For more than a century since its discovery, brain lateralisation was considered a uniquely human trait linked to the cognitive superiority of our species. We now know it is widespread in the animal kingdom, and even the tiny zebrafish exhibits lateralised behaviours and pronounced neuroanatomical asymmetries. The zebrafish is also the rock star of genetic manipulation studies thanks to its simple genome and transparent developing embryo, and it happens to be great with numbers. Maria Elena Miletto Petrazzini exploited these benefits to investigate the lateralisation of numerical abilities in zebrafish with potential implications for other vertebrates including humans. A Marie Skłodowska-Curie individual fellowship supported her work within the context of the NUMELAT project.
Asymmetry serves a purpose
Zebrafish have sophisticated mathematical abilities including perceiving cardinal numbers (e.g. the larger of two groups, as with adult males selecting groups of females) and ordinal numbers (e.g. the second exit in a corridor with many similar when the second leads to companions). The role of lateralisation in these numerical processes is unclear, as is the timing of appearance during development. Miletto Petrazzini exploited the observation that individual fish of a social species, when placed in a novel environment, spontaneously join the larger of two shoals to avoid possible predation. She compared the behaviour in this task of fish whose brain asymmetry she disrupted using genetic modifications to that of wild-type fish. Miletto Petrazzini found that the zebrafish could select the larger group even at 4 weeks of age and that loss of asymmetry impaired performance. “Compromised functional lateralisation has been associated with neuropsychiatric disorders and cognitive dysfunctions including dyscalculia in humans, but it is not known which is the cause and which the effect. Our finding supports the possibility that disrupted brain lateralisation could be causative in cognitive or behavioural dysfunctions,” expounds Miletto Petrazzini.
Fish join the club
So-called number neurons are found in the neocortex of humans and non-human primates and in crows that lack a neocortex. This suggests that the neural circuits for numerosity processing might be evolutionarily conserved and located in brain areas functionally homologous to the mammalian neocortex in other vertebrates. Miletto Petrazzini explains: “In collaboration with the University of Southern California, we adopted an in vivo imaging protocol called two-photon light-sheet microscopy or 2P-SPIM to monitor neural activation in zebrafish larvae. During presentation of numerical stimuli, we obtained the first hints of cells differently activated by the presentation of a different number of items which may resemble the number neurons described in humans, primates and birds.” This finding may extend the nascent list of vertebrates with number neurons, pointing to an evolutionarily conserved process.
Lateralisation impacts more than numerical processing
As if confirming her original hypotheses were not enough, Miletto Petrazzini made a serendipitous discovery. “During the development of an automated assay to assess adult numerical discrimination abilities using stimuli presentation on a computer screen, we found that some patterns of altered brain asymmetries affected more general learning skills and impulsive behaviour.” Miletto Petrazzini’s fellowship has helped her answer questions and pose more, also launching her career. In the future, expect evolving insight into the evolutionary origins of numerical competence, its neuroanatomical basis and the intriguing role of brain lateralisation in processing.
NUMELAT, lateralisation, zebrafish, number neurons, brain assymetry, dyscalculia, numerosity, two photon light-sheet microscopy, 2P-SPIM, in vivo imaging