Holography in quantitative biology

In recent years, scientists have learned to make light-sensitive molecular probes and incorporate them into biological tissue. When stimulated by the correct wavelength, the probes then make available for study targeted dynamic processes of living systems. In a typical experiment, static optical elements such as lenses are used to focus light on a region of tissue, where all photoactivity—wanted or unwanted—is then observed. A new, active element for optical imaging is the liquid-crystal spatial light modulator, used to tailor light’s distribution in such applications as optical tweezers and adaptive optics. Valentina Emiliani of the University of Paris Descartes and her colleagues are now using LC-SLMs to create holographic illumination to selectively activate biological probes in specific locations. One such probe is glutamate, a major neurotransmitter, caged within a photoactive molecule. Shown here are data recorded from cells within a slice of mouse brain tissue perfused with caged glutamate and illuminated in the green regions with a 6-micron-diameter spot (left) and, in the same location, with a holographic shape that coincides with just the dendrite of interest (right). Plotted below each image are glutamate-stimulated currents resulting from brief light pulses. The responses show a large reduction in unwanted signal when the LC-SLM is used. The researchers also simultaneously activated several precisely positioned spots on the same neuron, demonstrating highly controlled stimulation of different neural inputs. (C. Lutz et al., Nat. Meth. 5 , 821, 2008 http://dx.doi.org/10.1038/nmeth.1241 .)