A subjective distance between stimuli: quantifying the metric structure   of representations
  As subjects perceive the sensory world, different stimuli elicit a number of neural representations. Here, a subjective distance between stimuli is defined, measuring the degree of similarity between the underlying representations. As an example, the subjective distance between different locations in space is calculated from the activity of rodent hippocampal place cells, and lateral septal cells. Such a distance is compared to the real distance, between locations. As the number of sampled neurons increases, the subjective distance shows a tendency to resemble the metrics of real space. 
D. Oliva, I. Samengo, S. Leutgeb, S. Mizumori