Naked Science Forum
Life Sciences => Physiology & Medicine => Topic started by: sazr on 10/04/2019 00:50:42
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Do Rod photo-sensitive cells contribute at all to daytime vision?
If so, what exactly do they contribute? For example, they contribute towards big homogenous shapes, slow moving big objects, etc. I'm interested in whether this applies to any animal not just humans.
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Both rods and cones are effective during twilight.
Because the frequency sensitivity of rods is different from the three cones, it provides a degree of additional color discrimination in low light levels.
See: https://en.wikipedia.org/wiki/Color_vision#Physiology_of_color_perception
In terms of distribution on the retina, the center of vision (as used for reading or making out fine details) has a very high density of cone cells. There are few rods in this region - the highest density of rod cells is at about 20° from the center of vision, where there are fewer cone cells.
So rod cells, if they are effective at all in broad daylight (ie not saturated), will be more useful for peripheral vision.
See: http://hyperphysics.phy-astr.gsu.edu/hbase/vision/rodcone.html#c2
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Rods are far more numerous in the retina than their cone cousins. But other than sheer numbers, there are some important distinctions between the two photoreceptor types that, for want of a better phrase, turn light waves into brain waves.
First, the function of the two cell types differs. Cones come in three main "flavours", which respond best to three different wavelengths or colours of light: red, green and blue. This is achieved by loading the cones with a pigment called rhodopsin, the recipe for which is varied according to the cone colour type. The relative responses of different colour-sensing cones to the light falling upon each region of the retina produce the colour experience that the brain sees. So a surface that looks, say, yellow, is activating the red and green cones roughly equally, and the blue cones hardly at all. This combination of activity is interpreted by the brain as "yellow".
But sensing light like this is very light-hungry, so cones require bright conditions to work, which is why we see colours poorly, if at all, at night.
The other reason that we perceive colours poorly at night is that rods carry a different light-sensing pigment called scotopsin. This is excrutiatingly sensitive to light, making rods very light-responsive, especially at green wavelengths, but it comes in only a single flavour, so all rods can do is to detect a spot of light, which your brain thinks looks white. So rods cannot discriminate colour.
The distribution of the two cell types is also quite different. Cones are concentrated in the central part of the retina; this is the macula or "yellow spot" that we use for detailed vision, because the cones have small receptive fields, meaning that they are activated by light falling on only a small part of the retina. But elsewhere in the retina, cones are less abundant and rods - with bigger receptive fields and therefore poorer acuity - predominate.
So, at a very basic level, during the day and in brightly-lit conditions, the more-detailed cone-mediated vision dominates and the rods are left idle; at night, when there's insufficient light to drive cones, vision is chiefly rod-mediated, which comes at the cost of absent colour discrimination and lower acuity.
But, there is a clever trick that the retina can pull to extend the range of light levels over which it can operate in full colour mode: at low - but not dark - light conditions, cones can electrically couple themselves through cell-to-cell connections called "gap junctions" to nearby adjacent rods. This means that the electrical activity in the rod cell can be shared with the cone, facilitating its activity. So even though there may be insufficient light to activate a cone on its own, the input from the rod can help to push it over its threshold and activate it. This means you can still see in colour but at light levels below which a cone could not normally operate. The downside of doing this is that the retina has to surrender some acuity for the sake of preserving colour vision. This is one reason why people speculate that accidents are more common at daybreak and dusk.