Sensory processing and its perception require that local information would also be available globally.
Indeed, in the mammalian neocortex, local excitation spreads over large distances via the long-range
horizontal connections in layer 2/3 and may spread over an entire cortical area if excitatory
polysynaptic pathways are also activated. Therefore, a balance between local excitation and surround
inhibition is required. Here we explore the spatiotemporal aspects of cortical depolarization and
hyperpolarization of rats anesthetized with urethane. New voltage-sensitive dyes (VSDs) were used for
high-resolution real-time visualization of the cortical responses to whisker deflections and cutaneous
stimulations of the whisker pad. These advances facilitated imaging of ongoing activity and evoked
responses even without signal averaging. We found that the motion of a single whisker evoked a cortical
response exhibiting either one or three phases. During a triphasic response, there was first a cortical
depolarization in a small cortical region the size of a single cortical barrel. Subsequently,
this depolarization increased and spread laterally in an oval manner, preferentially along rows
of the barrel field. During the second phase, the amplitude of the evoked response declined rapidly,
presumably because of recurrent inhibition. Subsequently, the third phase exhibiting a depolarization
rebound was observed and clear, and ~16 Hz oscillations were detected.
Stimulus conditions revealing a net surround hyperpolarization during the second phase were also found.
By using new, improved VSD, the present findings shed new light on the spatial parameters of
the intricate spatiotemporal cortical interplay of inhibition and excitation.