Neural bases of quantity estimation of zebrafish

It is well recognized that non-human animals can discriminate collections of objects on the basis of their numerosity. Such a non-symbolic numerical ability is thought to be based on an evolutionarily conserved system for approximating numerical magnitude named Approximate Number System (ANS). Key to the present research is the use of zebrafish as a behavioural model for study the neural and molecular bases of the ANS. Zebrafish were presented with a visual stimulus consisting of a set of 3 or 9 elements (red dots), associated with food delivery. The dots changed in size, position and density from trial to trial but maintained their numerousness. After this habituation phase (twelve trials per day for four consecutive days), fish underwent a dishabituation test (one single trial) in which the stimulus was changed (i) in numerosity (from 3 to 9 or viceversa with the same overall surface), or (ii) in shape (from dots to crosses, with the same overall surface and numerosity), or (iii) in size (with the same shape and numerosity). In the control group fish were shown the same stimulus as during habituation. Thirty minutes after the dishabituation test, fish brains were dissected to quantify changes in the expression of the immediate early genes. RTqPCR revealed that c-fos transcription was altered in telencephalon and thalamus of fish tested with a change in numerosity but not (or not to the same extend) in fish tested with a change in shape with respect to fish of the control group. Similarly, c-fos mRNA was altered in the telencephalon of fish tested for change in dots size with respect to control fish. On the contrary, no significant alteration of c-fos was noticed in optic tectum, cerebellum and medulla oblongata. Our data suggest an involvement of telencephalon and thalamus in the estimation of numerosity in zebrafish.