Eyesight: More Reasons to Be Thankful
So much is going on in your body when you look at that sliced turkey and raise it to your salivating mouth, a human mind can only fathom bits and pieces of the story. Everyone knows the eye is the quintessential example of a complex organ, but Current Biology1 focused on one of the wonders that occurs after the signal leaves the optic nerve.
Alexander Thiele (U of Newcastle upon Tyne) started by saying that we move our eyes more often than our heart beats. Our eyes constantly jerk from side to side, without our even noticing. This means the brain has to constantly stabilize a shaky input. How does our brain cope with the constantly changing scene?
When you watch a music video you are inundated with a seemingly incoherent and rapid stream of visual scenes, changing on average every 2.3 seconds. Such a rate of change may appear fast to an old fashioned television consumer, but it is still about seven times slower than the rate of scene change imposed by rapid eye movements on the visual system. While the former may be tiring, the latter goes seemingly unnoticed. (Emphasis added in all quotes.)
Why does the eye make these constant movements, called saccades? “Saccades ensure that an attended object is foveated for high acuity processing,” Thiele writes. But how does the visual center in the brain, like a digital screen made of neurons, give us the impression that our field of view is steady? There are two possibilities. The conventional view is that the brain has enough processing overhead to constantly interpret the scene. Another, newer view is that the neurons compensate for the shifts in a pre-processing step. Neurons may be synchronized to the eye muscles so that they are prepared for the changes, kind of like a screen synchronized to move in step with a vibrating projector. Here it is in scientific jargon:
Of particular importance for such adjustments may be neurons in the lateral intraparietal, frontal, and even early visual areas which shift their receptive fields shortly before the occurrence of a saccadic eye movement, causing an internal re-mapping of visual space. These neurons signal that a saccade will bring an object into their field of view, even if that object has been removed from sight just before or during the saccade. Such a re-mapping could result in predictive adjustments in early cortical areas that prepare for scene changes, thereby minimizing their negative impact, and maximizing rapid information processing following a saccade. This requires a substantial amount of trans-saccadic integration.
Experimental tests so far have been unable to determine which method the brain uses. Either way, it’s amazing; these adjustments are made in fractions of a second. “Psychophysical studies have shown that human perceptual abilities are exquisite and extraordinarily fast when extracting information during rapid serial visual presentation of natural scenes,” Thiele said. His only mention of evolution was after noting the “surprising” evidence that our brain can detect and classify images even in the near absence of attention. “This suggests that the human visual system has evolved to rapidly extract information from highly variable natural scenes….”
1Alexander Thiele, “Vision: A Brake on the Speed of Sight,” Current Biology, Volume 15, Issue 22, 22 November 2005, Pages R917-R919, doi:10.1016/j.cub.2005.10.057
Just set aside that little piece of evolutionary fat, so you won’t lose your appetite. As you visually scan that plate full of colorful, fragrant, tasty food, think about this one of all those senses taking in those cues. You have a high-tech, integrated, super-fast, high-resolution video recording, processing, and display system, and it even has image stabilization. “The hearing ear, and the seeing eye, the Lord has made them both” (Proverbs 20:12). Give thanks, and enjoy.