Abstract
Can adult visual performance actually be improved by rearing in an altered visual environment? Recent demonstrations of plasticity in neural systems would suggest “yes”, however years of monocular deprivation studies suggest “no”. In those studies, despite a competitive advantage for the favored eye (resulting in widespread anatomical and physiological dominance in visual cortex), vision in that eye was not improved.
We have taken a different tact by looking at potential competition among neurons comprising different visual channels (e.g., color, motion, form) that co-exist in parallel from retina to cortex. How these pathways develop, and whether experience-dependent competitive interactions may play a role synapse formation (possibly as early as in the retina) have received little attention. This is due, at least in part, to the difficulty of stimulating or inactivating individual components of these parallel pathways.
Our animal model is the tree shrew, which possesses a 95% cone-dominated dichromatic retina. Red light rearing (RLR) deprives its SWS cones, but stimulates LWS cones normally. This in turn produces an imbalance in the activation of a now deprived “color” pathway compared to a normally stimulated “luminance” pathway (driven solely by LWS cones). We recorded retinal ganglion cell responses from optic tract fibers in normal and RLR adult shrews. (RLR shrews were reared from birth to 12- to 25-wks of age under Kodak 1A tungsten illumination, then housed in normal white light.) Receptive fields, the sustained/transient nature of the response, chromatic responsiveness and temporal modulation sensitivity functions (tMSFs) were determined. Our primary finding was that the tMSFs of RLR neurons peaked at higher temporal frequencies compared to normals (23Hz vs 12 Hz, t-test, p<0.01). This result held for sustained and for transient type neurons.
Our previous psychophysical testing of RLR shrews revealed enhanced high-frequency temporal vision, paired with poorer color vision. This combination cannot be attributed solely to a deprivation effect, but implies competitive interactions between neurons coding color and motion information during post-natal development. The observed shift in temporal tuning of retinal ganglion cells in RLR shrews argues for a retinal locus of this competition.