Stereotypy · Lobeworks/17

Stereotypy is the property of a nervous system whose cells and connections repeat closely from one individual of a species to the next, because a genetic programme builds them the same way each time; it is what lets the connectome of one animal stand for its whole species.

Stereotypy. Stereotypy is the property of a nervous system whose cells and connections repeat closely from one individual of a species to the next, because a genetic programme builds them the same way each time; it is what lets the connectome of one animal stand for its whole species.

Insects are strongly stereotyped. When the FlyWire brain was compared with itself and with the hemibrain of another fly, the number of cells of a cell type differed between the two sides of one brain by less than one on average, and any connection made of more than ten synapses was found again more than nine times in ten in the other hemispheres compared. Weak connections of a few synapses vary much more.

It sets what one map can say. A map of one fly describes any fly to the precision of its strong connections, and says little about its weak ones.

It separates real variation from error. Reconstruction mistakes and true individual differences both show up as disagreement between two maps, and stereotypy gives the baseline against which the two can be told apart.

It is much weaker in a human cortex. Wiring there is shaped far more by experience through synaptic plasticity, so a map of one person's cortex would describe that person, and a generic map would describe nobody well.

Questions: Can the connectome of one fly stand for all flies? For its strong connections, largely yes. Comparing two flies and the two sides of one brain, the number of cells of a type differed by less than one on average, and connections of more than ten synapses were found again more than nine times in ten. Weak connections of a few synapses vary much more, so a single map describes the species well only at the level of its strong wiring. Why would a human connectome describe one person rather than the species? Because the human cortex is far less stereotyped than an insect brain. Much more of its wiring is shaped after birth by experience, through synaptic plasticity, so two people differ in ways two flies do not. A generic human map would miss most of what makes any one cortex work, while the map of one fly describes most flies.