Introduction

Newborn primates see poorly. Their visual capacities improve over time, with a course that varies somewhat depending on the measure used to define visual function and the species studied. Many common measures of vision reach adult levels by the age of about 1 year in macaque monkeys and about 5 years in humans; during the period of maturation, performance typically improves roughly 10- to 30-fold. Figure 12.1 caricatures the effect on vision of two of these measures, spatial resolution and sensitivity to spatial contrast. The panel on the left shows a cityscape as seen by an adult, whereas the panel on the right shows the same scene transformed to represent the view of a newborn infant. The “infant view” has been spatially lowpass-filtered (blurred) and reduced in contrast.

Figure 12.1..  

Simulation of the visual worlds of an adult and young infant primate. To create the simulated image on the right, the image on the left was convolved with a Gaussian, the σ of which was 1°; the image contrast was reduced by a factor of 5. The angular width of the view shown is approximately 10°.

Figure 12.2 shows developmental measurements of spatial resolution (Kiorpes, 1992a; Movshon and Kiorpes, 1988) and contrast sensitivity (Boothe et al., 1988) taken from macaque monkey infants. Figure 12.2A shows grating activity data from a group of young monkeys tested cross-sectionally; Figure 12.2B shows a series of contrast sensitivity functions measured longitudinally in two representative individual animals. Figure 12.2B emphasizes that different animals develop at different rates, so in Figure 12.2C we show the range of sensitivity and resolution values measured across a population of six monkeys. Both resolution and sensitivity develop smoothly over the first 6 to 12 months of life. These functions mature somewhat more rapidly when measured electrophysiologically using a visual evoked potential (VEP) technique in monkeys (Skoczenski et al., 1995) and humans (Kelly et al., 1997; Norcia et al., 1990; Chapter 13; see also Peterzell et al., 1995). This discrepancy between techniques is not surprising given that the VEP signal arises from the summed activity of visual cortical neurons. As we discuss later, neurons in infant visual cortex are considerably more mature than behavior would suggest.

Figure 12.2..  

The development of spatial vision in infant macaque monkeys. A, Spatial resolution data from a set of 17 normal infant macaques (taken from Kiorpes, 1992a; Movshon and Kiorpes, 1988). The measure of spatial resolution was grating acuity, the highest spatial frequency at which a grating could reliably be distinguished from a uniform field of the same luminance. Animals younger than 16 weeks were tested using a forced-choice, preferential looking technique. Older animals were tested in a standard two-choice operant discrimination task. B, Spatial contrast sensitivity functions, measured using operant techniques, in two infant macaques at a range of ages (as indicated) (data from Boothe et al., 1988). Error bars indicate the standard error of the mean threshold determined by Probit analysis. C, The range of rates of development of spatial contrast sensitivity in six infant macaques. Each line represents the course of contrast sensitivity development in an individual monkey. The two plots indicate the horizontal and vertical positions of the peaks of the measured contrast sensitivity functions. (Redrawn from Movshon and Kiorpes, 1988.)

We want to understand the processes that limit visual development. In the first part of this chapter we will consider what aspects of visual system organization and function limit performance in newborn infants, and what factors develop to permit attainment of adult level of visual performance. We are also interested in the modifiable mechanisms that are responsible for the altered visual development that occurs when normal vision is disrupted, and in the second part of the chapter we will explore the neural factors responsible for this behavioral plasticity.