In Part 1 we introduced the 6th edition of Principles of Neural Science, and in Part 2 we highlighted Chapter 21 The Constructive Nature of Visual Processing. In this entry we tour the construction scene, beginning with Chapter 22 titled Low-Level Processing: The Retina, authored by Markus Meister and Marc Tessier-Lavigne.
What is the connotation of “low-level” visual processing, as distinct from intermediate or high level processing? It is not as much a value judgment or architectural design as it is the first level of neural processing of light winding its way through the visual system. While the eyeball gathers external light to be collimated by its optical elements, neural processing begins within the brain’s outermost visual circuit board known as the retina. Mesiter and Tessier-Lavigne write:
“The retina is the brain’s window on the world. All visual experience is based on information processed by this neural circuit in the eye. The retina’s output is conveyed to the brain by just one million optic nerve fibers, and yet almost half of the cerebral cortex is used to process these signals. Visual information lost in the retina – by design or deficiency – can never be recovered. Because retinal processing sets fundamental limits on what can be seen, there is great interest in understanding how the retina functions.”
After introducing the neurobiology of the retinal circuit in terms of photoreceptor cells that absorb light and convert it into signals within a neural network populated by bipolar, retinal ganglion, horizontal, and amarcrine cells, the authors note that this retinal circuit performs low-level visual processing which constitutes the initial stage in the formation and analysis of visual images. Spatial filtering is accomplished through lateral inhibition, and temporal filtering occurs in synapses and feedback circuits. It is this editing and neuro-modulatory process that is the first stage in deriving visual percepts from the pattern of light falling on the retina, and that will be useful in guiding behavior through what is selectively transmitted upstream. The processes of phototransduction, extraction, and compression convey raw information about spatial and temporal features axonally through the optic nerve on its journey to intermediate and higher levels or stages of visual processing. Essentially the optic nerve is the retina’s fiber optic cable conveying filtered information about external light to the rest of the brain. If, in the words of our colleague Dr. David Cook, vision is the consummation of the love affair between light and life, the retina is where the affair begins in earnest.
In this regard, the authors pose an interesting question: Why is it that the neural circuitry for light adaptation resides in the eye and not at the other end of the optic nerve? Because light adaptation has two important roles. One is to discard information about the intensity of ambient light while retaining information about object reflectances. The other is to match the small dynamic range of firing in the retinal ganglion cells to the large range of light intensities in the environment. These large gain changes must be accomplished with graded neuronal signals before action potentials are produced in optic nerve fibers. It is in the circuity of the retina that sparks of the love affair between light and life begin to fly.