Action potential detection by non linear microscopy

The central nervous system can process a tremendous amount of information, which is encoded in terms of spikes andtransmitted between neurons at synapses. A central question in neuroscience is how simple processes in neurons cangenerate cognitive functions and form complex memories like those experienced by humans and animals. In principle, ifone were able to record from all the neurons in a network involved in a given behavior, it would be possible toreconstruct the related computations. Unfortunately, this is not possible with current techniques for several reasons.Generally, the more precise the method of neuronal recording is (e.g. patch-clamp), the more limited the number ofsimultaneously recorded neurons becomes. Conversely, global recordings (e.g. field recordings) collect activity frommany neurons but lose information about the computation of single neurons.Current optical techniques for recording membrane potential (Vm) can potentially overcome these problems [1,2]. Mostapproaches to the optical recording of fast Vm events in neural systems rely on one-photon methods [3-7]. Thesemethods can be used to generate high signal-to-noise ratio (S/N) measurements of action potentials (APs) from subcellularregions in a single trial and sub-threshold events with averaging. However, in intact tissue slices theireffectiveness in detecting AP in multiple deep neurons is markedly limited because strong multiple light scattering blursthe images. In order to record deep Vm activity in intact systems maintaining a high spatial resolution, nonlinear opticalmethods are needed [8-11]. Fast second-harmonic generation (SHG) recordings of Vm have been achieved in modelmembranes [12], in Aplysia neurons in culture [13,14] and in intact mammalian neural systems [15,16]. The nextchallenge is to record multiple simple APs simultaneously from multiple neurons in intact systems.