Differential Changes of Magnocellular and Parvocellular Visual Function in Early- and Late-Onset Strabismic Amblyopia

PURPOSE. Studies in nonhuman primates show that monocular visual deprivation starting at different ages has different effects on cells in the parvocellular and magnocellular laminae of the lateral geniculate nucleus. The present study used color and luminance contrast sensitivity (CS) measurements to look for differences in parvocellular- and magnocellular-related visual function in human subjects with strabismic amblyopia. METHODS. Fifteen subjects with early- and 14 with late-onset strabismic amblyopia and similar ranges of visual acuity were studied, together with 15 subjects with normal vision. Contrast sensitivities were measured to an equiluminant (L-M conemodulated) grating with slow onset and an achromatic (LM cone-modulated) 0.8-cpd grating with rapid onset using an adaptive method. RESULTS. Luminance and color CS were lower in the amblyopic eyes than in the fellow eyes of all amblyopes. For luminance CS, this was due both to an increase in sensitivity of the fellow eye and to a reduction in sensitivity in the amblyopic eye. Color CS was greatly reduced in the amblyopic and fellow eyes of subjects with strabismic amblyopia of early- and late onset compared with subjects with normal vision. The reduction in color CS compared with luminance CS was significantly greater in eyes with late- rather than early-onset amblyopia. CONCLUSIONS. Parvocellular and magnocellular function are differentially affected in the amblyopic and fellow eyes of subjects with strabismic amblyopia. The difference is more marked in late-onset amblyopia than in early-onset amblyopia. (Invest Ophthalmol Vis Sci. 2006;47:4836‐4841) DOI:10.1167/ iovs.06-0382 T he central visual pathways in the primate contain two major anatomic subdivisions that transmit information from the retina to the visual cortex in parallel. The magnocellular (M) pathway has a synaptic relay in the magnocellular laminae of the lateral geniculate nucleus (LGN) and terminates mainly in layer IVc of the primary visual cortex, whereas the parvocellular (P) pathway has a relay in the parvocellular LGN laminae and terminates mainly in layer IVc. 1,2 Cells in the magnocellular pathway respond best to achromatic stimuli of low spatial and high temporal frequencies, whereas cells in the parvocellular pathway respond best to chromatic stimuli of high spatial and low temporal frequencies. 3‐7 It is thus possible to select stimuli that, at threshold, measure the sensitivity predominantly of one or the other pathway. By using such stimuli, the effects of amblyopia on the magnocellular and parvocellular pathways can be studied in humans. Studies in nonhuman primates have shown that monocular visual deprivation causes hypertrophy and shrinkage of neuronal cell bodies in the LGN. 8,9 After experimental monocular visual deprivation of early or late onset, the patterns of cell size change in the magnocellular and parvocellular LGN laminae of nonhuman primates differ, suggesting differential changes in sensitivity to deprivation with age in the two pathways and demonstrating the presence of two distinct periods of developmental sensitivity. 9 Changes in visual evoked potentials and in contrast sensitivity (CS) found in human subjects with strabismic amblyopia are different when the age of onset of amblyopia is before or after 18 months of age, indicating that there may also be more than one sensitive period for visual development in humans. 10 The stimuli used in that study were not selective for M and P pathways. The present study describes the psychophysical results of a similar study performed in subjects with strabismic amblyopia of early and late onset using stimuli biased to measure CS predominantly of either the M or the P pathway and to examine whether the two pathways are affected differently and whether the changes found differ between amblyopia of early or late onset.