The synthesis of multi-whisker receptive fields in subcortical stations of the vibrissa system

This study addresses the origins of multi-whisker receptive fields of neurons in the thalamic ventral posterior medial (VPM) nucleus of the rat. We sought to determine whether multi-whisker receptive field synthesis occurs in VPM through convergent projections from the principalis (PrV) and interpolaris (SpVi) nuclei, or in PrV via intersubnuclear projections from the spinal trigeminal complex. We tested these hypothesis by recording whisker-evoked responses in PrV and VPM before and after electrolytic lesion of the SpVi in ligthly anesthetized rats. Before the lesion PrV cells responded, on average, to 3.2 ± 1.2 whiskers but responsiveness was reduced to 1.07 ± 0.31 whisker after the lesion. A similar reduction of receptive field size was observed in VPM, where neurons responded, on average, to 2.94 ± 0.95 whiskers before the lesion and to 1.05 ± 0.22 whisker after the lesion. Thus, one can conclude that intersubnuclear projections mediate surround whisker receptive fields in PrV, and therefore in VPM. However, it has previously been shown that parasagittal brainstem transection, which severed ascending projections from SpVi, but left intersubnuclear connections intact, rendered VPM cells mono-whisker responsive. We wondered whether midline brainstem lesion modified receptive field properties in SpVi. In normal rats SpVi cells responded, on average, to 7.52 ± 4.25 whiskers, but responsiveness was dramatically reduced to 1.47 ± 1.07 whisker after the lesion. Together these results indicate that the synthesis of surround receptive fields in subcortical stations relies almost exclusively on intersubnuclear projections from the spinal trigeminal complex to the PrV.