Cognitive task demands modulate the sensitivity of the human cochlea

Abstract

Recent studies lead to the conclusion that focused attention, through the activity of corticofugal and medial olivocochlear (MOC) efferent pathways, modulates activity at the most peripheral aspects of the auditory system within the cochlea. In two experiments, we investigated the effects of different intermodal attention manipulations on the response of outer hair cells (OHCs), and the control exerted by the MOC efferent system. The effect of the MOCs on OHC activity was characterized by measuring the amplitude and rapid adaptation time course of distortion product otoacoustic emissions (DPOAEs). In the first, DPOAE recordings were compared while participants were reading a book and counting the occurrence of the letter "a" (auditory-ignoring) and while counting either short- or long-duration eliciting tones (auditory-attending). In the second, DPOAEs were recorded while subjects watched muted movies with subtitles (auditory-ignoring/visual distraction) and were compared with DPOAEs recorded while subjects counted the same tones (auditory-attending) as in Experiment 1. In both Experiments 1 and 2, the absolute level of the averaged DPOAEs recorded during the auditory-ignoring condition was statistically higher than that recorded in the auditory-attending condition. Efferent-induced rapid adaptation was evident in all DPOAE contours, under all attention conditions, suggesting that two medial efferent processes act independently to determine rapid adaptation, which is unaffected by attention, and the overall DPOAE level, which is significantly affected by changes in the focus of attention.

Keywords: DPOAE; MOC; corticofugal pathways; distortion product otoacoustic emission; human; medial olivocochlear efferents; selective auditory attention.

Figure 1

Averaged group DPOAE traces, Experiment 1 (top panel). The mean DPOAE trace measured in the auditory-ignoring (text reading) condition is shown in blue and the mean DPOAE trace recorded in the auditory-attending (tone-counting) condition is shown in red. Each trace is fitted with a two-component exponential for use in estimating the magnitude and time constants of the MOC-mediated rapid adaptation response. When the two DPOAE traces are normalized to the DPOAE onset level (bottom panel, and inset), it can be seen that the onset slope is relatively steeper and there is relatively more rapid adaptation for the auditory-ignoring condition (blue) compared with the auditory-attending (red) condition. The estimated time constants are 49 ms for the auditory-ignoring condition and 66 ms for the auditory-attending condition, differences that are statistically indistinguishable (paired t-test). The estimated difference in overall DPOAE level is ∼0.3 dB.

Figure 2

Scatter plot comparison of the absolute DPOAE level measured for each subject during the auditory-attending condition (counting tones) and DPOAE levels obtained during the auditory-ignoring condition (reading text) in Experiment 1.

Figure 3

Distortion product otoacoustic emission traces, averaged across participants, from Experiment 2 (top panel). The mean DPOAE trace recorded in the auditory-ignoring (movie/subtitle viewing) condition is shown in green, and the mean DPOAE trace recorded in the auditory-attending (tone-counting) condition is shown in red. Each trace is fitted with a two-component exponential. The two DPOAE traces normalized to onset level are shown in the lower panel (and inset). Consistent with the findings of Experiment 1, the DPOAE adaptation curve recorded during the auditory-ignoring condition is significantly higher (p < 0.001) in overall level compared with that recorded during the auditory-attending condition (red). The calculated time constants are 114 ms for the auditory-ignoring condition and 127 ms for the auditory-attending condition. The absolute difference between the two traces is ∼1 dB.

Figure 4

Scatter plot comparison of absolute DPOAE levels recorded from individual participants during auditory-attending (counting tones) and auditory-ignoring conditions (watching muted DVD, reading subtitles) conditions in Experiment 2.