
Extremely rare in humans.  Usually so much else is involved you'd
just have a mess to sort out.  Birds do all vision in the tectum,
don't they?  

-- 
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Gordon Banks  N3JXP      | "Skepticism is the chastity of the intellect, and
geb@cadre.dsl.pitt.edu   |  it is shameful to surrender it too soon." 