  Recent progress in neuroscience to image and investigate brain function has
been made possible by impressive developments in optogenetic and opto-molecular
tools. Such research requires advances in optical techniques for the delivery
of light through brain tissue with high spatial resolution. The tissue causes
distortions of the wavefront of the incoming light which broadens the focus,
thereby reducing the intensity and resolution especially in techniques
requiring focal illumination. Adaptive wavefront correction has been
demonstrated to compensate for these distortions. However, in many situations
iterative derivation of the corrective wavefront introduces time constraints
that limit its usefulness when used to probe living cells. Here we demonstrate
a direct and fast technique by working with a small set of Zernike modes and
demonstrate that corrections derived a priori can lead to significant
improvement of the focus. We verify this idea by the electrical response of
whole-cell patched neurons following two-photon photolysis of caged
neurotransmitters along its dendrites.In particular, we find that the
organization of the neuropile in the cortical region of rat brain slicesprovide
sufficient a priori information to preselect an effective wavefront correction.
