Synaptic Architecture of Cortical Columns

The visual cortex of higher mammals, including humans, is arranged as to achieve a
continuously varying map of features such as the orientation of contours in the environment.
Previous studies used intrinsic signal and two-photon imaging to examine the functional
composition of these cortical maps, but lacked the functional resolution to resolve the underlying
synaptic architecture. Here, we exploited recent advances in genetically encoded calcium
indicators to perform in vivo two photon imaging of dendrites and dendritic spines in an animal
with a mapped visual cortex. We found sharp orientation and direction tuning when we presented
drifting gratings and imaged synaptic calcium transients from large numbers of dendritic spines
in single neurons, obtaining synaptic maps of orientation preference. In addition, we
implemented a newly developed two-photon microscope that uses acousto-optical deflectors to
rapidly steer a pulsed laser in three dimensions. This technology allowed us to image 320 single
cells in an 800x800x200 micron three-dimensional volume, which yielded a three-dimensional
orientation map with single-cell resolution. In the future, we will perform fast, three-dimensional
imaging of a single cell and its entire dendritic tree to monitor functional properties of a cell’s
inputs and its somatic spiking output. These experiments will yield important insight into
synaptic integration and sensory processing in cortical maps and how such organizing principles
might be disrupted in disease states.