Figure 7.

Context-dependent reweighting of auditory spatial cues following a recurring developmental hearing loss in one ear. (A) Sound localization performance of ferrets reared with an earplug in one ear, either in the presence or absence of an earplug. Each symbol represents data from an individual animal. Although juvenile-plugged ferrets adapt to an earplug (see Figure 3), their performance improves when the earplug is removed, and approaches the mean performance level of controls under normal listening conditions (dotted black line). This means that juvenile-plugged ferrets adapt to an asymmetric hearing loss without compromising their ability to localize accurately when normal hearing becomes available. Error bars denote bootstrapped 95% confidence intervals, with solid black lines showing group means. (B) Context-specific reweighting of auditory spatial cues. Randomizing stimulus spectra across trials is known to degrade the usefulness of spectral cues, since it becomes unclear whether spectral features arise from the filtering effects of the head and ears or are instead properties of the stimulus itself. Whilst wearing an earplug, sound localization performance in juvenile-plugged ferrets declined as the amount of spectral randomization was increased, but this effect largely disappeared once the earplug was removed. Each line shows data for an individual animal, either with an earplug in place (solid, dark blue), following earplug removal (solid, pink), or after the reintroduction of an earplug (dotted, dark blue). (C) To quantify the effects of spectral randomization, slope values were calculated for the lines in (B) and are plotted for different hearing conditions. Each symbol shows data from an individual animal, with solid black lines indicating group means. Performance of juvenile-plugged ferrets was only impaired by randomization (negative slope values) when one ear was occluded. This means that the localization behavior of juvenile-plugged ferrets became more dependent on the spectral cues available to the contralateral ear whenever a unilateral earplug was present. The lack of effect of spectral randomization in the absence of the earplug suggests that the animals were relying on binaural cues when normal inputs were available. (D) Neural weighting index values are shown for neurons in the primary auditory cortex of juvenile-plugged ferrets, either in the presence or absence of a virtual earplug. Higher values indicate greater reliance on the spectral cues provided by the developmentally non-occluded ear. In juvenile-plugged ferrets, neural weighting index values change depending on whether a virtual earplug is present or not. Controls do not show the same effect, and are indistinguishable from juvenile-plugged ferrets under normal hearing conditions (dotted black line shows mean neural weighting index values for controls). This means that recurring monaural deprivation during development leads to cortical neurons weighting auditory spatial cues differently depending on whether a hearing loss is experienced, providing a possible neural basis for the cue reweighting observed behaviorally. Modified with permission from Keating et al. (2013a).