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Spatial Vision

Spatial vision refers to our ability to resolve or discriminate spatially defined features. The two primary measures of spatial vision are acuity and contrast sensitivity

Acuity

Static visual acuity (SVA) refers to the ability to resolve fine details, typically in high contrast stationary targets (e.g., black targets on a white background). A sample chart using letters as acuity targets is shown below.

High Contrast Letter Acuity Chart

Different studies provide estimates of infant acuity that range from about 20/400 to 20/800 (20/200 or worse defines legal blindness in adults). Infant acuity improves rapidly over the first 6 months to approximately 20/25, and more slowly thereafter, reaching adult levels (better than 20/20) around 7 years. These changes are shown in the graph immediately below.

 

A newborn infant's poor acuity is explained by the initial lack of cone development (see preceding section on Eyeball & Retina). Acuity improves as the cones elongate and migrate more closely together to form the fovea. This results in more refined input to the cortex, stimulating its further development.

Contrast Sensitivity

Acuity measures only the smallest detail that can be resolved, but not our ability to see larger targets. By measuring our ability to see objects of different size (i.e., of different spatial frequency), contrast sensitivity provides a more comprehensive test of spatial vision. It is measured by finding the lowest contrast needed to see light/dark gratings of varied fineness or spatial frequency. Spatial frequency refers to the number of light/dark cycles per degree (c/deg) of visual angle on the retina. As shown immediately below, gratings are usually defined by gradual sine-wave (i.e. sinusoidal) variations in luminance.

Low (left) & High (right) Spatial Frequency Sinusoidal Gratings

The contrast sensitivity function (CSF) depicts an observer's sensitivity (i.e. 1/contrast threshold) to sinusoidal bar gratings of widely varied spatial frequency. Adult contrast sensitivity is greatest to intermediate spatial frequencies (about 2 to 4 c/deg). Lower and higher spatial frequencies require more contrast to be detected, resulting in an inverted-U function (see graph below). The highest spatial frequency can be resolved only at very high contrast and corresponds to the observer's acuity level.

At birth, infants' sensitivity to fine, high-spatial frequency gratings, like their acuity, is very poor but improves steadily with age. Newborns can't resolve targets above 2 to 3 c/deg. The improving contrast sensitivity can be seen in the CSFs at birth, 3 months, and 6 months in the following figure. The CSF of an infant differs in 3 ways from that of an adult: 1.) its peak is lower, 2.) the function is shifted to the left, and 3.) the shape is a "low pass" rather than a "band pass" function.

Infant CSFs Compared to an Adult

As for acuity, the improvements in contrast sensitivity is due to the elongation, tighter packing of cones in the retina and the associated development of the visual cortex.

The effects of infants' improving contrast sensitivity, especially at higher spatial frequencies, can be see in the image processing simulation below. It provides a gray scale representation of how an infant might see contrast information at 3, 6, and 9 months compared to an adult. As can be seen, the infant is increasingly able to see fine-detail, low-contrast elements in their environment.

Simulation of Infant & Adult Contrast Perception

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