B.  Functional structure revealed by
        voltage sensitive dye imaging    

Voltage sensitive dye imaging revealed that two components of fluorescence changes were elicited by visual stimuli: local modulation of fluorescence changes (left panel, red region), and spread of fluorescent changes over one millimeter (right panel, active domain indicated by green region).  The active domains were stimulus specific (right panel). The local modulation coincided with activity spots revealed by intrinsic signal imaging. The result suggests the existence of functional domain whose area was larger than columns.