Adaptation I Flashcards
What characterises the luminance of a retinal image?
Surface illumination: the amount of light falling onto an object.
Relative surface reflectance (albedo): the proportion of light reflected back from the surface.
The retina is tasked with converting this image into a useable neural signal.
What characterises the output range of the retina?
Retinal ganglion cells have a limited response range (approx 1-300 strikes per second).
These have to accommodate large changes in mean luminance (10^9 range).
Maintain sensitivity to differences in luminance within scene (typically less than 10^3 range).
Outline the changes that the pupil undergoes in visual adaptation.
By controlling the pupil size, the amount of light entering is regulated.
Pupillary light reflex: high intensity light decreases pupil size, whereas low intensity light causes the pupil size to increase to allow more light in.
However this can only explain a 10-20 fold change in light intensity in the retina, failing to explain the other 8-fold.
In cats, vertical slits pupils are more effective (e.g. 135-fold change).
How can duplex retinal function increase the retina’s operating range?
Through having two receptor systems specialised for different lighting conditions, this increases the operating range of the retina.
One for scotopic vision: low light conditions, driven by rods, and one for photopic vision: high light conditions, mediated by cones.
Outline the functions of rods in the eye
Highly light sensitive, high convergence, and saturated under daylight conditions.
Outline the function of cones in the eye
Less light sensitive, low convergence, can respond in high light conditions.
How is the visual system adapted for scotopic conditions?
Spatial distribution of photoreceptors:
Rods: none at fovea, peak density at around 15 deg eccentricity
Cones: highest density at fovea, some in periphery of retina.
What are key features of the dark adaptation?
Increase in visual sensitivity over time.
Two distinct branches due to transition from cones to rods.
Within each branch there is significant adaptation, cones sensitivity adjustment takes 4-5 minutes, and rod sensitivity adjustment takes 20-30 mins.
How does dark adaptation change rod and cone sensitivity?
Bleaching/regeneration of photopigment - more rod and cones are gradually regenerated (increases ability to respond to light).
Rhodopsin recovery (rod pigment) slower than 3 types of cone photopigments (S,M,L).
What is ‘bleaching’ and ‘regeneration’ of photopigments?
Bleaching: light absorbed by rhodopsin causes retina to change shape and releases opsin (light sensitive protein).
Regeneration: slow conversion of all-trans retinal to its 11-cis form in pigmented epithelium; requires isomerase enzyme and ATP.
How do the mechanisms for light adaptation differ to that for dark adaptation?
In light adaptation, the changes are the same but in the opposite direction, i.e. pupil constricts to reduce light input, shift to photopic system (cones) and reduction in photoreceptor sensitivity.
What are some real life applications of light and dark adaptation?
Stargazers use red torches to avoid bleaching the rod system once the eye is dark adapted.
Some fighter pilots wear red glasses before a night flight to dark adapt before entering dark conditions, as rods aren’t sensitive to red light.
What are perceptual consequences of light/dark adaptation?
Perception of ‘lightness’ doesn’t simply reflect the absolute luminance of an object.
Luminance contrast is more important: the difference between the object’s luminance and its background.
Gain control: when the retina operating range shifts at the mean light level in the photopic range. Results in responses representing contrast not intensity.
What is Weber’s Law?
The just discriminable difference (JND) in luminance is a constant proportion of the background luminance.
Negative afterimages due to light/dark adaptation
- Colour opponency: complementary coloured aftereffects occur due to the opponent nature of chromatic pathways. (Red/Green and Blue/Yellow).
Van Lier, Vergeer and Anstis (2009): ‘filling in’ of afterimage.
