2015 Vision

in the visual system, acuity refers to the ability to discriminate fine details of the visual scene (a broader definition than was used in the somatosensory system)

3 forms are recognized, and they are primarily a function of the cone system:

1. SPATIAL ACUITY: ability to resolve 2 points in space
   
   • is a function of location: acuity at the periphery is relatively constant as a function of brightness, and is much lower than at the fovea (due to the differences in the distribution of rods and cones)
   • is a function of brightness: as brightness increases, the ability to resolve a gap increases at the fovea
     • i.e., in low light, vision is carried out by rods; as brightness increases more cones respond
   • assessed using SNELLEN eye charts, which contain square letters of the alphabet
     • measurement of acuity is made relative to ability of the normal population to distinguish the letter at 20 feet (20/20 vision is normal)
     • LOW ACUITY: standing at 20 feet, can only resolve letters that the normal population can resolve at 200 feet (20/200)

     • HIGH ACUITY: standing at 20 feet, can resolve a letter that the normal population can resolve at 15 feet (20/15)

2. TEMPORAL ACUITY: ability to distinguish visual events in time
   • there is a CRITICAL FUSION FREQUENCY (CFF) where a flashing light appears to be continuous rather than repetitive
   • the CFF is lower in rods compared to cones

     • FYI: in the early years of the motion picture industry, projector speeds were slower than the CFF of cones (<50 Hz), therefore the audience perceived the "flicker", and movies became known as "flicks"

3. SPECTRAL ACUITY: ability to distinguish differences in the wavelength of the stimuli
   • to elaborate on a point made earlier, far-red light can excite cones but not rods
   • therefore, in a room with a red light, actions requiring high acuity (reading, writing, etc.) can be performed without bleaching the rods, allowing a person to move into dim light conditions without losing the sensitivity provided by dark adaptation
     • two examples where this phenomenon is exploited: in a dark room (photography) and on aircraft carriers (pilots can perform regular duties in a red-lighted room at night, and still scramble to their planes on the dark deck, if needed)

• Acuity

• Receptor potential
• Retinitis pigmentosa
• Retinopathy

• Vitamin A deficiency
• Macular degeneration