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
