Figure 4.

Maladaptive effects of asymmetric hearing loss during development. (A) Example of a binaural interaction matrix recorded from a unit in the central nucleus of the inferior colliculus (ICc) in a normally reared rat. Contralateral sound level is plotted against ipsilateral level, with color denoting the number of spikes fired for each combination. Firing rates typically increase as the contralateral level is increased, but are suppressed when the ipsilateral level exceeds that in the contralateral ear. For each interaural level combination enclosed by the blue box, binaural suppression was quantified by comparing it with the linear sum of its monaural intercepts (e.g., blue cross relative to the sum of the red and green crosses). (B) To investigate the developmental effects of monaural deprivation, rats were reared with a hearing loss in one ear that was induced by ligation of the ear canal, which was reversed prior to electrophysiological experiments. Bilateral recordings were then performed in the ICc and primary auditory cortex (A1) of these animals and compared with data from sham-operated controls reared with normal hearing. (C,D) Examples of binaural interaction matrices from A1 (C) and ICc (D) in sham operated controls (left), and in ligated animals. For ligated animals, data are shown for the hemisphere ipsilateral (middle) and contralateral (right) to the ligated ear. Color scales and axis labels are identical to (A). (E,F) Ipsilaterally mediated suppression expressed as a function of ILD for A1 (E) and ICc (F) recordings. Data are shown for sham operated controls (open symbols) as well as ligated animals, both contralateral (gray) and ipsilateral (black) to the ligated ear. Asterisks denote significant differences between ligated animals and controls, with asterisk grayscale indicating the hemisphere in which the comparison was made. Error bars show SEMs. Ipsilaterally mediated suppression in A1 of monaurally deprived animals is increased in the hemisphere contralateral to the deprived ear (E), but not in the corresponding hemisphere of the ICc (F). Conversely, ipsilaterally mediated suppression is reduced in the ICc ipsilateral to the deprived ear (F), but this effect is not apparent at the level of A1 (E). These results suggest that monaural deprivation induces persistent changes in the strength of ipsilateral input, which acts to weaken the representation of the deprived ear at the level of the ICc and strengthen the representation of the intact ear at the level of A1. In both cases, this increases the relative strength of the intact ear, and produces maladaptive shifts in ILD sensitivity. Modified with permission from Popescu and Polley (2010).