A computational model illustrates how attenuation through an earplug could lead to the development of a neural correlate of phantom sounds.
a) Architecture of the model covering auditory nerve (bottom) and cochlear nucleus (middle) with projection neurons (PNs), narrow- (NBIs) and wide band inhibitor neurons (WBIs), 4 frequency channels are shown. Circles denote neurons, black lines excitatory and grey lines inhibitory connections. The strength of inhibition from WBIs and NBIs onto the PNs is determined by the gain factors gw and gn. b) Attenuation through an earplug is modelled by shifting AN rate-vs.-intensity functions to higher intensities, two different degrees of attenuation are shown (black line – normal, dark grey line –20 dB attenuation, light grey line –40 dB attenuation). c) The mean AN activity is reduced in proportion to the degree of attenuation. d) Attenuation reduces the mean activity of the principal neurons in the CN stage of the model (grey line). By increasing excitation and decreasing inhibition, homeostatic plasticity is able to restore the mean activity to its healthy target level (black line). e) As a side-effect of activity stabilization through homeostatic plasticity, spontaneous firing rates in the model PNs are increased in dependence upon the degree of attenuation. f) Average hearing thresholds of our participants with the earplug in place. g) After homeostatic plasticity has compensated for the earplug-induced decrease in mean activity, the PNs in the CN stage of the model display a pattern of increased spontaneous activity in the high frequency range.