Competition and convergence between auditory and cross - modal visual inputs to 12 primary auditory cortical areas

42 Sensory neocortex is capable of considerable plasticity after sensory deprivation or damage to 43 input pathways, especially early in development. Although plasticity can often be restorative, 44 sometimes novel, ectopic inputs invade the affected cortical area. Invading inputs from other 45 sensory modalities may compromise the original function or even take over, imposing a new 46 function and preventing recovery. Using ferrets whose retinal axons were rerouted into auditory 47 thalamus at birth, we were able to examine the effect of varying the degree of ectopic, cross48 modal input on reorganization of developing auditory cortex. In particular, we assayed whether 49 the invading visual inputs and the existing auditory inputs competed for or shared postsynaptic 50 targets, and whether the convergence of input modalities would induce multisensory processing. 51 We demonstrate that although the cross-modal inputs create new visual neurons in auditory 52 cortex, some auditory processing remains. The degree of loss of auditory input to MGN was 53 directly related to the proportion of visual neurons in auditory cortex, suggesting that the visual 54 and residual auditory inputs compete for cortical territory. Visual neurons were not segregated 55 from auditory neurons, but shared target space even on individual target cells, substantially 56 increasing the proportion of multisensory neurons. Thus spatial convergence of visual and 57 auditory input modalities may be sufficient to expand multisensory representations. Together 58 these findings argue that early, patterned visual activity does not drive segregation of visual and 59 auditory afferents, and suggest that auditory function might be compromised by converging 60 visual inputs. These results inform possible ways in which multisensory cortical areas may form 61 during development and evolution. They also suggest that rehabilitative strategies designed to 62 promote recovery of function after sensory deprivation or damage need to take into account that 63