NS 39 - Retinal Function + Visual Perception (ii)
Snellen Chart
tests visual acuity (ability to distinguish between 2 nearby points)
placed 20 ft away from patient
normal vision: 20/20
50% visual acuity: 20/40
poor visual acuity: 20/200
PhotoRs
detect photons (electromagnetic radiation)
long wavelength = red
short wavelength = violet
rods: light intensity (amplitude) - can detect a single photon (causes 2% reduction in dark current)
cones
rod-cone distribution varies across retina
more rods in peripheral areas
more cones in macula, only cones in fovea
consist of outer segment (R region, membranous photopigment containing plate/disc) + inner segment (mitochondria + other organelles)
each photoR synapses with a bipolar cell, with synapses with an axon
rods have low acuity because many rods synapse on a few bipolar cells which synapse on 1 ganglion cell -> larger receptive field
phototransduction in rods
= converting light to an electric signal
1) Rhodopsin activation
photosensitive pigment
light triggers conformational change
2) transducin activation
3) phosphodiesterase activation
4) decreased cGMP levels, Na+ channels close, less glutamate
glutamate inhibits bipolar cells via the Na+ leak
5) bipolar cells depol, release NT
6) AP generated in ganglion cell
colour + sharpness (wavelength)
lower sensitivity (100s of photons required for activation)
less pigment
high acuity
3 classes
short (blue)
medium (green)
long (red) 75% of all cones
decoding wavelength: brain compares relative stimulation to detect colour
rod hyperpolarises
Colour blindness
x-linked recessive
more common in men (possible heterozygosity + compensation in women)
most common: red-green
L + M pigment genes on x chromo
protanope (defective red cones)
deuteranope (defective green cones)
tritanope (defective blue cones)
Eye's sensitivity to light depends on amount of available pigment
dark adaptation
not enough photons to activate cones
20-30mins for rod rhodopsin to regenerate after being in bright area
light adaptation
rhodopsin bleaches
cone info only
scotopic vision: can't distinguish colours in dark
Vit A deficiency: night blindness
Visual processing
Cortical
reinversion
signals from left visual field brought to right visual cortex
fovea overrepresented on topographical map
certain peps of neurons respond to specific object orientation
hardwired, with some plasticity
visual exposure during development matters
parietal lobe: movement perception
temporal lobe: shape + colour perception