Inhibition and inhibitory plasticity in the mammalian auditory midbrain
Neuronal plasticity that involves large scale anatomical changes is restricted to the early period of development. Beyond this there is a "critical period" during which processes of formation of arborization patterns of afferent axons and of synaptic contacts form a basis for plasticity (for a review see Rauschecker, 1991). Although these findings set limits on the prospects for plasticity in the mature nervous system, a number of experimental paradigms have demonstrated considerable functional plasticity in the adult brain, These involve study of the topographic representations of the somatosensory and motor cortices (for reviews see Dykes, 1990; Kaas, 1991; Colford, 1995), the auditory cortex (Robertson and Irvine, 1989; Rajan et aI., 1993) and primary visual cortex (Kaas et aI., 1990; Gilbert and Wiesel, 1992) following a restricted nerve injury or a behavioural manipulation (Recanzone et aI., 1992a,b, 1993). These topographic representations have been chosen as appropriate models because they serve as scales against which any induced changes can be measured. Work in this laboratory over the past nine years has given emphasis to the role of inhibition in the early events following the loss (or inactivation) of a subset of the inputs to a brain area. The initial event is a disinhibition which allows expression (unmasking) of otherwise ineffective inputs.