The topographical representation of the large facial whiskers in the somatosensory cortex is known as the ‘barrel cortex’ where each whisker is represented as a single barrel in layer IV of the cortical column. Stimulation of a single whisker produces the greatest response in the anatomically corresponding barrel and forms the centre receptive field (CRF). However, other surround whiskers also generate lesser responses in the same barrel and form the surround receptive field (SRF). A central question for understanding the functional organisation of the cortex is to discover what pathways are responsible for the CRF and SRF. In this study we examined this question for layer V cells.

These experiments were conducted on six urethane (1.5g (kg body weight)^-1, I.P.) anaesthetised rats of both sexes, and after each experiment the rat was killed humanely. These experiments used a combination of global inhibition of cortical activity by the GABA_A agonist muscimol in conjunction with local reactivation by iontophoresis of the GABA_A antagonist bicuculline (Fox et al. 2003). This technique permits the receptive field properties to be studied in the absence of intracortical activity. Diffusion of muscimol through the cortical column was monitored by the loss of stimulated whisker activity at increasing depths after the application of 500 µM muscimol to the surface of the cortex. The loss of activity in the cortical column was ensured to a depth of 1500 µm before bicuculline was iontophoretically released from a multibarrelled recording electrode to locally reactivate neurones.

Preliminary data show reactivated Va cells have extensive SRFs in the absence of intracortical activity, whereas reactivated layer Vb cells show only CRFs in the absence of intracortical activity. If the data are reanalysed on the basis of response latency, then it is apparent that the short latency cells receive single whisker inputs, while long latency cells receive multiple whisker inputs. These data suggest that short latency response cells, mainly located in layer Vb receive their SRF input intracortically, while longer latency response cells receive their SRF input from a subcortical source or perhaps another cortical area (S II).

This work was funded by the MRC.