ReviewPlasticity and stability of somatosensory maps in thalamus and cortex
Introduction
Somatosensory maps are often studied both as a means of understanding how information is processed between cortical columns and as a means of measuring plasticity. The monkey hand representation (in the primary somatosensory cortex) and the rat whisker representation (in the barrel cortex) have received much attention in recent years. Both contain unitary representations of body locations: digits in the first case and whiskers in the second. The mapping of adjacent body areas presumably confers some computational advantage on the system, and yet the map is plastic enough to alter these relationships in response to changes in sensory experience or to nerve damage.
In this review, we focus on a number of recent papers concerned with the somatosensory map and plasticity of the somatosensory system. Although lesion-induced plasticity differs in many respects from experience-dependent plasticity, and neonatal plasticity may differ from that found in the adult, we have tended to be inclusive in this review in order to cover work published during the past year. We have split the reports into three sections. The first section concerns plasticity of a single whisker’s cortical representation, assessed by intrinsic signal imaging. The second section covers recent results on the location of plasticity within the somatosensory neuraxis, as assessed by single- and multi-electrode recording. In the third section, we review evidence on some of the cellular mechanisms that might underlie map plasticity in the cortex, such as coincidence detection of presynaptic and postsynaptic action potentials (or spikes).
Section snippets
Functional imaging of whisker representation
It has previously been found that stimulation of a single whisker elicits a response within the posterior medial barrel subfield that covers an area of approximately 2 mm2 as assessed with optical imaging of the intrinsic signal 1, 2. An area of 2 mm2 is roughly equivalent to the principal barrel-column for the stimulated whisker plus all of the surrounding barrels (8 or 9, depending on the location). This finding is consistent with the fact that neurones in the principal barrel-column plus all
The locus of sensory map plasticity
When changes in cortical somatosensory map representation were described in 1983 by Merzenich et al. [8] for adult animals, it was natural to assume that the changes described originated in the cortex. Subsequent work has shown that this certainly can be the case if plasticity is induced by changes in sensory experience. For example, a period of single-whisker experience leads to cortical expression of plasticity that depends on intracortical connections [9] but that is not accompanied by
Coincidence detection as a mechanism for map plasticity
Substantial progress has been made recently toward uncovering cellular mechanisms for experience-dependent plasticity in the barrel cortex [19••]. Experiments in two-week-old rat brain slices, using dual recordings from interconnected intrabarrel spiny stellate neurones, show that transmission between these neurones can be downregulated by pairing presynaptic and postsynaptic spike activity. Moreover, this form of LTD can be evoked only if presynaptic and postsynaptic spiking occur within a 10
Conclusions
The multiplicity of techniques being brought to bear on somatosensory cortex, from multi-electrode recording and intrinsic imaging through to dual intracellular recording in slices, is yielding a greater wealth of information on plasticity in this area than ever before. Recent studies on plasticity induced by nerve damage or nerve block show that the cortex is involved in changing subcortical representations. The exact nature of the thalamocortical interaction will need to be studied further in
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
•• of outstanding interest
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