Competition and cooperation in visual development

Abstract

SUMMARYVisual disorders of early childhood occur in a visual system which is still undergoing development. They are influenced by this development and may influence its sub sequent progress. This review describes experiments per formed to investigate visual development in the primate and in particular the way in which the central visual path ways related to the two eyes interact during both normal and abnormal development. There is evidence that these interactions play an important role in the developmental process and understanding them should allow better The effects of abnormal visual experience on develop ment were first shown experimentally in the classical stud ies of Wiesel and Hubel in the cat. These showed that closure of one eye in kittens caused marked changes in the structure and function of the visual pathways.3 , 4 Three major changes were described. Firstly the eye became amblyopic; secondly the closed eye became much less effective in driving cells in the visual cortex which became dominated by inputs from the other, open eye; and thirdly neuronal cell bodies in the lateral geniculate nucleus (L GN) which received input from the closed eye shrank when compared with cells in the LGN laminae related to the open eye. These studies were the first demonstration that abnormal visual experience, in the absence of direct anatomical interference, had a marked effect on the development of the structure and function of the visual pathways and they introduced two important concepts. Firstly they showed that the development of the visual system was only affected by abnormal visual experience during a restricted period early in life: the critical or sensi tive period. Secondly, because the closure of one eye has a much greater disruptive effect than the closure of both, they suggested that the changes produced by visual depri vation were due at least in part to a competitive interaction between the pathways related to the two eyes. 5The concept of competition in visual development was further investigated and developed by Guillery who stud ied the changes in LGN cell size produced by visual depri vation in kittens. He demonstrated that closure of both eyes had much less effect on LGN cell size than closure of one eye.6 He also studied the cells in the LGN related to the extreme temporal visual field, which is monocular because there is no corresponding area of retina in the other eye. He showed that these LGN cells in the monocu-

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

“…This confirms the findings of a previous study in humans 10 and anatomic studies of monocular deprivation in primates 9 indicating that the underlying pathophysiology is different in amblyopia of early and late onset. One possible explanation is that M pathways simply mature and lose plasticity earlier than P pathways.…”

“…8,9 In the primate, in addition to the relative shrinkage of deprived LGN cells compared with undeprived cells, an increase in the ratio of M to P cell size was observed for deprived eyes after long-term deprivation and undeprived eyes. The relative reduction of P compared with M function observed for amblyopic and fellow eyes in the present study parallels these changes and suggest a similar underlying pathophysiology in both instances.…”

“…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.…”

Amblyopia: neural basis and therapeutic approaches

“…In many children, amblyopia develops at an age in which the ocular dominance columns of V1 (46,68) are no longer affected, i.e., after the early sen sitive period (Table 1). It has been documented that morphologic changes occurring during the late sensitive period selectively affect cells of the parvocellular pathway (17,21,(69)(70)(71) . It seems reasonable to re commend that treatment for amblyopia should be individualized, as some children with amblyopia may not respond to occlusion because of morphophysiologic dissociation between the magno-and parvocellular pathways (72) .…”

“…After that, there is a significant reduction in neuroplasticity. Until the end of (17) (Table 1), there is macular maturation, optic nerve myelination, fusion of images for binocular vision (20) , formation of the ocular dominance columns in V1 by competition, and maturation of binocular connections by cooperation among afferents from both eyes (21) . Once past this period, even if the cause of the deficit is corrected, an 8-to 10-year-old child is likely to have persistent reduction in visual acuity and contrast sensitivity (22) .…”