ABSTRACT
To measure visual acuity is to measure the spatial limits of visual discrimination and involves the determination of thresholds. There are several different ways of classifying visual acuity:
minimum visible (‘is that object visible or not?’):
– requires detection of a visible stimulus against a uniform background
– can be achieved at visual angles of 1 second of arc or less
– changes in object size below 1 minute of arc visual angle are perceived as variation in contrast and not size
minimum resolvable (‘is that one object or two?’):
– a measure of the ability to distinguish two stimuli as being separate
– relies on detection of contrast differences between retinal ‘detecting units’
– greatest at the fovea and least in the periphery
– at threshold it corresponds to between 30 seconds and 1 minute of visual angle
minimum discriminate (Vernier acuity or hyperacuity):
– is the determination of the relative locations of two or more visible points
– threshold of hyperacuity is 3–12 seconds of visual arc
– possible despite a minimum cone separation at the fovea of 1 minute of arc due to cortical processing
stereoacuity:
– stereoacuity is the smallest binocular disparity that can be reliably detected
– threshold for stereoacuity is 3–12 seconds of arc, similar to hyperacuity.
