Static and dynamic posture control in postlingual cochlear implanted patients: effects of dual-tasking, visual and auditory inputs suppression

Abstract

Posture control is based on central integration of multisensory inputs, and on internal representation of body orientation in space. This multisensory feedback regulates posture control and continuously updates the internal model of body's position which in turn forwards motor commands adapted to the environmental context and constraints. The peripheral localization of the vestibular system, close to the cochlea, makes vestibular damage possible following cochlear implant (CI) surgery. Impaired vestibular function in CI patients, if any, may have a strong impact on posture stability. The simple postural task of quiet standing is generally paired with cognitive activity in most day life conditions, leading therefore to competition for attentional resources in dual-tasking, and increased risk of fall particularly in patients with impaired vestibular function. This study was aimed at evaluating the effects of postlingual cochlear implantation on posture control in adult deaf patients. Possible impairment of vestibular function was assessed by comparing the postural performance of patients to that of age-matched healthy subjects during a simple postural task performed in static (stable platform) and dynamic (platform in translation) conditions, and during dual-tasking with a visual or auditory memory task. Postural tests were done in eyes open (EO) and eyes closed (EC) conditions, with the CI activated (ON) or not (OFF). Results showed that the postural performance of the CI patients strongly differed from the controls, mainly in the EC condition. The CI patients showed significantly reduced limits of stability and increased postural instability in static conditions. In dynamic conditions, they spent considerably more energy to maintain equilibrium, and their head was stabilized neither in space nor on trunk: they behaved dynamically without vision like an inverted pendulum while the controls showed a whole body rigidification strategy. Hearing (prosthesis on) as well as dual-tasking did not really improve the dynamic postural performance of the CI patients. We conclude that CI patients become strongly visual dependent mainly in challenging postural conditions, a result they have to be awarded of particularly when getting older.

Keywords: auditory input; cochlear implanted patients; dual-tasking; posture control; visual input.

Figure 1

(A,B) Posture control of the CI patients in static conditions. (A) Limits of stability of the CI patients (filled histograms) compared to the controls (open histograms) in the eyes open (EO) and eyes closed (EC) conditions. The stability limits are expressed as the energy of the spectral power density recorded in the 0.05–0.5 Hz frequency range while the subjects were standing on the platform and asked to move voluntary as far as possible in the forward/backward and left/right directions, without moving the feet. ^****Significant differences (p < 0.0001) between the CI patients and the controls. ON indicates that hearing is present. (B) Postural Instability Index (PII) calculated from the 3D posturographic map of the center of pressure recordings obtained with the wavelet analysis. Recordings were made in subjects standing quietly on the platform with (EO) or without (EC) vision, with (ON) or without (OFF) hearing, and during dual-tasking (DT) with a concomitant cognitive task consisting of a visual (visu ST) or an auditory (audi ST) memory task. The PII values are expressed on the ordinates; significant differences between the CI patients (filled histograms) and the controls (open histograms) are indicated by asterisks (^*p < 0.05; ^**p < 0.01).

Figure 2

Posture control of the CI patients in dynamic conditions. The spectral power density peaks provide by the wavelet transform at the 0.5 Hz stimulus frequency of sinusoidal platform translation in the for-aft direction are expressed on the ordinates for each of the different experimental conditions: with (EO) or without (EC) vision, with (ON) or without (OFF) hearing, and during dual-tasking with a concomitant cognitive task consisting of a visual (visu ST) or an auditory (audi ST) memory task. Significant differences between the CI patients (filled histograms) and the controls (open histograms) are indicated by asterisks (^**p < 0.01; ^***p < 0.001; ^****p < 0.0001). Note the strong impact of eye closure in the CI patients.

Figure 3

Gain of the head, hip and knee body segments during sinusoidal platform translation at the 0.5 Hz frequency. The gains of the head (filled black squares), hip (open squares), and knee (filled gray squares) body segments, evaluated from motion analysis (Codamotion), were calculated as the ratio between platform displacement and displacement of the different body segments. Gains close to unity mean that the body segments move in phase and with the same amplitude as the platform; lower and higher gains for the head, for instance, indicate good and poor head stabilization in space, respectively. The gain values are expressed on the ordinates for each of the experimental condition tested: with (EO) or without (EC) vision, with (ON) or without (OFF) hearing, and during dual-tasking with a concomitant cognitive task consisting of a visual (visu ST) or an auditory (audi ST) memory task. Significant differences between the experimental conditions are shown by asterisks (^*p < 0.05; ^**p < 0.01; ^***p < 0.001).