Protracted Visual Functions Opening New Doors to Plastic Changes

Though so far we have concentrated on visual functions that have a short and well-defined matura-tional time course, other visual functions, such as visual acuity, long-range orientation integration, or orientation toward peripheral space, seem to have a protracted time course of development and a much longer period of sensitivity to altered visual experience.

Grating acuity or the capacity to discriminate sinusoidal gratings from noise is extremely low at birth; it develops rapidly during the first 6 months of life, but then continues to improve very slowly until about 6 years of age. Although adultlike performance is achieved around that age, it is not until about 10 years of age that visual acuity is no longer sensitive to altered visual experience. Early visual deprivation such as cataracts or strabismus results in enduring loss of visual acuity. Researchers have shown that tests of visual acuity just after corrective surgery indicate acuity within the range of newborns, even when deprivation lasted up to 9 months. Within an hour, the acuity of the treated eye was shown to improve dramatically to the level of a normal 6-month-old infant. These results highlight the triggering role of patterned visual input in the development of visual acuity. The timing of that patterned input is critical, however. Although the grating acuity of children treated within their first year for cataracts or strabismus continues to improve with age, it does not keep pace with normal development and falls below normal limits as development proceeds. Whereas this effect is quite similar for unilateral and bilateral deprivation, the effects of early unilateral deprivation appear to be more drastic later in life than those of bilateral deprivation. For example, acuity as measured by the ability to recognize or match letters (e.g., Snellen chart) is much worse after unilateral cataracts than bilateral cataracts. Additionally, the outcome after bilateral cataracts shows little sensitivity to the duration of deprivation. In contrast, whereas there is no effect of deprivation duration after unilateral cataracts if operated early (before 6 weeks of age), deprivation duration affects the outcome of recovery in cases of unilateral cataracts treated after 6 weeks. By this age, uneven competition between the two eyes appears to limit recovery. Accordingly, the patching strategy adopted after surgery has been found to be a major determinant of treatment outcome. These findings support the view that plastic changes in visual acuity are mediated by at least two separate mechanisms with different time courses. One mechanism is driven by the absolute amount of patterned input to which the visual system has access; this mechanism is present at the onset of development and probably throughout development. The other mechanism, which emerges only later in life, appears to be driven by the competition between the two eyes and quickly becomes the main predictor of recovery.

Another skill with a protracted time course of development is the capacity to integrate long-range spatial information. For example, the detection of contours that are defined solely on the basis of longrange orientation domain correlation has been found to improve throughout childhood, reaching adult levels around 14 years of age (Fig. 2). This skill requires the integration of visual information across space and is believed to rely in part on the integrity of long-range horizontal connections in layers 2 and 3 of the visual cortex. This skill is also jeopardized by alterations of visual experience such as in amblyopia. Little is known at this time about the time frame during which visual deprivation affects that skill, but its protracted period of development suggests that it may be influenced by altered visual experience throughout childhood.

A similarly protracted visual function is the ability to orient to peripheral information. Whereas the ability to detect large, bright peripheral events is present in the newborn, the automatic ability of adults to detect pinpoints of light at the far edge of the visual field develops slowly, reaching adult levels by 12-14 years of age. In accordance with its slow developmental time course, this skill is altered when deprivation occurs not only early in life but also at 6 years of age.

Figure 2 (a) Examples of stimuli used to study visual-spatial integration of long-range orientation. Subjects were presented with displays like the one in (a) and asked to delineate the elements that form a contour [answer given in (b)]. The difficulty of the task was manipulated by varying the spacing and numbers of elements used. (b) The contour present in (a) is indicated by arrows. (c) The developmental time course of this skill appears to be protracted, with adult performance only seen at 13-14 years of age. Reprinted from Kovacs, Kozma et al. (1999), Late maturation of visual spatial integration in humans. Proc. Natl. Acad. Sci. USA 96(21), 12204-12209, copyright 1999, National Academy of Sciences, USA.

Figure 2 (a) Examples of stimuli used to study visual-spatial integration of long-range orientation. Subjects were presented with displays like the one in (a) and asked to delineate the elements that form a contour [answer given in (b)]. The difficulty of the task was manipulated by varying the spacing and numbers of elements used. (b) The contour present in (a) is indicated by arrows. (c) The developmental time course of this skill appears to be protracted, with adult performance only seen at 13-14 years of age. Reprinted from Kovacs, Kozma et al. (1999), Late maturation of visual spatial integration in humans. Proc. Natl. Acad. Sci. USA 96(21), 12204-12209, copyright 1999, National Academy of Sciences, USA.

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