Light Levels & Logs, Ocular Anatomy

Retinal Anatomy Primer

Optic nerve ~ 5 deg (1500 microns)

• 15-20 degrees eccentric - distance from foveola

Foveola ~ 1 deg (= rod-free area)

Photoreceptors (#’s & Use)

6 million cones (max cone density 0 deg)

120 million rod (max rod density 20 deg)

optic nerve: nasal

Scotopic vision

Refers to vision at ‘low’ light levels

It is mediated by rod photoreceptors

Rods

• Have a visual acuity of ~ 20/200

Do not play a role in color vision

• Patients with only rods – Rod Monochromats

» Congenital; usually have nystagmus (no foveola, can’t focus)

Photopic vision

Refers to vision at ‘high’ light levels

It is mediated by cone photoreceptors (There are 3 kinds)

Cones

• Have a visual acuity of ~ 20/20

• Play a role in color vision

• Patients with only cones – No congenital cases of true ‘cone monochromats’

– Clinically: Cone monochromats (Patients that have 1 cone type + rods)

– Some eye diseases can present (sort of) like only cone function is present (have rods that don’t work)

» Retinitis pigmentosa (depending on stage)

» Congenital stationary night blindness (CSNB)

Mesopic vision

Refers to vision at ‘medium’ light levels – ‘Twilight’/Moonlight

It is mediated by rod and cone photoreceptors

• Cones are not functioning optimally though

– i.e. Wouldn’t be as good at visual acuity or color vision tasks

• Rods are not necessarily functioning optimally either

– i.e. Wouldn’t be as good at light detection

• But depends on the vision function of interest

– Depending on the vision function of interest, the best system (the one with the lowest threshold) would determine vision function

Visual angles

Objects in space, form a particular image size on our retina

Image size is discussed in terms of angle of subtense

Retinal landmarks can be discussed in angular terms or absolute distances

• Clinically, often assume the optic nerve head is 5 degrees in diameter

• So 1 degree is ~ 300 microns