Fig. 5.

Barrel cortex slices cut tangential to the pia containing only layer 4 indicate that excitation is confined to the stimulated barrel. A, Bright-field micrograph (top left) of the region of tangentially sliced barrel cortex imaged with the voltage-sensitive dye. Cyan outline indicates the extent of the stimulated barrel with the stimulation electrode visible in the center. Images of the peak voltage-sensitive dye signal (12 msec after stimulation) without (top right) and with (bottom left) barrel demarcation. The optical responses at the locations indicated bydots in the top left panel are plotted (bottom right), with responses from within the stimulated barrel colored blue, from the septum ingreen, and from the neighboring barrel inred. The spatial extent of the response decays rapidly outside of the stimulated barrel with very little in neighboring barrels. The shape of the optical dye response matches closely the shape of the barrel viewed in bright-field microscopy. Scale bar, 100 μm. B, Quantification of the spatial extent of the voltage-sensitive dye signal with respect to the barrel border at 0 μm (red line indicates septum). The voltage-sensitive dye signal is fitted with a sigmoidal curve, with half-maximal value at 6 μm and length constant 32 μm. C, The full-width at half-maximum of the voltage-sensitive dye signal shows a close correlation with the width of the stimulated barrels, as defined by bright-field microscopy.