Limits on short-term plasticity in somatosensory cortex
It is now clear, from a large number of studies, that disruption of a subset of the inputs to the sensory representations in cortex of adult manunals induces a rapid unmasking of larger neuronal receptive fields and an increased neuronal responsiveness. Within the somatosensory cortex, such demonstrations indicate that a capacity for short-term plasticity results from the unmasking of normally unexpressed inputs to a cortical locus. Despite the clear demonstration of such unmasking, an explanation for how this occurs is not immediately apparent. Aside from the studies directly demonstrating short-term plasticity, there have been anatomical (Landry and Deschenes, 1981; Garraghty and Sur, 1990), physiological (Snow et aI., 1988) and pharmacological (e.g. Dykes et al., 1984) demonstrations of cortical afferents that are not normally expressed as part of neural receptive fields. The rapid unmasking phenomena indicate that these inputs are inherently viable and thus must normally be actively suppressed. The only candidate source for an inhibition which can account for this suppression is that produced by the GABAergic interneurons located within a given cortical area. This short review will examine the implications that studies of short-term plasticity require for the nature of that inhibition, and the limitations of the capacity for plasticity that is dependent upon these normally unexpressed inputs.