The Greek historian Herodotus (whose historical accounts most people should be sceptical about) once wrote about the Egyptian Pharaoh, Psammeticus, who performed an experiment to determine where language came from. He gave two newborn children to a shepherd and demanded that while they were looked after no one may speak to them. Eventually one of the children, having grown a bit, was heard saying “bekos,” a word in Phrygian, but most contemporary thought is that the baby was actually imitating a sheep’s bleat. Although it isn’t an exact analogy it is similar to the topic of today’s paper. How do the neurons of the brain develop without visual input for early years of life?
Due to those pesky scientific ethics we can’t just blindfold two babies and see how their brains develop, but we can do it for mice. Almost all mammals are very vision dominant and rely heavily on sight. Previous experiments have shown that monocular deprivation (the closing of one eye) while the visual cortex is developing leads to the brain quickly redistributing its efforts by weakening cerebral response from the shut eye and strengthening it from the working one. Other, more physical alterations, happen to the dendrites and the axons that make up the neurons.
The layer 5 neurons were not observed to have any change in density for the first two weeks of optical impairment, but on day 28, the spines (the protrusions from the dendrite in no way related to the skeleton) were seen to have increased flexibility. If during day 25, as this is about when a mouse’s visual cortex is in peak development, the monocular deprivation occurs, the dendritic protrusions from layer 2 and 3 reduce in density significantly after 4 days.
In order to gain a better understanding of how both mono and binocular deprivation affect the brain’s development two-photon microscopy was performed on transgenic (genetically modified, in this case to contain a fluorescent yellow protein) mice. The evidence supports one of the main theories that in monocular deprivation the effects are induced by the changing competitive interaction between the working and covered eye. Unlike the possible spine elimination that can occur in the monocular case the binocular deprivation leads to spine strengthening of the layer 5 neurons. The need to strengthen the synapses after the loss of visual input is believed to be the cause.