The Effect of Side of Implantation on the Cortical Processing of Frequency Changes in Adult Cochlear Implant Users

Abstract

Cochlear implants (CI) are widely used in children and adults to restore hearing function. However, CI outcomes are vary widely. The affected factors have not been well understood. It is well known that the right and left hemispheres play different roles in auditory perception in adult normal hearing listeners. It is unknown how the implantation side may affect the outcomes of CIs. In this study, the effect of the implantation side on how the brain processes frequency changes within a sound was examined in 12 right-handed adult CI users. The outcomes of CIs were assessed with behaviorally measured frequency change detection threshold (FCDT), which has been reported to significantly affect CI speech performance. The brain activation and regions were also examined using acoustic change complex (ACC, a type of cortical potential evoked by acoustic changes within a stimulus), on which the waveform analysis and the standardized low-resolution brain electromagnetic tomography (sLORETA) were performed. CI users showed activation in the temporal lobe and non-temporal areas, such as the frontal lobe. Right-ear CIs could more efficiently activate the contralateral hemisphere compared to left-ear CIs. For right-ear CIs, the increased activation in the contralateral temporal lobe together with the decreased activation in the contralateral frontal lobe was correlated with good performance of frequency change detection (lower FCDTs). Such a trend was not found in left-ear CIs. These results suggest that the implantation side may significantly affect neuroplasticity patterns in adults.

Keywords: acoustic change complex; cochlear implant; frequency change detection; frontal lobe; standardized low-resolution brain electromagnetic tomography (sLORETA); temporal lobe.

FIGURE 1

(A) ERP waveforms with error bars for the right-ear CIs (red traces), left-ear CIs (blue traces), and the NH listeners (black traces) are displayed. The stimulus was a 1-s pure tone at 160 Hz that contained an upward frequency change of 50% in the middle of the stimulus. The N1 peak of the onset CAEP evoked by the tone onset and the N1’ of the ACC evoked by the 50% frequency change are marked. Note that the N1’ is the focus of this study. (B,C) The means and standard errors of the N1’ amplitude and latency for left- and right-ear CIs. There is no significant difference in the amplitude and latency of N1’ waves between right- and left-ear CIs.

FIGURE 2

Mean regional normalized sLORETA solutions modeling the distributed sources for ACC N1’ of NH listeners (A), right-ear CIs (B), and left-ear CIs (C). Yellow and blue colors represent increased and decreased current source density, respectively. Note that the NH listeners were stimulated binaurally and each CI ear was stimulated monaurally. There are differences in the activated brain regions among NH, right-, and left-ear CIs.

FIGURE 3

The activated brain regions displayed for individual CI users with different FCDTs. (A) good performer (SCI 50 R, 1.83%). (B) poorer performer (SCI45 R, 5.17%). (C) poorest performer (SCI55 L, 9.33%). The strong activation in the temporal lobe appears in the good performer with a low FCDT.

FIGURE 4

Hemispheric differences in the current source density values for right- and left-ear CIs. (A) The current source density values in four regions of interest (ROIs, right, and left temporal lobes and frontal lobes) for right- and left-ear CIs. For the right-ear CIs, the current source densities in the contralateral hemisphere (left temporal lobe and left frontal lobe) are larger than those in the ipsilateral (right) hemisphere. The contralateral-dominant pattern is not observed in the left-ear CIs. (B) The left-dominant pattern is obvious in the right-ear CIs but not visible in the left-ear CIs. The left/right (L/R) ratios of responses in both the temporal lobe and frontal lobe for the right-ear CIs are greater than 1, whereas the L/R ratios of responses in both the temporal lobe and the frontal lobe for the left-ear CIs are close to 1. There are significant differences in the ratio of L/R responses between right-ear CIs and left-ear CIs (*p < 0.05, **p < 0.01, t-test).

FIGURE 5

The correlations between the FCDT and the ratio of current source density (CSD) values in the temporal lobe and frontal lobe in the left and right hemispheres for the right-ear CIs (A,B) and left-ear CIs (C,D). Straight lines represent linear regression lines. Note that only the right-ear CIs in panel (A) shows a significant correlation, with good performers (low FCDTs) showing larger CSD ratios in the left hemisphere (p < 0.05).