It May Be Rocket Science After All …


A couple of months ago I blogged about duplex models of visual steams, the basic separation into a yin and yang of vision under different names.  If you’re enamored with neologisms, you’ll be drawn to a variety of terms that appear to introduce new concepts when in reality they’re re-packaging or only modestly extending pre-existing concepts.

More recently I made reference to researchers looking at what they term the PCR, or periphery to center ratio of the visual field.  Though it’s a bit of a re-package it is a very useful concept, not only with regard to dyslexia, but to visual processing styles in general.  The origin of this work is the Harvard-Smithsonian Center for Astrophysics.  In particular, Dr. Matthew Schneps began to speculate on why the preponderance of astronomers he knew were dyslexic, including himself.  This ties in to the idea that the Eides have helped popularize – that dyslexia might confer gifts or advantages that come with a utility cost in which reading is compromised.  But why?

This slide comes from a presentation by Dr. Schneps and his colleagues at the Laboratory for Visual Learning.  The two visual systems, constituting center and periphery, are contrasted in a fish-eye photograph of Harvard Yard covering 180 degrees in visual extent.  When you place your hand on a printed page and fixate the center, the visual angle is approximately 2 degrees.  The inset scales normally sized text to the visual angle subtended by your hand, when you fixate on the center circle.   If you extend your hand into the view of the yard, maintaining fixation on the center, the angle subtended is approximately 16 degrees.  The implication is that people differ dramatically in their proclivities for focused search tasks in the center of the field, vs. broad comparisons in the peripheral field, such as the wide expanse between the span of one’s hand and the rest of the scene in the yard.

Schneps and colleagues broadly categorize people into Low PCR, or Periphery to Center Ratio, vs. High PCR as follows:


The entire article in Mind, Brain and Education is worth reading: DYSLEXIA – VISUAL LEARNING & THE BRAIN – 2007

Though the authors acknowledge that it is difficult to precisely demarcate a center-periphery boundary, for the purposes of their discussion they characterize center as being within an 8 degree visual subtense, and outside 8 degrees as peripheral.  When conceptualized as separate systems, use of center vs. periphery introduces a trade-off between attention and working memory, broadly considered as visually sequential in the central field and spatially simultaneous in the peripheral field.  However, the key is attaining a healthy working balance between center and periphery depending on the nature of the visual task at hand.

The point here is that there is evidence that at least some people with dyslexia exhibit a visual bias favoring the periphery, characteristic of what they call a high PCR group.  For these individuals undergoing optometric vision therapy, emphasis might be made on become more efficient at visual processing in the central field.  So for example, with form fields, one would emphasize central fixation with awareness of the central portion and de-tuning the peripheral portion.

One of the newer modalities, the Sanet Vision Integrator offers the opportunity to manipulate divided attention, loading the center in favor of the periphery, with simultaneity in the center and scanning in the periphery.  This push-pull relationship between center and periphery, or the reciprocal inhibitory interactions, are deeply intertwined with attention mechanisms.  In addition, the central field is strongly associated with face recognition.  It is not difficult to see the extensions of the PCR theory to other clinical populations we deal with.  Though Dr. Schneps was struck by the skew toward dyslexia among his colleagues, it would not surprise me if those on the Autistic Spectrum are over-represented among his astrophysicist colleagues as well.  Understanding how we modify visuospatial performance may turn out to be part rocket science after all.

– Leonard J. Press, O.D., FCOVD, FAAO

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