Producing columnar depositions of neuroanatomical tracer in rat cortex

A new technique was developed to fill columnar regions of cortex with retrograde neuroanatomical tracer. This technique involved (1) standard iontophoresis to eject tracer from a micropipette in concert with (2) displacement of this micropipette radially through the cortex using a computer controlled microdrive. The displacement of the micropipette left an evenly dense deposit of tracer throughout the cortical thickness forming a column of neuroanatomical tracer. The diameter of the columnar injection site was reliably dependent on the iontophoretic current strength, the micropipette displacement parameters, and the nature of the tracer used. This simple technique provided very reproducible results. Several issues related to the interconnections of functionally identified columns of sensory and motor cortex could be addressed with this injection technique.