Psychophysical versus physiological spatial forward masking and the relation to speech perception in cochlear implants

Objectives: The primary goal of this study was to determine if physiological forward masking patterns in cochlear implants are predictive of psychophysical forward masking (PFM) patterns. It was hypothesized that the normalized amount of physiological masking would be positively correlated with the normalized amount of psychophysical masking for different masker-probe electrode separations. A secondary goal was to examine the relation between the spatial forward masking patterns and speech perception performance. It was hypothesized that subjects with less channel interaction overall (either psychophysically or physiologically) would have better speech perception ability because of better spectral resolution. Design: Data were collected for 18 adult cochlear implant recipients [N = 9 Clarion CII or HiRes 90K, N = 9 Nucleus 24R(CS)I. Physiological spatial forward masking patterns were obtained with the electrically evoked compound action potential (ECAP) through the implant telemetry system. PFM patterns were obtained using a three-interval, two-alternative forced-choice adaptive procedure. Both measures used a fixed probe electrode with varied masker location. For each subject, spatial forward masking patterns were obtained for three probe electrodes with five masker locations per probe. Results: On an individual basis, the correlation between ECAP FM and PFM was strong for 10 subjects (r = 0.68 - 0.85, p <= 0.02), moderately strong for two subjects (r = 0.54-0.55, p = 0.06-0.07), and poor for six subjects (r = 0.13-0.45, p > 0.14). Results across subjects and electrodes showed a highly significant correlation between ECAP FM and PFM (r = 0.55, p < 0.0001); the correlation was strongest for basal electrodes. There was no significant correlation between speech perception and ECAP FM or PFM. Subjects whose ECAP FM patterns correlated well with PFM patterns generally had the poorest speech perception and subjects with the poorest correlations had the best speech perception. Conclusions: ECAP FM and PFM patterns correlated well for two-thirds of the subjects. Although the group correlation was statistically significant, ECAP FM patterns only accounted for 30% of the variance in the PFM measures. This suggests that the ECAP measures alone are not sufficient for accurately predicting PFM patterns for individual subjects.