The Rods and Cones on Equal Terms

Question

Inside our eyes are two different kinds of receptors:

• The rod, in which the human eye has 120 million and work in low light
• The cone, in which the human eye has only 6 to 7 million of them, the receptor that brings us color.

Before you bring up the tetrachromats, yes, I have read about the humans who can see 100 million shades of colors, but for this question, I'm going some steps further.

Let's say that a predatory animal has equal number of rods and cones--say, 400 million each. What would color vision and night vision look like? Would it affect the eyes' attention to detail in any way?

Answer 1

Rods, while numerous, are interconnected so that receiving a photon on any one is perceptually indistinguishable from receiving a photon on any other in its group, increasing sensitivity at the cost of resolution. Without knowing how interconnected the predatory animal's rod cells are, or whether it has structures such as a tapetum lucidum, we cannot say if this animal's night vision is better, equivalent or worse than humans, who have approximately the same numbers of rod cells.

Cones are typically not interconnected, so that stimulation of each cone results in perception of a smaller point of light. Having far more cone cells than human eyes would mean that colour vision had significantly higher resolution than human eyes, quite possibly over a wider field of view than human eyes. Such an animal would likely not have a fovea, but would have vision of equal acuity across its entire field of view.

However such visual acuity would not be without cost. The optic nerves would be especially thick, reducing the potential for movement of the eyes within their sockets, though some animals such as owls already have such a limitation.

In addition, the processing requirements for such a large number of cones in the retina would mean that the animal's brain's visual cortex would necessarily be greatly enlarged. This would come at the cost of either less brain volume being available for other mental functions, making the animal more stupid and/or dextrous than other species with similar-sized brains, or at the cost of having a larger brain, and thus having a higher basal metabolism and a greater nutritional requirement for the proteins and lipids necessary for growing such a large brain.

Answer 2

This page says:

The retina of nocturnal animals is almost entirely composed of rods. The other type of vision cells, cones, is absent or almost absent, leaving nocturnal animals with virtually no color vision.... However, despite being more sensitive to light, the low number of cones means nocturnal animals have sacrificed visual acuity. They must get by with somewhat fuzzy, unfocused images. Only by greatly exaggerating the size of their eyes (and therefore the retinal image), can dark-adapted animals develop reasonable resolution to their images.

So I'd imagine that with a lot of rods and an equal number of cones, the eyes could catch a lot more detail.

The same page, in the ellipsis, also mentions that the decay rate of the photosensitive pigment is important:

The photosensitive pigment inside the rods, rhodopsin, is particularly sensitive to low levels of light. During the day, in a daylight adapted eye, the rhodopsin breaks down so rapidly, it is ineffective for visual perception. At night-time, in the rod-rich eyes of dark-adapted animals, rhodopsin is created faster than it breaks down. Therefore, the threshold of light needed to stimulate the eye is reduced. It is just a minute fraction of the light needed to activate a cone cell for vision during the day.