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Why have mammals evolved different ways of seeing?

24.02.2020

In a nutshell: Investigating four mammalian species could fill important gaps in our understanding of visual processing.

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Why have mammals evolved different ways of seeing?

Most visual processing is done in the brain, not in the eye. Information about what we see is transmitted from the retina to a part of the cerebral cortex called the visual cortex.

All mammalian species have a cerebral cortex that is organised in a similar way. However, their visual processing systems are quite different. Most of what we know about their structure and function comes from research into primates and rodents. Primates have the most complex visual systems, while rodents have the most basic.

Why have mammalian species evolved different solutions for carrying out the same task? To answer this question, it would help to study mammals with visual systems that are somewhere between the most and least complex. But how can researchers find the most relevant mammals to study from the thousands of species alive today?

Brain Function CoE investigators Michael Ibbotson and Young Jun (Jason) Jung, from the University of Melbourne, have proposed four mammal species that would be good targets for future study: a South-American rodent called an agouti, the fruit bat (or flying fox), the sheep, and the wallaby. They believe that these four species could fill important gaps in our understanding of visual processing.

Ibbotson and Jung shortlisted these species by focusing on a particular part of the visual system called the primary visual cortex, or V1. This part of the brain is the first region to receive visual information from the retina. V1 has brain cells that process basic information about an object, such as its orientation. Certain cells become more active in response to an object with vertical edges, for example, while other cells respond more to something with horizontal edges.

In some species – like primates, cats and ferrets – cells with the same orientation preference cluster together into columns. In other species, like rodents and rabbits, cells with the same preference are distributed randomly.

What determines how orientation-selective (OS) cells are organised in any one species? The Brain Function CoE investigators reviewed published literature on a range of factors to see if they could draw any conclusions. Unlike previous reviews, they looked across the entire visual system, from the retina to the visual cortex.

They found that mammals with eyes at the front of their heads, like humans, tend to have columns of OS cells, while animals with eyes at the side have a random arrangement of OS cells. They also found that species with more V1 cells tend to arrange their OS cells in columns. However, for these and other features, there were exceptions to the rule – species that didn’t fit the trend.

Based on what they found, the researchers proposed that studying the agouti, fruit bat, sheep and wallaby would reveal the most relevant information about how different parts of the visual system work together to influence visual processing.

Next steps:
The researchers plan to use state-of-art techniques to determine how OS cells are organised in the visual cortices of the four shortlisted animals. By understanding how the animals organise visual information, they hope to gain important insights into how brain connectivity controls visual processing.


Reference:
Ibbotson, M., & Jung, Y. J. (2020). Origins of functional organisation in the visual cortex. Frontiers in Systems Neuroscience. doi: 10.3389/fnsys.2020.00010


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