Functional columns in the primate prefrontal cortex revealed by optical imaging in vitro

Although the functional column has been implicated in the dorsolateral prefrontal cortex (DLPFC) of primates, its dynamics and even existence are still uncertain. We performed optical recording with a voltage-sensitive dye (RH482) in brain slices obtained from the principal sulcal region (area 46) of macaque monkeys. Columnar activity was evoked by electrical stimulation of the middle layer (lower layer III or layer IV); this activity consisted of two components: probably action potentials and excitatory postsynaptic potentials. The width of the columnar activity was saturated when the current intensity of stimulation exceeded a certain level, that is, approximately 900 μm at the peak with this intensity. The stimulation of different sites within the same slice activated different columnar activities with only slight overlaps. Furthermore, similar columnar activity appeared when a different site within the area with columnar activity was stimulated. These findings suggest that the primate DLPFC consists of functional columns formed by excitatory synaptic connections.