mgreen@cs.toronto.edu (Marc Green) (04/09/90)
In article <6404@ur-cc.UUCP> orin@cvs.rochester.edu (Orin Packer) writes: > > Stereo blindness is very common even though the person affected may > have otherwise unremarkable vision. Approximately 5% of the > population is unable to make use of stereopsis at all and another 25% > or so has some difficulty, such as being insensitive to just divergent > or just convergent disparities. > Medically, the term "stereoblindness" refers only to people with normal acuity in each eye, but no ability to use disparity cues. This is actually quite rare, usually developing primarily in people with a divergent strabismus (eye turn). Packer is confusing this with the more common condition, amblyopia, in which one eye is supressed by the other and loses function. Amblyopes lack stereo vision primarily because one eye simply doesn't work. When vision is restored to the bad eye, such as by placing a filter in front of the good eye, amblyopes may still exhibit some stereopsis. This usually develops as a result of strabismus or unequal refractor power in he two eyes. > The visual system undergoes a long period of calibration during early > development that involves refinement of the connections between > neurons in the visual cortex. This process is easily disrupted by > any problem that gives one eye an advantage over the other eye, such > as a congental cataract or an imbalance in the eye muscles, or even a > difference in the optical focus of the two eyes. Once the > binocular inputs to the cortical neurons are disrupted, they > apparently never recover even though visual acuity can usually be at > least partially restored by removing the underlying problem. Many studies on humans and animals show that the effects of depriving an eye can easily be reversed if done during the "critical period", when the visual system is plastic. In humans, the critical period for stereo vision extends to about age three while the critical period for visual acuity extends to about age 8. This means that correction of a strabismus by 3 will usually allow normal stereo vision and acuity to develop. After age 3, it is still possible to restore normal acuity, but stereo vision is degraded. Marc Green Trent University