Your Eye Works a Precision Jigsaw Puzzle

You have twin 125 megapixel video cameras in your eyeballs.  Each pixel, a rod or cone connected to a neuron, sees only a small bit of the total image.  How do these bits, like pieces of a jigsaw puzzle, fit together?  Scientists at the Salk Institute have found that they are finely tuned to fit together for optimum clarity.  Writing in PLoS Biology, they said,

All visual information reaching the brain is transmitted by retinal ganglion cells, each of which is sensitive to a small region of space known as its receptive field.  Each of the 20 or so distinct ganglion cell types is thought to transmit a complete visual image to the brain, because the receptive fields of each type form a regular lattice covering visual space.  However, within each regular lattice, individual receptive fields have jagged, asymmetric shapes, which could produce “blind spots” and excessive overlap, degrading the visual image.  To understand how the visual system overcomes this problem, we used a multielectrode array to record from hundreds of ganglion cells in isolated patches of peripheral primate retina.  Surprisingly, we found that irregularly shaped receptive fields fit together like puzzle pieces, with high spatial precision, producing a more homogeneous coverage of visual space than would be possible otherwise.  This finding reveals that the representation of visual space by neural ensembles in the retina is functionally coordinated and tuned, presumably by developmental interactions or ongoing visual activity, producing a more precise sensory signal.

In the discussion, they added, “The present results demonstrate that the visual representation in the primate retina is finely coordinated to achieve a homogeneous sampling of visual space.”  They pondered how this coordination is achieved.  Is there a one-to-one correspondence between the dendritic field (DF) and the receptive field (RF)?  Or are there overlapping layers of circuitry between that control the precision of the RF?  Bipolar cells may do this, they said.  Alternatively, inhibitory amacrine cells may tune the edges of RF shapes to prevent excessive overlap.  They also wondered how this precision is achieved during development.  Perhaps light produces cues that guide the RFs into position.  Either way, the implications are surprising.  It means that neurons don’t operate in isolation.  They follow a precision code:

The present results have surprising implications for how populations of neurons produce an efficient and complete representation.  Recorded in isolation, single neurons frequently exhibit irregular response properties, suggesting that large populations must rely on averaging or interpolation to produce accurate sensory performance or behavior (e.g., see [37�39]).  The present results, however, show that in a complete population, irregular features can be integral to a finely coordinated population code.  This suggests that the nervous system operates with a higher degree of precision than previously thought, and that irregularities in individual cells may actually reflect an unappreciated aspect of neural population codes (e.g., [40]).

This article was summarized on Science Daily, which stated, “scientists say their findings suggest that the nervous system operates with higher precision than previously appreciated and that apparent irregularities in individual cells may actually be coordinated and finely tuned to make the most of the world around us.”

1.  Gauthier, Field, Sher, Greschner, Shlens, Litke, and Chichilnisky, “Receptive Fields in Primate Retina Are Coordinated to Sample Visual Space More Uniformly,” Public Library of Science Biology, Vol. 7, No. 4, e63 doi:10.1371/journal.pbio.1000063.

There was not one mention of evolution in this paper.  It was all coordination, information, and encoding.  As Theophilus Designsky said, Nothing in biology makes sense except in the light of design.