Pushing the limits on speed of optical interrogation of large-scale neuronal circuits
Alipasha Vaziri (The Rockefeller University)
Knowledge on structural connectivity in neuronal circuits is necessary for understanding information representation and processing in local circuits. However, as some examples of well-?characterized neuronal architectures illustrate, structural connectivity alone it is not sufficient to predict how input stimuli are mapped onto activity patterns of neuronal populations and how the collective dynamics of all neurons in the network leads to behavior. Addressing this challenge has been hampered by lack of appropriate tools and methods that allow parallel and spatiotemporally specific application of excitation patterns onto neuronal populations while capturing the dynamic activity of the entire network at high spatial and temporal resolution. The combination of new optical excitation techniques, optogenetics and high speed functional imaging are providing new opportunities to address this question and move towards a dynamic map of neuronal circuits.
Most structural and functional imaging modalities, including two-?photon imaging, rely on the manipulations of light fields in the spatial domain. However, pulsed excitation sources such as femtosecond lasers also possess a spectral width that provide an additional degree of freedom that can be used to “sculpt” other spatial light distributions. This has been exemplified in the technique of temporal of focusing through which sheet-?like light distributions with an axial confinement close that of point scanning have been demonstrated. Using this approach we developed a two-?photon technique for brain-?wide calcium imaging which has allowed us to capture the activity of individual neurons within the densely packed head ganglia of C. elegans [1]. We demonstrate near-?simultaneous recording of activity of up to 70% of all head neurons. The application of this technique to other model organisms including rodents and zebrafish larvae is being currently pursued. In order to functionally image even larger brains and ultimately apply volumetric brain imaging in freely behaving animals we have recently established light-?field microscopy in combination with 3D deconvolution [2]. Thereby, we demonstrate intrinsically simultaneous volumetric Ca-?imaging in the entire brain of larval zebrafish during sensory stimulation. We are able to track the activity of 5000 neurons distributed throughout the brain at 20Hz volume rate. The simplicity of this technique and the possibility of the integration into conventional microscopes make it an attractive tool for high-?speed volumetric Ca-?imaging. These tools combined with high speed optogentic control of neuronal circuits [3, 4], advanced statistics tools and mathematical modeling and will be crucial to move from an anatomical wiring map towards a dynamic map of neuronal circuits.
