Frequency Map for the Human Cochlear Spiral Ganglion: Implications for Cochlear Implants

Abstract

The goals of this study were to derive a frequencyposition function for the human cochlear spiral ganglion (SG) to correlate represented frequency along the organ of Corti (OC) to location along the SG, to determine the range of individual variability, and to calculate an Baverage^frequency map (based on the trajectories of the dendrites of the SG cells). For both OC and SG frequency maps, a potentially important limitation is that accurate estimates of cochlear place frequency based upon the Greenwood function require knowledge of the total OC or SG length, which cannot be determined in most temporal bone and imaging studies. Therefore, an additional goal of this study was to evaluate a simple metric, basal coil diameter that might be utilized to estimate OC and SG length. Cadaver cochleae (n = 9) were fixed G24 h postmortem, stained with osmium tetroxide, microdissected, decalcified briefly, embedded in epoxy resin, and examined in surface preparations. In digital images, the OC and SG were measured, and the radial nerve fiber trajectories were traced to define a series of frequency-matched coordinates along the two structures. Images of the cochlear turns were reconstructed and measurements of basal turn diameter were made and correlated with OC and SG measurements. The data obtained provide a mathematical function for relating represented frequency along the OC to that of the SG.Results showed that whereas the distance along the OC that corresponds to a critical bandwidth is assumed to be constant throughout the cochlea, estimated critical band distance in the SG varies significantly along the spiral. Additional findings suggest that measurements of basal coil diameter in preoperative images may allow prediction of OC/SG length and estimation of the insertion depth required to reach specific angles of rotation and frequencies. Results also indicate that OC and SG percentage length expressed as a function of rotation angle from the round window is fairly constant across subjects. The implications of these findings for the design and surgical insertion of cochlear implants are discussed.

Integration of acoustic and electrical hearing

“…Their acoustic hearing is responsible for speech information only up to approximately 750 Hz, and the electrical hearing provides the remaining (higher) frequencies. This finding is particularly remarkable when one considers that the Iowa/Nucleus Hybrid electrode is only inserted into the very basal end of the cochlea, a region that normally responds to frequencies of 2,800 to 4,700 Hz and above [24][25]. Thus, the speech information presented through the implanted electrode is "shifted," not only in relation to the normal place-frequency map of the cochlea but also in relation to the acoustically presented information at the lower frequencies, with a potential "gap" in the middle of the cochlea where no information is presented.…”

Automated analysis of human cochlea shape variability from segmented μ CT images

“…Previous analyses include measurements of the length of the cochlea and the number of cochlear turns (Hardy, 1938;Kawano et al, 1996;Skinner et al, 2002;Escudé et al, 2006;Stakhovskaya et al, 2007;Erixon et al, 2009;Gunz et al, 2012;Shin et al, 2013;van der Marel et al, 2014), measurements of the heights, widths and sectional areas of the scala tympani (Zrunek et al, 1980;Zrunek and Lischka, 1981;Hatsushika et al, 1990;Gulya and Steenerson, 1996;Wysocki, 1999;Thorne et al, 1999;Biedron et al, 2010;Avci et al, 2014), the scala vestibuli (Zrunek and Lischka, 1981;Gulya and Steenerson, 1996;Wysocki, 1999;Thorne et al, 1999;Biedron et al, 2010) and the cochlea (Erixon et al, 2009;Shin et al, 2013), measurements of the radial component of the cochlear centerlines (also called cochlear curvature) (Cohen et al, 1996;Baker, 2008;van der Marel et al, 2014), and studies of the longitudinal component of the centerlines of the scala tympani (Avci et al, 2014) and the cochlea itself (Verbist et al, 2009).…”

Optimizing frequency-to-electrode allocation for individual cochlear implant users

“…One of these was the participant's everyday clinical map, usually the 3 default map, which was presented as a new map and trialed for at least six weeks so as 4 to reduce bias based on the idea that a new map would be better. The relationship 5 between electrode number and lower frequency boundary, for the default map, is a 6 fourth order polynomial function, which allocates a larger proportion of the frequency 7 range to the apical electrodes than the basal electrodes, consistent with a more rapid 8 decrease in pitch in the middle turn, as indicated by the spiral ganglion frequency-9 matched map (Stakhovskaya et al, 2007). The three alternative maps were a mapping 10 to the Greenwood function, using the function expressed as a proportion of cochlear 11 length (a=2.1; A=165.4; k=0.88) and data from table two of Kawano et al (1996) for whom the polynomial default frequency map may be inappropriate, and the 5…”