Perceptual Development: Visual Acuity

Visual acuity (VA) is a measure of spatial resolution of the visual system, which is determined at least in part by the ability of the eye to focus incoming rays of light directly onto the retina at the back of the eye. For mature, cooperative, verbal individuals, VA is typically measured by having the subject identify letters on an eye chart that is at a precise distance from the individual. The stimuli are black letters on a white background, yielding the highest possible contrast. In a clinical setting, VA is generally defined as a fraction with the denominator specifying the distance at which the subject sees what most individuals see at 20 feet. Normal VA of 20/20 can be achieved for most individuals naturally or by use of optical correction to focus the rays of light onto the back of the retina. The standard and familiar Snellen chart has been in use since the mid-1800s. The singular “E” at the top of this chart makes a good reference point: If this is the only letter that an individual can identify, his or her VA is approximately 20/200, or at 20 feet this individual can see what most people see at 200 feet. Typically, an individual with 20/200 or worse VA is considered “legally blind.”

Still, the question remains: How do you know what an infant can see? True, these youngest of humans will not be reading an eye chart or even identifying visual shapes. But researchers have devised some relatively simple yet sophisticated ways to determine what infants see. Behavioral and electrophysiological measures are the two most commonly used techniques for evaluating visual acuity in human infants. These techniques are quite different from each other, but both have their place in research as well as clinical applications. This entry describes the behavioral and electrophysiological measures of VA and newborns, along with the underlying physiological limitations relating to VA and newborns.