January 6, 2010 | David F. Coppedge

It Takes More than Eyes to See

We think of eyes as objects that see, but vision requires a whole system of parts.  One of the most important is the brain.  Without your thalamus, vision would be a hopeless jumble of jerky signals, reported scientists from the National Eye Institute.
    Writing in PNAS,1 Ostendorf, Liebermann and Ploner found that the human thalamus contributes to perceptual stability across a moving visual field.  “We continuously move our eyes when we inspect a visual scene,” they began.  “Although this leads to a rapid succession of discontinuous and fragmented retinal snapshots, we perceive the world as stable and coherent.  Neural mechanisms underlying visual stability may depend on internal monitoring of planned or ongoing eye movements.”  By distinguishing ongoing from planned eye movements, they were speaking of the continual oscillations the eye muscles perform unconsciously, called saccades, that aid in preventing saturation of the retina.  Receiving a combination of self-induced motions and automatic motions would seem to produce a hopeless jumble.  The thalamus helps sort it all out.
    The researchers studied a patient who had a stroke that affected the right lobe of his thalamus.  He had trouble monitoring his eye movements and locating visual stimuli.  Combined with research on macaques, this information indicated to the researchers that “the human brain draws on transthalamic monitoring signals to bridge the perceptual discontinuities generated by our eye movements.”
    The patient’s visual abilities were impaired, but not enough to cause a breakdown of visual stability in his everyday life.  Why?  The visual system is provided with backup systems for this important function:

Although proprioceptive information about eye position seems to play a negligible role for the maintenance of visual stability, a number of alternative corollary discharge pathways may help to partially compensate for the affected transthalamic pathway.  Visual stability plays a fundamental role for any kind of visually guided behavior and should therefore be maintained by robust and redundant mechanisms.  This is in keeping with the observation that only large and bilateral brain pathology may occasionally lead to the subjective experience of compromised visual stability…. Temporal contiguity of visual stimuli across saccades and constancy of relative positional information in a visual scene may thus mainly account for visual stability in most everyday situations.  Even a distorted and coarse internal monitoring signal might, under these circumstances, convey sufficient temporal and spatial information to complement the evaluation of visual reafferent information.
   ….impaired corollary discharge transmission comes at a perceptual cost: Whenever the brain starts to question the prior assumption of a stationary world, visual space perception across eye movements is compromised and affected by oculomotor noise.  Transthalamic monitoring signals may thus significantly contribute to the correct attribution of self-induced vs. externally imposed changes in the continuous flow of our sensory experiences.

Getting the visual scene right can actually affect our sense of self.  They noted in conclusion, “Central aspects of our self-conception may build upon the integration of such corollary discharge-transmitting loops and their disturbed functioning might contribute to the symptomatology in devastating diseases such as schizophrenia.”  The scientists did not attempt to explain how these integrated, coordinated, robust, overlapping systems might have evolved.

1. Ostendorf, Liebermann and Ploner, “Human thalamus contributes to perceptual stability across eye movements,” Proceedings of the National Academy of Sciences USA, December 28, 2009, doi: 10.1073/pnas.0910742107.

If you counted up the stories we have reported about integrated biological systems in which every component contributes to exquisite function, and the researchers who studied them avoided the topic of evolution, it would be a substantial list.  Even the subset of stories about the human eye would be substantial (three examples: 08/07/2008, 05/12/2005, 08/28/2003).
    The force of these stories is in the details.  We’re going to keep piling them on as long as we can till the weight of evidence shames the Darwinists into whimpering irrelevance.  We’d like to do more and do it better.  Have you considered supporting this resource?

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