Light detection and dark adaptation Flashcards
Where is the lateral geniculate nucleus located?
Thalamus
What carries the stimulus from the thalamus to the visual cortex?
The optic radiations carry the stimulus from the thalamus to the visual cortex
What is the foveal region specialised for?
High spatial resolution (visual acuity) and colour detection
Which specialised neuronal cells are present in the fovea to facilitate increased visual acuity?
Midget / Slow-transmitting ganglion cells
What do fast transmitting ganglion cells detect?
Luminance
Motion detection
Where are fast transmitting ganglion cells located in the retina?
Everywhere (apart from fovea), they dominate most of the retina
Where are slow transmitting ganglion cells located?
Fovea
Where in the retina is spatial resolution the highest?
Fovea, it declines towards the peripheral retina
What is flicker detection?
Ability to detect two stimuli separated in time
Which type of photoreceptor is involved in spatial resolution?
Cone are predominantly involved in spatial resolution at the fovea
Which type of photoreceptor is involved in flicker detection?
Rods
With regards to flicker detection, which test is used to predict outcomes of cataract surgery?
Critical Flicker Fusion Frequency Test
When is critical flicker fusion frequency (CFF) test useful to use a a predictor of cataract surgery outcomes?
In cases where there is comorbidity of macular disease in cataracts
How is CFF useful predicting outcomes in patients with cataract and macular disease?
CFF is unaffected by cataract but would be affected by foveal disease
Which type of photoreceptor is involved in colour detection?
Cone photoreceptor
What are the three types of cone photoreceptors?
Long wavelength (red) Medium wavelength (green) Short wavelength (blue)
What is chromaticity?
Refers to the ‘quality of the colour’ or colourdness
What factors influence chromaticity?
- Hue
- Saturation
- Intensity of light
How can colour vision be tested?
Farnsworth-Musel 100 hue test
At what age is peak ability to detect colour hues?
19
In which area of the brain does ‘shape processing’ occur?
Visual cortex
In which area of the brain does ‘recognition’ of shapes occur?
Infero-temporal cortex
What is prosopagnosia?
Inability to recognise familiar faces
How much quanta of light is required to strike the cornea for a signal to be detected?
50-150 quanta of light
What percentage of light striking the cornea reaches the photoreceptors?
10%
What other factors can ‘stimulate’ photoreceptors aside from a photon?
Random opening of ion channels
Stray or scattered light from the stimulus itself
What does the minimum visual stimuli for photoreceptors depend on?
Whether the stimulus is viewed in the dark or light
In the dark, are photoreceptors more or less sensitive to light stimulus?
More sensitive
In the dark: how long does it take for photoreceptors to reach the minimum light threshold for stimulation (i.e maximum sensitivity)?
30 minutes
What is the order of increasing photoreceptor sensitivity in dark adaptation?
- First, cones become more sensitive to light stimulus
2. Secondly, rods become more sensitive to light stimulus
How long does it take to regenerate rods in dark adaptation?
30 minutes
What is dark adaptation and regeneration of rhodopsin dependent on?
local concentration of 11 cis- retinal
Where is the light-dark cycle generated?
suprachismatic nuclei
Which hormone is involved in the light-dark cycle?
Melatonin
Where is Melatonin produced?
Pineal gland
Besides the pineal gland, where else can melatonin be produced?
Retina
Bone marrow
What is melatonin synthesised from?
Tryptophan
What is the function of melatonin?
Provides information to the rest of the body regarding the day and night, to allow organisation of physiological functions, thereby permitting seasonal or diurnal variations in immune defence functions, secretion of hormones, glucose regulation, secretion of ocular aqueous secretion etc
Is melatonin secreted mainly in the day or night?
Night
What controls the rhythm of secretion of melatonin?
Supra-chiasmatic nuclei
How can the diurnal variation of the levels of melatonin be investigated?
By measuring the levels of melatonin in plasma/saliva
or by measuring the hepatic metabolite of melatonin: sulphatoxymelatonin
How long is melatonin secreted for?
It adjusts to night length
How many quanta of light is required to induce a detectable response in photoreceptors?
120 quanta per second
If the stimulus is instantaneous, how much quanta of light is required to induce a detectible response in photoreceptors?
5-15 quanta of light
Is stimulation of a single photoreceptor sufficient to detect a stimulus?
No
How many photoreceptors must be stimulated in order to detect a stimuli?
10-15 rods, then the collective response must be summed up at the level of a bipolar cell or ganglion cell to induce a visual sensation
Define Ricco’s law:
The threshold intensity of a stimulus is inversely proportional to the area of stimulus (i.e. if the area of stimulus is large then a low intensity stimulus will suffice in generating a response)
What is Bloch’s law (Temporal summation)?
The intensity of the threshold stimulus is inversely proportional to the duration of the stimulus (i.e frequent rapid stimuli will generate the same level of response as if the stimulus was presented for a long period of time)
What is binocular summation?
Summation occurring from visual stimuli in both eyes
