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The advent of novel optogenetics technology allows the recording of brain activity with aresolution never seen before. The characterization of these very large data sets offers newchallenges as well as unique theory-testing opportunities. We have analyzed the space andtime correlations of a population of about ten thousand neurons in a region of the mousevisual cortex, computing correlations inside boxes of increasing size, which is equivalent tochanging the system size. The robustness of spatial correlations was tested using threedifferent methods: two for the connected correlation function and one for the densitycorrelation function. We similarly study time correlations in boxes of different sizes and therelationship between characteristic correlation lengths and times. Our results show that boththe correlation length and the correlation time are closely related, scaling according to thesystem size, and the fact that neuron activity is strongly correlated is influencing thecollective dynamics, similar to what happens in thermodynamic critical systems.