A primary objective of our lab is to understand
how the brain performs parallel distributed signal processing.
However, investigating information processing in the brain
has been severely hampered by the limited experimental methods
available for studying rapid signalling in 3D structures.
Functional optical imaging using 2-photon microscopy
(
Denk et al. 1990)
has been increasingly used to investigate neuronal signalling,
because it allows submicron resolution and good tissue penetration.
However, conventional 2-photon imaging using galvanometers
is too slow to capture the timing of fast neuronal signalling events
and, as for most optical microscopes, is limited to 2D imaging.
To overcome these limitations we have developed a new type of optical element
based on 4 acousto-optic deflectors that can focus and steer a laser beam at ~40 kHz.
The concept of a fast acousto-optic lens (AOL),
which was first proposed by Kaplan
(
Kaplan et al. 2001)
is shown in
Figure 1.
Our lab
(
Kirkby et al. 2010)
as well as Prof. Peter Saggau's lab
(
Duemani Reddy et al. 2008)
have developed AOLs for use with 2-photon microscopy.
This high speed 3D functional optical imaging technology
enables many points of interest to be measured at kHz rates
and is well suited for measuring distributed neuronal signalling.
Our 3D 2-photon AOL microscope
(
Figure 2)
has many new design features
that overcome several of the technical limitations of this technology.
This has enabled a 10-fold reduction in the laser power
required for imaging and an extended focussing depth range.
These features now make it useful for neuroscience research applications.
The image in
Figure 3
shows a cortical neuron and its dendritic processes filled with a fluorescent dye.
It illustrates the submicron resolution achievable when the AOL is used to focus over 100 microns.
This 2-photon AOL microscopy also provides a new way
to image signals in neural populations distributed in 3D space.
Our preliminary
in vitro and
in vivo data,
using calcium sensitive fluorescent dyes to monitor neural activity,
shows that signalling can be monitored simultaneously from more than 30 neurons
distributed in 3D space with a temporal resolution greater than 1 kHz.
This time scale is relevant for 'reading' spike timing-based neural population codes.
UCL Business
has filed two
patent applications
on this technology
is hoping to licence the design features of our 3D 2-photon AOL microscope.
This technology has several potential applications outside neuroscience and cell biology.
These include high capacity data storage, 3D lithography, laser tweezers
and cold atom traps for quantum physics experiments.
