**1. Introduction**

The conventional electrophysiological methods have been well used for the measurement of neural activities of vertebrate and invertebrate. The direct measurement from single neurons and neural areas is possible by the conventional methods using microelectrodes. However, the brain or central nervous system forms a network composed of so many neurons and

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their function is based on complex interactions among electric neural activities, intracellular calcium signals, intercellular communications by neurotransmitter, etc. The electrophysiological methods are not suitable for the simultaneous measurement from many neurons or neural areas. For multi-point measurement of neural activities, fluorescent-imaging technique using voltage-sensitive dyes or calcium-sensitive dyes can be a powerful technique. In this technique, the dyes, which change their fluorescent intensities due to the voltage or calcium ion concentration change [1], are loaded into the cells, and their fluorescence changes are acquired into a computer as a series of images.

The multi-point measurement using fluorescent-imaging technique has been reported in vertebrates [2–5] and invertebrates [6–16]. In invertebrates, for example, this technique has been applied to measure spatiotemporal neural activities involved in the olfactory processing of the land slug *Limax* [6, 7, 9–11, 13–16]. In *Lima*x, the procerebral (PC) lobe, which is the olfactory center located in the lateral part of each cerebral ganglion, spontaneously produces a periodic oscillation of local field potential (LFP) of about 1 Hz. The LFP oscillation is well synchronized over the entire PC lobe, but the phase of oscillation is advanced at the distal region, resulting in periodic propagating waves of neural activity from the distal to proximal regions. The previous studies showed that odor stimuli change the LFP frequency and the wave propagation speed.

In this article, we review the previous studies, as well as our recent studies, on the spatiotemporal neural activities of the land slug *Limax* using fluorescent-imaging technique.
