A FOCAL ZONE OF THALAMIC PLASTICITY

In this study, sensory maps in the thalamus were investigated by exami ning their volume and shape. We determined the forelimb representation in adult rats after the removal of hindlimb input by nucleus gracilis lesions. Three-dimensional reconstructions of thalamic sensory maps w ere obtained from a grid of electrode penetrations. We found that the volume of the shoulder sensory map contracted >50% at an acute time in terval (n = 6), followed by a robust volumetric sensory map expansion of 25% at 1 week (n = 8) and 1 month (n = 8) after lesion relative to controls (n = 8). The topology of the volumetric increase was scrutini zed by slicing functional maps in the coronal, sagittal, and horizonta l planes. The equivalence of such slices from each animal was establis hed by virtue of their distance from either a functional or neuroanato mical landmark. Surprisingly, all of the volumetric increase unequivoc ally occurred in a circumscribed coronal slice 300 mu m thick. This fo cal zone was located toward the rostral pole of the thalamic tactile r elay, the ventroposterolateral nucleus. Analysis in the sagittal plane revealed that, unexpectedly, the shoulder map volume expanded by supe rimposing its representation on that of the forepaw, via an advancemen t of the shoulder representation by 0.6 mm medially. We propose a ''ho t spot'' hypothesis in which focal zones of plasticity may not be spec ific to the thalamus but may have manifestations elsewhere in the nerv ous system, such as the cerebral cortex or dorsal column nuclei.