W19 Dark Adaptation

what are the 4 ranges of human function

duplex retina

PLR

photochemical adaptation

neural adaptation

what is the duplex retina

the 2 photoreceptor subsystems which doubles the rang eof light levels over which the visual system can operate

when are rods highly sensitive

dim light in low light levels

scotopic

when are cones highly sensitive

high light levels

photopic

what does the pupillary light reflex help with

deal with a range of luminance levels

what is the average size of the pupil in high light levels

2mm

what is the average size of the pupil in low light levels

8mm

which adaptation mechanisms do we require to allow us to function over different light levels

photochemical

neural

what is dark adaptation

increase in sensitivity in response to reduction in illuminance

how long does it take to dark adapt

50 mins

where does adaptation take place

in the retina pre chiasm

what is the usual RGC response limited to

500 spikes/second

what does adaptation allow for RGC

can respond to a large range of intensities

how to test for dark adaptation

by increasing the intensity / threshold of light until the px sees it

describe the dark adaptation function

rod-cone branch

cone branch

rod branch

absolute threshold

what is the rod-cone branch in the dark adaptation function

when rod sensitivity exceeds cone sensitivity

what is the cone branch in the dark adaptation function

when cones are more sensitive and dark adapt faster

what is the rod branch in the dark adaptation function

rods determine the threshold

what is the absolute threshold in the dark adaptation function

the dimmest light seen when dark adapted

why do cones dark adapt faster than rods

Cones regenerate their photopigment (opsins) more quickly than rods regenerate rhodopsin

what is a bleached photopigment

photopigment loses colour

what is an unbleached photopigment

an intact photopigment

what is photochemical adaptation

when the unbleached photopigment absorbs light

phototransduction

photopigment breaks down and is bleached

photopigment cannot absorb light

describe photopigment regeneration

all trans retinal diffuses into RPE

isomered into 11-cis

diffuses into photoreceptors

recombines with opsin

absorbs photon

how is photochemical adaptation investigated

using retinal densitometry

when does neural adaptation take place

within first 0.3 seconds of dark adaptation

what limits threshold during dark adaptation

concentration of unbleached photopigment

concentration of photoproducts bleaching

how does the concentration of 'unbleached photopigment' limit threshold

Bleached pigment can't absorb light, need unbleached pigment for vision

eg: in bright light more bleached pigments, cant absorb light, higher thresholds

what are the photoproducts of bleaching

metarhodopsin II

opsin

how does the concentration of 'photoproducts bleaching' limit threshold

bleaching the photopigment makes it temporarily nonfunctional

The breakdown products accumulate and must be cleared before new rhodopsin can form.

what did Dowling Rushton state

the log sensitivity during dark adaptation is linearly related to the concentration of bleached photopigment

what are the factors affecting dark adaptation function (DAF)

retinal location of target

target size

target wavelength

adapting light wavelength

adapting light intensity and duration

how does the retinal location of target affect the DAF

sensitivity improves with eccentricity

higher rod density in the periphery

no cones

how does the target size affect the DAF

Threshold for detecting a small target is higher in the early stages of dark adaptation (since rods are still regenerating), and it will take longer for the eye to become sensitive enough to detect small targets.

Larger targets are more likely to be detected early in the dark adaptation process, meaning they lower the detection threshold earlier, contributing to a quicker perception of the target.

how does target wavelength affect the DAF

Cone peak sensitivity longer wavelength than rods even in dark adapted eye

how does adapting light wavelength affect the DAF

Red adapting light will not bleach much rhodopsin bc rods insensitive to light

Rods remain sensitive when lights are off

how does adapting light intensity and duration affect the DAF

Longer / intense bleaching light results in: higher initial threshold, more prominent cone branch, later rod-cone break, longer time to reach final threshold

what are the factors which affect the rate of dark adaptation

age

retinal disease

systemic conditions

how does age affects rate of DA

miosis = retinal illuminance decraesed, threshold increased

night myopia = eye focuses 1m in the dark w no visual stimulus

low rate of photopigment regeneration = slower DA

examples of retinal disease which affect DA

AMD

open angle glaucoma

Vit A deficiency

RP

CSNB

how does AMD affect DA

photoreceptor death, thickened bruch's membrane = RPE & photoreceptor damage

how does OAG affect DA

RGC loss, coincides w area of max rod sensitivity, reduced scotopic sensitivity

how does vit A deficiency affect DA

part of rhodopsin, reduces conc, slows regeneration, raises threshold

how does RP affect DA

AR, AD, X-linked, shortening of rod and cone outer segments due to disc shedding

how to investigate causes of poor night vision

1. Goldmann adaptometer

2. Computer based

3. Photostress test