Daniel L. Adams, Ph.D.

Summary of current research

Misalignment of the optic axes (strabismus) is a common disease of the central nervous system.  In strabismus, normal stereovision is impossible because the eyes are not fused on a common target.  As a result, strabismic children have poor or absent depth vision.  In addition, many children with strabismus develop amblyopia, depriving them of sight in one eye.  If adults become strabismic through eye injury or disease, they complain of diplopia (double vision).  However, strabismic children rarely complain of diplopia because they suppress vision in the deviated eye.  Unfortunately, if one eye is consistently suppressed in favor of its dominant fellow, visual acuity can be reduced in the deviated eye permanently.  My current research at the Koret Vision Research Center was conceived in light of anatomical experiments on strabismic suppression that we completed here in 1999. We showed that regions of the striate cortex in strabismic monkeys that fall within suppression scotomas have reduced metabolic activity, suggesting that perceptual suppression is reflected in the activity of cells in striate cortex.

To pursue our investigation into strabismic suppression, we moved into a new space where we could create a laboratory suitable for physiological recording experiments in awake-behaving monkeys.  I participated in the architectural planning of the new laboratory and helped design and assemble the equipment necessary for the physiology experiments.  The training and psychophysical testing phases of these experiments are now well underway.  We will use multiple tetrodes to record simultaneously from neighboring ocular dominance columns within suppression scotomas of strabismic animals as they switch fixation from eye to eye.  We hope to learn about the physiological basis of strabismic suppression and how it is controlled by visual fixation behavior.  This study will enhance our knowledge of how neuronal activity in striate cortex both gives rise to- and gates visual perception. One also hopes that such knowledge will facilitate new approaches for the treatment and prevention of strabismus and amblyopia.

Good form vision in both eyes during development is critical for normal visual development.  Compromised vision in one eye, caused by a congenital cataract for example, can lead to irreversible blindness.  In this case, normal vision will not be restored by removal of the cataract alone, because the visual loss originates in the brain, not the eye itself.  This childhood condition is called deprivation amblyopia.  In striate cortex, deprivation amblyopia causes the geniculate afferents to re-wire such that the deprived eye's ocular dominance columns shrink.  The relinquishing cortical territory is taken over by the unencumbered eye.  In normal primates, retinal blood vessels cast shadows into the retinas that cause narrow regions of blindness in each eye called angioscotomas. I discovered that angioscotomas also cause geniculate afferents to re-wire such that the pattern of retinal blood vessels becomes represented in striate cortex.  Just as in deprivation amblyopia, cortical territory belonging to a deprived region of one eye is relinquished to the fellow, unshadowed eye.  This finding demonstrates that deprivation amblyopia is the result of a normal developmental process that occurs (albeit on a smaller scale) in all normal primates including humans.

Both strabismic and deprivation amblyopia occur as a result of abnormal visual experience during development.  However, the function of ocular dominance columns has been sought for more than 30 years, yet no theory of their function has reached the level of general acceptance.  While studying the representation of retinal blood vessels in striate cortex of squirrel monkeys, I observed that the expression of ocular dominance columns was capricious.  We interpreted the fact that ocular dominance columns were absent in one third of a normal population of a single species as evidence against their functional importance.  We are currently exploring the possibility that ocular dominance columns, and by extension, all cortical columns, might be trivial.