Short-term plasticity in adult somatosensory cortex
The primary sensory fields in cortex encode topographical representations of their peripheral epithelium (visual: retina; somatosensory: body surface; auditory: cochlear place or frequency) in which responsive neurons at a given locus or throughout a column have similar receptive fields, These representations provide useful model systems for the study of functional neuronal plasticity since they present an inbuilt scale upon which any changes can be measured. In the developing mammal, these representations have been shown to be capable of a remarkable degree of plasticity by virtue of the reorganization of the basic topography which follows from a changed peripheral input (Kelahan et aI., 1981; Spinelli and Jensen, 1979; Waite. 1984; Wall and Cusick, 1986) or from manipulation of central pathways (Roe et al 1990), Plasticity has also been demonstrated in the sensory cortex of adults where the removal of a restricted region of primary afferents can lead to the affected area of cortex gaining sensitivity to other regions (visual: Gilbert and Wiesel, 1992; Kaas et al 1990; Schmid et al 1995; 1996; auditory: Rajan et al 1993; Robertson and Irvine, 1989; somatosensory: see below). However, such changes in the organization of adult cortex must be considered fundamentally different in nature from those in neonates where there is a critical period for plasticity (Hubel and Wiesel, 1970; Wall, 1988). In the developing nervous system there are bases for mutability of the pattern of projections through disruption of the processes of initial connections, trimming of exuberant projections (e,g, Wall and Cusick, 1986) or alterations to the normal patterns of segregation of inputs (e.g. LeVay et al 1981). Such mechanisms of plasticity are not available in the adult brain where the connections are considered to be stable - although it does appear that some deprivation conditions may extend the developmental critical period (Daw et al 1992; Moore, 1993; Smith et al 1978), Nevertheless, significant alterations in the coding properties of neurons have been reported which have led some to challenge the concept of a stable adult central nervous system, It is, however, important to point out that the use of the term plasticity has itself undergone a transformation over the period of these studies. Whereas the term was once strictly used to describe cases of anatomical change, it is now widely used where there is a functional change with no apparent anatomical substrate. This functional plasticity encompasses cases where the outcome is thought to be an inherent property of the neural circuit (e.g. unmasking of previously ineffective inputs: Calford and Tweedale, 1991a; Turnbull and Rasmusson, 1990) and cases in which some change in synaptic efficacy is implicated (e.g. Glazewski et al 1996; Kano et al 1991). The majority of studies demonstrating a functional plasticity in the adult brain have been of the somatosensory system. Studies of such plasticity in the dorsal hom and thalamus predate those of somatosensory cortex. Nevertheless, the present chapter will concentrate on the effects as seen in cortex and will address the other levels of the pathway only when looking at possible explanatory mechanisms - an extensive review of the lower pathway effects is recommended (Snow and Wilson, 1991).