Update 'README.md'

README.md

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 Calcium fluorescence signals of hundreds of simultaneously imaged neurons in mouse retrosplenial cortex during a virtual navigation task (running on a linear treadmill track with head-fixed), from different experimental groups of animals: control mice, mice with unilateral hippocampus lesion (uni), and mice with bilateral hippocampus lesions (bi).
 
+Each element in the cell array corresponds to data from one animal with the suffix indicating experimental group.
+
 data structure explained below:
 
 **behavior**.