Image Processing in the Eye: Like "Magic" Evolution News & Views January 16, 2014 5:33 The amount of image processing going on in the eyeball is astounding. Did you know that the signal from your retina splits into some 20 channels that analyze the image before it reaches the brain? A pair of German neuroscientists, writing in Current Biology, describe as "magic" how two of those channels work at the neural level. They begin by commenting on how we enjoy the best of both design constraints: The visual system of primates, including that of humans, famously features both exquisite spatial acuity and a high temporal resolution. This dual focus on both 'sharpness' and 'speed' is made possible through different processing streams set up already in the retina. In a recent study, Puthussery et al. now show that key differences in the processing streams that are thought to underlie these visual abilities are already set up right after the first synapse of the visual system -- in retinal bipolar cells.
The retina breaks the visual world into several parallel representations prior to transmission to the brain. Each representation, or 'channel', is based on a different type of retinal ganglion cell that carries information about specific features of the visual scene -- such as edges, directed motion or 'color'. Of the 20 or so types of ganglion cells that exist in the primate retina, two in particular have attracted considerable attention since they were first described in the 1940s: the parasol and midget cells. (Emphasis added.)
The authors go on to describe how the "parasol ganglion cells" are responsible for "fast, low acuity" while the "midget ganglion cells" provide "slow, high acuity." What makes the difference? At least three factors are involved: (1) the way they are wired in parallel with the bipolar cells (between the photoreceptors and the amacrine cells), (2) the way they are wired in series with the amacrine cells (between the bipolar cells and the ganglion cells), and (3) the waveforms of the electrical output of the bipolar cells.
The new findings "build on a growing body of evidence that active conductances and spikes in bipolar cells present a fundamental ingredient for high-precision temporal processing in the visual system," they say.
There's not one word about evolution in this paper. But there is a subsection entitled, "The Design of a Retinal Bipolar Cell." Notice the design words and concepts:
The differential subcellular localization of conductances becomes of paramount importance when considering what bipolar cells actually do. Far from being simple passive 'cables' that relay the photoreceptor signal from the outer to the inner retinal layers, bipolar cells perform extensive signal pre-processing that goes beyond simply changing signal polarity to create the 'classical' light-ON and -OFF pathways. The bipolar cell signal is fundamentally shaped by a multitude of cellular mechanisms as it passes from the dendrites to the output synapses in the terminals. At the dendrites, bipolar cells receive glutamatergic inputs from photoreceptors, which depending on receptor types and other factors, can not only be excitatory or inhibitory but impart specific kinetic properties. This signal then 'trickles' through the soma into the largely passive axon. The axon branches into the terminal system consisting of 5 to 30 individual presynaptic compartments (boutons) within the retina's 'switchboard', the inner plexiform layer.
It is in the axon terminal system, where much of the 'magic' happens. Each bouton can be both pre- and postsynaptic to a vast barrage of inhibitory amacrine cell processes. Accordingly, the visual signal arriving at each terminal can be fundamentally transformed by a wide range of non-dendritic synaptic inputs.
There's the "magic" word -- as they use it, a synonym for complexity or technical wizardry that achieves wonderful things. The article becomes very technical at this point, but this excerpt gives you a sense of the exquisite detail involved in vision. Each detail has a functional purpose. And all this processing is going on before the signal even leaves the eyeball! There's even more wizardry in the visual cortex, another stage prior to the cerebrum where interpretation begins.
Magic in the Brain
Research from Ruhr-University Bochum gives a glimpse into more visual magic taking place in the visual cortex. Scientists there found "data compression mechanisms" that generate "efficiently compressed sensory information" to the cerebrum.
Dr. Dirk Jancke from the Institute for Neural Computation at the university explains:
"We intuitively assume that our visual system generates a continuous stream of images, just like a video camera. However, we have now demonstrated that the visual cortex suppresses redundant information and saves energy by frequently forwarding image differences."
The mechanisms are similar to movie compression algorithms. There's no need to keep sending unchanged parts of an image; the brain already has that information. By subtracting two sequential images, the visual cortex can forward just the parts that change. But those changes are time-dependent; if they occur in less than 30 milliseconds, the whole image is forwarded, experiments showed. "That changed when the time elapsing in the sequences was longer than 100 milliseconds," Jancke said. "Now, the neurons represented only those elements that were new or missing, namely image differences."
The eyeball is constantly looking for changes. "Very fast miniature movements" called saccades constantly scan the visual field, sending a flood of data to the visual cortex. In a fast-moving scene, nothing is lost, but the cerebrum would waste energy over "TMI" (too much information) when gazing at a tranquil scene, were it not for the visual cortex intervening with its data-compression mechanisms. "Thus, certain image sections stand out and interesting spots are easier to detect," the researchers believe.
There's another neat trick the visual cortex performs. Vision experts knew that the cerebrum performs "predictive coding," building up "short-term memory" so that it knows what it expects to see. That's why we can make sense of a scene of a horse running behind a picket fence, without having to figure out what animal reappears after every picket. Now, these researchers have found that predictive coding takes place in the visual cortex, too. "Our brain is permanently looking into the future and comparing current input with the expectations that arose based on past situations," Jancke says. Without this ability, we would be hopelessly confused in everyday experience.
Magic in the Mind
These new findings are not only exciting for design implications -- amplifying the enormous complexity in vision -- they also resurrect old philosophical issues. Do we sense the world as it really is? All this image processing we've been talking about intervenes between the "real world" and our perception of it. We're discovering that our interpretation of the world is many stages removed from the physics of photons impinging on the cornea. The same is true for all the other senses -- hearing, tasting, smelling, and feeling.
But if our "common sense" interpretations of the world are this far removed from reality, how much more the reasoning of the mind! On what basis can the materialist have confidence when saying, "I only believe what I can see"? As C.S. Lewis and others point out in the Argument from Reason -- see The Magician's Twin, edited by John West, Chapters 8-9 -- a materialist can have no confidence that the "convictions in a monkey's mind" are at all reliable or trustworthy. That "horrid doubt" troubled Darwin late in his life, as it should anyone who argues that the eye and brain are products of undirected natural processes.
Only intelligent design provides the grounds for reason that can be reliable and trustworthy. For that to be the case, the world must be fundamentally rational, and the senses and brain must be designed to provide reliable access to that world. Furthermore, only an immaterial mind can interact with other minds to discuss propositions based on reliable sense perception and the laws of logic. Intelligent design meets these criteria.
Armed with reliable sensory information from the world and the gift of rationality, the human mind is equipped to pursue knowledge of the good, the true, and the beautiful -- but equipment is not enough. The real value depends on how it is used.
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