SINGLE-CELL CORRELATES OF A REPRESENTATIONAL BOUNDARY IN RAT SOMATOSENSORY CORTEX

In primary somatosensory cortex (S1), the transition from one represen tation to the next is typically abrupt when assayed physiologically. H owever, the extent of anatomical projections to and within the cortex do not strictly respect these physiologically defined transitions. Phy siological properties, such as synaptic strengths or intracortical inh ibition, have been hypothesized to account for the functionally define d precision of these representational borders. Because these represent ational borders can be translocated across the cortex by manipulations or behaviors that change the activity patterns of inputs to the corte x, understanding the physiological mechanisms that delimit representat ions is also an important starting point for understanding cortical pl asticity. A novel in vivo and in vitro preparation has been developed to examine the cellular and synaptic mechanisms that underlie represen tational borders in the rat. In vivo, a short segment of the border be tween the forepaw-lower jaw representations in rat S1 was mapped using standard electrophysiological methods and was visibly marked using io ntophoresis of pontamine sky blue dye. Slices were then obtained from this marked region and maintained in vitro. Intracellularly recorded r esponses to electrical stimulation of supragranular cortex were obtain ed from single neurons near the border in response to stimulation with in the representational zone or across the border. Both excitatory and inhibitory responses were smaller when evoked by stimuli that activat ed projections that crossed borders, as compared with stimuli to proje ctions that did not. These findings indicate that intracortical networ k properties are contributing to the expressions of representational d iscontinuities in the cortex.