in spatial resolution. Resolution ability was not correlated with phoneme recognition in quiet or sentence recognition in quiet and background noise, consistent with a hypothesis that implantees rely on cues other than fine spectral detail to identify speech, perhaps because this detail is poorly accessible or unreliable.

FIG. 1.

The electrodes activated in the spectrally flat (SF) and spectrally non-flat (SNF) stimuli. The sizes of the arrows represent the loudness contributions of each active electrode.

FIG. 2.

Schematic of hypothetical neural response patterns evoked by A SNF1 and SNF3 stimuli in the same implantee, B SNF1 stimulus for two subjects P and Q where Q has more spread of neural response across cochlea, and C SNF1 (rippled curves) and SF (flat lines) stimuli for two subject R and S who have similar spatial specificity of neural response but S has a higher neural response variance over time (shown by hashed area).

FIG. 3.

Across-subject averages of current levels on the peak electrode (E14) of the SF and SNF stimuli. Error bars represent standard errors.

FIG. 4.

Psychometric functions constructed from the percent correct discrimination of SNF stimuli from the SF stimulus for the stimuli presented at 80% DR (upper panel) and 50% DR (lower panel). The numbers of SNF stimuli are also the distance between peak and valley electrodes (number of electrode spacings) in the corresponding stimulus. The unfilled circle shows the chance performance (25%) for a virtual SNF0 stimulus that was used for plotting purposes. The blue dashed line is the 50% discrimination performance.

FIG. 5.

The values of spatial resolution for each subject (per millimeter) obtained from stimuli presented at 80% DR (blue bars) and 50% DR (red bars). Subjects S4, S6, S7, and S8 were not tested at the lower (50% DR) level.